Page 418 - Fundamentals of Radar Signal Processing

FIGURE 5.38 Coincidence detection of target ranges in replicated range data.

The graphical interpretation suggests various methods to reduce the

sensitivity of the CRT to measurement errors. In one approach, exact
coincidence is not required to declare a target. Instead, a tolerance N is
T
established and a detection is declared if a detection occurs in all three PRFs at
some range bin n ± N . Depending on the range bin size and SNR, N will
T
t
T
typically be only 1 or 2 range bins. A more sophisticated version of this basic
idea is described in Trunk and Kim (1994). Their method combines a systematic
approach to clustering plausible ranges from each PRF into candidate target

ranges with a maximum likelihood calculation to recognize multiple target
situations.
In the last example three PRFs proved sufficient to resolve two different
range-ambiguous targets. In general, N PRFs are required to successfully
disambiguate N – 1 targets. If the number of targets exceeds N – 1, ghosts can
appear (Morris and Harkness, 1996). Ghosts are false targets resulting from

false coincidences of range-ambiguous data from different targets. The problem
is illustrated in Fig. 5.39, which repeats the example of Fig. 5.38 using only two
of the previous three PRFs. While targets will still be detected at the correct
bins n = 6 and n = 11, a third coincidence occurs between detections from
b
a
targets 1 and 2 at range bin n = 20, representing an apparent third target. Unless
c
additional data such as tracking information is available, the signal processor
has no way of recognizing that the last coincidence is among detections from

different targets. Thus, the processor will declare the presence of three targets
in this example, the two correct targets and one “ghost.” Use of a third PRF as in
Fig. 5.38 eliminates this ghost. More extensive discussion of ghosting in range
and Doppler and of ghosting due to false alarms is given in Alabaster (2012).

FIGURE 5.39 Formation of ghosts in range ambiguity resolution.

In a medium or high PRF mode the radar may also suffer velocity
ambiguities. This problem is identical to that of range ambiguities: given an