Page 75 - Fundamentals of Radar Signal Processing

function that has been reversed in time and conjugated; thus the impulse

response is “matched” to the particular transmitted waveform modulation. Pulse
compression is the process of designing a waveform and its corresponding
matched filter so that the matched filter output in response to the echo from a
single point scatterer concentrates most of its energy in a very short duration,
thus providing good range resolution while still allowing the high transmitted
energy of a long pulse. Figure 1.21b shows the output of the matched filter

corresponding to the LFM pulse of Fig. 1.21a; note that the mainlobe of the
response is much narrower than the duration of the original pulse. The concepts
of matched filtering, pulse compression, and waveform design, as well as the
properties of linear FM and other common waveforms, are described in Chap.
4. There it is seen that the 3-dB width of the mainlobe in time is approximately
1/β seconds, where β is the instantaneous bandwidth of the waveform used. This
width determines the ability of the waveform to resolve targets in range.

Converted to equivalent range units, the range resolution is given by

(1.35)

[This is the same as Eq. (1.2) presented earlier.]
Clutter filtering and Doppler processing are closely related. Both are

techniques for improving the detectability of moving targets by suppressing
interference from clutter echoes, usually from the terrain in the antenna field of
view, based on differences in the Doppler shift of the echoes from the clutter
and from the targets of interest. The techniques differ primarily in whether they
are implemented in the time or frequency domain and in historical usage of the
terminology.

Clutter filtering usually takes the form of moving target indication, or
MTI, which is simply pulse-to-pulse highpass filtering of the radar echoes at a
given range to suppress constant components, which are assumed to be due to
nonmoving clutter. Extremely simple, very low-order (most commonly first- or
second-order) digital filters are applied in the time domain to samples taken at a
fixed range but on successive transmitted pulses.
The term “Doppler processing” generally implies the use of the fast

Fourier transform algorithm, or occasionally some other spectral estimation
technique, to explicitly compute the spectrum of the echo data for a fixed range
across multiple pulses. Due to their different Doppler shifts, energy from
moving targets is concentrated in different parts of the spectrum from the clutter
energy, allowing detection and separation of the targets. Doppler processing
obtains more information from the radar signals, such as number and

approximate velocity of moving targets, than does MTI filtering. The cost is
more required radar pulses, thus consuming energy and timeline, and greater