TABLE OF CONTENTS Radars are of two general types: continuous wave called CW and pulsed. A CW radar transmits continuously and simultaneously listens for the reflected echoes. A pulsed radar, on the other hand, transmits its radio waves intermittently in short pulses, and listens for the echoes in the periods between transmissions.
Pulsed radars fall into two categories: (1) those that sense doppler frequencies and (2) those that do not. The former have come to be called pulse-doppler radars; the latter, simply pulsed radars. Here, though, pulsed will be used in a general sense to refer to any radar that transmits pulses.
In this chapter, we'll consider the advantages of pulsed transmission, characteristics of the pulsed waveform, and effects of pulsed transmission on transmitted power and energy.
With the exception of doppler navigators, altimeters, and VT proximity fuses, most airborne radars are pulsed. The chief reason is that with pulsed operation, one avoids the problem of the transmitter interfering with reception.
The transmitter's intended output the signal is not, of course, the problem. In doppler navigators, for example (Fig. 1) the doppler shift provides sufficient frequency separation to keep the transmitted signal from interfering with reception. And in altimeters (Fig. 2), where the doppler shift is usually near zero, interference from the transmitted signal is avoided by continuously shifting the transmitter's frequency. Because of the time the radio waves take to reach the ground and return, the frequency of the received signal lags behind the frequency of the transmitter; so, the signal...
