# Patent

## Title
Method for changing the frequency for sampling sonar wavefronts

## Abstract

A system and method for processing sonar information from an underwater hydrophone array, at frequencies having wavelengths less than two times a spacing between adjacent hydrophones. The method and system all for processing received underwater waveforms at high frequency limits beyond the mechanical limits of the array.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to underwater acoustic hydrophone arrays. More particularly to a method for processing received underwater sonar waveforms to thereby raise the high frequency limit of the array.
2. Description of the Related Art
The gathering of data in an acoustic environment by means of an array of electrically interconnected hydrophones is important for commercial and military purposes. Passive sonar arrays have been used for many years in applications such as geologic surveys and submarine detection and tracking. They are essentially an assembly of hydrophones held together by a strength member and moved through water by a platform such as a ship or submarine to detect sound signals in the water. Such sensors are useful for detecting the position of underwater sonar target contacts. Towed arrays of hydrophones are long, continuous, neutrally buoyant hose lines, often several hundred meters long, which periodically contain numerous hydrophone receivers. Typically, these elongated arrays have a number of hydrophone elements in modules or clusters, along with their associated electronics and control leads, contained within the hose-shaped structure that extends through the water. In order to gather data in an ocean environment, one end of a linear hydrophone array is coupled to a navigating vessel. The principle advantages of the towed array are that it is separated from noise generated by the towing platform, allowing for improved detection of low source level signals, and that it can be constructed so that it is significantly longer than the towing platform, allowing for detection of lower frequency signals and for better bearing accuracy.
The individual hydrophones cooperate or interact to provide a beam, which may be steered to determine the bearing from the array of various acoustic targets. The hydrophones are connected to suitable electronics on the towing vessel which indicates the bearing of an identified underwater target. Exemplary towed arrays are described in U.S. Pat. Nos. 4,554,650 and 5,412,621. In locating submerged bodies, sonar is either used passively, through reception of signals sent through water by the target, or actively wherein a pulse of sound is sent into the water by a sonar projector. The sound is then reflected back from the target and detected by the hydrophone arrays as an echo. This echo is received as acoustic energy by an acoustic transducer, converted into electrical energy, and read by a computer on the vessel to determine the location of objects within the water. As shown in U.S. Pat. No. 5,844,860, elongated, hose-like towed arrays of hydrophones attached to the rear end of towed bodies are commonly used for the acoustic sensing of moving objects within the ocean.
In a conventional towed array, the mechanical design of the array limits the array's performance in that both the highest frequency and the lowest frequency that the array can effectively process are determined by the mechanical design itself. The lowest frequency (fL) is determined by the total array length. The fL is the frequency with half-wavelength (λ/2) equal to the length of the acoustically active portion of the array. The highest effective frequency (fH) is determined by the spacing between the individual hydrophones. The fH is the frequency with half-wavelength (λ/2) equal to the spacing between the hydrophones or hydrophone cluster.
Changes in the array performance, in either the high frequency or the low frequency, require redesigning and remanufacturing of an array which is either longer, to achieve a lower fL, or which has reduced hydrophone spacing, to raise fH. However, solving these problems by merely changing the length of the array or the hydrophone spacing may have consequences which may be undesirable. For instance, raising fH by reduction in hydrophone spacing increases the array's weight, thus complicating ballasting and buoyancy of the array and increasing the cost. Further, fH cannot be raised above the limit determined by the physical size of the hydrophone clusters being used. In addition, lowering fL requires that the acoustically active portion of the array be lengthened, increasing the complexity of the stowing and handling equipment as well as limiting the maneuverability of the towing platform.
Accordingly, the present invention solves the above mentioned problems by providing a method to process the received waveforms over a particular period of time in such a manner that the array performs as if it were synthetically longer or as if the hydrophone spacing were made synthetically shorter, thus synthetically raising the high frequency limit. The telemetry sampling frequency (fS) is chosen based on sampling requirements for the highest effective received frequency fH. Typically fS is fixed at a multiple of fH such that the desired signal waveform is adequately represented by the sample.
SUMMARY OF THE INVENTION
The invention provides a method for processing sonar information from an underwater hydrophone array, which array has a known spacing between adjacent hydrophones, at frequencies having wavelengths less than two times a spacing between adjacent hydrophones, the method comprising:

    
    
        a) providing an underwater hydrophone array comprising a plurality of receiver hydrophones, which array has a known length and known spacing between adjacent hydrophones, wherein λDU is the upper design wavelength for the design upper frequency limit fDU for the array;
        b) providing forward motion for the hydrophone array underwater with a vessel at a velocity of the hydrophone array in water VS;
        c) selecting a synthetic upper frequency fSU, wherein the synthetic upper frequency is equal to or greater than the frequency of a signal to be measured;
        d) defining a synthetic hydrophone spacing ΔdSU, wherein the synthetic hydrophone spacing is equal to λSU/2 wherein λSU is the wavelength for the synthetic upper frequency fSU for the array;
        e) determining the time interval for the hydrophones to travel the distance of the synthetic hydrophone spacing;
        f) calculating the number of sonar signal wavefronts predicted to be encountered by each hydrophone in the time interval for the hydrophones to travel the distance of the synthetic hydrophone spacing;
        g) selecting a telemetry sampling rate for the predicted number of sonar wavefronts, wherein the telemetry sampling rate is selected such that each wavefront is sampled more than once;
        h) detecting samples of a first sonar wavefront with each of the hydrophones at a time t0 and summing and recording the samples at time t0 with a beamformer to provide a plurality of beam sets at a time t0;
        i) detecting samples of N subsequent sonar wavefronts with each of the hydrophones at ≧2N equally spaced times in a time interval Δt; summing and recording the samples from each of the hydrophones with a beamformer to provide beam sets for the number of samples taken in the time interval Δt; and
        j) combining the beam sets into an assembly of beams at a time t1=t0+Δt.
    
    


The invention also provides a system for processing sonar information from an underwater hydrophone array, which array has a known spacing between adjacent hydrophones, at frequencies having wavelengths less than two times a spacing between adjacent hydrophones, comprising:

    
    
        a) an underwater hydrophone array comprising a plurality of receiver hydrophones, which array has a known length and known spacing between adjacent hydrophones, wherein λDU is the upper design wavelength for the design upper frequency limit fDU for the array;
        b) a driver for providing forward motion for the hydrophone array underwater at a velocity of the hydrophone array in water VS;
        c) an arrangement for selecting a synthetic upper frequency fSU, wherein the synthetic upper frequency is equal to or greater than the frequency of a signal to be measured;
        d) an instrumentality for defining a synthetic hydrophone spacing ΔdSU, wherein the synthetic hydrophone spacing is equal to λSU/2 wherein λSU is the wavelength for the synthetic upper frequency fSU for the array;
        e) an implement for determining the time interval for the hydrophones to travel the distance of the synthetic hydrophone spacing;
        f) a calculator for calculating the number of sonar signal wavefronts predicted to be encountered by each hydrophone in the time interval for the hydrophones to travel the distance of the synthetic hydrophone spacing;
        g) a sampler for selecting a telemetry sampling rate for the predicted number of sonar wavefronts, wherein the telemetry sampling rate is selected such that each wavefront is sampled more than once;
        h) a detector for detecting samples of a first sonar wavefront with each of the hydrophones at a time to and summing and recording the samples at time t0 with a beamformer to provide a plurality of beam sets at a time t0; and for detecting samples of N subsequent sonar wavefronts with each of the hydrophones at ≧2N equally spaced times in a time interval Δt; and for summing and recording the samples from each of the hydrophones with a beamformer to provide beam sets for the number of samples taken in the time interval Δt; and
        i) a combiner for combining the beam sets into an assembly of beams at a time t1=t0+t.
    
    





BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically shows a towed array of hydrophones K1, K2, K3, K4 and an incident wavefront at angle with the array θ.
FIG. 2 shows a polar plot of the resulting beam set for a sonar wave front arriving from the 180 degree position according to Example 1.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
According to the present invention, a computer is input with a selected high frequency limit fH. The high frequency limit is determined by the upper limit of the frequency range that contains the signal of interest. Thus, if the signal of interest is bounded by the interval of 1 kHz to 6 kHz, 6 kHz might be selected as the high frequency limit fH. No mechanical design parameter, such as hydrophone spacing, is used in selecting fH since fH is an operational parameter rather than a design parameter. It may be changed at any time in response to changing requirements.
The speed of sound through water LS is sampled and input into a computer. Likewise, the velocity of the array through the water VS is sampled and input into the computer. Vs is a vector quantity so that a magnitude and direction are input to adequately define it.
With fH selected and LS measured, the half wavelength λ/2 can be calculated. The distance λ/2 is the maximum distance between samples that will adequately replicate the incoming waveform. Set fsu=fh.
The time interval Δt that the individual hydrophone takes to “sweep out” the distance λ/2 is calculated using the array's measured velocity VS. The time interval Δt is subdivided into N intervals where N is the number of wave fronts of frequency fH that the hydrophone encounters over the time interval Δt.
The telemetry sampling frequency fS is set to at least twice the high frequency limit fH such that data is sampled as each wave front encounters the hydrophone. Since the hydrophone has moved the distance (Δt*VS)/N, the sampling frequency fS is set in this manner so that two things are accomplished. First, each waveform is sampled more than once as it passes the hydrophone. This allows rejection of non-coherent noise in subsequent processing of the synthetic waveform when the waveforms are combined over the interval Δt. Second, since the hydrophone movement between samples is proportionate to the wavelength for fH, the coherent signal waveform is sampled N times over the distance λ/2. Thus the synthetic waveform is described at N points along the interval (0, λ/2).
At the beginning t0 of each interval Δt, a sample is taken at each hydrophone in the array. The samples are summed in a beamformer to provide a composite family of output signals (beam sets). These beam sets are recorded and stored. The process is repeated N equally spaced times. At the completion of N samples over the time interval Δt, the array has traveled λ/2 distance from the initial point. The database contains N families of beam sets, one from each of the N sample periods along the interval.
A transpositional function is applied to each of the N beam sets that “moves” it forward to position λ/2 and time t1=t0+Δt. The transposed data is combined so that all the beam sets taken over the interval are combined into one data family composed of beams that are synthesized at time Δt.
The transposed signals constitute a synthesized waveform made up of the N waveforms sampled at each time over the interval Δt. The output is a single synthesized waveform that would appear like a single wave front encountered at position 0+λ/2 and at time t1. Only coherent signal that was present in each wave front encountered in each of the N samples would be present in the synthetic wave front. Non-coherent signal (noise), since by its non-coherent nature only appears in one of the N samples, does not appear in the synthetic waveform. Thus the signal to noise ratio SNR of the synthetic signal is improved over the natural signal.
One application of this method and system might be for a towed array used to detect a submerged submarine. As an example, if the particular towed array is built with a mechanical upper frequency of 1 kHz (hydrophone spacing of 2.4 ft) is to be used to search for a submarine that emanates a signal in the range of 1 to 4 kHz, the upper frequency would be selected to cover the frequency band of interest by first selecting an fH of 4 kHz. The speed of sound in the water LS is measured as 4800 ft/sec and array velocity VS is measured at 5 knots in a direction of North. The half wavelength λ/2 is calculated as 0.6 ft. N is thus 480 and the time interval Δt is 0.0718 sec. The distance 0.6 ft and time 0.0718 sec are divided into 480 separate, equal intervals with a sample point at the beginning of each interval. Signal data is collected at each sample point, processed through a beamformer, and transposed to a position 0.6 ft from the start point and 0.0718 seconds from the start time. The synthetic waveform is assembled at that point and analyzed for signals of interest.
The Theory
The array movement is used to “sweep out” the interval spacing between hydrophones, thus synthesizing a higher hydrophone density and closer hydrophone spacing. Since the array hardware upper design frequency fDU for a particular towed array is based on having the hydrophone spacing equal to the half wavelength λDU/2 for that fDU, the concept of synthetically generating hydrophone density raises the upper frequency limit for the array above the mechanical design limit.
If an array is designed with hydrophone spacing ΔdDU, then the conventional array hardware upper design frequency fDU would satisfy the equation:

fDU=LS/ΔdDU 

wherein: LS=the speed of sound in water

ΔdDU=λDU/2 

The sampling frequency fS is a multiple of fDU, and selected so that adequate data is sampled. A number of 2.5 times fDU is a typical value. With the conventional approach, raising fDU entails lowering ΔdDU, i.e. adding hydrophones to the array. For a given array length, doubling the upper design frequency fDU requires halving the hydrophone spacing ΔdDU, thus doubling the hydrophone population.
Likewise, since the design frequency fDU doubled, the sampling frequency fS would also require doubling, usually entailing a change to the telemetry system.
The array's movement through the water is utilized to sample data at discrete intervals. The data is processed by standard beamforming techniques and “transposed” by mathematical means such that the all samples are taken over the interval Δt, where Δt is defined by the interval t0 to t1, is transposed to time t1. Careful selection of the discrete interval allows the increase in the subsequent synthetic upper frequency fSU by an integer multiple of the array hardware design upper frequency fDU, such that:

fSU=A*fDU where A=the desired integer multiple 

Referring to FIG. 1, there is shown a towed array of hydrophones K1, K2, K3, K4 and an incident wavefront at angle with the array θ.
The fSU is a function of the hydrophone spacing ΔdHU, array velocity VS, and the incident angle of the wavefront with the array θ.
wherein:


    
    
        LS=speed of sound in water
        VS=towed array velocity
        θ=incident angle of wave front
        KN=hydrophone at position N
        ΔdHU=design hydrophone spacing=λFU/2 for fDU 
        λSU/2=half wavelength for synthetic upper frequency fSU 
    
    


If we define a synthetic hydrophone spacing ΔdSU such that ΔdSU is equal to λSU/2 for fSU, then: ΔdSU=VSΔt where Δt is the time interval for the hydrophone to travel the distance ΔdSU. Since ΔdSU=λSU/2 where λSU/2 is the half wavelength for the synthetic upper frequency, then Δt=λSU/(2 Vs).
In the time interval Δt, the number of wave fronts passing the hydrophone KN as the hydrophone travels from position X0 to position Xλ/2 at speed Vs, would be defined by:

LS cos θ=(NXλSU+λSU/2)/Δt 

wherein Nx=integer number of wave fronts passing hydrophone KN in time Δt.

LS cos θ=[λSU/2(2NX+1)]/[λSU/(2 VS)]

2NX+1=LS cos θ/VS 

Thus: Nx=1⁄2[(LS cos θ/VS)−1]

Hydrophone KN would sample the wave train at the λsu/2 position on the Nth wave of the train. Using conventional sampling theory, fSH=fSU/N, a very small number of samples would be taken. But, if samples are taken at each NX over the interval t=(0, Δt) then fSH=fSU where fSH is a highly over-sampled space such that each wave form is sampled as it passes the hydrophone KX at the position Xn=dΔdSU/N. Thus the wave train is sampled sequentially such that each subsequent wave in the train is sampled at a position over the interval (0, λSU/2) such that Xn=nλSU/2N.
The conventional time domain beamforming equation can be written as:

Ar(t)=K=0ΣK=Z−1WKfK(t+τK,r) 

wherein: Z=the number of hydrophones in the array

    
    
        WK=shading factor
        τK,r=time delay applied to the Kth element data for the rth beam
    
    


With synthetic beamforming, the beamforming at each position XN would be:

Ar(t)X=K=0ΣK=Z−1WKfK(t+τK,r,X) 

wherein: τK,r,X=time delay applied to the Kth element data for the rth beam at position XN.

Over the sampling time interval (0,Δt), this results in a family of N beam sets, with each set identified with a particular time tn and position Xn that is contained with in the intervals (0,Δt) and interval (0, λSU/2). The beam sets are then transposed such that the composite beam set is formed at position XN and at time Δt. Thus, the composite time domain beamforming equation takes the form:

Ar(t)=X=0ΣX=NfX(ψK,r,X)K=0ΣK=Z−1WKfK(t+τK,r,X) 

Where: ψK,r,X=transpositional function applied to the Kth element data for the rth beam at the Xth position.

The system according to the invention comprises the underwater hydrophone array which is known in the art. The hydrophones of the array may be individual hydrophones or may be clusters of hydrophones. Hydrophone clusters may be arranged linearly or grouped in various geometric arrangements such as spherical, planar, conformal to the vessel. The hydrophone spacings for purposes of this invention are measured from the midpoint of one hydrophone cluster to the midpoint of the adjacent hydrophone cluster. The system includes a driver for providing forward motion for the hydrophone array underwater. The driver is typically a surface ship or a submarine. The array may be attached to the outer surface of the hull of the vessel, or it may be towed directly by the vessel, or it may be towed by the vessel via an intermediate platform. The system then includes an arrangement for selecting a synthetic upper frequency. Typically, such an arrangement comprises a determination of the bounds of the frequency of the sonar signal to be measured. These bounds are usually known to those skilled in the art. The synthetic upper frequency is selected to be equal to or greater than the upper bounds of the sonar signal to be measured by means of either manual input or by means of a computer algorithm.
The system then includes an instrumentality for defining a synthetic hydrophone spacing. Typically, such an instrumentality comprises a computer algorithm that takes input of the synthetic upper frequency and calculates the half wavelength for that frequency. The algorithm then sets the synthetic hydrophone spacing equal to the half wavelength.
The system then includes an implement for determining the time interval for the hydrophones to travel the distance of the synthetic hydrophone spacing. Typically, such an implement comprises a computer algorithm that takes as input the synthetic hydrophone spacing and a physical means for measuring the array velocity through the water. The computer algorithm calculates the time interval for the hydrophone to travel the synthetic hydrophone spacing.
The system then includes a calculator for calculating the number of sonar signal wavefronts predicted to be encountered by each hydrophone in the time interval for the hydrophones to travel the distance of the synthetic hydrophone spacing. Typically, such a calculator comprises a computer algorithm that calculates the number of wavefronts encountered by the hydrophone during the time interval for each of a family of expected incident anglesθ.
The system then includes a sampler for selecting a telemetry sampling rate for the predicted number of sonar wavefronts. Typically, such a sampler comprises a computer algorithm that takes as input the number of expected sonar signal wavefronts for the case of the limiting incident angle θ. The computer algorithm also takes as input the number of samples per wavefront that has been chosen. The algorithm calculates the sample rate and outputs that value to the control function for the telemetry to sample the sonar signal.
The system then includes a detector for detecting samples of a sonar wavefront with each of the hydrophones and summing and recording the samples with a beamformer to provide beam sets. Typically, such a detector comprises a plurality of hydrophones that convert the sonar signal to electrical energy. The telemetry system samples the electrical energy at discrete times. The samples are combined and transformed in the beamformer.
The system then includes a combiner for combining the beam sets into an assembly of beams. Typically, such a combiner comprises a computer algorithm that calculates the transformation and transposition functions to combine the beams formed during the time interval Δt at time t1 in position ΔtSU.
The following non-limiting examples serve to illustrate the invention. It will be appreciated that variations in proportions and alternatives in elements of the components of the invention will be apparent to those skilled in the art and are within the scope of the present invention.
EXAMPLE 1
This examples illustrates a method for determining a variable sample rate, dependent on array velocity and selected upper synthetic frequency. An upper synthetic frequency is chosen and a synthetic hydrophone spacing is calculated. The array velocity is used to calculate a time period needed for the hydrophone to travel the synthetic hydrophone distance. The number of sonar wave fronts encountered by the hydrophone during the time period is calculated. The sample rate is then calculated based on the number of wave fronts and the time period. A linear array is built with eight hydrophone groups equally spaced such that the array is 10.5 meters in length. The array is designed with a spacing between hydrophones of 1.5 meters. Since the upper design frequency FDU=LS/ΔdDU and the speed of sound through the water LS equals 1500 meters/sec, then FDU equals 500 Hz.
The sonar signal to be measured is in the range of 500 to 1000 Hz. To raise the frequency capability of this array to 1000 Hz, select a synthetic upper frequency equal to two times the fDU. fSU=A*fDU with A=2. The synthetic hydrophone spacing ΔdSU is set equal to the half wavelength λ/2 for FSU. λ/2=LS/2 FSU so λ/2 and ΔdSU are equal to 0.75 meters.
If the array is moved through the water at 10 knots (5.14 meters/sec) and ΔdSU=VSΔt then the time for a hydrophone to move from its position at time t0 a distance equal to ΔdSU is Δt=0.1459 seconds. To determine the number of wave fronts N the hydrophone will encounter over the time interval Δt, the equation N=1⁄2[(LS cos θ/VS)−1] is solved for cos θ=1, the limiting case. N=145 wave fronts encountered. The sample rate SR is then 2N/Δt=1994 samples per second. A transpositional function is applied to each of the beam sets that “moves” it forward to position λ/2 and time Δt. The transposed data is combined so that all the beam sets taken over the interval are combined into one data family composed of beams that are synthesized at time Δt. FIG. 2 shows a polar plot of the resulting beam set for a sonar wave front arriving from the 180 degree position.
EXAMPLE 2
In this example, a synthetic upper frequency of four times the design upper frequency is chosen. A linear array is built with hydrophone groups equally spaced such that the array is 105 meters in length. The array is designed with a spacing between hydrophone of 15 meters. Since the upper design frequency fDU=LS/ΔdDU and the speed of sound through the water LS equals 1500 meters/sec, then fDU equals 50 Hz. The sonar signal to be measured is in the range of 150 to 200 Hz. To raise the frequency capability of this array to 200 Hz, select a synthetic upper frequency equal to two times the fDU. fSU=A*FDU with A=4. The synthetic hydrophone spacing ΔdSU is set equal to the half wavelength λ/2 for fSU. λ/2=LS/2 FSU so λ/2 and ΔdSU are equal to 3.75 meters. If the array is moved through the water at 15 knots (7.71 meters/sec) and ΔdSU=VSΔt then the time for a hydrophone to move from its position at time to a distance equal to ΔdSU is Δt=0.486 seconds. To determine the number of wave fronts N the hydrophone will encounter over the time interval Δt, the equation N=1⁄2[(LS cos θ/VS)−1] is solved for cos θ=1, the limiting case. N=97 wave fronts encountered. The sample rate SR is then 2N/Δt=398.2 samples per second. A transpositions function is applied to each of the beam sets that “moves” it forward to position λ/2 and time Δt. The transposed data is combined so that all the beam sets taken over the interval are combined into one data family composed of beams that are synthesized at time Δt. A polar plot of the resulting beam set for a sonar wave front arriving from the 180 degree position is similar to that of FIG. 2.
EXAMPLE 3
A method for using a fixed sample rate is shown. An upper synthetic frequency is chosen and a synthetic hydrophone spacing is calculated. The array velocity is used to calculate a time period needed for the hydrophone to travel the synthetic hydrophone distance. The sample point nearest the synthetic hydrophone point is chosen and the distance between the two is calculated. An error function is calculated and applied to the sonar signal. A linear array is built with eight hydrophone groups equally spaced such that the array is 10.5 meters in length. The array is designed with a spacing between hydrophone groups of 1.5 meters. Since the upper design frequency fDU=LS/ΔdDU and the speed of sound through the water LS equals 1500 meters/sec, then fDU equals 500 Hz. The system has a sample rate of 3,000 samples per second. The sonar signal to be measured is in the range of 500 to 1000 Hz. To raise the frequency capability of this array to 1000 Hz, select a synthetic upper frequency equal to two times the fDU. fSU=A*fDU with A=2. The synthetic hydrophone spacing ΔdSU is set equal to the half wavelength λ/2 for fSU. λ/2=LS/2 FSU so λ/2 and ΔdSU are equal to 0.75 meters. If the array has a velocity VS through the water of 10 knots (5.14 meters/sec) and ΔdSU=VSΔt then the time for a hydrophone to move from its position at time t0 a distance equal to ΔdSU is Δt=0.1459 seconds. The number of samples available during the time period that the hydrophone travels the synthetic hydrophone distance ΔdSU, given that SR=3000, is equal to the sample rate times Δt. Thus N=round(SR*Δt)=437. The sample taken nearest the synthetic hydrophone point is taken at a distance from the origin equal to (NS*VS)/SR=0.7493 meters. Thus there is a 0.0007 meter error between the sample point and the synthetic hydrophone distance. The time difference between the two positions 1.36*10−5 is applied as an error correction. The correction factor is a positive value if the sample point is less than synthetic hydrophone point or negative if the sample point is greater than the synthetic hydrophone point.
This method further allows for corrections for measured speed. Assume that the actual array velocity VA through the water is 9 knots (4.63 meters/second) when the measured velocity VS is 10 knots. The sample taken nearest the synthetic hydrophone point is taken at a distance from the origin equal to (NS*VA)/SR=0.6751 meters. Thus there is a 0.0749 meter error between the sample point and the synthetic hydrophone distance. The time difference between the two positions 1.62*10−2 is applied as an error correction.
A transpositional function is applied to each of the beam sets that “moves” it forward to position λ/2 and time Δt. The transposed data is combined so that all the beam sets taken over the interval are combined into one data family composed of beams that are synthesized at time Δt. A polar plot of the resulting beam set for a sonar wave front arriving from the 180 degree position is similar to FIG. 2.
While the present invention has been particularly shown and described with reference to preferred embodiments, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended that the claims be interpreted to cover the disclosed embodiment, those alternatives which have been discussed above and all equivalents thereto.

## Claims

1. A method for processing sonar information from an underwater hydrophone array, which array has a known spacing between adjacent hydrophones, at frequencies having wavelengths less than two times a spacing between adjacent hydrophones, the method comprising:
a) providing an underwater hydrophone array comprising a plurality of receiver hydrophones, which array has a known length and known spacing between adjacent hydrophones, wherein λDU is the upper design wavelength for the design upper frequency limit fDU for the array; 
b) providing forward motion for the hydrophone array underwater with a vessel at a velocity of the hydrophone array in water VS; 
c) selecting a synthetic upper frequency fSU, wherein the synthetic upper frequency is equal to or greater than the frequency of a signal to be measured; 
d) defining a synthetic hydrophone spacing ΔdSU, wherein the synthetic hydrophone spacing is equal to λSU/2 wherein λSU is the wavelength for the synthetic upper frequency fSU for the array; 
e) determining the time interval for the hydrophones to travel the distance of the synthetic hydrophone spacing; 
f) calculating the number of sonar signal wavefronts predicted to be encountered by each hydrophone in the time interval for the hydrophones to travel the distance of the synthetic hydrophone spacing; 
g) selecting a telemetry sampling rate for the predicted number of sonar wavefronts, wherein the telemetry sampling rate is selected such that each wavefront is sampled more than once; 
h) detecting samples of a first sonar wavefront with each of the hydrophones at a time t0 and summing and recording the samples at time t0 with a beamformer to provide a plurality of beam sets at a time t0; 
i) detecting samples of N subsequent sonar wavefronts with each of the hydrophones at ≧2N equally spaced times in a time interval Δt; summing and recording the samples from each of the hydrophones with a beamformer to provide beam sets for the number of samples taken in the time interval Δt; and 
j) combining the beam sets into an assembly of beams at a time t1=t0+Δt.

2. The method according to claim 1, wherein step c) comprises selecting a synthetic upper frequency such that the synthetic upper frequency is an integer multiple of the hydrophone array design upper frequency limit fDU.

3. The method according to claim 1, wherein step c) comprises selectively varying the synthetic upper frequency.

4. The method according to claim 1, wherein step f) comprises determining an incident angle of the wavefronts with the array θ, and the speed of sound in water LS, and determine the number of wavefronts N according to the equation


N=1⁄2[(LS cos θ/VS)−1].

5. The method according to claim 1, wherein step g) comprises selecting a variable telemetry sampling rate.

6. The method according to claim 1, wherein step g) comprises:
i) calculating a time error between the time when each hydrophone reaches λSU/2 for the synthetic hydrophone spacing and the nearest wavefront sampling time; and 
ii) applying a correction factor for the time error for correcting a time delay, and moving the resulting combined beam to a position which corrects for the time error.

7. The method according to claim 1 comprising:
a) providing a towed underwater hydrophone array comprising a plurality of receiver hydrophones, which array has a fixed length and a fixed spacing between adjacent hydrophones, each spacing being fixed at a distance equal to ΔdDU=λDU/2 wherein λDU is the upper design wavelength for the upper design frequency limit fDU for the array, wherein fDU=LS/ΔdDU wherein LS=the speed of sound in the water; 
b) towing the array underwater with a vessel at a velocity of the hydrophone array in water VS; 
c) selecting a synthetic upper frequency fSU=A*fDU, wherein A is an integer; 
d) defining a synthetic hydrophone spacing ΔdSU=λSU/2 wherein λSU is the upper synthetic wavelength for the synthetic upper frequency fSU for the array; 
e) determining the time interval Δt for the hydrophones to travel the distance ΔdSU wherein Δt=λSU/(2*VS); 
f) calculating the number of wavefronts N encountered by each hydrophone in time interval Δt by the equation 


N=1⁄2[(LS cos θ/VS)−1]


wherein θ=the incident angle of the wavefronts with the array.

8. A system for processing sonar information from an underwater hydrophone array, which array has a known spacing between adjacent hydrophones, at frequencies having wavelengths less than two times a spacing between adjacent hydrophones, comprising:
a) an underwater hydrophone array comprising a plurality of receiver hydrophones, which array has a known length and known spacing between adjacent hydrophones, wherein λDU is the upper design wavelength for the design upper frequency limit fDU for the array; 
b) a driver for providing forward motion for the hydrophone array underwater at a velocity of the hydrophone array in water VS; 
c) an arrangement for selecting a synthetic upper frequency fSU, wherein the synthetic upper frequency is equal to or greater than the frequency of a signal to be measured; 
d) an instrumentality for defining a synthetic hydrophone spacing ΔdSU, wherein the synthetic hydrophone spacing is equal to λSU/2 wherein λSU is the wavelength for the synthetic upper frequency fSU for the array; 
e) an implement for determining the time interval for the hydrophones to travel the distance of the synthetic hydrophone spacing; 
f) a calculator for calculating the number of sonar signal wavefronts predicted to be encountered by each hydrophone in the time interval for the hydrophones to travel the distance of the synthetic hydrophone spacing; 
g) a sampler for selecting a telemetry sampling rate for the predicted number of sonar wavefronts, wherein the telemetry sampling rate is selected such that each wavefront is sampled more than once; 
h) a detector for detecting samples of a first sonar wavefront with each of the hydrophones at a time t0 and summing and recording the samples at time t0 with a beamformer to provide a plurality of beam sets at a time t0; and for detecting samples of N subsequent sonar wavefronts with each of the hydrophones at ≧2N equally spaced times in a time interval Δt; and for summing and recording the samples from each of the hydrophones with a beamformer to provide beam sets for the number of samples taken in the time interval Δt; and 
i) a combiner for combining the beam sets into an assembly of beams at a time t1=t0+Δt.

9. The system according to claim 8, wherein the arrangement c) comprises a selector for selecting a synthetic upper frequency such that the synthetic upper frequency is an integer multiple of the hydrophone array design upper frequency limit fDU.

10. The system according to claim 8, wherein the arrangement c) comprises a selector for selectively varying the synthetic upper frequency.

11. The system according to claim 8, wherein the calculator f) comprises a determiner for determining an incident angle of the wavefronts with the array θ, and the speed of sound in water LS, and determine the number of wavefronts N according to the equation


N=1⁄2[(LS cos θ/VS)−1].

12. The system according to claim 8, wherein sampler is capable of selecting a variable telemetry sampling rate.

13. The system according to claim 8, wherein sampler g) is capable of calculating a time error between the time when each hydrophone reaches λSU/2 for the synthetic hydrophone spacing and the nearest wavefront sampling time; and applying a correction factor for the time error for correcting a time delay, and moving the resulting combined beam to a position which corrects for the time error.

14. The system according to claim 8 comprising:
a) an underwater hydrophone array comprising a plurality of receiver hydrophones, which array has a fixed length and an equal fixed spacing between adjacent hydrophones, each spacing being fixed at a distance equal to ΔdDU=λDU/2 wherein λDU is the upper design wavelength for the upper design frequency limit fDU for the array, wherein fDU=Ls/ΔdDU wherein LS=the speed of sound in the water; 
b) a driver for towing the array underwater with a vessel at a velocity of the hydrophone array in water VS; 
c) an arrangement for selecting a synthetic upper frequency fSU=A*fDU, wherein A is an integer; 
d) an instrumentality for defining a synthetic hydrophone spacing ΔdSU=λSU/2 wherein λSU is the wavelength for the synthetic upper frequency fSU for the array; 
e) an implement for determining the time interval Δt for the hydrophones to travel the distance ΔSU wherein Δt=λSU/(2*Vs); 
f) a calculator for calculating the number of wavefronts N encountered by each hydrophone in time interval Δt by the equation 


N=1⁄2[(LS cos θ/VS)−1]


wherein θ=the incident angle of the wavefronts with the array.