Envisat In Flight Experience: FDIR and Lifetime Optimisation

The ENVISAT satellite, launched on March 1 st 2002 by Ariane V from French Guyana, is the largest and most advanced European Earth Observation Satellite in orbit. At the time of writing, Envisat has been returning high quality environmental data for over four years. The original design lifetime of five years will be reached in March 2007, but with the spacecraft in good health, lifetime extension is foreseen. Maximising life extension depends upon maintaining robust performance and safely decreasing the rate of usage of life limiting consumables. This paper presents recent work performed on the Envisat Power FDIR (Failure Detection Isolation and Recovery) software and on the orbit control strategy. The first part of this paper describes the results of trade-off studies that led to a modification of the orbit control constraints, aimed at reduced propellant consumption. This was achieved by revisiting some of the pre-launch operational constraints with the benefit of flight experience, and a careful trade off of all the dependent factors. The first manoeuvre to use the new, more efficient strategy, was executed in January 2006. The second part of the paper focuses on the enhancements made to the Power FDIR software to reduce the number of failure cases that could lead to battery overcharge. A number of special operations performed in 2004 and 2005 are described; including a major on-board software modification, aimed at optimising the power assembly FDIR and battery charge Management algorithms. These algorithms are now in place on-board, enhancing the robustness of the power subsystem FDIR.