Abstract:
A sonar system for detecting underwater acoustic signals includes a plurality of hydrophone units capable of converting acoustic impulses to electrical signals, the hydrophone units being substantially vertically oriented when deployed in a body of water, and the hydrophone units occupying at least some of the positions of an M×N horizontal array. Two-dimensional Chebyshev mathematical weighting is applied to the electrical signals from the individual hydrophone units such that each individual signal from each hydrophone unit is assigned a respective weighting number and a numerical value is assigned to each individual signal corresponding to the strength of the electrical signal as adjusted by the respective weighting number.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/727,382 filed Oct. 17, 2005, the contents of which are incorporated herein by reference. 

   STATEMENT OF GOVERNMENTAL INTEREST 
   This invention is made with Government support under NAVSEA Contract No. N00024-03-D-6606, awarded by the U.S. Navy. The Government has certain rights in the invention. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a hydrophone array, particularly hydrophone arrays for underwater acoustic sensing of subsurface marine vehicles. 
   2. Background of the Art 
   Undersea noise is well known to be highly dynamic and anisotropic. Surface-generated noise undergoes multipath propagation in the ocean waveguide arriving at the receiver at a distribution of elevation angles. Discrete noise contributions from anthropogenic and biologic sources add acute directionally in azimuth and elevation. Shallow water bathymetry also contributes by varying the distribution of the noise field across azimuth. Such anisotropy in the noise field may be exploited to increase the detection range of a passive sonar array. However, detection performance can be severely affected by nearby shipping noise through the array&#39;s beam response sidelobes. 
   The detection performance of low-frequency passive sonars, may be severely affected if deployed under heavy shipping conditions. Merchant ships, tankers and other anthropogenic undersea acoustic sources are very loud and they may affect detection performance even at long distances from the receiver. However, energy from the distant ships (which drops by about 6 dB/octave in source level with increasing frequency) undergoes water-column absorption and multiple bottom interactions while the energy from a nearby quiet signal will be less attenuated due to its shorter range. Therefore, at higher frequencies, clutter from distant shipping noise is expected to decrease more rapidly than the received energy level of the target. In addition, high frequency arrays are much smaller, which reduces cost and makes them attractive components of unmanned vehicles and expendable systems. 
   There is yet need for array designs and signal processing approaches to satisfy a number of requirements: a high-frequency passive sonar to mitigate clutter from shipping noise. The sonar system should be easy to deploy and the number of array elements should be minimized to reduce data rate and processor demand. To maximize the detection range of the system, a new design must feature a vertical aperture to exploit the ambient noise anisotropy. In addition, the system must be inexpensive for use as expendable units for multiple uses such as adjuncts to ocean observatories deployable from air or surface platforms. 
   SUMMARY OF THE INVENTION 
   A sonar system for detecting underwater acoustic signals is provided herein, the sonar system comprising: (a) a plurality of hydrophone units capable of converting acoustic impulses to electrical signals, said hydrophone units being substantially vertically oriented when deployed in a body of water, and said hydrophone units occupying at least some of the positions of an N×M horizontal array wherein N is the number of rows and M is the number of columns in the array; (b) means for applying a two-dimensional Chebyshev mathematical weighting to the electrical signals from the individual hydrophone units such that each individual signal from each hydrophone unit is assigned a respective weighting number and a numerical value is assigned to each individual signal corresponding to the strength of the electrical signal as adjusted by the respective weighting number; and (c) means for deploying said hydrophone units in the body of water. 
   The sonar system advantageously reduces beam side lobes which cause confusion of the acoustic signals and hamper accurate detection and directional localization of target marine vessels. Moreover, microprocessor demand is minimized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various embodiments are described below with reference to the drawings wherein: 
       FIG. 1  illustrates a hydrophone deployment system of the invention loaded into a sonobuoy; 
       FIG. 2  illustrates the deployed system in a body of water; 
       FIG. 3  illustrates a hydrophone array support. 
       FIG. 4A  illustrates an N×M array of hydrophones; 
       FIG. 4B  illustrates an N×M hydrophone array with vacant corner positions; 
       FIG. 5  is a diagram of a system for processing the electrical signals from the hydrophone units. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to  FIG. 1 , the hydrophone array deployment system  100  includes a float  102 , battery  108 , tether line  104 , electronics package  130 , array support  105 , hydrophone array  110 , and optionally weight  106 , loaded into a sonobuoy canister  101 . The sonobuoy canister  101  can be size A or smaller. 
   Referring also now to  FIG. 2 , the system  100  is shown deployed in a body of water  10 . Deployment can be accomplished by, for example, dropping system  100  into the water  10  by airplane, surface vessel, or any other suitable means. Jettison of the contents of the sonobuoy canister  101  can be accomplished by various means such as the impact of the sonobuoy into the water. Alternatively, a battery  108  (e.g., seawater activated battery) can power the ejection of the contents by, for example, firing a squib. Also, back plate  109  can be jettisoned (by water impact, firing a squib, etc.) to allow the hydrophone array to descend to the proper depth. Float  102  remains at the surface of the body of water and preferably includes a transmitter with antenna  103  and the appropriate battery-powered electronics for converting the electrical signals from the hydrophones into radio waves for wireless transmission to a remote receiver. The transmitter  103  is preferably powered by battery  108 . Tether  104  allows the hydrophone array  110  to descend to a predetermined depth. Tether  104  can typically range up to 250 feet in length, for example. Electronics package  130  increases the power of the electrical signal from the hydrophones for transmission to the transmitter and also includes a microprocessor for converting analog to digital signals and performing the calculations described below. Electronic package  130  can also include a compass to facilitate geographic orientation of the hydrophone array  110 . Weight  106  can optionally be included to facilitate deployment of the hydrophone array  110 . Support system  105  includes two to four rigid connecting rods  105   a  to maintain the hydrophone array  110  in a fixed orientation with respect to the electronics package  130 . 
   The system  100  is “passive.” Active sonobuoy systems emit acoustic signals into the water and listen for the return echo. Passive systems merely listen for sounds made by underwater craft, e.g., power-plant, propellers, door closings or other mechanically generated or human generated noise. 
   Referring now to  FIG. 3 , the hydrophone array  110  includes a plurality of hydrophone units  120  arranged parallel to each other in an array which is embedded in a polymeric encapsulant material  111 . The hydrophones are sized and spaced for specific target frequencies, e.g., 25 kHz, 20 kHz, 15 kHz. The hydrophones are vertically oriented when fully deployed. The polymeric encapsulant  111  is preferably an acoustically transparent polyurethane, i.e., having the same acoustic impedance as seawater (rho-c material). Polyurethane encapsulants suitable for use in the invention are commercially available from BF Goodrich Co. and other suppliers. 
   Referring now to  FIGS. 4A and 4B , hydrophone units  120  are deployed in an N×M array wherein N and M are the number of rows and columns, respectively, of positions for hydrophones in a rectangular grid. The distance between adjacent hydrophone units within a row or column is the same. N and M can be the same or different whole numbers and preferably each range from 6 to 10, more preferably 6 to 8, and most preferably 6 or 7. The more hydrophones in the grid the more accurate determination of the direction of the received acoustic signals. However, the more space is needed in the sonobuoy canister to store the array. In a preferred embodiment shown in  FIG. 4B  a nominal 6×6 grid is altered by leaving the corner positions vacant. The reason for this modification is explained below. 
   Various types of acoustic transducers can be used to detect acoustic waves transmitted through the water. For example, the acoustic transducer can comprise a tube formed at least in part of a piezo material. Piezo materials can be piezoelectric, which generate an electrical pulse or current upon receiving a mechanical impulse such as from an acoustic vibration, or piezoresistive, which change resistance upon receiving a mechanical impulse. Piezoelectric material can comprise an active polarized ceramic material, such as barium titanate or lead zirconate titanate (PZT). The piezoelectric material can, in another embodiment, be a piezoelectric polymer material, such as polyvinylidene fluoride (PVDF), or a piezo-rubber composite material. Piezoresistive materials include, for example, conductive elastomeric polymeric foams or rubbers which become more conductive when compressed. 
   Typically, hydrophones include a central or core conductor, an outer conductor, and a layer of piezo material disposed between, and in contact with, the core conductor and outer conductor in a coaxial configuration. When subjected to mechanical force the piezo material, such as polyvinylidene fluoride (PVDF) generates an electrical current which is carried by the conductors. Hydrophone units typically have a diameter of from about 1/10 inch to about ⅛ inch. Hydrophone units suitable for use in the invention are known and commercially available for example from Argotech Inc. of Fort Lauderdale, Fla. 
   The system  100  is deployed, for example, by launch from an airplane. When the sonobuoy enters the water the contents of the sonobuoy are ejected from the canister. The float remains on the water surface and the deployable array  110  drops to a predetermined depth. 
   A problem associated with sonar detection using such arrays is the presence of side lobes where partial constructive interference of incoming acoustic waves takes place. The side lobes cause confusion of the signals and it is desirable to suppress the side lobes. 
   Shading the conventional beamformer of an array can suppress the sidelobe levels. There is a vast variety of line-array (one dimensional) shading functions such as Hanning, Hamming, Blackman, Chebyshev, Gaussian, Kaiser, Bartlett, Hann, Nuttall, Blackman-Harris, modified Bartlett-Hanning, Tukey, Bohman, Parsen and, of course, uniform weighting to name a few. A significant feature of the present invention is the application of two dimensional (2-D) Chebyshev weighting. Full azimuthal coverage without ambiguity may be achieved by applying 2-D Chebyshev weighting to a grid-patterned array of vertical PVDF wires. The advantage of Chebyshev shading is that it offers the ability to control all sidelobes to any desired peak level. 
   A 2-D gridded array designed for 5 kHz must have an element spacing of half a wavelength at about 7 kHz to mitigate a backlobe ambiguity. In addition, the minimum array size is 6×6 hydrophone units. A smaller array results in the appearance of a backlobe that can&#39;t be mitigated with further reduction of hydrophone unit spacing. Applying 2-D Chebyshev weighting to this 6×6 array results in heavily shaded corners that hardly contribute to the array&#39;s performance. For example, the corner hydrophone units are shaded to 4% the value of any of the four center elements of a 6×6 array Chebyshev shaded to reduce the sidelobes to 40 dB below the level at the maximum response axis. Therefore, the corner hydrophone units may be omitted from the array to reduce the total number of hydrophone units to 32 in a “trimmed grid array.” Elimination of the corner units reduces the computational demand upon the microprocessor. At much lower frequencies, shaded beamforming would result in an omnidirectional beam pattern with no gain against noise. Instead, this array may be processes as a gradient sensor to form a cardioid beam pattern in azimuth to maintain a non-zero array gain. 
   The 6×6 trimmed grid array consists of 32 elements (hydrophone units) collecting time variability of the received pressure field. Fast Fourier Transform (FFT) of the element-level time series results in the spectral content of the pressure field p across the selected processing time,
 
 p   n (ω)=FFT( p   n ( t )),  (1)
 
where n is the element number from 1 to N=32, p is the dynamic pressure, which is a function of time t and angular frequency ω. A polynomial fit is applied to obtain a simple expression to estimate the flop count of each Fast Fourier Transform (FFT). The number of flops can be estimated using the “flops” command of Matlab (version 5.2).
 
   Element-based adaptive beamforming is applied to each frequency over L number of snapshots, 
                     R   nn     ⁡     (   ω   )       =       ∑     l   =   1     L     ⁢         p   nl     ⁡     (   ω   )       ⁢       p   nl   H     ⁡     (   ω   )                   (   2   )               
where R is the plane-wave reflection coefficient, where water depth H indicates complex conjugate, or Hermitian, transpose of the pressure spectrum. The minimum snapshot duration is approximately given by twice the group delay across the maximum dimension of the array. Since the 22 kHz grid array is about 0.122 meters in diameter, the minimum required snapshot duration is 0.16 ms. If the sample rate of this array is 44 kHz, the minimum snapshot duration correspond to an FFT size of eight samples. The minimum number of snapshots for an accurate estimation of the noise covariance matrix is equivalent to three times the number of array elements for full-rank element-level adaptive beam forming (ABF). Once the covariance matrix is properly estimated, singular value decomposition is applied to predict its eigenvalues and eigenvectors
 
                 R   =       ∑     n   =   1     N     ⁢       λ   n     ⁢     v   n     ⁢     v   n   H                 (   3   )               
where λ n  and ν n  are the nth eigenvalue and eigenvector, respectively.
 
   Since the conventional beam array (CBF) beamwidth of the 2-D Chebyshev-shaded 6×6 grid array is 33 degrees, the number of CBF beams is about 11. However, the beamwidth of an adaptive beamformer is much thinner and the number of beams will definitely increase for complete coverage. Each ABF beam is steered to,
 
 h =exp( ik ( x  cos(θ)cos(φ)+ y  sin(θ)cos(φ)+ z  sin(φ)))
 
where (x,y,z) are the coordinates of each array element, k is the wavenumber, and (θ,φ) are the beam steering directions in azimuth and elevation, respectively. In the case of the array of vertical wire elements, the elevation is fixed to φ=0. The minimum variance distortionless response (MVDR) beamformer weights with a white-noise gain constraint are given by the elements of,
 
                   w   =           (     R   +     ɛ   ⁢           ⁢   I       )       -   1       ⁢   h             h   H     ⁡     (     R   +     ɛ   ⁢           ⁢   I       )         -   1       ⁢   h         ,           (   4   )               
where I is the identity matrix, ε is the white noise constraint (usually set to −3 dB), and
 
   
     
       
         
           
             
               
                 
                   
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   The ABF beam spectra are given by,
 
 b   n (ω)=( wp   n (ω)) 2   (6)
 
where an inverse FFT is applied to generate ABF beam-space time series.
 
   If the noise field is cluttered with a non-stationary component caused by a dynamic shipping population, the fastest and nearest ships relative to the array may cross from one narrow ABF beam to others during the integration time required to estimate the full-rank element-level covariance matrix. If the ships are not moving, their energy may be suppressed with just one eigenvalue per ship. However, a moving ship that crosses from beam to beam over the integration time will require more than just one eigenvalue for complete suppression. 
   To address this problem, reduced-rank adaptive beamforming was introduced. It consists of selection of the 1&lt;L&lt;N largest eigenvalues in Eq. (3) to build a smaller L×L covariance matrix with the purpose of reducing the required number of snapshots and, consequently, reducing the integration time to limit the number of ABF beams ships can cross. The trade-off is that by reducing the size of the covariance matrix, the number of available eigenvalues to suppress the ships is also reduced and only the L loudest ships would be suppressed under the best scenario. Reduced-rank ABF may also be applied to beam-space time-series. Beam-based reduced-rank ABF requires applying a conventional beamformer to the array pressure spectra, 
   
     
       
         
           
             
               
                 
                   
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   where the steering vector is given by,
 
 h   mn (ω)= w   n exp( ik ( x   n  cos(θ m )cos(φ m )+ y   n  sin(θ m )cos(φ m )+ z   n  sin(φ m )))  (8)
 
and m represents the beam number from 1 to M. The weights, w n , of the 6×6 trimmed grid array are given by 2-D version of the Chebyshev weights of a line array with −40 dB sidelobe levels,
 
   
     
       
         
           
             
               
                 
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   More generally, however, for any size matrix w NM  is the inner product of the linear Chebyshev weights in both coordinates: w NM =cheb(N×1)×cheb(1×M) 
   The reduced-rank beam-space covariance matrix is given by, 
                     R     m   ⁢           ⁢   m       ⁡     (   ω   )       =       ∑     l   =   1     L     ⁢         b     m   ⁢           ⁢   l       ⁡     (   ω   )       ⁢       b     m   ⁢           ⁢   l     H     ⁡     (   ω   )                   (   10   )               
where 1&lt;L&lt;M is the number of snapshots. Singular value decomposition follows, as in Eq. (6) except in beam rather than element space, to obtain the L beam-space eigenvalues and eigenvectors of this covariance matrix. The beam-space pressure spectra are computed by applying the eigenvectors to the CBF vectors,
   d   m (ω)= v   m   b   m (ω),  (11) 
which is inverse Fourier transformed for the beam-space reduced-rank ABF time-series solution. These are the main operations for flop-count measurement of a weighted conventional beamformer and an element or beam reduced-rank adaptive beamformer.
 
   It can be seen from the array of weights w n  set forth in formula (9) above the weights of the corner positions (0.04) are relatively low with respect to the other. Hence, these corner positions of the 6×6 array need not be occupied by hydrophones. The elimination of the corner hydrophones reduces the number of required hydrophones from 36 to 32, with little loss of accuracy, thereby saving valuable space so as to allow the array to be packed within a size A sonobuoy canister. Moreover, the shaded beamforming described above with reduced number of hydrophone units reduces the computational demand on the microprocessor. 
   The final 6×6 grid array configuration is illustrated in  FIG. 4B . The on axis sidelobes are reduced to −40 dB. The sidelobes along elevation from the main lobe can&#39;t be suppressed through shading because the length of the vertical wires is uniform in thickness and sensitivity. These sidelobes could potentially limit the array&#39;s ability to fully exploit the vertical noise anisotropy. However, they do not degrade the array&#39;s ability to mitigate the nearby ship interference through the sidelobes. 
   Conventional computer control systems can be used to process the electrical signals from hydrophones  120  using the 2D Chebyshev weighting. Referring to  FIG. 6 , electrical signals S from the hydrophone units are sent to a microprocessor M which applies 2-D Chebyshev weighting C to the individual hydrophone signals, as described above, the strength of each signal being adjusted by the corresponding weighting value w n  to produce weighted signals W. Microprocessor M performs the beamforming computations of formula (11) above to provide accurate directional information regarding, for example, submarine vessels. 
   While the above description contains many specifics, these specifics should not be construed as limitations of the invention, but merely as exemplifications of preferred embodiments thereof. For example, while the invention herein is particularly advantageous for military applications and has been described in terms of detection of submarines, it can clearly be employed in any situation wherein acoustic detection is needed, such as oceanographic or other scientific studies, rescue operations, and the like. Those skilled in the art will envision many other embodiments within the scope and spirit of the invention as defined by the claims appended hereto.