The synthetic aperture radar (SAR) aboard Seasat in 1978 demonstrated a unique sensitivity to oceanic and geologic features imaged over a 100 km swath with 25 m resolution. The ability of the remote sensor to resolve the fine details of large environmental systems resulted in the orbiting of a similar system, the Shuttle Imaging Radar (SIR-B), aboard the Space Shuttle Challenger during October 1984. Coherent speckle noise observed for these Doppler imaging-radar systems is caused by random correlations of the illuminating radar chirp with the surface reflectance downrange. Radar speckle is similar to optical speckle in that respect, but it is also influenced by along-track sampling statistics. The Rayleigh statistics of coherent scattering and the Poisson statistics of radar pulse detection are employed to model the speckle observed in spatially random data samples. Speckle degradation of a radar scene may obstruct interpretations of scene detail, but it can also be useful in determining the spatial response of the remote sensor and scene correlator. Randomly speckled scenes that are otherwise featureless provide a white-noise input to the Doppler imaging process. Several such scenes have been processed with fast Fourier transform methods to estimate the point spread function and its Fourier-domain equivalent, the wavenumber response function. These measure-ments of spatial resolution are used to compare the Seasat SAR and the Challenger SIR-B remote sensors. In addition, two ground-scene correlators are compared in terms of the point spread estimate of spatial resolution for common input data from the SIR-B sensor.
David G. Tilley,
"Use Of Speckle For Determining The Response Characteristics Of Doppler Imaging Radars," Optical Engineering 25(6), 256772 (1 June 1986). https://doi.org/10.1117/12.7973904