Page 423 - Fundamentals of Radar Signal Processing

5.7.1 The DPCA Concept
MTI filtering and pulse Doppler processing provide an effective way to detect
moving targets whose Doppler shift is in the clear region of the spectrum on at
least one PRF. Airborne targets can generally be detected in this manner.
However, slow-moving ground targets having actual Doppler shifts only slightly
higher than the ground clutter or less will appear within the clutter spectrum or

on its skirts at all PRFs and are therefore very difficult to detect. Recall that
platform motion can substantially spread the ground clutter spectrum as
described in Eq. (5.72). This spread of mainlobe clutter exacerbates the
problem, raising the minimum velocity at which slow-moving ground targets can
be detected. This phenomenon is illustrated in Fig. 5.44, which shows the
spreading of the mainlobe clutter by the platform motion after the change in
Doppler center frequency has been removed. Because of this spreading, clutter

energy may compete directly with relatively slow-moving targets (“slow
movers,” typically surface targets such as vehicles on land and ships on the
sea), making MTI processing less effective and detection difficult. Processing
techniques intended to detect such “slow movers” from moving platforms are
referred to as ground moving target indication (GMTI) or surface moving
target indication (SMTI).

FIGURE 5.44 Illustration of the effect of a moving radar platform on the
Doppler spectrum and the detection of “slow movers.” The change in Doppler

center frequency has been removed.

Displaced phase center antenna (DPCA) processing is a technique for

countering the platform-induced clutter spectral spreading. By minimizing the
clutter spectral width, DPCA improves the probability of detection for slow-
moving targets. It is a special case of the more general space-time adaptive
processing (STAP) introduced in Chap. 9. Heuristically, the basic concept is to
create the effect of a stationary antenna even though the platform is moving
forward by electronically moving the receive aperture backward during
operation, thus avoiding the clutter spreading. More specifically, DPCA

processing attempts to compensate for aircraft motion by using multiple receive
subapertures to create carefully controlled multiple virtual phase centers such
that data received on one subaperture have the same virtual phase center as the
data received on a different subaperture some time later. By properly