In-Flight Demonstration of Formation Control based on Relative Orbital Elements S. D’Amico 1 , J.-S. Ardaens 1 , R. Larsson 2 1 German Space Operations Center, 82234 Wessling, Germany; 2 Swedish Space Corporation, Strandväg 86, P.O.Box 4207, Stockholm, Sweden 1 ABSTRACT The fundamental objective of the PRISMA mission is to respond to the increasing demand of autonomous formation flying and on-orbit servicing technology through the in-flight demonstration of novel guidance, navigation and control (GNC) techniques. This paper addresses one of the primary experiments conducted in the frame of the PRISMA mission to demonstrate broad autonomous formation keeping and reconfiguration capabilities on a routine basis using GPS navigation, relative orbital elements, and impulsive control. After a brief introduction of the adopted formation flying concept and its key algorithms, the paper focuses on the experiment planning, operations and its performance in orbit. The obtained results show the high readiness of the developed spaceborne GNC technology and pave the way for its adoption in future advanced multi-satellite missions for remote sensing. 2 INTRODUCTION This paper presents flight results from the first demonstration of autonomous formation keeping and reconfiguration based on relative orbital elements in low Earth orbit. The results have been obtained in the frame of the Spaceborne Autonomous Formation Flying Experiment (SAFE) [1] contributed by the German Space Operations Center (GSOC) to the Swedish PRISMA mission [2]. SAFE is intended to demonstrate fuel-efficient, collision free, accurate and robust relative motion control on a routine basis to fulfill the typical requirements of future distributed sensors for Earth observation like synthetic aperture radar interferometers and gravimeters. In this research a description in terms of relative orbital elements has been preferred to the canonical Cartesian parameterization. In contrast to the fast varying Hill variables, the use of orbital element differences simplifies the formation flying description and the satellite relative position computation. Here the method of relative eccentricity/inclination vector separation, first developed for the safe collocation of geostationary satellites, is generalized and applied to proximity operations of formation flying spacecraft in orbit [3]. The spontaneous geometrical representation offers a direct correlation between the relevant characteristics of the bounded relative motion in near circular orbit and the magnitude/phase of the relative eccentricity/inclination vectors. This aspect extremely simplifies the design of safe, passively stable formation-flying configurations. In particular minimum collision risk conditions can be guaranteed by imposing the (anti-)parallelism of the eccentricity and inclination vectors of the respective satellites, while J 2 -stable relative orbits are obtained by setting a specific nominal phase for the configuration. The adopted approach is shown to be suitable either for the realization of synthetic aperture radar interferometers with baselines below 1 km [4] or the application in longitude swap operations with along-track separations above 200 km [5]. This paper addresses the first case where an active relative orbit control strategy is necessary, in order to compensate for the main disturbance forces represented by Earth’s oblateness perturbations and differential aerodynamic drag. The required velocity budget for formation keeping and reconfiguration can be expressed in terms of relative orbital elements and is directly proportional to the relative eccentricity and inclination offsets. The proposed analytical feedback control law is adopted to maintain and reconfigure the PRISMA formation in a safe and fuel- efficient way on a routine basis during the technology demonstration. After a brief description of the relative orbital elements parameterization and the control strategy, the paper focuses on the second and final part of the SAFE experiment, executed in the time frame between 17 th March and 4 th April 2011. The flight results shown in the paper demonstrate the fulfillment with margins of the prescribed objectives and requirements which are at the basis of the guidance, navigation and control (GNC) system design [6,7]. 1