Navigation of Formation Flying Spacecraft using GPS: the PRISMA Technology Demonstration

Abstract

A fundamental need of spacecraft autonomous formation flying is the determination of the relative motion between individual satellites in near real-time. For formation-flying in low Earth orbit (LEO), differential GPS represents an ideal sensor which can be used to directly measure the relative positions and velocities to a high level of accuracy with low costs. This paper addresses the design, implementation and validation of the GPS-based on-board navigation system for the PRISMA technology demonstration mission. The objective of the navigation system is to provide in real-time absolute and relative orbit information for the PRISMA space segment which consists of two satellites flying in formation in LEO. The key drivers for the design of the navigation system are the accuracy requirements on the absolute and relative orbit determination and prediction. Furthermore, a high level of robustness and flexibility imposed by the numerous formation flying scenarios and related thrust activities is required during the mission lifetime of about eight months. The paper focuses on the description of the navigation software architecture and algorithms. In contrast to earlier approaches that typically separate the GPS-based navigation task into the independent reconstruction of absolute and relative states, here a single reduced-dynamic Kalman filter has been developed which processes pseudorange and carrier-phase data from both spacecraft in order to exploit the full GPS measurements information at all times. Emphasis is given to key functional and performance test results of the GPS hardware/software navigation system once integrated into the PRISMA spacecraft in its flight configuration.