interpretation seems to improve if both like- and cross-polarized signals are collected. SAR is particularly important for filling gaps in crop and other vegetation data sets collected by optical sensors in cloudy regions.

Research dating from the 1960s shows that Ka- and X-band images can be used to produce vegetation cover maps of both natural and cultivated surfaces (Ulaby et al., 1986a, and references therein), Most of the research between 1964 and 1984 concentrated on temperate agricultural applications, but since then, the focus in the United States has turned toward ecosystems and natural vegetation. The European and Canadian research communities have continued their work on crops. A review of some of the work on crops appears in Chapter 21 of Ulaby et al. (1986a,b); Anys and He (1995) and Foody et al. (1994) report more recent research. Much early work in the 1960s and early 1970s was based on extensive imagery acquired by the K a-band (35 GHz) like- and cross-polarized AN/APQ-97 side-looking airborne radar systems. Earlier validation can be found in Dellwig et al. (1975) and Banhart (1981, 1984). The extensive scatterometer work by Ulaby (1974, 1984) indicates that the Ku-band is the best frequency for crop discrimination, with the X-band a close second (Ulaby et al., 1986a,b, and references therein).

Because of their sensitivity to structural characteristics, multiparameter SARs offer a means to classify vegetation cover, as discussed in Chapter 2. SAR data can be used to detect deforestation and forest regrowth and to discriminate among up to 10 distinct vegetation types in a region, with an accuracy comparable to that attained with current electro-optical systems (approximately 89 percent). “Types” refers to vegetational communities having distinctive morphologies such as evergreen forest, deciduous forest, shrubland, marshland, and grassland. SAR is also sensitive to temporal-dynamic factors such as moisture content and freeze-thaw status. SAR is able to detect regional flood conditions, especially under variable canopies. For flooded forests, a lower-frequency (L- or P-band), HH-polarized design is preferred. For wetlands mapping, a higher-frequency, HH- or VV-polarized system is more suitable. The all-weather capabilities of SAR allow for repetitive coverage of flooded regions and provide a unique tool for use in disaster relief.

More recent research confirms SAR's sensitivity to forest biomass and plant moisture content, making it a useful tool either as a stand-alone sensor for vegetation applications or as a supplemental sensor. Biomass measurements are possible in both agricultural fields and natural vegetation such as forests and rangeland (Beaudoin et al., 1994; Dobson et al., 1995; Kasischke et al., 1995; Le Toan et al., 1992; Ranson and Sun, 1995; Rignot et al., 1995a; and Saatchi et al., 1995). Single-frequency, single-polarization SARs are sensitive to above-ground biomass differences in forests up to approximately 100 to 150 metric tons per hectare. Multichannel SAR systems, which include low frequencies (L-band at 24-cm wavelength and P-band at 65 cm) and a higher frequency (C-band at 6 cm or X-band at 3 cm), can be used to estimate biomass levels up to 250 to 300 metric tons per hectare. This biomass range includes all forests except mature old-growth forests in temperate regions and some tropical rain forests.

It is the committee's opinion that a small SAR mission focused on the above objectives would enhance the general understanding of SAR capabilities for monitoring rangelands, crops, and forests on a global basis if the data were made widely available for analysis. Such a mission



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