TABLE OF CONTENTS In order to clearly discuss how Doppler radar can be used for aircraft navigation and stabilization, the echo obtained by an aircraft Doppler radar in Chapter 3 was considered to be a sinusoid with a constant amplitude and frequency. The frequency of the echo was considered to be equal to that of the sinusoid transmitted by the Doppler radar transmitter displaced by an amount equal to the Doppler shift that would be obtained from an individual scatterer when it is at the center of the antenna beam. The spectrum was thus assumed to contain only one frequency in order not to obscure the basic theory of how an airborne Doppler radar can be used for navigation and stabilization.
As we discussed, the terrain is actually composed of many randomly distributed individual scatterers. As a given scatterer passes through the radar antenna beam, the reflection is actually a sinusoid that has been phase and amplitude modulated. The phase modulation is the result of the changing path length between the radar and the scatterer and the changing phase of the reflection from the scatterer as it passes through the radar antenna beam. [1] The amplitude modulation is a consequence of the scatterer passing through the radar antenna pattern and the changing scatterer reflectivity. The echo received by the radar from the ground is the sum of the reflections received from all the randomly distributed scatterers. Consequently, the spectrum of the received waveform is not concentrated at a single frequency.
