GB2070879A - Range finding apparatus - Google Patents

Abstract

Known range finding techniques using binocular vision are useful only in circumstances where the existance and position of an object whose range is to be determined is already known. This limitation is eliminated by the illustrated apparatus which receives infra-red or visible radiation through two apertures 2 and 3 and focusses the two respective fields of view at a common image plane 15. A photodetector 14 is located at the image plane and the two images are scanned vertically across this photodetector by a drum 11 carrying reflective surfaces 12. Because of the separation of the apertures 2 and 3 a target in the field of view gives rise to two signals from the sensor 14, the time separation between these signals depending on the range. A circuit 18 measures the time separation and actuates an alarm 25 if the range is within predetermined limits and if various other conditions pertain. The casing 19 rotates in azimuth and the azimuth bearing of the detected target is indicated to the operator by an indicator 26. <IMAGE>

Description

SPECIFICATION Range finding apparatus This invention relates to range finding apparatus and arose in the design of a passive system (i.e. one which does not emit radiation) which detects the existence of an object in its field of view and established its direction and range.

This invention provides apparatus for finding the range of an object comprising means for receiving radiation from the object at spaced positions on the apparatus, a radiation detector, and scanning means operative to deflect the said received radiation so that radiation received from the object at one of the said positions impinges first on the detector followed by radiation received from the object at another of the said positions thereby causing the detectorto produce signals whose timing depends on the range of the object.

A preferred form of the invention includes means for focussing radiation from the aforementioned positions onto a common focal plane. This forms two images of the same object under observation the spacing of these images depending on the range of the object. In this form of the invention the scanning means serves to deflect first one and then another of the images onto the detector, the latter preferably being located in the focal plane.

The apparatus preferably includes a focussing system such that the detector, at any one time, is capable of receiving radiation received from one direction at one of the said positions and from another direction at another of the said positions, these directions diverging away from the apparatus. This preferred feature gives the apparatus improved resolution.

In one form of the invention, to be described later, the aforesaid positions where radiation is received are defined by a housing which is adapted to rotate about an axis parallel to a line joining the said positions. This meansfhatthe apparatus scans a different part of a field of view during each scanning action.

An example of one way in which the invention can be performed is described hereinafter with reference to the accompanying drawings in which: Figure 1 is a schematic diagram illustrating the principle of operation of a range finding apparatus constructed in accordance with the invention; Figure2 is a schematic diagram similarto Figure 1 but illustrating a different apparatus which has diverging directions of observation; Figure 3 is a vertical cross section through the axis of the apparatus shown schematically in Fig. 2; and Figure 4 is a block diagram illustrating circuitry for processing signals produced by the apparatus shown in Fig. 3.

Referring to Figure 1, an apparatus constructed in accordance with the invention is shown schematically at 1. At a first time, t, it is sensitive to infra-red radiation received by a first aperture 2 in the direction d1 and through a second aperture 3 in a direction d2. d, and d2 are, in this form of the invention, parallel. The apparatus is designed to scan a field under observation in a vertical direction so that at a time t2 the directions d, and d2 have been replaced by d3 and d4 respectively. Thus an object at position 4will give rise to a signal in the apparatus 1 at two instants of time, t, and t2. The time interval between these instants is a function of the range of the object 4.

Therefore, by measuring the time interval t2-t, it is possible to obtain an indication of the range of the object. The fact that the aforementioned time interval is a function of range can easily be seen if one considers another object 5 at a closer range. It is clear that this will not be viewed by the aperture 2 until a later time, t3.

Figure 2 shows a modified arrangement 6 for which the directions d, and d2, instead of being parallel as in Figure 1, diverge by an angle 8. The same considerations apply as for Figure 1 except that the time intervals t2-t, and t3-t, are extended by the time required for the angle of observation to swing through the angle 0. This is taken into account when calculating the range. The divergance of directions d, and d2 as indicated in Figure 2 is an advantage since, if these directions are parallel as shown in Figure 1, the optical resolution of the apparatus may not be adequate to ensure that radiation from an object such as shown at 4 is never received by both apertures 2 or 3 at the same time.

Figure 3 shows the apparatus 6 in greater detail and it should firstly be explained that, apart from detecting the range of an object, using the technique of the present invention, it also detects the existance of the object, indicates its azimuth and operates an alarm if certain detected characteristics of the object indicate that it is hostile.

Referring now to Fig. 3, electromagnetic radiation, in particular infra-red radiation, enters the upper objective aperture 2 and is reflected from mirror 7 through a partially reflective and partiallytransmis- sive mirror 8 into an infra-red imager 9. The imager 9 includes a telescopic eye piece 10, a rotating drum 11 having six mirror surfaces, e.g. shown at 12, a lens 13 and a photosensitive device 14. The lenses 2, 10 and 13 form an image of a field under observation from the aperture 2 onto the plane indicated at 15. As the reflective surface 12 rotates in the direction indicated by the arrow, this image moves downwardly with respect to the photodetector 14 so that an output of the latter represents a vertical scan of the field of view.When the top of the image at 15 has reached the sensor 14 the next mirror comes into operation and a further vertical scan occurs.

The output of the sensor 14 thus consists of a series of scans. If an object, e.g. the object 4 illustrated in Figure 1, is present, a corresponding signal is emitted by the sensor 14 at an instant during each scan, which instant indicates the elevation angle of the object 4.

Superimposed on the image at 15, derived from the aperture 2, is another image derived from the aperture 3. Radialion received by the aperture 3 is reflected from a mirror 16 onto a mirror 17. It is then reflected from the mirror 8 and follows the same path as the radiation from mirror 7 so as to focus an image as viewed by aperture 3 at the plate 13; i.e.

superimposed on the image from aperture 2.

The angle of the mirror7 is set so that radiation arriving in direction d1 at the aperture 2 is brought to a focus on the sensor 14 at the same time as radiation arriving at the aperture 3 in a direction d2, which is not parallel to dt. Thus, each object under observation produces two signals at the output of the sensor 12 in a manner as described with reference to Figure 2. The time difference between these two signals is measured by circuitry 18, to be described later, and this is used to calculate the range of the object.

As the drum 11 rotates a casing 19 also rotates on bearings 20 by which it is mounted in a base 21. This rotation is effected by a motor 22 which drives the casing 19 through gear wheels 23 and 24. Thus, each time the superimposed images at 15 scan vertically past the sensor 14, the apertures 2 and 3 are facing in a slightly different azimuth direction. In this way the apparatus 6 scans both in azimuth and elevation.

Apart from a small recess in which mirror 16 is housed, the casing 19 is substantially symmetrical about a vertical axis x, which is also the optical axis and the axis of rotation. This casing 19 must be of substantial length in order to give an adequate base line b between the two apertures 2 and 3. Since the casing rotates about an axis along which the apertures 2 and 3 are spaced the apparatus occupies a relatively small area and, more important, there is no apparent movement of the apparatus which can be detected by hostile radar systems. This is a very significant advantage over an alternative possibility which would be for the axis of rotation to be perpen dicularto the base line between the apertures.

The output of the sensor 14 is fed to the control circuit 18 which, amongst other things, times the interval between the two signals received from a target 4 or 5. If the time interval indicates that the range of the target is within limits indicating that a threat exists, and if various other conditions are satisfied (as will be described later) the circuit 18 actuates an alarm 25 and also operates an indicator 26 which indicates by some suitable visual means the azimuth ofthetargetwhich has been detected.

In reality the single sensor 14 is constituted by a collection of individual sensors spaced in a direction perpendicular to the plane of Fig. 3. Only one, our a few of these sensors is illuminated (by infra-red radiation) when the detected target is small. A larger number is illuminated by a large target Figure 4 shows the sensor array 14 which has a number of parallel outputs 27. These are fed to a pulse forming circuit 28 which has an output line corresponding to each input line. The circuit 28 inspects the signal on each input line 27 and, when that signal exceeds a threshold level for a time period not in excess of a fixed value, it produces an output pulse on the corresponding output line.

When an input exceeds the threshold for longer than the fixed time period, indicating the presence of a target which has a relatively large height dimension, no pulse is produced at the output since such targets ca: be assumed nf i to be of interest. The outputs of the circuit 28 are passed to a range discriminating circuit 29 and to a target width discriminator 30. The range discriminator 29 times the interval between pulses at each of its inputs and passes these pulses to its corresponding input only when the interval between pairs of input pulses is within predetermined limits. In this way the circuit 29 removes signals arising from targets which are not within a predetermined range of the equipment.The target width discriminator 30 compares the signals on the individual lines at its input and produces a pulse at its output only when a pulse appears at a limited number of adjacent inputs. Thus, in the case of a wide target, which produces pulses at all the inputs of the circuit 30, no pulse will be passed to the circuit 31. For a narrow target, a pulse will be applied to perhaps only one of the inputs of the circuit 30 which will respond by applying a pulse to the circuit 31 thereby enabling the latter to produce an output at 32 as will now be described.

The circuit 31 times the signals from circuit 29 to obtain an indication of the range of a target and receives information on lines 33, from the drive unit 22, indicating the azimuth and elevation of the target. The circuit 31 further receives information on lines 34 defining characteristics (i.e. azimuth, elevation and range) of particular targets or types of target which are known not to present a threat. This information is contained within a store 35 which receives information from three sources as follows.

The store 35 firstly receives information from a manual input device 36 which can, for example, be a keyboard. This enables the store 35 to be loaded with information concerning elevation and azimuth values at which the detection of targets is not required. Secondly, the store 35 receives on lines 37 information relating to the position of targets detected during an initial period of operation of the apparatus. Any such targets can reasonably be assumed to be of a non-offensive nature and their detection during subsequent operation of the apparatus is not required. The store circuit 35 includes a timer which defines this initial operation period, and which allows information to be presented via lines 37 to the store 35 during this period.

Finally, the store 35 receives, on line 38, a logic signal whenever circuit 39 decides that a detected target is not offensive. On receipt of such a logic signal the timer in the store 35 is overriden thereby allowing the latter to accept the azimuth and elevation angles of the non-offensive target via the lines 37. The circuit 31 produces an output logic signal on line 32 only when a target has been detected which is within a range defined by circuit 29 as being of interest, is of a size not excluded by circuit 28 and 30, is not already recorded in store 35, and has a changing azimuth, elevation andlor range (i.e. it is moving).

The signal appearing on line 32 is applied to the azimuth indicator 26 which is thereby caused to indicate the azimuth as defined by the signals on line 33.

It is also applied to a "stop and look" activator 40.

This circuit 40 controls the drive 22 so as to inhibit rotation of the housing 1. The particular region where a target has been spotted is thus scanned quickly and repetitively by the rotation of the drum 11.

The logic signal on line 32 is also applied to a test circuit 39 which tests whether the signal at 32 persists after the stop and look activator has been operated. If it does persist, then a logic signal is produced on line 41 which actuates the alarm 25. If it does not persist a logic signal is fed via line 38 to the store 35 for the purpose previously described. Whatever decision is made by the test circuit 39, a signal is produced by OR gate 43 on line 44to reset the stop and look activator 40.

It will be noted that the apparatus does not indicate to the operator, the range of a detected target, though it does employ circuitry 29 and 31 to indicate range and uses this indication to suppress signals arising from targets outside a predetermined range "window".

Claims (5)

1. Apparatus for finding the range of an object comprising means for receiving radiation from the object at spaced positions on the apparatus, a radiation detector, and scanning means operative to deflect the said received radiation so that radiation received from the object at one of the said positions impinges first on the detector followed by radiation received from the object at another of the said positions thereby causing the detector to produce signals whose timing depends on the object.

2. Apparatus according to claim 1 including means for focussing radiation from said positions onto a common focal plane so asto form two images of the object, the spacing of the images depending on the range of the object and the scanning means being operative to deflect first one and then another of the images onto the detector.

3. Apparatus according to any preceding claim designed so that the detector is at any one time capable of receiving radiation arriving from different directions at respective different said positions; and in which said directions diverge away from the apparatus.

4. Apparatus according to any preceding claim in which the said different positions are defined by a housing which is adapted to rotate about an axis parallel to a line joining the said positions.

5. Apparatus substantially as described with reference to Figure 1 of the accompanying drawings and substantially as illustrated in Figure 1 of the accompanying drawings.