ESA GNC Conference Papers Repository
LEO GNC Design Evolution at Thales Alenia Space-France
First LEO GNC designed at Thales Alenia Space France was in the frame of PROTEUS development for CNES French Space Agency. The first application was JASON1 launched successfully by end-2001. Basic requirements put on the GNC were performance, robustness and versatility for ensuring PROTEUS to be an actual multi-purpose platform. The GNC proposed is integrating innovative solutions : a) based on STR and GPS in Normal Mode, secured by the association of a medium accuracy gyro and the implementation of a board/ground/board loop (GPS not in closed loop) b) implementing a very robust Safe Mode using magnetic actuation and a magneto-solar scheme for attitude/rate estimation. Transition Mode in between the two modes is through a board/ground loop for computation of the adequate guidance. Orbit Correction Mode is using an optimal set of 4 Hydrazine thrusters in off-modulation for large delta-V maneuvers. 3 satellites covering a wide range of inertia, a wide range of pointing (nadir with yaw steering, vertical, inertial) and a wide range of LEO orbits (drifting, SSO, polar) are currently in orbit and functioning perfectly; two others are in integration and will be launched in 2008 and 2009. The new generation of Thales Alenia Space-France LEO GNC, dedicated to PROTEUS MK2 platform, is highly inherited from PROTEUS design. Basic PROTEUS requirements remain applicable but additional driving specifications have to be accounted for. On customer side there is a strong request for a higher autonomy, an increase of the robustness, a maximum simplification of the board/ground link, an agility capacity for specific missions, an adaptation to a wider range of launchers (impact for GNC being angular rates at separation) and in some cases some attitude/convergence time requirements related to sun acquisition in Safe Mode. Strong internal industrial requirements are on development cost (design, manufacturing, validation), recurring cost, non-recurring cost in case of adaptation or equipment change, non-itar units, maximum design communality with concurrent GNC studies and developments at Thales Alenia Space-France (Formation Flying, Interplanetary, Telecom and HR Observation in GEO). In the mean time advances in the GNC equipments field allow positive upgrades of the overall GNC S/S. Some are about robustness, typical ones being STR tolerance to solar flares or lower GPS receiver S/N thresholds. Some are technological and programmatic as for example the development of multi-head STR assembly, or the development of European low/medium accuracy gyros (as SELEX MEMS gyro or SAGEM REGYS-20) with a reduced recurring cost. The Thales New Generation LEO GNC is then currently being designed in both top-down approach for what concern requirements and bottom-up approach for what concern running evolutions of units. In Normal Mode a gyroless scheme is implemented, based on a Multi Head STR assembly managed through a dynamics attitude estimator. The robustness of STR is now flight demonstrated and in the mean time increased performances are achieved. Gyros are then no longer necessary for nominal pointing. Coarse Gyros, supposed to be also low-cost, are however kept primarily for Transition Mode and for the slews that can be necessary for reaching the Orbit Correction Modes attitude. A high effort has been devoted to the maximization of autonomy and minimization of the ground workload. GPS is fully used in closed loop for both navigation and guidance. For what concern Safe Mode, a pure magneto-solar concept for attitude estimation and magnetic control is still the baseline. A specific mode using the chemical propulsion is an alternative, using in that case the coarse gyros for rates determination. This propulsion-based mode may be necessary regarding battery capacity for damping initial angular momentum at launcher separation. Also once in operation in case of emergency if payload or electrical subsystem require in the same way a short duration for pointing the sun. Transition Mode (between Safe Mode and Normal Mode) is fully redesigned in the direction of full autonomy. That is made possible using in closed loop the coarse gyros and GPS which get the capability to fix in a sun pointing attitude. The baseline PROTEUS MK2 GNC integrates an upgraded capacity regarding specific requirements that could be placed by some missions on agility or high performance pointing. The introduction of a high performance gyro in Normal Mode with a gyro-stellar scheme, the increase of the wheels capacity or of the number of the wheels, as well as the increase of the STR heads number is possible with a limited Non-Recurring effort. This is already anticipated in the design. The paper presents then the LEO GNC design evolutions at Thales Alenia Space-France and the associated rationales, starting from initial PROTEUS design and the resulting in-flight experience, detailing the PROTEUS MK2 GNC new generation design and its potential for evolution, and finally presenting the detailed design proposed for Sentinel-3 which is the first application of this new generation.