GB1572714A - Acoustic miss indicators for airborne targets - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ACOUSTIC MISS INDICATORS FOR AIRBORNE TARGETS (71) I, THE SECRETARY OF STATE FOR DEFENCE, LONDON, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to accoustic miss indicators for airborne targets.

For gun trials and for firing practice, airborne targets are used and projectiles are fired towards the target generally from the rear. A typical target has a torpedo-shaped body with fins at the tail and is towed by a cable fixed to an attachment point on the side of the body about mid-way along its length. A miss-distance indicator which is known and used with such a target comprises a transducer, typically a microphone, mounted on the target to detect shock waves from projectiles passing the target at supersonic speed and means connected to the transducer for estimating the miss-distance of the projectiles from the strengths of the detected shock waves. The estimated missdistances are encoded and transmitted to a receiving station. The known acoustic missdistance indicator gives no indication of the direction of miss.That is to say it produces the same indication whether a projectile passes to the left or the right or above or below the target.

It is an object of the present invention to provide a miss indicator which will provide some indication of the direction of miss.

According to the present invention there is provided a miss indicator for an airborne target comprising: a plurality of transducers which are arranged in a polygonal array around the longitudinal axis of the target when in use and detect shock waves from projectiles passing the target at supersonic speed, means connected to the transducers for deriving, from the relative times of detection of shock waves at the transducers, directional information from a determination of the order in which the transducers detect a shock wave wherein the means for deriving directional information includes warning means for distinguishing orderings inconsistent with detection of a single shock wave from a direction transverse to the longitudinal axis of the target and producing error indication information in response thereto which is in addition to and distinct from the directional information.

At least one of the transducers may also be connected to means for estimating the miss-distance of the projectiles from the strengths of the detected shock waves.

There are preferably four transducers in the array, and the array is preferably square, with one transducer above the longitudinal axis of the target, one below, and one to each side. In a towed target the transducers are preferably situated forward of the attachment point for the line by which the target is to be towed, and in any target they are preferably mounted at the forward end of the target, projecting forward.

The means for deriving directional information may comprise means for determining, for each shock wave, the first pair of adjacent transducers to detect the shock wave, so as to determine the quadrant through which the projectile passed.

An embodiment of the invention will now be described by way of example with reference to the drawings accompanying the Provisional Specification, of which: Figure 1 shows a target equipped with a miss-distance indicator according to the present invention, Figure 2 is a schematic circuit diagram of the indicator of Figure 1, and Figures 3 and 4 show in more detail some portions of the circuit of Figure 2.

The target shown in Figure 1 has a cylindrical body 1, a blunt nose 2 and tail fins 3. At about mid-way along the length of the body 1 there is a towing-line attachment point 4. The target is a coventional type of target. In use the target is towed from an aircraft by a towing line attached to the towing-line attachment point 4 and it travels nose-forwards below and to the rear of the aircraft with the towing-line attachment point upwards.

Mounted on the nose of the target, and projecting forwards, are four microphones 5, 6, 7 and 8 in a square array. Microphone 5 is mounted to be above the longitudinal axis of the target when the target is in its in-use orientation, microphone 6 is below the axis, and microphones 7 and 8 are mounted one on each side of the axis. The nose mounting illustrated has been found to be aerodynamically satisfactory, using bullet-shaped microphones, and has the advantage over, say, a tail mounting, that the microphones are not in the wake of the towing line, which give rise to a serious amount of noise aft of the towing line. The advantage of projecting forward of the nose is that all the microphones can receive shock waves from any side without being shadowed by the body of the target.

In the circuit of Figure 2 the microphones 5, 6, 7 and 8 are connected to a sector code generator 9 which is constructed to produce a sector signal, when a shock wave is detected, indicating which two adjacent microphones first detect the shock wave.

This sector signal is applied via a delay circuit 10 to a data multiplexer 11. One of the microphones 6 is also connected to a miss-distance code generator circuit 12, which is similar to the circuit on a conventional miss-distance indicator and produces a distance signal indicative of the missdistance, as deduced from the strength of the shock wave. This distance signal is also applied to the multiplexer 11, which is constructed to combine the sector signal and the distance signal and apply the combined signal to a modulator 13 which produces a modulated signal and applies it to a transmitter 14.

From the signal transmitted by the transmitter it is possible to determine not only the miss-distance. but also the quadrant through which each projectile passed, whether above and to the left, above and to the right, below and to the left or below and to the right. The quadrant information is only completely reliable when the projectiles are fired from directly behind the target, as is usually the case in gun trials. It has been found however that there is only a five per cent error rate when the projectiles are red at twenty degrees to the axis of the target, and it is expected that in practice most engagements will be from the rear and within twenty degrees of the axis.

The sector code generator is shown in more detail in figure 3. The amplified outputs of the microphones 5, 6, 7 and 8 of Figures 1 and 2 are connected via respective band-pass filters 15, 16, 17 and 18 to inputs of respective monostable circuits 19, 20, 21 and 22. The band-pass filters 15, 16, 17 and 18 have a pass band of three Hertz to nine Kilohertz and are included to remove components of shock-wave signals which vary with height, so that the shape of the signals applied to the monostable circuits 19 to 22 is approximately independent of the height at which the target is operating. The monostable circuits 19 to 22 give pulses of about five milliseconds duration.

The outputs of the monostable circuits 19 to 22 are connected to inputs of four NAND gates 23, 24, 25 and 26 as follows: the output of monostable circuit 19 is connected to inputs of gates 23 and 26; the output of monostable circuit 20 is connected to inputs of gates 24 and 25; the output of monstable circuit 21 is connected to inputs of gates 23 and 24 and the output of monostable circuit 22 is connected to inputs of gates 25 and 26. The outputs of gates 23 to 26 are connected via respective capacitors 27, 28, 29 and 30 to inputs of respective monostable circuits 31, 32, 33 and 34 which are set to produce inverted pulses of ten milliseconds duration.The outputs of the monostable circuits 31 to 34 are each connected to inputs of all of the gates 23 to 26 except the respective one; that is to say the output of monostable circuit 31 is connected to inputs of gates 24, 25 and 26, ie all except 23, the output of monostable circuit 32 is connected to inputs of all of the gates except 24 and so on.

The outputs of monostable circuits 31 to 33 are also connected to inputs of NAND gates 35 and 36 as follows: the output of monostable circuit 31 is connected to an input of gate 35; the output of monostable circuit 32 is connected to inputs of gates 35 and 36 and the output of monostable circuit 33 is connected to an input of gate 36. The outputs X and Y of gates 35 and 36 are the sector code outputs of the sector code generator.

When no shock waves have been detected the outputs of monostable circuits 19 to 22 are at logical '0', so the outputs of gates 23 to 26 are at logical '1'. The outputs of monostable circuits 31 to 34 are at logical '1' so the outputs X, Y of gates 35 and 36 are at logical '0'. When shock waves have been received at two adjacent microphones, such as microphones 5 and 7 in Figures 1 and 2 for example, within five milliseconds of one another, the respective ones of the monostable circuits 19 to 22, namely 19 and 21 in the example, are simultaneously at logical '1'. All the inputs of one of the gates 23 to 26. namely gate 23 in the example, are then simultaneously at logical '1' so the output of that gate drops to logical '0'.The input of the respective monostable circuit 31 to 34 (31 in the example) thereupon momentarily drops to logical '0', triggering it. The output of the respective monostable circuit 31 to 34 thus falls to logical '0'. Since the outputs of the monostable circuits 31 to 34 have the described connections to the inputs of the gates 23 to 26, when one of the monostable circuits 31 to 34 is triggered the inputs to the others are blocked. Thus only one of the monostable circuits 31 to 34 is triggered corresponding to the first pair of adjacent microphones to detect a shock wave. If the first pair is 5 and 7 then 31 is triggered, if 6 and 7 then 32, if 6 and 8 then 33 and if 5 and 8 then 34. The gates 35 and 36 derive a two-bit sector signal at their outputs X and Y.If the shot passes the target in either of the lower two quadrants the first adjacent pair of microphones to detect the shock wave will be 6 and 7 or 6 and 8, so monostable circuit 32 or 33 will be triggered.

In either case the Y output is at logical '1'. If the shot passes in either of the upper two quadrants the Y output will be '0'. Similarly the X output is '1' or '0' as the shot passed to the right or left respectively (assuming that it came from the rear of the target).

The sector code generator of Figure 3 also includes some provision for detecting spurious signals. The outputs of monostable circuits 19 and 20 are connected to inputs of a NAND gate 37 and the outputs of monostable circuits 21 and 22 are connected to inputs of a NAND gate 38. The inputs of monostable circuits 31 to 34 are all connected to inputs of a NAND gate 39. The outputs of the gates 37, 38 and 39 are connected to inputs of a NAND gate 40, the output of which is connected to an input of a monostable circuit 41.

Before any shock waves are detected the outputs of gates 37 and 38 are at logical '1' and the output of gate 39 at logical '0'. The output of gate 40 is thus at logical '1'. When a pair of adjacent microphones have detected a shock wave one of the inputs of gate 39 drops to logical '0' so its output rises to logical '1'. If the outputs of gates 37 and 38 are still at logical '1'. which is the case if no pair of opposite microphones have detected a shock wave, then the output of gate 40 drops to logical '0' and monostable circuit 41 is triggered. If on the other hand a pair of opposite microphones detect a shock wave before any pair of adjacent microphones, then the output of either gate 37 or gate 38 will be at logical '0' and the monostable circuit 41 will not be triggered.A pulse from the monostable circuit 41 thus indicates a permissible ordering of detections, and thus a prima facie reliable sector indication.

The circuit shown in Figure 4 combines the delay circuit 10 and the multiplexer 11 of Figure 2. The output of the distance code generator 12 of Figure 2 is connected to one input of an adder circuit 42, which provides the multiplexing function, and to an input of a monostable circuit 43. The pulse length of the monostable circuit 43 mainly determines the length of delay produced and therefore its best value depends on the duration of the output signal of distance code generator 12; about twenty milliseconds is a typical value.

The output of the monostable circuit 43 is connected to an input of a NAND gate 44, the other input of which is connected to the output of monostable circuit 41 of Figure 3.

The output of gate 44 is connected via two monostable circuits 45 and 46 in series to a parallel-load-triggering input of a five-stage shift register 47. The five parallel data inputs of the shift register 47 are connected to receive respectively constant '0', '1' and '0' signals and the X and Y outputs of the sector code generator of Figure 3.

The output of monostable circuit 43 is also connected via a monostable circuit 48 to a setting input of a bistable circuit 49. The non-inverted output of bistable circuit 49 is connected to one input of a NAND gate 50, the other input of which is connected to receive a clock pulse train. The output of gate 50 is connected to a clocking input of the shift register 47 and to an input of a count-5 circuit 51. The output of the count-5 circuit 51 is connected to the resetting input of the bistable circuit 49 and to an input of a monostable circuit 52. Outputs of the monostable circuit 52 are connected to resetting inputs of the shift register 47 and the count-5 circuit 51 respectively. The serial output of the shift register 47 is connected to an input of the adder 42.

When a shock wave is detected and the distance code generator 12 of Figure 2 produces an output the output of the sector code generator passes to the adder 42 and at the same time the monostable circuit 43 is triggered. If a pulse is also produced by the monostable circuit 41 of Figure 3 the output of gate 44 will fall to '0', triggering monostable circuit 45 which, after a delay, triggers monostable circuit 46 causing the shift register 47 to be loaded with the X and Y outputs of the sector code generator and a constant '010' code.

When monostable circuit 43 returns to its stable state, by which time the signal from the distance code generator should be finished, monostable circuit 48 is triggered, setting the bistable circuit 49 and opening gate 50. Clock pulses are then applied to the shift register 47 causing the five bits 'O1OXY' to be applied to the adder 42. When five clock pulses have been produced at the output of gate 50 the count-5 circuit 51 resets the bistable circuit 49 and triggers monostable circuit 52 which resets the count-5 circuit 51 and the shift register 47.

The adder 42 thus first receives the distance code and then the sector code. If there had been no signal from the distance code generator 12 the shift register 47 would never have been loaded or clocked, so no output would have been produced. Such a situation might occur if a shot passed wide of the target, since distance code generators are generally made to produce no output in response to shots whose calculated range is greater than some distance, typically 45 feet (14 metres). If no pulse had been produced by monostable circuit 41 of Figure 3, the shift register 47 would not have been loaded, though it would have been clocked.

Thus the output of the adder 42 would have been the distance code followed by five '0' bits. The lack of the constant '010' in the sector code bits would have indicated that no reliable sector information had been derived.

Some modifications of the embodiment described will be apparent to a person skilled in the art to which this invention relates. For example, instead of determining the first pair of adjacent microphones to detect the shock wave, the indicator could simply determine the first microphone to detect the shock wave. This would allow directional information in quadrant form to be provided using simpler electronic circuit ry, but at the cost of losing the protection against spurious signals. Alternatively, at the cost of more complex circuitry, direc tional information in octant form could be obtained by determining the order of detec tion within the first pair, or by determining the first pair and the third microphone to detect the shock wave.The method em ployed in the described embodiment is a compromise between desirable performance and simplicity of circuitry, bearing in mind the fact that airborne targets have to be to some extent expendable.

WHAT I CLAIM IS: 1. A miss indicator for an airborne target comprising: a plurality of transducers which are arranged in a polygonal array around the longitudinal axis of the target when in use and detect shock waves from projectiles passing the target at supersonic speed, means connected to the transducers for deriving, from the relative times of detec tion of shock waves at the transducers, directional information from a determina tion of the order in which the transducers detect a shock wave wherein the means for deriving directional information includes n'eans mean for distinguishing orderings . inconsEtent with detection of a single shock from & n a direction transverse to the longitudinal axis of the target and producing error indication information in response thereto which is in addition to and distinct from the directional information.

2. A miss indicator as claimed in claim 1 wherein the warning means is arranged to determine if a shock wave is apparently detected by a non-adjacent pair of transducers before being detected by an adjacent pair of transducers.

3. A miss indicator as claimed in either of the preceding claims mounted on a target having a towing-line attachment point and wherein the transducers are mounted forward of the towing-line attachment point.

4. A miss indicator as claimed in any of the preceding claims wherein the transducers are mounted on the nose of the target.

5. A miss indicator as claimed in any of the preceding claims wherein the transducers are four in number and mounted in a square array. indicator 6. A miss indicator as claimed in any of the preceding claims in combination with means connected to one of the transducers for deriving miss-distance information from the strength of detected shock waves.

7. A miss indicator as claimed in Claim 6 in combination with means for transmitting the miss distance information, the error indication information and the directional information to a receiving station.

8. A miss indicator substantially as herein described with reference to the drawings accompanying the provisional specification (Figures 1 and 2).

9. A miss indicator substantially as herein described with reference to the accompanying drawings (Figures 3 and 4).

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

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. resets the bistable circuit 49 and triggers monostable circuit 52 which resets the count-5 circuit 51 and the shift register 47. The adder 42 thus first receives the distance code and then the sector code. If there had been no signal from the distance code generator 12 the shift register 47 would never have been loaded or clocked, so no output would have been produced. Such a situation might occur if a shot passed wide of the target, since distance code generators are generally made to produce no output in response to shots whose calculated range is greater than some distance, typically 45 feet (14 metres). If no pulse had been produced by monostable circuit 41 of Figure 3, the shift register 47 would not have been loaded, though it would have been clocked. Thus the output of the adder 42 would have been the distance code followed by five '0' bits. The lack of the constant '010' in the sector code bits would have indicated that no reliable sector information had been derived. Some modifications of the embodiment described will be apparent to a person skilled in the art to which this invention relates. For example, instead of determining the first pair of adjacent microphones to detect the shock wave, the indicator could simply determine the first microphone to detect the shock wave. This would allow directional information in quadrant form to be provided using simpler electronic circuit ry, but at the cost of losing the protection against spurious signals. Alternatively, at the cost of more complex circuitry, direc tional information in octant form could be obtained by determining the order of detec tion within the first pair, or by determining the first pair and the third microphone to detect the shock wave.The method em ployed in the described embodiment is a compromise between desirable performance and simplicity of circuitry, bearing in mind the fact that airborne targets have to be to some extent expendable. WHAT I CLAIM IS:

1. A miss indicator for an airborne target comprising: a plurality of transducers which are arranged in a polygonal array around the longitudinal axis of the target when in use and detect shock waves from projectiles passing the target at supersonic speed, means connected to the transducers for deriving, from the relative times of detec tion of shock waves at the transducers, directional information from a determina tion of the order in which the transducers detect a shock wave wherein the means for deriving directional information includes n'eans mean for distinguishing orderings . inconsEtent with detection of a single shock from & n a direction transverse to the longitudinal axis of the target and producing error indication information in response thereto which is in addition to and distinct from the directional information.

2. A miss indicator as claimed in claim 1 wherein the warning means is arranged to determine if a shock wave is apparently detected by a non-adjacent pair of transducers before being detected by an adjacent pair of transducers.

3. A miss indicator as claimed in either of the preceding claims mounted on a target having a towing-line attachment point and wherein the transducers are mounted forward of the towing-line attachment point.

4. A miss indicator as claimed in any of the preceding claims wherein the transducers are mounted on the nose of the target.

5. A miss indicator as claimed in any of the preceding claims wherein the transducers are four in number and mounted in a square array. indicator

6. A miss indicator as claimed in any of the preceding claims in combination with means connected to one of the transducers for deriving miss-distance information from the strength of detected shock waves.

7. A miss indicator as claimed in Claim 6 in combination with means for transmitting the miss distance information, the error indication information and the directional information to a receiving station.

8. A miss indicator substantially as herein described with reference to the drawings accompanying the provisional specification (Figures 1 and 2).

9. A miss indicator substantially as herein described with reference to the accompanying drawings (Figures 3 and 4).