Theory and Measure of Certain Image Norms in SAR

This paper summarizes principal properties of SAR imagery of point and distributed objects. Against this background, the response of a synthetic-aperture radar (SAR) to the moving surface of the sea is considered. Certain conclusions are drawn as to the mechanisms of interaction between microwaves and the sea surface. It has been established for a "weli-behaved" SAR (as for other radars) that a principle of conservation of energy is satisfied. This means that the energy of the response (in the image) is constant under conditions of changing phase of the signal. Phase errors may arise systematically, such as focus mismatch to parameters appropriate to a specular scattering center, or randomly, as from complex motion of the sea surface. Of course, focus errors reduce the peak and spread the impulse response of the image of a point target. Focus errors do not, however, change the speckle spectrum of a truly random "uniform" Gaussian scene. Focus and speckle spectral tests may be used on selected SAR imagery for areas of the ocean. When this is done, it is observed that the fine structure of the sea imagery is sensitive to processor focus and adjustment. FÿRthermore, there is frequently correlation between nominally statisticly independent looks. In such cases, therefore, the ocean reflectivity mechanism must include point-like scatterers of sufficient radar cross section to dominate tihe return from certain individual resolution elements. FÿRthermore, both specular and diffuse scattering mechanisms are observed together, to a varying degree. The effect is sea state dependent, of course.