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    24-Jul-2014
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Index
  Acknowledgements and contact
List of figures and tables
List of tables
 Additional information on the GOMOS measurements
Periods of data unavailabilities
Evolution of the IPF
Glossary
List of abbreviations and acronyms
Product types and structure
Other sources of information
GOMOS-related ESA web pages
Publications (peer-reviewed publications and proceedings of conferences)
Secondary products
Scintillation and turbulence
Aerosols and PSC
Product validation
NO2 and NO3 measurements
O3 measurements
CAL/VAL activities
Retrieval and processing issues
Assimilation of GOMOS products
Mesosphere
Specific events
GOMOS-related theses
Other technical reports by members of GOMOS SAG, ESL and QWG
Level2 processing
How to
FAQ
Access tools
GOMOS products toolbox
EnviView
Data selection
PCD summary in the Level2 products
Obliquity
Star properties
Data availability
Presentation of the GOMOS products
Description of the products
Product content
Product structure
Time availability
Data size
Presentation of the instrument and the measurements
Scientific achievements
Validation results
Measurement characteristics
Accuracy
Occultation obliquity
Geographical and time coverage
Star characteristics
Mission planning
Modified mission scenario since August 2005
Instrument description and measurement principle
Calibration phase and monitoring activities
Measurement technique
GOMOS Product Handbook
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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

Channel

Spectral range

Spectral resolution

UV-VIS

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

PHOT 1

650 nm–700 nm

broadband

PHOT 2

470 nm–520 nm

broadband

data rate

226 kb / sec

mass

160 kg

power

200 W

operation

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.