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Ranging information is routinely used for orbit determination and spacecraft positioning. Currently, RF radio links are used to periodically measure radiometric observables from which range information is derived. With the advent of high-speed free space optical links, both in the near-Earth and deep space domains, techniques for deriving range information in bidirectional optical links and mixed RF-optical links are gaining relevance. This paper presents two methods for obtaining range information between a ground station and a spacecraft, which we term Synchronous Mode (SM) and Asynchronous Mode (AM). In the synchronous mode, the spacecraft ties the arrival of a Ranging Codeword (RCW) in the uplink with the departure of a RCW in the downlink. Phases of the uplink and downlink signals (and their associated time tags) are measured at the ground station and serve as observables for deriving the range information. The AM uses a similar principle, but one phase measurement (and time tag) is performed by the ground station uplink subsystem, while the second phase measurement is performed by the spacecraft and associated with a time tag generated by the station’s downlink subsystem. This method has the advantage of not requiring synchronicity between the uplink and downlink on board the spacecraft and, at the same time, avoids stringent clock requirements on the mission.
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