AAS 08-212 THE PRECISE AUTONOMOUS ORBIT KEEPING EXPERIMENT ON THE PRISMA FORMATION FLYING MISSION Sergio De Florio * *, Simone D’Amico* and Miquel Garcia Fernandez* This paper analyses the problem of the autonomous control of the Longitude of the Ascending Node (LAN) for a satellite in Low Earth Orbit (LEO) by means of along-track and anti-along- track velocity increments which adjust the semi-major axis. The problematic concerned with the possibility of generating the reference orbit on-board and with the estimation of the at- mospheric drag are considered. The Autonomous Orbit Keeping (AOK) experiment of the PRISMA formation flying mission will be the test platform of the theoretical achievement here exposed. PRISMA comprises a fully maneuverable micro-satellite (MAIN) as well as a smaller sub-satellite (TARGET) which are launched together in a clamped configuration and separated in orbit after completion of all checkout operations. The AOK on-board software shall demon- strate MAIN satellite autonomous orbit control using a guidance law for the orbits LAN and shall implement a deterministic control algorithm using along-track and anti-along-track ve- locity increments. Using GPS-based absolute navigation data, AOK shall command thruster activations in the orbital frame to autonomously control the orbit within a predefined window. The main requirement of the experiment is to demonstrate an orbit control accuracy of the osculating ascending node of 10 m (1σ). The paper shows results from real-world software simulations where the accuracy of the reference orbit is limited and GPS sensors and hydrazine actuators are accurately modeled. The fundamental approach on which the software design, validation and testing is based, is also explained. INTRODUCTION Autonomous navigation and orbit control is acquiring increasing interest as it can enhance mission perfor- mance and provide significant operations cost reduction. By controlling the spacecraft orbit to match a chosen reference, the fulfillment of strict requirements on different orbit parameters can be achieved in real time and with a significant reduction of ground operations. The main cost benefits deriving by the use of autonomous navigation and control are: relieve of ground station operational burden, reduction of planning and scheduling costs, use of less propellant than traditional orbit maintenance, need of smaller and lighter weight thrusters, easier scheduling ground station operations and data collection (Ref. 1). The benefits of this type of control can be fully exploited in remote sensing space missions where the satellites are generally placed in sun- synchronous, phased and frozen orbits: examples of missions of this type are TerraSAR-X (Ref. 2, 3 and 4) and ERS-1, ERS-2 (Ref. 5 and 6). This paper analyses the problem of the autonomous control of the longi- tude of ascending node for a satellite in LEO by means of adjustments of the semi-major axis. Establishing a reference orbit and the way of generating it is a key element of the orbit control system. The on-board generation by a numerical routine can be severely constrained by the limitation of the on-board computer. On the other hand generating the reference points with an analytical model can be precluded by the accuracy requirements of the grid that defines the nominal longitudes of the ascending node during the satellite life. Sending a reference orbit from the ground to the satellite keeps from facing all the problems concerned with on-board generation but reduces the autonomy and flexibility of the control system. As this type of orbit control is based on along-track and anti-along-track velocity increments which adjust the semi-major axis, a correct estimation of the atmospheric drag, the main non gravitational perturbation in LEO, is also essential to the end of a fine control. The autonomous orbit keeping experiment is the secondary objective of the German Aerospace Center (DLR) contributions to the PRISMA mission (Ref. 7 and 8). PRISMA is a micro-satellite mission created by the Swedish National Space Board (SNSB) and Swedish Space Corporation (SSC), which * Space Flight Technology Department, German Aerospace Center (DLR), D-82230 Wessling, Germany.