Real Time Precise GPS Constellation Orbits and Clocks Estimation using Zero-Difference Integer Ambiguity Fixing

D. Laurichesse, F. Mercier, J.P. Berthias

Abstract:	A method to solve the GPS zero-difference measurement equations with integer ambiguities has been recently introduced [1]. When the method is applied to data from a global network of GPS receivers it provides a consistent set of satellite orbits and clocks, which have an ‘integer’ property: phase residuals for any receiver computed using these orbits and clocks easily reveal integer ambiguities. The paper focuses on the application of this novel approach to the computation of real-time orbits and clocks for the GPS constellation. The benefit of using these products for realtime Precise Point Positioning (PPP) of user receivers (with integer ambiguity fixing) is demonstrated. In this method real-time corrections to extrapolated IGS IGU orbits are estimated at the same time as all other relevant parameters by a Kalman which processes measurements from a world-wide network of 50 IGS stations. The filter performs zero-difference ambiguity fixing in real-time. One month of data was processed (July 2007) to estimate the achieved precision. Relative to IGS final orbits, the 3-D precision of the real-time orbits is about 3 cm RMS. When these constellation orbits and clocks are used to perform real-time PPP for receivers outside of the reference network, the precision obtained using zero-difference integer ambiguity fixing is close to 2 cm RMS. This is about five times better than standard solutions which rely upon floating ambiguity fixing.
Published in:	Proceedings of the 2009 International Technical Meeting of The Institute of Navigation January 26 - 28, 2009 Disney's Paradise Pier Hotel Anaheim, CA
Pages:	664 - 672
Cite this article:	Laurichesse, D., Mercier, F., Berthias, J.P., "Real Time Precise GPS Constellation Orbits and Clocks Estimation using Zero-Difference Integer Ambiguity Fixing," Proceedings of the 2009 International Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2009, pp. 664-672.
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