The international team that investigates the problems with the Dutch space instrument HIFI on board the ESA space telescope Herschel has arrived at a complete and consistent failure scenario. A "chain of events" resulted in an overload in one of the power converters of the Local Oscillator Control Unit (LCU), which controls the signal artificially generated within the instrument, causing a permanent failure of one of the diodes.
Scientist of ESA are working on a draft report which is currently under review within the team, although some further consolidating investigations on this scenario are still on-going. But thanks to the capability to perform extended tests on fully representative hardware and software on the ground a clear picture has emerged. A single event upset in the Local Oscillator Control Unit (LCU) activated an emergency switch off. This switch was designed to protect the local oscillators against damage from a drop in the power supply from the spacecraft (28 V). But now the switch was activated while the power supply was still up, which resulted in an overvoltage spike. This caused an overload in one of the power converters, leading to a permanent failure of one of the diodes.
Although the detailed investigations have uncovered some marginal stress overloads of the diodes, laboratory tests have shown these diodes to be quite resilient against the short overvoltage spikes they are subjected to during nominal instrument operations. Therefore the investigation team is confident that the back up system can perform nominally for the remainder of the mission provided a number of corrective actions are implemented. Before the restart of the back up system, changes in the operation an protection logic of the instrument will be implemented in the on-board software to prevent re-occurrence of the sequence of events.
Birth of stars
HIFI – the Heterodyne Measuring instrument for the Far Infrared – is investigating the composition of interstellar gas clouds and measures, for example, how much carbon and water these contain. The wealth of detail HIFI is expected to deliver will tell us more about the birth and early development of stars and planets. HIFI shall also make measurements of the atmosphere of planets and comets in our solar system.
The far-infrared light observed by HIFI has a very high frequency, and is very hard to detect and process. Therefore in HIFI the sky signal (terahertz radiation) is mixed with the local oscillator signal that is generated within HIFI . The mixed signal has a much lower frequency but still contains all scientific information and can then be easily processed. HIFI was designed and built by a nationally-funded consortium led by SRON Netherlands Institute for Space Research. The consortium includes institutes from France, Germany, USA, Canada, Ireland, Italy, Poland, Spain, Sweden, Switzerland and Taiwan."