The primary scientific objective of SCIAMACHY is the global measurement of various trace gases in the troposphere and stratosphere, which are retrieved from the instrument by observation of transmitted, backscattered, and reflected radiation from the atmosphere in the wavelength range between 240 nm and 2400 nm. The large-wavelength range is also ideally suited for the determination of aerosols and clouds. The nadir and limb viewing strategy of SCIAMACHY yields total column values as well as profiles for trace gases and aerosols in the stratosphere. Additionally, this enables estimates of global trace gas and aerosol content and distribution in the lower stratosphere and troposphere.
The measurements obtained from SCIAMACHY enable the investigation of a wide range of phenomena which influence atmospheric chemistry such as measurement in the troposphere: biomass burning, pollution, arctic haze, forest fires, dust storms, industrial plumes; and measurement in the stratosphere: ozone chemistry, volcanic events and solar proton events.
In order to achieve the scientific objectives, measurements are performed by observing the atmosphere under different viewing angles. In nadir mode, the global distribution (total column values) of the atmospheric trace gases and aerosols are observed. Additionally, cloud measurements are obtained. In this mode, the instrument is scanning across-track, with a swath width of +500 km with respect to the subsatellite track. To obtain the altitude distribution of trace gases, SCIAMACHY performs observations in limb over an altitude range of 100 km, with a vertical resolution of 3 km. Starting at Earth's horizon, the atmosphere is scanned tangentially over a 1000-km-wide swath. After each azimuth scan, the elevation is increased until the maximum altitude of 100 km is reached.
Differential optical absorption spectroscopy is applied in sun and moon occultation measurements, where sun or moon are either tracked or a vertical scan over the complete sun/moon surface is performed. The obtained spectra can then be compared with suitable calibration spectra to yield the differential absorption of the atmosphere.
In the SCIAMACHY optical system, the light from the atmosphere is fed by the scanner unit (consisting of an azimuth and an elevation scanner) into the telescope, which directs it onto the entrance slit of the spectrometer. The spectrometer contains a predisperser which separates the light into three spectral bands followed by a series of dichromatic mirrors which further divide the light into a total of eight channels. A grating is located in each channel to diffract the light into a high-resolution spectrum which is then focused onto eight detectors. The predisperser also serves as a Brewster window to separate polarised light, a part of which is sensed by the polarisation measurement device (PMD). The output of the PMD is later used to correct for the polarisation effects. Light reflected off the slit is directed to the sun follower, which controls the scan mirrors in the Sun and Moon occultation mode. Each spectrometer channel is equipped with a detector module. The detector modules provide the detectors and their detector module electronics (DME). The DME controls the associated detector, reads out the integrated charge, amplifies the analogue signal and then digitises this signal. The digital signal of each channel is transferred to the science data processing unit (SDPU).
To achieve the required instrument performance the detectors of the optical assembly have to be cooled, in particular the IR-detectors. This cooling is provided by the SCIAMACHY radiant cooler which is coupled through a thermal bus unit (TBU) to the detectors of the optical assembly. The TBU is build up from two cryogenic heatpipes which cool the two NIR-detectors. A tube-shaped shroud which envelops the cryogenic heat pipes transports the heat load from the detectors of the other six channels. The SDPU controls the DME, acquires and processes science and auxiliary data, and transmits them to the S/C measurement data interface. The instrument control unit receives the macrocommands from the S/C, performs autonomously the overall management and control of the instrument, and feeds back telemetry information. The power, mechanism, and thermal control unit (PMTC) provides secondary power to all equipment and controls the scanners (via the encoder electronics), the different mechanisms, the calibration source, and the temperature of the optical bench and the detectors. Furthermore the PMTC acquires analogue housekeeping signals indicating the health and status of the instrument. a decontermination heater control module (DHCM) performs the decontamination on/off control and the thermal knife control.
SCIAMACHY is developed by a bilateral Dutch/German activity under NIVR and DARA contract.
