Galileo In-Orbit Satellite Clocks Performance Assessment at NTSC

National Time Service Center has developed the system time offset monitoring and forecast system based on UTC(NTSC) standard time scale in order to support time interoperability of BDS relative to other GNSSs since 2013. The monitoring results include UTC(NTSC)-GPST and UTC(NTSC)-GLONASST and UTC(NTSC)-BDT. With the development of Galileo satellite navigation, it is imperative to take Galileo system time offset monitoring into consideration. At present, the Galileo in-orbit satellites include 4 IOV (In-Orbit Validation) satellites and 8 FOC ( Full Operational Capability ) satellites. The complete Galileo constellation is expected by 2020. Therefore, the experiments of Galileo In-Orbit Satellite clocks assessment at NTSC have implemented for several months. This paper will present a scheme of Galileo in-orbit satellite clock assessment. The short-term performance specifications of all Galileo in-orbit satellites clocks are assessed by receiving the signal-in-space, which includes frequency stability, the consistency of the frequency stability over time. The method and process of absolute delay calibration of the receiver and associated antennas are discussed. The calibration results are given with which more accurate Galileo system time offset with respect to UTC(NTSC) are obtained preliminarily. The evaluation results show that in normal condition, frequency stabilities of Galileo satellite clock are all close to the level of e-13 and consistent over time when average times equal to 60s and 300s respectively. After satellite clock correction, the time difference between Galileo System Time(GST) and UTC(NTSC) which derived from each satellite change from -35 ns to -5 ns. The satellite clock stabilities are slightly improved by broadcasted clock error correction. The performance of two abnormal FOC satellites(E14 and E18) are also analyzed. The results will be very significant not only for Galileo system time offset monitoring but also for Galileo stand-alone positioning, multi-GNSS combination positioning, Integrity monitoring, the Galileo system time interoperability, etc.