ERS marked in many ways Earth observation at the European Space Agency. When ERS-1 was launched in 1991, it was one of the most sophisticated spacecraft ever built in Europe. It accomplished, together with its successor ERS-2, a major scientific mission covering a period of more than 20 years. During this extended period, the mission had to respond to new requirements while the actual capabilities of the ERS-2 spacecraft were declining. At the end of the mission, the mitigation of space debris had become a priority and posed a final challenge. Disposal operations were prepared to lower the orbit and to "passivate" the ageing spacecraft. The development and the execution of these operations are presented with a particular emphasis on the influence of the platform design and of its status at the time of execution. In order to give an overview of the platform status at the beginning of the disposal phase, its design is presented together with a short in-flight history of major failures and on-board software modifications. The ability of the spacecraft to comply with space debris mitigation recommendations is then analyzed before the development and the execution of the disposal operations is described in detail.
Since December 2010, a new French Law on Space Operations (LSO) must be complied with by any entity related to space activities. It imposes, for instance, that any satellite operator shall demonstrate his capability to control the space vehicle, whatever the mission phase from the launch up to its End Of Life (EOL). The French Space Agency (CNES) is currently operating several remote sensing satellites (the last one called PLEIADES 1A), among a larger fleet of satellites. In that context, CNES has decided to perform several specific studies in order to define satellite disposal operation plans in any situation. The typical scenario of EOL operations consists in implementing five phases : disposal orbital manoeuvres, fluidic passivation, electrical passivation, transmitter disconnection and EOL orbit computation. One shall point out that these satellites (except PLEIADES 1A and 1B) were not originally designed to conform with the constraints imposed by the LSO. Thus, detailed analysis were conducted to identify the on-board and the on-ground improvements, as well as the operations chronology, without compromising in any way the security in the conduct of the operations. Depending on the context (nominal, contingency or emergency case) and the satellite subsystems status, different scenarios have been defined and compared in terms of strategy of disposal orbital manoeuvres, on-board and on-ground software upgrades, cumbersome operations, complexity, feedback… This paper addresses this tradeoff, and moreover, it focuses on two main points : 1) the on-board software upgrades (monitoring parameters, Failure Detection Isolation Recovery strategy, chronology and constraints of modifications upload) and the equipment that must be switched off or for which configuration parameters must be tuned, 2) the different disposal orbit manoeuvre strategies : several Orbit Control Manoeuvres (OCM), one or two OCMs followed by an infinite-duration thrust, or an unique infinite-duration thrust. In conclusion, the generic principles applied in these studies could be considered as guidelines for any other satellite platform EOL operations.
Mitigating the effects of Single Event Upsets (SEU) is critical to ensuring spacecraft safety in low earth orbits. The memory of the Central Communications Unit on Envisat, launched in March 2002 for a nominal life of 5 years, is protected by EDAC ensuring thatany single bit flip identified during memory read instructions is corrected prior to usage by the processing unit. The memory is divided into two blocks (prime and redundant), each consisting of three pages. A memory scrubbing task runs in the background to correct the contents in the memory, and reports periodically the number of corrected SEUs. In November 2006, the memory scrubbing task began reporting SEUs at a very high rate which lead to a diagnosis of permanently faulty memory areas. Alas the limitations in the data available in the scrubbing report prevented assessment of the full extent of the problem. Fortunately, the internal prime memory block redundancy could be activated, masking the problem, and provided time for the development of a diagnostic routine. A similar behavior was then observed in another prime memory page in December 2007 while no further internal prime memory redundancy was available. The new diagnostic routine was used to obtain a detailed assessment of the affected memory areas. Furthermore, the memory fault statistics delivered by the scrubbing were no longer usable, the report being flooded by the permanent errors. The permanently corrupted memory areas also increased the risk of double event upset occurring which would have led to a mission interruption. A permanent swapping of the prime and the redundant memory blocks mitigated this risk, but at the cost of a potential loss in memory redundancy in case of an SEU occurring in the corrupted area. An extension to the Envisat operational lifetime was under discussion at the time of the second memory page anomaly and it was clear that failure to recover some memory redundancy could jeopardize approval. This was achieved by 1/modifying the Failure Detection Isolation and Recovery (FDIR) behavior in case of a double bit flip detection in redundant memory and, 2/improving the scrubbing task to make it tolerant to permanent errors. This paper presents the lessons learned from the Envisat prime memory anomalies and provides recommendations to the design of future LEO spacecraft data handling subsystems.
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