Page 329 - Radar Technology Encyclopedia

319 pulse repetition frequency, high (HPRF) PULSER

staggered PRFs. HPRF pulsed doppler radars at X-band typi- target may be unresolvable in doppler. Such conditions can
cally operate at PRFs from 100 kHz to over 300 kHz, at duty regularly occur for slowly moving and receding targets.
cycles up to 50%. Figure P30 is a range-doppler “map” for a typical MPRF
For nonclosing or slowly closing target conditions, the radar, showing the blind regions caused by the range and dop-
target signal must compete with sidelobe and backlobe clut- pler ambiguities. With limited clutter attenuation, the first
ter, and if sufficient clutter attenuation (CA) cannot be several range cells after the transmitted pulse will contain
achieved in the radar system, an alternate, medium PRF strong clutter, which may eclipse the target, adding to eclips-
(MPRF) waveform may have to be used. PCH ing by the transmitter itself. This creates blind range bands of
Ref.: Schleher (1991), pp. 65,66. width D , which cross the blind doppler bands of width D v
r
produced by blind velocities. To obtain a high probability of
PRF jitter (see staggered PRF).
detection in each observation t , bursts at different PRFs must
o
Low PRF (LPRF) describes a waveform whose unambigu- be used. The number of bursts needed depends on the fraction
ous range is greater than the maximum target range. Such of clear area F in range-velocity space:
a
waveforms are particularly suited to long-range airborne sur-
veillance and to surface-based radars that must detect low- æ D r öæ D v ö
F = 1 – ------ 1 – -----
a
altitude targets in land or sea clutter. Most of these radars è R øè v ø
u
b
employ moving-target detection (MTD) techniques, such as
MTI, pulsed doppler, or both, to detect the moving target
against a background of stationary clutter, but use of the
LPRF waveform allows only an ambiguous measurement of
target radial velocity. The LPRF radar must detect targets
having radial velocities from near zero to several times the
blind speed of the radar. To avoid loss of detection of targets
near the radar blind speed and its multiples, a pulse-to-pulse
stagger can be introduced in the pulse repetition interval.
Clutter cancellation is retained for clutter lying within the
unambiguous range. If clutter exists beyond the first range
ambiguity, another procedure, referred to as PRF-diversity
can be used. With PRF-diversity MTI, the PRF is held con- Figure P30 Blind regions in MPRF PD radar (from Barton,
stant until echoes from the longest-range clutter are received 1988).
and processed.
Since sidelobe clutter poses a major problem in MPRF
If target radial velocity information is required in addi-
radar, ensuring low antenna sidelobes is a priority. Use of
tion to target range (e.g., for target designation to a separate
pulse-compression techniques to narrow the width of the
tracking radar), PRF-stagger or PRF-diversity, when used in a
effective range resolution cell is another step that can be taken
pulsed doppler radar, can be used to resolve the doppler ambi-
to reduce the relative contribution of ambiguous range clutter.
guities. (See MTI; RADAR, pulsed doppler.) PCH
PCH
Ref.: Barton (1988), pp. 234,239.
Ref.: Barton (1988), pp. 260–262. Schleher (1991), pp. 68–71.
Medium PRF (MPRF) describes a waveform that is ambigu-
Staggered PRF is a technique employed in MTI and pulsed
ous in both range and doppler. Simply stated, use of MPRF is
doppler radars in which the radar’s interpulse period is altered
a compromise to ameliorate the clutter effects posed by the
on a pulse-to-pulse or burst-to-burst basis to allow detection
range-ambiguous HPRF waveform on the one hand, and
of targets whose velocities fall within the blind regions of the
those created by the velocity-ambiguous LPRF waveform on
radar’s velocity response that occur at multiples of the radar’s
the other.
PRF. In burst-to-burst stagger, also called “block” stagger, the
The MPRF waveform is especially important in airborne
PRF is changed after a number of radar pulses have been
intercept (AI) radar, where detection of slow-moving and
transmitted at a constant PRF. Pulse-to-pulse stagger gener-
receding targets is required in an often-severe look-down
ally provides a better velocity response than burst-to-burst
clutter environment. With a LPRF waveform, in the presence
stagger, but imposes more severe stability requirements on
of a broad clutter spectrum such as that produced with wind-
the system, especially the transmitter. Burst-to-burst stagger
blown rain or chaff, it may not be possible to realize a blind
allows multiple-time-around clutter responses (from clutter
speed that is high enough to reject clutter without rejecting a
beyond the first range ambiguity) to be canceled in a coherent
significant fraction of targets as well. At the PRFs typically
MTI system. PCH
employed by modern high-speed interceptor aircraft HPRF
Ref.: Schleher (1991), pp. 9, 390–392.
radar, sidelobe clutter from virtually all ranges folds into the
range interval corresponding to the interpulse period. If the PULSER. A pulser is a modulating device used in transmit-
sidelobe clutter amplitude in the target doppler resolution cell ters of pulsed radars to shape the transmitted pulse. The basic
is sufficiently strong (e.g., from short-range clutter) and the types are the cathode pulser, modulating-anode (or