Return to full science operations
15 Feb 2012
Following a permanent anomaly affecting the onboard solid-state mass memory system on Mars Express, efforts to implement a work-around are almost complete. Full science operations have been resumed and the potential mission lifetime remains unaltered.
Since the suspension of Mars Express science operations, the mission control team has been hard at work devising, testing and implementing a new method of commanding the spacecraft in order to mitigate the effects of a failure in the onboard data storage system.
A big problem
In mid-August 2011, an anomaly affecting the on-board Solid-State Mass Memory (SSMM) system on Mars Express caused the spacecraft to enter safe mode. Despite switching to the cold-redundant memory controller, the problem persisted; after several more transitions to safe mode, it was decided to suspend science observations on 16 October 2011, in order to conserve propellant supplies.
During normal spacecraft operations, telecommands for the platform and its instrument payload that have been received from the ground stations are stored in the Mission TimeLine (MTL), a special file in the SSMM that is then used to fill the platform's command cache. Errors during the transfer of data from the MTL to the command cache mean that the contents of the cache may be incomplete or incorrect; the spacecraft's supervisory systems therefore have to command a transition to safe mode and await intervention from the ground.
An elegant solution
In order to cope with anomalous operational scenarios, a hardware-based timeline store – the Short Mission TimeLine (S-MTL) – is available in the Data Management System (DMS), a special-purpose computer. The work-around involves using the S-MTL for normal operations, instead of the regular MTL.
However, the SSMM has a capacity of 1.5 GB and, although the MTL uses only a small part of that storage, it can hold thousands of commands – whereas the S-MTL can only store 117. Since it can take 50 or more commands to perform a single observation, just switching to the S-MTL is not a workable solution. Fortunately, the spacecraft designers incorporated the ability to use On-Board Control Procedures (OBCPs) – sets of spacecraft commands that function in a manner similar to programming language macros – a facility that has seen little use until now.
By loading OBCPs into the command cache and executing them using instructions from the S-MTL, it has been possible to replace the thousands of commands previously stored in the MTL with much shorter command sequences suitable for storage in the S-MTL. Stringent precautions have been taken to ensure that an SSMM anomaly, or any other unexpected occurrence, does not lead to the spacecraft being incorrectly commanded. The command sequences in the S-MTL are always designed so that the final instruction authorises execution of the entire sequence; an incompletely loaded sequence therefore cannot be executed. In addition, an unrecoverable error during the transfer of an OBCP to the command buffer will result in the associated S-MTL command sequence being aborted.
HRSC image of an area near the Martian North Pole, acquired using the newly developed FAST commanding method. Credit: ESA / DLR / FU Berlin (G. Neukum)
The new commanding system has been given the name 'File Activities on Short Timeline' (FAST). Although four SSMM anomalies have occurred since the introduction of FAST, they no longer trigger safe mode transitions and have only a minor impact on mission operations.
This has recently been tested in real life, as the spacecraft encountered an SSMM anomaly exactly like those in late 2011, but this time was able to complete the observation and return to Earth pointing without entering safe mode. This allowed a much faster recovery to nominal operations and avoided wasting precious propellant.
While full science operations have now been resumed, a number of tasks remain to be completed. Most important among these is the implementation of an OBCP scheduler. This will enable the spacecraft to operate autonomously for up to a week, compared to the few days that are possible with the current FAST system. Work is also in hand to resume operation of the Visual Monitoring Camera (VMC – the 'Mars webcam').
Enormous team effort
Completely redesigning the way in which Mars Express is controlled has involved an enormous amount of work for the mission control team at the European Space Operations Centre (ESOC), assisted by their counterparts at the European Space Astronomy Centre (ESAC), PI-teams, other ESA experts and partners in industry. Everyone involved with the mission is extremely grateful for their hard work.
Although the 'Express' in Mars Express highlights that the mission was developed in a short time and with a relatively modest budget, the ability to resume full operations after a very serious failure shows that the resulting design is both robust and flexible.
Mars Express has now been restored to full operational capability and its potential mission lifetime remains unchanged.
Mars Express Mission Manager
Directorate of Science and Robotic Exploration, ESA, The Netherlands
Last Update: 15 Feb 2012