Formation flight handling with 4 satellites – electronic earth observation as an application

Abstract

CNES Toulouse Space Center operates several satellites dedicated to earth observation of many kinds. Some of these observations, as the ones made by the ELISA satellites, require to fly in formation to fulfill its objectives. ELISA (ELectronic Intelligence by SAtellite) is a project sponsored by CNES and DGA (French Defence Procurements Agency). Its objective is to demonstrate the ability to locate radar equipment from space and to collect information on their technical characteristics. ELISA mission involves a fleet of four microsatellites which were launched in December 2011 and positioned just a few kilometers apart in a Low-Earth Orbit. They have been built around the “Myriade” micro spacecraft bus, developed by CNES for its science missions. CNES not only supervises the project but also operates the four satellites from the ground. Handling formation flying encompasses several challenges starting from the Launch and Early Orbit Phase to the management of the formation station-keeping itself. In addition to these obvious challenges the control center should be ready to face contingency situations in case of in-flight anomalies. The recovery or workaround solutions for potential issues have required that the operational people and satellite manufacturer use all their skills and imagination. This paper will first provide a brief presentation of the mission, of the operational organization, and of the satellites characteristics. Then it will present the orbital requirements for the mission and in particular the ones linked to the need to keep a specific satellite geometrical pattern. The strategy, used during the Low and Early Orbit Phase, to gather the 4 satellites and place them on theirs marks from their ejection orbit will be explained. In particular, it will be detailed how the inclination, the eccentricity and the drift of the satellites have been tuned to reach the final relative positions. The navigation principle and the adopted strategy to ensure the satellites geometrical pattern keeping during all the mission will be explained as well. Distinctive characteristics of this strategy are that it is focused on the steadiness of the geometry made by the satellites relative position, and that the maneuvers plan for station keeping has been designed to be as automated as possible. Furthermore, it will also be described in this paper how the station keeping strategy could be adapted to cope with imaginable critical failures which could impact the navigation, in particular failures linked to the AOCS and propulsion systems. In addition, it will be presented how the geometrical pattern is foreseen to be recovered, or adapted, if one satellite of the ELISA fleet had to be separated from the others. This separation could occur when one satellite needs to perform a maneuver to avoid debris, or another satellite external to the fleet, or in case one satellite would encounter a major failure preventing it to fulfill its mission or to keep its relative position. Finally, lessons learned by the applied strategies will be addressed as a conclusion.