Analysis of SBAS Ephemeris Correction and UDRE Generation Algorithm

Satellite based augmentation system (SBAS) improves the accuracy and integrity of user positioning by broadcasting ephemeris corrections and associated integrity parameters through geostationary earth orbit satellites in real time. Minimum variance estimation method is proposed to derive the GPS ephemeris corrections and a Chi-square test is performed on the residual errors of corrected ephemeris to meet the integrity requirement. Satellite user differential range error (UDRE) and scaling matrix contained in message type 28 (MT28) are calculated based on the covariance information with the assumption that one of the reference stations failed. Actual data collected by 27 stations spread over China are applied to verify the proposed algorithm. International GNSS Service precise clock and orbit products are firstly taken as reference to assess the accuracy of ephemeris corrections. Results indicate that an accuracy of 0.1 m for clock and 0.4 m for orbit corrections can be obtained. Then, corrections of GPS broadcast ephemeris are estimated, the integrity parameters including UDRE and MT28 are generated. Chi-square test results of the ephemeris remaining errors show no exceeding of the threshold which reveals the integrity of corrections. A clear dependence of UDRE on satellite elevation is presented. On board clock failure and satellite maneuvering were simulated to evaluate the performance of the proposed algorithm. Measurement errors introduced by satellite abnormity can be effectively detected and compensated by the algorithm. With ionospheric delay corrected by Klobuchar model and ephemeris corrections generated in this contribution, SBAS positioning results achieve the accuracies of 0.7 m in horizontal direction and 1.0 m in vertical direction, which show an improvement of 30% in horizontal direction and 40% in vertical direction compared with the GPS single point positioning results.