Page 236 - Radar Technology Encyclopedia

jamming, active jamming, cooperative 226

reradiated). The main types of active jamming are noise jam- and probability of false alarm, and L is the total of the radar
s
ming, deception jamming, and active decoys. system losses.
Ref.: Schleher (1986), p. 9. In angle jamming, the barrage scan-frequency jamming
technique is used, in which noise or repeater jamming is
Angle jamming is an “ECM technique, when azimuth and
amplitude-modulated with a band of audio frequencies cover-
elevation information from a scanning fire control radar
ing the possible conical-scan rate of the victim radar. A por-
present in the modulation components on the returning echo
tion of the resulting AM spectrum, at the radar’s actual scan
pulse is jammed by transmitting a jamming pulse similar to
frequency, then enters the tracking servo to cause tracking
the radar pulse but with modulation information out of phase
noise. The technique is less effective than inverse-gain jam-
with the returning target angle modulation information.” SAL
ming or AM at the actual scan frequency because the noise
Ref.: Johnston (1979), p. 52.
within the servo bandwidth is only a fraction of the total.
Barrage jamming is jamming with a bandwidth much wider However, when the radar uses conical scan on the receiving
than radar signal bandwidth. The most common form is bar- beam only (COSRO), the actual scan frequency may be
rage noise jamming; that is, noise jamming in which the jam- unknown to the jammer, and barrage scan-frequency jamming
ming bandwidth is designed to cover the entire tuning range may be the only practical countermeasure. DKB, PCH
of a victim radar, or to encompass several radars operating at Ref.: Barton (1988), pp.139–140, 494; Goj (1993), p. 45.
different frequency. This wideband feature distinguishes the
Bistatic jamming is created by reflection from sources dis-
barrage noise jammer from the spot noise jammer, whose
placed in space from the jamming transmitter. It can be cre-
bandwidth is designed to cover the radar’s instantaneous sig-
ated from a single jammer platform by illuminating an
nal bandwidth.
external object with a directional jamming beam. Then the
As does noise jamming in general, barrage noise jam-
radar sees the jamming source at or near the illuminated
ming realizes its effect by raising the receiver noise level in
objects. If these objects lie within the same resolution cell as a
the victim radar. While the radar’s system noise temperature
target, accurate tracking of the target becomes impossible.
T was, in the absence of jamming, determined only by the
s
receiver and the natural environment, the jammer raises this Possible external objects are chaff, towed or expendable
decoys, or the surface of the earth. The special case of bistatic
to a new value T =T + T , with the jammer temperature given
j
i
s
by jamming is use of the surface of the ground that results in
ground-bounce jamming. The radar ECCM against bistatic
P G A F 2
j r j
j
T = ----------------------------- jamming is basically better resolution. SAL
j
2
4pkB R L Ref.: Barton (1989), p. 502.
j j aj
where P G is the effective radiated power of the jammer, F is Blinking jamming is noncoherent multiple-source jamming
j j
j
the pattern propagation factor of the radar receiving antenna done with several (basically two) jamming transmitters by
in the jammer direction, B is the bandwidth of the jamming turning them on one at a time. This kind of jammer is consid-
j
signal, R is the jammer range from the radar, and L is the ered an effective electronic countermeasure against guided
j
aj
one-way propagation loss for the jammer. missiles using radar seekers. When the jamming transmitters
Barrage jamming can be very effective in screening quiet are located on different platforms the main aim of this jam-
(nonjamming) penetrator aircraft from detection. A typical ming is to make a missile wander from one target to another,
tactic is for one or more high-power barrage jammers to stand as a result going between the platforms without hitting them.
off at a range beyond the engagement range of the defensive To ensure the effectiveness of this jamming, the correct
weapon system, and to screen a high-speed, relatively low- choice of the commutation rate must be made. Too high a rate
RCS penetrator into its weapon release range. The range at causes tracking radar to average the data (and, therefore,
which the victim radar can detect the screened target, called angular error is minimized), and too low a rate allows the
the burnthrough range is given by the equation: radar to determine the real position of each target. The best
solution is when the rate is of the order of the tracking servo’s
1
---
æ 2 2 ö
4
ç P t G G sF R L aj ÷ bandwidth (about 0.1 to 10 Hz). SAL
j
av o
t
rj
R = ç ----------------------------------------------------- ÷ Ref.: Schleher (1986), p. 155; Maksimov (1979), p. 55.
bt
ç æ P G j ö ÷
j
è 4p ----------- D 1 () L s ø Chaff jamming is passive jamming using chaff as a decep-
è
ø 0
B
j
tive measure. (See CHAFF.) SAL
where P is the radar average power, t is the radar observa-
o
av
tion time, G is the radar transmit antenna gain, G is the Cooperative jamming is jamming that requires the coopera-
rj
t
radar receive antenna gain in the direction of the jammer, s is tion of two platforms, each possessing either a deception or
noise jammer. The typical example of cooperative jamming is
the penetrating target’s RCS, F is the radar propagation factor,

L is the one-way atmospheric attenuation suffered by the blinking jamming. Cooperative jamming requires a radio link
aj
jammer, P G /B is the jammer power spectral density (W/ between two platforms to assure that the emissions are syn-
j j
j
Hz), D (1) is the SNR required for the radar to detect the chronized. SAL
0
Swerling Case 1 target with a given probability of detection Ref.: Neri (1991), p. 385.