FAST: A new MEX Operations concept, quickly!

Abstract

Mars Express has been in orbit around Mars since 2003, relying on the Solid State Mass Memory (SSMM) to hold a “mission time-line” (MTL) of 3000 commands, refreshed daily, to execute the mission. The MTL schedules transmitter switching, spacecraft pointing and instrument operations. The original operations concept called for the MTL to be kept as fully loaded as possible, as early as possible. In 2011, Mars Express suffered from five SSMM-related anomalies, three of which put the spacecraft into safe mode. The safe-modes were caused by an inability of the MTL to refill its cache of commands, due to the SSMM anomalies. As each safe mode expends roughly 6 months' of fuel, the decision was taken to halt science and non-Earth pointing operations. Another MTL independent from the SSMM exists in processor RAM. This “short MTL” contains space for only 117 commands. A new concept, the File-base Activities with Short Timeline (FAST), was devised to restart science operations as soon as possible. The core of the concept relies on storing commands in the SSMM in discrete files that contain an entire “activity”. Critically, “an activity” always starts and ends with the spacecraft in a safe configuration. These command files, always fewer than 100 commands, are loaded into the short MTL in a just-in-time scheme, via “trigger” commands in the MTL, and activating the activity commands only if the file loads completely. This all-or-nothing approach provides robustness to possible further SSMM anomalies by preventing them from causing a safe mode. This paper will present the new operations concept, the additional safety mechanisms, the implementation approach onboard and on-ground, the challenges for team and knowledge management, as well as the achieved performance of the so-rehabilitated mission.