Instrument Components

© CNES

The components of IASI, along the light path through the instrument are:

• Scan mirror. It provides a swath of ±48.3 degrees perpendicular to the satellite track, with field motion compensation to stabilize scene during acquisition. It also views a calibration blackbody and the deep space, considered as a cold blackbody.
   
• Off-axis afocal telescope that transfers the aperture stop onto the scan mirror.
   
• Michelson interferometer, including a beamsplitter, a compensating plate and two silicon carbide corner cube mirrors: one corner cube is mounted on a mechanism and moves linearly by ±1 centimetre within 151 milliseconds, which corresponds to an optical path difference of 2 centimetres, taking into account the return light path. This path difference is necessary to obtain the specified spectral resolution. A 1.54 µm frequency-stabilized laser is injected into the interferometer. Stabilization is achieved by locking the frequency to an acetylene absorption line. This signal is used as a reference to sample the interferogram directly as a function of the optical path difference.
   
• A folding mirror directs the recombined beam onto the off-axis focusing mirror, which images the Earth at the entrance of the cold box.
   
• The cold box contains field and aperture stops, the field lens that forms the image of the pupil on the corner cubes, the dichroic plates that divide the whole spectrum range into three bands, aspheric lenses that image the field stop on the detector unit and the three focal planes, equipped with microlenses, detectors and preamplifiers. Radiometric performance requires cooling the cold box to below 100 Kelvin (-173° Celsius) using a three-stage passive radiator to reduce the instrument background and thermoelectronic detector noise.
   
• Interferograms are digitized and processed by the digital signal processing subsystem, which performs the inverse Fourier transform and radiometric calibration. This last is achieved by viewing the internal hot blackbody and cold deep space, every eight seconds for the radiometric calibration.

The IASI instrument also has an infrared imager to co-register soundings with AVHRR images (AVHRR is another instrument on the Metop satellite).