Abstract:
A sonar array comprising a support structure, an acoustic projector attached to the support structure, and a plurality of directional hydrophones attached to the support structure and arranged so as to surround the acoustic projector. The directional hydrophones are equidistantly spaced about the acoustic projector. In one embodiment, the array includes means attached to the support structure that allows the support structure to be connected to a device that effects course steering of the sonar array.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     CROSS TO OTHER PATENT APPLICATIONS 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention generally relates to a sonar tracking array, and more particularly to a sonar tracking array for an active sonar system. 
     (2) Description of the Prior Art 
     Highly accurate sonar systems are continuously needed in today&#39;s military and oceanographic environments. However, many conventional active sonar systems that operate at relatively high frequencies utilize an extremely large amount of components. Conventional high-channel count arrays, even if sparsely populated, contain a very high number of elements that significantly increase costs related to manufacturing, installation, maintenance and repair. Conventional short baseline tracking systems are unable to achieve adequate signal-to-noise ratio without the use of transponders and responders. The bearing measurement accuracy achievable with a sonar array is dependent on both the physical aperture of the array and the signal-to-noise ratio (SNR). The extent of the aperture available for mounting sonar arrays on submarines is limited. Thus, alternatively, narrow sonar beam-widths and correspondingly high angular resolution with a given fixed aperture can be achieved by operating at shorter wavelengths. However, high frequency (short wavelength) operation has a severe drawback in that sound propagation loss increases dramatically as described by R. J. Urick in “Principles of Underwater Sound”, McGraw Hill, New York, 1975, pages 99-102, which portion of a publication is incorporated herein in its entirety. 
     Receiver beamwidth is an expression of the angular sector within which the sonar tracking array responds to incident sounds. Outside the aforesaid angular sector, the response is severely attenuated. The 3 dB beamwidth of an array of sensors for a given uniformly shaded fixed aperture L is approximately represented by the equation 50 λ/L wherein λ is the wavelength in the transmission medium of the acoustic energy being generated. This concept is described by William S. Burdic in “Underwater Acoustic Systems Analysis”, Prentice-Hall, Englewood Cliffs, N.J., 1991, page 310, which portion of a publication is incorporated herein in its entirety. If the array elements are spaced one-half wavelength apart, the number of elements across the aperture is then represented by the equation 2 L/λ. Thus, for a square array, the channel count is proportional to L 2 . 
     What is needed is an improved sonar tracking array that is highly accurate but yet, is relatively less complex than conventional sonar tracking arrays. Another desirable feature of such an improved sonar tracking array is that it should have a relatively low per-unit-cost than conventional sonar tracking arrays. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a sonar tracking array that is highly accurate but yet, is relatively less complex than conventional sonar tracking arrays. 
     Another object of the present invention is to provide a sonar tracking system that can operate at higher frequencies with relatively fewer hydrophone channels and yet achieve sufficient directivity. 
     A further object of the present invention is to provide a sonar tracking array that has a relatively lower per-unit-cost than conventional sonar tracking arrays. 
     The present invention is directed to a sonar array that is suitable for mounting to the exterior of a submarine. The sonar array exhibits relatively high accuracy but has a relatively low component and channel count. In a preferred embodiment, the sonar array is mechanically course steered in order to maintain the target within the main lobe response of the sonar array. 
     The sonar array of the present invention comprises a support structure, an acoustic projector attached to the support structure, and a plurality of directional hydrophones attached to the support structure and arranged so as to surround the acoustic projector. The directional hydrophones are spaced about the acoustic projector. In one embodiment, the array includes means attached to the support structure that allows the support structure to be connected to a device that effects course steering of the sonar array. 
     The sonar array of the present invention comprises a support structure, an acoustic projector attached to the support structure, and a plurality of directional hydrophones attached to the support structure and arranged so as to surround the acoustic projector. The directional hydrophones are spaced about the acoustic projector. In one embodiment, the array includes means attached to the support structure that allows the support structure to be connected to a device that effects coarse steering of the sonar array. 
     In one embodiment, all of the hydrophones are center-spaced from the acoustic projector by substantially the same distance. 
     In one embodiment, the plurality of hydrophones comprises four hydrophones. 
     In one embodiment, the outer diameter of the acoustic projector and each hydrophone is about 6.0 inches. 
     In one embodiment, each pair of successive hydrophones are center-spaced from each other by about 8.5 inches. 
     In a related aspect, the present invention is directed to a sonar tracking system, comprising a sonar array comprising a support structure, an acoustic projector attached to the support structure, and four directional hydrophones attached to the support structure and arranged so as to surround the acoustic projector. The directional hydrophones are spaced about the acoustic projector. The sonar tracking system further includes a mechanical turret for steering the sonar array. The turret has inputs for receiving control signals that control the movement of the turret. The sonar tracking array further includes means for receiving and processing sonar signals received from the hydrophones, and means, responsive to the processed acoustic signals, for generating the control signals for input into the mechanical turret. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention are believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a side plan view of the sonar tracking array of the present invention; and 
     FIG. 2 is a block diagram of a sonar system that utilizes the sonar tracking array of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In describing the preferred embodiments of the present invention, reference will be made herein to FIGS. 1-2 of the drawings in which like numerals refer to like features of the invention. 
     Referring to FIG. 1, sonar tracking array  10  generally comprises support structure  12 , acoustic projector  14  and a plurality of directional hydrophones  16 . Acoustic projector  14  and directional hydrophones  16  are securely mounted to support structure  12 . However, acoustic projector  14  and hydrophones  16  are mounted to structure  12  in such a manner so as to enable projector  14  and hydrophones  16  to be dismounted for repair or replacement. In a preferred embodiment, hydrophones  16  are arranged so as to completely surround acoustic projector  14 . In one embodiment, support structure  12  is configured as a rectangular-shaped or square-shaped stainless steel frame. However, other suitable structural configurations can be used to fabricate support structure  12 . Furthermore, support structure  12  may be fabricated from other suitable materials. Support structure  12  is movably mounted to a submarine by use of a mechanically steered turret. However, this feature is discussed in detail in the ensuing description. 
     In one embodiment, the transducing face or surface of projector  14  has a generally convex, or more particularly, a parabolic shape. Acoustic projector  14  may have other geometrical configurations provided that acoustic projector is able to exhibit a beam pattern having the required width. In one embodiment, acoustic projector  14  has an outer diameter of about 6.0 inches. However, it is to be understood that acoustic projector  14  can have an outer diameter that is less than or greater than 6.0 inches. 
     In one embodiment, the transducing face or surface of each hydrophone  16  has a generally flat or planar shape. In another embodiment, each hydrophone  16  has a hemispherical geometry. However, it is to be understood that each hydrophone  16  may have other suitable geometrical shapes. In one embodiment, each hydrophone  16  has an outer diameter of about 6.0 inches. However, it is to be understood that each hydrophone  16  can have other outer diameters as well. In a preferred embodiment, each hydrophone  16  is center-spaced from the next or successive hydrophone  16  by a predetermined distance D 1 . Thus, when four hydrophones  16  are used, as shown in FIG. 1, the angular separation of each hydrophone is about 90 °. In one embodiment, the distance D 1  is about 8.5 inches. However, it is to be understood that distance D 1  can be greater or less than 8.5 inches. In one embodiment, the hydrophones are center-spaced from the acoustic projector by substantially the same distance. 
     The operational frequency range of sonar array  10  is between about 10 kHz and 100 kHz. The well known Product Theorem describes the combined overall response of sonar array  10 . The Product Theorem is described by R. J. Urick in “Principles of Underwater Sound”, McGraw Hill, N.Y., 1975, page 57, which portion of a publication is incorporated herein in its entirety. When comparing sonar array  10  to a fully populated, conventional sonar array, sonar array  10  reduces channel count by more than two orders of magnitude. 
     Acoustic projector  14  may be realized by a suitable commercially available acoustic projector manufactured by Edo Acoustics Corporation of Salt Lake City, Utah. Similarly, hydrophones  16  also may be realized by suitable commercially hydrophones manufactured by Edo Acoustics Corporation. 
     Referring to FIG. 2, there is shown system  100  which utilizes sonar array  10 . System  100  generally comprises receiver/amplifier  102 , signal processor  106 , control signal generator  112  and mechanical turret  116 . Acoustic signals  118  received from sonar array  10  are inputted into receiver/amplifier  102 . Amplified acoustic signals  120  are inputted into processor  106 . Processor  106  outputs processed acoustic signals  122  for input into peripheral sonar equipment (not shown) and processed acoustic signals  124  for input into control signal generator  112 . Generator  112  converts processed acoustic signals  124  into control signals  126 . Control signals  126  are inputted into inputs (not shown) of mechanical turret  116  so as to effect course steering of array  10  in order to maintain a tracked target in the main lobe of the response of array  10 . As a result, split beam processing can be implemented to provide accurate bearing estimates. The directional capabilities of hydrophones  16  contributes to the suppression of the level of the side-lobes. Stated another way, the invention provides the advantage of operation at higher frequencies with fewer hydrophone channels, yet achieving the same directivity. 
     The principals, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein should not, however, be construed as limited to the particular forms disclosed, as these are to be regarded as illustrative rather than restrictive. Variations in changes may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, the foregoing detailed description should be considered exemplary in nature and not limited to the scope and spirit of the invention as set forth in the attached claims.