The DESIS hyperspectral instrument is realized as a pushbroom imaging spectrometer spectrally sensitive over the VNIR range from 400 to 1000 nm with a spectral sampling distance of 2.55 nm employing a 2-dimensional back illuminated CMOS (Complementary Metal Oxide Semiconductor) detector array. The across direction of the array is used for the spatial resolution and the along direction for the spectral resolution (channels). The optical design is based on the Offner-type grating spectrometer widely used in hyperspectral imaging. The electronic shutter mechanism is realized as a rolling shutter, in which each channel collects light during the same period of time, but the time light collection starts and ends are slightly different for each channel. The Ground Sampling Distance (GSD) at nadir view depends on the flight altitude of the ISS and is about 30 m resulting in a swath width of about 30 km.
DESIS is equipped with a Pointing Unit (POI) consisting of two fixed and one rotating mirror in front of the entrance slit that allows, by rotating one mirror, a forward and backward viewing change up to ±15° w.r.t. the nominal (e.g. nadir) view. The POI can be operated in a static mode with 3° angle steps for the viewing direction and in a dynamic mode with up to 1.5° change in viewing direction per seconds. This allows - besides normal Earth data takes (Image Strip Mode) - acquiring stereo or Bidirectional Reflectance Distribution Function (BRDF) products (Image Stereo Mode). The POI can be also operated in calibration mode to minimize the external light fields and allowing on-board calibration measurements. The specifications of the MUSES platform and DESIS instrument are listed in Table 2 1.
Besides a pre-flight spectral, radiometric and geometric calibration and characterisation of the instrument in laboratory, the instrument contains an on-board calibration unit comprising different monochromatic and white light LEDs between 400 and 1000 nm. The calibration unit is located close to the POI mirror in front of the DESIS instrument, and allows the illumination of the full spectrometer FoV with color and white light. The calibration unit is characterised in laboratory and temperature controlled in orbit within 1 K.