IGS Bibliography BibDesk 131119.bib

IGS Real Time Infrastructure: From Pilot Project to Operational Service
L. Agrotis and M. Caissy and G. Weber and M. Ge and K. MacLeod and M. Hernández-Pajares
ESOC's RETINA System and the Generation of the IGS RT Combination
L. Agrotis and P. A. Sanz and J. Dow and R. Zandbergen and D. Svehla and A. Ballereau
Consistency evaluation of space geodetic techniques via ITRF combination
Z. Altamimi and X. Collilieux and L. Métivier
ITRF2008 plate motion model
Z. Altamimi and L. Métivier and X. Collilieux
IGS contribution to the ITRF
Z. Altamimi and X. Collilieux
We examine the contribution of the International GNSSService (IGS) to the InternationalTerrestrialReference Frame (ITRF) by evaluating the quality of the incorporated solutions as well as their major role in the ITRF formation. Starting with the ITRF2005, the ITRF is constructed with input data in the form of time series of station positions (weekly for satellite techniques and daily forVLBI) and daily Earth Orientation Parameters. Analysis of time series of station positions is a fundamental first step in the ITRF elaboration, allowing to assess not only the stations behavior, but also the frame parameters and in particular the physical ones, namely the origin and the scale. As it will be seen, given the poor number and distribution of SLR and VLBI co-location sites, the IGS GPS network plays a major role by connecting these two techniques together, given their relevance for the definition of the origin and the scale of the ITRF. Time series analysis of the IGS weekly combined and other individual Analysis Center solutions indicates an internal precision (or repeatability) <2mm in the horizontal component and <5mm in the vertical component. Analysis of three AC weekly solutions shows generally poor agreement in origin and scale, with some indication of better agreement when the IGS started to use the absolute model of antenna phase center variations after the GPS week 1400 (November 2006).
Quality assessment of the ITRF2008
Z. Altamimi and X. Collilieux and L. Métivier
ITRF2008 & the IGS contribution
Z. Altamimi
Accuracy Assessment of the ITRS 2008 Realization of DGFI: DTRF2008
Z. Altamimi and X. Collilieux and M. Seitz and D. Angermann and H. Drewes
  138  87-93  (2013)
The DTRF2008 is a realization of the ITRS computed by the ITRS Combination Centre at DGFI. It is based on the same input data as the ITRF2008. In order to assess the internal and external accuracy of DTRF2008 several validation procedures are applied which are based on comparisons with technique-only multi-year solutions (for assessing the internal accuracy) and comparisons with ITRF2008 and ITRF2005 (for assessing the external accuracy). The analysis is done separately for the four space-geodetic techniques GPS, VLBI, SLR and DORIS. The internal accuracy for station positions is between 0.6 and 3.3mm and the external accuracy between 7 and 10mm depending on the space technique. For the velocities the internal accuracy is between 0.25 and 1.0mm/a and the external between 0.2 and 2.0mm/a.
ITRF2008 ERP comparisons with IGS repro1
Z. Altamimi
Ionosphere Effect Mitigation for Single-frequency Precise Point Positioning
C.-O. Andrei and R. Chen and H. Kuusniemi and M. Hernandez-Pajares and J. M. Juan and D. Salazar
Precise Point Positioning (PPP) is a stand-alone positioning technique that has continuously increased the interest of the GNSS community in the last years as the accuracy of the precise satellite orbit and clock data products has achieved centimeter accuracy level. Combining these products with a dual-frequency GNSS receiver data, PPP is able to provide solutions at centimeter to decimeter level. Apart of the dual-frequency receiver users, there is a broad range of applications that use single-frequency receivers. For these users, ionosphere represents the most critical source of error. This paper investigates PPP performance using single-frequency code and carrier-phase data with different approaches to mitigate the ionosphere effect, such as broadcast ionospheric model, Global Ionospheric Maps or Wide-Area Real Time Kinematic derived ionospheric correction. Stations covering different latitudes and solar activity periods (high, medium, low) are considered. The results show that sub-meter positioning accuracy can be obtained.
Assessment of time-series of troposphere zenith delays derived from the Global Data Assimilation System numerical weather model
C.-O. Andrei and R. Chen
  13  109-117  (2009)
Troposphere zenith path delays derived from the Global Data Assimilation System (GDAS) numerical weather model (NWM) are compared with those of the International GNSS Service (IGS) solutions over a 1.5-year period at 18 globally distributed IGS stations. Meteorological parameters can be interpolated from the NWM model at any location and at any time after December 2004. The meteorological parameters extracted from the NWM model agree with in situ direct measurements at some IGS stations within 1 mbar for pressure, 3$\,^{\circ}$ for temperature and 13% for relative humidity. The hydrostatic and wet components of the zenith path delay (ZPD) are computed using the meteorological parameters extracted from the NWM model. The total ZPDs derived from the GDAS NWM agree with the IGS ZPD solutions at 3.0 cm RMS level with biases of up to 4.5 cm, which can be attributed to the wet ZPDs estimates from the NWM model, considering the less accurate interpolated relative humidity parameter. Based on this study, it is suggested that the availability and the precision of the GDAS NWM ZPD should be sufficient for nearly all GPS navigation solutions.
Methodology for the combination of sub-daily Earth rotation from GPS and VLBI observations
T. Artz and L. Bernhard and A. Nothnagel and P. Steigenberger and S. Tesmer
  86  221-239  (2012)
A combination procedure of Earth orientation parameters from Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI) observations was developed on the basis of homogeneous normal equation systems. The emphasis and purpose of the combination was the determination of sub-daily polar motion (PM) and universal time (UT1) for a long time-span of 13 years. Time series with an hourly resolution and a model for tidal variations of PM and UT1-TAI (dUT1) were estimated. In both cases, 14-day nutation corrections were estimated simultaneously with the ERPs. Due to the combination procedure, it was warranted that the strengths of both techniques were preserved. At the same time, only a minimum of de-correlating or stabilizing constraints were necessary. Hereby, a PM time series was determined, whose precision is mainly dominated by GPS observations. However, this setup benefits from the fact that VLBI delivered nutation and dUT1 estimates at the same time. An even bigger enhancement can be seen for the dUT1 estimation, where the high-frequency variations are provided by GPS, while the long term trend is defined by VLBI. The estimated combined tidal PM and dUT1 model was predominantly determined from the GPS observations. Overall, the combined tidal model for the first time completely comprises the geometrical benefits of VLBI and GPS observations. In terms of root mean squared (RMS) differences, the tidal amplitudes agree with other empirical single-technique tidal models below 4 μas in PM and 0.25 μs in dUT1. The noise floor of the tidal ERP model was investigated in three ways resulting in about 1 μas for diurnal PM and 0.07 μs for diurnal dUT1 while the semi-diurnal components have a slightly better accuracy.
Impact of different individual GNSS receiver antenna calibration models on geodetic positioning
Q. Baire and E. Pottiaux and C. Bruyninx and P. Defraigne and W. Aerts and J. Legrand and N. Bergeot and J. M. Chevalier
Instantaneous Cycle-Slip Correction for Real-Time PPP Applications
Real-time precise point positioning (PPP) is limited to only a few applications using a moving receiver because the quality of the solution is vulnerable to interruptions in signal tracking. A loss of lock on all GPS signals simultaneously implies that users may have to wait for several minutes before again obtaining cm-level precision. To avoid such a scenario, this paper proposes a method to instantaneously mitigate the impacts of signal interruptions and the resulting cycle slips. The approach is based on a time-differenced solution that allows for estimating the size of cycle slips in a least-squares adjustment. Once cycle slips are corrected, the PPP filter can be modified accordingly so as to prevent the occurrence of discontinuities in the positioning time series. The usefulness of the approach is demonstrated in selected applications such as geodynamics and car navigation.
Impact of SLR tracking of GPS
Y. Bar-Sever and J. L. Davis and R. Dach and C. Flohrer and T. Herring and J. Ray and J. A. Slater and D. Thaller
Near-simultaneous great earthquakes at Tongan megathrust and outer rise in September 2009
J. Beavan and X. Wang and C. Holden and K. Wilson and W. Power and G. Prasetya and M. Bevis and R. Kautoke
The Earth's largest earthquakes and tsunamis are usually caused by thrust-faulting earthquakes on the shallow part of the subduction interface between two tectonic plates, where stored elastic energy due to convergence between the plates is rapidly released. The tsunami that devastated the Samoan and northern Tongan islands on 29 September 2009 was preceded by a globally recorded magnitude-8 normal-faulting earthquake in the outer-rise region, where the Pacific plate bends before entering the subduction zone. Preliminary interpretation suggested that this earthquake was the source of the tsunami. Here we show that the outer-rise earthquake was accompanied by a nearly simultaneous rupture of the shallow subduction interface, equivalent to a magnitude-8 earthquake, that also contributed significantly to the tsunami. The subduction interface event was probably a slow earthquake with a rise time of several minutes that triggered the outer-rise event several minutes later. However, we cannot rule out the possibility that the normal fault ruptured first and dynamically triggered the subduction interface event. Our evidence comes from displacements of Global Positioning System stations and modelling of tsunami waves recorded by ocean-bottom pressure sensors, with support from seismic data and tsunami field observations. Evidence of the subduction earthquake in global seismic data is largely hidden because of the earthquake's slow rise time or because its ground motion is disguised by that of the normal-faulting event. Earthquake doublets where subduction interface events trigger large outer-rise earthquakes have been recorded previously, but this is the first well-documented example where the two events occur so closely in time and the triggering event might be a slow earthquake. As well as providing information on strain release mechanisms at subduction zones, earthquakes such as this provide a possible mechanism for the occasional large tsunamis generated at the Tonga subduction zone, where slip between the plates is predominantly aseismic.
Sea level variations at tropical Pacific islands since 1950.
M. Becker and B. Meyssignac and C. Letetrel and W. Llovel and A. Cazenave and T. Delcroix
Global and Planetary Change  80-81  85-98  (2012)
The western tropical Pacific is usually considered as one of the most vulnerable regions of the world under present-day and future global warming. It is often reported that some islands of the region already suffer significant sea level rise. To clarify the latter concern, in the present study we estimate sea level rise and variability since 1950 in the western tropical Pacific region (20$\,^{\circ}$S--15$\,^{\circ}$N; 120$\,^{\circ}$E--135$\,^{\circ}$W). We estimate the total rate of sea level change at selected individual islands, as a result of climate variability and change, plus vertical ground motion where available. For that purpose, we reconstruct a global sea level field from 1950 to 2009, combining long (over 1950--2009) good quality tide gauge records with 50-year-long (1958--2007) gridded sea surface heights from the Ocean General Circulation Model DRAKKAR. The results confirm that El Niño-Southern Oscillation (ENSO) events have a strong modulating effect on the interannual sea level variability of the western tropical Pacific, with lower/higher-than-average sea level during El Niño/La Niña events, of the order of $\pm$ 20--30 cm. Besides this sub-decadal ENSO signature, sea level of the studied region also shows low-frequency (multi decadal) variability which superimposes to, thus in some areas amplifies current global mean sea level rise due to ocean warming and land ice loss. We use GPS precise positioning records whenever possible to estimate the vertical ground motion component that is locally superimposed to the climate-related sea level components. Superposition of global mean sea level rise, low-frequency regional variability and vertical ground motion shows that some islands of the region suffered significant `total' sea level rise (i.e., that felt by the population) during the past 60 years. This is especially the case for the Funafuti Island (Tuvalu) where the ``total'' rate of rise is found to be about 3 times larger than the global mean sea level rise over 1950--2009.
GNSS absolute antenna calibration at the National Geodetic Survey
A. L. Bilich and G. L. Mader and C. Geoghegan
Precise Point Positioning: A Powerful Technique with a Promising Future
S. B. Bisnath and Y. Gao
Atmospheric & oceanic forcing of the rapid polar motion
C. Bizouard and L. Seoane
The rapid polar motion for periods below 20 days is revisited in light of the most recent and accurate geodetic and geophysical data. Although its amplitude is smaller than 2 mas, it is excited mostly by powerful atmospheric processes, as large as the seasonal ones. The residual amplitude, representing about 20% of the total excitation, stems from the oceans.Rapid polar motion has an irregular nature that is well explained by the combined influence of the atmosphere and the oceans. An overall spectrum reveals cycles principally at 20, 13.6 (fortnightly tidal period) and 10 days (corresponding to the normal atmospheric mode 1 3 ), but this is only an averaged feature hiding its strong variability over seasonal time scales. This explains why it is so delicate to determine an empirical model of the tidal effect on polar motion. The variability in both amplitude and phase of the 13.6-day term is probably caused by a lunar barometric effect, modulated by some sub-seasonal thermal processes. The irregularities of the prominent cycles of the short-term polar motion are well explained by the atmospheric and oceanic excitations. The oceanic variability reinforces the atmospheric one, as they were triggered by the same agent, maybe seasonal and inter-annual thermal variations.
High-frequency signals of oceans and atmosphere in Earth rotation
S. Böhm
The high resolution Water Vapour model on the area of Poland
J. Bosy and J. Kapłon and J. Sierny and W. Rohm and M. Ryczywolski and T. Hadaś and A. Oruba and K. Wilgan
Global Navigation Satellite Systems (GNSS) are designed for positioning, navigation and amongst other possible applications it can also be used to derive information about the state of the atmosphere. Continuous observations from GNSS receivers provide an excellent tool for studying the neutral atmosphere, currently in near real time. The Near Real Time neutral atmosphere and water vapour distribution models are currently obtained with high resolution from Ground Base Augmentation Systems (GBAS), where reference stations are equipped with GNSS and meteorological sensors. The Poland territory is covered by dense network of GNSS stations in the frame of GBAS system called ASG-EUPOS (www.asgeupos.pl). This system were established in year 2008 by Head Office of Geodesy and Cartography in the frame of EUPOS project (www.eupos.org) for providing positioning services. The GNSS data are available from 130 reference stations located in Poland and neighbour countries. The ground meteorological observations in the area of Poland and neighbour countries are available from ASG-EUPOS stations included in EUREF Permanent Network (EPN) stations, airports meteorological stations (METAR messages stations), and stations managed by national Institute of Meteorology and Water Management (SYNOP messages stations). The first part of the paper present the methodology of NRT GNSS data processing for ASG-EUPOS stations for Zenith Total Delay (ZTD) estimation. The second part is covering analysis of meteorological parameters interpolation methods for determination of Zenith Hydrostatic Delay (ZHD). The last part concerns the modelling of water vapour distribution over the area of Poland.
Verification of the meteorological observations on the EPN stations
J. Bosy and W. Rohm
The EUREF Permanent Network (EPN) consist of 200 stations, almost 40 % of these stations have meteorological equipment. The observations obtained from meteorological packages could be used to estimate the tropospheric delay. The fluctuations in meteorological parameters cause fast changes of the tropospheric delay, as a correlated value, and in consequence difficulties in GPS heights determination. The meteorological parameters from meteorological equipment on the EPN stations and the additional reference data from World Meteorological Organization were two in situ data sources. The values from WMO server consist of the daily mean value of the most important weather characteristics. Among these data also appears: the pressure, the temperature and the humidity. These data and values obtained from Global Pressure and Temperature (GPT) model were used as basis for the meteorological data verification on EPN/IGS stations. The accuracy and the reliability of the meteorological data sets were estimated. This paper presents the procedures and the results of meteorological data verification on the all EPN stations equipped with meteorological packages.
Atmosphere model on the area of GBAS system for real-time GNSS and meteorological applications
J. Bosy and W. Rohm and J. Kaplon and J. Sierny and T. Hadas
Global Navigation Satellite System (GNSS) has been designed for positioning, navigation, amongst other possible applications. A number of GNSS applications require precise positioning with centimetre accuracy in real time. Precise positioning in this mode is currently being implemented by two methods: differential RTK and autonomous PPP. These methods also allow determination of points coordinates changes (movements) and are therefore used in studies of crustal deformation, such as GNSS seismology or landslides monitoring. The positioning using RTK or PPP method can be supported by Ground Base Augmentation System (GBAS), which provide better results stability in the area of GBAS network. One of the elements supporting precise positioning, especially for height component is a model of the atmosphere, carried out for the GBAS network area. Since 2008 in the territory of Poland a Ground Base Augmentation System GBAS) called ASG-EUPOS is working. This system gathers permanently the GNSS data from 130 stations and meteorological (temperature, pressure and relative humidity) data from 17 stations. The average distance between GNSS stations is 70 km. The GNSS and meteorological data from the ASG-EUPOS network are the basis for building a model of the atmosphere in near real-time. The spatial structure and temporal behaviour of the atmosphere (mainly water vapour in the atmosphere) is modelled using the GNSS tomography method. Principal purpose of this paper is the presentation of the methodology of the integrated investigations for near real time (NRT) atmosphere model construction based on the GNSS and meteorological observations from ASG-EUPOS stations. Second aim covers the discussion of procedure and results of GNSS data processing and Numerical Weather Prediction models (NWP) derived products. The last aim covers the results of armosphere monitoring system based on the GNSS tomography method.
NRT atmosphere model based on the ground GNSS permanent networks for meteorological and positioning applications
J. Bosy and W. Rohm and J. Kapon and J. Sierny
Global Navigation Satellite System is designed for positioning, navigation, amongst other possible applications it can also be used to derive information about the state of the atmosphere, what is now recognized as GNSS meteorology. Particularly GNSS meteorology is the remote sensing of the atmosphere from satellite platform (GNSS radio occultation meteorology) and ground permanent stations (ground based GNSS meteorology). The ground based GNSS meteorology is investigated by research group of the Institute of Geodesy and Geoinformatics of Wroclaw University of Environmental and Life Sciences since 1995. The spatial structure and temporal behaviour of the troposphere (mainly water vapour in the troposphere) is modelled using the GNSS tomography method. It is important product for GNSS positioning services (in post-processing and real time mode) and meteorological application (additional meteorological monitoring tool). Since 2008 in the territory of Poland a Ground Base Augmentation System GBAS) called ASG-EUPOS is working. This system gathers permanently the GNSS data from 130 stations and meteorological (temperature, pressure and relative humidity) data from 17 stations. The average distance between GNSS stations is 70 km. Principal purpose of this paper is the presentation of the methodology of the integrated investigations for near real time (NRT) atmosphere model construction based on the GNSS and meteorological observations from ASG-EUPOS stations. Second aim covers the discussion of procedure and results of processing NRT ZTD, NRT SWD and COAMPS derived products . The last aim covers the preliminary results of troposphere monitoring system based on the GNSS tomography method.
The concept of Near Real Time atmosphere model based of GNSS and meteorological data from ASG-EUPOS reference stations
J. Bosy and W. Rohm and J. Sierny
GNSS meteorology is the remote sensing of the atmosphere (particularly troposphere) using Global Navigation Satellite Systems (GNSS) to deliver information about its state. The two currently available navigation satellite systems are the Global Positioning System (GPS) and the GLObal'naya NAvigatsionnaya Sputnikovaya Sistema (GLONASS) maintained by the United States and Russia, respectively. The Galileo navigation satellite system, which is under supervision of the European Space Agency (ESA), is expected to be completed within the time frame of a few years. Continuous observations from GNSS receivers provide an excellent tool for studying the earth's atmosphere. The GNSS meteorology has reached a point, where there is a need to improve methods not only to compute Integrated Water Vapor over the GNSS receiver, but also to investigate the water vapor distribution in space and time (4DWVD). Since 2008, the new national permanent GNSS network ASG-EUPOS (98 stations) has been established in Poland. 17 Polish stations equipped with GNSS receivers and uniform meteorological sensors work currently in the frame of the European Permanent Network. This paper presents the concept of the integrated investigations for NRT atmosphere model construction based on the GNSS and meteorological observations from ASG-EUPOS stations.
A strategy for GPS data processing in a precise local network during high solar activity
J. Bosy and M. Figurski and P. Wielgosz
  7  120-129  (2003)
This paper presents the analyses connected with reduction of errors from ionospheric refraction using GPS data from local satellite networks. This is particularly essential during rising solar activity. The Bernese GPS Software v. 4.2 was used, as an analytical tool. The test data included measurements from a geodynamic network SUDETES situated in the Sudety Mountains across the border between the Czech Republic and Poland. A local ionospheric activity model developed from a regional model augmented with data from a local network has been tested at three levels. The criteria included comparison with a global model, the success rate of ambiguity determination using the quasi-ionosphere free and wide-lane/narrow-lane strategies, and in the position domain through analysis of residuals. The results show that the local model increases the success rate for ambiguity determination for the wide-lane/narrow-lane strategy and is available sooner than the global models. The output of the SUDETES network processing including the models of local ionospheric and tropospheric activity have been used to process data from a number of relatively small networks situated in the Sudety mountains.
Integration and verification of meteorological observations and \{NWP\} model data for the local \{GNSS\} tomography
J. Bosy and W. Rohm and A. Borkowski and K. Kroszczynski and M. Figurski
Atmospheric Research  96  522 - 530  (2010)
\{GNSS\} meteorology applies the Global Navigation Satellite Systems (GNSS) to derive information about the state of the atmosphere (particularly troposphere). The tomography is one of the methods used in \{GNSS\} meteorology. The input data of \{GNSS\} tomography are the signal troposphere delays, results of \{GNSS\} data processing and additionally meteorological observations and Numerical Weather Prediction (NWP) models data. Different types of the input data have heterogeneous character, but give broad spectrum of information. The paper presents the methodology of integration and verification of GNSS, meteorological and \{NWP\} data for \{GNSS\} tomography. Because of the data inhomogeneity, original methods of meteorological data validation and \{NWP\} model calibration are proposed. In order to verify the ground meteorological observations, the differential wavelet method is proposed. The results of analysis show that pressure should be measured as accurately as possible (uncertainty below 1.0 hPa) with homogenous instruments and in the similar conditions. Whereas the temperature could be measured less accurately (uncertainty below 2.4 K), however usage of meteorological radiation shelters (e.g. Stevenson screens) or mathematical algorithms to mitigate the influence of direct insolation is a must. The integration procedure of the \{COAMPS\} \{NWP\} model shows that the model output parameters are squaring with the meteorological observations from the synoptic stations. The best match is found between pressures from both sources, a good match is also found in case of temperature, whereas relative humidity measurements are less consistent. Moreover, the presented methodology could be used for assimilation of meteorological observations into numerical weather models.
Land motion estimates from GPS at tide gauges: a geophysical evaluation
M. N. Bouin and G. Wöppelmann
Geophysical Journal International  180  193--209  (2010)
Space geodesy applications have mainly been limited to horizontal deformations due to a number of restrictions on the vertical component accuracy. Monitoring vertical land motion is nonetheless of crucial interest in observations of long-term sea level change or postglacial rebound measurements. Here, we present a global vertical velocity field obtained with more than 200 permanent GPS stations, most of them colocated with tide gauges (TGs). We used a state of the art, homogeneous processing strategy to ensure that the reference frame was stable throughout the observation period of almost 10 yr. We associate realistic uncertainties to our vertical rates, taking into account the time-correlation noise in the time-series. The results are compared with two independent geophysical vertical velocity fields: (1) vertical velocity estimates using long-term TG records and (2) postglacial model predictions from the ICE-5G (VM2) adjustment. The quantitative agreement of the GPS vertical velocities with the `internal estimates' of vertical displacements using the TG record is very good, with a mean difference of −0.13 $\pm$ 1.64 mm yr−1 on more than 100 sites. For 84 per cent of the GPS stations considered, the vertical velocity is confirmed by the TG estimate to within 2 mm yr−1. The overall agreement with the glacial isostatic adjustment (GIA) model is good, with discrepancy patterns related either to a local misfit of the model or to active tectonics. For 72 per cent of the sites considered, the predictions of the GIA model agree with the GPS results to within two standard deviations. Most of the GPS velocities showing discrepancies with respect to the predictions of the GIA model are, however, consistent with previously published space geodesy results. We, in turn, confirm the value of 1.8 $\pm$ 0.5 mm yr−1 for the 20th century average global sea level rise, and conclude that GPS is now a robust tool for vertical land motion monitoring which is accurate at least at 1 mm yr−1.
The re-analysis of the IGS tropospheric product
S. Byun and Y. Bar-Sever
A new type of troposphere zenith path delay product of the international GNSS service
S. H. Byun and Y. E. Bar-Sever
The International GNSS Service (IGS) has been producing the total troposphere zenith path delay (ZPD) product that is based on combined ZPD contributions from several IGS Analysis Centers (AC) since GPS week 890 in 1997. A new approach to the production of the IGS ZPD has been proposed that replaces the direct combination of diverse ZPD productswith point positioning estimates using the IGSCombined Final orbit and clock products. The new product was formally adopted in 2007 after several years of concurrent production with the legacy product. We describe here the advantages of the new approach for the IGS ZPD product, which enhance the value of the new ZPD product for climate studies.We also address the impact the IGS adoption in November 2006 of new GPS antenna phase center standards has had on the new ZPD product. Finally we describe plans to further enhance the ZPD products.
Coming Soon: The International GNSS Real-Time Service
M. Caissy and L. Agrotis and G. Weber and M. Hernandez-Pajares and U. Hugentobler
GPS World  23  52-58  (2012)
Innovation: Coming Soon
M. Caissy and L. Agrotis and G. Weber and M. Hernandez-Pajares and U. Hugentobler
GPS World      (2012)
The International GNSS Service has embarked on a project to provide a high-accuracy GPS satellite orbit and clock data service in real time. The service will also provide 1-Hz data streams of GPS and GLONASS data from a network of global continuously operating reference stations. The IGS real-time data and orbit and clock products will be of immense benefit for geoscience studies and a host of other science and engineering applications. A team of authors associated with this project discusses the genesis and status of the real-time service and the plans to provide an initial operating capability.
SHA: The GNSS Analysis Center at SHAO
J. Chen and B. Wu and X. Hu and H. Li
Lecture Notes in Electrical Engineering  160  213-221  (2012)
Today, most precise GNSS products, including orbits and clocks, are provided by the International GNSS Service (IGS) and its Analysis Centers (ACs). Each AC provides its products to the AC Coordinator (ACC) for combination. ACs develop their own software packages by implementing different strategies, which as a result improving the robustness of the combined products. Following the IGS AC strategy and to fulfill the requests of satellite missions in China, we set up the GNSS Analysis Center at Shanghai Astronomical Observatory (SHAO). Currently our GNSS routine analysis includes: Global GPS+GLONASS data processing, GLOBAL+CMONOC GPS data processing. In the first routine, we use ~110 global stations, of which ~50 have GLONASS observations, to derive the integrated and consistent GNSS products. In the second routine, we combine the IGS network used the first routine and the Crustal Movement Observation Network of China (CMONOC) network, GPS only solution is performed using ~300 stations. This paper introduces the details of the Analysis Center and presents the latest results.
Rotational evaluations of global geophysical fluid models & improvements in the annual wobble excitation
W. Chen and W. B. Shen
Evaluation of GPS orbit prediction strategies for the IGS Ultra-rapid products
K. Choi and T.-S. Bae and J. Griffiths and J. Ray
Quality assessment of GPS reprocessed terrestrial reference frame
X. Collilieux and L. Métivier and Z. Altamimi and T. van Dam and J. Ray
The International GNSS Service (IGS) contributes to the construction of the International Terrestrial Reference Frame (ITRF) by submitting time series of station positions and Earth Rotation Parameters (ERP). For the first time, its submission to the ITRF2008 construction is based on a combination of entirely reprocessed GPS solutions delivered by 11 Analysis Centers (ACs). We analyze the IGS submission and four of the individual AC contributions in terms of the GNSS frame origin and scale, station position repeatability and time series seasonal variations. We show here that the GPS Terrestrial Reference Frame (TRF) origin is consistent with Satellite laser Ranging (SLR) at the centimeter level with a drift lower than 1 mm/year. Although the scale drift compared to Very Long baseline Interferometry (VLBI) and SLR mean scale is smaller than 0.4 mm/ year, we think that it would be premature to use that information in the ITRF scale definition due to its strong dependence on the GPS satellite and ground antenna phase center variations. The new position time series also show a better repeatability compared to past IGS combined products and their annual variations are shown to be more consistent with loading models. The comparison of GPS station positions and velocities to those of VLBI via local ties in co-located sites demonstrates that the IGS reprocessed solution submitted to the ITRF2008 is more reliable and precise than any of the past submissions. However, we show that some of the remaining inconsistencies between GPS and VLBI positioning may be caused by uncalibrated GNSS radomes.
Consistency of crustal loading signals derived from models & GPS: A Re-examination
X. Collilieux and P. Rebischung and T. van Dam and J. Ray and Z. Altamimi
Evaluation of the scale rate of the GNSS terrestrial reference frame using satellite antenna z-offsets
X. Collilieux and R. Schmid
Strategies to mitigate aliasing of loading signals while estimating GPS frame parameters
X. Collilieux and T. van Dam and J. Ray and D. Coulot and L. Métivier and Z. Altamimi
Quality assessment of GPS reprocessed terrestrial reference frame
X. Collilieux and Z. Altamimi and L. Métivier and T. van Dam
Global sea-level rise and its relation to the terrestrial reference frame
X. Collilieux and G. Wöppelmann
  85  9-22  (2011)
We examined the sensitivity of estimates of global sea-level rise obtained from GPS-corrected long term tide gauge records to uncertainties in the International Terrestrial Reference Frame (ITRF) realization. A useful transfer function was established, linking potential errors in the reference frame datum (origin and scale) to resulting errors in the estimate of global sea level rise. Contrary to scale errors that are propagated by a factor of 100%, the impact of errors in the origin depends on the network geometry. The geometry of the network analyzed here resulted in an error propagation factor of 50% for the Z component of the origin, mainly due to the asymmetry in the distribution of the stations between hemispheres. This factor decreased from 50% to less than 10% as the geometry of the network improved using realistic potential stations that did not yet meet the selection criteria (e.g., record length, data availability). Conversely, we explored new constraints on the reference frame by considering forward calculations involving tide gauge records. A reference frame could be found in which the scatter of the regional sea-level rates was limited. The resulting reference frame drifted by 1.36 $\pm$ 0.22 mm/year from the ITRF2000 origin in the Z component and by −0.44 $\pm$ 0.22 mm/year from the ITRF2005 origin. A bound on the rate of global sea level rise of 1.2 to 1.6 mm/year was derived for the past century, depending on the origin of the adopted reference frame. The upper bound is slightly lower than previous estimates of 1.8 mm/year discussed in the IPCC fourth report.
SIRGAS Analysis Centre-IBGE: new processing and combination strategies and the influence of the global reference change in results
S. M. A. COSTA and A. L. D. SILVA and M. A. D. A. LIMA and N. J. D. M. JÚNIOR
Boletim de Ciências Geodésicas  18  63-85  (2012)
Currently, the SIRGAS (Geocentric Reference System for the Americas) is performed by a permanent GNSS network called SIRGAS- CON, where there are about 240 permanent stations in operation, distributed in South, Central Americas and Caribbean region. The SIRGAS Analysis Centers were established in order to systematically determine the SIRGAS-CON station coordinates, following the standards established internationally in order to support the maintenance of the system and the activities of the SIRGAS Working Group GT-I (Reference System). Since August 2008 the Coordination of Geodesy of the Brazilian Institute of Geography and Statistics - IBGE officially took over the activities of an analysis Center. This is a kind of daily work to which bigher and bigher dedication is given as the number of stations in the South American continent has been increasing rapidly in the recent years. Other results are coming out; the station coordinates time series, thus enabling the determination of the station displacements due to the movement of the earth crust, local movements such as subsidence and / or local crustal uplift caused by natural phenomena such as earthquakes, as well as seasonal effects caused by several factors. At the same time, IBGE also carried out the weekly combination of solutions from nine weekly SIRGAS Analysis Centers. This combination aims at comparing to the results to those obtained by DGFI (Deutsches Forschungsinstitut Geodätisches), which provides the final weekly solution from SIRGAS-CON network. Due to result accuracy, the change of any information in the processing can lead to certain changes in the coordinates and hence discontinuities in the time series of each station. Recently, on April 17, 2011 (GPS week 1632), the orbits (rapid and final fast), the corrections of the satellite clocks and model calibration of antennas provided by the International GNSS Service -- IGS, started to be referred to the new IGS realization, named IGS08. Since then, thereafter, the GPS processing using IGS products will have their results referred to this new reference system, which may cause discontinuities in the coordinates. The paper aims at presenting the processing strategy currently in operation, as well as a new strategy to improve the results. Another aim is to present some results of weekly processing and combination carried out by IBGE, and also to clarify the changes with the adoption of the new version of the Global Reference Network GNSS solutions, the IGS08.
Ionospheric Delay Forecast Using GNSS Data
M. Cueto and E. Sardon and A. Cezon and F. Azpilicueta and C. Brunini
The ionosphere affects signals broadcast by Global Navigation Satellite Systems (GNSS) by delaying the propagation of the code carried by the signal. In general ionospheric effects in mid-latitude regions are not severe, causing only gradual variations in ionospheric delays (except in the presence of magnetic storms) whose magnitude can be roughly predicted. However in the equatorial region ionospheric effects are more severe: added to the higher ionospheric delays in this area, the presence of the equatorial anomaly and other equatorial features complicate the ionospheric modeling and make more difficult the prediction of its main features and its effects on GNSS and other applications that depends on the ionospheric status. Nowadays ionospheric algorithms are mainly focused on post-processing and now cast estimation. However, an ionospheric predicting tool could be very useful for several applications, above all for unstable conditions and for the equatorial region: an algorithm capable of predicting the ionospheric behavior in advance could be used to set up early warnings for different uses, among others civil aviation based on Ground Based Augmentation Systems (GBAS) or Satellite Based Augmentation System (SBAS), the protection of valuable communication satellites from space weather adverse conditions, as well as other technologies affected by space weather, including geophysical exploration and protection of long distance pipelines, HF radio systems, communication and surveillance systems, spacecraft operations, defense needs and alarm systems for safety applications. Taking into account the aforementioned need, a new ionospheric forecasting tool based on the use of GNSS measurements has been developed. This tool provides predicted ionospheric delays for a set of Ionospheric Grid Point (IGP) located in the service area defined by the user. The ionospheric forecast algorithm is based on magicSBAS tool, which is an innovative tool developed by GMV which computes SBAS corrections and additional information required by a SBAS system in real-time to be broadcast to SBAS users. magicSBAS implements multi-constellation (GPS, GLONASS) state-of-the-art algorithms for precise orbit determination and time synchronization, ionospheric delay estimation, SBAS wide area correction computation and SBAS integrity determination. The tool uses as input GNSS raw data in several formats (such as Networked Transport of RTCM via Internet Protocol, NTRIP, Receiver INdependent Exchange, RINEX and European Geostationary Navigation Overlay Service, EGNOS) and provides as output SBAS information compliant with SBAS international standards (International Civil Aviation Organization Standards and Recommended Practices and Radio Technical Commission for Aeronautics DO-229D Minimum Operational Performance Standards) in two ways, as SBAS binary messages for GEO (Geostationary Earth Orbit) broadcast and as SBAS signal-in-space through Internet (SISNET). The forecast algorithm used for ionospheric delays prediction is based on the ionospheric delay estimation from previous epochs using GNSS data and the main dependence of ionospheric delays on solar and magnetic conditions. On account of the fact that the ionospheric behavior is highly dependent on the region of the Earth, different algorithmic modifications have been implemented in GMV´s magicSBAS ionospheric algorithms to be able to estimate and forecast ionospheric delays worldwide, adapting the ionospheric algorithms to the ionospheric characteristics at different latitudinal regions. The predicted ionospheric delays for different forecast periods have been compared with the ionospheric delay values estimated by magicSBAS for those predicted epochs. This paper shows how the new ionospheric delay forecasting tool is able to provide very good forecasting results for middle latitudes, and even for those equatorial latitudes where the ionosphere is much more complicated the results obtained are quite encouraging. Forecast periods from 1/2 hour to few hours are provided, showing that the prediction periods for which adequate forecast ionospheric results are achieved at middle latitudes are longer than those for equatorial latitudes, due to the higher (and less predictable) spatial and temporal variability in this region. Several examples of comparisons between predicted and estimated ionospheric delays for different locations using Global Navigation Satellite System data for different latitudinal regions and space weather conditions are also provided in this paper.
Ionospheric Depletion Detection Over the Indian Region using a Single Frequency Detection Algorithm
M. Cueto and I. Hidalgo and E. Sardon and G. Um and M. Bailey
The presence of plasma depletions in the equatorial regions represents a potential difficulty for the implementation of a SBAS system (such as GAGAN in the Indian region), due to the fact that these depletions can develop in small regions, and therefore may not always be detectable at ground system level. This paper presents a single frequency algorithm to detect plasma depletions at user level, improving the ionospheric contribution estimation and therefore, the user positioning and integrity. A description of the GAGAN-DDD Depletion Detection algorithm is provided in the following sections, as well as some examples of the performances achieved when using it.
M. Cueto and I. Hidalgo and E. Sardón and G. Um and M. Bailey
M. Cueto and I. Hidalgo and E. Sardón and G. Um and M. Bailey
Plasma depletions (or bubbles) are strong reductions in the ionospheric F-region plasma density due to the appearance of a Rayleigh-Taylor instability in the post-sunset, producing severe radio signal disruptions when crossing them. Most of the plasma depletions are confined on the Appleton Anomaly region, which also shows the presence of strong scintillation activity (having consequently a severe effect on the L band signals propagation). Therefore, the geographic latitude of the Indian region between 15º and 25º N is expected to be frequently affected by the presence of plasma depletions. Satellite Based Augmentation Systems (SBAS) support regional GNSS augmentation through the transmission of additional satellite-broadcast messages. In those systems, a network of double-frequency reference stations is used to estimate the ionospheric delay over the region. This ionospheric information is broadcast to the single-frequency SBAS users through an ionospheric model of grid vertical Total Electron Content (TEC) values, called Grid Ionospheric Vertical Delays (GIVD). In addition, the SBAS system also broadcasts for each grid point a conservative bound on the estimated GIVD accuracy, known as Grid Vertical Ionospheric Error (GIVE). Due to the small scale of the plasma depletion and the limited number of reference stations, these phenomena are difficult to detected by the SBAS ground systems, that mainly provide ionospheric large scale information. Therefore the ionospheric delay transmitted to the user in the SBAS message could not take into account the feasible presence of a depletion event in the satellite-user line of sight. In this case, a single frequency algorithm to detect the presence of plasma depletions at user level would improve the ionospheric contribution estimation and therefore, the user positioning and integrity. The GPS Aided Geo Augmented Navigation (GAGAN) is the SBAS for India being developed by Raytheon, ISRO and Airports Authority of India. This paper briefly provides a high-level description of the GAGAN Depletion Detection Algorithm developed by GMV and Raytheon to detect the existence of plasma depletions for single frequency users in the lines of sight of the incoming measurements for each satellite and epoch. The GAGAN Depletion Detection algorithm is mainly based on exploiting the properties of the difference between the ionospheric-large-scale-corrected code and the ionospheric-large- scale-corrected phase single frequency measurements (for each epoch and satellite): this combination allows us to remove the geometric and large-scale ionospheric contributions from the observables, focusing the analysis on the small-scale ionospheric variations as well as error & multipath contributions. Examples of plasma depletion detection using IGS Indian data for different conditions (solar activity, cycle slip presence, etc) are also provide.
Ionospheric Analysis in the Equatorial Region: Impact on GNSS Performances
M. Cueto and A. Cezon and S. Pineda and E. Sardon
At equatorial latitudes the ionospheric activity can become a limitation on GNSS augmentation systems. In order to analyze the equatorial ionospheric effects on satellite navigation systems, a 6-years dual frequency GPS dataset from a maximum of 42 IGS stations was processed. Maximum ionospheric TEC values and gradients, as well as rate of TEC (RoT) values were analyzed as a function of time, elevation and geomagnetic latitude. The mapping function distortion and the ionospheric scintillations behaviour were also studied in order to complete the analysis. Residual positioning errors may persist in regions of steep TEC gradients. At low latitudes (near the equatorial anomaly) and during periods of solar maximum an enhancement of large-scale TEC gradients is observed. In this paper, maximum temporal and spatial TEC gradients have been calculated and analyzed using IONEX and GILION TEC values. Ionospheric irregularities can be detected and characterized by calculating the rate of TEC. In this paper RoT values were computed by differencing the phase measurements over 60 seconds after cycle slips removing. Mean, RMS, maximum and minimum RoT values were analyzed for different hours, months, years, elevations and geomagnetic latitudes in order to characterize and quantify the ionospheric irregularities. Possible multipath effects will be taken into account when analyzing the results. At very low latitudes the sTEC computation error due to the mapping function could become significant. An analysis of the relation between GILION vertical and slant TEC values near the geomagnetic equator has been developed. GILION tool is based on a Kalman filter and allows us to estimate inter-frequency hardware biases and TEC values at each epoch. Finally, the impact of the ionospheric effects on GNSS performance is discussed in this paper. In particular, an analysis of the SBAS performances for Centre-South America is performed taking into account the ionospheric conditions in the region as output of the previous analysis. The objective of this paper is not only to go deeper in the ionospheric knowledge in the CAR/SAM (Centre-South America) region using a great amount of real data, but to extrapolate the conclusions of these studies to preliminary analyze the impact on future SBAS systems (like the Solución de Aumentación para Caribe, Centro y Sur América, SACCSA).
System dependence of GNSS receiver antenna calibration
R. Dach and M. Meindl and S. Schaer and S. Lutz and R. Schmid and G. Beutler
Impact of troposphere modeling on GNSS satellite antenna phase center pattern estimation
R. Dach and A. Jäggi and R. Schmid and S. Lutz and P. Steigenberger and G. Beutler
Improved antenna phase center models for GLONASS
R. Dach and R. Schmid and M. Schmitz and D. Thaller and S. Schaer and S. Lutz and P. Steigenberger and G. Wübbena and G. Beutler
Thanks to the increasing number of active GLONASS satellites and the increasing number of multi- GNSS tracking stations in the network of the International GNSS Service (IGS), the quality of the GLONASS orbits has become significantly better over the last few years. By the end of 2008, the orbit RMS error had reached a level of 3--4 cm. Nevertheless, the strategy to process GLONASS observations still has deficiencies: one simplification, as applied within the IGS today, is the use of phase center models for receiver antennas for the GLONASS observations, which were derived from GPS measurements only, by ignoring the different frequency range. Geo?? GmbH calibrates GNSS receiver antennas using a robot in the field. This procedure yields now separate corrections for the receiver antenna phase centers for each navigation satellite system, provided its constellation is sufficiently populated. With a limited set of GLONASS calibrations, it is possible to assess the impact of GNSS-specific receiver antenna corrections that are ignored within the IGS so far. The antenna phase center model for the GLONASS satellites was derived in early 2006, when the multi-GNSS tracking network of the IGS was much sparser than it is today. Furthermore, many satellites of the constellation at that time have in the meantime been replaced by the latest generation of GLONASS-M satellites. For that reason, this paper also provides an update and extension of the presently used correction tables for the GLONASS satellite antenna phase centers for the current constellation of GLONASS satellites. The updated GLONASS antenna phase center model helps to improve the orbit quality.
Evaluation of the impact of atmospheric pressure loading modeling on GNSS data analysis
R. Dach and J. Böhm and S. Lutz and P. Steigenberger and G. Beutler
  85  75-91  (2011)
In recent years, several studies have demonstrated the sensitivity of Global Navigation Satellite System (GNSS) station time series to displacements caused by atmospheric pressure loading (APL). Different methods to take the APL effect into account are used in these studies: applying the corrections from a geophysical model on weekly mean estimates of station coordinates, using observation-level corrections during data analysis, or solving for regression factors between the station displacement and the local pressure. The Center for Orbit Determination in Europe (CODE) is one of the global analysis centers of the International GNSS Service (IGS). The current quality of the IGS products urgently asks to consider this effect in the regular processing scheme. However, the resulting requirements for an APL model are demanding with respect to quality, latency, and---regarding the reprocessing activities---availability over a long time interval (at least from 1994 onward). The APL model of Petrov and Boy (J Geophys Res 109:B03405, 2004) is widely used within the VLBI community and is evaluated in this study with respect to these criteria. The reprocessing effort of CODE provides the basis for validating the APL model. The data set is used to solve for scaling factors for each station to evaluate the geophysical atmospheric non-tidal loading model. A consistent long-term validation of the model over 15 years, from 1994 to 2008, is thus possible. The time series of 15 years allows to study seasonal variations of the scaling factors using the dense GNSS tracking network of the IGS. By interpreting the scaling factors for the stations of the IGS network, the model by (2004) is shown to meet the expectations concerning the order of magnitude of the effect at individual stations within the uncertainty given by the GNSS data processing and within the limitations due to the model itself. The repeatability of station coordinates improves by 20% when applying the effect directly on the data analysis and by 10% when applying a post-processing correction to the resulting weekly coordinates compared with a solution without taking APL into account.
A review of GPS & GRACE estimates of surface mass loading effects
T. van Dam and X. Collilieux and Z. Altamimi and J. Ray
Quantifying load model errors by comparison to a global GPS time series solution
T. M. van Dam and X. Collilieux and P. Rebischung and J. Ray and Z. Altamimi
On the alternative approaches to ITRF formulation. A theoretical comparison.
A. Dermanis
The one-step approach for ITRF formulation is compared with the two-step technique where the ITRF parameters are first separately estimated for each space technique separately (stacking per tech-nique) and are then combined into the final ITRF es-timates (combination step). The comparison is achieved by splitting the one-step approach into two equivalent steps such that the first one is identical to the first step of the two-step approach. It is shown that the two approaches give equivalent results when the input covarariance matrices in the combination step of the two-step approach are the normal equation coeffi-cient matrices formulated in the corresponding stack-ing per technique first step. Furthermore it is shown that the model of the combination step can be signifi-cantly simplified by ignoring the parameters of refer-ence system transformation from the per technique estimates to those of the final ITRF parameter esti-mates.
Results from the reanalysis of Global GPS data in the IGS08 reference frame
S. D. Desai and W. Bertiger and J. Gross and B. Haines and N. Harvey and C. Selle and A. Sibthorpe and J. P. Weiss
GPS IIF yaw attitude control during eclipse season
F. Dilssner and T. Springer and W. Enderle
Updating the IGS processing standard: New GLONASS satellite antenna corrections for igs08.atx
F. Dilssner and R. Dach and R. Schmid and T. Springer and R. Zandbergen
GPS satellite antenna parameters from combined ground-based & space-borne data processing
F. Dilssner and M. Otten and T. Springer and C. Flohrer and D. Svehla and R. Zandbergen
The GLONASS-M satellite yaw-attitude model
F. Dilssner and T. Springer and G. Gienger and J. Dow
Advances in Space Research  47  160 - 171  (2011)
The proper modeling of the satellites' yaw-attitude is a prerequisite for high-precision Global Navigation Satellite System (GNSS) positioning and poses a particular challenge during periods when the satellite orbital planes are partially eclipsed. Whereas a lot of effort has been put in to examine the yaw-attitude control of \{GPS\} satellites that are in eclipsing orbits, hardly anything is known about the yaw-attitude behavior of eclipsing GLONASS-M satellites. However, systematic variations of the carrier phase observation residuals in the vicinity of the orbit's noon and midnight points of up to $\pm$27 cm indicate significant attitude-related modeling issues. In order to explore the GLONASS-M attitude laws during eclipse seasons, we have studied the evolution of the horizontal satellite antenna offset estimates during orbit noon and orbit midnight using a technique that we refer to as ``reverse kinematic precise point positioning''. In this approach, we keep all relevant global geodetic parameters fixed and estimate the satellite clock and antenna phase center positions epoch-by-epoch using 30-second observation and clock data from a global multi-GNSS ground station network. The estimated horizontal antenna phase center offsets implicitly provide the spacecraft's yaw-attitude. The insights gained from studying the yaw angle behavior have led to the development of the very first yaw-attitude model for eclipsing GLONASS-M satellites. The derived yaw-attitude model proves to be much better than the nominal yaw-attitude model commonly being used by today's GLONASS-capable \{GNSS\} software packages as it reduces the observation residuals of eclipsing satellites down to the normal level of non-eclipsing satellites and thereby prevents a multitude of measurements from being incorrectly identified as outliers. It facilitates continuous satellite clock estimation during eclipse and improves in particular the results of kinematic precise point positioning of ground-based receivers.
GPS IIF-1 satellite: Antenna phase center center & attitude modeling
F. Dilssner
Calculating the distances between satellites and user equipment is a basic operation for GNSS positioning. More precisely, these ranges are measured from the antenna phase centers of the satellites' transmitting antenna. However, phase centers vary among types and generations of spacecraft and, further, the calculation requires knowledge of a satellite's orientation or attitude. A researcher at the European Space Operations Center has analyzed the initial performance of the first GPS Block IIF space vehicle and found some expected - and unexpected - results.
The GLONASS-M satellite yaw-attitude model
F. Dilssner and T. Springer and G. Gienger and J. Dow
Estimation of phase center corrections for GLONASS-M satellite antennas
F. Dilssner and T. Springer and C. Flohrer and J. Dow
  84  467-480  (2010)
Driven by the comprehensive modernization of the GLONASS space segment and the increased global availability of GLONASS-capable ground stations, an updated set of satellite-specific antenna phase center corrections for the current GLONASS-M constellation is determined by processing 84 weeks of dual-frequency data collected between January 2008 and August 2009 by a worldwide network of 227 GPS-only and 115 combined GPS/GLONASS tracking stations. The analysis is performed according to a rigorous combined multi-system processing scheme providing full consistency between the GPS and the GLONASS system. The solution is aligned to a realization of the International Terrestrial Reference Frame 2005. The estimated antenna parameters are compared with the model values currently used within the International GNSS Service (IGS). It is shown that the z-offset estimates are on average 7 cm smaller than the corresponding IGS model values and that the block-specific mean value perfectly agrees with the nominal GLONASS-M z-offset provided by the satellite manufacturer. The existence of azimuth-dependent phase center variations is investigated and uncertainties in the horizontal offset estimates due to mathematical correlations and yaw-attitude modeling problems during eclipse seasons are addressed. Finally, it is demonstrated that the orbit quality benefits from the updated GLONASS-M antenna phase center model and that a consistent set of satellite antenna z-offsets for GPS and GLONASS is imperative to obtain consistent GPS- and GLONASS-derived station heights.
The International GNSS Service in a Changing Landscape of Global Navigation Satellite Systems
J. M. Dow and R. E. Neilan and C. Rizos
IGS/EPN Reference Frame Realization in Local GPS Networks
H. Drewes and J. Bosy and B. Kontny and A. Borkowski
  134  197-203  (2009)
Real-Time GNSS Activities at ESA
W. Enderle and L. Agrotis and R. Zandbergen and M. van Kints and J. Martin
GPS World      (2013)
The European Space Operations Centre has taken on the roles of real-time analysis center, data provider, and analysis-center coordinator for the International GNSS Service’s Real-Time Service, providing a number of products combining data streams from multiple sources. http://gpsworld.com/real-time-gnss-activities-at-esa/
Comparative testing of four ionospheric models driven with GPS measurements
J. Feltens and M. Angling and N. Jackson-Booth and N. Jakowski and M. Hoque and M. Hernández-Pajares and A. Aragón-Àngel and R. Orús and R. Zandbergen
In the context of the European Space Agency/European Space Operations Centre funded Study ``GNSS Contribution to Next Generation Global Ionospheric Monitoring,'' four ionospheric models based on GNSS data (the Electron Density Assimilative Model, EDAM; the Ionosphere Monitoring Facility, IONMON v2; the Tomographic Ionosphere model, TOMION; and the Neustrelitz TEC Models, NTCM) have been run using a controlled set of input data. Each model output has been tested against differential slant TEC (dSTEC) truth data for high (May 2002) and low (December 2006) sunspot periods. Three of the models (EDAM, TOMION, and NTCM) produce dSTEC standard deviation results that are broadly consistent with each other and with standard deviation spreads of ~1 TECu for December 2006 and ~1.5 TECu for May 2002. The lowest reported standard deviation across all models and all stations was 0.99 TECu (EDAM, TLSE station for December 2006 night). However, the model with the best overall dSTEC performance was TOMION which has the lowest standard deviation in 28 out of 52 test cases (13 stations, two test periods, day and night). This is probably related to the interpolation techniques used in TOMION exploiting the spatial stationarity of vertical TEC error decorrelation.
Comparative testing of four ionospheric models driven with GPS measurements
J. Feltens and M. Angling and N. Jackson-Booth and N. Jakowski and M. Hoque and M. Hernández-Pajares and A. Aragón-Àngel and R. Orús and R. Zandbergen
Radio Science  46  n/a--n/a  (2011)
In the context of the European Space Agency/European Space Operations Centre funded Study ``GNSS Contribution to Next Generation Global Ionospheric Monitoring,'' four ionospheric models based on GNSS data (the Electron Density Assimilative Model, EDAM; the Ionosphere Monitoring Facility, IONMON v2; the Tomographic Ionosphere model, TOMION; and the Neustrelitz TEC Models, NTCM) have been run using a controlled set of input data. Each model output has been tested against differential slant TEC (dSTEC) truth data for high (May 2002) and low (December 2006) sunspot periods. Three of the models (EDAM, TOMION, and NTCM) produce dSTEC standard deviation results that are broadly consistent with each other and with standard deviation spreads of ∼1 TECu for December 2006 and ∼1.5 TECu for May 2002. The lowest reported standard deviation across all models and all stations was 0.99 TECu (EDAM, TLSE station for December 2006 night). However, the model with the best overall dSTEC performance was TOMION which has the lowest standard deviation in 28 out of 52 test cases (13 stations, two test periods, day and night). This is probably related to the interpolation techniques used in TOMION exploiting the spatial stationarity of vertical TEC error decorrelation.
Combination of the reprocessed IGS Analysis Center SINEX solutions
R. Ferland
Consistency of the IGS contribution to ITRF2008
R. Ferland
The IGS-combined station coordinates, Earth rotation parameters & apparent geocenter
R. Ferland and M. Piraszewsk
The International GNSS Service (IGS) routinely generates a number of weekly, daily and sub-daily products. Station coordinates and velocities, earth rotation parameters (ERPs) and apparent geocenter are among these products generated weekly by the IGS Reference Frame Coordinator. They have been determined since 1999 by combining independent estimates from at least seven IGS Analysis Centers (ACs). Two Global Network Associate Analysis Centers (GNAACs) also provide independent combinations using the same AC weekly solutions and they are currently used to quality control the IGS combination. The combined solutions are aligned to an IGS realization (IGS05) of the ITRF2005 using a carefully selected set of the IGS Reference Frame (RF) stations (nominally 132). During the combination process, the contributing solutions are compared and outliers are removed to ensure a high level of consistency of the estimated parameters. The ACs and the weekly combined solution are consistent at the 1--2 and 3--4mm levels for the horizontal and vertical components. Similarly, the excess Length of Day (LOD), the pole positions and pole rates are consistent at the 10 ?s, 0.03--0.05mas and 0.10--0.20 mas/day levels, respectively. The consistency of the apparent geocenter estimate is about 5mm in the X and Y components and 10mm in the Z component. Comparison of the IGS-combined ERP estimates with the IERS Bulletin A suggests a small bias of the order of -0.04mas and +0.05mas (both $\pm$0.05 mas) in the x and y components.
Analysis methodology & recent results of the IGS network combination
R. Ferland and J. Kouba and D. Hutchison
The effect of using inconsistent ocean tidal loading models on GPS coordinate solutions
Y. Fu and J. Freymueller and T. Dam
  86  409-421  (2012)
We use up to a 6-year span of GPS data from 85 globally distributed stations to compare solutions using ocean tidal loading (OTL) corrections computed in different reference frames: center of mass of the solid Earth (CE), and center of mass of the Earth system (CM). We compare solution sets that differ only in the frame used for the OTL model computations, for three types of GPS solutions. In global solutions with all parameters including orbits estimated simultaneously, we find coordinate differences of ~0.3 mm between solutions using OTL computed in CM and OTL computed in CE. When orbits or orbits and clocks are fixed, larger biases appear if the user applies an OTL model inconsistent with that used to derive the orbit and clock products. Network solutions (orbits fixed, satellite clocks estimated) show differences smaller than 0.5 mm due to model inconsistency, but PPP solutions show distortions at the ~1.3 mm level. The much larger effect on PPP solutions indicates that satellite clock estimates are sensitive to the OTL model applied. The time series of coordinate differences shows a strong spectral peak at a period of ~14 days when inconsistent OTL models are applied and smaller peaks at ~annual and ~semi-annual periods, for both ambiguity-free and ambiguity-fixed solutions. These spurious coordinate variations disappear in solutions using consistent OTL models. Users of orbit and clock products must ensure that they use OTL coefficients computed in the same reference frame as the OTL coefficients used by the analysis centers that produced the products they use; otherwise, systematic errors will be introduced into position solutions. All modern products should use loading models computed in the CM frame, but legacy products may require loading models computed in the CE frame. Analysts and authors need to document the frame used for all loading computations in product descriptions and papers.
IGS reprocessing -- Summary of orbit/clock combination & first quality assessment
G. Gendt and J. Griffiths and T. Nischan and J. Ray
Effects of azimuthal multipath asymmetry on long GPS coordinate time series
S. Goebell and M. King
  15  287-297  (2011)
Carrier phase multipath is currently one source of unmodeled signals that may bias GPS coordinate time series significantly. We investigate the effect of simulated carrier phase multipath on time series of several sites covering the period 2002.0--2008.0 and spanning a range of observation geometries. High-, mid-, and low-latitude IGS sites are investigated as well as sites with significant signal obstructions. We examine the effect of multipath in different sectors of the sky, considering time-constant, horizontal reflectors at each of 0.1, 0.2, and 1.5 m below the antenna. The differences between a horizontally uniform multipath source are analyzed, and it is shown that positioning errors are generally larger when unmodeled carrier phase multipath is azimuthally heterogeneous. Using the adopted multipath model, height biases reach $\pm$1 mm in case of the symmetric multipath and $\pm$5 mm for the asymmetric multipath but this increases to being $\pm$10 mm in the worst case. In addition to mean bias, low-frequency variations in the bias also exist, including periodic signals and leading to velocity biases of up to $\pm$0.1 mm/year in the symmetric case and $\pm$1 mm/year in the asymmetric case over the considered period. In contrast to the generally slowly varying observation geometry that is typically experienced, we show the effects of an abrupt change in geometry due to receiver/antenna hardware changes; in the case considered, we see changed pattern of temporal variation in the bias in addition to an instantaneous offset.
IGS preparations for the next reprocessing and ITRF
J. Griffiths and P. Rebischung and B. Garayt and J. Ray
Subdaily alias & draconitic errors in the IGS orbits 2011
J. Griffiths and J. Ray
IGS reprocessed GPS satellite orbits
J. Griffiths
On the precision & accuracy of IGS orbits
J. Griffiths and J. Ray
In order to explore the precision and accuracy of International GNSS Service (IGS) orbits, we difference geocentric satellite positions midway between successive daily Final orbits for the period starting 5 November 2006, when the IGS switched its method of antenna calibration, through 31 December 2007. This yields a time series of orbit repeatabilities analogous to the classical geodetic test for position determinations. If we compare our average positional discontinuities to the official IGS accuracy codes, root-sum-squared (RSS) for each pair of days, we find the discontinuities are not well correlated with the predicted performance values. If instead the IGS weighted root-mean-square (WRMS) values from the Final combination long-arc analyses are taken as the measure of IGS accuracy, we find the position differences and long-arc values are correlated, but the long-arc values are exaggerated, particularly around eclipses, despite the fact that our day-boundary position differences apply to a single epoch each day and the long-arc analyses consider variations over a week. Our method is not well suited to probe the extent to which systematic effects dominate over random orbit errors, as indicated by satellite laser ranging residuals, but eclipsing satellites often display the most problematic behavior. A better metric than the current IGS orbit accuracy codes would probably be one based on the orbit discontinuities between successive days.
Assessment of the orbits from the 1st IGS reprocessing campaign
J. Griffiths and G. Gendt and T. Nischan and and J. Ray
Validating Earth orientation series with models of atmospheric & oceanic angular momenta
R. Gross
External comparison of EOP result
R. S. Gross
GNSS and heat waves: case studies for 2003 and 2007.
G. Guerova and J. Morland and T. Simeonov
Both climate models and observations indicate that globally the column-integrated atmospheric water vapour increases by about 7 % per 1? C increase of temperature. Over Europe however, no evident positive water vapour trend was found in the radiosonde record since 1988 despite the large positive trends over the North Atlantic. The temperature/water vapour feedback is tested for the 2003 heat wave in Switzerland and the 2007 heat wave in Bulgaria. In August 2003, the column-integrated water vapour, derived from GNSS station Payerne Switzerland, increased by 7 % while the temperature was on averaged 5? C higher than the 2001-2006 mean. This weak response of water vapour on temperature forcing was found to be due to the significant precipitation deficit in the 2003 spring i.e. more than 50 % below the 2002- 2006 mean. An increase of evapotranspiration observed in June 2003 facilitated soil moisture depletion and then the heating went in raising temperature hence the oc- currence of heat wave in August 2003. The temperature/water vapour feedback is further tested for the 2007 heat wave in Bulgaria when the temperature was on average 1.6? C higher than 1961-90 mean. During the July heat wave in Southeast Europe, water vapour decreased by 18 %, while temperature was 3? C above the 2001-2006 mean.
Impact of ambiguity resolution & orbit reprocessing on the global reference frame
B. Haines and Y. Bar-Sever and W. Bertiger and S. Desai and N. Harvey and J. Weiss
Improved models of the GPS satellite antenna phase- & group-delay variations using data from low-Earth orbiters
B. Haines and Y. Bar-Sever and W. Bertiger and S. Desai and N. Harvey and J. Weiss
Scientific Value of Real-Time Global Positioning System Data
W. C. Hammond and B. A. Brooks and R. Bürgmann and T. Heaton and M. Jackson and A. R. Lowry and S. Anandakrishnan
The Ionosphere: Effects, GPS Modeling and the Benefits for Space Geodetic Techniques
M. Hernández-Pajares and J. M. Juan and J. Sanz and A. Aragón-Àngel and A. Garcia-Rigo and D. Salazar and M. Escudero
The IGS VTEC maps: a reliable source of ionospheric information since 1998
M. Hernández-Pajares and J. M. Juan and J. Sanz and R. Orus and A. Garcia-Rigo and J. Feltens and A. Komjathy and S. C. Schaer and A. Krankowski
Modeling of the GIOVE-B clock as a tool for studying radiation pressure models
U. Hugentobler and O. Montenbruck and C. Rodriguez-Solano and P. Steigenberger
Impact of albedo modelling on GNSS satellite orbits & geodetic time series
U. Hugentobler and C. Rodriguez-Solano and P. Steigenberger and R. Dach and S. Lutz
Extension of the GPS satellite antenna patterns to nadir angles beyond 14$\,^{\circ}$
A. Jäggi and F. Dilssner and R. Schmid and R. Dach and T. Springer and H. Bock and P. Steigenberger and Y. Andres and W. Enderle
The absolute phase center model igs08.atx adopted by the International GNSS Service (IGS) in 2011 is based on robot calibrations for more than 200 terrestrial GNSS receiver antennas and consistent correction values for the GNSS transmitter antennas estimated from tracking data of the global IGS ground network. As the calibration of the satellite antennas is solely based on terrestrial measurements, the estimation of their phase patterns is limited to a nadir angle of 14$\,^{\circ}$. This is not sufficient for the analysis of spaceborne GPS data collected by low Earth orbiting (LEO) satellites that record - depending on the missions' orbital altitude - observations at nadir angles of up to 17$\,^{\circ}$. We use GPS tracking data from the LEO missions Jason--1/2, MetOp--A, GRACE, and GOCE to extend the IGS satellite antenna patterns to nadir angles beyond 14$\,^{\circ}$ using different processing strategies and GNSS software packages (BERNESE, NAPEOS). In order to achieve estimates that are consistent with the PCVs currently used within the IGS, GPS satellite orbits and clocks are fixed to reprocessed solutions obtained by adopting the IGS conventional values from igs08.atx. Due to significant near-field multipath effects arising in the LEO spacecraft environment, it is necessary to solve for GPS (nadir--dependent only) and LEO (azimuth-- and elevation--dependent) antenna patterns simultaneously. We compare and combine the results obtained with both software packages and derive the PCV extension proposed for igs08.atx.
Extending the GPS satellite antenna patterns of the IGS to nadir angles beyond using LEO data
A. Jäggi and R. Dach and H. Bock and G. Beutler and O. Montenbruck and R. Schmid and Y. Andres
Combining terrestrial & LEO data to extend the GPS satellite antenna patterns to nadir angles beyond
A. Jäggi and R. Dach and H. Bock and G. Beutler and O. Montenbruck and P. Steigenberger and Y. Andres
Extending the GPS satellite antenna patterns of the IGS to nadir angles beyond 14$\,^{\circ}$ using LEO data
A. Jäggi and R. Dach and H. Bock and G. Beutler and O. Montenbruck and R. Schmid
A New Global TEC Model for Estimating Transionospheric Radio Wave Propagation Errors
N. Jakowski and M. M. Hoque and C. Mayer
GPS Tracking of a Nanosatellite - The CanX-2 Flight Experience
E. Kahr and O. Montenbruck and K. O'Keefe and S. Skone and J. Urbanek and L. Bradbury and P. Fenton
Quality of GRACE orbits using the reprocessed IGS products
Z. Kang and B. Tapley and S. Bettadpur and H. Save
Modifications to GPS reference station antennas to reduce multipath
A. Kerkhoff and R. B. Harris and C. Petersen and A. Pickard
The National Geospatial Intelligence Agency's (NGA) GPS Monitor Station Network (MSN), consisting of 11 ground reference stations distributed throughout the world, is used collaboratively by NGA and the Air Force Operational Control Segment (OCS) to monitor the health of the GPS constellation, and to generate both broadcast and precise ephemeris products. Multipath caused by signal scattering off objects in the vicinity of the antennas at these stations continues to be a dominant error source in MSN measurement observables. Both hardware-based and processing-based techniques are implemented in the MSN to mitigate this effect, however further suppression of multipath is sought in order to improve system performance. This paper considers two different approaches to modifying the basic choke ring antenna design used in the MSN in order to reduce its reception of multipath. One approach consists of placing a large metallic ground plane directly beneath the antenna. This has the effect of shaping the antenna.
On the determination of antenna phase center corrections in a multi-GNSS multi-frequency approach
T. Kersten and S. Schön
Detection of offsets in GPS experiment (DOGEx)
M. King and S. Williams
Regional biases in absolute sea-level estimates from tide gauge data due to residual unmodeled vertical land movement
M. A. King and M. Keshin and P. L. Whitehouse and I. D. Thomas and G. Milne and R. E. M. Riva
Geophysical Research Letters  39  n/a--n/a  (2012)
The only vertical land movement signal routinely corrected for when estimating absolute sea-level change from tide gauge data is that due to glacial isostatic adjustment (GIA). We compare modeled GIA uplift (ICE-5G + VM2) with vertical land movement at ∼300 GPS stations located near to a global set of tide gauges, and find regionally coherent differences of commonly $\pm$0.5--2 mm/yr. Reference frame differences and signal due to present-day mass trends cannot reconcile these differences. We examine sensitivity to the GIA Earth model by fitting to a subset of the GPS velocities and find substantial regional sensitivity, but no single Earth model is able to reduce the disagreement in all regions. We suggest errors in ice history and neglected lateral Earth structure dominate model-data differences, and urge caution in the use of modeled GIA uplift alone when interpreting regional- and global- scale absolute (geocentric) sea level from tide gauge data.
Ocean tides in the Weddell Sea: New observations on the Filchner-Ronne and Larsen C ice shelves and model validation
M. A. King and L. Padman and K. Nicholls and P. J. Clarke and G. H. Gudmundsson and B. Kulessa and A. Shepherd
Journal of Geophysical Research: Oceans  116  n/a--n/a  (2011)
Ocean tides under the large Weddell Sea ice shelves are among the least well observed on Earth. Here we present new, spatially extensive observations of the vertical tidal motion of the Filchner-Ronne and Larsen C ice shelves using Global Positioning System (GPS) data spanning a few weeks to years. We pay particular attention to the major tidal constituents (M2, S2, O1, K1) as well as important GRACE aliasing periods (K2 and S1). We compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves. The root-sum-square errors are 7--8 cm (CATS2008a and TPXO7.2) and 11--12 cm (GOT4.7 and FES2004) with the energetic M2 (RMSE = 4--8 cm) and S2 (4--5 cm) generally dominating these statistics. The FES2004 K1 is particularly inaccurate near the Larsen C Ice Shelf, with errors approaching 20 cm, meaning that GRACE Release 4 estimates of mass change in the northern Antarctic Peninsula will be biased. We find tidal energy at 3, 4, 5, 6 and, weakly, at 7 cycles per day at all of our sites. The largest amplitudes within these bands are at M4, MO3 and SP3 and approach 30 mm, although significant spatial variations exist. We show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones. Comparing model estimates with our M4, MS4 and MN4 values shows that models do not accurately represent these terms.
Monument-antenna effects on GPS coordinate time series with application to vertical rates in Antarctica
M. King and M. Bevis and T. Wilson and B. Johns and F. Blume
  86  53-63  (2012)
We examine the electromagnetic coupling of a GPS antenna--monument pair in terms of its simulated affect on long GPS coordinate time series. We focus on the Earth and Polar Observing System (POLENET) monument design widely deployed in Antarctica and Greenland in projects interested particularly in vertical velocities. We base our tests on an absolute robot calibration that included the top ~0.15 m of the monument and use simulations to assess its effect on site coordinate time series at eight representative POLENET sites in Antarctica over the period 2000.0--2011.0. We show that the neglect of this calibration would introduce mean coordinate bias, and most importantly for velocity estimation, coordinate noise which is highly sensitive to observation geometry and hence site location and observation period. Considering only sub-periods longer than 2.5 years, we show vertical site velocities may be biased by up to $\pm$0.4 mm/year, and biases up to 0.2 mm/year may persist for observation spans of 8 years. Changing between uniform and elevation-dependent observation weighting alters the time series but does not remove the velocity biases, nor does ambiguity fixing. The effect on the horizontal coordinates is negligible. The ambiguities fixed series spectra show noise between flicker and random walk with near-white noise at the highest frequencies, with mean spectral indices (frequencies <20 cycles per year) of approximately −1.3 (uniform weighting) and −1.4 (elevation-dependent weighting). While the results are likely highly monument specific, they highlight the importance of accounting for monument effects when analysing vertical coordinate time series and velocities for the highest precision and accuracy geophysical studies.
ITRF2008 & IGS repro1 polar motion comparisons with AAM+OAM
J. Kouba
A simplified yaw-attitude model for eclipsing GPS satellites
J. Kouba
A simplified yaw-attitude modeling, consistent with Bar-Sever (1996), has been implemented and tested in the NRCan PPP software. For Block IIR GPS satellite it is possible to model yaw-attitude control during eclipsing periods by using the constant hardware yaw rate of 0.20/s. The Block IIR satellites maintain the nominal yaw attitude even during a shadow crossing (Y. E. Bar-Sever, private communication, 2007), except for the noon and shadow midnight turn maneuvers, both of which can be modeled and last up to 15 min. Thus, for Block IIR satellites it is possible to maintain continuous satellite clock estimation even during eclipsing periods. For the Block II/IIA satellites, it is possible to model satisfactorily the noon turns and also shadow crossing, thanks to the permanent positive yaw bias of 0.5, implemented in November 1995. However, in order to model the Block II/IIA shadow crossings, satellite specific yaw rates should be used, either solved for or averaged yaw-rate solutions. These yaw rates as estimated by the Jet Propulsion Laboratory (JPL) can differ significantly from the nominal hardware values. The Block II/IIA post-shadow recovery periods, which last about 30 min, should be considered uncertain and cannot be properly modeled. Data from post-shadow recovery periods should, therefore, not be used in precise global GPS analyses (Bar- Sever 1996). For high-precision applications, it is essential that users implement a yaw-attitude model, which is consistent with the generation of the satellite clocks. Initial testing and analyses, based on the IGS and AC Final orbits and clocks have revealed that during eclipsing periods, significant inconsistencies in yaw-attitude modeling still exist amongst the IGS Analyses Centers, which contribute to the errors of the IGS Final clock combinations.
Testing of the IERS2000 sub-daily Earth rotation parameter model
J. Kouba
IGS reference frame realization
J. Kouba and J. Ray and M. M. Watkins
Present & future IGS ionospheric products
A. Krankowski and M. Hernandez-Pajares and J. Feltens and A. Komjathy and S. Schaer and A. Garcia-Rigo and P. Wielgosz
The CNES Real-time PPP with Undifferenced Integer Ambiguity Resolution Demonstrator
D. Laurichesse
Impact of regional reference frame definition on geodynamic interpretations
J. Legrand and N. Bergeot and C. Bruyninx and G. Wöppelmann and M.-N. Bouin and Z. Altamimi
Journal of Geodynamics  49  116 - 122  (2010)
Ten years (1997--2006) of weekly \{GNSS\} solutions of 205 globally distributed stations have been used to investigate the impact of the reference frame definition on the estimated station velocities. For that purpose, weekly regional solutions (covering the European region) and global solutions have been, respectively, stacked to obtain regional and global velocity fields. In both cases, the estimated long-term solutions (station positions and velocities) were tied to the \{ITRF2005\} under minimal constraints using a selected set of reference stations. Several sets of global and regional reference stations were tested to evaluate first the impact of the reference frame definition on the global and regional velocity fields and later the impact on the derived geodynamic interpretations. Results confirm that the regional velocity fields show systematic effects with respect to the global velocity field with differences reaching up to 1.3 mm/year in the horizontal and 2.9 mm/year in the vertical depending on the geographical extent of the network and the chosen set of regional reference stations. In addition, the estimations of the Euler pole for Western Europe differ significantly when considering a global or a regional strategy. After removing the rigid block rotation, the residual velocity fields show differences which can reach up to 0.8 mm/year in horizontal component. In Northern Europe, the vertical ground motion is dominated by the Glacial Isostatic Adjustment (GIA). A proper modeling of this effect requires sub-mm/year precision for the vertical velocities for latitudes below 56$\,^{\circ}$. We demonstrate that a profile of vertical velocities shows significant discrepancies according to the reference frame definition strategy. In the case of regional solutions, the vertical modeling does not predict any subsidence around 52$\,^{\circ}$ as predicted by the global solution and previous studies. In summary, we evidence the limitation of regional networks to reconstruct absolute velocity fields and conclude that when geodynamics require the highest precisions for the GNSS-based velocities, a global reference frame definition is more reliable.
Spherical cap harmonic model for mapping and predicting regional TEC
J. Liu and R. Chen and Z. Wang and H. Zhang
  15  109-119  (2011)
An approach to modeling the regional ionospheric total electron content (TEC) based on spherical cap harmonic analysis is presented. This approach not only provides a better regional TEC mapping accuracy, but also the capability for ionospheric model prediction based on spectrum analysis and least squares collocation. Unlike conventional approaches, which predict the immediate TEC with models using current observations, the spherical cap harmonic approach utilizes models using past observations to predict a model which will provide future TEC values. A significant advantage in comparison with conventional approaches is that the spherical cap harmonic approach can be used to predict the long-term TEC with reasonable accuracy. This study processes a set of GPS data with an observation time span of 1 year from two GPS networks in China. The TEC mapping accuracy of the spherical cap harmonic model is compared with the polynomial model and the global ionosphere model from IGS. The results show that the spherical cap harmonic model has a better TEC mapping accuracy with smoother residual distributions in both temporal and spatial domains. The TEC prediction with the spherical cap harmonic model has been investigated for both short- and long-term intervals. For the short-term interval, the prediction accuracies for the latencies of 1-day, 2-days, and 3-days are 2.5 TECU, 3.5 TECU, and 4.5 TECU, respectively. For the long-term interval, the prediction accuracy is 4.5 TECU for a 2-month latency.
A Preliminary Study on Mapping the Regional Ionospheric TEC Using a Spherical Cap Harmonic Model in High Latitudes and the Arctic Region
J. Liu and R. Chen and H. Kuusniemi and Z. Wang and H. Zhang and J. Yang
The conventional ionosphere total electron content (TEC) models based on geodetic coordinates have asynchronous dimensional resolution, especially in the area close to the pole. This paper presents a novel spherical cap harmonic model for mapping the arctic regional ionospheric TEC in a spherical cap coordinate system. Utilizing a series of IGS (International GNSS Service) products, a set of dual-frequency GPS (Global Positioning System) data from IGS stations in high latitudes is processed and used to map the arctic regional TEC values with the spherical cap harmonic model and the conventional regional TEC models. Together with the global ionosphere mapping (GIM) model from IGS, the TEC mapping accuracies from these models are compared. The comparison results show that the spherical cap harmonic model has a better TEC mapping performance with more homogeneous accuracy distributions in both temporal and spatial domains for the arctic region. In addition, the spectrum components of the coefficient series of the spherical cap harmonic models are demonstrated in this paper.
A new automated cycle slip detection and repair method for a single dual-frequency GPS receiver
Z. Liu
This paper develops a new automated cycle slip detection and repair method that is based on only one single dual-frequency GPS receiver. This method jointly uses the ionospheric total electron contents (TEC) rate (TECR) and Melbourne--Wübbena wide lane (MWWL) linear combination to uniquely determine the cycle slip on both L1 and L2 frequencies. The cycle slips are inferred from the information of ionospheric physical TECR andMWWLambiguity at the current epoch and that at the previous epoch. The principle of this method is that when there are cycle slips, theMWWL ambiguity will change and the ionospheric TECR will usually be significantly amplified, the part of artificial TECR (caused by cycle slips) being significantly larger than the normal physical TECR. The TECR is calculated based on the dual-frequency carrier phase measurements, and it is highly accurate. We calculate the ionospheric change information (including TECR and TEC acceleration) using the previous epochs (30 epochs in this study) and use the previous data to predict the TECR for the epoch needing cycle slip detection. If the discrepancy is larger than our defined threshold 0.15 TECU/s, cycle slips are regarded to exist at that epoch. The key rational of method is that during a short period (1.0 s in this study) the TECR of physical ionospheric phenomenon will not exceed the threshold. This new algorithm is tested with eight different datasets (including one spaceborne GPS dataset), and the results show that the method can detect and correctly repair almost any cycle slips even under very high level of ionospheric activities (with an average Kp index 7.6 on 31 March 2001). The only exception of a few detected but incorrectly repaired cycle slip is due to a sudden increased pseudorange error on a single satellite (PRN7) under very active ionosphere on 31 March 2001. This method requires dual-frequency carrier phase and pseudorange data from only one single GPS receiver. The other requirement is that the GPS data rate ideally is 1 Hz or higher in order to detect small cycle slips. It is suitable for many applications where one single receiver is used, e.g. real-time kinematic rover station and precise point positioning. An important feature of this method is that it performs cycle slip detection and repair on a satellite-by-satellite basis; thus, the cycle slip detection and repair for each satellite are completely independent and not affected by the data of other satellites.
Comparison of different radiation pressure models for GNSS orbit determination
S. Loyer and F. Mercier and F. Perosanz and H. Capdeville
Zero-difference GPS ambiguity resolution at CNES--CLS IGS Analysis Center
S. Loyer and F. Perosanz and F. Mercier and H. Capdeville and J.-C. Marty
  86  991-1003  (2012)
CNES (Centre National d'Etudes Spatiales) and CLS (Collecte Localisation Satellites) became an International GNSS Service (IGS) Analysis Center (AC) the 20th of May 2010. Since 2009, we are using the integer ambiguity fixing at the zero-difference level strategy in our software package (GINS/Dynamo) as an alternative to classical differential approaches. This method played a key role among all the improvements in the GPS processing we made during this period. This paper provides to the users the theoretical background, the strategies and the models used to compute the products (GPS orbits and clocks, weekly station coordinate estimates and Earth orientation parameters) that are submitted weekly to the IGS. The practical realization of the two-step, ambiguity-fixing scheme (wide-lane and narrow-lane) is described in detail. The ambiguity fixing improved our orbit overlaps from 6 to 3 cm WRMS in the tangential and normal directions. Since 2008, our products have been also regularly compared to the IGS final solutions by the IGS Analysis Center Coordinator. The joint effects of ambiguity fixing and dynamical model changes (satellite solar radiation pressure and albedo force) improved the consistency with IGS orbits from 35 to 18 mm 3D-WRMS. Our innovative strategy also gives additional powerful properties to the GPS satellite phase clock solutions. Single receiver (zero-difference) ambiguity resolution becomes possible. An overview of the applications is given.
GNSS Absolute Antenna Calibration at the National Geodetic Survey
G. Mader and A. Bilich
Geocenter coordinates estimated from a combined multi-GNSS data analysis
M. Meindl and R. Dach and G. Beutler and S. Schaer and A. Jäggi
Real-time Combination of GNSS Orbit and Clock Correction Streams Using a Kalman Filter Approach
L. Mervart and G. Weber
Glacial isostatic adjustment & ITRF solutions
L. Métivier and X. Collilieux and Z. Altamimi
Online Precise Point Positioning: A New, Timely Service from Natural Resources Canada
Y. Mireault and P. Tétreault and F. Lahaye and P. Héroux and J. Kouba
Monico J F G. Posicionamento pelo GNSS: Descrição, fundamentos e aplicações.
J. F. G. Monico
It is a book in Portuguese language.
GPS Based Relative Navigation for the TanDEM-X Mission - First Flight Results
O. Montenbruck and M. Wermuth and R. Kahle
Apparent Clock Variations of the Block IIF-1 (SVN62) GPS Satellite
O. Montenbruck and U. Hugentobler and R. Dach and P. Steigenberger and A. Hauschild
Flight Characterization of New Generation Satellite Clocks
O. Montenbruck and P. Steigenberger and E. Schönemann and A. Hauschild and U. Hugentobler and R. Dach and M. Becker
Carrier Phase Differential GPS for LEO Formation Flying - The PRISMA and TanDEM-X Flight Experience
O. Montenbruck and S. D'Amico and J.-S. Ardaens and M. Wermuth
Apparent clock variations of the Block IIF-1 (SVN62) GPS satellite
O. Montenbruck and U. Hugentobler and R. Dach and P. Steigenberger and A. Hauschild
The Block IIF satellites feature a new generation of high-quality rubidium clocks for time and frequency keeping and are the first GPS satellites transmitting operational navigation signals on three distinct frequencies. We investigate apparent clock offset variations for the Block IIF-1 (SVN62) spacecraft that have been identified in L1/ L2 clock solutions as well as the L1/L5-minus-L1/L2 clock difference. With peak-to-peak amplitudes of 10--40 cm, these variations are of relevance for future precision point positioning applications and ionospheric analyses. A proper characterization and understanding is required to fully benefit from the quality of the new signals and clocks. The analysis covers a period of 8 months following the routine payload activation and is based on GPS orbit and clock products generated by the CODE analysis center of the International GNSS Service (IGS) as well as triplefrequency observations collected with the CONGO network. Based on a harmonic analysis, empirical models are presented that describe the sub-daily variation of the clock offset and the inter-frequency clock difference. These contribute to a better clock predictability at timescales of several hours and enable a consistent use of L1/L2 clock products in L1/L5-based positioning.
Rapid orbit determination of LEO satellites using IGS clock & ephemeris products
O. Montenbruck and E. Gill and R. Kroes
Different types of GPS clock and orbit data provided by the International GPS Service (IGS) have been used to assess the accuracy of rapid orbit determination for satellites in low Earth orbit (LEO) using spaceborne GPS measurements. To avoid the need for reference measurements from groundbased reference receivers, the analysis is based on an undifferenced processing of GPS code and carrierphase measurements. Special attention is therefore given to the quality of GPS clock data that directly affects the resulting orbit determination accuracy. Interpolation of clock data from the available 15 min grid points is identified as a limiting factor in the use of IGS ultra-rapid ephemerides. Despite this restriction, a 10-cm orbit determination accuracy can be obtained with these products data as demonstrated for the GRACE-B spacecraft during selected data arcs between 2002 and 2004. This performance may be compared with a 5-cm orbit determination accuracy achievable withm IGS rapid and final products using 5 min clock samples. For improved accuracy, high-rate (30 s) clock solutions are recommended that are presently only available from individual IGS centers. Likewise, a reduced latency and more frequent updates of IGS ultra-rapid ephemerides are desirable to meet the requirements of upcoming satellite missions for near real-time and precise orbit determination.
Apparent clock variations of the Block IIF-1 (SVN62) GPS satellite
O. Montenbruck and U. Hugentobler and R. Dach and P. Steigenberger and A. Hauschild
  16  303-313  (2012)
The Block IIF satellites feature a new generation of high-quality rubidium clocks for time and frequency keeping and are the first GPS satellites transmitting operational navigation signals on three distinct frequencies. We investigate apparent clock offset variations for the Block IIF-1 (SVN62) spacecraft that have been identified in L1/L2 clock solutions as well as the L1/L5-minus-L1/L2 clock difference. With peak-to-peak amplitudes of 10--40 cm, these variations are of relevance for future precision point positioning applications and ionospheric analyses. A proper characterization and understanding is required to fully benefit from the quality of the new signals and clocks. The analysis covers a period of 8 months following the routine payload activation and is based on GPS orbit and clock products generated by the CODE analysis center of the International GNSS Service (IGS) as well as triple-frequency observations collected with the CONGO network. Based on a harmonic analysis, empirical models are presented that describe the sub-daily variation of the clock offset and the inter-frequency clock difference. These contribute to a better clock predictability at timescales of several hours and enable a consistent use of L1/L2 clock products in L1/L5-based positioning.
Precision spacecraft navigation using a low-cost GPS receiver
O. Montenbruck and P. Swatschina and M. Markgraf and S. Santandrea and J. Naudet and E. Tilmans
  16  519-529  (2012)
Within the PROBA-2 microsatellite mission, a miniaturized single-frequency GPS receiver based on commercial-off-the-shelf (COTS) technology is employed for onboard navigation and timing. A rapid electronic fuse protects against destructive single-event latch-ups (SEL) and enables a quasi-continuous receiver operation despite the inherent sensitivity to space radiation. While limited to single-frequency C/A-code tracking with a narrow-band frontend, the receiver is able to provide precision navigation services through processing of raw GPS measurements on ground as well as a built-in real-time navigation system. In both cases, ionospheric path delays are eliminated through a combination of L1 pseudorange and carrier phase measurements, which also offers a factor-of-two noise reduction relative to code-only processing. By comparison with satellite laser ranging (SLR) measurements, a 0.3-m (3D rms) accuracy is demonstrated for the PROBA-2 reduced dynamic orbit determinations using post-processed GPS orbit and clock products. Furthermore, the experimental onboard navigation system is shown to provide real-time position information with a 3D rms accuracy of about 1 m, which notably outperforms the specification of the Standard Positioning Service (SPS). In view of their lower hardware complexity, mass budget and power requirements as well as the reduced interference susceptibility, legacy C/A-code receivers can thus provide an attractive alternative to dual-frequency receivers even for demanding navigation applications in low Earth orbit.
The International GNSS Service: Any Questions?
A. W. Moore
Mitigation of site specific errors
M. Moore and S. McClusky and P. Tregoning
On the effects of the ionospheric disturbances on precise point positioning at equatorial latitudes
B. Moreno and S. Radicella and M. C. de Lacy and M. Herraiz and G. Rodriguez-Caderot
In precise point positioning (PPP), the ionospheric delay is corrected in a first-order approximation from GPS dual-frequency observations, which should eliminate almost completely the ionosphere as a source of error. However, sudden plasma density variations can adversely affect the GPS signal, degrading accuracy and reliability of positioning techniques. The occurrence of plasma density irregularities is frequent at equatorial latitudes and is reflected in large total electron content (TEC) variations. We study the relation between large changes in the rate of TEC (ROT) and positioning errors in single-epoch PPP. At equatorial latitudes and during post-sunset hours, the estimated altitudes contain errors of several meters for a single-epoch position determination, and latitude and longitude estimates are also degraded. These results have been corroborated by the online CSRS-PPP (NRCan) program. Moreover, abrupt changes in the satellite geometry have been discarded as possible cause of such errors, suggesting an apparent relation between the occurrence of large ROT and degraded position estimates.
Hydrological deformation induced by the West African Monsoon: Comparison of GPS, GRACE and loading models
S. Nahmani and O. Bock and M.-N. Bouin and A. Santamaría-Gómez and J.-P. Boy and X. Collilieux and L. Métivier and I. Panet and P. Genthon and C. de Linage and G. Wöppelmann
Journal of Geophysical Research: Solid Earth  117  n/a--n/a  (2012)
Three-dimensional ground deformation measured with permanent GPS stations in West Africa was used for investigating the hydrological loading deformation associated with Monsoon precipitation. The GPS data were processed within a global network for the 2003--2008 period. Weekly station positions were retrieved with a repeatability (including unmodeled loading effects) of 1--2 mm in the horizontal components and between 2.5 and 6 mm in the vertical component. The annual signal in the vertical component for sites located between 9.6$\,^{\circ}$N and 16.7$\,^{\circ}$N is in the range 10--15 mm. It is consistent at the 3 mm-level with the annual regional-scale loading deformations estimated from GRACE satellite products and modeled with a combination of hydrological, atmospheric, and nontidal oceanic models. An additional 6 month transient signal was detected in the vertical component of GPS estimates at most of the West African sites. It takes the form of an oscillation occurring between September and March, and reaching a maximum amplitude of 12--16 mm at Ouagadougou (12.5$\,^{\circ}$N). The analysis of in situ hydro-geological data revealed a strong coincidence between this transient signal and peak river discharge at three sites located along the Niger River (Timbuktu, Gao, and Niamey). At Ouagadougou, a similar coincidence was found with the seasonal variations of the water table depth. We propose a mechanism to account for this signal that involves a sequence of swelling/shrinking of clays combined with local loading effects associated with flooding of the Niger River.
Development of data infrastructure to support scientific analysis for the International GNSS Service
C. Noll and Y. Bock and H. Habrich and A. Moore
  83  309-325  (2009)
The International GNSS Service provides data and products to support a wide range of global, multidisciplinary scientific research. The service has established a hierarchy of components to facilitate its mission: a globally distributed network of Tracking Stations, Data Centers, Analysis Centers, a Central Bureau, and a Governing Board. The Data Centers, in conjunction with the Central Bureau, serve as the primary means of distributing GNSS data, products, and general information to the user community through ftp and Web servers and email services. The requirements of analysis centers and the scientific community have evolved over the lifetime of the IGS, requiring enhancement and extension of the supporting data center infrastructure. The diversity of IGS data and products extends today from the realm of the real-time and near real-time to the long-term archive and thus forms a basis for multidisciplinary research spanning decades. Reliability of all components is a key requirement within the IGS and is accomplished through the geographic distribution of data centers and the creation of independent, redundant, parallel channels for the transmission of data and products. We discuss the development of the IGS data infrastructure, current status, and plans for future enhancements. Descriptions of IGS data and products and associated metadata are also included
Improvements in Space Geodesy Data Discovery at the CDDIS
C. Noll and N. Pollack and P. Michael
The Crustal Dynamics Data Information System (CDDIS) supports data archiving and distribution activities for the space geodesy and geodynamics community. The main objectives of the system are to store space geodesy and geodynamics related data products in a central data bank, to maintain information about the archival of these data, and to disseminate these data and information in a timely manner to a global scientific research community. The archive consists of GNSS, laser ranging, VLBI, and DORIS data sets and products derived from these data. The CDDIS is one of NASA's Earth Observing System Data and Information System (EOSDIS) distributed data centers; EOSDIS data centers serve a diverse user community and are tasked to provide facilities to search and access science data and products. Several activities are currently under development at the CDDIS to aid users in data discovery, both within the current community and beyond. The CDDIS is cooperating in the development of Geodetic Seamless Archive Centers (GSAC) with colleagues at UNAVCO and SIO. The activity will provide web services to facilitate data discovery within and across participating archives. In addition, the CDDIS is currently implementing modifications to the metadata extracted from incoming data and product files pushed to its archive. These enhancements will permit information about CDDIS archive holdings to be made available through other data portals such as Earth Observing System (EOS) Clearinghouse (ECHO) and integration into the Global Geodetic Observing System (GGOS) portal. This poster will present the prototype implementation of these GSAC web services at the CDDIS as well as plans for the metadata enhancements to facilitate cross discipline data discovery.
Updates to the IGS Data Center Infrastructure
C. Noll and M. Schmidt and P. Michael and Y. Lu
The global data centers are key components of the IGS infrastructure, providing access to a full suite of GNSS data, products, and information. These archives are essential tools for a diverse international scientific user community. Since the start of the IGS in 1992, the number of stations, types of station data and derived products has increased substantially, requiring modifications to data center operations. Updates to the archives occur due to new data and products, re-publishing of previously archived data and products, transmission of historic data, and the resulting re-distribution of these data and products from data center to data center. In addition, statistics on the usage of the data and products are important to the data centers as well as data and product providers. This poster will review the status of the IGS data center infrastructure and present ideas, both near-term and long term, for data center equalization and validation of data holdings, modernization of compression schemes, and reporting of usage statistics.
The crustal dynamics data information system: A resource to support scientific analysis using space geodesy
C. E. Noll
Advances in Space Research  45  1421 - 1440  (2010)
Since 1982, the Crustal Dynamics Data Information System (CDDIS) has supported the archive and distribution of geodetic data products acquired by the National Aeronautics and Space Administration (NASA) as well as national and international programs. The \{CDDIS\} provides easy, timely, and reliable access to a variety of data sets, products, and information about these data. These measurements, obtained from a global network of nearly 650 instruments at more than 400 distinct sites, include \{DORIS\} (Doppler Orbitography and Radiopositioning Integrated by Satellite), \{GNSS\} (Global Navigation Satellite System), \{SLR\} and \{LLR\} (Satellite and Lunar Laser Ranging), and \{VLBI\} (Very Long Baseline Interferometry). The \{CDDIS\} data system and its archive have become increasingly important to many national and international science communities, particularly several of the operational services within the International Association of Geodesy (IAG) and its observing system the Global Geodetic Observing System (GGOS), including the International \{DORIS\} Service (IDS), the International \{GNSS\} Service (IGS), the International Laser Ranging Service (ILRS), the International \{VLBI\} Service for Geodesy and Astrometry (IVS), and the International Earth rotation and Reference frame Service (IERS). Investigations resulting from the data and products available through the \{CDDIS\} support research in many aspects of Earth system science and global change. Each month, the \{CDDIS\} archives more than one million data and derived product files totaling over 90 Gbytes in volume. In turn, the global user community downloads nearly 1.2 Tbytes (over 10.5 million files) of data and products from the \{CDDIS\} each month. The requirements of analysts have evolved since the start of the CDDIS; the specialized nature of the system accommodates the enhancements required to support diverse data sets and user needs. This paper discusses the CDDIS, including background information about the system and its user communities, archive contents, available metadata, and future plans.
Multi- technique combination at observation level with NAPEOS
M. Otten and C. Flohrer and T. Springer and W. Enderle
Use of IGS products in TAI applications
G. Petit and E. F. Arias
The Bureau International des Poids et Mesures (BIPM) is in charge of producing International Atomic Time TAI. In this aim, it uses clock data from more than 60 laboratories spread worldwide. For two decades, GPS has been an essential tool to link these clocks, and products from the International GNSS Service (IGS) have been used to improve the quality of these time links since its creation in the early 1990s. This paper reviews the various interactions between the IGS and time activities at the BIPM, and shows that TAI has greatly benefited from IGS products so that their availability is now an essential need for the quality of TAI links. On the other hand, IGS has also benefited from introducing time laboratories equipped with highly stable clocks in its network of stations. In the future, similar products will be needed for an ensemble of satellite systems, starting with GLONASS and GALILEO. It will be a major challenge to the IGS to obtain a consistent set of products, particularly for what concerns satellite clocks and inter-system bias values.
Higher-order ionospheric effects on the GPS reference frame and velocities
E. J. Petrie and M. A. King and P. Moore and D. A. Lavallée
Journal of Geophysical Research: Solid Earth  115  n/a--n/a  (2010)
We describe how GPS time series are influenced by higher-order ionospheric effects over the last solar cycle (1995--2008) and examine implications for geophysical studies. Using 14 years of globally reprocessed solutions, we demonstrate the effect on the reference frame. Including second- and third-order ionospheric terms causes up to 10 mm difference in the smoothed transformation to the International Terrestrial Reference Frame (ITRF) 2005, with the Z translation term dominant. Scale is also slightly affected, with a change of up to ∼0.05 ppb. After transformation to ITRF2005, residual effects on vertical site velocities are as high as 0.34 mm yr−1. We assess the effect of the magnetic field model on the second-order term and find a time-varying difference of 0--2 mm in the Z translation. We also assess the effect of omitting the third-order term. We find that while the second-order term is responsible for almost all the Z translation effects, it is the combination of the second- and third-order terms that causes the effect on scale. Comparison of our GPS reprocessing with ITRF2005 suggests that GPS origin rates may vary with time period. For example, we find Z translation rates of −0.82 $\pm$ 0.17 mm yr−1 for 1995--2008 and 0.17 $\pm$ 0.24 mm yr−1 for 1995--2005. If GPS were to contribute to origin rate definition for future ITRFs, higher-order ionospheric corrections would need to be applied due to their effect on translation parameters during solar maximum.
A Review of Higher Order Ionospheric Refraction Effects on Dual Frequency GPS
E. Petrie and M. Hernández-Pajares and P. Spalla and P. Moore and M. King
  32  197-253  (2011)
Higher order ionospheric effects are increasingly relevant as precision requirements on GPS data and products increase. The refractive index of the ionosphere is affected by its electron content and the magnetic field of the Earth, so the carrier phase of the GPS L1 and L2 signals is advanced and the modulated code delayed. Due to system design the polarisation is unaffected. Most of the effect is removed by expanding the refractive index as a series and eliminating the first term with a linear combination of the two signals. However, the higher order terms remain. Furthermore, transiting gradients in refractive index at a non-perpendicular angle causes signal bending. In addition to the initial geometric bending term, another term allows for the difference that the curvature makes in electron content along each signal. Varying approximations have been made for practical implementation, mainly to avoid the need for a vertical profile of electron density. The magnetic field may be modelled as a tilted co-centric dipole, or using more realistic models such as the International Geomagnetic Reference Field. The largest effect is from the second term in the expansion of the refractive index. Up to several cm on L2, it particularly affects z-translation, and satellite orbits and clocks in a global network of GPS stations. The third term is at the level of the errors in modelling the second order term, while the bending terms appear to be absorbed by tropospheric parameters. Modelling improvements are possible, and three frequency transmissions will allow new possibilities.
A first look at the effects of ionospheric signal bending on a globally processed GPS network
E. Petrie and M. King and P. Moore and D. Lavallée
  84  491-499  (2010)
This study provides a first attempt at quantifying potential signal bending effects on the GPS reference frame, coordinates and zenith tropospheric delays (ZTDs). To do this, we homogeneously reanalysed data from a global network of GPS sites spanning 14 years (1995.0--2009.0). Satellite, Earth orientation, tropospheric and ground station coordinate parameters were all estimated. We tested the effect of geometric bending and dTEC bending corrections, which were modelled at the observation level based, in part, on parameters from the International Reference Ionosphere 2007 model. Combined, the two bending corrections appear to have a minimal effect on site coordinates and ZTDs except for low latitude sites. Considering five days (DOY 301--305, 28 October--1 November 2001) near ionospheric maximum in detail, they affect mean ZTDs by up to ~1.7 mm at low latitudes, reducing to negligible levels at high latitudes. Examining the effect on coordinates in terms of power-spectra revealed the difference to be almost entirely white noise, with noise amplitude ranging from 0.3 mm (high latitudes) to 2.4 mm (low latitudes). The limited effect on station coordinates is probably due to the similarity in the elevation dependence of the bending term with that of tropospheric mapping functions. The smoothed z-translation from the GPS reference frame to ITRF2005 changes by less than 2 mm, though the effect combines positively with that from the second order ionospheric refractive index term. We conclude that, at the present time, and for most practical purposes, the geometric and dTEC bending corrections are probably negligible at current GPS/reference frame precisions.
High-accuracy subdaily ERPs from the IGS
J. Ray and J. Griffiths
Consistency of crustal loading signals derived from models & GPS: Inferences for GPS positioning errors
J. Ray and X. Collilieux and P. Rebischung and T. van Dam and Z. Altamimi
Status of IGS orbit modeling & areas for improvement
J. Ray and J. Griffiths
Why does the IGS care about EOPs?
J. Ray
Current positioning accuracy using space geodesy
J. Ray
Current positioning accuracy using space geodesy
J. Ray
Status of IGS core products
J. Ray and J. Griffiths
Future improvements in determinations of Earth orientation parameters
J. Ray
IGS reprocessed polar motion estimates from the ITRF2008 combination
J. Ray
Earth Rotation Parameters
IGS reprocessed polar motion estimates
J. Ray
Status & prospects for IGS polar motion measurements
J. Ray and R. Ferland
Preliminary analysis of IGS reprocessed orbit & polar motion estimates
J. Ray and J. Griffiths
Anomalous harmonics in the spectra of GPS position estimates
J. Ray and Z. Altamimi and X. Collilieux and T. van Dam
Prior studies of the power spectra of GPS position time series have found pervasive seasonal signals against a power-law background of flicker noise plus white noise. Dong et al. (2002) estimated that less than half the observed GPS seasonal power can be explained by redistributions of geophysical fluid mass loads. Much of the residual variation is probably caused by unidentified GPS technique errors and analysis artifacts. Among possible mechanisms, Penna and Stewart (2003) have shown how unmodeled analysis errors at tidal frequencies (near 12- and 24-hour periods) can be aliased to longer periods very efficiently. Signals near fortnightly, semiannual, and annual periods are expected to be most seriously affected. We have examined spectra for the 167 sites of the International GNSS (Global Navigation Satellite Systems) Service (IGS) network having more than 200 weekly measurements during 1996.0--2006.0. The non-linear residuals of the weekly IGS solutions that were included in ITRF2005, the latest version of the International Terrestrial Reference Frame (ITRF), have been used. To improve the detection of common-mode signals, the normalized spectra of all sites have been stacked, then boxcar smoothed for each local north (N), east (E), and height (H) component. The stacked, smoothed spectra are very similar for all three components. Peaks are evident at harmonics of about 1 cycle per year (cpy) up to at least 6 cpy, but the peaks are not all at strictly 1.0 cpy intervals. Based on the 6th harmonic of the N spectrum, which is among the sharpest and largest, and assuming a linear overtone model, then a common fundamental of 1.040 $\pm$ 0.008 cpy can explain all peaks well, together with the expected annual and semiannual signals. A flicker noise power-law continuum describes the background spectrum down to periods of a few months, after which the residuals become whiter. Similar sub-seasonal tones are not apparent in the residuals of available satellite laser ranging (SLR) and very long baseline interferometry (VLBI) sites, which are both an order of magnitude less numerous and dominated by white noise. There is weak evidence for a few isolated peaks near 1 cpy harmonics in the spectra of geophysical loadings, but these are much noisier than for GPS positions. Alternative explanations related to the GPS technique are suggested by the close coincidence of the period of the 1.040 cpy frequency, about 351.2 days, to the ``GPS year''; i.e., the interval required for the constellation to repeat its inertial orientation with respect to the sun. This could indicate that the harmonics are a type of systematic error related to the satellite orbits. Mechanisms could involve orbit modeling defects or aliasing of site-dependent positioning biases modulated by the varying satellite geometry.
Analysis effects in IGS polar motion estimates
J. Ray
Status of IGS Ultra-Rapid products for real-time applications
J. Ray and J. Griffiths
Geodetic techniques for time & frequency comparisons using GPS phase & code measurements
J. Ray and K. Senior
We review the development and status of GPS geodetic methods for high-precision global time and frequency comparisons. A comprehensive view is taken, including hardware effects in the transmitting satellites and tracking receiver stations, data analysis and interpretation, and comparisons with independent results. Other GPS techniques rely on single-frequency data and/or assume cancellation of most systematic errors using differences between simultaneous observations. By applying the full observation modelling of modern geodesy to dual-frequency observations of GPS carrier phase and pseudorange, the precision of timing comparisons can be improved from the level of several nanoseconds to near 100 ps. For an averaging interval of one day, we infer a limiting Allan deviation of about 1.4*10-15 for the GPS geodetic technique. The accuracy of time comparisons is set by the ability to calibrate the absolute instrumental delays through the GPS receiver and antenna chain, currently about 3 ns. Geodetic clock measurements are available for most of the major timing laboratories, as well as for many other tracking stations and the satellites, via the routine products of the International GPS Service.
IGS reference frames: Status & future improvements
J. Ray and D. Dong and Z. Altamimi
The hierarchy of reference frames used in the International GPS Service (IGS) and the procedures and rationale for realizing them are reviewed. The Conventions of the International Earth Rotation and Reference Systems Service (IERS) lag developments in the IGS in a number of important respects. Recommendations are offered for changes in the IERS Conventions to recognize geocenter motion (as already implemented by the IGS) and to enforce greater model consistency in order to achieve higher precision for combined reference frame products. Despite large improvements in the internal consistency of IGS product sets, defects remain which should be addressed in future developments. If the IGS is to remain a leader in this area, then a comprehensive, long-range strategy should be formulated and pursued to maintain and enhance the IGS reference frame, as well as to improve its delivery to users. Actions should include the official designation of a high-performance reference tracking network whose stations are expected to meet the highest standards possible.
Reinforcing & securing the IGS reference tracking network
J. Ray
IGS/BIPM pilot project: GPS carrier phase for time/frequency transfer & timescale formation
J. Ray and K. Senior
Dependence of IGS products on the ITRF datum
J. R. Ray and P. Rebischung and R. Schmid
Throughout its nearly two decades, the International GNSS (Global Navigation Satellite Systems) Service (IGS) has sought to align its products closely to successive realizations of the International Terrestrial Reference Frame (ITRF). This has been disruptive for IGS users at times, especially during the 1990s when some radical ITRF datum choices were adopted. During the past decade, IGS impacts due to ITRF updates have been smaller and mostly caused by errors in the results from the contributing space geodetic techniques. Frame orientations (rotations) are purely conventional, so the IGS relies on the ITRF via a subset of reliable, globally distributed stations. Except for the period when ITRF93 was used, this procedure has worked well. The IGS origin in principle could be self-reliant or contributory to ITRF by direct observation of a frame origin aligned to the long-term center of mass of the entire Earth system. In practice, however, GNSS-based results have been less reliable than those from satellite laser ranging (SLR). So the ITRF origin, based on SLR only, has been adopted historically. Until the transition from ITRF2005 to ITRF2008, there have sometimes been significant origin shifts as SLR results have evolved. However, the present stability of the ITRF origin may finally have reached the few-mm level. In many respects, the IGS dependence on the ITRF scale is most subtle and problematic. In addition to an overall Helmert alignment of the IGS frame to match the ITRF scale (and other datum parameters), since 2006 the IGS calibration values for the GNSS satellite antenna z-offsets depend directly on the same ITRF scale (due to high correlations if the IGS frame scale is not fixed). We therefore face a non-linear situation to maintain full consistency between all IGS products and the ITRF scale: each IGS frame contribution to ITRF based on one set of antenna calibrations must be used, together with frames from other techniques, to determine an updated ITRF and new antenna calibrations, which are then no longer strictly consistent with the starting IGS frame. One can hope that the process will iteratively converge eventually. But large shifts in the ITRF scale, such as the -1 ppb change from ITRF2005 to ITRF2008, are highly disturbing, much more so than the associated rotational or translational shifts. Only SLR and very long baseline interferometry (VLBI) have been considered reliable and accurate enough to be used for the ITRF scale. But experience and theoretical studies have shown that neither is accurate to better than about 1 ppb. Note in particular that a 1 ppb uncertainty in the GM constant fundamentally limits the possible scale agreement between SLR and VLBI to no better. Consequently, the authors strongly urge that the ITRF scale hereafter be fixed conventionally to the ITRF2008 scale indefinitely until it is convincingly shown that VLBI and/or SLR can determine the ITRF scale within 0.5 ppb. If this is not done, the IGS might maintain its own ITRF2008 scaled frame to minimize future operational dislocations.
Dependence of IGS products on the ITRF datum
J. R. Ray and P. Rebischung and R. Schmid
Geocenter motion estimates from the IGS Analysis Center solutions
P. Rebischung and X. Collilieux and Z. Altamimi
IGS08: the IGS realization of ITRF2008
P. Rebischung and J. Griffiths and J. Ray and R. Schmid and X. Collilieux and B. Garayt
  16  483-494  (2012)
On April 17, 2011, the International GNSS Service (IGS) stopped using the IGS05 reference frame and adopted a new one, called IGS08, as the basis of its products. The latter was derived from the latest release of the International Terrestrial Reference Frame (ITRF2008). However, the simultaneous adoption of a new set of antenna phase center calibrations by the IGS required slight adaptations of ITRF2008 positions for 65 of the 232 IGS08 stations. The impact of the switch from IGS05 to IGS08 on GNSS station coordinates was twofold: in addition to a global transformation due to the frame change from ITRF2005 to ITRF2008, many station coordinates underwent small shifts due to antenna calibration updates, which need to be accounted for in any comparison or alignment of an IGS05-consistent solution to IGS08. Because the heterogeneous distribution of the IGS08 network makes it sub-optimal for the alignment of global frames, a smaller well-distributed sub-network was additionally designed and designated as the IGS08 core network. Only 2 months after their implementation, both the full IGS08 network and the IGS08 core network already strongly suffer from the loss of many reference stations. To avoid a future crisis situation, updates of IGS08 will certainly have to be considered before the next ITRF release.
IGS08: Elaboration, consequences & maintenance of the IGS realization of ITRF2008
P. Rebischung and B. Garayt and R. Schmid and J. Ray and X. Collilieux
Impact of solar radiation pressure modeling on GNSS-derived geocenter motion
C. Rodriguez-Solano and U. Hugentobler and P. Steigenberger and M. Fritsche
Non-conservative GNSS satellite modeling: long-term behavior
C. Rodriguez-Solano and U. Hugentobler and P. Steigenberger and K. Sosnica and M. Fritsche
Solar radiation pressure & attitude modeling of GNSS satellites
C. Rodriguez-Solano and U. Hugentobler and P. Steigenberger
Earth radiation pressure model for GNSS satellites
C. Rodriguez-Solano and U. Hugentobler and P. Steigenberger
Impact of Albedo Radiation on GPS Satellites
C. J. Rodriguez-Solano and U. Hugentobler and P. Steigenberger
  136  113-119  (2012)
GPS satellite orbits available from the International GNSS Service (IGS) show a peculiar pattern in the SLR residuals at the few centimeter level that is related to radiation pressure mismodeling. Part of the mismodeling may be attributed to neglecting the solar radiation reflected and reemitted from the Earth, the albedo radiation, as most IGS analysis centers do not yet take into account this radiation pressure component. In this study the relative importance of different albedo model constituents is analyzed. The impact of nine albedo models with increasing complexity is investigated using 1 year of global GPS data from the IGS tracking network. The most important model components are the solar panels of the satellites while different Earth radiation models have a minor impact on orbits at GPS altitudes. Albedo radiation has the potential to remove part of the anomalous SLR residual pattern observed by Urschl et al. (Calibrating GNSS orbits with SLR tracking data. Proceedings of the 15th International Workshop on Laser Ranging, 2008) in a Sun-fixed reference frame.
Impact of albedo radiation on GPS satellites
C. J. Rodriguez-Solano and U. Hugentobler and P. Steigenberger
GPS satellite orbits available from the International GNSS Service (IGS) show a peculiar pattern in the SLR residuals at the few centimeter level that is related to radiation pressure mismodel-ing. Part of the mismodeling may be attributed to neglecting the solar radiation reflected and re-emitted from the Earth, the albedo radiation, as most IGS analysis centers do not yet take into ac-count this radiation pressure component. In this study the relative importance of different albedo model constituents is analyzed. The impact of nine albedo models with increasing complexity is inves-tigated using one year of global GPS data from the IGS tracking network. The most important model components are the solar panels of the satellites while different Earth radiation models have a minor impact on orbits at GPS altitudes. Albedo radiation has the potential to remove part of the anomalous SLR residual pattern observed by Urschl et al. (2008) in a Sun-fixed reference frame.
Impact of albedo modelling on GPS orbits
C. J. Rodriguez-Solano
Some studies have suggested the importance of modelling the Earth radiation that reaches GPS satellites, an effect that is not currently included by most of the Analysis Centres that contribute to the computation of the IGS (International GNSS Service) Final Orbits. It is also thought that Earth radiation could be partially responsible for the observed bias between SLR (Satellite Laser Ranging) and GPS (Global Positioning System) measurements, known as the GPS -- SLR orbit anomaly. Furthermore this bias sets the actual limits for the accuracy that can be achieved in the computation of the GPS orbits. In this thesis several models of different complexity are developed, in particular models for Earth radiation that reaches the satellites and models of the satellite structure that interact with the radiation coming from the Earth. The complete development of these models is given in the thesis together with deep analysis about the differences of the models. For the interested person, the programs for the computation of the acceleration due to Earth radiation pressure are also provided. The computed acceleration is also introduced in the computation of GPS orbits. First the perturbing acceleration is used for a very simple study case where a general understanding of the effect of Earth radiation on the orbits can be acquired. Second the perturbing acceleration is introduced in the Bernese GPS Software for the computation of GPS orbits as done by CODE (Center for Orbit Determination in Europe). The effects on the orbits are studied per revolution and per year. The computed orbits using the Bernese GPS Software are compared with SLR measurements, to have an external validation. Doing that a reduction of the GPS -- SLR orbit anomaly is obtained, leading to a potential subcentimeter accuracy of the orbits. Finally as Earth radiation and satellite models of different complexity were tested, it is possible to give a recommendation of the key factors for an adequate but simple modelling of Earth radiation pressure for GPS satellites.
The verification of GNSS tropospheric tomography model in a mountainous area
W. Rohm and J. Bosy
Advances in Space Research  47  1721 - 1730  (2011)
The \{GNSS\} (Global Navigation Satellite System) has not been developed as a meteorological data source provider, but with a careful and sophisticated processing strategy it might be used as one. The term \{GNSS\} tomography refers to the usage of the ray traced \{GNSS\} signal as scanning rays in the tomographic model input. The model is divided into a number of voxels. The system is inverted and value of refractivity is obtained. Typically, as in the most of the inverse processing, there is a problem of the undetermined system and as a consequence the cofactor matrix is close to singular. To avoid singularity additional conditions or constrains should be added to the system. Here, additional parameters are derived with the help of the air flow analysis in the Sudety mountains (south-west region of Poland), and special Slant Wet Delay (SWD) trimming procedure. The flow's synthetic parameters like the Bruint-Väisälä frequency and the Froude number are determined. This way the type of the flow is recognized and the analysis of the impact of orographic barrier has been quantified. The \{SWDs\} from the \{GNSS\} observations were tested against, \{SWD\} from raytracing through the \{COAMPS\} model field. The modified \{GNSS\} tomography model was tested for the real \{GNSS\} observations delivered from the \{GNSS\} network Karkonosze located in the Sudety mountains and compared with the \{COAMPS\} model. The solution shows a considerable improvement in comparison with plain tomographic model results.
Local tomography troposphere model over mountains area
W. Rohm and J. Bosy
Atmospheric Research  93  777 - 783  (2009)
The term \{GNSS\} meteorology refers to the utilization of the Global Navigation Satellite System's (GNSS) radio signals to derive information about the state of the troposphere. \{GNSS\} tomography allows to resolve the spatial structure and temporal behavior of the tropospheric water vapor. This paper presents the verification of \{GNSS\} tomography over dense local \{GNSS\} network. The paper addresses the problem of obtaining a stable tomographic solution from an ill-conditioned system of linear equations. The main interests are in suitable horizontal and vertical resolution in given conditions. Here the Moore--Penrose pseudo inverse of variance--covariance matrix is used. The minimum constraints solution is obtained with no additional assumptions. The results are validated with the help of simulated weather conditions. Three various scenarios are tested. As general output of this paper the optimal model construction scheme is presented with possible further improvements. The verification of the tomography model based on the local \{GPS\} KARKONOSZE, situated in the Karkonosze mountains area in Poland.
GGOS-D: homogeneous reprocessing and rigorous combination of space geodetic observations
M. Rothacher and D. Angermann and T. Artz and W. Bosch and H. Drewes and M. Gerstl and R. Kelm and D. König and R. König and B. Meisel and H. Müller and A. Nothnagel and N. Panafidina and B. Richter and S. Rudenko and W. Schwegmann and M. Seitz and P. Steigenberger and S. Tesmer and V. Tesmer and D. Thaller
  85  679-705  (2011)
In preparation of activities planned for the realization of the Global Geodetic Observing System (GGOS), a group of German scientists has carried out a study under the acronym GGOS-D which closely resembles the ideas behind the GGOS initiative. The objective of the GGOS-D project was the investigation of the methodological and information-technological realization of a global geodetic-geophysical observing system and especially the integration and combination of the space geodetic observations. In the course of this project, highly consistent time series of GPS, VLBI, and SLR results were generated based on common state-of-the-art standards for modeling and parameterization. These series were then combined to consistently and accurately compute a Terrestrial Reference Frame (TRF). This TRF was subsequently used as the basis to produce time series of station coordinates, Earth orientation, and troposphere parameters. In this publication, we present results of processing algorithms and strategies for the integration of the space-geodetic observations which had been developed in the project GGOS-D serving as a prototype or a small and limited version of the data handling and processing part of a global geodetic observing system. From a comparison of the GGOS-D terrestrial reference frame results and the ITRF2005, the accuracy of the datum parameters is about 5--7 mm for the positions and 1.0--1.5 mm/year for the rates. The residuals of the station positions are about 3 mm and between 0.5 and 1.0 mm/year for the station velocities. Applying the GGOS-D TRF, the offset of the polar motion time series from GPS and VLBI is reduced to 50 μas (equivalent to 1.5 mm at the Earth's surface). With respect to troposphere parameter time series, the offset of the estimates of total zenith delays from co-located VLBI and GPS observations for most stations in this study is smaller than 1.5 mm. The combined polar motion components show a significantly better WRMS agreement with the IERS 05C04 series (96.0/96.0 μas) than VLBI (109.0/100.7 μas) or GPS (98.0/99.5 μas) alone. The time series of the estimated parameters have not yet been combined and exploited to the extent that would be possible. However, the results presented here demonstrate that the experiences made by the GGOS-D project are very valuable for similar developments on an international level as part of the GGOS development.
Correlated errors in GPS position time series: Implications for velocity estimates
A. Santamaría-Gómez and M.-N. Bouin and X. Collilieux and G. Wöppelmann
Correlated errors in GPS position time series: Implications for velocity estimates
A. Santamaría-Gómez and M.-N. Bouin and X. Collilieux and G. Wöppelmann
This study focuses on the effects of time correlation in weekly GPS position time series on velocity estimates. Time series 2.5 to 13 years long from a homogeneously reprocessed solution of 275 globally distributed stations are analyzed in terms of noise content and velocity uncertainty assessment. Several noise models were tested, including power law and Gauss-Markov processes. The best noise model describing our global data set was a combination of variable white noise and power law noise models with mean amplitudes of ~2 mm and ~6 mm, respectively, for the sites considered. This noise model provided a mean vertical velocity uncertainty of ~0.3 mm/yr, 4--5 times larger than the uncorrelated data assumption. We demonstrated that correlated noise content with homogeneously reprocessed data is dependent on time series length and, especially, on data time period. Time series of 2--3 years of the oldest data contain noise amplitude similar to that found for time series of 12 years. The data time period should be taken into account when estimating correlated noise content, when comparing different noise estimations, or when applying an external noise estimation to assess velocity uncertainty. We showed that the data period dependency cannot be explained by the increasing tracking network or the ambiguity fixation rate but is probably related to the amount and quality of recorded data.
Correlated errors in GPS position time series: Implications for velocity estimates
A. Santamaría-Gómez and M.-N. Bouin and X. Collilieux and G. Wöppelmann
Journal of Geophysical Research: Solid Earth  116  n/a--n/a  (2011)
This study focuses on the effects of time correlation in weekly GPS position time series on velocity estimates. Time series 2.5 to 13 years long from a homogeneously reprocessed solution of 275 globally distributed stations are analyzed in terms of noise content and velocity uncertainty assessment. Several noise models were tested, including power law and Gauss-Markov processes. The best noise model describing our global data set was a combination of variable white noise and power law noise models with mean amplitudes of ∼2 mm and ∼6 mm, respectively, for the sites considered. This noise model provided a mean vertical velocity uncertainty of ∼0.3 mm/yr, 4--5 times larger than the uncorrelated data assumption. We demonstrated that correlated noise content with homogeneously reprocessed data is dependent on time series length and, especially, on data time period. Time series of 2--3 years of the oldest data contain noise amplitude similar to that found for time series of 12 years. The data time period should be taken into account when estimating correlated noise content, when comparing different noise estimations, or when applying an external noise estimation to assess velocity uncertainty. We showed that the data period dependency cannot be explained by the increasing tracking network or the ambiguity fixation rate but is probably related to the amount and quality of recorded data.
Biases in GNSS analysis
S. Schaer
Reliability of GPS data for geodynamic studies case study: Sudeten area, The Bohemian Massif
V. Schenk and Z. Schenková and J. Bosy and B. Kontny
A reliability of site movement assessments determined from GPS data monitored during eight two-day epoch measurements on the regional geodynamic EAST SUDETEN network (the Bohemian Massif, Central Europe) is discussed in details. Statistical tests of site positions processed by the BERNESE GPS software, their linear approximations for site movement velocity assessments and an establishment of probabilistic thresholds for reliability of the GPS data for regional geodynamic studies are delivered. The thresholds define necessary observation periods for annual epoch measurements performed on the networks with aim to obtain reliable movement estimates for geodynamic studies.
IGS phase center model igs08.atx -- Current status & future improvements
R. Schmid
Updated phase center corrections for satellite & receiver antennas
R. Schmid and X. Collilieux and F. Dilssner and R. Dach and M. Schmitz
Interactions of the IGS reprocessing & the IGS antenna phase center model
R. Schmid and P. Steigenberger and U. Hugentobler and R. Dach and M. Schmitz and F. Dilssner
Generation of a consistent absolute phase center correction model for GPS receiver and satellite antennas
R. Schmid and P. Steigenberger and G. Gendt and M. Ge and M. Rothacher
The development and numerical values of the new absolute phase center correctionmodel forGPS receiver and satellite antennas, as adopted by the International GNSS (global navigation satellite systems) Service, are presented. Fixing absolute receiver antenna phase center corrections to robot-based calibrations, the GeoForschungsZentrum Potsdam (GFZ) and the Technische Universität München (TUM) reprocessed more than 10 years of GPS data in order to generate a consistent set of nadir-dependent phase center variations (PCVs) and offsets in the z-direction pointing toward the Earth for all GPS satellites in orbit during that period. The agreement between the two solutions estimated by independent software packages is better than 1 mm for the PCVs and about 4 cm for the z-offsets. In addition, the long time series facilitates the study of correlations of the satellite antenna corrections with several other parameters such as the global terrestrial scale or the orientation of the orbital planes with respect to the Sun. Finally, completely reprocessed GPS solutions using different phase center correction models demonstrate the benefits from switching from relative to absolute antenna phase center corrections. For example, tropospheric zenith delay biases between GPS and very long baseline interferometry (VLBI), as well as the drift of the terrestrial scale, are reduced and the GPS orbit consistency is improved.
IGS Tide Gauge Benchmark Monitoring Pilot Project (TIGA): scientific benefits
T. Schöne and N. Schön and D. Thaller
The establishment of a long-term stable global reference frame is important for studying sea level records for, e.g., climate-related studies. GPS stations connected to the tide gauge benchmarks provide the necessary technique. However, the analysis of existing GPS solutions showed inconsistencies within the time series especially for the height component. To solve related issues, in 2001 the IGS Tide Gauge BenchmarkMonitoring Pilot Project was established. The aim is the processing and re-processing of GPS data of stations at or near tide gauges in order to provide homogeneous and high-quality estimates of the vertical motion. A second objective is the establishment, maintenance and expansion of existing network of GPS stations at tide gauges. During the recent years six different analysis centers have processed overlapping GPS at tide gauge networks and are providing individual solutions allowing now to provide a combined solution. The ansatz for the combination is explained and quality measures are given. In addition, on the basis of the reconstruction of sea level anomalies, the benefit of using the combined TIGA solution is demonstrated.
The 2008 DGFI realization of the ITRS: DTRF2008
M. Seitz and D. Angermann and M. Bloßfeld and H. Drewes and M. Gerstl
  86  1097-1123  (2012)
A new realization of the International Terrestrial System was computed at the ITRS Combination Centre at DGFI as a contribution to ITRF2008. The solution is labelled DTRF2008. In the same way as in the DGFI computation for ITRF2005 it is based on either normal equation systems or estimated parameters derived from VLBI, SLR, GPS and DORIS observations by weekly or session-wise processing. The parameter space of the ITRS realization comprises station positions and velocities and daily resolved Earth Orientation Parameters (EOP), whereby for the first time also nutation parameters are included. The advantage of starting from time series of input data is that the temporal behaviour of geophysical parameters can be investigated to decide whether the parameters can contribute to the datum realization of the ITRF. In the same way, a standardized analysis of station position time series can be performed to detect and remove discontinuities. The advantage of including EOP in the ITRS realization is twofold: (1) the combination of the coordinates of the terrestrial pole---estimated from all contributing techniques---links the technique networks in two components of the orientation, leading to an improvement of consistency of the Terrestrial Reference Frame (TRF) and (2) in their capacity as parameters common to all techniques, the terrestrial pole coordinates enhance the selection of local ties as they provide a measure for the consistency of the combined frame. The computation strategy of DGFI is based on the combination of normal equation systems while at the ITRS Combination Centre at IGN solutions are combined. The two independent ITRS realizations provide the possibility to assess the accuracy of ITRF by comparison of the two frames. The accuracy evaluation was done separately for the datum parameters (origin, orientation and scale) and the network geometry. The accuracy of the datum parameters, assessed from the comparison of DTRF2008 and ITRF2008, is between 2--5 mm and 0.1--0.8 mm/year depending on the technique. The network geometry (station positions and velocities) agrees within 3.2 mm and 1.0 mm/year. A comparison of DTRF2008 and ITRF2005 provides similar results for the datum parameters, but there are larger differences for the network geometry. The internal accuracy of DTRF2008---that means the level of conservation of datum information and network geometry within the combination---was derived from comparisons with the technique-only multi-year solutions. From this an internal accuracy of 0.32 mm for the VLBI up to 3.3 mm for the DORIS part of the network is found. The internal accuracy of velocities ranges from 0.05 mm/year for VLBI to 0.83 mm/year for DORIS. The internal consistency of DTRF2008 for orientation can be derived from the analysis of the terrestrial pole coordinates. It is estimated at 1.5--2.5 mm for the GPS, VLBI and SLR parts of the network. The consistency of these three and the DORIS network part is within 6.5 mm.
Introduction to timescales
K. Senior
Status & prospects for combined GPS LOD & VLBI UT1 measurements
K. Senior and J. Kouba and J. Ray
A Kalman filter was developed to combine VLBI estimates of UT1-TAI with biased length of day (LOD) estimates from GPS. The VLBI results are the analyses of the NASA Goddard Space Flight Center group from 24-hr multi-station observing sessions several times per week and the nearly daily 1-hr single-baseline sessions. Daily GPS LOD estimates from the International GNSS Service (IGS) are combined with the VLBI UT1-TAI by modeling the natural excitation of LOD as the integral of a white noise process (i.e., as a random walk) and the UT1 variations as the integration of LOD, similar to the method described by Morabito et al. (1988). To account for GPS technique errors, which express themselves mostly as temporally correlated biases in the LOD measurements, a Gauss-Markov model has been added to assimilate the IGS data, together with a fortnightly sinusoidal term to capture errors in the IGS treatments of tidal effects. Evaluated against independent atmospheric and oceanic axial angular momentum (AAM + OAM) excitations and compared to other UT1/LOD combinations, ours performs best overall in terms of lowest RMS residual and highest correlation with (AAM + OAM) over sliding intervals down to 3 d. The IERS 05C04 and Bulletin A combinations show strong high-frequency smoothing and other problems. Until modified, the JPL SPACE series suffered in the high frequencies from not including any GPS-based LODs. We find, surprisingly, that further improvements are possible in the Kalman filter combination by selective rejection of some VLBI data. The best combined results are obtained by excluding all the 1-hr single-baseline UT1 data as well as those 24-hr UT1 measurements with formal errors greater than 5 ?s (about 18% of the multi-baseline sessions). A rescaling of the VLBI formal errors, rather than rejection, was not an effective strategy. These results suggest that the UT1 errors of the 1-hr and weaker 24-hr VLBI sessions are non-Gaussian and more heterogeneous than expected, possibly due to the diversity of observing geometries used, other neglected systematic effects, or to the much shorter observational averaging interval of the single-baseline sessions. UT1 prediction services could benefit from better handling of VLBI inputs together with proper assimilation of IGS LOD products, including using the Ultra-rapid series that is updated four times daily with 15 hr delay.
Results from the new IGS time scale algorithm
K. Senior and J. Ray
Status & prospects for combined GPS LOD & VLBI UT1 measurements
K. Senior and J. Kouba and J. Ray
Results from the new IGS time scale algorithm
K. Senior and J. Ray
A Kalman filter to combine VLBI UT1 & GPS LOD estimates
K. Senior and J. Kouba and J. Ray
Characterization of periodic variations in the GPS satellite clocks
K. Senior and J. Ray and R. Beard
The clock products of the International Global Navigation Satellite Systems (GNSS) Service (IGS) are used to characterize the timing performance of the GPS satellites. Using 5-min and 30-s observational samples and focusing only on the sub-daily regime, approximate powerlaw stochastic processes are found. The Block IIA Rb and Cs clocks obey predominantly random walk phase (or white frequency) noise processes. The Rb clocks are up to nearly an order of magnitude more stable and show a flicker phase noise component over intervals shorter than about 100 s. Due to the onboard Time Keeping System in the newer Block IIR and IIR-M satellites, their Rb clocks behave in a more complex way: as an apparent random walk phase process up to about 100 s and then changing to flicker phase up to a few thousand seconds. Superposed on this random background, periodic signals have been detected in all clock types at four harmonic frequencies, n 9 (2.0029 $\pm$ 0.0005) cycles per day (24 h coordinated universal time or UTC), for n = 1, 2, 3, and 4. The equivalent fundamental period is 11.9826 $\pm$ 0.0030 h, which surprisingly differs from the reported mean GPS orbital period of 11.9659 $\pm$ 0.0007 h by 60 $\pm$ 11 s. We cannot account for this apparent discrepancy but note that a clear relationship between the periodic signals and the orbital dynamics is evidenced for some satellites by modulations of the spectral amplitudes with eclipse season. All four harmonics are much smaller for the IIR and IIR-M satellites than for the older blocks. Awareness of the periodic variations can be used to improve the clock modeling, including for interpolation of tabulated IGS products for higher-rate GPS positioning and for predictions in real-time applications. This is especially true for high-accuracy uses, but could also benefit the standard GPS operational products. The observed stochastic properties of each satellite clock type are used to estimate the growth of interpolation and prediction errors with time interval.
Developing an IGS time scale
K. Senior and P. Koppang and J. Ray
Empirical modeling of solar radiation pressure forces affecting GPS satellites
A. Sibthorpe and J. Weiss and N. Harvey and D. Kuang and Y. Bar-Sever
NAPEOS: The ESA/ESOC tool for space geodesy
T. Springer and F. Dilssner and D. Escobar and D. Svehla and C. Flohrer and R. Zandbergen
Multi-technique reprocessing & combination using space-ties
T. Springer and F. Dilssner and D. Escobar and M.Otten and I. Romero and J. Dow
GNSS-specific local effects at the Geodetic Observatory Wettzell
P. Steigenberger and U. Hugentobler and R. Schmid and U. Hessels and T. Klügel and M. Seitz
GNSS antenna array at the Geodetic Observatory Wettzell
P. Steigenberger and U. Hugentobler and R. Schmid and U. Hessels and T. Klügel and M. Seitz
Potential use of atmospheric & ocean angular momentum forecasts for polar motion prediction
B. E. Stetzler and B. Luzum and J. Cline
Prediction of SBAS Integrity Performance using PORIMA Algorithm
H. Su
The paper presents an independent method using the PORIMA algorithm to predict SBAS integrity performance regarding protection levels and to provide timely warnings when SBAS integrity performance declines for an assigned flight phase in close future. The PORIMA is a user integrity monitoring and prediction algorithm using IGS ultra-rapid product. The algorithm uses differences between the GPS broadcast ephemeris and the precise predicted orbits (IGS ultra-rapid) to determine the thresholds of measurement errors due to satellite orbit and satellite clock errors as well as troposhreic, ionospheric and multipath errors and compute the related protection levels for a user location in real-time. The accuracy of a GPS predicted orbit can reach a level of about 24 $\pm$ 6 mm, much higher than the accuracy of the broadcast ephemeris, therefore the predicted GPS orbit is able to be used as reference for analysis of the performance of the GPS broadcast ephemeris both in real time and in advance. The exact impact of GPS SIS (Signal-in-Space) errors on a user location can be determined in advance and therefore the GPS integrity performance in terms of protection levels can be predicted earlier than the actual SBAS real-time results. The system fault models are considered. The errors such as tropospheric and multipath etc. are corrected according to SBAS models and the ionospheric effects are calculated using the broadcast Klobuchar model. With civil dual frequency code measurements, the impact of ionospheric errors would even be smaller.
A New User Integrity Prediction and Monitoring Algorithms for Aviation Application
H. Su and W. Ehret
The paper presents new user integrity prediction and monitor algorithms for aviation applications - PORIMA ( Precise Predicted Satellite Orbit based Receiver Integrity Monitoring Algorithm). Using the PORIMA for GNSS SIS integrity monitoring and satellite failure detection, the thresholds of user satellite measurement errors can be determined based on the differences between the broadcast ephemeris and the precise predicted orbits and mapped to user locations. The actual user satellite measurement errors at each epoch are obtained from the measurement residuals of the user navigation solution or PVT (Position, Velocity and Time Solution). If the satellite measurement errors are larger than the thresholds, the related satellites are labeled as ``bad'' satellites and those measurements will be removed from the navigation solutions or PVT. After removing ``bad'' measurements, the navigation solution at that epoch will be performed again and the related protection levels are calculated as well. In such a way, the satellite failures such as satellite maintenance anomaly, satellite manoeuvre, satellite clock jump and signal distortion etc. can be easily detected. The paper also presents an approach to use PORIMA for GNSS SIS integrity prediction. As IGS GPS ultra-rapid orbits are predicted for at least 24 hours in advance and the broadcast ephemeris transmitted by GPS satellites are updated at least in two-hourly intervals. Therefore using PORIMA, the actual broadcast ephemeris and the precise predicted orbits, the user integrity performance in the next six or more hours in terms of protection levels can be predicted. Since precise predicted GNSS orbits are used as reference, the user receivers are able to determine the exact impact from GNSS SIS errors on the user navigation performance. Therefore using PORIMA, the users can achieve better performance of the integrity monitor and satellite failure detection than using RAIM. In fact the performance is near SBAS levels. Other errors such as ionospheric, tropospheric and multipath effects etc. can be corrected using SBAS model (ref. RTCA MOPS D229). With civil code measurements from L2 or L5, the performance of PORIMA can even be further improved.
Vertical deformations from homogeneously processed GRACE and global GPS long-term series
V. Tesmer and P. Steigenberger and T. Dam and T. Mayer-Gürr
  85  291-310  (2011)
Temporal variations in the geographic distribution of surface mass cause surface displacements. Surface displacements derived from GRACE gravity field coefficient time series also should be observed in GPS coordinate time series, if both time series are sufficiently free of systematic errors. A successful validation can be an important contribution to climate change research, as the biggest contributors to mass variability in the system Earth include the movement of oceanic, atmospheric, and continental water and ice. In our analysis, we find that if the signals are larger than their precision, both geodetic sensor systems see common signals for almost all the 115 stations surveyed. Almost 80% of the stations have their signal WRMS decreased, when we subtract monthly GRACE surface displacements from those observed by GPS data. Almost all other stations are on ocean islands or small peninsulas, where the physically expected loading signals are very small. For a fair comparison, the data (79 months from September 2002 to April 2009) had to be treated appropriately: the GPS data were completely reprocessed with state-of-the-art models. We used an objective cluster analysis to identify and eliminate stations, where local effects or technical artifacts dominated the signals. In addition, it was necessary for both sets of results to be expressed in equivalent reference frames, meaning that net translations between the GPS and GRACE data sets had to be treated adequately. These data sets are then compared and statistically analyzed: we determine the stability (precision) of GRACE-derived, monthly vertical deformation data to be ~1.2 mm, using the data from three GRACE processing centers. We statistically analyze the mean annual signals, computed from the GPS and GRACE series. There is a detailed discussion of the results for five overall representative stations, in order to help the reader to link the displayed criteria of similarity to real data. A series of tests were performed with the goal of explaining the remaining GPS--GRACE residuals.
GNSS satellites as co-locations for a combined GNSS & SLR analysis
D. Thaller and K. Sosnica and R. Dach and A. Jäggi and M. Mareyen and B. Richter and G. Beutler
Precipitable water vapor estimates from homogeneously reprocessed GPS data: An intertechnique comparison in Antarctica
I. D. Thomas and M. A. King and P. J. Clarke and N. T. Penna
Journal of Geophysical Research: Atmospheres  116  n/a--n/a  (2011)
Homogeneously reprocessed GPS data offer the possibility of an accurate, stable, and increasingly long-term record of integrated precipitable water vapor (PW) of particular value in data sparse regions. We present such a global reanalysis of GPS data, focusing on 12 Antarctic sites. We show stepwise improvements of GPS zenith total delay (ZTD) estimates upon adoption of each of (1) absolute antenna phase centre variations, (2) VMF1 tropospheric mapping functions, and (3) an accurate model of a priori zenith hydrostatic delay (ZHD) from observed surface meteorological data. The cumulative effect of these three additions to the analysis is a systematic decrease in the magnitude of GPS estimates of ZTD by an average of ∼11 mm ZTD (∼1.8 mm PW). The resultant GPS PW data set for 2004 shows a mean bias to radiosonde measurements of -0.48 mm PW. Our conclusion is that, in Antarctica at least, a proportion of the widely observed bias between GPS and radiosonde measurements can be explained by earlier GPS analysis deficiencies. We also compare our GPS PW measurements with AIRS and MODIS level 2 PW products. The GPS agreements with AIRS and MODIS are comparable. Reanalyzed GPS gives typically larger measurements than AIRS with a mean site bias of 0.58 mm PW and a mean rms of 1.24 mm PW. By contrast, the GPS measurements are typically smaller than those from MODIS, with a mean site bias of -0.35 mm PW and rms of 1.42 mm PW. PW estimates from reprocessed GPS solutions using state-of-the-art models now have greater potential for assimilation into regional or global numerical weather models.
Widespread low rates of Antarctic glacial isostatic adjustment revealed by GPS observations
I. D. Thomas and M. A. King and M. J. Bentley and P. L. Whitehouse and N. T. Penna and S. D. P. Williams and R. E. M. Riva and D. A. Lavallee and P. J. Clarke and E. C. King and R. C. A. Hindmarsh and H. Koivula
Geophysical Research Letters  38  n/a--n/a  (2011)
Bedrock uplift in Antarctica is dominated by a combination of glacial isostatic adjustment (GIA) and elastic response to contemporary mass change. Here, we present spatially extensive GPS observations of Antarctic bedrock uplift, using 52% more stations than previous studies, giving enhanced coverage, and with improved precision. We observe rapid elastic uplift in the northern Antarctic Peninsula. After considering elastic rebound, the GPS data suggests that modeled or empirical GIA uplift signals are often over-estimated, particularly the magnitudes of the signal maxima. Our observation that GIA uplift is misrepresented by modeling (weighted root-mean-squares of observation-model differences: 4.9--5.0 mm/yr) suggests that, apart from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high.
Systematic Errors in Global Radiosonde Precipitable Water Data from Comparisons with Ground-Based GPS Measurements
J. Wang and L. Zhang
Journal of Climate  21  2218--2238  (2008)
A global, 10-yr (February 1997--April 2006), 2-hourly dataset of atmospheric precipitable water (PW) was produced from ground-based global positioning system (GPS) measurements of zenith tropospheric delay (ZTD) at approximately 350 International Global Navigation Satellite Systems (GNSS) Service (IGS) ground stations. A total of 130 pairs of radiosonde and GPS stations are found within a 50-km distance and 100-m elevation of each other. At these stations, 14 types of radiosondes are launched and the following 3 types of humidity sensors are used: capacitive polymer, carbon hygristor, and goldbeater's skin. The PW comparison between radiosonde and GPS data reveals three types of systematic errors in the global radiosonde PW data: measurement biases of the 14 radiosonde types along with their characteristics, long-term temporal inhomogeneity, and diurnal sampling errors of once- and twice-daily radiosonde data. The capacitive polymer generally shows mean dry bias of −1.19 mm (−6.8%). However, the carbon hygristor and goldbeater's skin hygrometers have mean moist biases of 1.01 mm (3.4%) and 0.76 mm (5.4%), respectively. The protective shield over the humidity sensor boom introduced in late 2000 reduces the PW dry bias from 6.1% and 2.6% in 2000 to 3.9% and −1.14% (wet bias) in 2001 for the Vaisala RS80A and RS80H, respectively. The dry bias in Vaisala radiosondes has larger magnitudes during the day than at night, especially for RS90 and RS92, with a day--night difference of 5%--7%. The time series of monthly mean PW differences between the radiosonde and GPS are able to detect significant changes associated with known radiosonde type changes. Such changes would have a significant impact on the long-term trend estimate. Diurnal sampling errors of twice-daily radiosonde data are generally within 2%, but can be as much as 10%--15% for the once-daily soundings. In conclusion, this study demonstrates that the global GPS PW data are useful for identifying and quantifying several kinds of systematic errors in global radiosonde PW data. Several recommendations are made for future needs of global radiosonde and GPS networks and data.
A near-global, 2-hourly data set of atmospheric precipitable water from ground-based GPS measurements
J. Wang and L. Zhang and A. Dai and T. Van Hove and J. Van Baelen
Journal of Geophysical Research: Atmospheres  112  n/a--n/a  (2007)
A 2-hourly data set of atmospheric precipitable water (PW) has been produced from the zenith path delay (ZPD) derived from ground-based Global Positioning System (GPS) measurements. The PW data are available every 2 hours from 80 to 268 International GNSS Service (IGS, formally International GPS Service) ground stations from 1997 to 2004. The accuracy of the IGS ZPD product is roughly 4 mm. An analysis technique is developed to convert ZPD to PW on a global scale. Special efforts are made on deriving surface pressure (Ps) and water-vapor-weighted atmospheric mean temperature (Tm), which are two key parameters for converting ZPD to PW. Ps is derived from global, 3-hourly surface synoptic observations with temporal, vertical and horizontal adjustments. Tm is calculated from NCEP/NCAR reanalysis with temporal, vertical and horizontal interpolations. The derived Ps and Tm at the GPS location and height have root-mean-square (rms) errors of 1.65 hPa and 1.3 K, respectively. A theoretical error analysis concludes that typical PW error associated with the errors in ZPD, Tm and Ps is on the order of 1.5 mm. The PW data set is compared with radiosonde, microwave radiometer (MWR) and satellite data. The GPS and radiosonde PW comparisons at 98 stations around the globe show a mean difference of 1.08 mm (drier for radiosonde data) with a standard deviation of differences of 2.68 mm, which corresponds to mean percentage difference and standard deviation of 5.5% and 10.6%, respectively. The bias is primarily due to known dry biases in the Vaisala radiosonde data. The RMS difference between GPS and radiosonde/MWR data ranges from 1.2 mm to 2.83 mm. The latitudinal and seasonal variations of PW derived from the GPS data agree well with that from International Satellite Cloud Climatology Project (ISCCP) data if the ISCCP data are sampled only at grid boxes containing GPS stations. The large difference between GPS and ISCCP data in the subtropics is interesting, but is not easily explained. The comparisons did not reveal any systematic bias in GPS PW data and show that a RMS difference of less than 3 mm between GPS-derived PW and other data sets is achieved. The comparison study also illustrates the value of GPS-estimated PW for examining the quality of other data sets, such as those from radiosondes and MWR. Preliminary analysis of this data set shows interesting and significant diurnal variations in PW in four different regions.
Carrier-phase inter-frequency biases in GLONASS receivers
L. Wanniger
The frequency division multiplexing of the GLONASS signals causes inter-frequency biases in the receiving equipment. These biases vary considerably for receivers from different manufacturers and thus complicate or prevent carrier-phase ambiguity fixing. Complete and reliable ambiguity fixing requires a priori information of the carrier- phase inter-frequency bias differences of the receivers involved. GLONASS carrier-phase inter-frequency biases were estimated for 133 individual receivers from 9 manufacturers. In general, receivers of the same type and even receivers from the same manufacturer show similar biases, whereas the differences among manufacturers can reach up to 0.2 ns (more than 5 cm) for adjacent frequencies and thus up to 2.4 ns (73 cm) for the complete L1 or L2 frequency bands. A few individual receivers were identified whose inter-frequency biases behave differently as compared to other receivers of the same type or whose biases vary with time.
Real-time Clock and Orbit Corrections for Improved Point Positioning via NTRIP
G. Weber and L. Mervart and Z. Lukes and C. Rocken and J. Dousa
Networked Transport of RTCM via Internet Protocol (Ntrip) … IP-Streaming for Real-Time GNSS Applications
G. Weber and D. Dettmering and H. Gebhard and R. Kalafus
Characterizing GPS Block IIA shadow and post-shadow maneuvers
J. Weiss and Y. Bar-Sever and W. Bertiger and S. Desai and N. Harvey and A. Sibthorpe
Terrestrial reference frame realization from combined GPS/LEO orbit determination
J. P. Weiss and W. Bertiger and S. D. Desai and B. J. Haines and N. Harvey
TerraSAR-X precise orbit determination with real-time \{GPS\} ephemerides
M. Wermuth and A. Hauschild and O. Montenbruck and R. Kahle
Advances in Space Research  50  549 - 559  (2012)
For active and future Earth observation missions, the availability of near real-time precise orbit information is becoming more and more important. The latency and quality of precise orbit determination results is mainly driven by the availability of precise \{GPS\} ephemerides and clocks. In order to have high-quality \{GPS\} ephemerides and clocks available at real-time, the German Space Operations Center (GSOC) has developed the real-time clock estimation system RETICLE. The system receives data streams with \{GNSS\} observations from the global tracking network of the International \{GNSS\} Service (IGS) in real-time. Using the known station position, \{RETICLE\} estimates precise \{GPS\} satellite clock offsets and drifts based on the most recent available ultra rapid predicted orbits provided by the IGS. The clock offset estimates have an accuracy of better than 0.3 ns and are globally valid. The latency of the estimated clocks is approximately 7 s after the observation epoch. Another limiting factor is the frequency of satellite downlinks and the latency of the data transfer from the ground station to the operations center. Therefore a near real-time scenario using \{GPS\} observation data from the TerraSAR-X mission is examined in which the satellite has about one ground station contact per orbit or respectively one contact in 90 min. This test campaign shows that precise orbits can be obtained in near real-time. With the use of estimated clocks an orbit accuracy of better than 10 cm (3D-RMS) can be obtained. The evaluation of satellite laser ranging (SLR) observations shows residuals of 2.1 cm (RMS) for orbits using \{RECTICLE\} and residuals of 4.2 cm (RMS) for orbits using the \{IGS\} ultra rapid ephemerides and clocks products. Hence the use of estimated clocks improves the orbit determination accuracy significantly (∼factor 2) compared to using predicted clocks.
Rates of sea-level change over the past century in a geocentric reference frame
G. Wöppelmann and C. Letetrel and A. Santamaria and M.-N. Bouin and X. Collilieux and Z. Altamimi and S. D. P. Williams and B. M. Miguez
Geophysical Research Letters  36  n/a--n/a  (2009)
The results from a carefully implemented GPS analysis, using a strategy adapted to determine accurate vertical station velocities, are presented. The stochastic properties of our globally distributed GPS position time series were inferred, allowing the computation of reliable velocity uncertainties. Most uncertainties were several times smaller than the 1--3 mm/yr global sea level change, and hence the vertical velocities could be applied to correct the long tide gauge records for land motion. The sea level trends obtained in the ITRF2005 reference frame are more consistent than in the ITRF2000 or corrected for Glacial-Isostatic Adjustment (GIA) model predictions, both on the global and the regional scale, leading to a reconciled global rate of geocentric sea level rise of 1.61 $\pm$ 0.19mm/yr over the past century in good agreement with the most recent estimates.
Geocentric sea-level trend estimates from GPS analyses at relevant tide gauges world-wide
G. Wöppelmann and B. Martin Miguez and M.-N. Bouin and Z. Altamimi
Global and Planetary Change  57  396--406  (2007)