The Navy Vol_73_No_3 Jul 2011 - Navy League of Australia

An RAN S-70 Seahawk. The updated RAN Seahawks have new EW intercept systems for passive searching as well as for the
aircraft’s self protection. Four receivers are mounted in such a way as to cover 360 degrees. They can provide an extension
of the ship’s own EW capability. (Mark Schweikert)
RADAR NOISE JAMMING
A transmission at the frequency of a radar
modulated by noise has the well known effect
of saturating the radar receiver and causing
its display to be covered by a continuous noisy
paint, which obscures targets the radar would
otherwise see. Certainly, a surveillance radar
is disrupted and confused in its operation
by this action, but its loss is not total as the
noise jammer identifies the presence of a
warship, which might otherwise be doubtful.
More seriously, the jamming signal can act
as a homing beacon for missiles, with most
anti-ship missiles today having a home-onjam
capability.
Against gunnery radars, when the shells
have no homing ability, the effect of the noise
jammer is to deny range information to the
radar and seriously reduce or destroy the
lethality of the gun, which needs accurate
ranging. Naval gunfire is not a serious threat
to ships in modern times. Missiles are, and
the use of a noise jammer can be dangerous,
since missiles do not need accurate ranging
but they do need to select a warship target.
Thus, on balance the noise jammer, which was
really the first ECM device to be conceived, is
too dangerous for ships and has been largely
replaced by other more effective and more
subtle devices.
The noise jammer in its original simple form
is actually rather a blunt instrument, which,
without some thought, can rebound on the
user. Its prime contribution in denying range
information to a radar is not an important
requirement and its role in naval warfare is
now not very significant. However, modern
forms of noise jamming using pulses of noise
in configuration with false target generators
play a useful role in causing confusion.
COMMUNICATION JAMMING
Communications jamming with a noisemodulated
signal is quite different in its
significance and potential. The message
content of the enemy signal can be totally
obscured and a definite advantage gained
without incurring any potential risk to the
jamming ship or shore base in the simplest
cases. However, if a communication signal
is jammed it cannot usually be read by the
jamming ship. It could be tactically more
valuable to read an enemy’s signals than
to stop him reading them, so this must be
considered within the framework of command
and control. If the enemy signal is encrypted,
so that it cannot be read quickly or easily,
then jamming can be more profitable and
all that is given away is the presence of a
jamming warship.
To avoid jamming, the enemy transmission
could employ ‘agility’ — that is frequency
hopping over quite a wide band. To jam
this form of signal, either an agile jammer
or a broad band jammer would be required.
This introduces undesirable features of
considerable cost in the jammer and the
chance of it interfering with own ship’s
communications. So a decision, based on the
balance of advantage must be made, as in
most applications of EW. The stage has now
been reached that, while communications
jamming can be very effective, it is no longer
a simple and inexpensive operation against
modern frequency agile communications
systems.
ELECTRONIC FALSE TARGETS
It can be dangerous to employ simple,
continuous noise jamming against a
surveillance radar, with the main attendant
risk of providing a home-on-jam capability for
missiles. It is possible to confuse the radar
operator, who is trying to select a target for
his missile, with a great number of false
targets around the ship. These targets can be
generated electronically and injected into the
radar through its sidelobes as well as its main
lobe. The appearance of these false targets
on the radar display can be very realistic
and the operator is certain to be confused to
some extent. But he knows a warship target
is there, somewhere, in the midst of all the
painted targets and he can fire a missile.
Now, the radar in the eye of the missile will
look for a target to lock to, and it will search
the area in front of it. If the ship is within the
search area of the missile it is most likely
to be selected because the missile cannot
lock onto an electronic false target. This is
because they have no physical reality in
space and they will disappear when the
missile radar stops scanning and attempts to
lock on. So, while the surveillance radar may
be confused, there is not much prospect of a
reduction in lethality of the missile.
These sophisticated and elegant electronic
false target generators are quite expensive
and unfortunately their real application is
limited to confusion in radar surveillance.
This may be useful in causing a short delay,
while the radar operator decides what to do.
The Israeli C-Pearl ESM receiver with a Millimetre Wave
(MMW) frequency receiver below. C-Pearl (minus the
MMW set) is fitted to the RAN’s four FFGs. C-Pearl is
usually mounted on the highest point of the ship. The saw
tooth arrangement below is to prevent the ship’s own
active radars from blanketing the receiver with ‘noise’
when operating.
He is exposed, while so doing, thus, allowing
the ship more time to take defensive action
and deploy decoys, but it is mainly a source of
confusion for gaining a tactical advantage, Of
course, if the missile uses infra-red homing,
THE NAVY VOL. 73 NO. 3 07