Quantitative estimation of tropical forest cover by SAR

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 (σ⁰), while in some cases, disturbed areas (nonforest and regenerating forest) exhibit changes that appear to be associated with soil moisture variations. To counteract σ⁰ distortions caused by topography, change detection based on ratios of intensity images (or differences of log images) provides a more useful discrimination approach than σ⁰ 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 σ⁰ 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