1.2.3 Calibration phase and monitoring activities
As defined by CEOS, the calibration is the process of quantitatively defining the system response to known, controlled signal inputs. After the ENVISAT launch, between March 2002 and November 2002, specific observations in monitoring mode were performed in order to verify the nominal operation for all the components of the instrument (command and control system, pointing system, rallying, centering, tracking, and acquisition phases, thermic and opto-mechanic behaviour, mirror behaviour, pixel response non-uniformity,…). The correct answer of the instrument to several anomalies was checked (impossible tracking, non detected star) as well as the general data handling. The outputs of this calibration phase allowed to confirm that most of the in-flight performances were consistent to the ground measurements performed before the launch, during the dedicated ground characterisation phase. It had also highlighted a high sensitivity of the CCD measurements to the proton radiation, and the subsequent need to calibrate the DC at each orbit for correction. This is achieved by using one specific occultation performed in full dark limb condition, the instrument looking at a so-called “dark sky area” (or DSA), where no input flux is expected at the entrance of the instrument.
The Quality assessment of the instrument and of the products is crucial for the mission success. It is a continuous task, implying various aspects such as the performance assessment and the anomaly investigation, the calibration activities and the delivery of auxiliary data files, the daily and long-term monitoring of the instrument health, and the daily and long-term monitoring of the products. Daily and monthly reports on these aspects are made available to the user community at the following addresses:
(current day and access to archives) and
(previous month and access to archives).
From 1 May 2003, the operations of the instrument during the star rallying phase had been affected by an anomalous behaviour of the mirror drive unit at low azimuth angles, due to Voice Coil Saturation (excessive current drawn by a coil in the mirror drive unit). As a first bypass solution, the field of view for the star selection had been reduced. However, this reduction of the field of view resulted in the unavailability of the highest quality occultation opportunities and the anomalies were still degrading. After detailed analysis and further testing by the Anomaly Review Board (ARB) and the Envisat Mission Manager, it was decided to switch operations to the redundant "B" ICU (instrument control unit) and MDE (mechanism drive electronics). Nominal measurements with the redundant side and data dissemination resumed on 19 July 2003, using the complete GOMOS field of view [-10.8°, +90.0°] for the star selection.
Between January 2005 and July 2005, a failure in the telescope elevation drive, the so called "elevation voice coil", which is part of the Steering Front Mechanism, prevented the nominal operations of the instrument. The anomaly occurs during the rallying of the telescope in the preparation for the star observation. Based on a series of tests, the ARB recommended to limit the rallying time by operating the instrument in a reduced horizontal azimuth range. Nominal operations resumed in September 2005 in the azimuth range [-5°, 20°]. The GOMOS Quality Working Group was solicited to advise about the choice of the boundaries of the azimuth range, in order to achieve the optimum of scientific return from the reduced operation scenario.