Analysis of SBAS Orbit and Clock Corrections for GPS and their Applicability to Today’s Mass Market Multi-GNSS Personal Navigation

Mojtaba Bahrami, Geraint Ffoulkes-Jones, Qiang Zhang

Abstract:	Satellite-Based Augmentation Systems (SBAS) are designed to generate and broadcast rapid integrity information and orbit and clock corrections for GPS/GNSS satellites-in-view with the aim of improving users positioning accuracy and integrity in real-time. In order to explore and characterize SBAS broadcast orbit and clock corrections, a consistent long-term analysis and comparison of GPS broadcast orbit and clock errors and associated SBAS orbit and clock corrections is presented. The metric used is the Signal-In-Space (SIS) Range Error (SISRE) which is an overall measure of the fidelity of the orbit and clock information broadcast in navigation messages. In this study, nominal SISREs are estimated by comparing both broadcast GPS orbits and clocks; and corrected GPS orbits and clocks using SBAS corrections, against precise orbit and clock products provided by the National Geospatial-Intelligence Agency (NGA). In both cases SISREs are estimated based on the local average values in the geographical area covered by the respective SBAS systems, specifically the European EGNOS and American WAAS system coverage areas. It is shown that SBAS orbit and clock corrections were beneficial for Block IIA satellites before GPS modernizations. However, analysis results in this study indicate that the benefits of SBAS corrections are diminishing by continuing replacement of old Block IIA satellites by new generation satellites, mostly due to better on-board clocks and better radial estimation by modernized and improved GPS Control and Space Segments. The evaluation also shows that SISREs decreased with the new generation of GPS satellites, and in 2016 GPS daily RMS SISREs due to broadcast orbit and clock errors are typically stable at decimeter level (less than 0.5 meters). It is shown that the scatter of SBAS-corrected broadcast GPS orbit and clock errors within each satellite Block exceeds the scatter of GPS-only broadcast orbit and clock errors for new generation of satellites. That indicates that as of 2016 the benefits of applying SBAS corrections to broadcast GPS orbits and clocks are marginalized. Results also show that SBAS-corrected broadcast GPS orbits and clocks introduce GPS satellite Block-specific systematic biases of up to 0.7 meters in orbit and/or clock. The apparent Block-specific systematic biases need to be investigated as combing SBAS-corrected GPS ephemerides for different satellite/Block might degrade the accuracy of the navigation solution.
Published in:	Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016) September 12 - 16, 2016 Oregon Convention Center Portland, Oregon
Pages:	2766 - 2776
Cite this article:	Bahrami, Mojtaba, Ffoulkes-Jones, Geraint, Zhang, Qiang, "Analysis of SBAS Orbit and Clock Corrections for GPS and their Applicability to Today’s Mass Market Multi-GNSS Personal Navigation," Proceedings of the 29th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2016), Portland, Oregon, September 2016, pp. 2766-2776. https://doi.org/10.33012/2016.14679
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