A proposal has been made to use the Tracking and Data Relay Satellite System (TDRSS) as an orbiting element for a very long baseline interferometry (VLBI) demonstration. The TDRSS is a satellite system designed to coherently track and relay data between other satellites and the central ground station. This system could also be used to coherently observe a celestial radio source. A ground-based frequency standard would be used to coherently drive the spacecraft receiver local oscillator and transmitter. The data will be telemetered to a ground station, where it will be recorded on a Mark III terminal.