Statistics – Applications
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.g41c0633r&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #G41C-0633
Statistics
Applications
1229 Reference Systems, 1240 Satellite Geodesy: Results (6929, 7215, 7230, 7240), 1241 Satellite Geodesy: Technical Issues (6994, 7969), 1244 Standards And Absolute Measurements
Scientific paper
Since November 2000 the International GNSS Service (IGS) has produced Ultra-rapid (IGU) products for near real-time and real-time applications. They include GPS orbits, satellite clocks, and Earth rotation parameters for a sliding 48-hr period. The first day of each update is based on the most recent GPS observational data from the IGS hourly tracking network. At the time of release, these observed products have an initial latency of 3 hr. The second day of each update consists of predictions. So the predictions between about 3 and 9 hr into the second half are relevant for true real-time uses. Originally updated twice daily, the IGU products since April 2004 have been issued four times per day, at 3, 9, 15, and 21 UTC. Up to seven Analysis Centers (ACs) contribute to the IGU combinations: Astronomical Institute of the University of Berne (AIUB), European Space Operations Center (ESOC), Geodetic Observatory Pecny (GOP), GeoForschungsZentrum (GFZ) Potsdam, Natural Resources Canada (NRC), Scripps Insitution of Oceanography (SIO), U.S. Naval Observatory (USNO). This redundancy affords a high measure of reliability and enhanced orbit accuracy. IGU orbit precision has improved markedly since late 2007. This is due to a combination of factors: decommissioning of the old, poorly behaved PRN29 in October 2007; upgraded procedures implemented by GOP around the same time, by SIO in spring 2008, and by USNO in June 2008; better handling of maneuvered satellites at the combination level starting June 2008; and stricter AC rejection criteria since July 2008. As a consequence, the weighted 1D RMS residual of the IGU orbit predictions over their first 6 hr is currently about 20 to 30 mm (after a Helmert transformation) compared to the IGS Rapid orbits, averaged over the constellation. The median residual is about 15 to 20 mm. When extended to the full 24 hr prediction period, the IGU orbit errors approximately double. Systematic rotational offsets are probably more important than random errors due to limitations in EOP predictions, especially UT1, reaching up to about 35 mm RMS (equatorial at GPS altitude) about the Z axis. The observed orbits in the first half of each IGU update have WRMS residuals of about 10 to 12 mm. Note that while the precision of the Rapid orbits is around 7 to 9 mm (compared to the IGS Finals) discontinuities between successive daily orbits imply an inaccuracy in the IGS orbits of at least 21 mm WRMS. So it is likely that the observed IGU orbits are nearly comparable in accuracy to the Rapids and that the current IGU orbit predictions are not worse by more than a factor of two or so. Only four ACs (ESOC, GFZ, NRC, USNO) contribute estimates of the satellite clocks, which limits the robustness and quality of the IGU clock products. Because the stochastic component of clock variations is not predictable, errors for the second-half IGU clock predictions grow quickly to the same level as the broadcast navigation values. But the IGU observed clocks have typical errors just about double that of the Rapids. The scatters of precise point positions using the IGU observed products are only slightly greater than for the Rapids.
Griffiths Jonny
Ray Jaideep
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