Quantitative estimation of tropical forest cover by SAR

The potential of spaceborne synthetic aperture radar (SAR) for monitoring t ropical forest areas is assessed, using three ERS images from the Tapajos r egion of Amazonia gathered in 1992 and a single JERS-1 image of the same ar ea acquired in 1993, The multitemporal ERS-1 data indicate that primary for est areas display a very stable radar backscattering coefficient (sigma(0)) , while in some cases, disturbed areas (nonforest and regenerating forest) exhibit changes that appear to be associated with soil moisture variations, To counteract sigma(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 sigma(0) variations in single i mages. Change detection techniques are compared, and their ability to class ify 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 regi on is detected in the ERS-1 images. This figure may be improved by more fre quent image acquisition, but there are fundamental limitations in using C-b and data since the effects of soil moisture changes on sigma(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 th e correction of topographic distortions is likely to present problems.