Time Synchronization and Performance of BeiDou Satellite Clocks in Orbit

The time model of Beidou satellite clocks is analyzed. The general relations of satellite clocks with the system time are studied. The error sources of two-way radio time transfer between satellites and uplink stations are analyzed. The uncertainty of type A is about 0.3？ns in Beidou system. All the satellite clocks in orbit of Beidou satellite navigation system are evaluated by the clock offsets observed by the two-way radio time transfer. The frequency stabilities at a sample time of 10000？s and 1 day for all the satellite clocks are better than . It means that the performance of Beidou satellite clocks in orbit is consistent with the ground test, and the results in orbit are a little better than those in ground vacuum. 1. Introduction Beidou satellite navigation system began to provide regional service since December 2012. The constellation of Beidou system is constituted of 14 satellites in orbit: 5GEO satellites, 5IGSO satellites, and 4MEO satellites. Table 1 shows the basic information of the Beidou satellites. Service area now covers latitude and longitude . Practical operational accuracy of Beidou system is better than 10？m (95%) in horizontal and 15？m (95%) in vertical [1]. Table 1: Basic information of Beidou satellite clocks. As we all know that time synchronization of satellite clock plays a significant role in satellite navigations, accurate and reliable satellite clock offset parameters are the base of PNT service. Time synchronization error of satellite clock is mainly caused by the time transfer from the master station and its offset prediction. The clock prediction error depends on its frequency instability. Then the measurement, prediction and evaluation, of satellite clocks are very important for a satellite navigation system. GPS operates a worldwide monitoring stations network, and includes six USAF stations, eleven NGA stations, and two IGS stations. Geodetic receivers are equipped in these stations to monitor the performance of satellite clocks. On 28 May, 2010, the first Block IIF satellite, designated SVN62/PRN25, was launched containing three atomic frequency standards, one DCBFS serial number 1010 (Cs 1010) and two RFS serial numbers 27 and 14 (Rb 27 and Rb 14). The frequency stabilities of SVN62 Cs 1010 and Rb 27 are, respectively, and at 1 day in orbit. And the frequency stability of GPS Block IIR Rb is about at 1 day for October 2010 (all using NGA data) [2–4] while the frequency stability at 1 day for Galileo satellite clock is about for GIOVE A Rb clocks and for GIOVE B PHM [5, 6]. How about the performance of Beidou