SBAS GEO Satellite User Range Error and Position Augmentation Research

Satellite Based Augmentation System (SBAS) improves the positioning accuracy and integrity by broadcasting ephemeris corrections and associated integrity parameters through Geostationary Earth Orbit (GEO) satellites. SBAS GEO satellite can also be used as a ranging source together with Global Positioning System (GPS) satellites to improve the system performance. User range error (URE) of the GEO satellite and their effect on positioning results are investigated. URE of SBAS GEO and GPS satellite is determined by weighting the observation residuals which are derived with fixed station coordinates. SBAS messages are applied to correct the orbit and clock errors contained in broadcast ephemeris and the ionosphere delay. Ranging data from GEO satellite is engaged in the SBAS positioning process to explore the impact on positioning accuracy, integrity and availability. SBAS messages broadcast by Wide Area Augmentation System (WAAS), BeiDou Satellite Based Augmentation Systems (BDSBAS), GPS Aided Geo Augmented Navigation (GAGAN) and MTSAT (Multi-functional Transport Satellite) Satellite based Augmentation System (MSAS) and real data from International GNSS Service (IGS) stations are applied to perform the assessment. European Geostationary Navigation Overlay Service (EGNOS) and System for Differential Corrections and Monitoring (SDCM) are not included because of the absence of the ranging capability. It was found that WAAS GEO satellite has the best performance with ranging accuracy better than 1.6m. The 99.9% error bound is less than 6.8m while the broadcast User Differential Range Error (UDRE) for the GEO satellite is 7.5m, which meets the integrity requirement. The 3 GEO satellites of BDSBAS show ranging biases of 14.32m, 12.64m and 17.44m respectively, and the accuracy is better than 2.9m. After removing the bias, the related 99.9% error bound is 8.60m, 7.80m and 11.60m which suggests an UDRE of 11~12. An URA of 15 is broadcast in message type 9 for the BDSBAS GEO satellites. URE of the GAGAN GEO satellite is better than 13.9m and for MSAS it is better than 3.2m. The UDRE of GAGAN and MSAS is 14. URE of GPS satellite after augmented by SBAS is also calculated for comparison purpose. Ranging accuracy of GPS is 0.60m, 0.53m, 0.21m and 0.34m for WAAS, BDSBAS, GAGAN and MSAS respectively. WAAS GEO satellite is selected to perform the positioning analysis whose UDRE is less than 14 so that it can be weighted properly in the solution. Engagement of GEO satellite in SBAS positioning will lead to lower Position Dilution of Precision (PDOP) and reduce the protection level especially for the blockage circumstance. The system availability of Localizer Performance with Vertical guidance 200 (LPV200) approach is improved from 99.984% to 99.997% with collaboration of 3 GEO satellites' observation. With sufficient GPS satellites, the combination of GEO satellites will decrease the positioning accuracy because of the relative larger range error. Results suggest that SBAS GEO ranging data should be included in the SBAS solution for aviation users.