GB2046550A - LIDAR system - Google Patents

Abstract

A doppler LIDAR guidance system for a missile uses a CO2 laser with optical superheterodyne reception achieved by feeding a part of the output signal from laser 13 from beam divider 13b through a modulator 17b to a mixer 17a for recombination with the reflected signal 14a. The transmitted signal carries IFF interrogation information (Fig. 2, not shown) and is modulated and spatially deflected at 18 to allow tracking of the target 12 by a tracking computer 21. The beam 14 initially forms a guide beam for a first flight phase of the missile and may thereafter serve as a target illuminating marker with reflected and modulated radiation being detected by an on-board missile system. A further embodiment (Fig. 1b, not shown) has a second high stability laser forming a local oscillator for the mixer 17a. The transmitted beam thus performs the function of guiding the missile, target illumination and IFF interrogation. <IMAGE>

Description

SPECIFICATION A CO2 laser doppler LIDAR for target location and missile guidance.

This invention relates to a target locating and mis sile guiding system using a CO2 laser doppler LIDAR.

Target locating and missile guidance systems are known and at least two CO2 lasers are required, one for the locating and the otherforthe guiding operation. The known systems require complex arrange ments for transferring the direction of the located target to the missile guidance system and for provid ing in addition an IFF signal. The sensitivity of the arrangements or systems is generally inadequate. In a two phase guidance system the missile in the first phase is guided by an IR or heat seeking unit which locks onto the target but this is subject to countermeasures.

This invention seeks to provide a system enabling small moving targets, including missiles to be detected, tracked, identified and countered even when close to the ground.

According to the invention there is provided a target locating and projectile guidance system using a CO2 laser doppler LIDAR with an optical superheterodyne receiver, wherein the emitted locator beam, following target acquisition, tracks the target and further provides an IFF carrier signal, missile guidance beam and target marking means.

With passive known systems using heat seeking apparatus as example, only targets against ground background are detectable and even so they often do not stand out clearly enough. Preferably the locator beam guides the missile during the initial flight phase, the target-tracking flight phase being controlled by an IR target seeking head which is receptive to modulated laser radiation from the locator beam. In this invention use of a laser doppler LIDAR eliminates factors of uncertainty and the system has a CO2 laser with optical superheterodyne reception.

The arrangement provides for evaluation of the duration and frequency shift of the laser radiation reflected from a moving target whereby the direction, distance and velocity of the target can be determined.

Further features of the invention are described in conjunction with the accompanying drawings showing embodiments by way of example only. In the drawings: Figure la shows schematically the arrangement of a CO2 doppler laser LIDAR with homodyne reception, Figure 1b shows schematically the arrangement of a CO2 doppler laser with superheterodyne reception, Figure 2 shows a block diagram of the'lFF system, Figure 3 shows schematically the arrangement of the guidance system for superheterodyne reception, and Figure 4 shows schematically the arrangement of the direct guidance receiving system.

The use of a high-performance CO2 laser in the system to which the invention relates, in place of Nd: YAG lasers hitherto used for military purposes enables the wave length range to be extended to the mid-infrared range, so that atmospheric penetration by the laser is more satisfactory and the system is compatible with the IR thermal imaging apparatus (FLIR) now in use. Furthermore, the increased wave length makes su perheterodyne reception possible, that is relative speeds can be measured by doppler shift evaluation.

In this selected wave length range optical counter-measures are difficult because the CO2 laser radiation in these systems can be reduced to a power density of 0.1 W/cm2 and danger to personnel is completely avoided. The acquisition and missile guiding capacity of the system of the invention is not reduced up to a nominal range of about 5 km and using 5W laser. Using a CO2 wave guide laser it is now possible to reduce the overall volume and weight of the apparatus considerably but maintaining the required high degree of mechanical stability.

This is a precondition for frequency stability in the emitted radiation when using superheterodyne reception. In the superheterpdyne reception (wave length of 10.6 ) a reference frequency having a fixed frequency difference frbm the signal frequency is superimposed, in the receiving detector on the received signal. This can be done by means of a separate beam derived from the transmission beam for homodyne reception or by means of a separate "local oscillator" laser beam for heterodyne reception. Homodyne reception makes it possible to dispense with an additional laser.

The electrical signal occurring at the detector output and at the intermediate frequency is subject to narrow-bandwidth filtration and amplification. In the reception of the radiation reflected back from a moving target an additional frequency shift through doppler effect occurs.

The system of the invention operates as follows: The marksman directs the aiming device 10, atthe firing station 11, approximately in the direction of an assumed or observed target 12 and then switches to active location, the control of the entire system is then taken over by the control unit 20. The CO2 wave guide laser 13 is activated and emits a locater beam 14, on which is superimposed, by a mod ulatorldeflector 18, a spatial modulation suitable for target scanning and tracking, as well as a time modulation suitable for distance measurement and IFF and a receivable signal forthetarget seeking head of a missile which is thus guided towards the target.

The location beam 14 is reflected back from the target 12 to a receiving lens 15 and a detector 16 with superimposed radiation from a local oscillator 17 mixed at 17a (in the case of heterodyne reception) or from the laser transmitter 13 via beam divider 13b (in the case of homodyne reception). In the latter case a frequency shift is effected by a modulator 17b. The position of the difference frequency between the emitted and the received radiation 14, 14a, which is determined in a known manner, provides an indication of the radial movement of the target For measuring the distance of the target the laser beams 14 is amplitude-modulated and the phase difference between the transmitted and the received modula- tion evaluated by the units 18 and 19.In the evaluation unit 19 the values determined are passed on to the tracking computer 21 and to the control unit 20.

In certain cases it may be of advantage for a pulse system to be used with a time and distance gate being set which enables the closeness of the locating apparatus to be screened out.

From the evaluation unit 19 the values determined are fed to a tracking computer 21 which trackes the target by means of a modulator/deflector 18. At the same time, through the control unit 20 and the 1FF unit 22 associated therewith, an IFF verification is effected. The IFF unit 22 has an IFF code generator 22a and an IFF receiver 22b. The associated IFF responder unit 23of a "friend" target comprises an IFF decoder 23a, and IFF answering code generator 23b, a modulator driver 24a and a modulator unit 24b, which precedes the IFF responder reflector 24c and is opened synchronously by the answering code pattern. The attack station therefore automatically receives the IFF code and the reflector cannot respond to a spurious laser system.

The laser signal reflected back from atarget equipped wich an IFF responder is always clearly identifiable, as the reflection is several orders of magnitudes greater than a diffusely reflected target signal. Owing to the small divergence of the reflected ray (0.1 mrad) it is only receivable at the attack station itself which makes it easier to preserve secrecy of the IFF answering code.

Figures 3 and 4 show the system applied to a guided missile 30. The firing point or the aiming and sighting point 11 has, as previously described, the locating and guidance beam 14. The missile 30 has at the rear a guide beam receiver 31.This can be arranged, as shown in Figure 3, for direct reception, or else arranged for superheterodyne reception, as in the'embodiment of Figure 1. It may also comprise, as shown in Figure 4 of a simple responder and the position of the projectile is then evaluated by means of the reflected beam or signal atthefiring point and the necessary control commands are transmitted separately.

The nose of the missile has a target seeking head 32. Figure 4 shows the system arranged for direct reception bythetargetseeking head but as shown in Figure 1 superheterodyne reception is also possible.

The target seeking head has a receiving lens 45 and a detector 41 for the signals reflected from the target, which in the second guidance phase transmit signals di rect to the control system 33.

The signal evaluation unit 49 only responds to signals which are modulated according to the modulation pattern applied to the modulator/deflector 18. This ensuresthatthe IR seeking head is responsive only to the beam 14 serving also as a target illuminator. This type of target iliumination generally marks the geometrical centre of the target, which is a more favourable location from the point of view of the weapon effect than the thermal centre of the target found by normal IR target seeking heads. In addition, counter-measures are made more difficult.

In the system described the locater beam transmitted is further activated following acquisition of a target which is then automatically tracked in attack ing mode, which at the same time serves as an IFF interrogater with a missile guide beam forming a target marker. In one example, described in the foregoing, the locator beam serving as a guide beam controls the first flight phase of missile, which may also be designated as the firing phase, while the second target seeking phase is guided by the IR target seeking head 12, which is receptivetothe modulated laser beam reflected from the target.

Claims (8)

1. Target location and projectile guidance system using a CO2 laser doppler LIDAR with an optical superheterodyne receiver, wherein the emitted locator beam, following target acquisition, tracks the target and further provides an IFF carrier signal, missile guidance beam and target marking means.

2. A system in accordance with Claim 1, wherein the locator beam guides the missile during the initial flight phase,thetarget-tracking flight phase being controlled by an IR target seeking head which is receptive to modulated laser radiation from the locator beam.

3. A system in accordance with Claim 1 or 2, wherein the optical superheterodyne reception is provided by a homodyne or heterodyne detector.

4. Apparatus forming the system of any preceding Claims 1 to 3, wherein the transmitter comprises a frequency-stable CO2 wave guide laser including a modulator and an optical transmission system with which are associated a control means and an IFF unit, and the receiver comprises an optical pick-up system, a beam divider, a modulatorto produce the necessary frequency shift of the laser beam in order to provide for homodyne reception and a beam mixer th rough which the reflected parts of the locator and guide beam and a beam from the transmitter are fed to a detector from which the output signals are conveyed to an evaluation unit to determine azimuth, elevation, distance and target movement values and to an IFF receiver unit the signals being passed to a control means and tracking computer which are operatively coupled to the missile and guidance system.

5. Apparatus in accordance with Claim 4, modified insofar as the beam divider and the modulator are replaced by a laser forming a local oscillator laser.

6. Apparatus in accordance with Claim 5, wherein the local oscillator laser has a fixed and accurately defined frequency difference from the main laser.

7. System for target acquisition and missile guidance constructed and arranged to function substant tially as herein described with reference to and as shown in the accompanying drawings.

8. Apparatus for guiding a missile to a target constructed and arranged to function substantially as herein described with reference to and as shown in the accompanying drawings.