Research on LEO Satellites Time Synchronization with GPS Receivers Onboard

Precise relative navigation of spacecraft is required for its critical movement, such as rendezvous and formation flying-key aspects of many current and future space missions. Potential applications of interest include the capabilities to detect and track slowly moving ground vehicles (ground moving target indication (GMTI)) and perform synthetic aperture radar (SAR) imaging, with the requirement to provide GMTI and SAR data to users in a timely manner. Extensive research has been carried out on terrestrial applications of global positioning system (GPS) time transfer. For low earth orbit (LEO) satellites, such missions can use the GPS signals for relative positioning and data time tagging. This paper focuses on linking these two key applications - the use of GPS in LEO for relative navigation and precise formation flying, and time and frequency transfer between LEO satellites. As an example, the research investigates co-orbiting satellites A and B of gravity recovery and climate experiment (GRACE) at a separation of about 200 km. The observations of GPS receivers onboard both GRACE A and B satellites are transferred into receiver independent exchange format (RINEX) format (1 Sept. 2003). The orbit of both satellites is then computed using the zero-difference precise point positioning technique. The RMS orbital difference between the results obtained and the precise orbits from GFZ is below 0.07 m. Two methods are proposed to compute the time difference between GPS receiver onboard satellites A and B respectively. One uses onboard GPS RINEX observations and the GRACE orbit from GeoForschungsZentrum Potsdam (GFZ) which has a large relative latency, the times between A and B in multi-channel common-view mode are compared. Another method computes the clocks of A and B by use of GPS observation onboard and the computed orbit. The times between A and B are then compared. Results indicate that a RMS accuracy of 2-3 nanoseconds (ns) can be achieved. This suggests that GPS has the capabilities of high-precision time transfer between LEO satellites.