GB2151871A

GB2151871A - Laser weapon detector

Info

Publication number: GB2151871A
Application number: GB08425590A
Authority: GB
Inventors: Clive Ian Coleman
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1984-01-13
Filing date: 1984-10-10
Publication date: 1985-07-24
Also published as: GB2151871B; GB8425590D0

Abstract

Apparatus for detecting an imminent attack by a pulsed laser beam or by a weapon directed by such a pulsed laser beam A-B-C comprises three or more directional detectors (1, 2, 3) arranged to detect light in respective different directions OA, OB, OC atmospherically scattered from the pulse when it is near-incident. By measuring the times of arrival of the pulse of scattered light, the orientation and/or position of the beam can be derived. <IMAGE>

Description

SPECIFICATION Light detector The present invention relates to the detection of light beams and in particular to the detection at a given location of a pulsed light beam directed towards, but not incident upon said location. The invention is particularly applicable to the detection of laser beams of the type used for sighting and range-finding.

An object of the present invention is to provide means for detecting a near-incident pulsed laser beam.

According to one aspect of the present invention, apparatus for detecting a pulsed laser beam comprises directional detector means sensitive in three or more defined coplanar directions to light atmospherically scattered from said beam, and timing means linked to said detector means and arranged to compare the times of arrival of said scattered light from at least three of said directions and thereby derive the orientation and/or the position of said beam.

Preferably said timing means is arranged to compare the time interval between the arrival of scattered light from a first and a second of said directions with the time interval between the arrival of scattered light from said first and a third of said directions. The orientation of the beam with respect to the first direction can then be derived from the equation: t3 2sinO sin(0+)[cos0-cos(0+oz)] t2 - sin(20+sinzx-sin(0+o)] where t3 is said time interval corresponding to the arrival of light from said third direction, t2 is the corresponding time interval for the second direction, o is the angle subtended by said first and second directions (assumed equal to the angle subtended by the second and third directions) and a is the angle subtended by the beam and said first direction.

The range R (that is the distance from the intersection of the beam with the first direction) is then given by the expression: R = ct3 sin(20+a) 2sinB[cosB -cos(0+a)j where c is the velocity of light.

Alternatively the range R can be found from t2 by using the expression: R = ct2sin(()+a) sin(3+sina-sin(0+a) Preferably said detector means is sensitive in five or more (preferably eight or more) directions distributed regularly about an arc of substantially 360 .

The required directional sensitivity may be achieved by providing a multiplicity of optical focussing means, each oriented in one of said directions and arranged to focus scattered light onto a suitable detector element.

Apparatus in accordance with the invention may be mounted on a military vehicle or aircraft and used to detect a near-incident laser beam from an enemy range finder or laser weapon.

According to another aspect of the invention a method of detecting an imminent attack on a given location by a laser beam or by a weapon directed by a laser beam, involves the detection at said location of light atmospherically scattered from said laser beam.

Preferably, if said beam is pulsed, said scattered light is detected from three or more known directions and the position and/or orientation of the beam are calculated from the times of arrival of scattered light along said directions. Appropriate evasive or countermeasure action may then be taken.

The weapon may in some cases be identified by measuring one or more of the parameters; wavelength, pulse length, pulse profile, pulse power (from the scattering intensity) and pulse repetition frequency.

An embodiment of the invention will now be described byway of example with reference to Figures 1 to 3 of the accompanying drawings, of which: Figure la is a diagram illustrating the derivation of the orientation and position of a pulsed light beam from the times of arrival of scattered light from three directions; Figure ib is a typical plot of the outputs of directional light detectors located at 0 and directed along lines OA, OB and OC of Figure 1.

Figure2 is a plot of the terms (sepa + tana -1 and tc/tB as a function of a in Figure 1; Figure 3 is a plot of tb and tc against miss distance M for a detector in accordance with the invention, and Figure 4 is a sketch perspective view of a compact apparatus in accordance with the invention suitable for mounting on a tank.

Referring to Figure 1, a light beam travelling in the direction AC undergoes scattering by atmospheric aerosol particles in its path. A group of light detectors are located at 0 and are sensitive in directions OA, OB, OC, OD, OE . . . OM, ON. The orientations of the sensors are regularly spaced apart by an angle e (û < 90 ) so that at least three sensors pick up scattered light from the beam, namely those pointing in directions OA, OB and OC. The outputs of these sensors are shown in plots A, B and C of Figure 1 (a). The first sensor (plot A) to pick up scattered light starts a clock which measures the time intervals tB and tc before the arrival of scattered light from B and C respectively.

The range R and the angle a which determine the position and orientation of the light beam with respect to the group of sensors can be determined from tB and tc as follows: If c is the velocity of light, ctC = AB + BC + OC - R and ct5 = AB + OB - R But AB OB BC OB -- = ---- = - sin # sin α' sin # sin(2#+α) OC OB R OB -- = --- and -- = - sin (#+α) sin (2#+α) sin (#+α) sinα

This expression may be simplified to give:

If, for example, α = 45 , expression (1) simplifies to:

Expression (2) is plotted in Figure 2 as a function of a and may be stored in appropriate look-up tables in a memory to enable a to be determined from the ratio tc/tB.Preferably more than one group of the three sensors is chosen to measure to and te, so that a value of or a greater than about 300 can be measured, in order to ensure maximum accuracy.

Since R sin# AB = --- sin(#+α) and R sin α OB = --- ; sin(#+α) sin# sinα ctB = R --- + --- - 1 sin(#+α) sin(#+α) This expression can be simplified to: ctB sin (#+α) (3) .. R = --- sin #+sinα-sin(#+α) The alternative expression for R in terms of to can be derived similarly and is: ctc sin (2#+α) (4)..R = ---- 2 sin#[cos#-cos(#+α)] When û = 45 this expression reduces to: (5)..R = ct, (seccw + tancu -1)-1 and the term (seca + tana-1)1 is also plotted in Figure 2.

In Figure 3 the arrival times tB and to (in nanoseconds) at different values of a (in degrees) are plotted against the miss distance M, in metres (Figure 3a) which is the perpendicular distance to the light beam ABC from the origin 0. The plots relate to a 45 angular spacing (û) between the sensors. It can be seen that tB and to will typically be of the order of one microsecond at a range of about 1 km.

The arrangement shown in Figure 4 comprises a mirror 4 in the form of an inverted truncated octagonal pyramid having faces 5 inclined at 45 . In front of each of these is mounted a baffle 1 (only two of which are shown, for the sake of clarity), a lens 2 and optical detector 3 being mounted in the optical path of each baffle 1. The baffles, detectors, lenses and mirror are mounted in a common protective housing (not shown). Thus each baffle 1 forms part of a highly directional optical detection system, the angle û between adjacent optical axes being precisely 45 . Each detector element 3 is connected to a timing circuit 6 which is programmed to calculate with the aid of appropriate look-up tables the range and/or orientation of any near incident laser beam from an enemy laser.

The system described so far is adequate for many ground-based applications, where likely laser paths will generally not be too far out of the horizontal plane. However, for certain applications (e.g. in airborne use), an extra dimension of information is required. This can be provided simply by substituting a vertical detector array for each single element detector (with appropriate optical modifications); with suitable inerpolative signal processing, angular resolution in elevation can potentially be derived to an accuracy better than the angle subtended by an element ofthe array.

Claims

1. Apparatus for detecting a pulsed laser beam comprising directional detector means sensitive in three or more defined coplanar directions to light atmospherically scattered from said beam, and timing means linked to said detector means and arranged to compare the times of arrival of said scattered light from at least three of said directions and thereby derive the orientation and/orthe position of said beam.

2. Apparatus according to Claim 1 wherein said timing means is arranged to compare the time interval t2 between the arrival of scattered light from a first and a second of said directions with the time interval t3 between the arrival of scattered light from said first and a third of said directions and is arranged to derive the ratio of : t2, look-up means being provided to give the orientation of said beam for any given value of said ratio.

3. Apparatus according to Claim 2 arranged to derive the orientation of said beam from the expression: t3 2sin# sin(#+α)[cos# - cos(# + α)] -- = --- t2 sin(2#+α)[sin#+sinα-sin(#+α)] where t2 is said time interval corresponding to the arrival of light from said third direction, t2 is the corresponding time interval for said second direction, (3 is the angle subtended by said first and second directions and is equal to the angle subtended by said second and third directions, and a is the angle subtended by said beam and said first direction.

4. Apparatus according to Claim 3 wherein the angle # is 45 .

5. Apparatus according to Claim 2 arranged to derive the range of said beam from its derived orientation with respect to one of said directions and the time intervals between the arrival of scattered light from two of said directions.

6. Apparatus according to Claim 5 arranged to derive the range from the expression: R = ct3sin(2(3+a) 2sinB[cosB - cos(8+a)l where: c is the velocity of light, R is the distance from the intersection of said beam with said first directions, a is the angle subtended by said beam and said first direction, and û is the angle subtended by said first and second directions and is equal to the angle subtended by said second and third directions.

7. Apparatus according to Claim 6 wherein the angle û is 45 , means are provided for calculating the expression: (seca + tana 1)-i and look-up means are provided for deriving the range from the expression: R = ct3(seca + tana-1)-' where R is the distance from the intersection of said beam with said first direction.

8. Apparatus according to Claim 5 arranged to derive the range from the expression: ct2 sin(û+a) sinû+sina-sin(û+a) where: c is the velocity of light, R is the distance from the intersection of said beam with said first directions, a is the angle subtended by said beam and said first direction, and û is the angle subtended by said first and second directions and is equal to the angle subtended by said second and third direction.

9. Apparatus according to any preceding Claim wherein said detector means is sensitive in five or more directions distributed regularly about an arc of substantially 360 .

10. Apparatus according to Claim 9 wherein said detector means is sensitive in eight or more directions distributed about an arc of substantially 360 .

11. Apparatus according to any preceding Claim incorporating means for determining one or more of the parameters: wavelength, pulse length, pulse profile, pulse power and pulse repetition frequency of the pulses of said beam.

12. Apparatus substantially as described hereinabove with reference to Figure 4 of the accompanying drawings.

13. A method of detecting an imminent attack on a given location by a laser beam or by a weapon directed by a laser beam, comprising the detection at said location of light atmospherically scattered from said laser beam.

14. A method as claimed in Claim 13 of detecting imminent attack by a pulsed laser weapon or by a weapon directed by a pulsed laser beam, wherein said scattered light is detected from three or more known directions and the position and/or orientation of the beam are calculated from the times of arrival of scattered light along said directions.