Mediumhigh altitude 220 km spatially scanning systems

The NASA Multichannel Ocean Colour Sensor (MOCS)199 measures radiance in 20 contiguous l5-nm wide bands between 400 and 700 nm. It has a spatial resolution of 4 x 2 mrad, corresponding to 20 x 10 m at the surface, and a field of view of 17.1°, corresponding to a swath width of

— 1.5 km, when viewed from a plane at 5 km altitude.

The Daedalus Airborne Thematic Mapper has eleven channels, with five broad, approximately contiguous, bands in the visible region covering the range 420 to 690 nm, and another six bands in the infrared.1470 It has a particularly wide field of view (86°), and an angular resolution of 2.5 mrad, corresponding to a geometric resolution at sea level of 10 m square from a plane at 4 km altitude. An updated version, the AZ-16, is now available with a slightly increased field of view (90°) and 16-bit rather than 8-bit resolution.

A great leap forward in airborne remote sensing has been the development of what is known as hyperspectral imagers. These use diffraction gratings to spectrally disperse the collected light across linear, or two-dimensional, photodetector arrays containing large numbers of elements. In this way measurement can be carried out in 200 to 300 wavebands, with bandwidth 3 to 10 nm, across the spectrum, thus providing something very close to a continuous reflectance spectrum. An early and notable example was the NASA Airborne Visible and Infrared Imaging Spectrometer (AVIRIS),975 which became operational in 1987 and is still in use today. Its optical system is of the whiskbroom type illustrated in Fig. 7.2b, using line arrays of silicon and indium alloy detectors to provide 224 contiguous spectral bands covering the visible/infrared region from 0.4 to 2.5 mm, with —10 nm resolution.46 There are 677 pixels across the swath, and a complete spectrum is obtained for each pixel. Data encoding is 16 bit. AVIRIS is designed primarily to be flown at —20 km in a high altitude ER-2 aircraft: with its angular resolution of 1 mrad, its ground-instantaneous-field-of-view is 20 m. The total field of view is 34°, giving a swath width of 11 km. When flown in the Twin Otter aircraft, at 4 km above the ground, each ground pixel is 4 m square, and the swath is 2 km wide.

Commercial hyperspectral imagers for airborne use are now available. The CASI 1500 (Compact Airborne Spectrographic Imager), for example, is of the pushbroom type, and covers a spectral range of 650 nm, adjustable between 380 and 1050 nm, with up to 288 spectral bands.613 The total field of view is 40° across-track, over 1500 pixels. Data encoding is 14 bit.

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