12.1 INTRODUCTION

Radar ground return is described by ? ⁰, the differential scattering cross section, or scattering coefficient (scattering cross section per unit area), rather than by the total scattering cross section ? used for discrete targets.1 Since the total cross section ? of a patch of ground varies with the illuminated area and this is determined by the geometric radar parameters (pulse width, beamwidth, etc.), ? ⁰ was introduced to obtain a coefficient independent of these parameters.

Use of a differential scattering cross section implies that the return from the ground is contributed by a large number of scattering elements whose phases are independent. This is primarily because of differences in distance that, although small fractions of total distance, are many wavelengths. Superposition of power is possible for the computation of average returns. If this condition is not applicable to a particular ground target, the differential-scattering-cross-section concept has no meaning for that target. For example, a very-fine-resolution radar might be able to resolve a part of a car; the smooth surfaces on the car would not be properly represented by ? ⁰. On the other hand, a coarser radar might look at many cars in a large parking lot, and a valid ? ⁰ for the parking lot could be determined.

If a region illuminated at one time by a radar contains n scattering elements and the above criterion is satisfied so that power may...