Workshop 2018
João Francisco Galera Monico - São Paulo State University contribution to the IGMA Trial Project – PDOP and broadcast ephemeris accuracy assessment - October 29, 2018 • 246 Views
In this presentation it is described the São Paulo State University – Unesp preliminary contribution to the International GNSS Monitoring and Assessment (IGMA) trial project. Unesp contributions are given in two approaches. The first one was towards the PDOP calculation and the development of a methodology to create the “.pdop” files. The second approach is related to the assessment of the quality of the broadcast navigation message from Galileo and GPS compared to the final International GNSS Service (IGS) ephemeris and between Unesp results and other analyzes center. For PDOP a tool called UNESPDOP was created with the option of using almanac or broadcast ephemeris data for both, GPS and Galileo, with a grid using the fixed angle method with 5-degrees spacing and the following boundaries: Latitude 10 to -40-degrees; Longitude -30 to 80-degrees. The time sample is given in epochs of 5 minutes. For the test week proposed by IGMA (week 1956) the average PDOP for Galileo standalone in the 7 days was 12.13 with an average standard deviation (SD) of 79.63 and an average of visible satellites of 5.34. For GPS, the average value of PDOP was 1.81 with an average SD of 0.33 and visible satellites average of 9.66. For the combination of both systems, the average was 1.41, the average of the SD was 0.18 and the average visible satellite was 15.01. The average improvement from using Galileo only to GPS and Galileo is of the order of 80% for the week. And for GPS only in comparison to GPS and Galileo it reached the order of 22%. For the broadcast ephemeris assessment, it was considered the initial test proposed by IGMA working group, assessing results of a common day (DOY 100 of 2018) without antenna offset to correct the difference between the center of mass (CoM) and the center of phase (CoP). The workgroup provided the broadcast and final ephemeris files in order to hand-care the results comparisons from the different analyses centers. Unesp results were compared with those from the Research Institute of Geodesy, Topography and Cartography (RIGTC), Institut Cartogràfic i Geològic de Catalunya (ICGC) and Deutsches Zentrum für Luft- und Raumfahrt (DZL) and with the final ephemerides from IGS. When comparing to ICGC, the results for Galileo satellites had a mean of zero and the SD of the order of 0.10 m for each of the X, Y and Z coordinates and a mean of 0.01 ns with a SD of 0.35 ns for the clocks. For GPS, the discrepancies of each of the X Y and Z coordinates had a mean of zero with a SD of about 0.11 m. For the clock it showed a mean of 0.01 ns with a SD of 1.51 ns. Unesp and ICGC results agreed quite well for both systems with few centimeters in SD. Considering the assessment with RIGTC and DZL the results agreed better for Galileo and the GPS results presented higher discrepancies. When comparing against the IGS GPSfinal ephemeris, it was obtained a mean of the order of -0.01 m for each X, Y and Z coordinates and 4.41 ns for the clocks. The SDs are of the order of 0.25 m for X, Y and Z and of 0.27 ns for the clocks. For Galileo, the average is of the order of 0.01 m for X, Y and Z and 408 ns for the clocks. The SDs are of about 0.46 m for X, Y and Z and 0.54 ns for the clocks. It is important to highlight that here, the IGS coordinates are in the CoM and the broadcasted in the CoP. The next steps will be towards the implementation of the antenna offset to correct the difference between the CoM and the CoP to make a more reliable comparison between final IGS and broadcast ephemeris, whose results will be presented in the workshop. The accuracy assessment has shown that even though the different analyzes centers are probably using the same algorithm described in the literature, different results have been obtained. The workshop will be a good opportunity for discussing such issues.

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