Statistical characterization of the signal-in-space errors of the BDS: a comparison between BDS-2 and BDS-3

The BeiDou Navigation Satellite System (BDS) is experiencing a transition from regional (BDS-2) to global (BDS-3) service capability, and both BDS-2 and BDS- 3 jointly provide primary navigation services. Using approximately 5-years of ephemerides since January 2016 from the International Global Navigation Satellite Systems (GNSS) Service (IGS) and precise products from Wuhan University (WHU), we provide an assessment of signal-in-space (SIS) errors (SISE) for both BDS-2 and BDS-3 broadcast ephemerides. Due to the new inter-satellite links ( ISLs) adopted by the BDS-3 satellites, the percentages of ages of data, ephemeris (AODEs) and ages of data, clock (AODCs) shorter than one hour are 96.48% and 95.78%, respectively, compared to 72.16% and 79.25% of BDS-2. The broadcast orbit performance is also improved by the ISLs. The 3D root mean square errors (RMSEs) of the geosynchronous earth orbit (GEO) broadcast orbits are 6.89 m and 2.45 m for BDS-2 and BDS-3, respectively, while they are 2.04 m and 0.75 m for the inclined geosynchronous orbit (IGSO) and 1.99 m and 0.47 m for the medium earth orbit (MEO). It is found in this contribution that the orientations implied in the BDS-2 and BDS-3 orbital realizations of the terrestrial reference frame are more scattered than those of other navigation systems (i.e., GPS, GLONASS and Galileo). The non-radial orbit errors of BDS- 3 (BDS-2) could be improved by approximately 0.2 m (0.1 m) when the estimates of the rotation parameters were considered. It is discovered that a linear pattern in the satellite laser ranging (SLR) residuals is characterized by BDS broadcast orbits, and orbit models might need to be improved. In terms of the broadcast clock error, the averages of the standard deviations (SD) are 0.4 m and 0.2 m for BDS-2 and BDS-3, respectively. However, significant satellite-dependent nonzero clock bias is also observed for the BDS-3 satellites, and this bias could be partly reduced using the differential code bias (DCB) product. The change of correlation between orbit radial component and clock errors from - 0.30 to 0.31 is found for BDS-2 GEO satellites when the reference point of ephemerides was switched from antenna to center of mass. Meanwhile, the reference point switch induces a change of correlation between along-track and clock components from positive to negative for BDS-2 IGSO and MEO. Compared to the moderate correlation of 0.13-0.41 between orbit radial component and clock errors of GEO or IGSO, both BDS-2 and -3 MEO satellites show the least averaging correlations of no more than 0.1. The global average user range error (URE) of BDS-3 (BDS-2) is approximately 0.6 m (1.0 m), which is dominated by satellite-dependent clock bias. When these biases are removed from the clock residuals, the URE can be reduced by 50% and 15% for BDS-3 and BDS-2, respectively.