Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENI'ION
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Modern military aircraft are designed to have as low
a radar return or cross~section as possible in an attempt
to enable them to evade interception and attack by a
radar defence system during a combat operation. It is
possible to reduce the radar return or cross-section of
an aircraft by modifying its shape r by having the
aircraft made from materials which do not reflect radar
and by using a material which absorbs electromagnetic
radiation to cover those parts which have to be made of a
material which reflects electromagnetic ra~iation.
In spite of the best endeavours which have been made
so far all aircraft still provide some radar return and
the particular return or radar signature which is
obtained from an aircraft is distinctive of aircraft of
that particular type. Modern radar systems analyse the
radar return which is obtained and use this to give an
indication of the type of aircraft. For example, modern
radar compares the radar signature of an aircraft at two
or more different frequencies and analyses the return for
different polarisation states. Phased array radars are
particularly discriminating. Typically, at least part of
the radar return is sampled to monitor changes that take
place as part of its threat assessment. A radar defense
system is usually programmed with the radar signatures of
that country's own, and other friendly aircraft and also
programmed with the radar signatures of those aircraft
which are operated by potential enemies, The radar
defence system is sometimes controlled so that it will
only launch an attack on aircraft which has been
identified as having a radar signature corresponding to
that of a potential enemy. It is relatively easy for any
country to be able to get information on the radar
signature of aircraft operated by its potential enemies
whilst these aircraft are taking part in trainin
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exercises. Thus, at present, it is virtually irnpossible to keep
the radar signature of any aircraft secret and thereby prevent it
being recordecl and built into a radar defence system of a
potential enemy as an enemy aircraft.
~UMMARY OF THE _NVENTIt?N
According to this invention an aircraft includes a radar
reflector arranged whereby when said radar reflec~or is deployed
the radar signature of ~he comblnation of said aircraft and said
radar reflector is yreater than that of said aircraft alone.
When the aircra~t has A very low radar cross-section so
that, on its own it provides a very low radar return/ the present
invention causes the aircraft and radar reflector combination to
have a significant radar cross-section and 50 to give a
significant radar return. The return from the radar reflector
swamps the radar signature of the aircraft alone. This firstly
lulls a potenkial enemy into a false sense of security, since the
potential enemy believes that the aircraft has a significant radar
cross-section and so can be readily identified by its radar
defence system and secondly it ensures that the radar signature
that is programmed into a radar defence system of a potential
enemy correspQnding to tha~ particular varie~y of aircraft is that
of the aircraft plus its radar reflector. Thus, the potential
enemy does not have any record of the radar signature of the
aircraft alone. The aircraft whils~ exercising always has the
radar reflector deployed and it is only when the aircraft is on a
combat operation that the radar xeflector is not deployed so
taking a potentlal enemy by surprise both in the smallness of the
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radar cross-section of the aircraft and the corresponding weakness
in the radar return that is obtained from it and by the particular
radar signature being one which is not programmed in t;o their
radar de~ence sy~tem.
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Another way in which the present invention is used
is to have the radar reflector modifying the existing
radar slgnature of an aircraft In this way, it is
possible to have the combination of aircraft and radar
reflector provide a slgnature which is different to that
of the aircraft alone. When arxanged in this way the
aircraft normally would have the radar reflector not
deployed so that, whilst exercising the aircraft would
give its "normal" radar signature and then only deploy
the radar reflector when on a combat operation so that,
under these circumstances, it provides a signature which
is not already programmed into the enemy's radar defence
system and so does not enable the enemy's radar defence
system to identify whether the aircraft is a friend or
foe.
It is very much preferred that the radar signature
of the combination of the aircraft and radar reflector
corresponds to that of an aircraft of a potential enemy.
In this way, the enemy's radar defence system picks up an
attacking aircraft but identifies it as one of its own
and so does not attack it.
Preferably the radar reflector includes an array of
trihedral re-entrant corner reflecting elements oriented
relative to one another so that a radar beam incident
upon the array is reflected from more than one reflecting
element at the same time. In this way the beams
reflected from the more than one element interfere with
one another to provide a fine structure in the reflected
signal which corresponds closely to that reflected from
an aircraft as a result of interference between
reflections from different parts of it.
Modexn radar systems often use a particular
polarisation state for the radar signal partly to
discriminate a true echo from a potential attacker or
target from background clutter caused, for example, by a
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rough sea or surrounding surface features and partly to
be able to identify an attacker or target from a signal
generated by electronic countermeasures fitted to the
aircraft or by echoes given off a radar reflecting decoy
like chaff. Conventionally chaff is formed by thin
strips of metallised foil of dipole length which are
ejected from an aircraft. The response from a cloud of
chaff is isotropic. The echo from a real aircraft is
invariably polarised in some way or another as a result
of multiple reflections from two or more surfaces of the
aircraft. Thus when a polarised radar signal interacts
with an aircraft the return echo and hence its radar
signature will vary with the polarisation state of the
radar used. In view of this it is possible for a radar
system to discriminate between a false echo generated by
chaff or decoys and that resulting from a real aircraft
by monitoring its polarisation state.
Accordingly it is preferred that the radar reflector
also includes dihedral re-entrant corner reflecting
elements.
A radar reflection from a dihedral re-entrant corner
is strongly polarised in the plane containing the
dihedral corner between the two surfaces which make up
the dihedral element, By mixing different types of
reflecting elements and varying their mutual orientation
and phase relationship it is possible to tailor the radar
signature of the reflector so that it corresponds to that
of an aircraft. Additional lateral plates may be
included on one or both faces of each dihedral element
which extend normally or at an angle to the faces.
Naturally when it is desired that the radar
signature of the combination of the aircraft and the
reflector has a particular configuration, the reflection
from ths radar reflector and its location on the aircraft
must be arranged so that the reflection from the
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reflector interferes with that from the remainder of the
aircraft to give the required radar signature for the
combinationO In some instances it may be desirable to
have more than one radar reflector located on the
aircraft so that the relative location of the two or more
provides a particular phase relationship between them.
Modern radar is often of the so-called agile type
which rapidly switches between differ~nt frequencies as
it scans. This rapid switching between frequencies is
partly to overcome the effect of electronic counter
measures and partly to help it to discriminate between a
cloud of chaff ejected from a potential target as a decoy
and the target. In practice it is very difficult to
attempt to match the length of the pieces of chaff with
the dipole length of the wavelengths of an agile radar
system.
A radar reflector with reflecting elements of
constant size would give a proportionally different
response to radar at different frequencies. It is
therefore preferred that the radar reflector includes an
array of trihedral re-entrant corner reflecting elements
with the size of some of the elements being different to
that of other of the elements in the array.
When a radar reflector includes trihedral reflecting
elements of different size it provides a radar reflection
which corresponds to that of an aircraft over a wide
range of incident radar wavelengths.
The different size of the trihedral re-entrant
corner reflecting elements may be created by having the
separator plate located between each pair of faces in
each dihedral element being offset from the centre of the
corner between adjacent faces of each dihedral element to
provide a large and a small trihedral re-entrant corner
reflecting element in each dihedral element.
Alternatively each separator plate is located
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substantially centrally along the corner between the two
faces of each dihedral element and it is the size of the
faces that make up each dihedral element that differs.
When the radar reflector ls foldecl from sheet material it
may be folded from a sheet of material the size of which
tapers so that the si2e of each trapezium-shaped plate
increases from one end to the other to provide a
reflector in which the size of each trihedral re-entrant
corner reflecting element is different from one another.
Reflectors can also be made so that the largest size of
reflecting element is in the middle of the reflector or
at both ends of the re~lector so that it is "waisted" in
the middle.
The or each radar reflector may comprise a number of
trihedral re-entrant corner reflecting element strings
with the reflecting elements in each string being located
on the surface of a solid of revolution having an axis
and the strings being connected together to form a single
reflector with the axes of the strings being clisplaced
from one another.
The axes of the Strings of reflecting elements may
each be generally straight and in this case their axes
may be arranged substantially parallel to one another or
may bé arranged at an angle to one another. When the
axes of the strings of reflecting element arrays are
generally parallel to one another the return echoes from
the strings reinforce one another and so reflect a
greater quantity of radar signal to provid~ a strong
echo.
Alternatively the radar reflector may comprise a
single string of trihedral re--entrant corner reflecting
elements arranged so that their origins are located on
the surface of a solid of revolution having a curved
axis. The curved axis may comprise a circle and in this
case the reflector occupies a toroidal volume.
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Preferably, however, the axis of the solid of revolution
does not lie in a single plane and preferably it
comprises a helix or sinuous path around the surface of a
sphere, elipsoid or other solid having a curved outer
surface.
With such a radar reflector the individual
reflecting elements that reflect an incident beam do not
all receive the incident radar wavefront at the same
instant and therefore a phase difference exists betwee~
their reflections. This gives rise to constructive and
destructive interference between the overlapping
reflected beams and this generates a fine structure in
the radar signature of the reflector which closely
resembles that obtained from an extended object such as
an aircraft. Thus the radar return obtained from such a
radar reflector mimics that obtained from an aircraft
closely.
One tell-tale radar analysis of an aircraft involves
monitoring the radar echo for any doppler shift, not just
to indicate movement of the aircraft, but to recognise a
particular aspect of the aircraft. The radar return from
an aircraft engine intake fan or any other rotating part
may cause a doppler shift when head-on or tail-on to a
radar source. The nature of this doppler behaviour is
not related to the speed of the target, but more to the
geometry and speed of engine parts. It is not a
consistent doppler shift such as that related to target
velocity, but a mixed doppler return of faster and slower
indications. The object of the aircraft designer is to
attempt to minimise this mixed doppler return but it is
difficult to eliminate i~ entirely and accordingly this
is often used as a way of discriminating between a real
target and a decoy.
~hen a radar reflector having reflecting elements
arranged ~ith their origins on the surface of a solid of
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revolution is rotated it provides such a mixed radar
return. The rotation may be provided by mounting at
least part of the reflector for rotation and providing a
drive to ro-tate it. It is not necessary for the
reflector to spin at the same speed as a gas turbine to
get a sufficiently confusing effect. With such an
arrangement it is possible to mimic the radar signature
of a real aircraft so that it is substantially
indistinguishable.
Preferably the or each radar reflector is arranged
to fold so that all of its elements are substantially
flat. When folded a reflector has a very small radar
cross-section and thus, in this state is not dep]oyed.
The or each reflector may be spring biassed into its or
their erected and deployed position so that it or they
can be erected automatically. It is also possible to
change the shape and configuration of the radar reflector
by folding part of it so that it changes its radar
signature. In thi way the or each refl~ctor can have two0 or more different reflective states which it can deploy.
BRIEF DESCRIPTION OF_THE DRAWINGS
Particular examples of an aircraft and a radar
reflector for use with an aircraft having a low radar
signature is shown in the accompanying drawings, in
which:-
Figure 1 is a perspective view of an aircraft;
Figure 2 is a plan of a main part of a first exampleof reflector;
Figure 3 is a side elevation of the main part of the
first reflector;
Figure 4 is a plan of one segment of the main part
of the first reflector with attached separator plate
drawn to a larger scale;
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Figure 5 is a cross-section through part of the
first example of reflector in an assembled condition and
drawn to a larger sca].e;
Figure 6 is a cross-section drawn to an even larger
scale through part of the material used to form the first
example of reflector;
Figure 7 is a perspectlve view of a second example
of reflector;
Figure 8 is a scrap perspective view of a
modification of the second example of reflector;
Figure g is a scrap perspective view of another
modification of the second example of reflector;
Figure 10 is a perspective view of a third example
of reflector; and,
Figures lla to lld are a series of plans uf strips
before folding to make other examples,
DESCRIPTION OF PREFERRED EXAMPLES
Figure 1 shows an aircraft 1 having two radar
reflectors used to provide the significant radar return
for the aircraft 1 which, otherwise, would have a very
low radar cross-section. The return from the reflectors
2 is used to mask the radar signature of the aircraft 1.
The reflectors 2 are preferably covered by radar
transparent radomes ~not shown~. Particular examples of
reflectors suitable for use as the or as one of the
reflectors 2 are descr.ibed alone subsequently,
The first example of reflector is made from a core
of rigid foam material 11 having metallised polymer film
12 bonded onto both faces as shown in Figure 6. A main
part 13 is formed by six identical elements T2 to T7
having one curved side and arranged so that alternate
elements are laterally reversed as shown in Figure 2.
Two further identical elements Tl and T8, again laterally
reversed relative to one another and again having one
curved face are arranged at the ends of the elements T2
13~
to T7. The main part 13 is arranged to fold along ~he
lines between adjacent elements Tl to T8 with the folds
Fl, F3, F5 and F7 being in one direction and the
remaining folds belng in the opposite direction. The
S elements are folded so as to lie at 90 to their
neighbours as shown in Figure 5. Separator plates S are
hinged onto one side of each of the elements Tl to T8 as
shown in Figure 4O As the elements are foIded the
separator plates S are also folded so as to lie
substantially normally to a particular element upon which
they are located again as shown most clearly in Figure 5.
Springs 14 of rubber, or rubber-like elastomeric material
are threaded through slots 15 and holes 16 and tend to
pull the separator plates and main part 13 into their
assembled position. However, the rubber or rubber-like
elastomeric springs 14 can be stretched to allow the
reflector to fold flat.
When in its erect position~ a pair of re-entrant
trihedral reflecting elements are formed between each
adjacent pair of elements Tl to T8 and the corresponding
separator plate S. There is one re-entrant trihedral
reflector formed on each side of each separator plate S.
In Figure 2 the chain dotted line shows the final
position of the separator plates S on one side of the
elemen*s whilst the do~ted line shows the final position
of the separator plates S on the opposite side of the
elements. The curved edges on the elements Tl to T8 and
separator plates S provide a cylindrical outer profile to
the assembled reflector.
A second example shown in Figure 7 comprises an
upper section 21 which is essentially a standard
trihedral re-entrant corner reflector such as described
in British patent specification 681,666 and a lower
section 22 which is formed by a series of
35 trapezium-shaped plates 23, 24, 25, 26, 27, 28, 29, 30,
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31 and 32 which are an~led at 90 to one another and form
a series of nine dihedral reflecting elements. Typically
the reflector i5 folded from a strip of sheet aluminium
with the separator plates connected to it with blind
rivets (not shown).
A modification of this example is shown in Figure 8.
In this modification additional lateral plates 33 are
located on the trapezium-shaped plates 28 and 29. Such
lateral plates may be included in all or selected ones of
the plates 23-32. Again they may be connected by blind
rivets. The radar reflection from the second example is
polarisation dependent.
A further modification is shown in Figure 10. In
this modification separator plates 34 in the trihedral
re-entrant corner upper section 21 are displaced from a
central location shown in Figure 7 to an off-centre
location.
In the third example three standard trihedral
re-entrant corner reflectors 41 such as those described
in British patent specification 681,666 are connected
together side-by-side by links 42 with their axes
parallel to one another. Each standard reflector 41 is
optimised to give its maximum reflection in a direction
away from its neighbours to maximise the reflections from
the complete reflector.
The strips of material shown in Figure 11 are marked
with lines indicating where they are folded. The solid
lines represent a fold with the arris facing out of the
plane of the paper and the dotted lines indicating a fold
with the arris facing into the plane of the paper.
Folding such strips to provide trapezium-shaped faces so
that an angle of 90 is included between adjacent
trapezium-shaped faces provides the basic element of a
trihedral re-entrant corner reflector as described in
British Patent Specification 681,666 and as shown in
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Figures 7 and 10. Separator plates (not shown in this
Figure) are installed between each palr of adjacent
trapezium-shaped plates and are typically located
centrally along the corner extending between adjacent
S trape~ium-shaped plates but they may be offset and
located towards one side edge or the other side edge of
the strip. In this way, trihedral re-entrant corner
reflecting elements of different size can be produced.
Figure lla illustrates a blanlc for forming a
}0 reflector which is made from a blank the sides of which
are curved. When this blank is folded as indicated in
Figure lla a generally cork-screw or helically shaped
radar reflector is produced. The origins of the
trihedral reflecting elements lie on the surface of a
solid of revolution having a helical axis.
Figure llb illustrates a blank in which different
and unrelated curves are used for each side of the blank.
The radar reflector resulting from this ~orms a tapering
cork-screw with different sized reflecting elements
located along its length.
Figure llc is folded to produce a reflector which is
shaped like a helix which reverses its direction of
rotation half way along. The origins of its trihedral
reflecting elements lie on the surface of a solid of
revolution which has a helical axis which again reverses
its direction of revolution half way along.
Figure lld is folded to produce a reflector which is
similar to that shown in Figure llc but the pitch of
which varies.
Multiples of any of the structures shown in Figure
11, or combinations of them may be used in combination to
give complete structures similar to that shown in Figure
10 .
