Page 414 - Fundamentals of Radar Signal Processing

outside the MLC region sets a minimum PRF. The maximum allowable duty

cycle of the radar transmitter and the pulse length (derived from the required
range resolution when a simple pulse is used) establish a maximum PRF.
Candidate PRFs are often adjusted so that there are an integer number of range
bins within the unambiguous range for each PRI; this eases ambiguity resolution.
In addition to these practical considerations, the set of PRFs used for
disambiguating range and velocity have to satisfy the “decodability constraints.”

(5.123)

where R and F are the maximum range and Doppler shift coverage of
ua
Db
interest. These constraints ensure that the ambiguity resolution algorithms
discussed next can provide a unique solution within that coverage area.

Additional constraints for robust operation include decodability and blindness
“margins” in range and Doppler and minimum transmit time. Generally, the best
PRF sets are obtained using advanced constrained search techniques rather than
the simpler major-minor or M-of-N methods.
Extensive discussion of PRF selection considerations and examples with
an emphasis on airborne multimode radars is given in Alabaster (2012). PRF

selection may be less complex in radars with less varied missions and less
complicated environments. For example, weather radars typically map weather
conditions in their surrounding region using fewer and less complex PRF sets. In
weather radar, volume clutter is the target of interest. Ground clutter is minimal
because the system is not generally down-looking (although airborne weather
radars may be). Weather radars typically require relatively long unambiguous
ranges and therefore low PRFs to provide adequate area coverage. The WSR-

88D radar used by the U.S. National Weather Service for long-range weather
observation uses PRFs of 322 Hz to 1282 Hz, giving unambiguous ranges of 466
to 117 km. The RF is approximately 3 GHz, so the corresponding unambiguous
velocity intervals are ±8 m/s (about ±18 mph) and ±32 m/s (about ±72 mph).
They are well short of the approximately ±100 mph velocity interval considered
adequate by meteorologists. Consequently, ambiguous windspeed measurements

are a common problem in weather radar. Because weather radars are interested
in continuous reflectivity fields (wind flows, storm cells, etc.) distributed in
three dimensions instead of discrete targets, they can also take advantage of
special ambiguity resolution methods that rely on continuity of the measured
velocities and other special features of the measured data; some of these
techniques are described in Doviak and Zrni (1993). Another approach
combines pulse pair processing with two staggered PRFs to compute two

autocorrelation values that can be combined to extend the unambiguous range.