Ultra-precise clock synchronization of remote atomic clocks with PRARE onboard ERS-2

Most precise time transfer and remote synchronization of highly stable clocks (Cs, Rb, H-maser) is realizable by two-way satellite systems. Compared to the at present largely used one-way systems (GPS, GLONASS), they offer to synchronize the involved clocks without precise knowledge of the satellite's position. The paper presents a validation test for two-way clock comparison with the help of the PRARE instrument (Precise Range and Range Rate Equipment) onboard ERS-2 and with two of the PRARE system stations, which are provided with atomic clocks. PRARE is a new spread-spectrum microwave system designed mainly for highly accurate ranging purposes operating with a space-segment, which is orbiting the Earth onboard ERS-2 since April 1995, and with dedicated ground station transponders. A "Common View" ground-clock synchronization scenario has been performed, using the PRARE instrument and simultaneously GPS time receivers connected to an atomic clock at each ground station site. By computing and comparing the results respectively the accuracy for the PRARE instrument has been estimated. The achievable synchronization accuracy for the PRARE equipment in this "Common View" mode is around 1 ns, due to the sub-optimum design of the existing PRARE system for time purposes. The experiment turned out that the PRARE system combined with a highly stable frequency source onboard a suitable satellite is qualified to be a basic element for a future GNSS II.