On the problems of system time generation for a future GNSS2

摘要

For any satellite navigation system clock synchronzation to a system common time is a requirement of utmost importance. There are some features, being of particular interest, which are studied here. For clock comparisons one- and two-way methods can be applied. In the present conventional systems GPS & GLONASS systems, only the first method is actively in use. Improvements in several areas are expected in scenarios applying two-way and inter-satellite-links (ISL). For generation of the system time scale (STS) with one or some chosen types of clocks the ensemble solution is the most stable and reliable. There are several and sometimes different constraints for the reallization of a STS for navigation or timing users. In this paper we discuss the impact of clock comparison measures and of STs generation peculiarities on the development of a 2~(nd) generation Global Navigation Satellite System (GNSS2). We give simulation results of a STS and induced navigation/timing user clock error in a possible GNSS2 scenario, defined by (Case 1) 15 Inclined Geosynchronous Satellites (IGSO) equipped with a RAFS (space Rb) each + 3 Geostationary Satellites (GEO) equipped with an H-Maser, and 4 ground stations operating with a Cs-frequency standard, where both inter-satellite and two-way links are adopted. It will be shown that for the navigation user the error contribution due to the clocks can reach a value of about 0.7 ns (1 sigma), with an update interval of 30 min in this particular scenario. Satellite constellations equipped with H-Maser give better results. For time dissemination purposes, the UTC steering policy will be addressed and an error reaching 27 ns (1 sigma) is expected in the above scenario. Finally we also investigated a constellation using Ultra Stable Quartz Oscillators on-board the satellites (Case 2). We show that such a configuration could also satisfy the needs of navigation/timing users.