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1.3.2 Modified mission scenario since August 2005

Following VCCS ARB outputs, the horizontal azimuth range of instrument operations has been reduced to 25° after August 2005 in order to limit the risk of VCCS failures. Instead of operating in the nominal azimuth range (between -10° and 90°), GOMOS has been operating in the azimuth range between -5° and 20° between September 2005 and February 2006. The choice of these limits was a trade-off between the number of possible occultations and the verticality of occultations. However, for given fixed limits of the azimuth window, the latitude coverage is strongly changing during the year. It has thus been suggested to change the lower and upper limits of the azimuth window while keeping its amplitude fixed to 25°. At ARB's request, the QWG produced a mission scenario with boundaries of the azimuth range changing every week in order to optimize the mission objectives (global coverage, validation campaigns objectives, minimum azimuth limit not lower than -5° to avoid the vignetting effects) and taking into account the reduced number of stars likely to be observed. This new mission scenario has been in operation since February 2006. The monitoring activities allowed to confirm that this reduced and varying horizontal azimuth range has no impact on data quality of profiles. However, as it results in a loss of about 50% in the number of occultations compared to a mission planning with a complete horizontal azimuth range. Possibilities of  progressively broadening the azimuth window by step of 5° will be investigated.