Page 351 - Fundamentals of Radar Signal Processing

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Blind speeds could be avoided by choosing the PRF high enough so that the

first unambiguous velocity exceeds any actual velocity likely to be observed for
targets of interest. Unfortunately, higher PRFs also correspond to shorter
unambiguous ranges. It is frequently not feasible to operate at a PRF that allows
unambiguous coverage of both the range and velocity intervals of interest. For
example, suppose a designer requires unambiguous range of 100 km and at least

±112.5 m/s of unambiguous velocity coverage, corresponding to a 225 m/s blind
speed. Figure 5.11 shows that the maximum RF at which this is possible is 1
GHz. If the radar is required to be at X band (10 GHz), the combination of 100
km unambiguous range coverage and 112.5 m/s unambiguous velocity coverage
is not obtainable and some ambiguity must be accepted in range, Doppler, or
both.
The use of staggered PRFs or staggered PRIs is an alternative approach
that raises the first blind speed significantly with only a modest reduction in the

unambiguous range (Levanon, 1988; Schleher, 2010). PRF staggering can be
performed on either a pulse-to-pulse or CPI-to-CPI basis. The latter case is
common in airborne pulse Doppler radars and is deferred to Sec. 5.3.8. Pulse-
to-pulse stagger varies the PRI from one pulse to the next within a single CPI or
dwell. One common approach is to cycle through a set of P preselected PRIs

from one pulse to the next, repeating when all of the PRIs have been used.
Figure 5.12 illustrates the pulse timing sequence for a case with P = 2. The
resulting slow-time data for a given range bin is then passed through an MTI
filter. As will be seen, this process has the advantage of achieving increased
Doppler coverage with a single dwell.

FIGURE 5.12 Pulse sequence timing for two staggered PRIs.

One disadvantage of staggering is that the slow-time data are now a

nonuniformly sampled sequence, making coherent Doppler filtering impractical
and complicating analysis. Another is that ambiguous mainlobe clutter can cause
large pulse-to-pulse amplitude changes as the PRI varies since the range of the
second-time-around clutter that folds into each range cell will change as the PRI
changes. Consequently, pulse-to-pulse PRI stagger is generally used only in low