GB2192061A - A phased array sonar system - Google Patents

Abstract

A sonar system comprises a phased array of transducers T0 to Tp-1 for receiving an acoustic wave from a source at a bearing theta , each transducer of the phased array generating a respective signal a in response to the acoustic wave, means S theta to Sp-1 for sampling the respective signals, means IF0 to IFp-1 for interpolating between the samples, a store for storing the interpolated samples, and combining means C0 for combining a selection of interpolated samples to form a beam indicative of the acoustic wave received by the transducers. The interpolation means IF0 to IFp-1 comprises a FIR digital filter arrangement for operation as an interpolation filter. This gives good beam acuity and permits steering of beams formed by the system. <IMAGE>

Description

SPECIFICATION Sonar systems This invention relates to sonar systems for sensing acoustic waves.

A known type of sonar system employs a technique known as beamforming. The sonar system comprises a phased array of transducers arranged for receiving acoustic waves from a distant source which may be located at a bearing angle 6 in relation to the phased array of transducers. By the time acoustic wave fronts reach the phased array of tranducers, they can generally be considered to be plane wave fronts.

A given plane wavefront will arrive at each transducer of the phased array as a function of time which is dependent upon the relative positions of the time transducers in the phased array. A delay is associated with each transducer and respective delays are arranged to delay the signals generated by respective transducers in response to the plane wavefront in order that the signals from each transducer arrive at a summing device contemporaneously that is, the signals are brought into phase.

The bearing of the source can be determined by analysing the amount of delay required by the respective delays to achieve the aforementioned conditions at the summing device. A known technique of beamforming involves sampling the signals from each transducer at a predetermined sampling rate and storing the samples in a store. Samples corresponding to a given plane wavefront can then be selected from the store in order to determined the bearing of the source producing the wavefront and to determine a summed beam representative of the plane wavefront.

Such known beamforming techniques are disadvantageous in that a relatively large amount of hardware is required for sampling the signals at an adequately high rate and for storing the samples. In order to alleviate this disadvantage, it is known to obtain a more accurate resolution of time delay by effectively increasing the number of samples by interpolating between samples. The interpolation is carried out by computational methods which involve using a linear or a low order polynomial fit to model the signal behaviour between the samples.

This method of interpolation is problematic in that the computations can give rise to interpolated samples which are not representative of what true samples would be. In addition, such methods limit the sonar system to resolving wavefronts from fixed directions and the acuity of beam location is also relatively limited.

It is an aim of the present invention to provide a sonar system which can provide more representative interpolation samples at a lower sampling rate while providing improved beam location acuity, and which enable beams formed by the system to be steerable.

According to the present invention there is provided a sonar system comprising a phased array of transducers for receiving an acoustic wave from a source, each transducer of the phased array generating a respective signal in response to the acoustic wave, sampling means for sampling the respective signals to provide samples of the respective signals, interpolation means for interpolating between the samples to provide interpolated samples, a store means for storing the interpolated samples, and combining means for combining a selection of interpolated samples to form a beam indicative of the acoustic wave received by the transducers, wherein the interpolation means comprises a digital filter arranged for operation as an interpolation filter.

Embodiments of the present invention are advantageous because they permit real time steering of relatively high resolution beams for fine bearing estimation. This is achieved due to an effective increase in sampling rate without actually increasing the direct sampling rate of the signals from the transducers. Further, the signal-to-noise ratio can be improved.

In a preferred embodiment of the present invention, the digital filter may be controlled to provide only interpolation samples which correspond to a beam which is required to be formed. This enables the size of the store to be reduced. In addition, a control means may be provided for controlling the generation of interpolation samples so that a moving source may be tracked by the sonar system.

A different interpolation filter and sampling means may be associated with each of the transducers, and the sampling means is preferably equipped to provide a digital output.

The digital filter may be in the form of a finite impulse response (FIR) filter having (B) sets of (M) multiplication co-efficients for generating (B) interpolation samples in response to (N) samples provided by the sampling means for each transducer of the array. The (M) multiplication co-efficients may be stored in a read only memeory (ROM) of the (FIR) filter or may preferably be down-loadable to a random access memory (RAM) thereby enabling a direct method of steering beams formed by the sonar system.

The co-efficients of the (B) sets of (M) multiplication co-efficients may be weighted in order to provide independent tapering and independent control of main load beam width verses side-load level.

The invention will now be further described by way of example with reference to the accompanying drawings, in which like reference numerals designate like elements, and in which: Figure 1 shows a schematic diagram of a sonar system embodying the present inven tion; Figure 2 shows a schematic diagram of a finite impulse response (FIR) filter which may be employed in embodiments of the present inventions; and Figure 3 shows a signal waveform produced by a transducer of the sonar system, a sampled waveform, and on interpolated sampled waveform.

Fig. 1 illustrates a sonar system having a line array of (P) transducers To to Top~, for receiving acoustic waves, adjacent transducers of which are separated by a distance d.

In response to receiving acoustic waves, the transducers To to Tp~1 each provide a signal "a" which is fed to a respective sampling device So to Sup~,. Respective sampling devices So to Sup~, sample the respective signals "a" at a relatively low sampling rate, for example, a minimum sampling rate of twice the signal "a" bandwidth. Sampling devices SO to also contain means for converting the sampled signals "a" to a digital sampled outputs (see sampled waveform "b" of Fig. 3).

Such a low sampling rate is not sufficient on its own to enable a high angular resolution between beams formed by the sonar system.

The sonar system of Fig. 1 is provided with a different finite impulse response (FIR) filter IFo to IFp, coupled to each of the sampling devices So to Sup~,. Respective FIR filters IFo to IFp~, receive respective digital sampled outputs "b" from respective sampling devices So to and and interpolate between the samples of the outputs "b" to provide B interpolated samples (see waveform "c" of Fig. 3) for every N samples provided by the sampling device.

A matrix of data is therefore provided having B interpolated samples for each of the P transducers To to Tp~,. The interpolated samples of data are stored in a memory which may either form part of the FIR filter, or alternatively, a separate memory array may be provided which is represented by the rectangular array of squares referenced T,, Top~, and to to tB 1 of Fig. 1.

The interpolated samples B are stored in respective locations of the separate memory array which locations are designated according to the time of the sample (to to tub.1), and in respect of which transducer P the sample is derived from.

In order to provide a beam output which is indicative of an acoustic wave received by the transducers from a bearing 6, a combining means C, is provided which is operative for selecting those interpolated samples from the memory array which combine to form the required beam, and for combining the samples to form the required beam.

The relationship between the interpolated samples which combined to form a beam representative of an acoustic wave received from bearing 6 is given by the relationship: 6--Sin-1 Ct Pdfs Where: C=the velocity of propagation of the acoustic wavefront; t=the value of interpolated sample to to tB~ for the pth transducer; P=transducer index where P is 0, 1, 2, 3 d=spacing between adjacent transducers; and fs=inverse of the time interval between successive interpolated samples.

Since the values t and P are of discrete values then the sonar system can form beams form bearings having discrete values of 6. The number of values of 6 and their angular resolution is increased due to the fact that the FIR filter provides interpolated samples which are highly representative of samples between the sampled values of the digital sampled output "b". Hence, where L is the interpolation factor of the FIR filters IFo to lFp1, the number of samples in the sampled output "b" can be increased by a factor of L to produce B interpolated samples so that the resolution of the sonar system can be increased in order to enable beam forming for beams from any bearing angle.

The formula given above assumes a linear array of transducers, but it is within the scope of the present invention for a non-linear and/or three dimensional array of transducers to be employed. In this case, a different formula for the relationship between interpolated samples and bearing angle 6 is derivable.

Fig. 2 illustrates an example of an FIR filter (the structure and operation of which is known in the art) which may be used in embodiments of the present invention.

A digital sampled output "b" are fed through serially coupled delays D10 D11 D12 D, and is also fed directly to a summ- ing device 30. The delays delay the digital sampled output b by successive amounts and supply a delayed signal to respective multipliers M, M2....Mn.

Multiplication coefficients m, m2 mn are associated with respective multipliers the values of which can be varied at input Mo.

Respective delayed digital sampled outputs are multiplied with respective coefficients at respective multipliers to provide signals which are combined in the summing device 30 to provide the interpolated samples B illustrated by the waveform C in Fig. 3.

The number n of delays is chosen so that the FIR filter has an impulse length of M coefficients at the chosen sampling rate of the sampling device. Thus each interpolated sample can be evaluated from the sampled signal by one coefficient multipliation by each multiplier and then summing the result from each of the multipliers M, M2....Mn at the summing device 30 independently of the interpolation factor L. In general each FIR filter will require B sets of M coefficients to form the B interpolated samples.

The coefficients can exist in ROM in the filter hardware but in a preferred arrangement, the sets of coefficients will be down-loadable to RAM since they may be more easily modified to allow for varying the bearing angle from which the beams formed originate.

The coefficients may be weighted to correct for geometrical changes in the array and the beams formed can have independent tapering and hence, independent control of main load beam width verses side-load level is attainable.

Embodiments of the present invention also permit beams to be scanned. In this case, the bearing 6 can be changed in a regular manner so as to in effect rotate the phased array so the array receives a notional continuous broadside beam.

Such scanning arrangement would have the advantage that the sonar system could be used for general surveillance.

Claims (10)

1. A sonar system comprising a phased array of transducers for receiving an acoustic wave from a source each transducer of the phase array generating a respective signal in response to the acoustic wave, sampling means for sampling the respective signals to provide samples of the respective signals, interpolation means for interpolating between the samples to provide interpolated samples, a store means for storing the interpolated samples, and combining means for combining a selection of interpolated samples to form a beam indicative of the acoustic wave received by the transducers, wherein the interpolation means comprises a digital filter arranged for operation as an interpolation filter.

2. A sonar system as claimed in claim 1, wherein the digital filter is arranged to provide only interpolation samples which substantially correspond to the beam indicative of the acoustic wave.

3. A sonar system as claimed in claim 1 or 2, wherein a control means is provided for controlling the digital filter thus enabling interpolation samples to be provided which are indicative of a moving source.

4. A sonar system as claimed in any one of the proceeding claims, wherein for each respective transducer there is associated a respective interpolation means and a respective sampling means.

5. A sonar system as claimed in any one of the preceeding claims, wherein the sampling means is equipped to provide a digital output.

6. A sonar system as claimed in any one of the preceeding claims wherein, the digital filter is in the form of a finite impulse response (FIR) filter having (B) sets of (M) multiplication co-efficients for generating (B) interpolation samples in response to (N) samples provided by the sampling means for each transducer of the array.

7. A sonar system as claimed in claim 6, wherein the (M) multiplication co-efficients are stored in a read only memory (ROM) of the (FIR) filter.

8. A sonar system as claimed in claim 6, wherein the (M) multiplication co-efficients are down-loadable to a random access memory (RAM) thereby enabling a direct method of steering beams formed by the sonar system.

9. A sonar system as claimed in any one of claims 6 to 8 wherein, the co-efficients of the (B) sets of (M) multiplication co-efficients are weighted in order to provide independent tapering and independent control of main load beam width verses side-load level.

10. A sonar system substantially as hereinbefore described with reference to the accompanying drawings.