6.1.1 The Problem

Chapter 5 showed that near the Earth's surface, beside the direct scanner-target ray, we must also consider the horizon, and the indirect ray reflected from the sea surface. Chapter 4 introduced the concept of a 'multipath factor', M, accounting for these effects and converting one-way transmission from free space to Earth's surface conditions, except for atmospheric attenuation, which is accounted separately as loss factor L _A. These terms feature in the radar range equation, Eq. (4.6). M is a power ratio. Other authors sometimes use a voltage ratio, F, called a pattern propagation factor; M = F ² numerically.

At shorter ranges an interference region exists where the indirect ray can be nearly as strong as the direct ray. Because the relative phasing changes with range it is impermissible simply to add the ray powers. The indirect ray may either reinforce or partially cancel the direct ray, the resultant being bigger or smaller than either ray alone, the numerical value of M varying between almost 4 and 0. Working in decibels, the multipath factor is added to the direct ray dBW and lies between almost 6 and - ? dB.

At long range the direct and indirect rays merge. At broadcast-band radio frequencies, considerable signal diffracts round the Earth, permitting reception well beyond the horizon. The effect is negligible at optical frequencies. Radar occupies an intermediate position,...