EVALUATION OF RELATIVE NAVIGATION ALGORITHMS FOR FORMATION-FLYING SATELLITES

Goddard Space Flight Center is currently developing advanced spacecraft systems to provide autonomous navigation and control of formation flyers. This paper discusses autonomous relative navigation performance for formations in eccentric, medium and high altitude Earth-orbits using Global Positioning System (GPS) Standard Positioning Service (SPS) and intersatellite range measurements. The performance of several candidate relative navigation approaches is evaluated. These analyses indicate that the relative navigation accuracy is primarily a function of the frequency of acquisition and tracking of the GPS signals. A relative navigation position accuracy of 0.5 meters root-mean-square (RMS) can be achieved for formations in medium-attitu de eccentric orbits that can continuously track at least one GPS signal. A relative navigation position accuracy of better than 75 meters RMS can be achieved for formations in high-altitude eccentric orbits that have sparse tracking of the GPS signals. The addition of round-trip intersatellite range measurements can significantly improve relative navigation accuracy for formations with sparse tracking of the GPS signals. 1 – INTRODUCTION Formation-flying techniques and satellite autonomy will revolutionize space and Earth science missions and enable many small, inexpensive satellites to fly in formation and gather concurrent science data. The Guidance, Navigation, and Control Center (GNCC) at Goddard Space Flight Center (GSFC) has successfully developed high-accuracy autonomous satellite navigation systems using the National Aeronautics and Space Administration’s (NASA’s) space and ground communications systems and the Global Positioning System (GPS) (References 1 and 2). Recently, the GNCC has leveraged this experience to develop advanced spacecraft systems that provide autonomous navigation and control of formation flyers. To support this effort, the GNCC is assessing the relative navigation accuracy achievable for proposed formations using GPS and intersatellite range measurements. Several universities and corporations are developing GPS transceivers that support this tracking concept for NASA and the Air Force Research Laboratory; these include Johns Hopkins Applied Physics Laboratory, International Telephone and Telegraph, Honeywell, Motorola, Jet Propulsion Laboratory, Cincinnati Electronics, and Stanford University (Reference 3). This paper evaluates the performance of several candidate relative navigation algorithms for missions with more than two vehicles maintaining a relatively tight formation, in a relatively eccentric orbit. High-fidelity simulations were performed to study two proposed formation-flying missions. One is a mission designed to study the Earth’s aurora. This medium-altitude formation consists of four satellites maintained in eccentric Earth orbits of approximately 500x7000 kilometer altitudes. To support autonomous planning of the formation-flying maneuvers to maintain the initial 10-kilometer separation at apogee, the total relative position and velocity accuracy must be about 100 meters and 20 millimeters per second, respectively. Later in the mission, when the separation is reduced to about 500 meters, the total relative position accuracy requirement reduces to 5 meters.