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6.3 Periods of data unavailabilities

Since the launch of ENVISAT in March 2002, some instrument failures specific to GOMOS occurred during operations. Some of the major anomalies prevented the measurements. Also, it may have been necessary to switch-off the instrument for the time needed to characterize the anomaly, to estimate its impact on the operations and on the measurements, and to either solve the problem or find a bypass or workaround solution.

From 1 May 2003, measurement failures had been occurring due to the progressive reduction of the instrument Field of View on the nominal side. Nominal operations resumed on 19 July 2003 after the switch to the redundant System Control Unit.

A failure of the Elevation voice-coil system (telescope elevation drive for the rallying of the telescope to prepare for a star observation) prevented the nominal operations between 24/01/2005 – 18/07/2005. Nominal operations resumed after the implementation of a new mission scenario, taking into account a reduced azimuth Field of View (the last systematic failure occurred on 26/01/2006). GOMOS has been operating in the azimuth range between -5° and 20° (instead of between -10.8° and 90.0° for the nominal case) between September 2005 and February 2006. Since February 2006, time-varying limits of the azimuth window are used in order to optimize the mission objectives, while its range has been kept to 25°. It is estimated that this reduced azimuth range results to a loss of about 50% in the number of measured occultations.