11.1 INTRODUCTION

A radar detects or tracks a target, and sometimes can identify it, only because there is an echo signal. It is therefore critical in the design and operation of radars to be able to quantify or otherwise describe the echo, especially in terms of such target characteristics as size, shape, and orientation. For that purpose the target is ascribed an effective area called the radar cross section. It is the projected area of a metal sphere which would return the same echo signal as the target had the sphere been substituted for the target.

Unlike the echo of the sphere, however, which is independent of the viewing angle, the echoes of all but the simplest targets vary significantly with orientation. As such, one must mentally allow the size of this fictitious sphere to vary as the aspect angle of the target changes. As will be shown, the variation can be quite rapid, especially for targets many wavelengths in size.

The echo characteristics depend in strong measure on the size and nature of the target surfaces exposed to the radar beam. The variation is small for electrically small targets (targets less than a wavelength in size) because the incident wavelength is too long to resolve target details. On the other hand, the flat, singly curved and doubly curved surfaces of electrically large targets each give rise to different echo characteristics. Reentrant structures like jet engine intakes and exhausts generally have large echoes,...