Page 329 - Fundamentals of Radar Signal Processing

and clutter are significant interference sources. The moving targets appear in the

spectrum at Doppler shifts consistent with their radial velocities relative to the
radar.
Doppler processing is most often of interest when the relative amplitudes
of the clutter, target, and noise signals are as shown: the target returns are above
the noise floor (signal-to-noise ratio SNR 1), but weaker than the clutter
(signal-to-clutter ratio SCR 1). In this case, targets cannot be detected reliably

based on amplitude in the slow-time domain alone because the presence or
absence of the target makes little difference to the total signal power. Doppler
processing is used to separate the target and clutter signals in the frequency
domain. The clutter can be filtered out, leaving the target return as the strongest
signal present, or the spectrum can be computed explicitly so that targets outside
of the clutter region can be located by finding frequency components that
significantly exceed the noise floor.

In this chapter the two major classes of Doppler processing, moving target
indication (MTI) and pulse Doppler processing, are described. In the
terminology used here MTI refers to the case where the slow-time signal is
processed entirely in the time domain. Pulse Doppler processing refers to the
1
case where the signal is processed in the frequency domain. As will be seen,
MTI processing produces limited information at very low computational cost;
pulse Doppler processing requires more computation but produces more
information and greater signal-to-interference ratio (SIR) improvement. Only
coherent Doppler processing using digital implementations is considered since
this is the approach taken in most modern radars. Good general references
include Richards (2010) and Schleher (2010). Alternative systems using
noncoherent Doppler processing and implementations based on analog

technologies are described in Eaves and Reedy (1987), Nathanson (1991), and
Schleher (2010).

5.1 Moving Platform Effects on the Doppler Spectrum

The notional Doppler spectrum of Fig. 5.1 represents a very simple case. While
it is realistic for some scenarios, the Doppler spectrum for a given range bin can
be greatly complicated by factors such as a moving radar platform, or range and
Doppler ambiguities caused by aliasing of the target signatures.

The effect of the PRF in the Doppler dimension is straightforward. The
PRF establishes the width of one period of the Doppler spectrum as discussed
above. Clutter or target signals having Doppler shifts outside of the range ±
PRF/2 will alias into that interval. Figure 5.2 illustrates how the spectrum of
Fig. 5.1c might look if the PRF were reduced by 40 percent. The clutter
spectrum is unchanged but now represents a larger fraction of the total spectrum

width. That is, the clear region is a smaller percentage of the spectrum width