TIME SYNCHRONIZATION IN ORBITING VLBI

At present it is not feasible for orbiting radio telescopes to include sufficiently stable on-board oscillators (e.g., H masers) to permit time synchronization to be achieved in the same way as in ground-based VLBI. A two-way radio link with a stable oscillator on the earth will enable phase coherence to be maintained between the spacecraft receiver and ground radio telescopes, but lack of precise orbit knowledge prevents maintaining an accurate clock on the spacecraft. Since the space telescope signals must be recorded on the ground for later correlation, they can be labeled with ground-clock time corrected for transmission time, with the latter derived from the two-way link. The absolute timing error then corresponds to the orbit uncertainty, but remains constant as long as continuous contact is maintained. To allow for occasional dropouts of communication, as well as to allow for periods of nonvisibility from earth stations and transfer between different earth stations, it is necessary that the two-way link have a time ambiguity greater than the orbit uncertainty. Achieving this while maintaining the very high precision needed to ensure phase coherence requires multiple-frequency signals on the two-way link.