THE PRECISE AUTONOMOUS ORBIT KEEPING EXPERIMENT ON THE PRISMA MISSION

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 (RO) on-board and with the estimation of the atmospheric 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 AOK experiment represents a substantial contribution to the state of the art and paves the way to the accurate and autonomous orbit control of LEO satellites on a routine basis. The main requirement of the experiment is to demonstrate an orbit control accuracy of the osculat-ing 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