|
With hyperspectral pushbroom imaging spectrometers on Earth observation satellites it is possible to detect and identify
dedicated ground pixels by their spectral signature. Conventional time consuming on-ground processing performs this
selection by processing the measured hyperspectral data cube of the image. The Imaging Spectral Signature Instrument
(ISSI) concept combines an optical on-board processing of the hyperspectral data cube with a thresholding algorithm, to
identify pixels with a pre-defined and programmable spectral signature, such as water, forest and minerals, in the ground
swath.
The Imaging Spectral Signature Instrument consists of an imaging telescope, which images an object line on the entrance
slit of a first imaging spectrometer, which disperses each pixel of the object line into its spectral content and images the
hyperspectral image on the spatial light modulator. This spatial light modulator will be programmed with a spatial
transmission or reflection behavior, which is constant along the spatial pixels and along the spectral pixels identical to a
filter vector that corresponds to the spectral signature of the searched specific feature. A second inverted spectrometer reimages
the by the first spectrometer dispersed and by the spatial light modulator transmitted or reflected flux into a line
of pixels. In case the spectral content of the ground scene is identical to the searched signature, the flux traversing or
reflecting the spatial light modulator will be maximum. The related pixel can be identified in the final image as a high
signal by a threshold discriminator.
A component test setup consists of an imaging lens, two Imspector™ spectrographs, a spatial light modulator, which is a
programmable transmissible liquid crystal display and a CCD sensor as a detector.
A mathematical model was developed for the instrument and its performance was evaluated in order to compare different
concept variations. All components were measured and characterized individually, and the results were used in the
simulations. Performance was then analyzed by means of radiometric throughput and spatial and spectral resolutions.
The simulations were performed at wavelengths of 450 nm to 900 nm. The throughput was found to be between 1% and 4.5%.
|
