6.1 INTRODUCTION

In Chapter 3 the reader was acquainted with the electromagnetic basis of radar echoes and in Chapters 4 and 5 with ways the echo may be predicted. As sound as those prescriptions may be, however, they are merely collections of formulas and equations that may not help us understand the echoing properties of targets of interest. In this chapter we present examples of the characteristics of both simple and complex targets.

By simple we mean metallic objects having elementary shapes, such as a sphere, a circular cylinder, or a flat plate. Simple targets may be arranged in a hierarchy according to the strength and frequency dependence of the echo, and this ordering gives us some insight into their relative importance. The hierarchy of scattering mechanisms is particularly useful when we approach the task of reducing the echo from complex targets. By complex we mean objects whose surfaces are not good conductors, whose composition is not uniform or homogeneous, whose profiles defy mathematical description, or combinations of all of these.

All targets of tactical, strategic, or civilian importance are complex targets; for example, missiles, fighters, bombers, jet liners, ships, sailboats, automobiles, trucks, birds, insects, brush, and trees. We attempt to detect and track military targets in the interest of national defense, civilian targets in the enforcement of drug and traffic laws, and sometimes we track natural targets, like birds and bugs, because clouds or swarms of them happen to be in the scene at the...