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2.3 Instrument Description
2.3.1 Science Requirements and Instrument Concept
To detect all species listed in table 2-1, it is essential that SCIAMACHY continuously observes the wavelength ranges 214-1773 nm, 1934-2044 nm and 2259-2386 nm. The retrieval of the amount of constituents depends on the ability to measure their absorptions precisely. Retrieving total column concentrations of minor trace gases with an accuracy of 1-5 % – or 5-10% for their profiles – requires observing intensity changes of 10-3-10-4 with respect to the optical depth. This can only be achieved with an instrument providing measurements with a high signal-to-noise ratio and a good radiometric calibration.
To fulfil the mission objectives with respect to spatial resolution and coverage, it is necessary to observe the scattered and reflected solar photons in nadir and limb direction as well as the light transmitted through the atmosphere in solar and lunar occultation geometry (Burrows and Chance 1991, Bovensmann et al. 1999). For calibration and monitoring purposes the extraterrestrial solar and lunar irradiance above the atmosphere has to be determined. As total column amounts and height resolved profiles are required, SCIAMACHY alternately observes the atmosphere in limb and nadir viewing. Combining both geometries of a single orbit for the same volume of air allows the study of tropospheric properties. Global coverage has to be obtained within 3 days in limb or nadir mode.
These requirements, together with the accommodation on the ENVISAT platform, were translated into an instrument concept providing spectroscopy capabilities from the UV via VIS and Near Infrared (NIR) to SWIR with
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moderately high spectral resolution
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high radiometric accuracy
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high spectral stability
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high dynamic range
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Viewing geometries must account for field of views in nadir, limb (in flight direction), sunrise and moonrise direction. Additional access to the sun around occurrence of the sub-solar condition, i.e. the sun reaches highest elevation above the instrument, is needed. Maintaining thermal stability at several temperature levels is a prerequisite for achieving high radiometric and spectral accuracy. This is ensured by thermal control systems. Finally, instrument control must be executed continuously in a highly autonomous manner with the ability to react to a wide variety of operations conditions. This includes not only measurement data relevant parameters as e.g. line-of-sight, signal-to-noise levels and spectral sampling but also the tasks of overall instrument command & control. All SCIAMACHY instrument requirements were finally documented in the SCIAMACHY Instrument Requirements Document (SIRD, DARA 1998) (table 2-3).
|Instrument Dimensions |
|Optical Assembly ||109 cm 65 cm 101 cm |
|Electronic Assembly ||82 cm 90 cm 28 cm |
|Radiant Cooler Assembly ||51 cm 91 cm 62 cm |
|Total Mass ||215 kg |
|Power Consumption ||140 W |
Table 2-3: SCIAMACHY instrument physical characteristics
Conceptually, SCIAMACHY is a passive imaging spectrometer, comprising a mirror system, a telescope, a spectrometer, and thermal and electronic subsystems. Functionally, three main blocks, the Optical Assembly (OA), the Radiant Cooler Assembly (SRC) and the Electronic Assembly (EA) can be identified. The instrument is located on the upper right (i.e. starboard, referring to nominal flight direction) corner of the ENVISAT platform with the OA mounted onto the front and the EA mounted onto the top panel. The Radiant Reflectance Unit (RRU) of the SRC points sideways into open space away from any heat source. Interfaces with the ENVISAT platform exist for the provision of on-board resources. These include power and command interfaces in one direction. In the other direction measurement data and HK telemetry from SCIAMACHY are routed into the overall ENVISAT data stream for downlinking.