Analysis of satellite-induced factors affecting the accuracy of the BDS satellite differential code bias

Differential code bias (DCB) is one of the main error sources of the positioning, navigation and timing services and slant total electron content extraction. The DCB can usually be estimated together with ionospheric model coefficients from the data of International GNSS Service stations. However, the precision of the BeiDou Navigation Satellite System (BDS) satellite DCBs determined with GNSS data is still lower compared with that of GPS. Apart from the sparsely distributed tracking stations of BDS, the quality of BDS observables also matters. Moreover, the determined DCB will have different characteristics for geostationary orbit (GEO), inclined geosynchronous orbit (IGSO) and medium earth orbit (MEO) satellites because of different observation times, geometric structures and orbit repeat periods of the different orbit types. In this study, we mainly analyze the factors affecting the accuracy of BDS satellite DCBs. First, a piece-wise local ionospheric model based on a two-step method is proposed to obtain the BDS DCBs. Second, the impact of BDS satellite-induced code bias variations on the ionospheric observable and DCB determination is analyzed in detail. Finally, we focus on the analysis of the cause of different day-to-day scattering results for GEO, IGSO and MEO satellite DCBs. The performance of the proposed algorithm and the satellite-induced factors affecting the accuracy of BDS DCBs are assessed by about 3 months of GNSS data in 2015 from 42 MGEX stations. Results show that the proposed algorithm is able to estimate BDS satellite DCBs precisely. The DCB day-to-day stability of BDS satellites is improved by 40.6% compared with DCB products of Institute of Geodesy and Geophysics (IGG), China. After the satellite-induced bias is eliminated by a correction model, the precision of BDS DCBs improves. In particular, the value of B1–B3 DCB day-to-day scattering for MEO satellites decreased by 28.4%. In addition, we determined that the lower DCB day-to-day stability of MEO satellites compared with that of IGSO satellites may be mainly attributed to the different satellite orbit repeat periods.