GB2181238A - Automatically locating the position of a sound source - Google Patents

Abstract

A method of locating the position of a source of sound, the method comprising the steps of feeding to signal processing means a plurality of signals representative of sound impulses detected from a plurality of detector clusters each of which includes a plurality of sound detectors arranged in a predetermined pattern. The signals are processed to generate data on the time and angle of arrival of each of the signals relative to respective detector clusters. Comparative associations are then made between different parts of the data to identify sets of signals originating from a common sound source. A sound location system for carrying out the method is also described.

Description

SPECIFICATION Improvements in or relating to a method of locating the position of a sound source The present invention relates to a method of locating the position of a sound source and more particularly to a sound location system for detecting sound emissionsfrom, and locating the positions of, sources of quasi-impulsive sounds.

The invention finds application as a sound location system for locating guns and mortars firing at high or low rates, the system also finding application in sonar, seismic and like sound location processes.

The basis of gun sound location is the measurement of characteristics (such as the time of arrival) of sound 'breaks' which are received at microphones distributed in some known configuration, and the calculation from the resulting data ofthe position of each gun, probably in Cartesian map co-ordinates.

In a future battle the rate offiring of these guns may be as high as 10 per second. To achieve sufficient location accuracy, an array length of approximately 15 to 20 kilometres or more is typically required. The speed of sound is such that it takes almost 60 seconds to travel 20 km. Thus, in orderto collect data on sound emissions from a single gun from all microphones in a 20 km array, one needs to store that data for unto two minutes. As a result, the memory oft processorwill accumulate about 1200x B breaks, where B is the number of microphone positions in the array. There are a number of methods of handling this sortofdata, which fall into one oftwo groups: array processing or single source estimation methods.The former group accepts data in unprocessed form and estimates source positions by various means of cross-correlation. Thistype of processing isapplicabletocontinuousorquasi- continuous signals and so is not particularlyapp- ropriateto gun location systems. The latter group of processing methods will only accept data that has been previously sorted into associated sets, but is appropriate to impulsive waveforms. The basic procedure is to take these breaks from each microphone and to associate them with breaks from all the others, so thatthe computation ofindividual gun positions may be carried out.

The conventional sound location systems rely on a large measure of human intervention during the process of sound detection and source location, for both pattern recognition and time measurements. It is not feasible to consider manual systems for processing at the high rate offiring given above. Furthermore, in addition to the large number of breaks in the data file, there will also be acoustic clutter, multi-path and noise, against which to discriminate. Accordingly, the present invention has as one objective the prov ision of a substantially automatic sound location systemforlocating guns and mortarsfiring at high rates, the system also finding application insubstan- tiallyautomaticsonarand seismic location pro cesses.

According to the present invention there is prov ided a method of locating the position of a source of sound,the method comprising the steps offeeding to signal processing means a plurality of signals representative of sound impulses detected from a plurality of detector clusters each of which includes a plurality of sound detectors arranged in a predetermined pattern, processing the signals to generate data on the time and angle of arrival of each of thesignalsrelativeto respectivedetectorclusters and making comparative associations between different parts of the data to identify sets of the signals originating from a common sound source.

In a preferred embodiment the method comprises the step of processing data representative of the associated sets of signals originating from the common sound source to provide a more accurate estimate ofthe location of the common sound source.

According to the present invention there is also provided a sound location system comprising a plurality of detector clusters each of which includes a plurality of sound detectors arranged in a predetermined pattern, the detector clusters being positioned at spaced apart locations and arranged to feed signals representative of detected sound impulses to signal processing means, the signal processing means being operative to identify signals from different detector clusters which correspond to a common sound source thereby providing data for identifying the location of each sound source.

In embodiments of the present invention the signal processing means is adapted to process the signals representative of detected sound impulses to generate data on the time and angle of arrival of each of those signals relative to respective detector clusters and to make comparative associations between different parts of the data to identify sets of signals originating from a common source.

In a preferred embodiment ofthe present invention the signal processing means is adapted to processthe associated sets of signals originating from a common source to provide a more accurate estimate of common sound source location.

In a preferred embodiment the signal processing relating to the generation of the data representative ofthetimeand angle of arrival ofsoundimpulsesto each detector cluster is carried out in a respective cluster processor, transmission means being provided for feeding the data from each cluster processor to a further processor for making the comparative associations between parts ofthe data to identify sets ofthe signals originating from a common source.

In a preferred embodimentofthe presentinvention thefurther processor, or an additional processor coupled to said further processor, is employed to process data representative of the associated sets of signals to provide a more accurate estimate of the location of the common sound source.

In one embodiment ofthe present invention the sound detectors are microphones, the method being employed as the basis of an automatic sound location system for locating guns or mortars.

The present invention will be described further, by way of example, with reference to the accompanying block diagram which illustrates an automatic sound location system according to one embodiment ofthe present invention.

The concept behind the system design in the drawing is that of utilising athree-levelfiltering and breakassociation process. Ratherthan using a single array processing algorithm, small and large scale microphone arrays are connected to give the potential for a distributed process.

Overthe large array distances being considered, cross-correlation between waveforms is thought to be impractable because modal and transmission distortions are both large and unpredictable. This is one ofthe main reasons why the distributed method of processing has been developed, since little advantage is seen in performing the full array processing if the signals are not reasonably coherent. But, over distances ofthe order oftens of metres, forthe small arrays, the processing advantage to be gained from cross-correlation is available as the signals are reasonably coherent.

Referring to the drawing a hostile battery 2 emits a variety of impulsive sounds which together with extraneoussounds are picked up bya pluralityofspaced microphone clusters 4, 6, arranged to define an ac oustic array.

A cluster processor 10 receives signals from the microphone cluster 4, the cluster 4 comprising typically three microphones. The microphones in cluster4 are arranged in a pre-determined shape with dimensions of a fewtens of metres. To provide 360 degree azimuth coverage, the shape adopted forthe three microphones is that ofan equilaterai triangle.

The cluster processor 10 receives the signals and cal culatesthe mean time of arrival and the angle of arrival ofthe acoustic wavefronts and estimates the errors associated with them. The precise method of accomplishing these calculations is not specified here, but the methods used must be compatible with the type and range of acoustic signatures likelyto be encountered. Over a distance of a few tens of metres, the acoustic signatures are likelyto be highly correla ted,and thusthe calculation ofangle and time of arrival can be enhanced by the use of crosscorrelation techniques.

The angle and time of arrival of a breakaredepen- dent on such environmentally related factors as topography, wind velocities, airtemperature, humidity and pressure, and mode of acoustic propagation all represented by the block 3 in the drawing. It is pos sibletotakeinto accountthe local values of some these factors and to provide some degree of correction fortheir effects. This correction is provided as data fed to the cluster processor 10 from a MET DATA block 12. Physically, the cluster processing can take place either at the cluster4 itself (i.e. at a remote, unmanned location), or at a central position where subsequent processing is taking place.An advantage of theformerarrangementisthatasignificantamount owt data reduction can be done before transmission by a transmitter 14 back to the central position, which may be a distinct advantage when radio transmission in adverse circumstances is required.

Signals from the transmitter 14, signals from a plurality of other transmitters 8 associated with other cluster microphones 6 located at other points in the acoustic array and electronic counter measure (ECM) signals, if any, are received at a receiver 16.

Demodulation takes place in the receiver 16 and the demodulated signals fed to a coarse processor 18.The coarse processor 18 is thereby supplied with the information on time and angle of arrival of each break calculated by the cluster processors associated with the cluster microphones 4, 6. The cluster microphones 4,6 are distributed over a distance of perhaps 15 to 25 km, depending on their number and the rangeoverwhich location is required.Theshapeof the acoustic array does not have to be predetermined, except in the sense that the width and depth (relative to the Forward Edge Battle Area, FEBA, orsimilar reference line) must be chosen two give the required location accuracy across the areas under surveillance.

Before being passed to a fine processor 20, the data from each cluster microphone array 4,6 must be correctly associated with matching data on the same break from the other clusters. the method of accomplishing this will generally consist of sequential testing ot sets of breaks from the channels to find these associations.

The method described above has a significant effect on the speed of processing. The reason for the term 'coarse processing' is that association relies on the use of trigonometry and time of arrival com- parisonsto locate the sources ofsound. Because of errors, individual vectors or time differences will not agree exactly and so some degree oftolerance or coarseness must be allowed. The effect ofthis is: (a) estimation of position may not be good enough except with high signal to noise ratios, and (b) if several sources are grouped together, there will inevitably be some confusion between their breaks.

Ifthesignalto noise ratio is fairly high, sufficient spatial resolution may be available from the coarse processor 18 to enable the next stage, of fine processing, to be dispensed with. Afurtherconsidera- tion is that confusion between adjacent breaks is arguably not a significant problem since resolution between guns within a battery is probably not required. A method of discriminating between the sets of confused data isto compare the mean square errors computed in the fine processing stage and to assume thatthe lowest values are given by the correctly associated sets. Clearly,the processing load is quickly increased if several combinations of break data have to be combined to find the correct ones.

The fine processor 20 receives the associated arrival times of angles and combines them in a mannerwhich is deemed to be optimal. The precise method of accomplishing this is not specified here, but is typically a method in which the mean square error in times ofarrival or angles of arrival (or a combination ofthetwo) isminimised byadjustingthe hypothesised position of the source. At this stage, it is also possible to include fine corrections of effects due to wind, topography etc. represented bythepre- sence of METDATA block 22.

The system according to the present invention is based on the concept of distributed processing. It is distinctly different from array processing in that it takes advantage ofthe impulsive nature ofthe sounds but does not need to rely upon thecorrela- tion between signals across the full array width, which is likely to be low.

Distributed processing has the following advantages: (a) the system is modular, and can be adapted to cope with different requirements, and also changes as battle progresses (such as failures in some ofthe microphones), (b)thearray geometry does not haveto be a simple linear one, but can be arranged to give the necessary location accuracies across the designated areas of surveillance, (c) the array geometry is tolerant of changes in planned position, caused by the need to avoid physical obstructions or acoustic dead areas, and may be operated with semi-random distribution of clusters, if was desirable, (d) the cluster processing allows valuable data reduction to be performed before transmission by radio from the microphones to the central processor.This helps with such aspects as ECCM and covertness, (e) two degrees of accuracy are available: highest from the fine processor, and lower, but perhaps adequate enough, directly from the coarse processor.

Whereasthesystem described above performs the functions of break detection and location,the signal processing means may also be adapted by suitable algorithms to enable the system to search over limited arcs or areas ofthe battlefield to be as insensitive as possible to the precise location of each microphone in the array of clusters and locate shell bursts.

Whereas the system has been described above in relation to gun sound location the system may also be used for processing other types of signal, such as from seismic or sonar arrays.

Whereas in the embodiment described above each Cluster Microphone comprises three microphones arranged in a predetermined pattern, in otherembodiments of the present invention two, four or more microphones may be used in a predetermined arrangementto define each cluster microphone.

Claims (13)

1. A method of locating the position of a source of sound, the method comprising the steps offeeding to signal processing means a pluralityofsignalsre- presentative of sound impulses detected from a plurality of detector clusters each of which includes a plurality of sound detectors arranged in a predetermined pattern, processing the signals to generate data on the time and angle of arrival of each of the signals relative to respective detector clusters and making comparative associations between different parts of the data to identify sets of signals originating from a common sound source.

2. A method of locating the position of a source of sound as claimed in claim 1 wherein data representative of the associated sets of signals originating from the common sound source is processed to provide a more accurate estimate of the location ofthe common sound source.

3. Asound location system comprising a plurality of detector clusters each of which includes a plurality of sound detectors arranged in a pred etermined pattern, the detector clusters being positioned at spaced apart locations and arranged to feed signals representative of detected sound impulses to signal processing means, the signal processing means being operative to identify signals from dif ferentdetectorclusterswhich correspond to a common sound sourcethereby providing datafor identifying the location of each sound source.

4. Asound location system as claimed in claim 3 wherein the signal processing means is adapted to process the signals representative of detected sound impulses to generate data on the time and angle of arrival of each of those signals relative to respective detector clusters and to make comparative associations between different parts of the data to identify sets of signal originating from a common source.

5. A sound location system as claimed in claim 4 wherein the signal processing means is adapted to process the associated sets of signals originating from the common source to provide a more accurate estimate ofthe location ofthe common sound source.

6. Asound location system as claimed in claim 5 wherein the signal processing relating to the generation of the data representative of the time and angle of arrival of sound impulses to each detector cluster is carried out in a respective cluster processor, transmission means being provided for feeding the data from each cluster processor to a further processor for making the comparative associations between parts ofthe data to identify sets of the signals originating from the common sound source.

7. Asound location system as claimed in claim 6 wherein the further processor, or an additional pro cessor coupled to said further processor, is employed to process data representative ofthe associated sets of signals to provide a more accurate estimate ofthe location of the common sound source.

8. A sound location system as claimed in any one of claims 3to 7 wherein means is provided for adjusting the data in dependence on one or more of known environmentally related factors in the group including topography, wind velocity, airtemperature, humidity and temperature, and mode of acoustic propogation.

9. A sound location system as claimed in any one of claims 3to 8wherein the sound detectors are microphones.

10. A method of locating the position of a source of sound, the method being substantially as here it before described with reference to, and as illustrated in, the accompanying drawing.

11. A sound location system substantially as hererin before described with reference to, and as illustrated in,the accompanying drawing.

12. An automatic sound location method of locating guns or mortarsfiring at high rates, the method being as claimed in claim 1 or claim 2 or claim 10.

13. An automatic sound location system for locating guns or mortars firing at high rates, the system being as claimed in any one of claims 3 to 9 or claim 11.