Measurement systems general considerations

Remote sensing photometers used in low-altitude aircraft normally have a fixed direction of view. They measure upward radiance at a series of points along a linear track determined by the flight path of the aircraft. For two-dimensional mapping, the aircraft must traverse the area of interest many times. The spectral distribution of the radiance at each point is determined using interference filters or a spectroradiometer.

With increasing altitude the area that it is practicable to 'view' increases. To take advantage of this, remote sensing from satellites and higher-altitude aircraft uses spatially scanning photometers, which collect information, not just from the thin line immediately below the trajectory of the satellite or aircraft, but from a broad swath of the Earth's surface, which can be anything from a few kilometres to hundreds of kilometres wide. There are two ways of achieving this, sometimes referred to respectively as 'pushbroom' scanning and 'whiskbroom' scanning.1231 In a pushbroom scanner the instrument optics form an image of a thin transverse strip of the Earth's surface extending right across the swath at right angles to the line of flight, and present it across a line array of photo-detector elements. Each of these detector elements thus receives radiation from one of the elements of area at the surface, which between them form the strip across the swath.

Fig. 7.2 Imaging spectrometry in remote sensing. (a) Pushbroom scanner with two-dimensional array of photodetectors. (b) Whiskbroom scanner with line array of photodetectors. (After HIRIS Instrument Panel Report. NASA, 1987.)

In fact, what is required for each element of area at the surface is a sample not of the total radiation but of the radiant flux in each of some specified set of spectral wavebands. The radiation from the transverse strip at the surface can, by means of dichroic beam splitters (partially reflective mirrors that reflect radiation below a certain wavelength and let the longer wavelength energy through), be subdivided into a number of separate fluxes each of which can be directed through a different spectral filter onto its own line array of photodetector elements. A more radical solution is to spectrally disperse the radiant flux with a prism or grating and form an image on a two-dimensional array of photodetectors, one axis corresponding to position of the individual surface element across the swath, the other to wavelength (Fig. 7.2a). In principle, in this way, a complete spectrum of the flux from each element of area can be obtained.

Remote sensing radiometers of the whiskbroom type scan by means of a rotating or oscillating mirror, which ensures that the direction of viewing is continually moving from one side of the swath to the other, at right angles to the satellite/aircraft trajectory (Fig. 7.2b). The scene reflected in the mirror is viewed through a telescope with a very small acceptance angle (0.002-0.05°) so that at any given instant only a small unit element of the Earth's surface is in view. The flux collected by the telescope can be partitioned into a set of spectral bands using beam splitters and filters, and a corresponding set of photodetectors, or be dispersed with a grating or prism onto a line array of detector elements to provide a more complete spectral distribution.

With either the pushbroom or whiskbroom type of radiometer, a series of contiguous but non-overlapping, square image elements, corresponding to a linear sequence of elements of area at the surface, extending across the swath, is recorded. By the time the next side-to-side scan begins the satellite or aircraft has moved on so that the next left-to-right sequence of image elements adjoins, but does not overlap, the previous sequence. The accumulated set of image elements constitutes a mosaic corresponding to that strip of the Earth's surface defined by the path of the satellite/aircraft and its maximum lateral angle of view.

For each image element, i.e. for each element of surface, the photometer has recorded a set of radiance values corresponding to the number of spectral bands with which it operates. The radiance values are encoded digitally, and a key parameter for any remote sensing instrument is the number of bits to which digitization is carried out: this can vary from 6 bits giving a measurement range of up to 64 different radiance values, to 16 bits giving 65 536 different values. This information, after being relayed to a receiving station in the case of a satellite, is stored on tape or other medium. The data can then be used, after appropriate computer processing, to prepare a map of any part of the Earth's surface that lay beneath the satellite's orbit, or aircraft's path. The continuous swath covered by a satellite is broken down for mapping purposes into individual sections that might be some hundreds of kilometres long.

Each small element of the map, referred to as a pixel, corresponds to one of the square unit elements of surface viewed by the scanning photometer. The intensity, or the colour within a selected colour scale, of each pixel in the final map displayed on the computer screen can be directly related to the radiance value in any one of the spectral bands, or can be determined by the value of some parameter (e.g. chlorophyll concentration) derived by calculation from more than one of the spectral radiance values.

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