Optical Ranging for Space-to-Ground Links

Ranging information is routinely used for orbit determination and spacecraft positioning. Currently, RF radiolinks are used to periodically measure radiometric observables from which range information is derived. Withthe advent of high-speed free space optical links, both in the near-Earth and deep space domains, techniques forderiving 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 (AM) and asynchronous mode (SM). In the synchronous mode, the spacecraftties the departure 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 stationand serve as observables for deriving the range information. The AM uses a similar scheme, but one phasemeasurement is performed by the ground station uplink subsystem, while the other is performed by the spacecraft.To avoid stringent clock requirements on the mission, time tags in the AM continue to be generated by the groundstation, which is assumed to have an excellent frequency and timing distribution system.