GB2158316A - Optical communications and missile guidance systems - Google Patents

Abstract

Information transmitting method using a laser beam projector which forms part of an optical missile guidance system on board a ship say. For transmitting information to another ship the same beam projector 4 is used to scan a field containing a detector 7 on board the other ship so that the detector receives successive glimpses of the beam at times dependent on the information to be transmitted. Alternatively, after acquisition of the detector in this way, the projector can be aligned by feedback and the beam modulated using conventional amplitude or pulse modulation. The specification also discloses developments of the missile guidance systems described in GB applications 2113939A and 2133652A for example blanking of the projector beam as it is scanned over the target to prevent detection of the projector source by the enemy target. <IMAGE>

Description

SPECIFICATION Communications and missile guidance systems Our UK patent application No's 2,113,939A and 8330302 disclose optical missile guidance systems wherein a laser beam is scanned over a field-of-view in accordance with a scan pattern or raster which is such that a missile within the field-of-view glimpses the laser beam twice during the execution of each raster and the time between these glimpses is indicative of the relative position of the missile within the field-of-view. Application No 8330302 further discloses a means of passing steering and/or command signals to the missile, this involving the adjustment of endof-line delay times within the raster execution, i.e. delay times between each two consecutive raster line in accordance with the information to be transmitted.

One object of the invention is to extend the possible uses of such a guidance system and hence to render its installation, on board a ship say, more cost effective.

Another object is to provide a means of secure communication, between two ships or between a ship and an aircraft say, which makes use of missile guidance equipment already installed on board the ship.

A third object is to provide a means whereby the command station of a missile guidance system as disclosed in the aforementioned patent applications is rendered somewhat less vulnerable to detection.

According to one aspect of the invention, there is provided a method of passing information from a transmitting station to a receiving station, on board respective ships for example, in which method a laser beam projector sited at the transmitting station and forming part of an optical missile guidance system is used to project a laser beam towards a laser radiation sensitive element at the receiving station and said laser beam is controlled so as to cause said sensitive element to produce a signal containing said information.

According to another aspect of the invention, there is provided an optical missile guidance command installation including a laser beam projector for projecting a laser beam and for scanning said beam over a field-ofview, and control means for controlling said projector to guide a missile towards a target, characterised in that the installation is provided with further beam projector control means operable, as an alternative to the firstmentioned control means, for receiving an information bearing signal and for so controlling the beam projector that a remote laser radiation sensitive element positioned for receiving said beam will respond to the beam to reproduce said signal.

According to a third aspect of the invention there is provided an optical missile guidance system including a laser beam projector for projecting a laser beam and scanning said beam over a field-of-view, the installation including means for modulating said beam so that, while it is being scanned over a selected portion of said field-of-view or would have been so scanned, the beam intensity is at least reduced relative to the intensity for the remainder of the field-of-view.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing, in which: Figure 1 is a diagrammatic view of two ships communicating with each other by a method according to the first aspect of the invention, and Figure 2 is a diagram of a target used on board each ship of Fig. 1.

In Fig. 1, two ships 1 and 2 with a line of sight from one to the other are fitted with respective scanning laser beam projectors 3 and 4, respective thermal imagers 5 and 6, and respective electro-optical 'targets' 7 and 8, each of which targets comprises some form of infra-red emissive marker, a hot-wire for example, detectable by the thermal imager on board the other ship and an adjacent electrooptical sensor able to detect the laser emission from the other ship.

The laser information field, i.e. the field scanned by the laser beam, on board ship 1 will be normally locked onto the target on ship 2 while the system is in its electro-optical automatic tracking mode (described later).

Should this prove difficult, in rain at night for example, the thermal imager can be used to provide an optical sight line.

The laser beam projector and thermal imager on board each ship are positioned adjacent one another and are coupled together so that the area imaged by the thermal imager is at least approximately boresighted with the area scanned by the laser beam.

When the ship 1 wishes to communicate with ship 2, it uses its thermal imager 5 to find the target marker of ship 2 and then the boresight of the imager 5 is aligned with or aimed at that marker. The target sensor of ship 2 will then be within the area scanned by the laser beam and will glimpse the beam each time the scan pattern is executed so producing a series of electrical pulses. The scan pattern executed by the laser beam is such that the time interval between two consecutive glimpses of the laser beam by the target sensor on ship 2, and hence also the time between two consecutive pulses produced by the sensor, are indicative of the position of the target sensor relative to the area scanned by the laser beam as described in patent application No's 2,1 13,939A and 8330302.The pulses, or signals encoded with positional information which can be derived from those pulses, are then retransmitted back to ship 1 from ship 2. Since, in the illustrated case, the ship 2 is equipped in the same way as the ship 1, the retransmission would be done probably by the ship 2 using its own laser beam projector and thermal imager to set up a similar line of communication back to the target sensor on board ship 1. However, the re-transmission could be by way of some alternative form of signal communication which may be available.

The re-transmitted information is then used within ship 1 to correct for any errors in the relative positioning of the laser beam scan and the target sensor of ship 2. For example, if the original pulses themselves are simply retransmitted, then appropriate processing equipment on board ship 1 is used to compare the actual timing of the pulses with that predicted. To overcome any such relative position errors, the laser beam projector can be physically moved relative to the thermal imager or preferably, since then no mechanical adjustments are involved, the various end-ofline delays incorporated in the scan pattern can be adjusted as described in the aforementioned patent applications so that at least the apparent relative position becomes correct.

Having obtained the correct actual or apparent relative positioning of the laser beam raster, the communication proper can commence. This is done, as described in patent application No 8330302, by causing the laser beam projector on ship 1 to execute a set quence of scan patterns, the sequence possibly continuing to include a pattern similar to that previously executed so that a continuing check can be kept on the relative positioning of projector and target sensor, but also including one or more scan patterns for which the end-of-line delays are so set that the betweenpulse intervals measured in ship 2 carry the information to be communicated.After the initial position correction, and at intervals or throughout the signal communication, the two ships may exchange 'hand-shake' signals to confirm verification of received signals and verify proper operation of the equipment.

As shown in Fig. 2, the target on board each ship can comprise a support frame 20, which could be position-stabilised by say a gyroscopic stabilising arrangement (not shown) with an electro-optical sensor element 21 positioned at the centre of the frame and one or more infra-red emissive marker elements 22, for example infra-red hot wire devices. The target is best positioned relatively high up in the ship's superstructure, say at or near the top of a mast as shown in Fig.

1.

If, as is preferred, the laser beam projector on board each ship comprises an acoustooptic deflector arrangement for causing the beam to execute the scan pattern, there becomes easily available a somewhat modified form of communication. An acousto-optic deflector simply deflects the laser beam in accordance with the deflection control signals applied to it. In order to execute a scan pattern, the control signals are made to have an appropriate repetitive waveform. Thus, when the projector is to be used for communication, this repetitive waveform could be replaced by a control signal which simply maintains the laser beam aimed at the target on board the other ship. Initial aiming may again be achieved by use of a thermal imager on board ship 1 to find and give approximate alignment with the target of ship 2.Fine adjustment can then be carried out by feedback, from ship 2 to ship 1, of the amplitude of the signal generated by the target sensor of ship 2, the deflection control signals and hence the beam direction being adjusted to achieve a peak in this amplitude.

The information to be passed to the ship 2 is transmitted as an analogue amplitude modulation or as any suitable form of pulse modulation, for example pulse position or pulse width modulation, of the laser beam.

The modulation is introduced into the beam by any of various known techniques, for example by making use of the amplitude modulation capability of the acousto-optic deflector arrangement itself.

Instead of maintaining the aim of the laser beam by feedback of the received beam amplitude from ship 2 to ship 1, it could be done entirely on board the ship 1 say by monitoring the radiation backscatter from the target of ship 2.

As described earlier, the laser beam projector is also usable as described in the aforementioned patent applications as part of an optical missile guidance system, the laser beam then being scanned over a field-of-view containing a missile and an enemy target and the missile comprising means for sensing and timing successive glimpses of the laser beam and for using such time measurements to guide itself within the field-of-view and eventually onto the enemy target. As will be appreciated, the laser beam may also be detectable by the enemy target thereby alerting the enemy to the impending threat and also possibly disclosing the position from which the beam originates. To avoid this the co-ordinates of the enemy target position, supplied by whatever apparatus is used to detect and track the enemy target (a radar system perhaps or a thermal imager), are fed to the laser beam transmitter which uses them the ensure that no laser power is transmitted to the enemy target, for example the transmitter can so amplitude modulate the beam that while the enemy target position itself is being scanned or would have been scanned, the laser beam is turned off. Meanwhile the missile is guided at least initially along an off target axis trajec tory. Eventually the missile has to be moved into alignment with the target axis, i.e. the line of sight from the beam projector to the enemy target, at which point the amplitude modulation can cease so that the missile can continue to receive guidance from the projector.This at least reduces the time during which the enemy target can detect the beam.

Alternatively, the missile could be provided with a homing head or seeker. In this case the laser beam projector, with the amplitude modulation to avoid its location by the target maintained, is used only to guide the missile along its initial off-axis trajectory and then, by physically moving the projector or by adjusting the scan pattern end-of-line delays, to slew the missile onto the target slightline.

After some fixed flight time, or when the missile has determined that the laser beam intensity has become less than a predetermined threshold, it changes over to guidance by its homing head. The advantage of the latter system is that, although the missile requires a homing head, since the missile is guided at least approximately towards the target by the laser beam projector, that homing head can be a much lighter and less expensive device than would be the case if it were the sole means of guiding the missile.

Fuzing of the missile may be initiated by the lapse of some predetermined flight time or by the loss of the laser beam or it may be done positively by commands transmitted as variations of the laser beam scan pattern as described earlier.

A further method of avoiding or reducing the chance of disclosure of the laser projector position, which method is particularly suitable for gun-launched projectiles having a terminal guidance capability, is to leave the laser projector switched off for the initial or ballistic phase of the projectile trajectory and to switch it on only during the terminal phase when it is needed.

As mentioned, a missile guidance system might comprise a thermal imager for detecting and tracking an incoming target and a laser beam projector for guiding missiles to the target, the projector being controlled by the imager so as to maintain the beam scanned area properly positioned with respect to the target. The thermal imager and beam projector are preferably assembled on the same servo-base. This permits the use of an inner loop solid state electronic correction of the laser field angles for correcting any servo errors and for the target not being centred within the field-of-view of the imager. This in turn permits the servos to be deliberately misaligned thereby permitting earlier illumination of the missile without effecting the information received thereby.

Throughout this specification, the term missile also includes the so-called GLGP's (gun launched guided projectiles).

Claims (4)

1. A method of passing information from a transmitting station to a receiving station, on board respective ships for example, in which method a laser beam projector sited at the transmitting station and forming part of an optical missile guidance system is used to project a laser beam towards a laser radiation sensitive element at the receiving station and said laser beam is controlled so as to cause said sensitive element to produce a signal containing said information.

2. An optical missile guidance command installation including a laser beam projector for projecting a laser beam and for scanning said beam over a field-of-view, and control means for controlling said projector to guide a missile towards a target, characterised in that the installation is provided with further beam projector control means operable, as an alternative to the first-mentioned control means, for receiving an information bearing signal and for so controlling the beam projector that a remote laser radiation sensitive element positioned for receiving said beam will respond to the beam to reproduce said signal.

3. An optical missile guide system including a laser beam projector for projecting a laser beam and scanning said beam over a field-of-view, the installation including means for modulating said beam so that, while it is being scanned over a selected portion of said field-of-view or would have been so scanned, the beam intensity is at least reduced relative to the intensity for the remainder of the fieldof-view.

4. A method, installation or system substantially as hereinbefore described with reference to the accompanying drawings.