The most basic version of this transponder is designed to lock onto a constant carrier uplink tone and to subsequently generate a constant carrier downlink town that is phase-coherent with the uplink.
In this way, the relative velocity between the earth station and the transponder can be established to great precision.

The applications range from spacecraft tracking, tracking of small celestial bodies to observation of planetary dynamics.

Advanced versions of this transponder can include features such as pseudo-noise sequence ranging, data transfer, telemetry, command-and-control functions, landing beacon.