The GOMOS measurement principle is based on stellar occultation. Its main measurement mode is the occultation mode. During this mode the instrument is 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 was 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 whether, the instrument is pointed towards "dark space" or towards uniform limb, the dark current or the photo response 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): 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.