Page 131 - Fundamentals of Radar Signal Processing

FIGURE 2.18 Three scans or CPIs, each having 10 samples of a unit mean
Swerling 4 power sequence.

2.2.8 Effect of Target Fluctuations on Doppler Spectrum
A common operation in radar signal processing is computing the discrete-time
Fourier transform (DTFT) of the data in a particular range bin for one CPI. The
DTFT is a coherent combination of measurements, usually over a sufficiently

short CPI that the target echo RCS and thus amplitude do not decorrelate
significantly. As will be seen in Chap. 4, the series of samples within a CPI for
a constant-velocity target will form a discrete-time sinusoid. Thus, the usual
model for the DTFT of a target is an aliased sinc function [also called an asinc,
dsinc (digital sinc), or Dirichlet function] with its mainlobe centered at the
appropriate frequency and with sidelobes that peak 13.2 dB below the mainlobe
peak and decay at frequencies further from the mainlobe.

In cases where there are significant RCS fluctuations within the CPI, the
amplitude and phase of the target data will vary within the CPI, so that the input
to the DTFT is no longer a discrete sinusoid with a constant complex amplitude.
Figure 2.19 illustrates the resulting effect on the DTFT. The gray spectrum is
that of an unwindowed zero-frequency sinusoid, modeling the returns from 20
pulses echoed from a stationary 10 × 5 m simulated many-scatterer target

viewed at a constant aspect angle. The black line is the spectrum of data