2.1 INTRODUCTION

The basic physics governing the prediction of radar maximum detection range, for a specified target under free-space conditions with detection limited by thermal noise, has been well understood since the earliest days of radar. The term free space implies (in the present context) that a spherical region of space, centered at the radar and extending to considerably beyond the target, is empty except for the radar and the target. ( Considerably as used here can be precisely defined for specific radars, but a general definition would be lengthy and not very useful.) It also implies that the only radar-frequency electromagnetic waves detectable within this region, other than those emanating from the radar itself, are from natural thermal and quasi-thermal noise sources, as described in Sec. 2.5.

Although this condition is never fully realized, it is approximated for some radar situations. Under many non-free-space conditions and with radically non-thermal forms of background noise, the prediction problem is considerably more complicated. Complications not considered in early analyses also result from modification of the signal and noise relationship by the receiving-system circuitry ( signal processing).

In this chapter the free-space equation will be presented, basic signal processing will be discussed, and some of the most important non-free-space environments will be considered. The effect of some common nonthermal types of noise will be considered. Although it will not be feasible to consider all the possible types of radar situations, the methods to be described...