Infrared and microwave radiometers

These instruments record the thermally emitted radiation produced by all natural surfaces. This thermal energy is a function of the physical temperature of the surface and a term known as the emissivity, which defines how much of the radiation is emitted at a given wavelength. Emissivity is the converse of the albedo, or

Band number Spectral range (mm) Resolution (m)

reflectivity of a surface. In the case of microwave radiometers, the emissivity is a function of the properties of the subsurface snowpack, allowing valuable information to be obtained not just from the surface, but from a depth down to 20 m or more in dry firn.

The most widely used IR radiometer is the advanced very high resolution radiometer (AVHRR). It views an area that is 2400km in width (the swath) as it moves along its orbit, with a resolution of about 1.1km at nadir (directly below). There has been continuous coverage by at least one AVHRR instrument since 1978. Currently four satellites carrying AVHRR are operational and the data are, in general, freely available to the scientific community. The AVHRR provides a useful complement to the higher resolution visible sensors, such as the Landsat ETM, as it has a wide swath width and moderate resolution, providing daily (or better, particularly at high latitudes) coverage of cloud-free areas of sea and land ice. The instrument has been used to examine sea-ice extent, although cloud discrimination is a serious problem for this application and, as a consequence, passive microwave radiometers are preferred for this application, despite their much poorer resolution (see below). The AVHRR has also been used to map the margins, surface characteristics and morphology (such as albedo, flow stripes and surface temperature) of the ice sheets (Fujii et al., 1987; Scambos & Bindschadler, 1991; Steffen et al., 1993; Stroeve et al., 1997). Landsat TM and ETM offer a similar capability for smaller ice masses such as valley glaciers.

As mentioned earlier, clouds are transparent in the microwave part of the spectrum, which means that microwave imaging systems have the advantage over visible or IR sensors (such as Landsat TM, SPOT (Systeme pour l'Observation de La Terre), AVHRR and ASTER) that they can offer day/night coverage even in cloudy conditions. In the polar regions, where cloud cover is

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