Quality assessment of GPS, Galileo and BeiDou-2/3 satellite broadcast group delays

Abstract

The quality of broadcast group delays (BGDs) transmitted in the navigation messages of the Global Positioning System (GPS), BeiDou-2/3 and Galileo is assessed by comparison with multi-GNSS differential code bias (DCB) products generated at the Chinese Academy of Sciences (CAS). An automatic DCB (AutoDCB) realignment procedure is presented to handle discontinuities within individual BGD/DCB series. The analysis is performed over a four-year period starting from January 2014. For GPS, the consistency between broadcast timing group delays (TGDs) and CAS MGEX DCBs is 0.25 ns, and the corresponding values are 0.05, 0.3 and 0.45 ns for L1C/A, L2C and L5Q inter-signal corrections (ISCs), respectively. The best agreement between GPS L2C ISCs and CAS MGEX C1W-C2L DCBs is emphasized. The inconsistency between broadcast and post-processed C2I-C6I DCBs (=TGD1) is found to decrease from 1.5 to 0.45 ns across the entire BeiDou-2 constellation since late July 2017, indicating a significant quality improvement of BeiDou-2 transmitted TGD1 parameters. A systematic offset of around 4.0 ns is found between TGD1 reference datums of BeiDou-3 and BeiDou-2 constellations, which needs to be removed in the combined BeiDou-2/3 B1I single-frequency or B1I+B3I dual-frequency standard positioning applications. Aside from the increasingly improved repeatability of broadcast E1-E5a/5b delays, an overall agreement at the level of 0.1–0.4 ns is achieved between Galileo transmitted value and DCB metadata from European GNSS Agency (GSA) for the in-orbit validation (IOV) satellites. In the comparison of Galileo BGDs and CAS MGEX DCBs, the consistency is at the level of 0.4 and 0.3 ns for IOV and full operation capability (FOC) satellites, respectively. The requirement of the routine assessment of broadcast group delays is also emphasized, in particular for newly launched satellites of new constellations.