Cracking the GPS - SLR Orbit Anomaly

Marek Ziebart, Ant Sibthorpe and Paul Cross, Yoaz Bar-Sever and Bruce Haines

Abstract:	The global scientific community has come to rely heavily upon GPS as a tool to observe, monitor and model Earth system processes such as sea level rise, post-glacial rebound, plate tectonics and the hydrological cycle. These processes have signals varying at the level of millimetres per year. In addition GPS data is used as one of the corner stones for the realisation of the global international terrestrial reference frame. Any systematic errors or biases in the estimates of the GPS satellite orbits will alias into these analyses. The principal independent test of GPS orbit accuracy has been laser tracking of SVN35 and SVN39. A comparison of altitudes derived from both satellite laser ranging data and from GPS orbits (estimated using phase and pseudorange data) shows a consistent and intractable bias of 4-5 cm. Laser ranging accuracy is at the level of 5-6 mm and hence it is suspected that the problem lies in some aspect of the GPS orbit estimation process. Whilst the bias may seem small it has some strong knock-on effects in terms of our ability to predict the satellite orbits – an error of 5cm in height results in a mis-modelling of the gravity field of the order of 2.1E-09 m/s2 and an along track prediction error of circa 2m. In addition, as the square of the orbital period is proportional to the cube of the semi-major axis (Kepler’s third law) it is clear that a small height error can map into a significant orbital period error. In this study a team of scientists from University College London and the NASA Jet Propulsion Laboratory have sought to resolve the GPS orbit anomaly. We have applied a new generation of high precision nonconservative force models in the orbit determination process. These force models cover two effects: Planetary Radiation Pressure (PRP) and Antenna Thrust (AT). PRP here is the combined effect of radiation both emitted (Long Wave – LW) and reflected (Short Wave – SW) by the Earth. AT is the reaction force on the satellite due to the energy transmitted in the L-band carrier waves. In the paper we describe the data sources and modelling techniques applied. Orbit determination tests were carried out using JPL’s GIPSY OASIS II software. We present initial results for five days of precise orbits. In these tests the SLR bias is reduced by 2.1 cm when using the new force models. In addition it has come to our notice that the accepted figure for the offset between the GPS SV centre of mass and the laser retro-reflector array may be in error by 11 mm for one satellite and 13 mm for the other. Combined with our force modelling result we can posit a reduction in the SLR bias of 32 – 34 mm, which is a significant reduction.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	2033 - 2038
Cite this article:	Ziebart, Marek, Sibthorpe, Ant, Cross, Paul, Bar-Sever, Yoaz, Haines, Bruce, "Cracking the GPS - SLR Orbit Anomaly," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 2033-2038.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In