LEURRE POUR SYSTEMES RADAR DOPPLER

DECOY FOR DECEIVING DOPPLER RADAR SYSTEMS

Abrégé français

La présente invention concerne un leurre pour tromper des systèmes de radar Doppler. Ce leurre comprend un réflecteur d'angle dont au moins une des surfaces (1) est disposée pour pouvoir obtenir une réflectivité variable pour la radiation radar à fréquence de modulation, qui dans la radiation réfléchie entraîne des bandes latérales Doppler d'étendue habituelle pour l'application radar.

Abrégé anglais

The present invention relates to a decoy for deceiving Doppler radar systems. The decoy comprises a corner reflector, where at least one of the surfaces (1) is arranged to be able to obtain a varying reflectivity for radar radiation with a modulation frequency, which in the reflected radiation causes Doppler sidebands of an extent that is usual for the radar application.

Revendications

6
Claims:
1. A decoy for deceiving Doppler radar systems, characterised in
that it comprises a comer reflector where at least one of the surfaces (1) is
adapted
to be able to obtain a varying reflectivity for radar radiation with a
modulation fre-
quency which in the reflected radiation causes Doppler sidebands of an extent
that
is usual for the radar application.
2. The decoy as claimed in claim 1, characterised in that the modu-
lation frequency is adapted to be variable.
3. The decoy as claimed in claim 2, characterised in that the modu-
lation frequency is adapted to be randomly variable.
4. The decoy as claimed in any one of claims 1-3, characterised in
that the surface (1), whose reflectivity can vary, comprises a conducting
surface
having a slotted pattern, said surface being separated from a second
conducting
surface via a dielectric, an element with a varying impedance being connected
across the respective slot, said elements being fed by a varying voltage, so
that a
varying reflectivity in the surface will be achieved.
5. The decoy as claimed in any one of claims 1-3, characterised in
that the surface (1), whose reflectivity can vary, comprises a non-reflecting
surface
provided with a check pattern of lines arranged so close together that, if
they are
electrically interconnected in the crossing points, the check pattern reflects
the
incident radar radiation, and that each crossing point of the check network is
provided with a switching element which alternatingly can electrically connect
the
lines and electrically disconnect the same.
6. The decoy as claimed in claim 5, characterised in that the
switching element comprises four diodes (5) in a diode bridge conducting
current
from one conductor to three other conductors, and that the check pattern of
lines is
adapted to be supplied with a square wave voltage between two opposite sides,
viz.
between the side from which direction the diode bridge conducts current and
the
opposite side of the check pattern of lines.
7. The decoy as claimed in claim 6, characterised in that the perma-
nently reflecting surfaces (2) comprise a reflecting foil and the surface or
surfaces

7
(1) having a variable reflection comprise a line-etched dielectric, where the
diode
bridges are arranged in the crossing points of the lines.
8. The decoy as claimed in any one of the preceding claims, characterised
in that, especially for use as air-borne decoy for protecting the aircraft,
all
surfaces are made of a flexible, foldable material, and that the decoy in the
storage
state is folded before being put into use.
9. The decoy as claimed in claim 8, characterised in that it is
enclosed by a flexible closed casing (7) of the balloon type and provided with
an
inflation device, which in operation transforms it from the storage state to
the state of
operation.
10. The decoy as claimed in claim 9, characterised in that the infla-
tion device uses a light inert gas, such as helium, which gives an extended
time of
function in its action as an air-borne decoy.

Description

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Decoy for deceiving Doppler radar systems
The present invention relates to a decoy for deceiving Doppler radar systems.
Decoys in all forms have constituted and still constitute an important
component for
deceiving the many sensor systems of war, anything from the eyes of the
individual
soldier to the ground or air-bome radar system.
Great efforts have been devoted especially to decoys for deceiving radar
systems
since the object to be protected, in many cases an aircraft, is of
considerable military
value. Chaff (bundles of strips) has previously been used as decoy for
deceiving
radar. If the metallised strips are of a length which is suitably adapted to
the radar
frequency of the radar that is to be misled, a strong resonance is obtained.
The
strips that are dispersed from aircraft in bundles then cause echoes that can
mislead
the radar or conceal the aircraft.
The introduction of pulsed Doppler radar dramatically reduced the capability
of chaff
to influence the radar. A pulsed Doppler radar uses the Doppler effect (phase
vari-
ation from pulse to pulse in the radar echo) to distinguish reflecting objects
moving
fast in relation to the radar station and stationary objects. As a result,
ground clutter
and also chaff that is almost immobile in relation to the ground can be
rejected. The
use of Doppler radar systems for rejecting ground echoes therefore renders the
capability of the bundle of strips of effective misleading impossible.
Other passive methods for confusing radar use reflectors of different kinds,
for
instance comer reflectors or Luneburger lenses to produce powerful echoes from
small objects. To produce the necessary Doppler frequency that permits
detection in
a Doppler radar, these must then be hauled or accommodated in small decoy air-
craft which can separate from the object to be protected. This requires
aerodynami-
cally well designed units and, moreover, in many cases restrictions in the
flight
appearance.
Modem decoy solutions often consist of active jamming transmitters which are
launched from the aircraft or hauled thereby. A pure amplification and
transmission
of the radar pulse cannot be canied out with isotropic transmitting and
receiving
antennae owing to insufficient insulation (results in so-called feedback).
Other active
solutions using e.g. microwave memory and delayed transmission result in
distortion
of the pulse shape. Narrow band jamming as well as wide band jamming are
known.

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Equipment for jamming by narrow band noise is sensitive to a frequency change
of
the radar and requires equipment for searching over the frequency band for the
new
frequency. Wide band noise requires high power output. All in all, active
decoys will
necessarily be relatively expensive and complicated equipment.
The present new passive decoy solution eliminates all the restrictions that
are con-
nected with traditional passive and active decoys. Such a decoy in the form of
a
modulated comer reflector has a combination of properties which is new in the
context and which comprises:
= Not filterable in a Doppler radar system,
= reflects any wave form correctly,
= isotropic radiation diagram,
= low power consumption (almost passive)
= size and price at a level allowing launching of showers (5 -10 pieces) at a
time
(may be regarded as a modem form of Doppler chaff).
These decoys should be usable in different contexts, for instance:
= Launching of decoys for misleading enemy radar missiles, air-bome or ground
fire-control radar,
= mass-launching of decoys for masking flight operations against air-bome or
ground reconnaissance radar,
= placing of decoys on the ground for activation in and thus masking of low
altitude flying operations in prepared corridors,
= placing of decoys on the ground close to objects to be protected to render
dis-
covery of these objects by using high-resolution mapping radar impossible.
The desired properties are achieved in the invention by designing it as is
apparent
from the accompanying independent claim. Suitable embodiments of the invention
are defined in the remaining claims.
The invention will now be described in more detail with reference to the accom-
panying drawings, in which:
Fig. 1 illustrates a corner reflector where one of the three surface planes
constitutes a modulatable plane of reflection,

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Fig. 2 shows the composition of the modulatable plane of reflection in the
form
of a wire structure which in the crossing points is connected by a diode
structure, and
Fig. 3 shows an activated decoy for air-bome application with protective
casing
and box for support electronics and battery.
The decoy consists of a radar-cross-section-modulated comer reflector
according to
Fig. 1, where two surfaces 2 are metallised and thus fully reflective. The
reflection of
the third surface 1 may be varied, which implies that the total decoy surface
is modu-
lated. The radar-cross-section-modulation will be seen in all directions of
incidence
except in parallel incidence with the modulated surface.
Such a radar-cross-section-modulation involves an amplitude modulation of the
pulse train of the radar, which generates symmetric Doppler sidebands on both
sides of the base frequency. The base frequency is the Doppler-shifted radar
frequency. The sidebands are separated with modulation frequency. After
launching,
the decoy will quickly assume wind velocity, and therefore the Doppler
frequency will
be low compared with aircraft. Since the modulation is carried out as a square
wave
variation, this implies for all pulsed Doppler radar systems (LPD, MPD and HPD
systems) that a plurality of modulation tones, above as well as below ground
retums,
are to be found in the passband active for the radar. Besides, if the
modulation
frequency is varied (swept), said tones will migrate in a natural fashion in
the field of
analysis of the radar.
A launching situation which is suitable for an aircraft is when tuming through
the
0-Doppler (transverse course relative to lobe direction), since a Doppler
radar will
then be forced to reject also the target, and the probability of relocking on
the decoy
is great. By sweeping the modulation frequency, also the probability of
penetrating a
narrow Doppler filter of the homing type for semiactive radar missile
increases.
Besides, the possibility of analysing and rejection of the decoy based on the
meas-
ured frequency will be prevented. Therefore, the modulation frequency should
suita-
bly be swept in the typical Doppler area close to the 90-degrees-sector
position, for
instance from 0 to 9 kHz on X-band. The sweeping velocity should correspond to
a
typical aircraft operation seen in Doppler frequency, for instance 3 kHz/s on
X-band.
A further convenient launching procedure involves the increasing of the
distance
uncertainty of the radar by active noise, whereupon the noise jamming is
interrupt-
ed at the time of launching, and the radar locks on the decoy.

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In contrast to many other repeater jamming systems, reflection against the
decoy
takes place without the pulse form and the wave form otherwise changing. This
implies that radar systems having different wave form techniques (for
instance,
different pulse compression techniques) will receive echo retums which conform
with
the retums from physical targets. Thus, such echo retums cannot be readily
distin-
guished as false ones.
The controllable surface may consist of lines in a check pattem according to
Fig. 2,
where each cross 4 in the check pattern is connected by a switching element.
The
switching element may consist of a diode bridge 5. The diodes can be PIN
diodes.
When the surface is supplied with a square wave voltage 3 with modulation fre-
quency, the line pattem will be interconnected and the surface reflective in
forward
voltage. In reverse voltage, the line pattern will be broken and the surface
assumes
a significantly lower reflection coefficient.
The diode bridge 5 according to Fig. 2 may consist of four diodes, where the
diodes
are arranged such that, in forward voltage, current is conducted from the
upper arm
into the three other arms. In this position, both vertical and horizontal
lines will thus
be conducting and the surface as such will be strongly reflecting. In reverse
voltage,
all diodes, however, will be operated in reverse voltage and no current flows
in the
line pattem. The surface will assume a pattem of dipoles which, if they are
shorter
than half a wavelength of the incident radar frequency, give the surface its
low
reflection. It should be noted that this special diode constellation means
that the
entire surface can be operated by a very simple feeding network that does not
interfere with the conductor network for radar-cross-section-modulation.
The decoy can be optimised for various frequency ranges. The following dimen-
sioning can be suitable for X-band:
= Distance between switching elements 7 - 10 mm,
= controllable surface 30 * 30 cm,
= number of switching elements 900,
= power consumption <1,5 W.
This results in a decoy surface corresponding to about 10 m2.

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Decoys of the type that is intended to be launched from aircraft should be
charge-
able in spaces for standard-type launchers. For this reason, both the two
conductive
surfaces and the modulating surface can be made of a flexible, foldable
material,
e.g. a foil-prepared fabric or a line-etched flexible dielectric. To the
latter, the diode
5 bridges have been applied by automatic soldering. The surfaces and the
support
electronics with battery are packed in a box of the size 100 - 200 cm3. In the
launching moment, a gas cartridge is activated, which develops a protective
casing 7
(balloon, cf. air bag) which in tum fixes the reflector planes according to
Fig. 3. The
support electronics and the battery 6 constitute a stabilising weight, such
that the
modulating surface 1 after stabilisation is vertical and thus minimises the
risk of
situations with radar reflection below a low modulation index. The gas
cartridge can
suitably contain some light inert gas, for example helium, which extends the
time of
function in the air.
The design of decoys for ground use can be made considerably simpler with
rigid
planes of reflection and a simple plastic cover as radome. The basic rules for
inter-
ference action against Doppler radar follow the above description in all
essentials.
Attack and reconnaissance systems which utilise the fact that different ground
elements within the main lobe of the antenna get a varying Doppler frequency
for
Doppler beam sharpening can also be interfered with by the proposed decoy. A
random frequency control should then suitably be selected to interfere with
the
Doppler fiitration of the radar. By arranging a number of decoys around ground
objects which deserve protection, information on details may be concealed and,
consequently, identification and combating can be rendered difficult.
Above an embodiment of the invention is discussed, in which the controllable
surface comprises lines in a check pattem. An altemative way of producing this
surface is to use a conducting surface having a slotted pattern being
separated from
a second conducting surface via a dielectric. (In a similar way as a printed
circuit
with a metallised surface on both sides.) Across the respective slot an
element with
a varying impedance is connected, e.g. a diode. If the diodes are fed by a
varying
voltage, a varying reflectivity in the surface will be the result. The
function will be the
same as for the embodiment of the decoy discussed above.

Dessin représentatif