ESA GNC Conference Papers Repository
Autonomous Orbit Determination and Station Keeping in GEO
Station keeping manoeuvres management from the ground represents a significant work load for Telecom operators: from ranging campaign used for orbit determination to manoeuvres planning, implementation, uploading, execution monitoring and calibration; this concerns especially electrical propulsion based satellites requiring more than 400/year long duration (up to 2h) manoeuvres because of their low thrust, and in case of large satellite fleet. To handle this situation, operators have developed automated layers to their flight dynamics ground software. The use of an Autonomous Orbit Determination and Station Keeping system is an extension of this logic with additional advantages such as a reduction of ground equipments dependency and cost, a reduction of routine controller staff, and a limitation of ground operations errors. The Autonomous Orbit Determination and Station Keeping Control for Competitive Telecom Missions ESA project must be understood in this context. The objective of this activity is to develop and evaluate the performances of algorithms for the planning and execution of station keeping manoeuvres for fully chemical and for hybrid chemical / electrical platforms, using optical navigation or GNSS. It is a natural follow-on to two previous ESA studies: Feasibility of GNSS sensors for AOCS Applications in GEO and Higher Altitudes and GEO Orbit Determination Using an APS-Based Navigation Sensor. In order to have a good ephemeris predictability (especially for collision risks purpose), the maneuver planning on-board algorithm is similar to the one implemented into the Flight Dynamic ground-software. For the pure chemical platforms, inclination is controlled by North manoeuvres and longitude / eccentricity / semi-major axis are controlled by East/West manoeuvres. For the hybrid platform, inclination and a part of the eccentricity are controlled by pairs of electrical North and South manoeuvres, whereas longitude / semi-major axis and the remaining part of the eccentricity are controlled by chemical East / West manoeuvres. The performance evaluation was performed on a closed-loop functional software simulator (FVB) and also on a closed-loop real-time test bench (RTB); the RTB includes a MOSAIC GPS receiver and a LEON-based hardware representative of future On-Board Computers. Both the environment software and the prototype software implementing the SK algorithms were auto-coded from SIMULINK models. Tests on the FVB confirmed that optical based sensors accuracy is not compatible with station keeping requirements; this solution has thus been discarded and not tested on the RTB; tests performed on the FVB with the MOSAIC and the LION GNSS receivers shown that both are compatible with autonomous SK algorithm, with an advantage to the LION. Long duration simulations were performed on the FVB using a simplified navigator instead of the MOSAIC or the LION software: these tests showed that SK performance was very sensitive to radial position and to along-track velocity accuracies. Two real-time tests were performed successfully on the RTB, one for each propulsion type, covering a full manoeuver cycle (7 days).