TABLE OF CONTENTS 
By sensing doppler frequencies, a radar not only can measure range rates, but also can separate target echoes from clutter, or produce high resolution ground maps. Since these are important functions of many of today's airborne radars, one of the keys to understanding their operation is a good understanding of the doppler effect.
Accordingly, in this chapter, we will look at the doppler shift more closely first, in terms of the compression or expansion of wavelength and, second, in terms of the continuous shift of phase. We will then pinpoint the factors which determine the doppler frequencies of the return from both aircraft and the ground. Finally, we will consider the special case of the doppler shift of a target's echoes as observed by a semiactive missile.
The doppler effect is a shift in the frequency of a wave radiated, reflected, or received by an object in motion. As illustrated in Fig. 1, a wave radiated from a point source is compressed in the direction of motion and is spread out in the opposite direction. In both cases, the greater the object's speed, the greater the effect will be. Only at right angles to the motion is the wave unaffected.
