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The Compact Airborne Spectographic Imager (CASI-2)

The CASI instrument

The scan head, control rack and display screen of the Itres CASI-2

The Itresexternal linkCompact Airborne Spectrographic Imager (CASI-2) contains a two-dimensional CCD array-based pushbroom imaging spectrograph. The CASI-2 offers a multispectral mode (programmable bands) and a hyperspectral data cube mode. It produces digital geocoded imagery for map registrations, GIS integration, and generates multiple value-added information products from the same set of image data. It operates over a spectral range between 405nm and 950nm and has a 54.4° field of view across-track. The CASI-2 is a high spatial and spectral resolution remote sensing sensor. Pixel resolutions can vary from sub-metre to 10 metres. A fully detailed technical explanation of the CASI-2 is available to download.

The Compact Airborne Spectrographic Imager (casi) sensor produces a ground spatial resolution where pixel width and length are essentially independent of each other. The along-track spatial resolution or pixel length is a function of the integration time and the speed of the instrument over the ground. The required integration time is itself determined by the signal level and the number of channels requested by the user. For a given light level input to the instrument, the number of channels requested by the user determines the minimum integration time and, for an average aircraft speed over the ground, the equivalent pixel length in metres. The across-track spatial resolution or pixel width is a function of the CCD detector element size, focal length of the imaging lens, which for a given casi are both constant, and the altitude of the aircraft. Clearly, as the aircraft altitude increases so does the pixel width and consequently also the total swath width. Due to the optical characteristics of this 'push-broom' sensor, which samples every scene pixel along a scanline in the same ratio as the fixed, and equal inter-detector dimension, all pixels in a scanline are of equal width. Depending on the priority the user places on either a specific ground resolution (altitude) or specific number of bands, formulae can be used to explore the trade-off in flight/sensor parameters for a particular mission. Choosing a specific ground spatial resolution and a requirement for near square pixels will determine the maximum number of bands allowable in spatial mode. Conversely choosing a specific number of bands in a casi bandset will determine the pixel length and, in retaining near-square pixels, determine pixel width and hence the altitude the aircraft must be flown at. For Spectral Mode and EnhancedSpectral Mode, where minimum integration times are much longer, it is sometimes impossible to attain square pixels and aspect ratios of 2:1 or more may result.