Multiple GNSS inter-system biases in precise time transfer

Abstract

Inter-system bias (ISB) is important for the fusion of multiple global navigation satellite system data. They should be estimated or corrected for precise position, navigation, and timing solutions. In this study, ISB estimation models are developed for the global positioning system, GLONASS, BeiDou Navigation Satellite System, and Galileo systems. Six stations, with three different types of receivers, are used to analyze the ISB characteristics of different systems. A pair of time-link stations, which have a short baseline and are connected to a common external time frequency from the laboratory, are used to analyze the performance of three different estimation strategies (epoch-wise variable, 30 min piece-wise constant, and daily constant). These stations are also used to evaluate the influence of ISB constrains on the precise estimation of time transfer, coordinates, and the troposphere. The results illustrate: (a) ISBs variation exists between different systems, and the same types of receivers also exhibit large differences between the different stations due to the receiver hardware delay; (b) the precision of the different measurements and their related errors, the estimated ISBs that exhibit large differences between pseudorange and carrier phase observations, and the different ISB estimation strategies can also cause differences in the final fusion results; and (c) using the estimated ISB as constraints can improve the precise estimation of time transfer, coordinates, and the troposphere in the real-time solution mode.