The potential of spaceborne synthetic aperture radar (SAR) for monitoring tropical forest areas is assessed, using three ERS images from the Tapajos region of Amazonia gathered in 1992 and a single JERS-1 image of the same area acquired in 1993. The multitemporal ERS-1 data indicate that primary forest areas display a very stable radar backscattering coefficient (/spl sigma//sup 0/), while in some cases, disturbed areas (nonforest and regenerating forest) exhibit changes that appear to be associated with soil moisture variations. To counteract /spl sigma//sup 0/ distortions caused by topography, change detection based on ratios of intensity images (or differences of log images) provides a more useful discrimination approach than /spl sigma//sup 0/ variations in single images. Change detection techniques are compared, and their ability to classify primary and disturbed forest is quantitatively assessed, assuming that a land cover map inferred from a 1992 Landsat thematic mapper (TM) image is correct. Even in the best case, less than 50% of the disturbed forest region is detected in the ERS-1 images. This figure may be improved by more frequent image acquisition, but there are fundamental limitations in using C-band data since the effects of soil moisture changes on /spl sigma//sup 0/ are masked once even comparatively low levels of standing biomass are present. At the longer wavelength of JERS-1, much better discrimination is possible, but the correction of topographic distortions is likely to present problems.
['K. Grover', 'S. Quegan', 'C. da Costa Freitas']
Quantitative estimation of tropical forest cover by SAR