Rain Clutter Statistics.

Abstract : Measurements of rain clutter were taken with a frequency-agile S-band radar for the purpose of studying the Doppler power spectra and the variation of backscattered power with space, time, and frequency. The clutter was found to be nonhomogeneously distributed in both space and time, but the relation of mean backscattered power to frequency seemed to involve only a single parameter having a spatial and temporal variation. Although the radar was looking into the surface wind, a significant portion of the backscattered signal energy was always found to lie in the negative frequency range, suggesting a wind shear, estimated to be about 8 m/s per km. The Doppler power spectra were sometimes found to have multiple peaks at both positive and negative Doppler frequencies, and from measurements of the peak widths it appears that they were produced by horizontal bands or clumps of clutter whose vertical extent was of the order of 200 m. Another possibility for the production of multiple spectral peaks is Bragg scattering from turbulent eddies. However, using statistical methods which are valid in the presence of calibration errors and noise, it was established that the data favored the vertical-stratification mode over the Bragg-scattering model. The nonhomogeneity of the clutter and the existence of Doppler spectra with multiple peaks have serious implications for the performance and design of detectors.