GPS-BASED NAVIGATION AND GUIDANCE FOR THE PRISMA FORMATION FLYING

Abstract

The Swedish National Space Board has recently endorsed the PRISMA technology demonstration mission devoted to the in-flight validation of sensor technologies and navigation guidance and strategies for spacecraft formation flying and rendezvous. PRISMA comprises a fully maneuverable micro-satellite (the MAIN spacecraft) as well as a smaller sub-satellite (TARGET) that will be released after initial commissioning. The mission is scheduled for launch in 2008 and will be implemented by the Swedish Space Corporation. Dedicated sensor packages and experiments will be contributed by the German Aerospace Center (DLR), the Technical University of Denmark (DTU) and Alcatel, France. Among the experiments to be performed in the area of Guidance, Navigation & Control are: Autonomous Formation Flying, Homing and Rendezvous, and precision 3D Proximity Operations including final Approach/Recede manoeuvers. Within the PRISMA project, DLR has assumed responsibility for providing an GPS-based onboard navigation system offering absolute and relative orbit information in real-time. In addition, DLR will implement one of two sets of guidance and control strategies for fuel-efficient and safe formation flying at representative spacecraft separations of 100 – 2000 m. For redundancy and coverage purposes each satellite will carry two independent GPS receivers that are usually operated in a hot-cold configuration. Increased flexibility for handling non-zenith pointing attitudes is provided by multiple GPS antennas, which can be selected by ground command. In accord with the envisaged application range, high-grade single-frequency receivers have been adopted for the mission. The Phoenix GPS receivers to be flown on PRISMA are based on a commercial-off-the shelf-hardware platform which has been qualified by DLR for use in low Earth orbit through a series of environmental tests. DLR’s proprietary receiver software is optimized for orbital and high dynamics applications and has demonstrated its outstanding performance in extensive signal-simulator tests and flight experiments. With a code tracking accuracy of better than 0.5 m and a carrier-phase accuracy of better than 1 mm at 45 dB-Hz, the Phoenix receiver outperforms most other industrial GPS receivers for space applications. Furthermore, the low weight (50 g) and power consumption (0.8 W) of the Phoenix receiver board makes it ideally suited for use on the TARGET satellite with its tight onboard resources. Using an ionosphere-free code-carrier combination, the absolute positions of the two PRISMA spacecraft can individually be determined with an accuracy of better than 0.5 m in a post-facto reconstruction. Subject to proper multipath avoidance and good common satellite visibility, the relative position can, furthermore, be derived with centimeter accuracy. For onboard usage, a dedicated navigation system on the MAIN satellite will process local GPS measurements and raw measurements transmitted from the TARGET spacecraft by a inter-satellite radio link. Using a sophisticated Kalman filter, real-time navigation accuracies of 2 m (absolute) and <0.1 m (relative) are expected. A prototype implementation of the real-time navigation system has earlier been tested in ESTEC’s radio navigation laboratory and demonstrated a millimeter-level relative navigation accuracy for up to four formation flying spacecraft. While this accuracy is not expected to be achievable in the present mission due to multipath effects, coverage problems and heavy maneuver activity, PRISMA mission will nevertheless provide an optimum test bed for the in-flight performance assessment of carrier-phase differential GPS. In addition to the GPS-based navigation system of the PRISMA mission, DLR will contribute with one of the two sets of Autonomous Formation Flying guidance and control functions - the Spaceborne Autonomous Formation Flying Experiment (SAFE). SAFE complements other PRISMA experiment sets and focuses on autonomous and fuel-optimized formation flying at representative distances of 100 to 2000 m. The eccentricity and inclination vector separation developed for this purpose ensures a maximum operational safety in contingency cases (maneuver execution errors, data and communication losses, etc.) and is ideally suited in the framework of future bi-static radar missions. Following a brief introduction to the PRISMA mission, the paper presents the GPS hardware, the architectural design of the navigation system and results of earlier hardware-in-the-loop tests. Thereafter the guidance strategy for autonomous formation flying is addressed. Special attention is, furthermore, paid to the onboard processor environment which is used for implementing the navigation and guidance functions and to the employed software development and testing philosophy.