Autonomous Formation Flying for the PRISMA Mission

Abstract

PRISMA is a technology demonstration mission for satellite formation flying and in-orbit-servicing. The space segment comprises the fully maneuverable MAIN mini-satellite as well as the smaller TARGET satellite in a low Earth orbit at 700 km altitude. A key mission objective is to demonstrate on-board a fully autonomous, robust, safe and precise formation flying of spacecraft. This is accomplished by spaceborne GPS-based navigation, guidance, and control functionalities for the maintenance of the relative motion between the two spacecraft. An innovative estimation approach employs a common Kalman filter for the absolute states of MAIN and TARGET which allows to fully exploit the interdependency of absolute and relative navigation without the need for an explicit relative state. As a result, the onboard navigation system provides absolute and relative orbit information in real-time with a position accuracy of 2 m and 0.1 m, respectively. The formation control achieves accuracies of a few tenths of meters with minimum usage of the onboard thrusters. Following an introduction to the PRISMA mission, the navigation, guidance and control concept is detailed with emphasis on GPS-based navigation, relative orbital elements parameterization, and impulsive orbit control based upon a relative eccentricity and inclination vector separation strategy. The paper derives realistic estimates of the expected relative orbit control performances based upon real world simulations using typical GPS receiver and propulsion system characteristics.