Page 73 - Fundamentals of Radar Signal Processing

FIGURE 1.20 Example of effect of adaptive beamforming. (a) Map of received
signal power as a function of angle of arrival and signal Doppler shift. (b)
Angle-Doppler map after adaptive processing. (Images courtesy of Dr. W. L.
Melvin. Used with permission.)

Pulse compression is a special case of matched filtering. Many radar
system designs strive for both high sensitivity in detecting targets and fine range

resolution (the ability to distinguish closely spaced targets). Upcoming chapters
show that target detectability improves as the transmitted energy increases, and
that range resolution improves as the transmitted waveform’s instantaneous
bandwidth increases. If the radar employs a simple, constant-frequency
rectangular envelope pulse as its transmitted waveform the pulse must be
lengthened to increase the transmitted energy for a given power level. However,
lengthening the pulse also decreases its instantaneous bandwidth, degrading the

range resolution. Thus sensitivity and range resolution appear to be in conflict
with one another.
Pulse compression provides a way out of this dilemma by decoupling the
waveform bandwidth from its duration, thus allowing both to be independently
specified. This is done by abandoning the constant-frequency pulse and instead

designing a modulated waveform. A very common choice is the linear frequency
modulated (linear FM, LFM, or “chirp”) waveform, shown in Fig. 1.21a. The
instantaneous frequency of an LFM pulse is swept over the desired bandwidth
during the pulse duration; the frequency may be swept either up or down, but the
rate of frequency change is constant.