Tracking systems

This invention relates to systems for tracking projectiles using optical means.

On release of a ground or air launched guided missile which is intended to intercept an aerial target, for example, it is necessary to track the missiles position relative to the target up to the point of impact.

It is known to use a scanned laser beam to illuminate a friendly missile and thereby track the missile. For example, in GB-A-2,113,939 receivers on board a missile deduce the missile's position relative to a ground-based tracking system by decoding a modulation superimposed on the scanning beam. This method requires sophisticated electronic circuitry to be carried onboard the missile for the purpose of decoding the modulation. A further disadvantage is the lack of a communications link from the missile back to the tracking system.

This invention aims to provide a two-way communications link between a tracking station and a friendly missile, the link being realizable using comparatively simple components.

According to the present invention a tracking system comprises:

a transmitter and a receiver, the transmitter including means for producing a scanned optical beam for the illumination of a projectile;

and a plurality of transponders for mounting on the projectile, each transponder comprising a detector/emitter pair for detecting the scanned beam and for emitting a further optical beam in response thereto for reception by the receiver.

The means for producing a scanned optical beam may comprise a laser coupled to a mechanical scanning arrangement (eg. as described in our co-pending Application EP92301208 published as European Patent Application publication number 50026B) or to an acousto-optic deflector cell.

An acousto-optic deflector cell may comprise any suitable material which exhibits an acousto-optic effect ie. diffraction of light by acoustic waves. One example of such a material is tellurium dioxide. Acoustic waves are usually coupled into the cell via a piezoelectric transducer, for example, which is bonded to one face of the cell.

As is known, an acousto-optic deflector cell may be configured to receive a beam of laser light, of frequency f say, and in response to a high frequency drive signal applied to the cell (in the MHz to GHz range), some of the light emerging from the cell is deflected to form a so-called “first order beam”. The angle of deflection of this beam with respect to the undeflected zero order beam is substantially proportional to the frequency of the drive signal, (fac) which initiates a sound wave which propagates across the cell. Hence by varying the drive signal frequency in some controllable manner, a beam may be scanned in a single plane. Two dimensional scanning may be achieved by incorporating a second deflector cell through which the first order beam produced by a first cell is arranged to pass as described in GB-A-2,113,939.

The transponders may comprise a single solid-state photodetector element located adjacent to a laser diode. Preferably the divergence of the beam emitted by the laser diode is small in order to minimize the risk of battlefield eye damage.

Some embodiments of the invention will now be described, by way of example only, with reference to the drawing which is a schematic representation of a tracking system in accordance with the invention.

FIG. 1 shows a schematic representation of a tracking system in accordance with the present invention.

In the drawing, a ground-based transceiver 1 comprises a laser 2 for producing an optical beam, an acousto-optic deflector cell arrangement 3 for scanning the beam in two dimensions and a digitally-controlled voltage to radio frequency (RF) converter 4 which provides a drive signal for the cell arrangement 3 . Digital control of the drive signal enables rapid scanning of the beam.

The transceiver 1 also incorporates a detector element 5 for receiving optical radiation from an external source. The output from the detector 5 and a signal from the voltage to RF converter 4 are both coupled to a computing unit 6 .

Also shown in the drawing is a missile 7 which is being tracked by the ground-based transceiver 1 . To the rear of the missile 7 are mounted four equi-spaced hemispheres 8 (only two being shown in the figure).

On the outer surface of each hemisphere 8 are located a plurality of transponders 9 , each transponder 9 being composed of a photodetector element 10 and a laser diode 11 . The photodetector element 10 and laser diode 11 of each transponder 9 are mounted close to one another and aligned so that they are “looking” in the same direction, the beam divergence of the laser diode 11 being at least equal to the field of view of the photodetector element 10 .

In operation, the transceiver 1 begins to scan an area containing the missile 7 . To ensure that the scanned beam emitted from the transceiver 1 illuminates at least one transponder 9 , the transponders 9 are sufficient in number and so disposed around the hemishpere 8 that the fields of view of each of them overlap. When the scanned beam illuminates one of the photodetector elements 10 mounted on a hemisphere 8 , the adjacent laser diode 11 is immediately switched on, sending a beam back to the transceiver 1 . This beam is detected by the detector element 5 which sends an output signal to the computing unit 6 . From information transmitted by the voltage to RF converter 4 , the computing unit 6 is able to deduce the scan co-ordinates of the beam from the laser 2 at the moment it receives the signal from the detector element 5 . Hence, the angular location of the missile 7 relative to, say, the boresight of the laser 2 is known. By monitoring instantaneous values of angular location, the missile 7 can be tracked.

In an alternative embodiment, each detector/emitter pair forming a transponder 9 is mounted within the body of the missile and communciates with the missile's surface via fiber optic links. In a further embodiment, the transponders 9 incorporate means for modulating the beam emitted by the laser diodes 11 . This modulation could be used to convey IFF information to the ground-based transceiver 1 .