Ground-to-space time and frequency transfer based on the “Virtual clock” method

We describe China Area Positioning System (CAPS) to transfer Time and Frequency between ground and space using a round-trip link via a transparent transponder onboard a satellite. The purpose is generation of navigation signals from ground, which appear to come from a non-physical “virtual” on-board clock, and its time modulation being close to UTC at the satellite transponder output. The system is synchronised to a high performance ground atomic clock via satellites in geosynchronous (GEO) and in inclined geo synchronous (IGSO) orbits is presented.