GB2231219A - Rangefinder - Google Patents

Description

3 X:2 I- e CC/3256 Rangefinder.

There exists a need for a passive rangefinder suitable for use during covert, stealthy, battlefield operations. The traditional optical rangefinder requires a view of the target from two positions separated by a baseline, range then being determined from the angle between the two sightlines after they are set by operator manipulation. Submarine periscopes view from a single position and find range by comparing target widths under two different magnifications; this also requires operator manipulation. An object of this invention is to provide a rangefinder which computes range automatically without operator intervention.

According to the present invention, a rangefinder comprises a plurality of radiation detector elements, focussing means associated with the detector elements to produce in respect of each detector element a beam, within which the respective detector element is responsive to a target source, the arrangement being such as to provide a plurality of at least three beams, means responsive to a range dependent characteristic of a target image as determined by different combinations of the detector elements when a target passes through the associated beams to produce a target range indication. The range dependent characteristic may be the angular velocity of the target travelling through the beams.

There may be three beams with asymmetric angular spacing and means provided for timing successive interceptions of the three beams.

Two of the three beams may be parallel so providing a range-independent velocity reference indication.

The detector elements associated with the three beams may be arranged within the field of view of a single lens, the spacing between the parallel beams being provided by prisms in the path of one of the parallel beams. The prisms are preferably penta-prisms providing 900 path deflection.

A respective pair of divergent beams providing target transition times between the beams may be arranged to diverge from each of two locations, the locations being at effectively different target distances.

The detector elements may be at a forward one of the locations, the forward location providing a pair of divergent beams directly and the other pair of divergent beams diverging fror) the forward location and, after reflection, effectively from a rear location at a greater target distance than the forward location. The two pairs of divergent beams preferably have equal divergence angles and are arranged symmetrically in a horizontal plane.

The beams of each pair of beams may be provided by a single detector and respective focussing means, successive interception of the beams of the pair providing successive outputs from the single detector. Alternatively, the beams of each pair of beams may be provided by respective detector elements and single focussing means.

According to another aspect of the invention, the range dependent characteristic is the target image extent. In this case, a first array of detector elements may provide a plurality of closely spaced beams diverging from a forward one of two locations, a second array of detector elements providing a second plurality of closely spaced beams effectively diverging from a rear one of the two locations, and means being provided for determining the range of a target from the relative extent of the target images on the first and second arrays.

According to a further aspect of the invention, there may be means for scanning the divergent beams through a target position so t that the target intercepts the divergent beams successively to provide detector element outputs and range determination. In this case the rangefinder may be mounted in a periscope for rotation about an axis parallel to the bisector of the divergent beams, and including periscope reflectors for scanning the beams in a horizontal plane.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which:- Figure 1 is.a perspective view of a three-beam rangefinder in accordance with the invention; Figure 2 is a plan view of the system shown in Figure 1; Figure 3 is a plan view of a four-beam rangefinder; Figure 4 is a plot of received signal power against time for the detectors in a four-beam system; Figure 5 shows in plan the geometry of a four-beam system using folded beams; Figure 6 is a side view of the system shown in Figure 5; Figure 7 shows the images received at two detector arrays positioned at different optical path lengths from the target; and Figure 8 is a diagram of a range-finding periscope using forward and rearward beams.

Referring to Figures 1 and 2, three infra-red detector elements 1, 2, 3 are positioned in the focal plane of a lens 7, each detector element having a separate field of view V, 2', 3' respectively, constituted by the projection of the detector element in space by the optical system and shown as 'beams'. The beams 1' and 2' associated with detectors 1 and 2 respectively, diverge at a known azimuth angle 8. The beam 3' lies parallel to but spaced from the beam 2' of detector 2. The angular divergence of the three beams is thus asymmetric. Two penta-prisms 9, 10, direct radiation in the third beam 3' along a stepped path so permitting the third detector 3 to be mounted close to the other detectors. In the arrangement shown, the third detector 3 is'mounted in the same vertical line as the second detector 2, alignment of the respective beams into a horizontal plane being achieved by tilting one of the penta-prisms, but, subject to being accommodated by the same lens 7, the position of detector 3 is arbitrary and can be chosen for convenience.

As a target, such as a tank 20, passes through any of the beams, the infra-red radiation it emits causes a signal at the corresponding detector. Since the target is not a point source, and the 'beam' is not a vertical line, the signal has a significant duration, increasing through one or more maxima and then trailing off. A timing signal is therefore derived from the passing of the signal through a threshold value. If the target moves through all the beams the detectors will all generate essentially the same signal, but at different times. Referring to Figure 2, the range R to be measured is the distance of the target from an antitank missile launcher 8 along its firing line. This firing line must be in the same general direction and preferably quite close to the lines of sight of the detectors.

The second and third beams 2' and 3' are separated by a known distance x in the direction perpendicular to the range R. The velocity of the target in this direction, i.e. the tangential velocity, is given by Vt = X TX where TX is the time between detection in the third and second beams 319 2@. The angular velocity is given by W = 0 3 TO where T. is the time between detection in the second and first beams 2' V.

The range R can then be calculated from j R It:- AT 0 W @Tx (1) This equation (1) makes an approximation that the target crosses the angled fields of view 4 & 5 at the same distance from the detectors 1 and 2, this being the case in fact only when the tank is moving at right angles to the bisector of angle 8. For a target travelling at any angle with respect to the beams, it can be shown that the range R along beam V is given exactly by

R = x T TX tan 8 The determinations of Tx, Te and R are made by a processor 4 which receives the outputs from the detectors. The range information is then fed to the missile launcher control 5. Clearly, the system depends on the target maintaining the same line and speed through the three fields of view and through the firing line. For this reason the distance should not be very great from the firing line to the first encountered field of view which, it will be appreciated, could be, as here, the 'parallel' beam 3', or with an opp osite target direction, the angled beam V.

Accuracy of alignment (i.e. parallelism) of the second and third beams 2' and 3' is (in this embodiment) important for accurate ranging so the use of penta-prisms which produce a 900 deflection irrespective of their orientation is preferred. Alternatively, though, the detector element 3 could be replaced by a separate lens and detector element, spaced from the first two elements 1, 2, and at the position of the prism 10, which would remove the need for the prisms to change the direction of radiation. The system is simplified by having two beams parallel, but this is not absolutely essential. If beam 3' is inclined to beam 1' at an angle ee. (rather than 8), the range in the direction of beam V is given by R = Tax Tx[tan 8 + Te,Tx (tan 8 - tanOK)] 1 At long ranges with widely spaced fields of view, the three-beam system may be configured as an active system using three lasers, each to direct radiation to a respective field of view and produce reflections onto a single detector. At short ranges this configuration is less practical because of the difficulty of distinguishing close and even overlapping signals.

A different embodiment of the invention is shown in Figure 3. A pair of detectors 11, 12 are positioned one behind the other at a separation y with lenses (not shown) which produce 'beams', i.e. focus radiation from two spaced fields of view 13, 14, onto the forward detector 11, and from two more-widely spaced fields of view 15, 16 onto the rear detector 12. The angular separation of both pairs of beams 13, 14 and 15, 16 is the same and is a known angle e. The two pairs of beams are arranged symmetrically in a horizontal plane. As in the previous embodiment, the angular velocity of the target 20 is given by

W = _yt_ Range where Vt is the velocity tangential to the range. detector Wf = it R For the forward and for the rear detector W r = it- R+y 1 The range can thus be determined from the ratio of the two angular velocities as:- R = y Wf Mr - 1 TrITf - 1 (2) The problem of measuring the time delays Tf and Tr to sufficient accuracy is eased by using differential measurements, as indicated on Figure 4.

6 ti is the time delay between crossing the leading rear beam 16, and leading forward beam 14. St2 is the time between crossing the trailing forward beam 13 and trailing rear beam 15.

The above equation (2) can be rearranged as yT Tr - Tf Referring briefly to Figure 4, this shows the timing of the signals arising from the various beams 16, 14, 13 and 15, the signals being designated by a primed version of the associated beam reference. The timing instant is detected as the point at which the signal amplitude increases through a threshold value To. From Figure 4, Tr - Tf = 5 t 1 + it 2 therefore from the above expression for R, we have:- R = y Ti Sti +&t 2 (3) It is not essential for the 6oresights of the two detectors to remain accurately in line (though they should always be parallel), since an increase of St, goes with a decrease OfS t2 and thus misalignment will not affect the range calculation. Equation (3) is always valid, with one St going negative if a boresight shift causes beams to cross. Of greater importance is the vertical beam displacement since ideally the waveforms received at each detector element are identical. This will not be the case however if the radiation is received from different parts of the target. For this reason a wide vertical field of view for each element is preferred.

It can be shown that equation (3) is exactly true for any crossing angle 0 made by the tank with the beams. The four-beam system can also be used to calculate that crossing angle, according to the equation:- tan t9 - 9 ticot (612) t2 + Sti (4) where 0 is the angle between the line of travel of the target 20 and a perpendicular to the range R, i.e. the angle off the tank track shown in Figure 3.

In general the range R will be considerably greater than the baseline (separation distance y), so the ratio Wf/Wr will be close to unity. The greater the difference between the angular velocities, the more accurate the range determination can be. If the difference is only small, high accuracy is required of the detectors. However, increasing the difference means increasing the baseline y. This can be done without the practical inconvenience of very widely spaced detectors by folding the optical path of the outer radiation beams as shown in Figure 5.

A single detector 21 is mounted in the forward position, the rear position being a virtual one created by using plane mirrors 24, 25. Radiation from the outer fields of view 15,16 is thus folded first at mirror 24 then again towards the detector 21 at mirror 25. Figure 6 shows the arrangement viewed from the side. The outer beams (one shown) are folded downwards, while the inner beams (one shown) strike the detector 21 after passing beneath the mirror 25. The

1 effective baseline y' could clearly be increased even further over the actual dimensions of the device, by repeating the folding several times.

It will be appreciated that each pair of beams may be provided by a single detector and separate lens systems or vice versa - as in Figure 1.

An alternative way of deriving range is to measure the magnification factor-between the image received directly, that is from the front beam, and the image received via the folded path. This method requires an array of detector elements at the focal plane so that a two dimensional image is formed and not simply a single point "image". Figure 7 shows the image plane where two arrays 35 and 37 of detector elements 33 are used, each element producing a corresponding 'beam' in an array of closely spaced beams. The path lengths of the direct beams, e.g. 14, from the forward location, and the folded beams, e.g. 16, effectively from the rear location, are different, so the two images are different sizes, the direct beam image 30 being larger than the folded beam image 31. The two images are correlated to determine the magnification factor, and hence the range of the target.

As shown in Figure 7 the detector elements 33 are vertical strips which are 'scanned' by the target image 30, 31 as it moves at a velocity corresponding to the target angular velocity. At any instant the length of the image is determined by the number of detector elements energised, this number being compared for the two images. To avoid confusion there is an array (35, 37) for each image, as shown in Figure 7 - providing forward and rear beam arrays appearing in side view as in Figure 6.

In this image comparison scheme the actual extent of the image is measured directly by the number of detector elements activated - or more reliably by the identity of the first and last elements activated. In an alternative approach the angular velocity difference as seen by the forward and rearward detector arrays (disposed as in Figure 7) is exploited. Each array then comprises only two vertical strip elements spaced apart horizontally and, in each array, the time lapse between their energisations is inversely proportional to their angular velocities. The range is then given by:

Tr/Tf - 1 where y is the effective displacement of the detector arrays, and Tr and Tf are the measured time lapses for the rear and forward arrays.

An image correlation version of the three beam rangefinder shown in Figure 1, is also possible. In this case, a focal plane array of detector elements is of sufficient extent to accept target images from the three beams simultaneously. The relative positions and spacings of the images, and hence the range, are determined by image correlation techniques.

The above embodiments rely on target motion to provide a scanning effect, but for certain platforms, periscopes for example, and for stationary targets, the effect of target motion might more suitably be achieved by scanning the detector array, sweeping its beams through the target. Such an arrangement is shown in-Figure 8 where a periscope shown very diagrammatically at 39 protruding from a hull 41 comprises an optical head 21 having an optical arrangement comparable to that of Figures 5 and 6. Two forward beams 13 and 14 are produced by twin focussing on to a single detector element and similarly two rear beams 15 and 16 are produced by twin focussing on to a further single detector element. The optical system is then scanned about an axis parallel to the bisectors of the beams pairs. The twin lens system may be replaced by a single focussing arrangement and twin detector elements for each pair of beams.

In this arrangement the rear beams 15 and 16 are doubly folded at reflectors 25, 24, 45 and 44, in addition to the basic periscope reflectors 47 and 49.

Again, the twin beam arrangement (i.e. twin forward and twin rear) could be replaced by the broad beam and detector array of i i Z Figure 7, using image magnification or relative angular (target) velocities. In the latter case, of course, the perceived angular velocity of a target would very largely result from azimuth scanning of the periscope although it would still appear different for the front and rear beams.

While the above embodiments have been descibed as employing infra-red sensitive detectors, it will be apparent that any radiation appropriate to the circumstances visible or invisible, could be employed, the detector elements being selected accordingly.

Claims (17)

1. A rangefinder comprising a plurality of radiation detector elements, focussing means associated with said detector elements to produce in respect of each detector element a beam, within which the respective detector element is responsive to a target source, the arrangement being such as to provide a plurality of at least three beams, means responsive to a range dependent characteristic of a target image as determined by different combinations of said detector elements when a target passes through the associated beams to produce a target range indication.

2. A rangefinder according to Claim 1, wherein said range dependent characteristic is the angular velocity of the target travelling through said beams.

3. A rangefinder according to Claim 2, wherein said beams include three with asymmetric angular spacing and means are provided for timing successive interceptions of said three beams.

4. A rangefinder according to Claim 3, wherein two of said three beams are parallel so providing a range- independent velocity reference indication.

5. A rangefinder according to Claim 4, wherein the detector elements associated with said three beams are arranged within the field of view of a single lens the spacing between said parallel beams being provided by prisms in the path of one of the parallel beams.

6. A rangefinder according to Claim 5, wherein said prisms are pentaprisms providing 900 path deflection.

7. A rangefinder according to Claim 2, wherein a respective pair of divergent beams providing target transition times between the beams -I Q are arranged to diverge from each of two locations, said locations being at effectively different target distances.

8. A rangefinder according to Claim 7, wherein said detector elements are at a forward one of said locations, the forward location providing a said pair of divergent beams directly and the other pair of divergent beams diverging from said forward location and, after reflection, effectively from a rear location at a greater target distance than said forward location.

g. A rangefinder according to Claim 8, wherein the two pairs of divergent beams have equal divergence angles and are arranged symmetrically in a horizontal plane.

10. A rangefinder according to any Claims 7, 8 and 9, wherein the beams of each said pair of beams are provided by a single detector and respective focussing means, successive interception of the beams of the pair providing successive outputs from said single detector.

A rangefinder according to any Claims 7, 8 and 9, wherein the beams of each said pair of beams are provided by respective detector elements and single focussing means.

12. A rangefinder according to Claim 1, wherein said range dependent characteristic is the target image extent.

13. A rangefinder according to Claim 12, wherein a first array of detector elements provides a plurality of closely spaced beams diverging from a forward one of two locations, a second array of detector elements provides a second plurality of closely spaced beams effectively diverging from a rear one of said two locations. and means are provided for determining the range of a target from the relative extent of the target images on the first and second arrays.

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14. A rangefinder according to Claim 13, wherein said first and second arrays of detector elements are disposed at the forward location and said second plurality of beams are reflected so as to diverge from the rear location.

15. A rangefinder according to Claim 8 or Claim 9 including means for scanning said divergent beams through a target position so that said target intercepts said divergent beams successively to provide detector element outputs and range determination.

16. A rangefinder according to Claim 15 mounted in a periscope for rotation about an axis parallel to the bisector of said divergent beams, and including periscope reflectors for scanning said beams in a horizontal plane.

17. A rangefinder substantially as hereinbefore described with reference to Figures 1 and 2, Figures 3 and 4, Figures 4, 5 and 6, Figure 7 or Figure 8 of the accompanying drawings.

Published 1990 atThe Patent office, State House. 6671 High Holborn. London WC1R 4TP-Firther copies maybe obtainedfromThe Patent OfficeSales Branch. St Mary Cray, Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd. St Mary Cray, Kent. Con. 1,87 I-