Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to the jamming of radar apparatus,
particularly radar apparatus in airborne interceptors.
Present air defence systems make use of two principles of control of
interceptors. The "close ground control" principle requires that each inter-
ceptor be carefully controlled by radar apparatus and associated equipment on
the ground, with almost all navigational and other control information trans-
mitted from ground to the interceptor. "Loose control" air defence systems
make use of only limited ground control and the interceptors in such loose
control systems complete their missions more or less independently with only
rudimentary control information from the ground. In general, loose control
systems have greater immunity from electronic counter measures directed
against
them by attacking bombers or missiles.
The available electronic counter measures for jamming interceptor
radar apparatus are not completely satisfactory. Simple electronic noise jam-
ming is ineffective against "home-on-jam" interceptors which have equipment
providing directional. control by making use of the jamming signal instead of
the radar reflection signal. Another type of jamming device makes use of an
inverse gain repeater which repeats the signal transmitted by the interceptor
radar set so as to create distorted "echo" signals in the radar receiver which
"unlock" the radar. However, such inverse gain repeaters are as presently
used ineffective when used against monopulse radar sets and against Lobe-on-
Receive Only radar equipment. A number of other specialized jamming techniques
are available but these are at best effective only against specific types of
interceptor radar systems. Chaff provides relatively little protection against
presently known radar techniques. While aerodynamic decoys are often
effective,
in practice it may be impossible to carry such decoys in sufficient number to
ward off all interceptors when flying through heavily defended zones.
The present invention provides an electronic jamming technique which
would combine the masking properties of a conventional jammer with the main
advantage of decoys, i.e. the possibility of diverting the interceptor from
the attacking bomber or missile to the decoy. Basically, the decoy is provided
by creating a source of radio waves distant from the attacking aircraft carry-
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ing the jamming equipment. This source is created by scattering techniques,
and preferably by scattered radiation from the surface of the earth.
The present invention therefore provides, a method of jamming radar
apparatus comprising directing a narrow jamming radio beam towards some scatt-
ering surface (Ordinarily the surface of the earth). After scattering, the
jamming radiation appears to an interceptor as though it were emenating from
the scattering surface. Therefore, the interceptor radar apparatus, instead
of homing on the attacking flying object, will in most cases be rendered in-
effective by the jamming or diverted towards the scattering source and will
miss the bomer or missile.
In general the interceptor will not be flying directly beneath the
attacking aircraft and therefore will not intercept directly the narrow beam
of jamming radiation transmitted from the attacking aircraft towards the
surface
of the earth. However, a number of side lobes will, in general, characterize
the antenna pattern for the jamming apparatus and it is therefore necessary to
attenuate the side lobes sufficiently so that no significant source of radia-
tion from the direction of the attacking object is detected by the
interceptor.
The invention will now be described with reference to the accompany-
ing drawings in which:
F'igure 1 is a schematic diagram illustrating a configuration of an
attacking aircraft and an interceptor and in which the attacking aircraft is
shown employing the jamming method according to the invention, and
Figure 2 is a diagram for use in deriving the desired ratio of jamm-
ing power of the target aircraft jamming transmitter to the peak power of the
interceptor radar tra.nsmitter.
In Figure 1 an attacking aircraft, for example a bomber 11, is shown
flying on a course within the principal lobe 15 of an antenna 14 of an inter-
ceptor radar unit 12, in an interceptor 10. The sizes of the aircraft and
inter-
ceptor in the drawing; are exaggerated relative to the distances one would
expect
them to be from one another and from the ground. The interceptor radar unit 12
is assumed to be of conventional design and mounted in conventional manner in
the interceptor 10, which is searching for attacking missiles or bombers. The
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13 4 1 5 5 8
jammer may be, for example, of the noise or repeater type. Because the antenna
14 is directed to that part of the sky in which the bomber 11 is located, the
radiation transmitted by the interceptor radar unit 12 includes a beam 37
strik-
ing the bomber 11 and reflected back to the interceptor radar unit 12. In the
absence of jamming, the interceptor 10 would respond to this target echo
signal
and align itself witla. the beam 37, thereafter maintaining the alignment in
res-
ponse to the target signal until close enough to destroy the bomber 11.
According to the invention, the bomber 11 is provided with a suit-
able jamming device 13 of conventional design which is provided with a narrow
beam antenna 28 which radiates a narrow beam of radiation 21 having a
frequency
corresponding to the frequency of the interceptor radar unit 12. The beam 21
is directed towards the ground and scattered by a scattering surface which
will
ordinarily be the surface 19 of the earth. The scattered radiation is directed
upwards in all directions, representative scattered rays being shown by the
reference numerals 22, 23, and 24. In particular, a ray 27 of scattered rad-
iation will fall within a side lobe 17 of the antenna 14 and will therefore
effectively jam the interceptor unit, provided the ray 27 is of sufficiently
high power. The interceptor unit 12 is thus unable to distinguish the echo
signal produced by reflection of its own transmitted signal from the bomber 11
because of the effect of the jamming signal transmitted via ray 27, and will
therefore be unable to provide proper guidance to the interceptor 10. If the
radar unit 12 is switched from a "search" mode of operation to a "home on jam"
mode, this will result in the interceptor's alignment with the ray 27, i.e.
the jamming signal in the radar unit 12 will cause the interceptor 10 to "home
on to" (direct itself towards) the source ar' jamming radiation. This source
of course appears to the radar to be situated on the surface of the earth 19,
and the interceptor will accordingly follow a suicide flight path towards the
ground.
The method according to the invention is not 100% effective; it works
best when the attacking aircraft flies at a lower altitude than the intercep-
tor and at long and medium ranges. When the attacking aircraft flies at an
altitude higher thari that of the interceptor, the interceptor radar may pick
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up jamming radiation from the side lobes of the jamming radar antenna which
are of higher power than rays reflected from the ground. Accordingly, if the
interceptor switches 'to a home-on-jam mode of operation, it will fly towards
the attacking aircraft rather than towards the ground. At very short ranges
the
method according to the invention may not be effective because the echo signal
may be much stronger than the reflected jamming radiation from the terrain, or
because the jamming radiation received directly from the attacking aircraft is
more powerful than that received from the terrain.
In general, the particular choice of jamming frequency range will be
determined by the known characteristics of the enemy's interceptor radar
units.
However, the choice of power output for the jamming device depends upon the
scattering properties of the earth's surface and upon the echo area presented
by the attacker as well as upon the characteristics of the interceptor radar
unit. In order to ensure adequate scattered jamming power, a theoretical
analysis reveals that it is necessary that the ratio of jamming power radiated
by the transmitter of the target aircraft to the peak power of the
interceptor's
radar transmitter 'be at least as great as given by the following equation,
the
derivation of which is set out in Appendix A.
P. _ KGAL .(D2 + 2 H.H - H 2)3/2
p 167rsH 1 J 4 ...--J . . . . . . . . . ( I )
r D
where Pj is the mean jamming power within the interceptor receiver's band
width.
Pr is the peak power of the interceptor's radar transmitter
A is the echoing area of the target (attacking aircraft)
Hi, H. are the altitudes of the interceptor and the jamming target air-
craft, respectively
D is the horizontal distance between the target and the interceptor
K is the jamming-to-signal ratio required for effective masking
G is the main beam power gain of the interceptor's antenna
L is the main lobe to side lobe power ratio for a side lobe of the
interceptor's antenna in the direction of the apparent source of
scattered jamming radiation at the earth's surface.
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s is the "scatter loss factor", i.e. that fraction of the energy
transmitted in the downwardly direction which is scattered upwards.
In order that the jamming signal emanating from the jamming trans-
mitter side lobes be smaller than that generated by the terrain scatter, analy-
sis reveals that it is necessary that the side lobe gain in the jammer's
antenna
in any direction be less than the value given by the following equation; the
derivation of which is set out in Appendix B:
2
8sHi=....__ D ~ .......... (II)
~
nL (D2 + 2 HjHi - Hj2)3/2
-T-1 where is the side lobe gain in the direction of the interceptor,
and each of the other symbols have the same meaning as in Equation
(I).
The significance of equations (I) and (II) can best be appreciated
by a discussion of exemplary configurations of the interceptor and the attack-
ing aircraft.
Example 1
Assume that the interceptor is equipped with an antenna of the type
characterized by a principal lobe which falls off rather sharply to about 27
decibels below the beam centre value at just over 4 degrees off the beam
centre, a first rather broad side lobe which peaks at about 6 degrees off the
beam centre at about -- 18 decibels, and a second side lobe which peaks at -
33
decibels at 17 degrees off the beam centre.
Assume further that the interceptor is flying at an altitude of
40,000 ft. and the attacking missile or bomber is flying at 10,000 ft. in such
a configuration, assuming values of 5/K_ = 0.01, G= 1,000 and A = 100 ft.2 it
can be shown using equation (I) and (II) that the maximum jamming power
required
for tne jamming transmitter amounts to only 3.5% of the interceptor radar
trans-
mitter power.
Exa.mple 2
Assume that both attacking aircraft and interceptor are flying at
10,000 ft.; otherwise, conditions are assumed to be the same as in Example 1.
With this corifiguration, the jamming power requirement as determined
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by equations (I) and (II) amounts to 5% of the interceptor radar power at a
range of five miles, and increases rapidly at closer ranges. At more distant
ranges the power requirement is below the 5% figure. The jamming technique
according to the inverition may therefore fail at some close range at which
the
jamming power is insufficient.
Considering the above examples, it appears that the attacking air-
craft ought to maintain a low altitude. However, in so doing it is necessary
to avoid creating a ground source of scattered radiation too close to the
attacking aircraft, and the jamming transmitter might therefore be directed to
a point on the earth's surface not directly beneath the attacking aircraft.
This point should be selected so that if the interceptor follows a "home on
jam" suicide path, the attacking aircraft does not at any time fall within the
principal lobe of the interceptor antenna pattern.
In most tactical situations, the interceptor will try to lock-on its
antenna to the target as early as possible. In the case of the present inven-
tion, therefore, the jamming signal being sufficiently strong at lock-on
ranges,
the interceptor would lock-on to the ground "decoy" and thereafter follow a
descending intercept course. With the angle a(Fig.l) always greater than the
interceptor's antenna beamwidth, the real target would never be seen again,
unless the interceptor changed back to the search mode at a sufficiently short
range. This would never happen in the case of a homing missile, and is, in
fact, unlikely even in the case of a manned interceptor, especially when on a
collision course or in automatic "jam-track" mode. Accordingly, the attacking
aircraft should fly at sufficiently high altitude to ensure that the angle a
be
at all times greater than the interceptor antenna beamwidth.
The jamming transmitter may be of conventional design, having regard
to the assumed interceptor radar characteristics in individual cases. A re-
peater jammer which re-radiates the signal transmitted by the interceptor
radar would be most effective in some instances, as for example against a pul-
sed doppler radar uni.t. In other cases, for example, in most conventional
manned interceptors, a regenerative repeater jamming transmitter may be prefer-
able. An illustrative example of the design of a repeater jammer follows.
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I :~~ ~ ~ D :) 8
Example 3
The power flux from the interceptor's transmitter is given by
pr Ga ...........(III)
where Ga is the antenna gain at angle a off beam centre, and all other
symbols are as defined with reference to equation M.
The input power Pin at the jammer receiver is therefore
Pin - Aj .........(IV)
where A. is the effective cross-sectional area of the jammer receiver
antenna.
The cross-sectional area for an antenna having a beam width 0 of 60
and operating at a wavelength 01- of 3 cm., assuming an antenna gain G of
about
25000, is given by the equation
02
A. G ?~ '~ .., 1 ft.2 .......(V)
~K ~ 1~$0
Combining equations (III), (IV) and (V), it follows that the ratio
of jailing power PJ , t;o received interceptor radar power Pin is given by the
e quat ion
Pj = 8.3 x 1010 Pj D2 (VI)
.....
Pin Pr Ga
If it is further assumed that at worst, Pj = Pr, then the ratio given
by equation (VI) yields a value of about 125 decibels at range 20 miles. In
other woids, the jamming transmitter amplifier output must be 125 decibels
above the input signal received from the interceptor radar transmitter. This
is a realistic possibility having regard to presently available equipment.
Appendix "A"
Consider the jamming aircraft 11 and an interceptor 10 in the config-
uration shown in the drawing.
If the jamming power radiated downwards in the narrow beam 21 is P
(within a bandwidth corresponding to that of the interceptor's receiver), the
total power scattered from the illuminated area of the ground is sPj, where s
is a "scatter loss factor" depending on the frequency, type of the terrain,
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13 4155 8
polarization, etc.
Assuming, for the lack of any better information, Lambert's law for
the distribution of the scattered energy, the power flux density at a unit dis-
tance from the illuminated ground area in the direction of the interceptor 10
(along ray 27) is:
(R) = (0) cos -------(A-i)
where (0) is the flux in the upward vertical direction.
The elementary ring area AA' BB' (Fig 2) is given by
ds = 2-z A'0' dR = 2x sin RdR
Thus the total power scattered
SPj = 2 (R) ds = 2 x (0) cos R sin RdR
R =0
_ -x (o )
Hence
(0) _ spj ------(A-2)
and
(R) - sP- cos ------(A-3)
At the interceptor's antenna the power fl.ux is therefore
(R) = sp. cos -------(A-4)
where r is the distance from the illuminated ground area to the interceptor
10.
As
r' = D2 +Hj2 +2 DHj cos (a+ R)
cos (a + R) = Hi - H- -----(A-5)
D
cos 13 = Hi
r
Rewriting one obtains
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(R) = s Pj Hi (A-6)
~
(D2 + 2 Hj Hi - Hj2)>~ .....=
If the interceptor's radar power transmitted is Pr, antenna gain,G
and the radar is looking towards the target (the worst case from the point of
view of the jamner) the power flux at the interceptor's antenna due to the
target echo is:
7_
PrG A ........(A-7)
y1~ (4 -x D~)2
where A is the echoing area of the jamming aircraft.
If the interceptor's receiving antenna gain in the directions of the
ground and the target; is G(a) and G respectively, jamning can be effective
when
4 (R) G (a) = K G .......(A-8)
r
where the "jamming masking factor" K~ 1.
From (A-6), (A-7) and (A-8), and putting
L = G .......(A-9)
Ga7
one finally obtains the ratio of the janm:ing and radar powers required for
effective jamming:
P (D2 + 2 Hj Hi - Hj2)3/2
~ L .........(A-10)
r [KG LISHiJ A
In this formula, L simply represents the main-lobe to side-lobe ratio
for a side-lobe in the direction of the illuminated ground area as seen by the
interceptor. It has to be evaluated from the radar antenna radiation pattern
for any angle a, involved, determined by the equation:
Sill a = IIj sin R . . . . . . . . . (A-11)
D
Appendix "B"
An ancillaxy requirement is for the jamming signal in the intercep-
tor's receiver causeci by the jammerts side lobes in the direction of the
inter-
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13 41558
ceptor to be smaller than that generated by the ground scatter.
The pot,rer flux F at the interceptor 10 due to the side lobe at the
jamming transmitter is given by:
F = P~'/ ..........(B-1)
where ! is the jammer's antenna side-lobe gain in the direction of the
interceptor, and other symbols are as previously used.
In the worst case, when the interceptor's antenna is looking towards
the jamming aircraft, the signal at the receiver's input due to (B-1) is pro-
portional to G, whereas that due to scatter is proportional to G(a) (these
tems are defined in Appendix "A"), therefore, (from B-1 and A-6):
2 sP H. G(a,) Pr ........ (B-2)
2 (D2 + 2H Hi - Hj2)3 2 ~ D'
or
~ < 8 sHi D2
~''i L .........(B-3)
(D + 2H Hi - 2)5/2
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