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1.2 Instrument description and measurement principle

1.2.1 Presentation of the instrument and its components

GOMOS is a medium resolution spectrometer measuring in the ultraviolet, the visible and in the infrared and using the stellar occultation technique.

The four spectrometers of the instrument cover the spectral ranges: 248-371 nm, 387-693 nm, 750-776 nm and 915-956nm. This wavelength coverage allows monitoring O3, NO2, NO3, atmospheric density from Rayleigh extinction and aerosols (UVIS measurements), O2 and H2O (IR measurements) from the upper troposphere to the mesosphere. The integration time of the spectrometers' measurements is 0.5s.

GOMOS is also equipped with two fast photometers sampling at the frequency of 1kHz in the ranges 644-705nm and 466-528nm. Their measurements are used to correct perturbations from scintillation effects and to determine vertical profiles of temperature of high resolution (200 m).

The instrument parameters are summarized in Table 1.1 .

Instrument parameters

Optical performance parameters


Spectral range

Spectral resolution


250 nm–675 nm

1.2 nm

IR 1

756 nm–773 nm

0.2 nm

IR 2

926 nm–952 nm

0.2 nm


650 nm–700 nm



470 nm–520 nm


data rate

226 kb / sec


160 kg


200 W


continuously over full orbit

Table 1.1: Summary of GOMOS instrument parameters.


A block diagram of the instrument is given in Figure 1.1 . The optic detector system is also illustrated in Figure 1.2.

Figure 1.1: Block diagram of the GOMOS instrument.

Figure 1.2: GOMOS detector system.

Due to the requirement on operating also on fainter stars, the sensitivity requirement to the instrument is very high. A large telescope (30cm x 20cm aperture) and detectors with high quantum efficiency and very low noise allow to collect sufficient signal and to achieve the required signal to noise ratios.

The large scanning mirror is controlled by the star tracker. The acquisition of a star includes three phases. The first phase is a rallying phase during which the telescope mechanism is directed towards the expected position of the star. Subsequently the acquisition procedure enters into detection mode, where the SATU star tracker output signal is pre-processed for spot presence survey and for the location of the most illuminated couple of adjacent pixels. The Most Illuminated Pixel (MIP) defines the position of the first SATU centering window. The second phase is then initiated. It is the centering phase during which  the SATU output signal is pre-processed for spot presence survey over the maximum of 10x10 pixel field. The third phase is then the tracking phase. The star tracker is sensitive inside the range 600nm-1000nm; it is able to keep the stellar spectrum fixed with an accuracy of 1 pixel. The star tracker may follow the star down to 5-20 km, depending on the star, the state of the atmosphere and the illumination conditions.