Precise in-orbit relative navigation technique for rendezvous mission of CubeSats using only GPS receivers

This paper proposes a global positioning system (GPS) based relative navigation algorithm for CubeSats that perform autonomous rendezvous and docking missions. To realize precise relative navigation using only GPS, an algorithm is developed to improve the differential GPS (DGPS) performance by reducing the integer ambiguity search space of carrier-phase DGPS. To this end, a Hatch filter is used to improve the pseudorange noise performance, and range-domain DGPS-based single-frequency relative navigation is realized. Because GPS measurements are transmitted intermittently using an inter-satellite link, orbit propagation is performed using the Hill-Clohessy-Wiltshire equation. Moreover, to improve the performance of the propagation error accumulated over time, an in-orbit velocity moving average filter is incorporated. Because the rate change of relative motion in the local-vertical-local-horizontal coordinate system on the orbit is small, the noise level for the relative velocity and overall relative navigation system performance are improved. To demonstrate the usefulness of the proposed method, software-in-the-loop simulation and processor-in-the-loop simulation-based real-time implementation is realized on the onboard computer of a reference CubeSat (SNUGLITE-III A, target) and thruster-equipped CubeSat (SNUGLITE-III B, chaser), and their performances are evaluated.