Close Proximity Operations for a 3 Nanosatellites Formation in LEO
Authors:   Cnes, Toulouse, France, Delavault Stephanie , Laurent Lorenzo , Valenzuela Irene 
Published: January 2024

In recent years, CNES has conducted several mission concept studies requiring the deployment of multiple satellites to constitute large-scale radio interferometers. ULID (Unconnected L-band Interferometer Demonstrator) is the most recent concept that involves three nanosatellites flying in close formation and that was proposed in 2017 to demonstrate the monitoring of moisture and ocean salinity with unprecedented accuracy and thus prepare the future enhancement of the SMOS program capabilities. The distributed instrument to be flown on a 600 km 6h/18h quasi-synchronous orbit requires to maintain a quasi-constant 40 meters distance between any pair of satellites using GNSS data for relative navigation and a single electric thruster for position control. Mission design studies and preliminary GNC analyses were conclusive enough to allow the transition into phase A [1]. However, budget restrictions in 2021 led to the project interruption at the end of phase A and most of GNC activity was stalled. Fortunately, ULID is expected to be reactivated in the next few years with de-scoped objectives through a low cost demonstrator that would focus on the formation flying technologies to be matured for the future deployment of radio interferometers.
The paper focuses on the challenges of the ULID mission concept and describes the results of the additional activities conducted in the perspective of this demonstrator to consolidate the formation flying control strategies, the GNC architecture and autonomy approaches for the following mission phases: formation acquisition, station-keeping and reconfiguration in case of contingency.
The first section presents the challenges of maintaining the formation during the interferometer measurements and the main design choices. The optimal instrument configuration is obtained when the positions of the 3 satellites projected on the TN plane (plane perpendicular to the radial axis) form a perfect equilateral triangle that rotates at the orbital period. Such a configuration is achievable by special settings of the relative orbital elements that bring the three satellites in a common local plane. Unfortunately such a configuration does not guarantee the absence of collisions in case any satellite starts drifting under the effect of the differential perturbations and a trade-off has to be found between the safe distance at nodal crossings and the deformation of the interferometer geometry. Each satellite carries a GNSS receiver and a single electric thruster as well an ISL equipment to exchange data with its companions. Station keeping can then be performed autonomously by maintaining within specific bounds the satellite relative orbital elements that are defined with respect to some appropriate reference position (anchor point). Different anchor point options have been proposed and analyzed: (1) the anchor point is virtual and obtained via a weighted combination of all satellites configurations (ex: barycenter of the formation) – in this case, all satellites are rotating around this anchor point and the control is perfectly identical, (2) the anchor point corresponds to the position of one specific satellite that can be regarded as the formation master – in that case, the two companions are rotating around the master. Despite the apparent architecture asymmetry, the second option has been selected since it allows to minimize the number of maneuvers and the propellant budget. In nominal behavior, the master role is actually transferred periodically to another satellite to balance the propellant usage but this change of responsibility can be also triggered in case of thruster deficiency from one of the satellites.
The second section describes the generic GNC architecture and details its different components. It is focused on the control module which objective is specified by three categories of parameters: (1) the anchor point identifier, (2) a set of 6 relative orbital elements including the two components of the relative eccentricity vector, the 2 components of the relative inclination vector and finally the tangential offset/drift parameters that are set to zero in the station-keeping phase, (3) the control tolerance associated to each relative orbital element. To minimize propellant consumption, the control module commands only normal and tangential maneuvers that need to be executed at the optimal relative argument of latitude. Sending the maneuver command at the right time requires therefore a specific module: the maneuver scheduler.
The third section describes the formation acquisition approach and the control paradigm that is based on the same maneuver control/scheduling module used during the station keeping phases. The paper first mentions the conservative approach that assumes the satellites to be initially in a trailing configuration and that consist in performing two independent and sequential rendezvous with the first and third satellites to get close to the anchor point (central satellite). Next, it details a more time efficient approach that includes a simultaneous transfer of the two satellites to the final configuration through at least an intermediate stable configuration with larger inter distances. Several options are analyzed to illustrate the trade-off between formation acquisition duration versus propellant budget considering identical safety requirements.
The fourth section presents the FDIR approach designed to eliminate the collision risk in case of contingencies and maintain some payload operation in degraded mode. The first contingency case concern the loss of communication with one satellite with or without notification. If the communication is not re-established before a given time lapse, a separation is triggered and is executed in a coordinated fashion to reach a stable configuration that is free of collision for an extended period of time. The second contingency case concern some orbital debris avoidance that obliges to raise the semi-major axis of the whole formation and which execution is foreseen in a coordinated way. The paper proposes and analyzes several options to perform these coordinated reconfigurations taking into account the criteria of safety, propellant budget and formation reacquisition time.
The last section provides full details of the control performances that can be achieved in station keeping mode and their sensitivity to environment perturbations, navigation uncertainties and propulsion characteristics. These results obtained in simulations are presented and discussed for the two cases of anchor point specification: formation barycenter and master satellite position.
[1] Close Satellite Formation Flying for ULID mission. 11th IWSCFF, Milano, 7-10 June 2022.