GB2191580A - Gun sound ranging systems - Google Patents

Abstract

The system comprises an array of microphone stations each connected by a digital data link to a store (24) in a command post (6). The battlefield area is divided into a number of cells and the expected time delays between the time of arrival of a gun break at each microphone station of the array for each of the cells are computed (10) and stored in a memory (16). Digital representations of the received waveforms from each microphone station are read out with the stored time delays for each cell in turn to a fast correlator (28) which computes the correlation coefficient. All the cells are scanned in turn by means of a scan generator (18) which also controls the addressing of a display store (38) which stores brightness levels corresponding to the calculated correlation coefficients. The display store (38) is connected to a VDU (40). <IMAGE>

Description

SPECIFICATION Gun sound ranging systems The present invention relates to gun sound ranging systems and, more particularly, to a system which is capable of providing a completely automatically updated display of hostile gun battery positions within a predetermined battlefield area.

Gun sound ranging systems have been in use since 1914to locate the positions of any gun batteries by means of the differences in time of arrival of gun shot sound at several spaced microphone stations. These relative time differences allow the position of the gun to be calculated.

One existing gun sound ranging system utilises a linear array of seven microphone stations extending about 15 kilometres in a line parallel to theforward edge ofthe battle area (FEBA). Each microphone station is connected to a respective channel of a pen recorder at a rear command post or processing station by means of a radio link. In orderto avoid an excessive amount of output from the pen recorder, it is only activated in response to a signal from an advance post indicating that a gun has fired in the battlefield area.

The sound signal received byeach microphone station is referred to as a "gun break". It is represented buy a complex sound signature with a typical duration of a 1/4 second. In the present system the time delays between corresponding parts of the gun break recorded by pairs of microphone stations manually measured. This is a skilled operation asthe operator mustselectcorresponding parts of the gun break signal to measure the time delay between. These measured time delays are then input to a computer at the command post which uses them to compute the position ofthe gun.

The above described system hasthedisadvantage that it is slow, that is one gun location maytake 3 minutes to locate. This is clearly impractical when a gun barrage may produce a rateofgunfire of upto4 gunshots persecond.The system also requires the presence of an operator in an advance post near the FEBAto activate the system when a gun is fired, as well as an operator at the command post to make the measurements from the output of the pen recorder.

Reliability of the system is also dependent upon the accuracyofthe measurements made bytheoperator on the pen recorder output. Since the gun break has a complex signature, it is difficu It to automate this measurement stage.

The present invention is therefore intended to solve these technical problems of slowness and requirement for skilled operator intervention.

The present invention accordingly provides a gun sound ranging system comprising an array of microphone stations each connected by means of a digital data linkto a processing station, the processing station comprising meansforcalculating from the positions of the microphone stations an expected set oftime delays between the arrival of a gun sound at the respective microphone stations from a gun fired from each of a plurality of cells in a predetermined battlefield area, a first memory for storing a said set oftime delays for each cell.

a second memory for continuously storing the data representing sound signals received in a preceding fixed time interval by each microphone station, means for repeatedly scanning the first memory and outputting address information to said second memory so that for each celtin turn, the data in the second memoryforeach microphone station is read out simultaneously after being delayed by an amount corresponding to the stored time delay for that station in the set of delays for that cell so that if a gun had fired in that cell the sound signal from that gun would be output simultaneously from the first memory for each microphone station, correlator means for receiving said signals read out from said second memory and correlating them to produce a correlation coefficient, and display means adapted to display information relating to the levels of the correlation coefficients.

The above defined system is advantageous in that it can be operated completely automatically and there is no requirement for operator intervention.

The measurementofthe relevant time delays between a gun break received at each microphone station is eliminated by the present system which effectively tests in turn the hypotheses that a gun has fired in each one of the plurality of cells. With modern processing equipment a large area of the battlefield can be scanned repeatedly in this way very rapidly. The initial calculation ofthe sets oftime delays for storage in the first memory requires a considerable computational effort. However, since it only needs to be carried infrequently, as when the array of microphone stations is first set up, it imposes a relatively low overhead on the operation ofthe system.

Preferably the display comprises a video display unit representing a map ofthe predetermined area covered by the cells, the brightness of an area of the display representing a particular cell corresponding to the level of the correlation coefficient for that cell.

Such a display could be arranged to permanently show at low intensity superficial features such as rivers, roads, urban areas and woodlands.

Superimposed on the display, bright spots would appearatthecalculated battery location points each time a gun fired.

In a preferred embodiment, the brig ht spots, the level of which represents the magnitude of the correlation coefficient are allowed to decay with a time constant which may be adjustable.

Consequently, each time the same gun fires the bright spot is enhanced in brightness. Spots representing gun battery locations that are calculated as a result of erroneous information will therefore rapidly decay whereas those which have been correctly computed will be reinforced in brightness.

In a preferred embodiment, the calculating means is provided with an input for meteprological information which can affect the time of arrival of a sound signal at the microphone stations from the predetermined area. By taking into accountthe meteorological conditions such as air density, wind velocity at various heights overthe battlefield area, and the vertical temperature gradient, all of which can affectthe speed of sound and the length of the path over which the sound wave travels since most energy reaches the microphone stations by refraction through layers of the air ratherthan directly, the overall accuracy of the system can be considerably enhanced with respect to prior art systems which took no account of the meteorological conditions whatsoever.

In a further' preferred embodiment, afurther memory is provided in which a portion ofthesound signal from at least one microphone station can be stored when a correlation coefficient exceeds a predetermined threshold, and means for enabling the display of said stored signal via the display means. By storing waveforms which produce high correlation coefficients, there is a high probability that this waveform will be the best record of the gun break. Display of the complex gun breaksound signal can allow identification of the gun which produced that particular gun break by a skilled operator on the basis of the characteristics of the gun break signature. This allows additional information on the type of gun to be produced from the system.

Agun sound ranging system embodying the invention will now be described, byway of example only, with reference to the accompanying diagrammatic drawings, in which: Figure lisa plan view of a battlefield area in which the system is deployed; and Figure 2 is a block diagram of the command post or processing station of the system.

The gun sound ranging system comprises an array of microphone stations 2 which are distributed in a random manner over a portion of the battlefield adjacent to the FEBA. They are arranged to receive sound from a battlefield area beyond the FEBA where enemy gun batteries may be located. In the example shown in Figure 1, the array consists of seven essentially randomly distributed microphone stations 2. The array of microphone stations for use at any particular time in the system may be selected from a larger array of microphone stations which are already in position. Dormant microphone stations 4 may be located as shown and a particular set of microphone stations may be brought into operation for use as the array at a particulartime.Each microphone station is provided with a digital data linkto a command post or processing station 6 located in a relatively safe rearposition.The microphone stations are preferably provided with solar batteries so that they can be operated for longperiods without attention. Each microphone station includes an anologueto digital converter and a radio transmitterfortransmitting backto the command post 6. Relays 8 may be distributed over the battlefield area for receiving and re-transmitting the signals from the microphone stations if a direct transmission to the command post 6 is not possible.

Again the relays are preferably solar battery powered.

The type of data links used between the microphone stations 2 and the command post 6 are not material for the purposes ofthe present invention provided that digital representations ofthe gun breaks can be accurately received by the command post 6. A 1 6kb/s digital data link has been found to be suitable for this purpose. For use with this digital data link, the microphone stations are provided with a data memory so that, after Ato D conversion, the data can be intermittently transmitted atfourtimes the real time rate. The data link is preferably such as to allow each ofthe microphone stations 2, 4to be commandable by the command post 6 allowing any microphone station 2, 4to be brought into operation orturned off as required in dependence upon whether it is to be used in the microphone station array ofthe system.

The microphone stations are randomly distributed overthe battlefield area as opposed to the linear arrangement adopted by the prior art system already outlined. This eliminates the mirror effect possible with the prior art system where gun shots originating from behind the FEBAfrom a friendly gun could appearto originate from in front ofthe FEBA at a position corresponding to its reflection in the line of the microphone stations. This can result in a waste of effort in trying to eliminate imaginary guns. The position of each microphone station in the array must be accurately known to the processing station. Any suitable surveying method can be used for this purpose.It is suggested that the microphone stations be deployed by using a vehicle equipped with a position and azimuth determining system so that by recording the exact position ofthevehicle when the microphone station is deployed, an initial estimate of the position can be obtained. The accuracy ofthis position can then be improved by radio ranging to the microphone station from two independent positions. The radio ranging equipment can use the data transmitting equipment already at the microphone station to form a transponder. Other surveying techniques can readily be used. The exact surveyed positions ofthe microphone stations to be used in the array are fed to a calculating means 10 at the command post 6 via an input 12 as shown in Figure 2.

The equipment atthe command post or processing station 6 will now be described with reference to Figure 2. The calculating means 10 has a further input 1 4for meteorological data. The function of the calculating means 10 is to compute from its inputs a set oftime delays for each of a plurality of cells defined in a predetermined area of the battlefield beyond the FEBA. Atypical sizeforthis area would be 20km wide and 20km deep. By selecting a cell size of a 1 00m square, 40,000 cel Is will exist within this predetermined area. Thus a set of time delays is computed by the calculating means for each ofthese 40,000 cells. The time delays are computed each relative to the next adjacent microphone station. The calculating means is a computer operating under software control. The program for computing the time delays takes into account the expected sound path from the cell to each microphone station and the speed of sound.

Both the path length and the speed of sound will vary in dependence on the meteorological conditions in a known manner. The output of the calculating means is a set of seven relative time delays for each cell and preferably an indication ofthe relative signal level of the gun break at each microphone station for a gun shot from each cell. This is referred to as a significance value. The calculating means has effectively transformed the input information on the cell and microphone array positions and the meteorological data into a set of data for each cell.

The transform output is stored in a first memory or transform store 16.

In the present example the transform store contains for each cell seven delay values each represented by two bytes and seven significance values, each of one byte. A memory of 840 bytes is therefore needed to contain the tableforthe example given. Read out from the transform store 16 is controlled by a scan generator 18 which provides address inputs to the store 16so that the stored information for each cell in turn can be read out in parallel on an output 20 to an address generator22.

The data input from the microphone stations is fed on seven channels to a second memory or main data store 24. The store 24 is sized to store all the data received from each of the seven microphone stations over a preceding real time interval, in this example, of substantially 33 seconds. For this purpose a 245Kb memory is appropriate. The memories are scrolled in real time so that the older information is lost as new information is input. Read out from the main data store 24 is controlled by address inputs generated by the address generator 22.

The main data store 24 is repeatedly read out, in a mannerto be described in more detail later, on a seven channei output 26 to a correlator28which repeatedly correlates the seven signals read to it.

Each of the signals read to it represents approximately 30 seconds a digital representation of an input sound waveform from each of the microphone stations. The calculated correlation coefficient output as a result of each correlation process is fed to a processor 30 where it is compared with a predetermined threshold. lfthecorrelation coefficient exceeds the threshold a signal is sent on line 32 to a special waveform store or memory 34 which signal causes one of the waveforms fed to the correlator 28 to be stored in the memory 34. The processor 30 converts the correlation coefficient into a brightness level which is passed on line 36 to a display memory 38.The location at which this brightness level is stored is determined by an address fed by the scan generator 18so that the address in the display store corresponds to the cell forwhich the set oftime delays was output by the memory 16. Therefore, as the scan generator 18 causes each cell to be scanned the brightness level corresponding to the correlation coefficientforthat cell is stored at the appropriate address in the display store 38 of at least40kb. The display store may also contain display information to enable a diagrammatic map of the battlefield area to be represented on a VDU display 40 connected to the display store 38.The brightness levels in the display store are controlled by the processor 30 to decay with a predetermined time constant which may be adjusted under software control. Each time the cell is scanned a new brightness level is added to that already present in the store 38. Therefore the store acts as an integrating memory.

The VDU display 40 may be a CRT display or a Plasma display or any other suitable type of visual display.Asoftwarecontrol unit 42 is provided which is responsive to operator input via a keyboard 44. Via the keyboard 44, the operator may instruct the display of the waveform (if any) stored in memory 34 for any selected cell area identified on the VDU 40.

This operation is controlled by an output on line 44to the special waveform store 34 which output controls the required stored waveform to be fed out on line 46 to the display 40 in place ofthe input from the main displaystore 48. The software control unit42 is connected by a databus 48 to the display 40 so that control ofthe display can take place and information obtained from the display can be passed via a line 50 to other systems such as gun laying systems. The operator may also be able to perform such operations as zooming in on a particular area ofthe display in order to provide greater detail orto select the main display, a waveform display or a display of status information which can be provided to the software control unit 42 via an input 52.

In order to provide greater security of operation a standby command post 6a may be provided in parallel so that control can be switched between the command posts 6 and 6a iffailure of one ofthem occurs.

The operation of the command post or processing station will now be described in more detail.

At the start of operation of the system with a new configuration of the microphone station array or when an update of meteorological information is available on input 14, which is typically at two hour intervals, the calculating means carries out the already described transform calculation and stores the set oftime delays, and significance values for each cell in the transform store 16. This operation may take some tens of minutes.

During operation of the system, the object is to repeatedly scan the cells of the battlefield area and testwhetherany received signal corresponds to a gun shot from a gun battery in that cell. The scan generator 18 ensures that the time delay information and significance values for each cell are fed outto an address generator22 in turn. The address generator 22 controls the read out of signals from the main data store 24 so that the stored signals from each ofthe microphone stations are read out with a relative delay which compensates for the different positions ofthe microphone stations. This is done by using the relative time delay information for each microphone station to produce an appropriate shift in the addresses generated for reading the received data from that microphone station to the correlator 28.

Therefore if a gun shot had originated in the cell under consideration, the read outwaveformswould be appropriately time shifted so that the gun breaks read out to the correlator 28 would be simultaneous.

Therefore, the output of the correlator 28 will be a high correlation coefficient when a gun shot has originated from the cell currently being scanned. If there has been no gun shot in that cell then the correlation coefficient will be relatively low either because only noise is being received from the microphone stations or because they are receiving a gun breakfrorn a different cell and the time delays read out from the transform store do not bring the gun breaks into alignment. It will be appreciated that the maximum correlation coefficient for a cell will occurwhen the read out signals each contain the whole of the gun break. When only a portion ofthe read out signals contains part ofthe gun breakthe correlation coefficient will be lower.

The fast correlator 28 effectively forms the product of all corresponding ordinates in the waveforms fed to itfrom the store 24. Since the waveforms already exist as digitised samples, the seven fold multiplication can be performed digitally. In the present example six fast sixteen bit digital floating point multipliers are needed and each multiplication must be completed in 1 OOns. An appropriate strategy for carrying outthis seven fold multiplication isto first generate the products of pairs of ordinates then form the products of pairs of these products and finally multiply the last pair of products. This multiplication tree thus has three sequential stages and an individual sevenfold product takes 300nS to complete. The throughput, however, is still one sevenfold corelation per lOOnS.The final product represents an instantaneous correlation coefficient.

As the correlation coefficient is re-computed for each time that cell is scanned, a definite correlation peak will occur if a gun shot originated from that cell. The significance values stored in transform store 16may also be introduced into the correlation process. The correlation coefficients are stored in the processor 30 and are updated typically one per second. As previously described the correlation coefficients are used to compute a brightness level for store in the display store 38. These brightness levels decay if the correlation coefficient is not maintained art a high level.This will allow high correlation coefficients resulting from spurious noise conditions to decay from the display quickly whereas if there is a gun located in a particular cell that is repeatedly firing, this will be shown as a continually reinforced bright spot on the display. In some circumstances, a cluster of dots on the display may occur even though there may only be one gun. For example this effect could occur if meteorological conditions were unstable overthe area.

When the correlation coefficient exceeds the predetermined threshold, the waveform from the main store 24 is stored in the store 34so that it can be subsequently displayed under operator control to allow identification of the type of gun from the gun break signature.

In a modified design the correlator 28 may be replaced by a set of parallel correlators which each only usethe input signalsfrom asubsetofthe microphone stations. Thus, in place of a single correlation step using all seven microphone stations, a set of parallel correlators can each perform the correlation on a respective subset of, for example, four of the seven channels. This would require a set of 35 correlators connected in parallel. This obviously increases the space and power consumption requirements of the system but would improve the discrimination. This is called subset correlation. The use of the significance value for a microphone station in the correlation calculation can also improve discrimination by reducing the effect of high noise input from a microphone that would be in sound shadowfrom a particular cell being scanned.

Claims (7)

1. A gun sound ranging system comprising an array of microphone stations each connected by means of a digital data linkto a processing station, the processing station comprising means for calculating from the positions of the microphone stations an expected set oftime delays between the arrival of a gun sound at the respective microphone stations from a gun fired from each of a plurality of cells in a predetermined battlefield area, a first memory for storing a said set oftime delays for each cell, a second memory for continuously storing the data representing sound signals received in a preceding fixed time interval by each microphone station, means for repeatedly scanning the first memory and outputting address information to said second memory so that for each cell in turn, the data in the second memory for each microphone station is read out simultaneously after being delayed by an amount corresponding to the stored time delayfor that station in the set of delaysforthat cell, so that if a gun had fired in that cell the sound signal from that gun would be output simu Itaneously from the first memory for each microphone station, correlator means for receiving said signals read out from said second memory and correlating them to produce a correlation coefficient, and display means adapted to display information relating to the levels ofthe correlation coefficients.

2. A system according to claim 1,whereinthe display means comprises a video display unit representing a map of the predetermined area covered by the cells, the brightness of an area ofthe display representing a particular cell corresponding to the level ofthe correlation coefficientforthat cell.

3. A system according to claim 2, where the video display unit permanently shows at low intensity physical features within the area covered by the cells.

4. A system according to claim 2 or 3, wherein the brightness of an area ofthe display representing a particular cell decays with a time constant which may be adjustable.

5. A system according to any one of the preceding claims, wherein the calculating means is provided with a n i n an inputformeteorological information which can affectthetimeof arrival of a sound signal at the microphone stations from the predetermined area.

6. A system according to any one ofthe preceding claims, comprising a further memory in which a portion ofthe sound signal from at least one microphone station can be stored when a correlation coefficient exceeds a predetermined threshold, and means for enabling the display of said stored signal via the display means.

7. A gun sound ranging system substantially as herein described with reference to the accompanying drawings.