Radar systems are normally divided into two operational categories (purposes) based on their

Figure 1-2.-True and relative bearings.

method of transmitting energy. The most common method, used for applications from navigation to fire control, is the pulse-modulation method. The other method of transmitting is continuous-wave (CW). CW radars are used almost exclusively for missile guidance.

Pulse Modulation

In the pulse method, the radar transmits the RF in a short, powerful pulse and then stops and waits for the return echo. By measuring the elapsed time between the end of the transmitted pulse and the received echo, the radar can calculate a range. Pulse radars use one antenna for both transmitting and receiving. While the transmitter is sending out its high-power RF pulse, the antenna is connected to the transmitter through a special switch called a duplexer. As soon as the transmitted pulse stops, the duplexer switches the antenna to the receiver. The time interval between transmission and reception is computed and converted into a visual indication of range in miles or yards. Pulse-radar systems can also be modified to use the Doppler effect to detect a moving object. The Navy uses pulse radars to a great extent.

Continuous Wave

In a CW radar the transmitter sends out a "continuous wave" of RF energy. Since this beam of RF energy is "always on", the receiver requires a separate antenna. One disadvantage of this method is that an accurate range measurement is impossible because there is no specific "stop time". This can be overcome, however, by modulating the frequency. A frequency-modulated continuous wave (FM-CW) radar can detect range by measuring the difference between the transmitted frequency and the received frequency. This is known as the "Doppler effect". The continuous-wave method is usually used by fire-control systems to illuminate targets for missile systems.