Graphics Programs Reference
In-Depth Information
TABLE 1A.1. PRF ambiguities.
PRF
Range Ambiguous
Doppler Ambiguous
Low PRF
No
Ye s
Medium PRF
Ye s
Ye s
High PRF
Ye s
No
Radars can utilize constant and varying (agile) PRFs. For example, Moving
Target Indicator (MTI) radars use PRF agility to avoid blind speeds. This kind
of agility is known as PRF staggering. PRF agility is also used to avoid range
and Doppler ambiguities, as will be explained in the next three sections. Addi-
tionally, PRF agility is also used to prevent jammers from locking onto the
radarÓs PRF. These two latter forms of PRF agility are sometimes referred to as
PRF jitter.
Fig. 1A.1 shows a simplified pulsed radar block diagram. The range gates
can be implemented as filters that open and close at time intervals that corre-
spond to the detection range. The width of such an interval corresponds to the
desired range resolution. The radar receiver is often implemented as a series of
contiguous (in time) range gates, where the width of each gate is matched to
the radar pulsewidth. The NBF bank is normally implemented using an FFT,
where bandwidth of the individual filters corresponds to the FFT frequency
resolution.
1A.2. Range and Doppler Ambiguities
As explained earlier, a pulsed radar can be range ambiguous if a second
pulse is transmitted prior to the return of the first pulse. In general, the radar
PRF is chosen such that the unambiguous range is large enough to meet the
radarÓs operational requirements. Therefore, long-range search (surveillance)
radars would require relatively low PRFs.
The line spectrum of a train of pulses has envelope, and the line
spectra are separated by the PRF, , as illustrated in Fig. 1A.2. The Doppler
filter bank is capable of resolving target Doppler as long as the anticipated
Doppler shift is less than one half the bandwidth of the individual filters (i.e.,
one half the width of an FFT bin). Thus, pulsed radars are designed such that
sin
xx
f r
2 v rmax
λ
f r
=
2 f dmax
=
----------------
(1A.1)
where
is the maximum anticipated target Doppler frequency,
is the
f dmax
v rmax
maximum anticipated target radial velocity, and
is the radar wavelength.
λ
 
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