Analysis and Correction of the Inter-frequency Clock Bias for BeiDou Satellites

Abstract

BeiDou provides triple-frequency signals for all operational satellites. However, an apparent inconsistency between triple-frequency carrier phases has been discovered. The satellite clock products derived from B1/B2 carrier phase observations cannot be used for B1/B3 based precise point positioning (PPP) without careful consideration of the inconsistency. The apparent inconsistency is known as inter-frequency clock bias (IFCB). Datasets collected at 37 globally distributed stations on fourteen consecutive days are employed to analyze the IFCB of BeiDou GEO (Geostationary Orbit), IGSO (Inclined Geosynchronous Orbit) and MEO (Medium Earth Orbit) satellites. A weighted epoch-differenced approach is proposed to estimate the IFCB. The results indicate that the IFCB is time- and satellite-dependent, and is irrelevant to the antenna types, station locations, satellite elevation and azimuth angles. The IFCB of GEO and IGSO satellites has a significant daily period. The IFCB varies within a range of –4 to +4 cm, and the RMS (root mean square) values of IFCB are smaller than 2 cm. There is a high correlation between IFCB of two adjacent days for part of GEO and IGSO satellites, and the RMS values of IFCB differences between two adjacent days for these satellites are usually smaller than 0.5 cm. When the extracted IFCB of the first day is used to correct the B1/B3 ionosphere-free carrier phase observations of the second day, the BeiDou PPP based on B1/B3 achieves higher positioning accuracy and smaller observation residuals.