σ?? Signature of the Amazon Rain Forest Obtained from the Seasat Scatterometer

The radar response from the Amazon rain forest has been studied to determine the suitability of this region for use as a standard target to calibrate spaceborne scatterometers. Backscattering observations made by the Seasat-1 scatterometer system (SASS) show the Amazon rain forest to be a homogeneous, azimuthally isotropic radar target which is insensitive to polarization. The variation with angle of incidence may be adequately modeled as σ^??(dB) = aθ + b. Typical values for the incidence-angle coefficient a are 0.07-0.15 dB/degree. No change in the relations was observed over the 99 days of the Seasat mission. A small diurnal effect occurs, with measurements at sunrise being 0.5-1 dB higher than the rest of the day. For a fixed incidence angle and time of day the standard deviations ranged from 0.6 dB at the extremes to 0.1 dB at the antenna pointing angle of 44°. Because of its consistent response the Amazon forest appears to be a suitable target for calibrating spaceborne scatterometers. Further research is needed to check for seasonal effects at other times of year.