Page 240 - Radar Technology Encyclopedia

jamming, noise jamming, self-screening [-protection] 230

ming pulses of constant amplitude and repetition frequency
(regular pulse noise jamming). Amplitude
T on
Noise jamming is the most common type of ECM tech-
nique, and it is effective against most types of radars. One of
T off
the advantages of this type of jamming is that, in comparison
with deception jamming, very little needs to be known about
the characteristics of the victim radar to be effective. The
quality of jamming noise and its effectiveness against a radar
depends on the methods used for generation, modulation and t
amplification of transmitted power. The electronic devices
emitting noise jamming are called noise jammers. The most Figure J6 On-off jamming plus signal at receiver input (after
Lothes, 1990, Fig. 6.6, p. 182).
common types of such jammers are frequency-swept noise
jammers, narrowband spot jammers, barrage noise jammers, Sometimes this kind of jamming is called AGC deception.
and blinking noise jammers. This type of jamming is more SAL
effective against search radars than against tracking radars. A Ref.: Lothes (1990), pp. 167–204.
major operational form of noise jamming is stand-off jam-
Passive jamming is jamming employing confusion reflectors
ming. Sometimes this kind of jamming is also called denial
to deceive a radar by returning spurious and confusing sig-
jamming or obscuration jamming. SAL
nals. Practically passive jamming is performed with chaff and
Ref.: Barton (1988), p. 139; Barton (1991), pp. 5.22, 12.5; Maksimov (1979), passive decoys. SAL
pp. 30–46; Johnston (1979), p. 63; Schleher (1986), pp. 117–137.
Ref.: Johnston (1979), p. 64; Maksimov (1979), pp. 63–76.
The jamming-to-noise ratio (JNR), or J/N, is the ratio of
Pulse jamming is jamming using the modulation of a carrier
jamming power received by the radar to background noise
with pulses of various widths and repetition rates. Depending
level in the absence of the jammer:
on whether the law of pulse parameters variations is random
2 2
J P G A F F p or regular random pulse jamming and regular pulse jamming
j
j r j
---- = ------------------------------
N 2 is distinguished. SAL
4pR kT L
j s
Ref.: Johnston (1979), p. 65; Barton (1988), p. 490.
where P is the jammer power, G is the jammer antenna gain,
j
j
A is the radar receiving aperture area, F is the jammer pat- Repeater jamming is a deception jamming technique based
r
j
tern-propagation factor (including allowance for radar side- on confusion or deception of a victim radar, causing its equip-
lobe level), F is the polarization factor, R is the jammer ment to present false information. Typically, it sends back an
p
j
range, k is Boltzmann’s constant, T is the radar system amplified version of the signal received from the radar that is
s
receiving temperature, and L is the total loss between the stronger than the radar signal and so captures the radar’s
point where P is measured and the point where T is mea- range gate. Then the deception signal is progressively delayed
s
j
sured. DKB and the range gate walks off the actual target. The electronic
device employing such a technique is called a repeater jam-
obscuration jamming (see noise jamming).
mer. If the jammer uses phase information of the received sig-
Off-line jamming is “the jamming from a station away from nal in creating false targets, it is called a coherent repeater
the vicinity of the target.” Sometimes this jamming is called jammer. SAL
off-target or off-board jamming. SAL Ref.: Barton (1988), p. 490; Johnston (1979), pp. 56,62.; Schleher (1986),
Ref.: Johnston (1979), p. 64. p. 11.
On-line jamming is “the use of the jamming device in line Scan-frequency jamming is jamming against conical-scan
with the target and the radar set.” This type of jamming may and other sequential scanning radars in which the errors are
be either active or passive. SAL introduced by amplitude modulation of a noise or repeater
jammer. When the scanning frequency is exactly known, the
Ref.: Johnston (1979), p. 64.
jamming modulation is approximately the same as the
On-off jamming is pulse jamming that attacks mainly the
antenna-scanning frequency, and this kind of jamming is
AGC function of radar receiver and forces the AGC to adjust
termed scan-frequency selective jamming. When the scanning
gain on the basis of jamming bursts rather than on the target
frequency is not known, scan-frequency barrage jamming
return signal. The jammer burst typically consists of a group
can be employed, which can be either noise or swept scan-fre-
of transponder or repeater pulses coinciding with target return
quency jamming. Against the monopulse radar, this type of
pulses. Jamming bursts of duration T followed by quiet jamming is ineffective because it uses an amplitude modula-
on
periods of duration T , and this causes the gain-controlled tion that does not much affect this type of radar. SAL
off
amplifier to cycle between the state when the amplifier output
Ref.: Barton (1989), pp. 493–496; Maksimov (1979), pp. 49–53.
is at limit level (then all signal modulation is lost as a result of
output limiting) and the state when amplifier gain is so low Self-screening [-protection] jamming is designed to protect
that the output signal is too small to be of any use (Fig. J6). the vehicle or platform that carries the jammer from engage-