Broadcast ephemeris SISRE assessment and systematic error characteristic analysis for BDS and GPS satellite systems

This study comprehensively evaluated the signal-in-space range errors (SISREs) and investigated the systematic error characteristics for GPS and BDS broadcast ephemeris. The analysis reveals that on March 28, 2021, the satellite antenna model used by GPS ground operation centre to generate GPS II series navigation ephemeris changed from the manufacturer model to the International GNSS Service (IGS) model. This transition is carefully considered in our GPS SISRE evaluation. Experimental statistics indicate that the SISRE for GPS IIR, IIF, and III satellites is approximately 0.50, 0.46, and 0.34 m, respectively, with a value of 0.44 m for the overall constellation. With enhancements in inter-satellite links, BDS-3 GEO, IGSO, and MEO satellites exhibit SISRE of 1.20, 0.62, and 0.46 m, respectively, representing improvements of 14.3, 27.5 and 48.8 % over their BDS-2 counterparts. The ephemeris performance of BDS-3 MEOs is comparable to GPS. However, the BDS orbit quality significantly degrades over a 3-day period following manoeuvre operations and during eclipse seasons. Furthermore, the systematic error characteristics of broadcast ephemeris are analysed based on satellite laser ranging (SLR) checks and Helmert transformation. The SLR residuals of nearly all of the BDS-2/3 satellites tracked by the International Laser Ranging Service vary linearly with the Sun elongation angle. Moreover, the Z-geocentre component of the GPS/BDS orbit-realises frame exhibits obvious annual periodicity. These indicate that the GPS/BDS broadcast orbit models need further improvement. Compared with GPS, the BDS broadcast orbit-realised frame can better maintain the X- and Y-geocentric components but presents significant systematic scale bias and rotation errors. These errors for BDS are attributable to the inconsistency in the radial disturbance models between broadcast and Multi-GNSS Experiment precise orbits as well as prediction errors of the Earth rotation parameter used. The overall SLR residual mean for BDS-3 CAST and SECM MEOs are approximately 7.8 and −4.7 cm, respectively, with a standard deviation of approximately 9.9 cm, 3–4 times better than that of the BDS-2 satellites. The SLR residual dispersion of BDS-3 MEO C43 and C44 satellites is significantly larger than that of other BDS-3 MEOs, likely because of the inaccurate official coordinates of laser retroreflector arrays.