GB1598064A - Distance-measuring apparatus - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO DISTANCE-MEASURING APPARATUS (71) We, BRITISH AEROSPACE, a Corporation established under the Aircraft and Shipbuilding Industries Act 1977, of Brooklands Road, Weybridge, Surrey, KT 13 OSJ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention is concerned with improvements in or relating to apparatus for the measurement of distances, such apparatus being capable of giving an output signal when a target body is within a predetermined range of distances from the apparatus. Apparatus of this kind will be referred to herein as "distance-measuring apparatus of the type specified".

According to one aspect of the invention, there is provided distancemeasuring apparatus comprising a source of visible or infra-red radiation, transmitting means for transmitting the radiation away from the apparatus in a radiation beam, receiving means spaced from the transmitting means and having a beam of vision which intersects the radiation beam, the receiving means being operable to detect said radiation when reflected from a target body which lies within the space defined by the intersection of the radiation beam and the beam of vision, generating means responsive to the receiving means and arranged to generate an electric signal on reception of such radiation by the receiving means and a first electrical filter arranged to receive said signal and to pass that signal only when the rate of rise of that signal exceeds a predetermined value.

Preferably, the transmitting means includes modulating means for modulating the radiation emitted by said source.

The modulating means may comprise a pulse-type modulator, for example a chopper disc arranged to periodically interrupt the radiation emitted by the said source.

Conveniently, in such case, the apparatus includes an electrical filter arranged to receive the electrical output signal of an electric cell forming the generating means and to pass that signal only when that signal corresponds to the receipt by the receiving means of radiation which has been modulated by the modulating means.

The apparatus may include an amplifier arranged to amplify the electrical output signal of the electric cell, the amplifier being provided with automatic gain control.

The apparatus may include a demodulator arranged to demodulate the electrical output signal of the electric cell, to form a second electrical signal the presence of which indicates the presence of the target body within the said space.

The apparatus may include a threshold gate arranged to receive the second electrical signal and to pass that signal only when the amplitude of that signal exceeds a predetermined value.

The invention also includes a fuze system for a missile, the fuze system including distance-measuring apparatus according to the invention.

The invention further includes a missile which is provided with a fuze system according to the invention.

One embodiment of the invention will now be described by way of example, reference being made to the accompanying drawings of which: Figure 1 is an incomplete longitudinal cross-section of a guided missile provided with a fuze system, which system includes distance-measuring apparatus of the type specified and according to the invention; Figure 2 is an end elevation, partially broken away, taken along the line Il-li of Figure 1; Figure 3 is a side elevation which is similar to Figure 1 (but is to a smaller scale) and which illustrates the operation of the arrangement; Figure 4 is a block circuit diagram of the fuze system; Figure 5 is a diagram illustrating the operation of the arrangement, and Figures 6 and 7 are graphs which illustrate the operation of the arrangement.

Referring to Figures 1--3, the guided missile I is provided, near one end of the missile, with three similar systems 2, each of which comprises apparatus for generating a pair of radiation beams 3 and 4. Each system 2 comprises a sealed optical unit, and includes a source of visible or infra-red radiation together with transmitting means for forming the relevant pair of radiation beams 3 and 4.

The source of visible or infra-red radiation may have any suitable form but is conveniently a tungsten-filament electric lamp 5 which operates at a temperature of about 2,700 K. The lamp may be a 150-watt lamp intended to be energised from a 21.6-volt supply, and actually energised by a silver-zinc battery 6. The lamp 5 is preferably of the kind provided with an internal elliptical reflector 7 (Figure 2), the filament 8 of the lamp 5 being located at one focus of the reflector 7.The reflector 7 thus collects radiation from the filament 8 over approximately a hemisphere, such radiation being directed, by the reflector 7, into a solid angle of approximately one steradian, which angle extends from the image of the filament, which image is formed at the second focus of the reflector 7; by this means, the source can be efficiently matched to a near--f/l lens system.

In a modification of the invention, the source may be a gas-discharge lamp.

The radiation from the lamp 5 is thus directed towards a chopper disc 12 which is rotated by an electrical hysteresis motor 13 energised from an electrical alternator (not shown) with which the missile 1 is provided. The chopper disc acts as a pulse modulator, and modulates the radiation from the lamp 5 at a frequency of 5 kc/s. The chopper disc may be 10 cms. in diameter, may have 25 blades, and may be driven at a speed of 12,000 revolutions per minute. Using an electrical hysteresis motor supplied from a 200-volt, 3-phase, 400 c/s supply, the chopper disc 12 can be accelerated to its operating speed in about 4 seconds.

The missile 1 is conveniently provided with an initiating circuit (not shown in the drawings) which is arranged both to energise the hysteresis motor (so causing the chopper disc 12 to be accelerated to its operating speed), and also to connect the battery 6 to the lamp 5, at a convenient time interval before the missile is required to be capable of being exploded by the fuze system.

The modulated radiation from the chopper disc 12 is received by a mirror 14 (Figure 2), whence the radiation is reflected to a pair of lenses 15 and 16. The overall arrangement is such that each of the systems 2 generates a pair of radiation beams 3 and 4, the beams of each pair of beams being angularly spaced by 31", and each pair of beams being angularly separated by 31".

Where the image of filament 8, produced by the reflector 7, is approximately 9 mmx6 mm in size, and where the lenses 15 and 16 have a focal length of about 10 cms., the angular size of each of the radiation beams 3 and 4 may be 6"x4".

The radiation beams 3 and 4 pass to the exterior of the missile 1, by way of plate-glass windows 17 provided in the skin of the missile.

The guided missile 1 is provided, near the other end of the missile, with three similar systems 19, each of which comprises a sealed optical unit which includes receiving apparatus having a pair of beams of vision, one such beam of vision being indicated at 20 in Figures 1 and 3. Each system 19 comprises an electric radiationsensitive cell 21 arranged to receive, through a suitable optical filter 22, radiation received by either one of a pair of lenses (of which one is indicated at 23, Figure 1), each of the lenses 23 being associated with a corresponding one of the beams of vision 20.

Each cell 21 may be of any suitable form, but is conveniently a lead-sulphide photoelectric cell having a sensitive surface of 6 mm2. Means (not shown) is preferably provided, to control the temperature of the cells 21 to +5"C; alternatively, variations of the electrical output signals of the cells 21 due to temperature variations may be compensated for, by providing the amplifier 36 (discussed below) with automatic gain control.

In modification of the invention, the cell 21 may be a photo-voltaic cell.

The beams of vision 20 extend to the exterior of the missile 1, by way of plateglass windows 24 provided in the skin of the missile.

The optical filter 22 may be a silicon cut-on filter.

The spectral sensitivity of each system 19 is determined by the cell 21, the filter 22 and the window 24. Conveniently, each system 19 is arranged to respond to radiation within a wavelength band of 1.1 microns to 2.7 microns, referred to a 10 /O transmission factor.

The angular width of each beam of vision 20 may be approximately 60.

As indicated in Figures 1 and 3, the systems 2 are spaced from the systems 19 by a distance of 2.25 feet (68.6 cms), and the arrangement is such that each of the radiation beams 3 and 4 is associated with a separate corresponding one of the beams of vision 20.

Figure 3 shows one corresponding pair of such beams, and it will be seen that the two beams of the pair are arranged to intersect over a space or region 28 which is indicated by diagonal shading lines.

It will thus be clear that when a target body (not shown) is within one of the six spaces 28 which are located at 310-intervals around the underside of the missile 1, radiation from the relevant radiation beam 4 will be reflected, by that body, along the relevant beam of vision 20 and towards the receiving apparatus located within the relevant system 19.

The space 28 is of greatest width where the relevant radiation beam, 3 or 4, fully intersects the relevant beam of vision 20. In Figure 3, this position is indicated by the chain line 29 which is conveniently arranged to be at a distance of 14 feet (4.3 metres) from the missile 1.

At a larger distance from the missile, indicated by the chain line 30 (Figure 3), each pair of corresponding beams intersect by only 10% of the greatest possible area. The line 30 may be at a distance of 30 feet (9.1 metres) from the missile 1.

Referring to Figure 4, the electrical output signal from each of the cells 21 is separately amplified by a corresponding pre-amplifier 34. Such separate amplification is preferred, at this stage, to allow for differences in characteristics of the individual cells 21.

The output signals from the amplifiers 21 are combined, and are supplied to an electrical bandpass filter 35 which may be arranged to pass electrical signals within a frequency band of 5 kc/s+500 c/s.

The output of the filter 35 is amplified by an amplifier 36 which, as indicated by the feedback line 37, may be provided with automatic gain control for the purpose (as mentioned above) of reducing the effect of temperature variations of the cells 21.

Notes of the Design of the Filter 35 and Amplifier 36 The optimum signal-to-noise ratio in the amplifier 36 is obtained with a bandwidth 1 7rh where h is the signal rise time.

The minimum rise time is dependent upon the overlap of the beams 4, 20 (Figure 3) (which overlap reduces to zero at about 30 ft), and is also dependent upon the target extent, as well as the relative velocity of the missile 1 and the target.

Assuming a 6 ins. target extent to be the least that need be considered, and a maximum relative velocity of 1,500 feet per second, the minimum likely rise time is 1/3 millisecond, giving an optimum signal-to-noise bandwidth of 1 kc/s.

The filter 35 will therefore have a centre frequency of 5 kc/s, and a bandwidth (between 3 dB points) of 1 kc/s. Below 4.5 kc/s, the filter will cut off, at 25 dB per octave, to reject background radiation which is not chopped by the chopper disc 12.

The frequency content of signals from background sources is defined by the missile velocity, the width of the beam of vision 20, and the range of the target from the missile. Assuming a missile velocity of 1,500 feet per second, and a detector to source range of 30 feet, the pulse duration from a discrete ground source will be 2 milliseconds, and the fundamental of this pulse will have a frequency of approximately 250 c/s. It is therefore desirable for the system to give strong rejection of 250 c/s signal components. Assuming the filter 35 to cut off at 25 dB per octave below 4.5 Kc/s, 250 c.p.s. signals will be attenuated by a factor of 105; with longer range or lower missile velocity, the attenuation will be proportionally greater.

The output of the amplifier 36 is supplied to a demodulator 38 of which the output is supplied to a high-pass filter 39 arranged to pass electrical signals of frequencies greater than 800 c/s.

The high-pass filter 39 acts to discriminate between real and false targets.

Thus, the high-pass filter 39 acts to suppress electrical signals which are generated by the cells 21 in response to the receipt of radiation reflected by a body within the relevant space 28, unless the rate of change of those signals is sufficiently great to indicate that the body is a target body and is not, for example, the ground or fog or cloud.

The action of the filter 39 is illustrated by Figure 5. It will be seen that, if the missile 1 passes along a trajectory 42, over a target body 43 and above the ground 44, then the electrical output signals of the demodulator 38 have the forms of the curves A and B respectively, the curve A representing the output signal of the demodulator 38 when the target body 43 is detected by the apparatus being described, and the curve B representing the output signal of the demodulator 38 when the ground 44 is detected by the apparatus being described.The curve A differs from the curve B, in that the rise (C) of the curve A is relatively abrupt, and changes in the output of the demodulator 38 which are appropriately abrupt (thus indicating the presence of a target body) are transmitted by the high-pass filter 39 and are capable of operating the fuze system of the missile 1, to initiate explosion of the missile. As stated above the filter 39 provides similar discrimination against a false target constituted by fog or cloud.

Notes on the Design of the Demodulator 38 and Filter 39 It is suggested that signals due to fog must be reduced by a factor of 104. Figure 5 shows that the signal rises as the extended ground target moves up the beam overlap, over approximately 10 feet along the beams. At the probable dive angle of 30 of the missile 1, this corresponds to 17 feet along the trajectory 42, or a time interval of 12 milliseconds at the maximum missile final velocity of 1,500 feet per second.

The system sensitivity is designed to trigger explosion of the missile at 30 feet from a target of low "reflectivity"; with a I kc/s bandwidth of the filter 35, a 1millisecond rise time is considered to be the maximum duration required.

Therefore, there is a 12-to-1 ratio between the minimum extended-ground-signal rise-rate and the maximum target-signal rise-rate with a 300 dive angle. A high-pass filter 39 with a slope of 20 dB per octave below 800c/s will attenuate extended sources by a factor of greater than 104.

The output of the filter 39 is supplied to a threshold gate 45 (Figure 4) which is arranged to pass the output of the filter 39 only when the amplitude of that output exceeds a predetermined value.

The output of the threshold gate 45 is capable of passing, via a firing-pulse inhibitor 46, to a firing-pulse generator 47 to operate the generator 47 and thereby to initiate explosion of the missile 1. The generator 47 may be of generally known form, and may include a silicon-controlled-rectifier switch.

The firing-pulse inhibitor 46 is capable of preventing a signal being passed to the firing-pulse generator 47 from the threshold gate 45, if any one of three conditions is not fulfilled.

Firstly, the missile 1 is provided with an altitude switch 50 arranged to respond to the altitude of the missile and arranged to cause the firing-pulse inhibitor 46 to act to inhibit the explosion of the missile unless the altitude of the missile is less than a predetermined value.

Secondly, the missile is provided with a time-delay switch 51 which is responsive to the switching-on of the fuze system and which is arranged to cause the firing-pulse inhibitor 46 to act to inhibit the explosion of the missile until the fuze system has been switched on and has been allowed a predetermined period of time to attain steady-state conditions.

Thirdly, the fuze system includes a sun-operated gate 52 which is arranged to monitor the electrical output signals from the cells 21 and to operate if any one of those output signals exceeds a predetermined magnitude corresponding to the reflection of sunlight (for example, from the ground) to any one of the cells 21.

Operation of the gate 52 is arranged to cause the firing-pulse inhibitor 46 to act to inhibit explosion of the missile while sunlight is reflected to any one of the cells 21.

Operation It is believed that the operation of the arrangement will be substantially clear from the above description. Briefly, if a target body is present within any one of the spaces 28 defined by the intersection of the radiation beams 3, 4 and the beams of vision 20, then radiation from the relevant radiation beam (which radiation is pulse modulated by the relevant chopper disc 12) will be reflected within the relevant beam of vision 20 and will be received by the relevant cell 21 which will generate a corresponding electrical output signal. This electrical output signal will be passed by the filter 35 (which is arranged to exclude signals corresponding to radiation which has not been pulse-modulated at the frequency of operation of the relevant chopper disc 12), and thereafter demodulated by the demodulator 38.If the target body represents a true target, as compared with a false target (e.g., fog or cloud, or the ground), then the rate of rise (with time) of the output signal of the demodulator 38 will be sufficiently great to permit that output signal to be passed by the highpass filter 39. If the output signal is large enough, it will be passed by the threshold gate 45 and will operate the firing-pulse generator 47 to initiate explosion of the missile, provided that the firing-pulse inhibitor 46 does not operate (as described above) to inhibit explosion of the missile.

Objects which are not within any one of the spaces 28 defined by the intersection of the radiation beams 3, 4 and the beams of vision 20, may reflect or radiate radiation to one or more of the cells 21, provided that such objects are located within one or more of the beams of vision 20. However, the corresponding electrical output signals from the cells 21 will be effectively suppressed by the band-pass filter 35.

The response of the apparatus of Figures 14 is illustrated by Figure 6, which is a graph of (the amplitude of the output signal of the demodulator 38) plotted against (the range, in feet, of a target body from the missile 1).

It should be noted that, when the spaces 28 are arranged to be elongated (as shown in Figure 3), the apparatus is capable of discriminating between target bodies which are moving (relatively to the missile 1) in a direction substantially perpendicular to the missile axis 48 (Figure 3) and which will therefore remain within one of the spaces 28 for a relatively long period of time, and target bodies which are moving (relatively to the missile 1) in a direction substantially parallel to the missile axis 48 and which will therefore remain within one of the spaces 28 for a relatively short period of time.

Modifications In a first modification of the invention, the systems 19 of receiving apparatus may be so modified, that the beams of vision 20 are supplemented by optical lobes indicated by the arrows 54 (Figure 3). The apparatus will then respond to the presence of a target body within spaces additional to the spaces 28 (Figure 3), and the overall response of the apparatus of Figures 1--4 will then become modified as indicated by the broken line 55 of Figure 6.

In a further modification of the invention, the systems 19 of receiving apparatus and the associated apparatus of Figure 4 are arranged to respond also to the phase of the modulation of the radiation received by the cells 21. Thus, each of the electric motors 13 (Figure 2) may be arranged to supply an electrical signal which represents the phase-angle of the modulation by the associated chopper disc 12, and this electrical signal is compared with the electrical output signal of the relevant cell 21 in a suitable gate circuit, the arrangement being such that the electrical output signal of the relevant cell 21 is suppressed if the phase-relationship of the two signals is unsatisfactory.

In another modification of the invention, the electrical output signals of the cells 21 are arranged to be compared, thus providing an indication of the variation of the position of a target body relatively to the missile. To facilitate such a comparison, the position of one or more of the radiation beams 3 and 4 and the beams of vision 20 may be arranged to vary, for example the beam may be arranged to rotate or oscillate relatively to the missile 1.

Further Modifications of the Invention In certain of the other applications of the invention, as mentioned in the preceding section, the distance-measuring apparatus may be provided with a source in the form of a semiconductor-type light-emitting diode or in the form of a laser, and the sensitive cell may be in the form of a photo-diode, or in the form of a transistor, or in the form of a photo-multiplier.

Further Notes Upon the Operation of the Missile Fuze-system of Figures 14 Part I-Characteristics of Target Bodies There is no readily available information upon the reflection characteristics of various target bodies to visible and infra-red radiation. However, a formula has been derived to permit the calculation of values of a reflection loss factor r, and it has been found experimentally that, with one or two exceptions, r lies within the range of 0.1 to 1.0 and is substantially independent of aspect.

In order to assess expected target signals from apparatus according to the invention, a simple experimental form of apparatus according to the invention was built. In this apparatus, the source consisted of the system described above but used a 6-inch focal-length powered mirror with an aperture of 2 inch diameter, giving basically a 3.50 angular beam-width. It was necessary to use the mirror a little off-axis, but the resulting blurring of the beam appeared small. The source was chopped at 800 c/s.

The detector consisted of a lead-sulphide cell and a 4-inch-focal-length, 2inch-diameter aperture, single-element glass lens giving a spot size of less than half a millimetre with parallel radiation. The cell was in series with a l-megohm load resistor, with a 120-volt polarizing battery across the cell and the load. The cell output was monitored using an 800 c/s narrow-band amplifier with an input impedance of 500 kilo-ohms. The polarizing and amplifier input arrangement thus corresponded to those described above.

An expression for the expected signal was developed as follows. It was assumed that most surfaces would give a specular reflection of the source, such reflection being confined to a relatively narrow range of incidence, and that, outside this range, reflection would be diffuse. Over the range of angles of diffuse reflection, it was assumed that it could be characterised by assuming a loss on reflection by a factor r, and the residual radiation would be uniformly scattered into a hemisphere. Although these assumptions are obviously approximate they result in an inverse-square law, and in an independence of incidence-angle which, for the majority of surfaces measured, was reasonably confirmed outside the region of specular reflection.

The following symbols are used: N=source radiance Am1=source mirror aperture area source mirror focal length f2=detector lens focal length R=range r=loss factor on reflection at the target As=lamp source area Am2=detector-lens aperture area N.E.P.=cell-noise equivalent power (800 c/s, 1 c/s, 2700"K black-body radiation, 1.1 microns-2.7 microns For the condition where the detector and source beams (which have the same angular beam width of 3.5 ) precisely overlap at the range quoted, R, the r.m.s.

signal-to-noise ratio in a bandwidth of Ic/s at 800 c/s is given by: S = NasAmlAm2r R=-= N 27rR2(f)2N.E.P.

Measurements were made with the source and detector placed at opposite ends of a 2-foot base-line, with beams arranged to overlap at a 20-foot range.

Estimates based on earlier laboratory calibrations gave the following values of N, As and N.E.P.

N.As=12.5 Watts/steradian N.E.P.=2x 10-10 watts.

With this system the above expression becomes S -=4.4x 104r N with a 1 c.p.s. bandwidth. Under the conditions of measurement the cell noise (converted to a 1 c/s bandwidth at 800 c/s) was 10-6 volts r.m.s. so that S 4.4x 104x 1O-6 (where S is in volts).

=23 S The measurements made were not intended as a comprehensive or wellbalanced survey, but as a series of spot checks and, as a result, surfaces and conditions which it was thought might be difficult have been emphasised.

For instance, in general, angles of incidence giving specular reflection were avoided and surfaces were chosen having finishes likely to give a low level of diffuse reflection. With the apparatus available, it was not possible to make measurements on grass and concrete at incidences less than 70 , whereas 20 to 40 is the most likely incidence range for these surfaces if the system is required to respond to them.

The following Table 1 is a table of relevant measurements of the reflection of radiation by different target surfaces. The table lists the background-irradiation estimates and measurements which have been made in the proposed spectral band of 1.1-2.7 microns, normalised to 2 microns.

TABLE 1 Target Measurements All measurements were at a range of 20 feet.

Incidence Signal Surface Type Angle (mV) r Dry concrete (three samples) 70" 22 0.5 12 0.28 12.5 0.29 Concrete with a pool of water lying on it 70" 2.3 0.05 Grass 80" 5.0 0.12 Stones 70" 24 0.55 Skin of Aircraft Having a Bright Metal Finish Main fuselage 0 1000 Not applicable (predominantly specular reflection).

Rear of nose 20 15 0.34 45" 20 0.46 Nose 30 3.9 0.09 Tail 50 10.0 0.23 70" 3.6 0.08 Skin of Aircraft Having a Gloss Paint Finish (except radome) Main fuselage 0 165 Not applicable predominantly specular).

(White paint) 5 75 1.7 20 82 1.9 45" 64 1.5 Matt Black Painted Radome 30 35 0.8 Skin behind nose 80" 40 0.92 Underside of wing (White paint) 80O 5.3 0.12 From Table 1, it will be seen that the conditions in which values of r are less than 0.1 are few, even in this type of sample, and are special. For instance, pools of water will only lie on horizontal surfaces, and targets with horizontal components are unlikely. Targets consisting only of a highly reflecting surface, such as aircraft having bright metal finish are also unlikely.

In the three cases where measurements were made on the same surface at several angles of incidence, it will be seen that the dependence on incidence is not great.

To determine the effect of incidence in more detail, measurements were made in the laboratory with the apparatus just described and using two targets, a gloss white-paint finish and a matt black-paint finish; a sheet of white blotting paper, this being a good uniformity diffusing surface, was used as a control surface. The results of these measurements are shown in Figure 7.Values of r, estimated as above for these surfaces at 450 incidence, were Blotting paper 1.3 Gloss white-paint 1.2 Matt black-paint 0.3 The fact that r exceeded unity in the case of some measurements on the aircraft having a gloss paint finish is considered to be due to cumulative errors in various conversion factors used in estimating N and N.E.P. in the relevant spectral band, and error in estimating As, r measured in this way also includes an allowance for chopping efficiency and component transmission loss.

Part lI-Characteristics of Background Radiation The field of view of the systems 19 of receiving apparatus will sweep across a range of non-target objects as the missile approaches the target body. Generally, such objects will be outside the 30-foot fuze-trigger range and so will not reflect chopped-source radiation into the receiving apparatus, but such objects will produce a varying irradiance at the receiving apparatus, due to reflected sun and sky radiation which will cause a fluctuating output from the cells 21. The two types of background radiation are discussed below under the headings "Unchopped background" and "Chopped background".

Unchopped Background The sources of background radiation to which the receiving apparatus is sensitive are direct sun, specular reflection of the sun, and scattered sun-reflection from ground and water. It will be seen below that background objects have temperatures producing radiation in the lead-sulphide band which is extremely small compared with reflected solar radiation.

Sun The irradiance (at a wavelength of 2 microns) which gives a response from a lead-sulphide cell equal to that from the sun, assuming a l.l-micron cut-on filter and assuming attenuation by one standard air mass, is 3.5x10-3 watts/cm2. This gives a reasonable approximation for elevations of the sun greater than 30 , and reduces rapidly at elevations less than this.

The optical configuration of this fuze is such that the detectors will not look directly at the sun.

Specular Sun Reflection Assuming all the radiation from the sun is reflected into the detector, then with a surface reflectivity of 0.1, the signal-to-(peak) noise ratio is 107.

The probability of the sun being specularly reflected into the detectors may be considered as the ratio of the total field of view of the detectors to one hemisphere, there being no preferred direction of reflection. This gives a probability of I in 100.

Assuming all the radiation from the sun is reflected into a detector with a surface reflectivity of 0.1, the signal-to-(peak) noise ratio will be 107. Since the dynamic range of the target signal is 104, then the probability of the sun signal appearing in this range is one in 103. A method of inhibiting the fuze with signals greater than the upper limit is described above, and the overall probability of initiation of explosion by specularly reflected sun is 1 in 105.

Scattered Sun Reflections from the Ground Excluding the atmospheric attenuation laws, the irradiance at the detector from the extended ground-reflecting source remains constant as the missile approaches the ground, and no signal change will occur at the detector. In practice, shadows and reflective-type variations at the ground surface will give rise to changes of irradiance at the detector as these surfaces sweep through its' field of view.

The signal duration is a function of the direction and the rate of turn of the sight-line between detector and edge of irradiance change and the angular beamwidth.

An indication of the effect of waveform can be obtained by taking the angular response of the detector as a half-sine-wave, 6" between 10% points. A source at 30-foot range requires a 3-foot missile-travel to pass between 10% points and, at the highest missile final phase velocity of 1500 feet per second this takes 2 milliseconds.

The input pulse consequently appears as a half-cycle of a 250 c/s fundamental frequency. This is taking the most extreme case from the point of view of producing the highest fundamental frequency, in that it assumes: (a) a point source (b) the highest relevant missile velocity (c) the minimum range. (Inside the 30-foot range the fuze is designed to trigger.) This frequency is directly proportional to the missile velocity and inversely proportional to the range of the target body.

The pass-band of the filter 35 incorporates filters which cut-on at 25 dbs per octave, with a lower-frequency "3 db point" at 4.5 kc/s, so that a frequency of 250 c/s is attenuated by a factor of 105.

Chopped Background In general it is a requirement that the fuze should trigger on the first object to pass through the space 28 of overlap between the radiation beams and the beams of vision.

Two exceptions can occur, fog and ground: It is possible that triggering by the ground may be required to produce an aerial explosion, and in such cases the ground-rejection filter 39 may be switched out, but, in general, both fog and ground signals will be rejected by virtue of their slow-rise characteristics compared with actual targets, by the filter 39.

The comparative rise-time of ground and target signals are illustrated in Figure 5, and it will be seen that, in entering a fog layer, the rise characteristics of the signal will be similar to those for the ground, but the rise-time will be longer, because fogs have no accurately defined edge. In the ground-rejection case, it will be seen from Figure 5 that the rise takes place over approximately 10 ft. along the beams, which, at a dive angle of 30 , corresponds to 17 feet along the trajectory 42.

The fastest rise, at the highest final-phase velocity of the missile of 1,500 f.p.s., will therefore take 12 milliseconds, corresponding to a fundamental frequency of 40 c/s and such rise will consequently be attenuated by the filter 39 which cuts of, at 20 dB/octave below 800 c/s, by a factor of 10-4.

Measurements have been made at Hatfield in a fog of a thickness occurring only a few times in a year. Visibility during the fog was estimated at approximately 200 ft., corresponding to an extinction coefficient to 7x10-4 per cm. Maximum signals obtained, factored to the system proposed, were 3x 103 above the peak noise level of the system. With a filter factor of the high-pass filter of greater than 10-4, the signal level from fog will be lost in the system noise level.

A number of measurements have been made of the attenuation characteristics of fog. These show that the expectation of fog visibilities equal to that of the actual fog in which measurements were made is very low and visibility down by a factor of 2 were never recorded. This is confirmed by the deductions from 10 years of records.

The scattering properties of water droplets depend on the ratio of their diameter to the wavelength of the radiation considered. For diameters of a few times the wavelength concerned scatter is predominantly forward. The sizes of drops in fogs and clouds appear to depend considerably upon the rate of updraught and upon the number of condensation nuclei present, the more nuclei are present or the faster the up-draught, the greater the tendency towards a lot of small drops rather than a few large ones.

Over the range of fogs and clouds for which details of distributions of drop size are available, drops with diameters greater than 5 microns predominate, and the diameters of the drops are generally greater than 10 microns.

It is considered that drop diameters in fog and cloud are sufficiently larger than the wavelength used here, for back-scatter not to depend critically on dropdistribution, but to be effectively proportional to extinction coefficient (i.e.

inversely proportional to visibility), so that since extinction coefficients greater than that of the fog in which measurements were made have negligibly low probability fog signals greater than that recorded also have negligibly low probability of occurring.

In all of these cases, the signal is expected to be attenuated below the peak noise of the system by the high-pass filter 39.

To summarise, information available from large numbers of measurements of fog and cloud drop-distributions, and statistical data available on the incidence of various visibilities, suggest that the measurement made is representative of severe fog or cloud, and that signals will not be critically dependent on drop-distribution within the limits found in fog and cloud, drop diameters being predominantly more than 5 times the wavelength, so that the signals recorded during this measurement can be taken as representative of the highest to be expected. After attenuation by the high-pass filter 39, these signals would be below the noise-level of the system.

It is therefore considered that the system has sufficient protection against the possibility of triggering on scatter from fog or cloud.

Attenuation in Fog Attenuation of target signal expected in the thickest fog, taken as that quoted above with an extinction coefficient of 7x 10-4 per cm., over the 60 ft. path length associated with a target at 30 ft. range, is approximately a factor of 0.4.

Part Ill-Sensitivity of the System The expected signal from the system can be derived from the expression given above: S NAsAmlAm2r N 27rR2(f,)2(N.E.P.) The measured values of r in Table I will be used, and N.As. taken as 12.5 Watts/steradian. S/N thus represents the ratio of (r.m.s.) signal-to-(r.m.s.) noise in a I c/s band at a frequency 800 c/s. This gives S/N in the condition where the target is at the position where the beams overlap precisely; and this is at a distance R from the system.

The values of the parameters above for the proposed system are as follows:- Am, = 78.5 cm2 Am2 = 28.3 cm2 R = 420cms f, = 10 cms N.E.P. = 2x10-'0watts From the expression above, S -13rx105.

N To convert this result to the ratio of (peak) signal-to-(peak) noise for a characteristic target-signal envelope-waveform at the proposed chopping frequency of 5 Kc/s, and with the proposed lKc/s bandwidth, factors are necessary for the following considerations: (a) The cell time-constant and the spectrum of the cell-noise power are such that both signal and noise are reduced in going from 800 c/s to 5 Kc/s.

Measurement of the ratio of signal at 5 Kc/s to that at 800 c/s, made on cells at 150C, gave factors from 7 to 10. Noise levels were down by a factor of 5. Based on these measurements, a reduction factor of signal-to-noise ratio of 2 is assumed.

(b) r.m.s. noise amplitude in the 1 Kc/s bandwidth is 30 times that in the I c/s bandwidth, (c) Peak signal amplitude is 2.8 times the r.m.s. value.

(d) The level at which there is an expectation of one noise peak during the 35 seconds that the fuze may be active is 5 times the r.m.s. noise level.

(e) Losses in the optical system due to a peak transmission of the filter less than 100%, and due to reflection losses at glass surfaces additional to those in the laboratory apparatus used to measure r, are estimated to attenuate the signal by a factor of 0.7.

(f) The 1/3 millisecond rise time signal due to a 0.5 foot target at 30-foot range, with a missile velocity of 1500 considered the most adverse condition, will be attenuated by a factor of 3 with the amplifier pass-band of 1 Kc/s taken. Signals with rise-times of a millisecond or greater will be attenuated by a factor near 1.5.

(g) Over the temperature range of the cells 21 of-400C to 500C the ratio S/N will change by a factor of 1.4.

These factors are tabulated below to give the overall factor by which the signal-to-noise ratio operated above must be divided to give the (peak) signal-to (peak) noise ratio.

(a) signal-to-noise ratio reduction at 5 Kc/s =2 (b) r.m.s. noise amplitude conversion to a 1 Kc/s bandwidth =30 (c) conversion to peak signal =0.36 (d) r.m.s. to peak noise level conversion =5 (e) losses in the optical system =1.5 (f) signal shape factor 1.5 to 3.0 (g) cell temperature variation =1 to 1.4 Therefore, the overall factor is =243 to 680 Applying these factors, the signal-to-noise ratio becomes 5,300 r to 1,900 r.

Values of r obtained in measurements are given in Table 1 above, and appear generally to lie between 0.1 and 1.0. Assuming this, the range of signal-to-(peak) noise ratio becomes 5,300 to 190.

Referring to Fig. 6 this is the spread of possible signal-to-noise ratio for a target at the range corresponding to the peak of the curve (except that, strictly, the most adverse shape-factor applies only at the extreme range).

At 30-foot range the signal-to-noise ratio will be reduced by a factor of 10.

The r.m.s. noise-level of available cells 21 is consistently in the range 6 to 8 microvolts+l0%, in a 50 c/s band at a frequency of 800 c/s, when polarized as described above. Converted to peak noise-level in a 1 Kc/s bandwidth at 5 Kc/s using the factors in (a), (b) and (d) this becomes 30 microvolts.

The range of the target signal expected at 30 feet is therefore 16 mV to 0.6 mV.

The particular beam-convergence (and thus range-characteristic) chosen here is, of course, not optimised and can only be optimised as a result of a lethality study.

If greater range is required, this will involve decrease of the beamconvergence and will result in a lower signal at the range of peak but a more gradual signal fall-off.

With the beam-convergence chosen here, a system-threshold of 1 millivolt will give a high probability of triggering out to 30 ft. miss distances.

This threshold is 33 times the peak noise-level.

WHAT WE CLAIM IS: 1. Distance-measuring apparatus comprising a source of visible or infra-red radiation, transmitting means for transmitting the radiation away from the apparatus in a radiation beam, receiving means spaced from the transmitting means and having a beam of vision which intersects the radiation beam, the receiving means being operable to detect said radiation when reflected from a target body which lies within the space defined by the intersection-of the radiation beam and the beam of vision, generating means responsive to the receiving means and arranged to generate an electric signal on reception of such radiation by the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (1)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   (b) r.m.s. noise amplitude in the 1 Kc/s bandwidth is 30 times that in the I c/s bandwidth, (c) Peak signal amplitude is 2.8 times the r.m.s. value.

   (d) The level at which there is an expectation of one noise peak during the 35 seconds that the fuze may be active is 5 times the r.m.s. noise level.

   (e) Losses in the optical system due to a peak transmission of the filter less than 100%, and due to reflection losses at glass surfaces additional to those in the laboratory apparatus used to measure r, are estimated to attenuate the signal by a factor of 0.7.

   (f) The 1/3 millisecond rise time signal due to a 0.5 foot target at 30-foot range, with a missile velocity of 1500 considered the most adverse condition, will be attenuated by a factor of 3 with the amplifier pass-band of 1 Kc/s taken. Signals with rise-times of a millisecond or greater will be attenuated by a factor near 1.5.

   (g) Over the temperature range of the cells 21 of-400C to 500C the ratio S/N will change by a factor of 1.4.

   These factors are tabulated below to give the overall factor by which the signal-to-noise ratio operated above must be divided to give the (peak) signal-to (peak) noise ratio.

   (a) signal-to-noise ratio reduction at 5 Kc/s =2 (b) r.m.s. noise amplitude conversion to a 1 Kc/s bandwidth =30 (c) conversion to peak signal =0.36 (d) r.m.s. to peak noise level conversion =5 (e) losses in the optical system =1.5 (f) signal shape factor 1.5 to 3.0 (g) cell temperature variation =1 to 1.4 Therefore, the overall factor is =243 to 680 Applying these factors, the signal-to-noise ratio becomes 5,300 r to 1,900 r.

   Values of r obtained in measurements are given in Table 1 above, and appear generally to lie between 0.1 and 1.0. Assuming this, the range of signal-to-(peak) noise ratio becomes 5,300 to 190.

   Referring to Fig. 6 this is the spread of possible signal-to-noise ratio for a target at the range corresponding to the peak of the curve (except that, strictly, the most adverse shape-factor applies only at the extreme range).

   At 30-foot range the signal-to-noise ratio will be reduced by a factor of 10.

   The r.m.s. noise-level of available cells 21 is consistently in the range 6 to 8 microvolts+l0%, in a 50 c/s band at a frequency of 800 c/s, when polarized as described above. Converted to peak noise-level in a 1 Kc/s bandwidth at 5 Kc/s using the factors in (a), (b) and (d) this becomes 30 microvolts.

   The range of the target signal expected at 30 feet is therefore 16 mV to 0.6 mV.

   The particular beam-convergence (and thus range-characteristic) chosen here is, of course, not optimised and can only be optimised as a result of a lethality study.

   If greater range is required, this will involve decrease of the beamconvergence and will result in a lower signal at the range of peak but a more gradual signal fall-off.

   With the beam-convergence chosen here, a system-threshold of 1 millivolt will give a high probability of triggering out to 30 ft. miss distances.

   This threshold is 33 times the peak noise-level.

   WHAT WE CLAIM IS:

   1. Distance-measuring apparatus comprising a source of visible or infra-red radiation, transmitting means for transmitting the radiation away from the apparatus in a radiation beam, receiving means spaced from the transmitting means and having a beam of vision which intersects the radiation beam, the receiving means being operable to detect said radiation when reflected from a target body which lies within the space defined by the intersection-of the radiation beam and the beam of vision, generating means responsive to the receiving means and arranged to generate an electric signal on reception of such radiation by the

   receiving means and a first electrical filter arranged to receive said signal and to pass that signal only when the rate of rise of that signal exceeds a predetermined value.

   2. Apparatus according to Claim 1, wherein the transmitting means includes modulating means for modulating the radiation emitted by the said source.

   3. Apparatus according to Claim 2, wherein the modulating means comprises a pulse-type modulator.

   4. Apparatus according to Claim 3, wherein the pulse-type modulator comprises a chopper disc arranged to periodically interrupt the radiation emitted by the said source.

   5. Apparatus according to Claim 2, 3 or 4 in which the generating means comprises an electric cell and including a further electrical filter arranged to receive the electrical output signal of the electric cell and to pass that signal only when that signal corresponds to the receipt by the receiving means of radiation which has been modulated by the modulating means.

   6. Apparatus according to Claim 5, wherein the apparatus includes an amplifier arranged to amplify the electrical output signal of the electric cell, the amplifier being provided with automatic gain control.

   7. Apparatus according to Claim 5 or Claim 6, which includes a demodulator arranged to demodulate the electrical output signal of the electric cell, to form a second electrical signal the presence of which indicates the presence of the target body within the said space.

   8. Apparatus according to Claim 7, in which the first electrical filter is arranged to receive the second electrical signal and to pass that signal only when the rate of rise of the signal exceeds a predetermined value.

   9. Apparatus according to Claim 7 or Claim 8, which includes a threshold gate arranged to receive the second electrical signal and to pass that signal only when the amplitude of that signal exceeds a predetermined value.

   10. Apparatus according to any one of Claims 5 to 9 wherein the electric cell comprises a photoelectric cell.

   12. Apparatus according to Claim 11, wherein the photoelectric cell is of the lead-sulphide type.

   13. Apparatus according to any preceding Claim, wherein the angular width of the radiation beam is approximately 60.

   14. Apparatus according to any preceding Claim, wherein the angular width of the beam of vision is approximately 6".

   15. Apparatus according to any preceding Claim, wherein each transmitting means is arranged to transmit the said radiation away from the apparatus in at least two angularly spaced radiation beams.

   16. Apparatus according to any preceding Claim, wherein there are at least two of the said sources, each source being associated with a separate corresponding transmitting means and a separate corresponding receiving means, the overall arrangement being such that there are at least two of the said spaces and each of the receiving means is operable to detect the presence of the target body within a given one of the said spaces.

   17. Apparatus according to Claim 16, wherein the said spaces are arranged to extend through an angle of approximately 1800 relatively to a predetermined axis of the apparatus.

   18. A fuze system for a missile, the fuze system including apparatus according to any one of the preceding Claims.

   -19. A fuze system according to Claim 18, the fuze system including firing means operable to initiate explosion of the missile in response to the presence of the target body within the said space.

   20. A fuze system according to Claim 19, which includes inhibiting means for preventing operation of the firing means in response to the existence of predetermined conditions.

   21. A fuze system according to Claim 20, in which the inhibiting means includes means responsive to the altitude of the missile and arranged to prevent operation of the firing means when the missile is above a predetermined altitude.

   22. A fuze system according to Claim 20 or Claim 21 in which the inhibiting means includes means responsive to the switching-on of the fuze system and arranged to prevent operation of the firing means until the fuze system has been switched on and has attained predetermined steady-state conditions.

   23. A fuze system according to any one of Claims 2022, in which the inhibiting means include means responsive to radiations from the sun and arranged to prevent operation of the firing means by such radiation

   24. A missile provided with a fuze system according to any one of Claims 1823.

   25. Distance-measuring apparatus substantially as specifically described herein with reference to any one, or any combination of, the accompanying drawings.

   26. A fuze system for a missile, substantially as specifically described herein with reference to any one, or any combination of, the accompanying drawings.