Page 226 - Fundamentals of Radar Signal Processing

CHAPTER 4

Radar Waveforms

4.1 Introduction
A radar transmits a waveform typically modeled as

(4.1)

The term Ω in the argument of the sine function is the carrier radar frequency
(RF) in radians per second. The term a(t) represents amplitude modulation of
the RF carrier; in a pulsed radar, this is typically just a rectangular function that

pulses the waveform on and off. The term θ(t) models any phase or frequency
modulation of the carrier. It can be zero, a nonzero constant, or a nontrivial
function. The overbar on denotes that the signal is on a carrier, i.e., it has not
yet been demodulated. Figure 4.1 illustrates three example waveform types
common in pulsed radar. The simple pulse is simply a constant-amplitude burst
at the RF frequency. The frequency of the linear frequency modulated (LFM)
pulse increases at a constant rate during the time the pulse is on. LFM pulses can
also have decreasing frequency during the pulse. The third example is a binary

phase-coded pulse. In this waveform, the frequency is constant but the absolute
phase of the waveform changes from zero to π radians several times within the
pulse. That is, the value of θ(t) changes between the constants zero and π at
specific times within the pulse.