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Cluster rescue

Mission extended until December 2012

  Artist's impression of the Cluster satellites studying the effects of solar wind.

This year a set of crucial science instruments on one of the European Space Agency's Cluster satellites, built by Airbus Defence and Space, stopped responding after more than 10 years in orbit. As part of its customer support activities, Airbus Defence and Space specialists Gunther Lautenschläger and Heiner Sondermann were on hand to offer assistance and advice to ESA leading to a full recovery at the beginning of June.

A Cluster satellite during integration in Airbus Defence and Space Friedrichshafen’s cleanrooms in 1998. The Cluster mission got underway with a pair of dual Soyuz launches in summer 2000. Since then, the four identically designed satellites have been measuring the effects of the solar wind on the Earth’s magnetic field, providing an exceptional insight into the largely invisible interaction between the Sun and Earth. The nominal Cluster mission was planned to last until 2005. Due to the extraordinary measurements, the mission has been extended three times and is now planned to last until December 2012.

Of the 11 instruments on board each of the satellites, five comprise the Wave Experiment Consortium (WEC), which is used for measuring electrical and magnetic fields. All sensors have to be working together for the measurements to give a consistent view. On 5 March, the WEC instrument package on board the third Cluster satellite, known as Samba, was switched off as a reaction to an on-board monitor alarm and failed to switch on again.

Cutaway of Cluster spacecraft main equipment platform, showing the STAFF (1), EFW (2), DWP (3), WHISPER (4) and WBD (5) instruments – all of which comprise the WEC (WAVE Experiment Consortium) package. ( © ESA)

Standard recovery procedures were carried out by controllers at ESOC (ESA’s European Space Operations Centre in Darmstadt, Germany), but all to no avail, and the instruments also proved unresponsive. Heiner Sondermann, Airbus Defence and Space’s Cluster electrical system engineer, says: “With neither a response from the instrument nor any information on the instrument’s power consumption, suspicions led to either an instrument malfunction or an electrical short circuit.” A permanent failure of the instrument would have been a significant decrease of the science return and virtually be the end of the Cluster mission.

ESOC also commanded the redundant power supply, but the Latching Current Limiter (LCL) within the Power Distribution Unit (PDU) tripped off immediately. This is a precaution designed to protect the satellite main bus against short circuits from the instrument.

Engineers working in Cluster's dedicated control room at ESOC.  (© ESA/J. Mai)Over the next several weeks, ESA’s Cluster control team at ESOC worked closely together with Airbus Defence and Space's specialists, the WEC scientists and manufacturer, and other ESA teams. Airbus Defence and Space systems engineer Gunther Lautenschläger proposed that ESOC make use of a dormant on-board software module to measure the WEC instrument inrush current. This software was last used after launch 10 years ago to monitor the proper deployment of the antenna and instrument booms!

This current profile, obtained via telemetry, ruled out a short circuit on the instrument side, leading to the assumption that the five power switches inside the WEC instrument package must be locked in the ‘closed’ position and hence the inrush current of a simultaneous switch on of five instruments exceeded the LCL’s nominal trip-off current.

The only chance was to switch on the WEC instrument package by simultaneously switching nominal and redundant satellite power supply; excluded by the design of the PDU as well by the On-Board Computer Bus protocol. After exploring several potential solutions, which had to be discarded, the ESA and Airbus Defence and Space specialists found a ‘back door’.

Airbus Defence and Space expert Gunther explains: “The idea was born from reading an operational constraint in the PDU user manual and trying to split one command into two half commands by switching on both the main and redundant LCLs simultaneously – something which never normally happens! Based on this idea, the two of us came up with a potential recovery procedure, which ultimately proved successful.”

   Gunther Lautenschläger, Airbus Defence and Space's Cluster project manager, in the ESOC control room in Darmstadt during the launch campaign in 2000 As reported by ESA: finally, on 1 June, in a very tense mission control room, a series of commands was radioed up. To immense relief, these flipped the power switches to ‘on’ and the recalcitrant WEC came back to life. Cluster has since returned to normal operation and measures are being taken to prevent this failure from happening again.

“When everything goes as planned, flying a mission can be routine,” said ESA’s Manfred Warhaut, Head of Mission Operations. “But when unexpected trouble occurs, and there’s nothing in the manuals, you really want to have an experienced and talented team on hand to solve the problem.”

Heiner concludes: “When you know what you are doing, you can sometimes take the risk of going ‘off road’ – luckily it paid off!”

SatelliteScienceCluster II