A real-time kinematic GPS sensor for spacecraft relative navigation

The concept and prototype implementation of a spaceborne relative navigation sensor based on a pair of GPS receivers is presented. It employs two individual receivers exchanging raw measurements via a dedicated serial data link. Besides computing their own navigation solution, the receivers process single difference measurements to obtain their mutual relative state. The differential processing provides a high level of common error cancellation while the resulting noise is minimized by appropriate use of carrier phase measurements. A prototype relative navigation sensor making use of the above concepts has been built up based on the GPS Orion 12 channel L1 receiver and qualified in hardware-in-the-loop tests using a GPS signal simulator. It provides a relative navigation solution with representative r.m.s. accuracies of 0.5 m and 1 cm/s, respectively, for position and velocity. For ease of use the relative state is provided in a co-moving frame aligned with the radial, cross-track and along-track direction. The purely kinematic nature of state estimation and the small latency make the system well suited for maneuvering spacecraft, while the minimalist hardware requirements facilitate its use on microsatellite formations.