Method For Determining Satellite Clock Offset Based On Two-way Time Transfer Between Satellite And Ground

High precision time reference and strict time synchronization between ground station andsatellite must be maintained in satellite navigation and positioning system. Therefore, the methodfor determining the satellite-ground clock offset of great significance to improve the precision ofsatellite navigation position and engineering applications. When the clock of ground receiver issynchronized with the system time, the determination of satellite clock offset is virtuallyidentical to the determination of satellite clock offset. At present, one-way time transfer andtwo-way time transfer is commonly used to determinate the satellite-ground clock offset. Amongthem, two-way time transfer is one of the methods used by Beidou Navigation Satellite Systemin China. Because of the correlation between up and down path, various errors impact on thepropagation path is weakened in two-way time transfer. At the same time, the determination ofclock offset is independent of the satellite orbit.Based on two-way time transfer between satellite and ground, the method for determiningthe satellite-ground clock offset of GEO, IGSO and MEO relative to ground station has beenstudied in this paper, the data processing program has been written, simulation data has beencalculated and analyzed. The main research contents are as follows:(1)The basic principle of two-way time transfer between satellite and ground wasintroduced, the causes and the method of various error sources are analyzed in detail, the formulaof satellite-ground clock offset was deduced. Based on the data of GEO, IGSO and MEO relativeto Xi’an Station and Beijing Fangshan station one day (24hours), the impact of ionosphere, theSagnac effect and satellite motion in the satellite-ground time comparison were calculatedrespectively, and their impact characteristics between different satellites and ground stationswere analyzed. The observational data of two-way time transfer between satellite and groundwas simulated by adding the influence of ionosphere, Sagnac effect, satellite motion and0.3nsrandom error based on the satellite-ground clock offset. The satellite-ground clock offset of GEO,IGSO and MEO relative to Xi’an Station and Beijing Fangshan station one day (24hours) wascalculated based on the simulation data, RMS of satellite-ground clock offset of three satelliteswas approximately0.2ns. Simulation results show that two-way time transfer between satelliteand ground was an effective method for determining the satellite-ground clock offset.(2) In the case of the regional fabric station, MEO would be appear invisible arcs, therefore,the satellite-ground clock offset couldn’t be determined by two-way time transfer betweensatellite and ground at this time. For above problems, the method for determining the satellite-ground clock offset by two-way time transfer combining satellite-ground andinter-satellite was proposed. When MEO was in the invisible arc of regional station, thesatellite-ground clock offset was calculated by the two-way time transfer combiningsatellite-ground and inter-satellite. Based on the data of satellite-ground clock offset in visiblearc of regional station, the satellite-ground clock offset of MEO in the invisible arc of regionalstation was forecasted by quadratic polynomial, and the satellite-ground offset calculated bycombination method was tested by the results of quadratic polynomial forecasts. RMS ofsatellite-ground clock offset in visible arcs was approximately0.2ns, RMS of inter-satellite clockoffset in invisible arcs was approximately0.3ns~0.5ns. In above case, RMS of satellite-groundclock offset of MEO in the invisible arcs calculated by the two-way time transfer combiningsatellite-ground and inter-satellite, was approximately0.4ns, RMS of the results of quadraticpolynomial forecasts was approximately0.5ns. The simulation results show that two-way timetransfer combining satellite-ground and inter-satellite was an effective method for determiningthe satellite-ground clock offset of MEO in the invisible arc of regional station.Satellite clock offset accurately determined is not only beneficial to establish the systemtime which is high accuracy and required by satellite navigation systems, but also important toimprove the accuracy of satellite navigation and positioning.