Multi-GNSS clock combination with consideration of inconsistent nonlinear variation and satellite-specific bias

Abstract

As part of the International GNSS Service (IGS) multi-GNSS Pilot Project (MGEX), precise orbit and clock products for multi-GNSS constellations have been submitted by several analysis centers (ACs) since 2012. Based on the 30 s satellite clocks from 6 MGEX ACs, the multi-GNSS clocks are combined and the consistency of the AC clocks is assessed in this study. Usually, a linear transformation between the combined and AC solution is used for clock combination, and the clock residuals of the AC solution w.r.t the combined solution are used to determine the weights of the AC. However, any inconsistent satellite-specific bias or nonlinear variations in the clocks induced by the AC’s processing strategy can contaminate the linear transformation as well as the determination of the weight. In this study, the analysis center and satellite-specific bias (ASB) of the MGEX AC clock solutions is first identified and estimated by using observations from globally distributed stations. Moreover, the clock solutions with nonlinear variations induced by the reference clock or nonconstant intersystem bias (ISB) are corrected by aligning the clocks to the selected reference solution before clock combination. With the correction of the ASB, the root-mean-square of the clock residuals decreases significantly and reaches 14-26, 37-91, 33-48 and 12-44 ps for GPS, GLONASS, BDS-2 and Galileo, respectively. In general, the consistency of AC solutions w.r.t the combination reaches 8-16, 27-58, 13-27 and 9-36 ps for GPS, GLONASS, BDS-2 and Galileo in terms of the standard deviation. Finally, the individual AC and combined orbit as well as clock solutions of different constellations are assessed by precise point positioning, and the combined multi-GNSS solutions show competitive performance with the best AC solution in terms of both the positioning accuracy and stability of the reference frame parameters. However, inconsistent scale parameters of both the AC and combined solutions are identified and require more investigation.Graphical