WO2018185477A1

WO2018185477A1 - Deployable radar decoy

Info

Publication number: WO2018185477A1
Application number: PCT/GB2018/050895
Authority: WO
Inventors: Ioannis Vagias; David BACCI
Original assignee: Cranfield University
Current assignee: Cranfield University
Priority date: 2017-04-03
Filing date: 2018-03-29
Publication date: 2018-10-11
Anticipated expiration: 2019-10-03
Also published as: GB201707142D0; GB2561252A

Abstract

A passive radar decoy (301) for deploying from a magazine on an aircraft, the decoy comprising an elongate body (302) having at least one radar reflective surface (316, 320) and a plurality of fins(304) that project from the elongate body.

Description

Deployable Radar Decoy

This invention relates to a deployable radar decoy which may be used to lure radar trackers and missile seekers missiles away from a dispensing aircraft.

An aircraft that is being tracked by a radar missile seeker may use various decoy devices to lure the seeker's boresight away from it. Such decoy devices may operate by presenting a larger radar cross section than the aircraft, such that the seeker locks onto the decoy and not the aircraft.

One type of decoy uses a receiving antenna, a wideband receiver, a digital radio frequency memory, a modulator/synthesiser, a transmitter and a transmitting antenna. Radar signals are detected by the receiving antenna and the decoy transmits manipulated signals back to the seeker which emulate a false target ahead or behind the aircraft. Such devices are expensive, rely on a large number of interrelated components functioning correctly and are typically towed by the aircraft on a tow line, restricting the distance between the aircraft and the decoy, and also the manoeuvrability of the aircraft.

Alternatively, actively emitting decoys may be expended permanently from an aircraft. Such decoys emit a signal for a short period of time, as on-board battery power permits, as it falls to the ground. Thus, actively emitting decoys require an on-board power supply. This can be provided with an on-board battery, however this has a limited energy, and thus restricts the time period in which the decoy can emit a sufficiently powerful signal to serve as a decoy. An active decoy can be powered from an aircraft, however this limits the distance which can be created between the decoy and the aircraft, as a power-supply link must exist between the decoy and the aircraft. It also required a power-supply link to be stored on the aircraft.

Aircraft also deploy chaff as a radar countermeasure, which are small pieces of radar-reflective material, such as aluminium, dropped from aircraft to scatter radar signals. Chaff only scatters radar signals and does not create plausible decoy targets. The present invention seeks to provide an improved deployable radar decoy.

According to a first aspect of the present invention, there is provided a passive radar decoy for deploying from a magazine on an aircraft, the decoy comprising an elongate body having at least one radar reflective surface and a plurality of fins that project from the elongate body.

According to a second aspect of the present invention, there is provided a method of deploying a passive radar decoy, the decoy having an elongate body with at least one radar reflective surface and a plurality of fins that project from the elongate body, the method comprising deploying the decoy from a magazine on an aircraft.

It can be seen that the present invention comprises a passive decoy which is stored in and deployed from a magazine on an aircraft. The radar decoy has an elongate body with one or more radar reflective surfaces. The decoy also has fins that project from the body.

The skilled person will appreciate that the radar reflective surface provides a lure to radar missile seekers, which takes the missile seekers away from the aircraft.

As the decoy is passive it does not require a power source. The decoy is hence not required to be linked to an aircraft for a power source or have a low, time-limited source of power from a battery. It may be expended permanently from the aircraft and provide a radar cross section for an extended period of time.

Furthermore the plurality of projecting fins help to permit the decoy to glide, thus, for example, enabling the decoy to emulate the trajectory of an aircraft. These features help to provide a more realistic radar decoy than those found in the prior art, thus increasing the chance that the decoy will lure a radar missile seeker away from an aircraft.

Also, the decoy's use of a (e.g. pre-existing) magazine on the aircraft to store and deploy the decoy is particularly convenient and may, for example, mean that it is unnecessary to provide an additional, dedicated deployment mechanism for the decoy.

The aircraft may be any suitable and desired aircraft, e.g., a plane or a helicopter. In one embodiment the aircraft is a (e.g. manned or unmanned) fixed wing aircraft. Preferably the aircraft employs radio-frequency signature reduction techniques. For example, preferably the aircraft projects an average radar cross section of less than 0.03 m² in the X-band of radar frequencies (i.e. between 8 GHz and 12 GHz). The elongate body is preferably substantially cylindrical, e.g. comprises a cylindrical section for at least the majority of its length (in the direction in which it is elongate). An approximately cylindrical body is aerodynamic. Preferably the elongate body comprises a nose cone. The nose cone may have any suitable and desired shape. In a preferred embodiment the nose cone has a spherically blunt tangent ogive shape. This shape helps to maximise the available volume for the accommodation of the reflective surface(s) (e.g. corner reflectors) and helps to minimise the drag at transonic and subsonic flight regimes, and helps to allow safe separation from the deploying aircraft. Preferably the elongate body comprises one or more (e.g. a plurality of)

aerodynamic lifting surfaces. Preferably the aerodynamic lifting surfaces are arranged to generate one or more of a pitching, a rolling and a yawing moment when the radar decoy is in flight. The decoy is preferably stored in a magazine on-board an aircraft, e.g. in a flare dispenser, from which it can be released into the airstream below or behind the aircraft, depending on the position and orientation of the flare dispenser. The decoy may be stored in a container (e.g. in the magazine) with the same dimensions as an aircraft's stored flares, so as to be easily mounted on existing aircrafts. The

Typhoon aircraft, for example, stores cylindrical flares of length 200 mm and diameter 55 mm in its magazines.

According to an embodiment of the present invention, the at least one radar reflective surface comprises retroreflectors, e.g. corner reflectors. Preferably the retroreflectors comprise trihedrals and/or dihedrals. Retroreflectors have a large radar cross section from a range of incident angles and can fit into a compact decoy design. Preferably the facets of the retroreflectors are arranged perpendicularly to each other. This helps to maximise the radar cross section of the at least one radar reflective surface.

The radar reflective surface(s) (e.g. the retroreflectors) may comprise any suitable and desired material. Preferably the radar reflective surface(s) comprise a conducting material, e.g. a highly conductive material such as metal. In one embodiment the radar reflective surface(s) comprise copper, e.g. solid copper or a copper plated material (e.g. copper plated polymer). Copper is highly conductive and therefore highly reflective; copper is also relatively inexpensive and relatively dense.

According to an embodiment of the present invention, the elongate body of the decoy comprises a rigid body. The rigid body helps the decoy to maintain its shape and structure when deployed, including when deployed from high-speed aircraft, e.g. preferably the decoy is strong enough to withstand the aerodynamic forces and moments during free flight. Preferably, the elongate (e.g. rigid) body (which is, e.g., cylindrical with a nose cone) comprises a (e.g. non-conducting) two-way radio-frequency translucent (e.g. transparent) material, with the radar reflective surface arranged inside the two-way radio-frequency translucent material, e.g. the two-way radio-frequency translucent material of the elongate body comprises a cover (e.g. sleeve) around the radar reflective surface, e.g. forming the external surface of the elongate body. Preferably the two-way radio-frequency translucent material (e.g. the cover) of the elongate body comprises a cylindrical section and a nose cone section, which may, e.g., be formed as separate sections. Preferably the elongate body (e.g. the two-way radio-frequency translucent material thereof and/or the radar reflective surface) is hollow. The two-way radio-frequency translucent (e.g. transparent) material causes minimal attenuation to incident and reflected radar. This helps the reflective surface inside to be visible to radar. This helps the decoy have a large radar cross section, enhancing its ability to lure radar seeking missiles away from aircraft. Preferably the two-way radio-frequency translucent (e.g. transparent) material is translucent (e.g. transparent) to radio- frequency waves.

The (e.g. non-conducting) two-way radio-frequency translucent (e.g. transparent) material may comprise any suitable and desired material. Preferably the two-way radio-frequency translucent (e.g. transparent) material comprises a high density polymer, e.g. a polyoxymethylene, e.g. an acetal (e.g. ertacetal). Polyoxymethylene is translucent to radio-frequency waves. The (e.g. non-conducting) two-way radio-frequency translucent (e.g. transparent) material may be translucent (e.g. transparent) to any suitable and desired range of radio (i.e. radar) frequencies, e.g. e.g. between 5.25 and 36 GHz, e.g. between 8 GHz and 12 GHz. According to an embodiment of the present invention, the decoy comprises a plurality of (e.g. modular) sections. Preferably each section comprises its own at least one radar reflective surface, e.g. comprising retroreflectors as described above. This allows the design on each section to be different, e.g. to have a (e.g. different) radar cross section that may be chosen as a function of azimuthal and elevation angle. Thus preferably the radar reflective surface(s), e.g. the

retroreflectors, in each section are arranged to present a different radar cross section (to the radar cross section of the reflective surface(s) in the other section(s)), e.g. they are offset from and/or rotated with respect to each other. In a preferred embodiment the decoy comprises four sections. The, e.g. modular, sections may be separately or (preferably) integrally formed.

Preferably, a first section of the decoy comprises a nose cone section at the front of the decoy. Preferably the first section comprises retroreflectors, e.g. trihedrals. This allows the front-facing side of the decoy to be radar reflective, helping the decoy to reflect radar incident from ahead of the decoy.

Preferably, a second section of the decoy (e.g. between the first and third sections) comprises a body section comprising trihedrals, e.g. arranged with alternate trihedrals symmetrically inverted relative to each other. Preferably, a third section (e.g. between the second and fourth sections) comprises a body section comprising dihedrals and trihedrals. Preferably, a fourth section (e.g. adjacent the third section) comprises a body section comprising dihedrals, trihedrals and a recess. The recess is situated at the rear of the decoy and may be used for receiving an ejection cartridge for ejecting the decoy from the aircraft. The rear-facing side of the decoy (e.g. the recess) preferably comprises a radar reflective surface, which helps the decoy to reflect radar incident from behind the decoy.

In a preferred embodiment the plurality of projecting fins comprises a plurality of canard fins (located towards the front of the decoy) and/or a plurality of tail fins (located towards the rear of the decoy). Preferably the decoy comprises only a plurality of tail fins (and thus, in some embodiments, does not comprise canard fins).

Preferably, one of the sections, e.g. the second section, comprises the canard fins (when provided). However, as indicated above, embodiments are also envisaged in which the decoy does not comprise canard fins, e.g. it only comprises tail fins.

Preferably, one of the sections, e.g. the fourth section, comprises the tail fins (when provided). According to one embodiment, (e.g. each of) the plurality of projecting fins are deployable. This enables, e.g. when the decoy is stored in the magazine on the aircraft, the deployable fins to be stored in the body of the decoy. This helps to reduce the volume of space required to store the decoy on board the aircraft and/or to maximise the area of the radar reflective surface for a decoy that is to be stored in a (e.g. standard size) magazine. Preferably the deployable fins are arranged to be deployed when the decoy is released or ejected from the aircraft, i.e. such that they project from the elongate body of the decoy. ln another embodiment (e.g. each of) the plurality of projecting fins are fixed (i.e. not deployable). Thus preferably the plurality of projecting fins form a solid unit with the elongate body. In a particularly preferred embodiment the part of the elongate, e.g. metal, body forming the radar reflective surface is integrally formed with the plurality of projecting fins. Preferably the radar reflective surface is formed on a surface of the elongate body, e.g. as a portion of the integrally formed part of the elongate body. Preferably the two-way radio-frequency translucent material (e.g. cylindrical and nose cone) cover surrounds the radar reflective surface of the integrally formed part of the elongate body, e.g. with the plurality of projecting fins projecting through the two-way radio-frequency translucent material cover. Thus preferably the cylindrical cover and the nose cone cover are formed separately from the radar reflective surface.

When the decoy comprises both canard fins and tail fins, preferably the canard fins are offset from the tail fins around the azimuthal axis of the decoy. Preferably, there are four canards fins and four tail fins, e.g. with the canard and tail fins being offset from each other by 45 degrees, e.g. such that the plurality of fins (considering the canard and tail fins together) are arranged at regular positions around the azimuthal axis.

According to an embodiment of the invention, the plurality of projecting fins comprise trapezoidal flat plates. Preferably the leading and/or trailing edges of the plurality of projecting fins are tapered.

According to an embodiment of the invention the plurality of projecting fins comprise radar reflecting surfaces. This helps to increase the radar cross section of the decoy.

According to an embodiment of the invention, one or more of the plurality of projecting fins are deflected from parallel to the main (e.g. cylindrical) axis of the decoy's elongate body, i.e. the axis parallel to the direction in which the elongate body is extended longitudinally. Preferably, the deflection angle (between the fin and the direction parallel to the main axis) is less than or equal to 2 degrees. More preferably, the deflection angle is less than or equal to 1 degree, e.g. approximately 0.25 degrees. When the fins comprise canards and tails, preferably the canards are deflected. Optionally, the tail fins could be deflected as well as, or instead of, the canard fins. Deflecting the fins may help the decoy to spin when it is deployed, e.g. to produce its own Doppler return that may mimic that of a jet engine compressor or turbine. Deflecting the fins helps to provide one or more of (preferably all of) a pitching, a rolling and a yawing moment.

The plurality of projecting fins may comprise any suitable and desired material, e.g. the same as the body and/or the reflective surface(s). In a preferred embodiment the plurality of projecting fins comprises copper, e.g. copper plated, e.g. high density polymer (e.g. acetal (e.g. ertacetal)) covered in (e.g. a thin layer of) copper.

According to an embodiment of the present invention, the decoy is arranged to glide autonomously and passively when deployed. Preferably, the centre of pressure of the decoy is located at or behind the centre of gravity of the decoy, i.e. the centre of gravity is as far, or further, from the rear of the decoy than the centre of gravity. This helps maintain the stability of the decoy when the decoy is gliding. A stable glide helps the decoy to maintain altitude and aircraft-like speeds for a longer duration than otherwise.

Gliding at, e.g., aircraft-like speeds aids radar reflected from the decoy to be imparted with a Doppler shift representative of such speeds. This allows the decoy to be a more plausible target for radar-seeking missiles compared to a decoy which does not glide. A gliding decoy may thus be able to deceive more sophisticated radar trackers than a decoy which does not glide.

Maintaining altitude, or minimising the rate at which altitude is lost, helps the decoy to be a more plausible target for radar seeking missiles for longer than decoys which lose altitude more rapidly. The longer period of time at a higher altitude increases the probability that a radar-seeking missile will target the decoy instead of the aircraft, and also help to increase the separation of the aircraft from the decoy to reduce the impact on the aircraft of the missile exploding when targeting the decoy. The decoy may have any suitable and desired mass. In a preferred embodiment the decoy has a mass greater than 1 kg, between 1 kg and 3 kg, e.g. between 1 kg and 2 kg, e.g. between 1 kg and 1.5 kg. A decoy with a greater mass has a greater inertia and therefore may be able to mimic the flight of an aircraft more accurately. The greater mass also helps to stabilise the decoy and thus avoid inertia coupling (also known as cross-coupling) which may act to destabilise the decoy.

The mass of the decoy may be distributed in any suitable and desired way, e.g. so that the centre of pressure of the decoy is located at or behind the centre of gravity of the decoy. In a preferred embodiment the decoy comprises counterweights, e.g. arranged to configure the mass distribution of the decoy. This helps to configure a suitable position for the centre of gravity and helps to increase the mass of the decoy, both of which help to increase the stability of the decoy in flight, e.g. to improve the flying and/or gliding ability of the decoy.

Preferably the counterweights (e.g. comprising insertable sections) are located between the radar reflective surface(s) and the (e.g. hollow) cover of the elongate body, e.g. in the void formed by the retroreflectors. Thus preferably the

counterweights are at least translucent (e.g. transparent) to radio frequencies. The counterweights may be located in any suitable and desired part of the decoy, e.g. in any of the sections and/or the nose cone.

The counterweights may be made from any suitable and desired (e.g. radio- frequency translucent) material. In a preferred embodiment the counterweights are made from a high density polymer, e.g. a polyoxymethylene, e.g. an acetal (e.g. ertacetal). Preferably the counterweights are manufactured separately from the radar reflective surface and/or separately from the cover of the elongate body, and, e.g., then assembled. According to an embodiment of the present invention, the method comprises releasing the decoy from the aircraft when the aircraft is moving. As discussed below, there are a number of different ways in which the decoy may be ejected and released from the aircraft, e.g. depending on the threat the aircraft is under and/or the engagement geometry. According to an embodiment of the present invention, the decoy may be released passively or ejected with a force (e.g. applied by an ejection cartridge) from the magazine. In one embodiment the decoy comprises a (e.g. pyrotechnic) propulsion mechanism (e.g. at its base (tail end)) that is energised when the decoy is released or ejected from the aircraft. This may help to separate the decoy from the aircraft while mimicking the flight of an aircraft.

Preferably the decoy is released or ejected into the free stream of the aircraft.

Preferably the decoy is ejected backwards from the aircraft, e.g. at a velocity of between 20 ms^"1 and 40 ms^"1 , e.g. approximately 30 ms^"1.

Once released or ejected from the aircraft, the decoy then preferably deploys its projecting fins (when the fins are deployable) and begins to glide. This is a quick and simple method of deploying the decoy, requiring minimal components. When the decoy comprises deployable fins, preferably the fins are deployed to project from the elongate body after the decoy is released or ejected from the magazine.

According to an embodiment of the present invention, the decoy is released or ejected from the aircraft's magazine while attached to a tow line. The decoy is then towed from the aircraft using a tow line, e.g. until aerodynamic stabilisation is achieved. Towing the decoy has the advantage of keeping the altitude and speed of the decoy at the same values as the aircraft's, increasing the probability that a radar seeking missile will lock onto the decoy. On the other hand, a tow line restricts the manoeuvrability of the aircraft and must be stored on the aircraft along with the decoy when not in use. It also restricts the maximum distance between the decoy and the aircraft.

Preferably, the decoy is released from the tow line, e.g. after a period of time of being towed while attached to the tow line. Deploying the decoy with the tow line can help the decoy achieve a stable glide. Releasing the decoy then allows the decoy and aircraft to take divergent paths and removes the restriction on the aircraft's manoeuvrability as well as the separation between the decoy and the aircraft. The aircraft may store one or a plurality of decoys for deploying. When the aircraft carries a plurality of decoys (in accordance with the present invention) these may be deployed individually or simultaneously (e.g. in an array). Therefore in a preferred embodiment the method comprises deploying an array of passive radar decoys, each decoy having an elongate body with at least one radar reflective surface and a plurality of fins that project from the elongate body, the method comprising deploying each decoy substantially simultaneously from respective magazines on an aircraft. In an embodiment of the present invention the decoy's length is less than 1 m, preferably less than 60 cm, more preferably less than 40 cm, even more preferably less than 30 cm and most preferably less than 21 cm, e.g. approximately 20 cm.

In another embodiment of the present invention the decoy's diameter less than 30 cm, preferably less than 20 cm, more preferably less than 10 cm, even more preferably less than 8 cm and most preferably less than 6 cm, e.g. approximately 5.5 cm.

In an embodiment of the present invention, the fins project from the elongate body by less than 10 cm, e.g. less than 5 cm, e.g. approximately 2 cm. Thus preferably the decoy's wingspan (i.e. of the projecting fins) is less than 50 cm, preferably less than 30 cm, more preferably less than 20 cm, even more preferably less than 15 cm, most preferably less than 10 cm, e.g. approximately 9.5 cm. In an embodiment of the present invention, the thickness of the projecting fins is between 0.5 mm and 5 mm, e.g. between 1 mm and 3 mm, e.g. approximately 2 mm.

In an embodiment of the present invention, the decoy is reflective to

electromagnetic waves over a plurality of IEEE (Institute of Electrical and

Electronics Engineers) frequency bands, preferably comprising 5.25 GHz to 36 GHz, e.g. between 8 GHz and 12 GHz.

Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures 1 a, 1 b, 2, 3a, 3b and 3c show a deployable radar decoy in accordance with an embodiment of the invention;

Figures 4a and 4b show a deployable radar decoy in accordance with another embodiment of the invention;

Figure 5a and 5b show an aircraft with an on-board magazine;

Figure 6 is a graph of the pitching moment against the incidence angle of the deployable radar decoy shown in Figures 1 a, 1 b, 2, 3a, 3b and 3c;

Figure 7a is a graph of the load factor against the time from tow release of the deployable radar decoy shown in Figures 1 a, 1 b, 2, 3a, 3b and 3c;

Figure 7b is a graph of the Mach number against the time from tow release of the deployable radar decoy shown in Figures 1 a, 1 b, 2, 3a, 3b and 3c;

Figure 7c is a graph of the altitude against the cross range of the deployable radar decoy shown in Figures 1 a, 1 b, 2, 3a, 3b and 3c;

Figure 7d is a graph of the altitude against the down range of the deployable radar decoy shown in Figures 1 a, 1 b, 2, 3a, 3b and 3c;

Figure 8 is a graph of the maximum value of the radar cross section of the deployable radar decoy shown in Figures 1a, 1 b, 2, 3a, 3b and 3c as a function of the azimuthal angle; and

Figure 9 is a graph of the maximum value of the radar cross section of the deployable radar decoy shown in Figures 1a, 1 b, 2, 3a, 3b and 3c as a function of the elevation angle.

An aircraft that is being tracked by a radar missile seeker may use various decoy devices to lure the seeker's boresight away from it. Such decoy devices may operate by presenting a larger radar cross section than the aircraft, such that the seeker locks onto the decoy and not the aircraft. A preferred embodiment of a deployable radar decoy in accordance with the present invention will now be described. Figure 1 a shows a side view of an embodiment of a deployable radar decoy 301 for launching from an aircraft. Figure 1 b shows a perspective view of the decoy 301. The decoy 301 has a elongate cylindrical body 302 and a nose cone 307 having a spherically blunt tangent ogive shape. This shape of the nose cone maximises the available volume for the accommodation of corner reflectors and minimises drag. The nose shape is efficient for supersonic and subsonic glide. The elongate cylindrical body 302 and the nose cone 307 have an outer casing (made from ertacetal, which is substantially transparent to radio frequencies) that covers an internal reflective surface and counterweights (as will be described below with reference to Figures 2, 3a, 3b and 3c).

Four tail fins 304 are attached to the rear end of the elongate cylindrical body 302. The tail fins 304 are trapezoidal flat plates of 2 mm thickness, tapering at their leading and trailing edges. The tail fins 304, which each project by 2 cm from the outer circumference of the elongate cylindrical body 302, are each spaced at 90 degree intervals around the circumference of the elongate cylindrical body 302 and may be deflected (e.g. in the same direction) by approximately 0.25 degrees (not shown) from the axis parallel to the direction in which the elongate cylindrical body 302 is extended longitudinally. The tail fins 304 are made from ertacetal covered in a thin layer of copper.

The length of the decoy 301 is 20 cm, the diameter of the elongate cylindrical body 302 is 5.5 cm, and the wingspan of the decoy 301 is 9.5 cm. Figure 2 shows the decoy 301 with the outer casing of the elongate cylindrical body 302 and the nose cone 307 (as shown in Figures 1a and 1 b) removed. From this, it can be seen that the decoy 301 comprises a radar-reflective surface (which in use, as shown in Figures 1 a and 1 b, is encased by the substantially radar transparent covering of the elongate cylindrical body 302). The radar-reflective surface comprises a series of retroref lectors (made from copper or copper plated ertacetal) arranged in four modular sections. A number of radar transparent counterweights (made from ertacetal) are housed in some of the retroreflectors, as will be described below. Figures 3a, 3b and 3c show the decoy 301 with both the outer casing of the elongate cylindrical body 302 (as shown in Figures 1a and 1 b) removed and the counterweights (as shown in Figure 2) removed, so that the retroreflectors can be seen clearly. The facets within each retroreflector are arranged perpendicularly to each other. The first section 307 is a nose cone section. The first (nose cone) section 307 has a radar reflective surface comprising four trihedral-shaped retroreflectors 308. Each of the retroreflectors houses a counterweight 310, with these being contained within the outer casing of the nose cone section 307.

The second section 309 comprises a body section with symmetrically inverted trihedral-shaped retroreflectors 312 inverted relative to each other. Each of the retroreflectors houses a counterweight 314, with these being contained within the outer casing of the elongate cylindrical body 302.

The third section 31 1 comprises a body section with dihedral and trihedral-shaped retroreflectors 316. Two of the four recesses for the retroreflectors 316 house a counterweight 318, with these being contained within the outer casing of the elongate cylindrical body 302.

The fourth section 313 comprises a body section comprising dihedral and trihedral- shaped retroreflectors 320 and a rear facing recess 315 having a 90 degree internal angle. Two of the four recesses for the retroreflectors 320 house a counterweight 322, with these being contained within the outer casing of the elongate cylindrical body 302. The tail fins 304 are attached to the body section between the recesses of the retroreflectors 320.

The rear facing recess 315 can contain an ejection cartridge, which can be used to eject the decoy 301 from a flare dispenser of an aircraft's magazine (as will be described with reference to Figures 4a and 4b below). This allows the decoy 301 to be deployed rapidly and regardless of the aircraft's orientation and current acceleration. The rear facing recess 315 has a radar reflective surface, which increases the radar cross section of the decoy 301 when radar is incident from behind the decoy 301.

Figures 4a and 4b show a deployable radar decoy 201 according to another embodiment of the invention. The decoy 201 shown in Figures 4a and 4b comprises a radar-reflective surface encased by a substantially radar transparent body covering made from ertacetal. The radar-reflective surface comprises a series of retroreflectors made from copper or copper plated ertacetal. The decoy 201 comprises four modular sections.

The first section 207 is a nose cone section. The nose cone of the nose cone section 207 comprises radar transparent material and has a power shape of y = x^{0 5}. The shape of the nose cone maximises the available volume for the

accommodation of corner reflectors and minimises drag. The nose shape is efficient for supersonic and subsonic glide. The first section 207 has a radar reflective surface comprising trihedral-shaped retroreflectors. The radar reflective surface is within the nose cone section 207.

The second section 209 comprises a body section with dihedral and trihedral- shaped retroreflectors. A radar transparent body covering encases the

retroreflectors.

The third section 21 1 comprises a body section with symmetrically inverted trihedral-shaped retroreflectors inverted relative to each other. A radar transparent body covering encases the retroreflectors. The fourth section 213 comprises a body section comprising dihedral and trihedral- shaped retroreflectors and a recess 215. The recess 215 can contain an ejection cartridge, which can be used to eject the decoy 201 from a flare dispenser of an aircraft's magazine (as will be described with reference to Figures 5a and 5b below). This allows the decoy 201 to be deployed rapidly and regardless of the aircraft's orientation and current acceleration. The recess 215 has a radar reflective surface, which increases the radar cross section of the decoy 201 when radar is incident from behind the decoy 201.

The second section 209 also comprises four deployable canard fins 203, and the fourth section also comprises four deployable tail fins 204. The fins 203, 204 are made from ertacetal covered in a thin layer of copper.

The canard fins 203 and tail fins 204 are stored inside the cylindrically shaped elongate body of the decoy 201 when the decoy 201 is stored in a magazine. The canard fins 203 are each spaced at 90 degree intervals around the second section 209. The tail fins 204 are each spaced at 90 degree intervals around the fourth section 213. The canard fins 203 and tail fins 204 are azimuthally offset from each other by 45 degrees around the body of the decoy 201.

The canard fins 203 and tail fins 204 are trapezoidal flat plates, of 0.5 mm thickness. The wingspan of the decoy 201 is 155 mm.

The canard fins 203 can be deflected by approximately 6 degrees (not shown) from the axis parallel to the direction in which the elongate body of the decoy 201 is extended longitudinally.

The length of the decoy 201 is 20 cm, and the diameter of the body of the decoy 201 is 5.5 cm.

The decoys 301 , 201 shown in Figures 1 a, 1 b, 2, 3a, 3b, 3c, 4a and 4b are radar reflective to electromagnetic waves from 5.25 GHz to 36 GHz. This covers the IEEE (Institute of Electrical and Electronics Engineers) frequency bands generally used by missile seekers.

Figures 5a and 5b each show an aircraft 101 in flight. A magazine 103 is attached to the wing of the aircraft 101. The magazine 103 is capable of being used for deploying flares 105 from the aircraft 101. The magazine 103 can, for example, face downwards (as shown in Figure 5a) or rearwards (as shown in Figure 5b) relative to the body of the aircraft 101. The magazine 103 has a bank of cylindrical flare dispensers 107. The deployable aircraft decoys 301 , 201 shown in Figures 1 a, 1 b, 2, 3a, 3b, 3c, 4a and 4b are shaped to fit inside one of the cylindrical flare dispensers 107. The aircraft 101 can then deploy the decoys 301 , 201 in the same manner in which a flare 105 is deployed form the aircraft 101.

In one embodiment a decoy 301 , 201 is attached to the aircraft 101 using a tow line. The tow line is sufficiently thin to be stored in the magazine 103, but is capable of withstanding the aerodynamic forces placed on it due to the deployment of the decoy 301 , 201. When the decoy 301 , 201 is deployed from the aircraft 101 , the tow line tows the decoy 301 , 201 behind the aircraft 101. The tow line comprises a conductive material so to present a radar cross section itself.

The retroreflectors reflect incident radar back in their incidence direction, to increase the perceived radar cross section of the decoy in the direction of the radar source.

By increasing the radar cross section of the decoy 301 , 201 above that of the deploying aircraft 101 , the decoy 301 , 201 can lure missile seekers away from the aircraft 101. A missile may thus initiate its warhead when its proximity fuse senses the presence of the decoy 301 , 201 rather than the aircraft 101. Alternatively, if the missile seeker engages the decoy and then attempts to engage the aircraft, the missile will be required to carry out a high g-force manoeuver, starving the missile of kinetic energy and increasing the probability of the missile missing the aircraft 101.

When deployed, after the decoy has been released or ejected from the magazine 103, the tail fins 304, 204 (and canard fins 203 when present) are used by the decoy 301 , 201 to facilitate the decoy gliding autonomously and passively in air, at subsonic and supersonic speeds. To facilitate stable autonomous gliding the centre of pressure of the decoy 301 , 201 is located rearward of the centre of gravity of the decoy.

Deflecting the tail fins 304, 204 and/or the canard fins 203 by approximately 0.25 degrees causes the path of the decoy 301 , 201 to curve through the air. This curving path mimics the path of an aircraft turning in the air.

In the embodiment shown in Figures 4a and 4b, storing the canard fins 203 and tail fins 204 inside the elongate body of the decoy 201 allows the decoy 201 to fit inside the flare dispensers 107 in the magazine 103, while maximising the radar reflective surface area of the deployed decoy. Upon deployment of the decoy 201 , the canard fins 203 and tail fins 204 are deployed from the elongate body of the decoy. ln both embodiments the canard fins 203 and/or the tail fins 304, 204 allow the decoy 301 , 201 to glide. The canard fins 203 and/or the tail fins 304, 204 each have radar reflective surfaces. Utilising a tow line when deploying the decoy 301 , 201 allows the decoy 301 , 201 to stabilise after deployment. A tow line can also be used to pull the decoy 301 , 201 by the aircraft 101 , allowing the decoy to maintain a higher speed and not be slowed down by air resistance. The tow line may subsequently be cut to allow the distance between the decoy 301 , 201 and the aircraft 101 to increase

If the decoy is made of conductive material, the tow line will backscatter incident radiation from a missile seeker. This increases the overall radar cross section of the aircraft 101 and decoy 301 , 201 , and the missile seeker will aim towards the centroid of the combined backscatter. As the missile seeker closes in, it will eventually aim towards the decoy 301 , 201 , as the decoy 301 , 201 has a larger radar cross section.

In another embodiment the decoy 301 , 201 is released or ejected from the magazine 103 without being attached to a tow line. This allows the decoy 301 , 201 to be fully separated from the aircraft 101.

A number of graphs showing the behaviour of the deployable radar decoy 301 (as shown in Figures 1a, 1 b, 2, 3a, 3b and 3c) in flight will now be described. Figure 6 is a graph of the pitching moment coefficient (CM) of the decoy 301 plotted against the angle of attack (AOA), in degrees, of the decoy 301. The data 401 shows the pitching moment coefficient as a function of angle of attack for the decoy 301 travelling at speeds of between Mach 0.2 and Mach 1.3. The data 401 was collected from simulations of the decoy 301. The results suggest the decoy 301 is stable when gliding.

Figures 7a and 7b are graphs showing the load factor (the ratio of the lift of the decoy 301 to its weight) and the Mach number as a function of time from tow release of the decoy 301 , respectively, for a decoy 301 that is released at 7,620 m (25,000 feet) at Mach 0.9 (these figures are typical during within or beyond visual- range (WVR or BVR) engagements). Figures 7c and 7d are graphs showing the altitude against the cross range and the down range respectively, for the same decoy 301 as in Figures 7a and 7b (i.e. having the same initial conditions). The simulation data shows that, once released from the tow, the decoy 301 glides autonomously and displays characteristics similar to that of an aircraft beyond 2 seconds after at the tow is cut. This increases the likelihood of missile seekers locking on to the decoy 301 , and to remain locked on for a longer time period than a decoy which does not display these characteristics.

Figure 8 shows a graph of the maximum value of the radar cross section of the decoy 301 as a function of the azimuthal angle, for an interrogating radar of frequency 9.55 GHz (X-band). Figure 9 shows a graph of the maximum value of the radar cross section of the decoy 301 as a function of the elevation angle, for an interrogating radar of frequency 9.55 GHz (X-band). These graphs show that owing to the placement of the retroreflectors on the decoy 301 , the decoy 301 generates broad radio frequency visibility for both the elevation and azimuthal angles. Thus even when the decoy 301 spins in flight, and so an incoming missile will see the decoy 301 in varying geometries, the radar cross section is maintained to mimic those of an aircraft.

The values shown in Figures 8 and 9 for the radar cross section of the decoy 301 demonstrates that they are of comparable or greater magnitude than the radar cross section of a fast jet that employs low observable coatings or a low reflective layout using faceting and/or contour shaping.

Thus it will be seen the embodiments of the present invention provide an improved method for decoy that is passive and glides autonomously. It will be appreciated by those skilled in the art that the embodiments described above are merely exemplary and are not limiting on the scope of the invention.

Claims

1. A passive radar decoy for deploying from a magazine on an aircraft, the decoy comprising an elongate body having at least one radar reflective surface and a plurality of fins that project from the elongate body.

2. A passive radar decoy as claimed in claim 1 , wherein the elongate body comprises one or more aerodynamic lifting surfaces, wherein the aerodynamic lifting surfaces are arranged to generate one or more of a pitching, a rolling and a yawing moment when the radar decoy is in flight.

3. A passive radar decoy as claimed in claim 1 or 2, wherein the elongate body of the decoy comprises a rigid body.

4. A passive radar decoy as claimed in claim 1 , 2 or 3, wherein the elongate body comprises an external surface comprising a two-way radio-frequency translucent (e.g. transparent) cover, and wherein the radar reflective surface is arranged inside the cover.

5. A passive radar decoy as claimed in claim 4, comprising one or more counterweights arranged between the radar reflective surface and the cover.

6. A passive radar decoy as claimed in claim 5, wherein the one or more counterweights comprise a two-way radio-frequency translucent (e.g. transparent) material.

7. A passive radar decoy as claimed in any one of the preceding claims, wherein the decoy comprises a plurality of (e.g. modular) sections.

8. A passive radar decoy as claimed in claim 5, wherein each section comprises its own at least one radar reflective surface.

9. A passive radar decoy as claimed in any one of the preceding claims, wherein the at least one radar reflective surface comprises retroreflectors, e.g. corner reflectors.

10. A passive radar decoy as claimed in claim 9, wherein the retroreflectors comprise trihedrals and/or dihedrals.

1 1. A passive radar decoy as claimed in any one of the preceding claims, wherein the plurality of projecting fins are deployable.

12. A passive radar decoy as claimed in claim 1 1 , wherein the deployable fins are arranged to be deployed after the decoy is released or ejected from the aircraft.

13. A passive radar decoy as claimed in any one of the preceding claims, wherein the plurality of projecting fins comprises a plurality of tail fins.

14. A passive radar decoy as claimed in any one of the preceding claims, wherein the plurality of fins comprise trapezoidal flat plates.

15. A passive radar decoy as claimed in any one of the preceding claims, wherein one or more of the plurality of projecting fins are deflected from parallel to the main axis of the elongate body.

16. A passive radar decoy as claimed in any one of the preceding claims, wherein the decoy is arranged to glide autonomously and passively when deployed from the aircraft.

17. A passive radar decoy as claimed in any one of the preceding claims, wherein the decoy comprises a tow line for attaching to the aircraft.

18. A passive radar decoy as claimed in any one of the preceding claims, wherein the decoy is arranged to be stored in the magazine, e.g. in a flare dispenser.

19. A passive radar decoy as claimed in any one of the preceding claims, wherein the decoy has a length of less than 1 m, has a diameter less than 30 cm and/or has a wingspan less than 50 cm.

20. A passive radar decoy as claimed in any one of the preceding claims, wherein the at least one radar reflective surface is reflective to electromagnetic waves over a plurality of IEEE frequency bands, preferably comprising 5.25 - 36 GHz.

21. A method of deploying a passive radar decoy, the decoy having an elongate body with at least one radar reflective surface and a plurality of fins that project from the elongate body, the method comprising deploying the decoy from a magazine on an aircraft.

22. A method as claimed in claim 21 , comprising storing the decoy the magazine on-board the aircraft, e.g. in a flare dispenser.

23. A method as claimed in claim 21 or 22, comprising deploying the decoy from the aircraft when the aircraft is moving.

24. A method as claimed in claim 21 , 22 or 23, comprising deploying the decoy from the aircraft's magazine while the decoy is attached to a tow line.

25. A method as claimed in claim 24, comprising subsequently releasing the decoy from the tow line.

26. A method as claimed in any one of claims 21 to 25, comprising deploying the plurality of fins to project from the elongate body after the decoy has been deployed from the aircraft.