DUAL-FREQUENCY MILLIMETER WAVE AND LASER RADIATION RECEIVER

DUAL-FREQUENCY MIT XIMETER WAVE AND LASER RADTATTON RECEIVER

BACKGROUND OF THE INVENTION

This invention relates to dual-frequency receivers, responsive to both

millimeter band radiation and to laser radiation.

Dual-frequency detectors responsive to both microwave and infra-red

radiation are found in the target seeking systems of aircraft, both piloted aircraft and

guided missiles. To obtain an unrestricted field of view, such sensors are generally located

at the nose of the aircraft or missile. The prior art teaches to provide reflector systems to

provide directivity and gain for detectors of both wavelengths. Such reflector systems are

generally of concave or Cassegrain configuration designed on classical optical principles.

To obtain maximum resolution and sensitivity, the aperture of the reflecting system should

be as large as possible, and in a missile seeker system generally occupies as much of the

missile cross-section as possible. This means that both wavebands have to use the same

aperture, and such systems are commonly referred to as common aperture receivers.

In a conventional reflecting system consisting of purely reflecting elements,

all wavelengths are brought to focus at the same point. While U.S. Patent No. 4,282,527

teaches wavelength discrimination by providing a fiber optic cable coaxially with a

microwave waveguide at the focus of a reflecting system to guide infra-red radiation and

microwave radiation to respective sensors in different locations remote from the focus, the more usual approach is to incorporate one or more dichroic elements in the reflecting

system so as to bring different wavelengths to a focus on different detectors at different

points in space. Examples of such prior art arrangements are disclosed in U.S. Patent No.

5,373,302; No. 5,327, 149; No. 5,214,438; No. 5,130,718; and No. 3, 165,749.

It is also known to provide dual mode detectors where the reflecting system

only reflects and focusses infra-red radiation. Microwave radiation is sensed by an array

of antennas located either on the surface of a principal infra-red radiation reflector or else

behind a principal dichroic reflector which is transparent to microwaves. Such

arrangements are disclosed in U.S. Patent No. 4,477,814 and No. 5,307,077, respectively.

In these prior art arrangements, the component parts of the dual mode

detectors, that is to say, the respective detectors responsive to different wavebands and the

reflecting systems associated therewith, have been in fixed spatial relationship to each

other. Beam steering has involved physically steering the reflecting system and both

detectors. This has necessitated the provision of correspondingly robust gimbal

arrangements and the need for relatively powerful actuators to overcome the inertia of the

reflectors and detectors.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved dual mode

tandem seeker configuration in which the disadvantages of the prior art are at least

ameliorated. The present invention pi ovides a tandem laser and millimeter wave receiver for a

moving vehicle and for providuig directional information of a target to a guidance system

for the vehicle. The invention comprises a primary reflector comprising a peripheral

region transparent to laser radiation and reflective to millimeter wave radiation; a

secondary reflector reflective to millimeter wave radiation and arranged to receive and

reflect millimeter waves reflected from the primary reflector; annular laser receiver means

comprising a plurality of discrete laser receiver means arranged in a ring behind the

peripheral region to receive laser radiation passing through the peripheral region;

millimeter wave receiving means arranged to receive the millimeter waves reflected from

the secondary reflector; processor means arranged to receive signals from the annular laser

receiver means and the millimeter wave receiving means and to generate therefrom target

position signals; and means to fasten the primary reflector, the secondary reflector, the

annular laser receiver means, the millimeter wave receiving means, and the processor

means together within the envelope of the vehicle.

The present invention provides full use of the common receiver aperture

areas for the laser and millimeter wave collectors with a common diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of non-limiting

example only, with reference to the drawings in which: FIG. 1 shows a schematic perspective view of a first dual-frequency seeker

for detecting radar radiation in the millimeter radar band and laser radiation in the infra¬

red band;

FIG. 2 shows a simplified longitudinal section through FIG. 1;

FIG. 3 shows simplified longitudinal section through part of FIG. 1 at right

angles to FIG. 2;

FIG. 4 is a simplified version of FIG. 2 on a reduced scale illustrating the

overlap of the fields of view of two laser detectors; and

FIG. 5 shows a block diagram of a guidance system.

[ 0

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1, 2 and 3, a radome 1 transparent to radiation of

both millimeter and infra-red wavelengths covers the forward end of the body of a missile

10. Only the forward extremity of the missile is shown in the drawing. An annular array

15 of discrete laser receivers 4 is located within the radome 1. Receivers 4 are mounted in

an annular support 41 which is secured to the missile body 10. Receivers 4 are thereby

immovably attached (strapped down) to the missile body 10. The receivers per se are of

the type described in U.S. Patent No. 5,784, 156, the contents of which are hereby

incoφorated by reference herein. As will be described in more detail below, receivers 4

0 are configured as a staring array looking generally ahead in the direction of travel of the

missile. A concave reflector 2 has an annular peripheral dichroic region 22, an inner

reflective region 20 and a central aperture 24. Dichroic region 22 transmits infra-red laser

radiation and reflects microwave radiation.

A strap-down microwave detector, shown generally by 5, is mounted on a

first housing 44 containing microwave receiver components. Housing 44 is itself mounted

on a second housing 46 containing common processing circuitry for the microwave

receiver and the laser receivers. Second housing 46 is attached to the missile body 10.

Microwave detector 5 is generally cylindrical in shape and consists of a

forward portion 52, an intermediate portion 54 and a rearward portion 56. Forward

portion 52 receives incident microwave radiation. Intermediate portion 54 is of smaller

diameter than forward portion 52 and rearward portion 56.

Reflector 2 is mounted to detector 5 via a gimbal arrangement 90 arranged

within an aperture 24 at the center of the reflector 2. The gimbal arrangement consists of

an annular ring-shaped member 91 having a first pair of pivots 92 arranged on the opposite

sides of the radially outer surface thereof by which ring 91 is coupled to reflector 2, and

a second pair of pivots 94 on opposite sides of the radially inner surface thereof by which

ring 91 is coupled to detector 5.

A convex reflector 3 has a reflective surface 30 which reflects microwave

radiation. Convex reflector 3 is attached to, and maintained in alignment with, the forward

portion 52 of detector 5 by supports 6. Referring to FIG. 2, elevation adjustment of reflector 2 is effected by a first

actuator 81 mounted on the rearward portion 56 of detector 5 and coupled via a first

mechanical linkage 82 to the rear surface of reflector 2. It will be seen that an incremental

movement of linkage 82 by acmator 81 will result in an incremental tilt of reflector 2 in

elevation. FIG. 3 shows the corresponding arrangement for adjusting azimuth consisting

of a second actuator 81 ' and second linkage 82' arranged orthogonal to the elevation

adjustment arrangement shown in FIG. 2. Second acmator 81 ' and second linkage 82'

of FIG. 3 perform the same functions as first actuator 81 and first linkage 82 of FIG. 2

but in respect of azimuth instead of elevation, and will therefore not be described further.

In the present embodiment, concave reflector 2 consists of a polycarbonate

concave substrate 26 which is transparent to infra-red laser radiation. The concave surface

of the substrate is coated with a thin layer of a metal which, in the present embodiment,

is gold. The metal coating is continuous in inner region 20. In outer dichroic region 22,

the metal coating is not continuous, metal being present in a pattern to leave areas of the

substrate exposed. The dimensions of the metal-coated regions and exposed regions are

such that infra-red laser radiation passes through outer dichroic region 22 with minimal

attenuation, while microwave radiation is substantially totally reflected. The design and

construction of such patterns exhibiting dichroic behavior are well known to those skilled

in the art and will therefore not be described further.

Concave reflector 2 may be moved from a position in which it detects

radiation incident from the boresight axis 6 to a position, shown by dashed lines and indicated by primed numbers 2' . where it detects radiation from an off-boresight direction

as indicated by chain dashed lines 16'.

Gimbal 90, actuators 81, 81 ' and associated linkages 82, 82', microwave

detector 5 and secondary reflector 3 are arranged in the radially central region of primary

reflector 2 and, as such, do not obscure the field of view of laser receivers 4. This means

that by making the entire outer perimeter of primary reflector 2 transparent to infra-red

laser light, an unobstructed field of view for receivers 4 is obtained without physically

mounting receivers 4 on the same gimbaled platform as the concave reflector 2. This

allows laser receivers and microwave receiver to respond simultaneously to radiation from

the same field of view.

As shown in FIG.4, beams 42 of receivers 4 overlap. Receivers 4 are

arranged so that their combined fields of view cover a greater field of view than that of a

single receiver and provide complete coverage of a predetermined field ahead of the

missile. The overlap between the fields of the individual receivers is such that each point

within the predetermined field lies within the field of view of at least two different laser

receivers. It will be seen that convex reflector 3 does not obscure the field of view seen

by the laser receivers.

Radiation received by the receivers is fed via fiber optic cables to a signal

processing arrangement which determines the direction of incident laser radiation from the

relative amplitudes of radiation received at different receivers. Such techniques are well

known to the skilled person, inter alia, from U.S. Patent No. 5,784, 156; No. 4,825,063; and No. 4,674,874, the contents of which are hereby incoφorated herein by reference, and

will not be described further.

The sensors disclosed in U.S. Patent No. 5,784, 156 are arranged at the

exterior of a missile and thus look directly at their respective field of view. In the present

invention, it will be appreciated that receivers 4 do not look directly at their respective

fields of view, but look through both dichroic region 22 of reflector 2 and radome 1.

Because radome 1 is in a fixed spatial relationship to detectors 4, any distortion in the

wavefront of radiation incident on receivers 4 caused by its passage through radome 1

which would make radiation appear to come from a different direction from its true

bearing can be compensated for during calibration. Further wavefront distortion may arise

from its passage through dichroic region 22 of reflector 2. Because reflector 2 is movable,

there is a possibility that distortion may vary with reflector position. If any such distortion

has a significant effect on accuracy, it may be necessary to provide compensation.

This can be effected in a number of different ways according to operational

requirements.

If it is not necessary to use the laser receivers and microwave receiver

simultaneously, then the reflector can be set to a predetermined position, e.g., aligned with

the missile boresight, during laser reception. The system can then be calibrated with the

reflector in the predetermined position. Alternatively, if simultaneous operation is

required, then the laser receivers can be calibrated with the reflector in a number of

different positions. The orientation of the reflector will, of course, be available from the circuitry which steers the reflector. Information about the radar reflector position can be

used by the laser receiver processing circuitry to provide appropriate compensation, e.g. ,

by using a look-up table and inteφolation. Such compensation techniques are well known

to the skilled person and will not be described further.

Referring now to FIG. 5, laser receivers 4 provide signals via fiber optic

cables 402 to laser processing circuit 40. Only two laser receivers 4 are shown in the

interest of clarity. The laser processing circuit 40 processes the signals on cables 402 and

obtains therefrom information regarding the azimuth and elevation of a target in the

manner described in U.S. Patent No. 5,784,156. This information is supplied via line 404

to a first input of common sensor processing circuit 60.

Received signals from millimeter wave radar receiver 5 are supplied via line

502 to millimeter wave processing circuit 50. Information about target bearing is fed via

line 504 to a second input of common sensor processing circuit 60. Signals from common

sensor processing circuit 60 are fed via line 522 to antenna steering circuit 52 which steers

the steerable reflector 2. Information about the position of reflector 20 is fed from antenna

steering circuit 52 back to common sensor processing circuit 60 via line 524. An output

602 of common sensor processing circuit 60 provides steering commands to an autopilot

70 which steers the missile.

The components associated with the generation and transmission of

millimeter wave radar signals are conventional. They have therefore been omitted in the

interest of clarity as they are not relevant to the present invention. This tandem configuration of a gimbaled millimeter wave primary reflector

and non-gimbaled laser receiver allows both the amount of electrical connections required

and the gimbaled mass to be reduced, thereby reducing the demands on the actuators which

steer the gimbaled reflector and providing instantaneous reception of laser pulses from the

entire sensor field of view.

While the above-described embodiment utilizes a Cassegrain-type reflector

system, the invention is not limited thereto and may equally well be performed using other

reflector configurations. For example, the microwave detector may be placed at the focus

of the reflector system, the receiver beam being steered by moving the reflector system and

receiver as a unit while the laser receiver array remains strapped down to its platform.

Numerous modifications are possible within the scope of the invention.

Instead of a pattern of conductors, the dichroic region 22 may consist of a material such

as indium tin oxide which possesses intrinsic dichroic properties, i.e. , is transparent to

infra-red radiation and reflects microwave radiation.

Alternatively, the dichroic region 22 may consist of a continuous layer of

metal, the layer being sufficiently thin to allow a significant amount of infra-red laser

radiation to pass therethrough and sufficiently thick to reflect microwave radiation. The

dichroic region may also be implemented using alternative frequency-selective resonant

structures known per se which exhibit the necessary dichroic properties. While in the described embodiment, the fixed convex reflector 3 of the

Cassegrain reflector is attached ta the receiver it may alternatively be attached directly to

the radome 1.

While the embodiment has a central aperture through which microwave

radiation passes, this is not essential. In arrangements where the microwave receiver and

gimbal lie wholly behind the primary reflector, and the substrate is intrinsically transparent

to microwaves, the aperture may consist of an uncoated portion of the substrate, rather

than a physical aperture er se. The transparent substrate need not be polycarbonate, but

may be any other material having the necessary dimensional stability and transparency.

It is evident that it is only necessary for the substrate to be transparent to those

wavelengths to which the receivers 4 therebehind are to respond. The transparency of the

substrate to other wavelengths is irrelevant to the invention.

While the above described non-limiting embodiment has described the

invention as applied to a missile, the invention is not limited thereto. It may be equally

well applied to other guided munitions as well as other fields where sensors of two

different wavelengths need to pass through a common aperture.

Thus for use in space applications beyond planetary atmospheres, it is not

necessary to provide a radome, which is provided only for aerodynamic streamlining.

It will be seen that the present invention provides a dual frequency seeker that

uses a tandem placement of the energy gathering systems for both laser and millimeter

wave radiation. A feature of the invention is that the forward energy gathering system (the concave reflector) is constructed to be transparent to the frequency of the rear energy-

collecting system (the laser detectors). The energy for the rear system passes through the

forward system and is collected by the rear system. This arrangement allows the

simultaneous use of the entire aperture for each of the frequencies. The other components

of the seeker need not be placed in a tandem configuration. They can be placed in various

configurations as long as the energy gathering systems are in tandem configuration. The

tandem placement allows more streamlined dual sensor configurations allowing each of the

sensors to perform to their own technical limit within the full diameter of the missile

preserving each sensor's maximum detection ranges and aerodynamic performance.

Common use of radomes, mounting structures and processors can significantly reduce the

number of components while providing increased system performance. Parts count and

complexity can be reduced, yielding lower costs.