The RTS Products consist of GNSS satellite orbit and clock corrections to the broadcast ephemeris. The RTS correction streams are formatted according to the RTCM SSR standard for State Space Representation and are broadcast using the NTRIP protocol. RTS corrected orbits are expressed within the International Terrestrial Reference Frame 2008 (ITRF08).
The product streams available in the RTS are combination solutions generated by processing individual Real Time solutions from participating Real-time Analysis Centers (RTAC). The effect of combining the different RTAC results is a more reliable and stable performance than that of any single AC's product. Operational responsibility for the official combination products lies with the IGS Real Time Analysis Center Coordinator (RTACC).
The official products currently include corrections to the GPS satellite orbits and clocks:
IGS01/IGC01: Single epoch GPS combination is a single-epoch combination solution produced using software developed by ESA/ESOC. The solution epochs in this product are completely independent of each other, which has the advantage that the full accuracy is available as soon as product generation starts. The combination process removes a common offset from all satellite clocks at each epoch, computed by processing pairs of Analysis Center solutions, in order to align the clocks in each contributing solution. The aligned clocks are then screened for outliers and combined by calculating a weighted average clock value for each satellite. The orbit states are combined by using the average value from all contributions. The product is offered with the orbit solution referred to the antenna phase center under the designation IGS01. A parallel product in center of mass coordinates is designated IGC01. Analysis Center RTCM stream decoding and combination solution encoding uses BKG's BNC software.
IGS02: Kalman fIlter GPS combination is a Kalman filter combination produced using BKG's BNC software. The Kalman filter solution requires a few minutes convergence time to reach full accuracy. Once converged, the accuracy is maintained unless there is a reason to restart the software. A mechanism is in place to avoid publishing results during the convergence period. The orbit information in IGS02 is extracted from one of the incoming ultra-rapid solutions. In the Kalman Filter approach satellite clocks estimated by individual Analyses Centers (ACs) are used as pseudo observations within the adjustment process. Each observation is modeled as a linear function of three estimated parameters: AC specific offset, satellite specific offset common to all ACs, and the actual satellite clock correction which represents the result of the combination. These three parameter types differ in their statistical properties. The satellite clock offsets are assumed to be static parameters while AC specific and satellite specific offsets are stochastic parameters with appropriate white noise. The solution is regularized by a set of minimal constraints. A recursive algorithm is used to detect orbit outliers. The greatest difference between AC specific and mean satellite positions is computed. If this is greater than a threshold, then corrections of the affiliated AC are ignored for the affected epoch.
The IGS is working on including solutions for additional constellations, starting with GLONASS. A third combination product, IGS03, containing GPS and GLONASS corrections is offered as an experimental product:
IGS03: Kalman filter GPS+GLONASS combination is a Kalman filter combination produced using BKG's BNC software and follows the same approach as IGS02. The major difference from IGS02 is that GLONASS corrections are included in addition to GPS. While clocks are combined for both systems in a Kalman filter approach, orbits are extracted from one of the incoming correction streams and checked for gross errors. Contributing analysis centers use orbit information either from IGS Ultra Rapid products (IGV), or disseminate their own orbit estimates. Note that a major part of the real-time IGS reference stations nowadays supports GLONASS.
The RTS products are disseminated in the form of RTCM SSR streams. The technical content of the RTS products is described in the Table below. The products, designated at IGS01/ICG01 and IGS02, contain corrections only for the GPS satellites. The experimental product, designated at IGS03, contains corrections for GPS and GLONASS.
|Stream Name||Description||Ref Point||RTCM Messages||Provider / Solution ID||Bandwidth kbits||Software|
|IGS01||Orbit/Clock Correction, Single-Epoch Combination||APC||1059 (5),1060 (5)||258 / 1||1.8/sec||ESA/ESOC|
|IGC01||Orbit/Clock Correction, Single-Epoch Combination||CoM||1059 (5),1060 (5)||258 / 9||1.8/sec||ESA/ESOC|
|IGS02||Orbit/Clock Correction, Kalman Filter Combination||APC||1057 (60), 1058 (10), 1059 (10)||258 / 2||0.6/sec||BKG|
|IGS03||Orbit/Clock Correction, Kalman Filter Combination||APC||1057(60), 1058(10), 1059(10), 1063(60), 1064(10), 1065(10)||258 / 3||0.8/sec||BKG|
|APC: Antenna Phase Center CoM: Center of Mass, (not compliant with current RTCM-SSR standard). The figures in brackets next to each RTCM message ID denote the message sample interval in seconds.|
The RTCM v3 streams listed above may be used to support development and testing of real-time Precise Point Positioning (PPP) and related applications.
|RTCM||v3 Message Types|
|1019||GPS Broadcast Ephemeris|
|1020||GLONASS Broadcast Ephemeris|
|1045||Galileo Broadcast Ephemeris|
|1057||GPS orbit corrections to Broadcast Ephemeris|
|1058||GPS clock corrections to Broadcast Ephemeris|
|1059||GPS code biases|
|1060||Combined orbit and clock corrections to GPS Broadcast Ephemeris|
|1061||GPS User Range Accuracy|
|1062||High-rate GPS clock corrections to Broadcast Ephemeris|
|1063||GLONASS orbit corrections to Broadcast Ephemeris|
|1064||GLONASS clock corrections to Broadcast Ephemeris|
|1065||GLONASS code biases|
|1066||Combined orbit and clock corrections to GLONASS Broadcast Ephemeris|
|1067||GLONASS User Range Accuracy|
|1068||High-rate GLONASS clock corrections to Broadcast Ephemeris|
Orbit corrections are provided as along-track, cross-track and radial offsets to the Broadcast Ephemeris in the Earth-centered, Earth-fixed reference frame. After applying corrections, the satellite position is referred to the 'ionospheric free' phase center of the antenna (for the APC streams) or to the satellite Center of Mass (CoM streams). Clock corrections are given as offsets to the Broadcast Ephemeris satellite clock corrections.
The SSR format provides for the dissemination of signal code biases. These would be the biases to apply to the pseudo ranges for the signals that are processed in generating the RTS solution. Because of the complexities of the combination, and the fact that different Analysis Centers could potentially use different signals, it was decided to set the signal biases to zero in the current product streams. The user is encouraged to use other sources for signal bias solutions, if these are needed.
RTS also provides real-time access to broadcast ephemeris. Two streams are generated which carry only ephemeris data and no observations. Incoming ephemeris are checked for plausibility then merged, encoded and uploaded to NTRIP broadcasters with a high repetition rate.
RTCM3EPH: Broadcast ephemeris for GPS, GLONASS and Galileo satellites are produced using BKG’s BNC software. The data stream is derived from a major part of receivers in the real-time IGS global network and is encoded in RTCM Version 3 messages. The complete set of messages is repeated every five seconds.
RTCM3EPH01: Broadcast ephemeris for GPS using DLR’s RETICLE software. The data stream is also derived from the real-time IGS global network and encoded in RTCM Version 3 messages with a five seconds repetition rate.
|Stream Name||Description||RTCM Messages||Supported GNSS||Bandwidth kbits||Software|
|RTCM3EPH||Broadcast Ephemeris||1019(5), 1020(5), 1045(5)||GPS, GLONASS, Galileo||6.0/sec||BKG/BNC|
Global IGS RTS product streams refer to the International Terrestrial Reference Frame 2008 (ITRF2008). Other product streams can refer to Regional Reference Frames such as ETRF2000, GDA94, NAD83, SIRGAS95, or SIRGAS2000. Applying orbit and clock corrections from regional product streams in a real-time PPP solution automatically leads to regional coordinates. The PPP client would not need to transform coordinates because that is already done on the server side. More information on Regional resources can be found at http://igs.bkg.bund.de/ntrip/orbits#Transformations.