Performance Assessment of Galileo In-Orbit Satellite Clocks

Abstract

At present, the Galileo in-orbit satellites include four in-orbit validation (IOV) satellites and eight full operational capability (FOC) satellites. The accuracy of Galileo positioning is better than 2 m. Thirty FOC satellites are expected to be launched completely by 2019, and a complete constellation of full operational capability will be achieved by the time. The quality of Galileo satellite clock affects the performance of navigation and reliability. Based on this, a solution of satellite clock assessment is given, the standard time and frequency signal of National time service center of Chinese Academy of Sciences are taken as references. The performance of all Galileo in-orbit satellites clocks is assessed by receiving the signal in space and using the way of dual-frequency ionosphere-free combination, which includes frequency deviation, frequency drift, frequency stability, and the consistency of the frequency stability over time. At the same time, the uncertainty of relative frequency, frequency stability, and time offset of Galileo satellites clocks is analyzed. The evaluation results show that the orbit satellite clock frequency stability is (τ = 60 s and τ = 300 s) at e−13 level, the uncertainty of frequency stability is in the order of e−14, and the stability is consistent over time. When the satellite clock time is corrected to the Galileo system time (GST), the time offset of GST relative to UTC (NTSC) is between −40 and −70 ns, and the satellite clock stability after clock correction is slightly improved. The uncertainty of measurement of system time offset is 3.37 ns. Research methods and achievements of this paper can be the significant references with respect to Galileo stand-alone positioning, multi-GNSS combination positioning, integrity monitoring, the Galileo system time interoperability, etc.