Objectives 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 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.
Methods SBAS messages broadcast by wide area augmentation system (WAAS), BeiDou SBAS (BDSBAS), GPS aided GEO augmented navigation (GAGAN) and multi-functional satellite 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.
Results WAAS GEO satellite has the best performance with ranging accuracy better than 1.6 m. The 99.9% error bound is less than 6.8 m while the broadcast user differential range error (UDRE) for the GEO satellite is 7.5 m, which meets the integrity requirement. The 3 GEO satellites of BDSBAS show ranging biases of 14.32 m, 12.64 m and 17.44 m respectively, and the accuracy is better than 2.9 m. After removing the bias, the related 99.9% error bound is 8.60 m, 7.80 m and 11.60 m which suggests an UDRE of 11-12. User range accuracy (URA) of 15 is broadcast in message type 9 for the BDSBAS GEO satellites. URE of the GAGAN GEO satellite is better than 13.9 m and MSAS is better than 3.2 m. 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 0.60 m, 0.53 m, 0.21 m and 0.34 m 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) service is improved from 99.984% to 99.997% with collaboration of 3 GEO satellites' observation.
Conclusions With sufficient GPS satellites, the combination of GEO satellites will decrease the positioning accuracy because of the relative larger range error, while with less available satellites, combining the GEO satellite data effectively reduces the protection level and improves the system availability. Results suggest that SBAS GEO ranging data should be included in the SBAS solution for aviation users.
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