Signal-in-Space accuracy research of GPS/BDS in China region

As one of the most important performance indicator of satellite navigation, Signal-In-Space Accuracy (SISA), which is usually characterized by Signal-In-Space Range Error (SISRE), directly affects the positioning accuracy, integrity, and other performance indicators. This paper analyzes SISA of GPS and BDS using SISRE based on the Worst User Location (WUL). Considering that the signal coverage in China region is irregular, the traditional geometric method is not applicable, therefore this paper proposes Geometric Approximation Algorithm (GAA) that can obtain accurately the WUL in global region and China region. The calculation interval is set at 30 s, and the time span is from January 11, 2015 to November 21, 2015, then we separately calculate the difference of broadcast ephemeris and precise ephemeris of GPS and BDS to get orbit errors and clock errors. Finally, we form data sets of global region and China region, thus we can calculate the RMS URE using their data sets. The results shows that, in China region, the average RMS URE of 32 GPS satellites is 0.68 m, and, average RMS URE of 13 BDS satellites is 1.38 m. We also find that average RMS URE based on WUL is greater than that based on integration method by about 16% for GPS and 28 % for BDS.