The modulation characteristics of a radar signal are dictated by the function of the radar. We will consider pulse, pulse-Doppler, and continuous-wave (CW) radars in acquisition, guidance, and fusing applications. Typical pulse radars output very short, fixed-frequency signals (pulses). As shown in Figure 1, pulse modulation is characterized by the pulse width (or pulse duration), interval, and amplitude. The pulse interval is the time from the leading edge of one pulse to the leading edge of the following pulse. The pulse interval in a signal is usually identified in terms of the pulse-repetition frequency (PRF) or pulse-repetition interval (PRI), but is also sometimes called the pulse-repetition time (PRT). The pulse width and repetition rate are the same whether measured at the transmitter output, at the target, or at the receiver as long as the radar and the target are not moving, but the pulse amplitude changes a great deal.
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