An innovative operations concept for close formation flight: the ESA CompSAR case study

The CompSAR mission study took place in ESA concurrent design facility for the assessment design of a Passive SAR on a Small Companion Satellite flying in formation with Sentinel-1 with the goal of measuring costal currents using the back-scattered signal of the active instrument. The study highlighted the operational risks and challenges associated with close, lose formation flight. Three major areas of trade off were identified: Orbit Determination and Reconstitution, Data Downlink and TT&C scenario, and Formation Flight strategy. While for the first two areas the mapping of mission requirements to available technologies and infrastructures permits to identify adequate solutions, for the Formation Flight, a further level of analysis is required, detailing the choices for general navigation strategy implementation, nominal formation maintenance, and contingency handling. Although operational solutions for the nominal scenario are straightforward if not simple, the non-nominal scenarios bring about the fundamental problem of a possible collision risk in a relatively short delay. A careful choice of the mission geometry can provide risk mitigation, but, especially for the contingency scenarios, innovative operational strategies need be devised. To this end, three measures are identified as greatly beneficial to relax safety related constraints and increase the scientific return by simply enabling robust and safe formation flight: ad-hoc design of orbit formation configuration, on-board relative navigation, and increased ground visibility (and commandability).