Formation flying interferometry: from the coarse control to the science mode


Frenkiel R. *, Pirson L. **, Thevenet J. ***

Thales Alenia Space, France



Interferometric missions with an instrument distributed on several vehicles in orbit allow deep space observations with an accuracy never reached, but demand a formation flying Guidance Navigation and Control (GNC) with a ultra-high level of precision. One ESA mission of that class, dedicated to exo-planets detection and characterization is nowadays identified: DARWIN. The different GNC stages, from the initial closed loop engagement at the end of deployment to the final control of the Optical Path Difference (OPD) and intensity mismatch used in science mode will be described.
First, a focus on the Navigation Process Unit (NPU) based on the Radio-Frequency (RF) sensor and the modes developed to achieve the coarse formation acquisition, the FDIR Collision Avoidance Mode (CAM) and the recovery after a failure will be given. The RF subsystem is developed by TAS under a CNES contract. Its aim is to provide a coarse relative position sensor and the Inter-Satellite Link (ISL), necessary for the finest mode. It covers a 30 km distance range, but will be adapted up to a 100 km range. It has 3 functional modes and is bi-frequency. The coarse mode covers all the directions of space and is based on the received powers measurements of several antennas scattered around each vehicle. The interferometric mode Before Ambiguity Raising (BIAR) measures the widelane phase shift and provides intermediate accuracy measurements in a privileged direction. The interferometric mode After Ambiguity Raising (AIAR) measures the carrier phase and provides 1 cm on inter-vehicles distance and 1 deg on Line of Sight accuracy measurements in the same direction, after a GNC maneuver. The two last modes require a triplet of antennas forming two orthogonal baselines. The CAM and the reconfiguration are based on the RF coarse mode. The CAM is used in a decentralized way to assure the robustness to formation level failures, whereas the reconfiguration is centralized in one vehicle to have a better coordination. The first control stage is based on the fine RF mode. These modes are generic to all formation flying missions.
Then, an overview of the EMMA configuration of DARWIN, a novel concept proposed by TAS, will be described from the system and GNC points of view. The Darwin GNC contains two more control stages in addition to the first one based on the RF sensor. The second stage based on optical coarse and fine metrology and ionic or Field Electrical Emission Propulsion (FEEP) actuates the Collecting Spacecrafts (CS) relatively to the Beam Combiner Spacecraft (BCS) and acts on the formation's geometry. The third stage, internal to the BCS, acts directly on the scientific beam, adding an optical path with an Optical Delay Line (ODL) and changing its orientation with corrective tip/tilt mirrors based on piezo-actuators. The joint use of these two stages allows to meet the science requirements imposed on the OPD (<1nm) and the intensity mismatch.


Radio-Frequency Sensor; Collision Avoidance; Reconfiguration; Darwin Mission; Emma; Guidance Navigation Control

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