Time synchronization in orbiting VLBI

Orbiting very long baseline interferometry based on radio transmission of a reference signal from an Earth-bound oscillator is considered. This method provides the possibility of forming a connected-element interferometer between the spacecraft and an Earth radio telescope co-located with the reference oscillator. The reference signal from the spacecraft to the Earth is retransmitted and the round-trip time is continuously measured by assuming reciprocity of the path; the time at the spacecraft is always known relative to a clock on the Earth. The author is concerned with the design of such a round-trip reference system and its accuracy limitations. Spacecraft and Earth station models, continuous contact and simple-frequency timing, ambiguity-free timing link design, and the onboard synthesizers are discussed. A design example is presented.>