Page 397 - Fundamentals of Radar Signal Processing

FIGURE 5.29 Blind zones using two PRIs and a “1 of 2” detection rule. Left:
Overlay of the two blind zone maps of Fig. 5.28. Right: Resulting blind zone
map using “1 of 2” detection logic.

A problem with a “1 of N” rule is that a single false alarm would be

accepted as a target. A more conservative “2 of 2” rule would mean a target
was detectable only if it was in the clear on both PRIs. The effective blind zone
map would be black in all of the black and gray regions of the left image in Fig.
5.29, with the result that very few combinations of range and velocity would
correspond to detectable targets. Thus, there is a tradeoff between the range-
velocity coverage area in which targets are detectable and the immunity to false

alarms and other error sources. Using values of N larger than two offers more
compromise options and better results. Figure 5.30 shows an exceptionally good
blind zone map for a system using eight PRFs with a “3 of 8” detection rule. The
radar operates at 10 GHz with a 1-μs pulse length, blanking of 10 near-in clutter
cells, a ±17 m/s mainlobe clutter spread, and a 10-ms CPI. An evolutionary
algorithm was used to select PRIs that maximized the detectable region within
the range and velocity limits shown; the result was the set {51, 53, 60, 63, 67,

84, 89, 93} μs (Davis and Hughes, 2002). Nearly all of the range-velocity space
shown is in the clear except for the first range and Doppler blind zones. “M of
N” detection logic is discussed further in Chap. 6.