Minimize Baseline

Operational modes

The main mode of operation of GOMOS is the occultation mode. During this mode the instrument is, as a result of a macrocommand, autonomously acquiring and tracking stars as they set through the atmosphere. The occultation mode has a specific submode called "fictive star" submode. In this submode the instrument is scanning the limb along a programmable trajectory as if a star were present. This submode can be used for limb sounding under bright limb conditions.The sequence of stars to be used for occultation was generated by a set of CFI software and was provided in form of a timeline of occultation.

Nominal measurements were regularly interrupted by three types of monitoring activities, namely:

  • The uniformity mode (UNI): in this mode the CCD sensors are read out in a non-binned, i.e., pixel by pixel, mode. Depending on weather, the instrument is pointed towards "dark space" or towards uniform limb, the dark current or the photoresponse uniformity of the CCD sensors is characterised. In this mode the tracking function is disabled. The spectrometer integration time is programmable in the range of 0.25 s to 10 s
  • The spatial spread mode (SSM): in this mode the instrument operates as in the uniformity mode, but with the tracking function active. Thus, the optical transfer function of the instrument can be monitored in this mode by observing a star outside the atmosphere
  • The linearity mode (LIN): in this mode the sensors are operated in binned mode as in occultation mode but with variable integration times (in the range 0.25 s to 10 s). This mode is used to monitor the linearity of the detection chains during the instrument lifetime by observing stars outside the atmosphere with variable integration times. 

Typical Routine Calibration sequence 

Seven consecutive orbits per month were acquired with the instrument in different monitoring modes.:

  • 1 orbit in SSM mode. The brightest stars have to be chosen (this cannot be done automatically by Starsel). At least 6 stars in dark limb (including at least 3 in eclipse) and 3 stars in bright limb.
  • 2 orbits in UNI mode, one at 10 km and one at 30 km altitude. The start times and the durations are always the same. The integration time is 0.5 s except for the last observation of the orbit. The azimuth is always equal to 30°. The location of the SATU windows is always the same.
  • 2 orbits in LIN mode, from 300 to 100 km altitude. The brightest stars have to be chosen (this cannot be done automatically by Starsel). At least 6 stars in dark limb (including at least 3 in eclipse) and 3 stars in bright limb. The integration time is 0.25 s for the first orbit and 0.5 s for the second one.
  • 2 orbits in OCC with as much Dark Sky Area (DSA) acquisitions as possible. The altitude range is 300 to 200 km for the first orbit and 300 to 0 km for the last orbit.

Due to different needs, this sequence changed during the mission.