Frequency division beamforming for sonar arrays

Abstract

A sonar array for forming multiple transmit and/or receive acoustic beams by a frequency beamforming technique. In frequency division beamforming, beam steering direction is made a function of frequency by driving (receiving) each element of a uniformly spaced line array by a signal which leads or lags its nearest neighbor by a fixed phase shift, Δφ. This permits scanning a transmit and/or receive beam through a range of angles by changing its frequency. The same principle can be used to form multiple simultaneous transmit and/or receive beams. This is accomplished by transmitting a wide bandwidth signal and receiving the echoes through a spectrum analyzer. Each frequency bin of the spectrum analyzer then corresponds to a beam pointing in its own unique direction. Advantages of such a sonar system include improved cost, weight, and size versus performance for a variety of systems including ahead looking sonars and bottom mapping sonars.

What is claimed is: 
     
       1. A sonar system for forming a steerable underwater acoustic beam, the system comprising:
 an arraya pair of acoustic transducerstransducer arrays; and 
 a beamforming system that associates a signal to each of the transducers transducer arrays to form an acoustic beam with a direction wherein the signal is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer and wherein the direction of the acoustic beam is determined by the frequency of the signals, wherein the beamforming system is adapted to vary the frequency of the signals so as to permit steering of the acoustic beam, and wherein the pair of arrays is arranged in a quasi-side scan configuration with the axis of each array rotated relative to a fore-aft axis of a platform. 
 
     
     
       2. The sonar system of  claim 1 , wherein the beamforming system comprises a transmitter that supplies signals to the array arrays so as to form a transmitted acoustic beam. 
     
     
       3. The sonar system of  claim 1 , wherein the beamforming system comprises a receiver that receives signals from the array arrays that results from a received acoustic beam. 
     
     
       4. The sonar system of  claim 1 , wherein the beamforming system comprises a transmitter that supplies signals to the array arrays so as to form a transmitted acoustic beam, and a receiver that receives signals from the array arrays that results from a received acoustic beam. 
     
     
       5. The sonar system of  claim 1 , wherein a formula cos θ=(Δφ/2π)(c/fd) represents a relationship between the direction of the acoustic beam and the frequency, where θ represents a direction angle relative to a plane defined by the transducers transducer arrays, Δφ represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers, wherein the phase shift Δφ is selected to be a substantially constant value and the direction angle θ is varied by varying the frequency f about a center frequency f 0 . 
     
     
       6. The sonar system of  claim 5 , wherein the phase shift Δφ is selected such that a signal associated with a given acoustic transducer is a simple linear combination of signals proportional to cos ωt and sin ωt, where ω=2πf and t represents time. 
     
     
       7. The sonar system of  claim 6 , wherein the phase shift Δφ between the adjacent acoustic transducers is selected to be approximately π/2 radian such that repeating sets of four acoustic transducers can be associated by a sequence of signals proportional to cos ωt, sin ωt, −cos ωt, and −sin ωt. 
     
     
       8. The sonar system of  claim 6 , wherein the phase shift Δφ between the adjacent acoustic transducers is selected to be approximately 3π/4 radian such that repeating sets of eight acoustic transducers can be associated by a sequence of signals proportional to cos ωt, −1/√{square root over (2)} cos ωt+1/√{square root over (2)} sin ωt, −sin ωt, 1/√{square root over (2)} cos ωt+1/√{square root over (2)} sin ωt, −cos ωt, 1/√{square root over (2)} cos ωt—1/√{square root over (2)} sin ωt, sin ωt, and −1/√{square root over (2)} cos ωt−1/√{square root over (2)} sin ωt. 
     
     
       9. The sonar system of  claim 6 , wherein the frequency f of the signals is varied in a range of approximately 0.75f 0  to approximately 1.25f 0 . 
     
     
       10. An underwater sonar system comprising:
 an arraya pair of acoustic transducerstransducer arrays; and 
 a beamforming system that simultaneously associates signals with a range of frequencies to the transducers transducer arrays wherein a signal to a given transducer is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer, wherein the phase shifted signals with the range of frequencies form an acoustic signal with a range of directions, wherein a given direction within the range of directions corresponds to a specific frequency of the signals within the range of frequencies, and wherein the pair of arrays is arranged in a quasi-side scan configuration with the axis of each array rotated relative to a fore-aft axis of a platform. 
 
     
     
       11. The sonar system of  claim 10 , wherein the beamforming system comprises a broadband transmitter that simultaneously supplies signals with a range of frequencies to the array so as to form transmitted acoustic signals with a range of directions. 
     
     
       12. The sonar system of  claim 10 , wherein the beamforming system comprises a receiver having a spectrum analyzer that simultaneously processes signals from the array arrays that result from received acoustic signals from a range of directions. 
     
     
       13. The sonar system of  claim 10 , wherein the beamforming system comprises a broadband transmitter and a receiver having a spectrum analyzer wherein the broadband transmitter simultaneously supplies signals with a range of frequencies to the array arrays so as to form transmitted acoustic signals with a range of directions and wherein the spectrum analyzer simultaneously processes signals from the array arrays that result from received acoustic signals from a range of directions. 
     
     
       14. The sonar system of  claim 10 , wherein a formula cos θ=(Δφ/2π)(c/fd) represents a relationship between the direction of the acoustic signal and the frequency, where θ represents a direction angle relative to a plane defined by the transducers, Δφ represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers, wherein the phase shift Δφ is selected to be a substantially constant value and the direction angle θ is varied by varying the frequency f. 
     
     
       15. The sonar system of  claim 14 , wherein the phase shift Δφ is selected such that a signal associated with a given acoustic transducer is a simple linear combination of signals proportional to cos ωt and sin ωt, where ω=2πf and t represents time. 
     
     
       16. The sonar system of  claim 15 , wherein the phase shift Δφ between the adjacent acoustic transducers is selected to be approximately π/2 radian such that repeating sets of four acoustic transducers can be associated by a sequence of signals proportional to cos ωt, sin ωt, −cos ωt, and −sin ωt. 
     
     
       17. The sonar system of  claim 15 , wherein the phase shift Δφ between the adjacent acoustic transducers is selected to be approximately 3π/4 radian such that repeating sets of eight acoustic transducers can be associated by a sequence of signals proportional to cos ωt, −1/√{square root over (2)} cos ωt+1/√{square root over (2)} sin ωt, −sin ωt, 1/√{square root over (2)} cos ωt+1/√{square root over (2)} sin ωt, −cos ωt, 1/√{square root over (2)} cos ωt−1/√{square root over (2)} sin ωt, sin ωt, and −1/√{square root over (2)} cos ωt−1/√{square root over (2)} sin ωt. 
     
     
       18. A method of using an underwater sonar system arranged on a platform and having an array a pair of acoustic transducers transducer arrays, the method comprising:
 associating signals having a frequency component to the transducers pair of transducer arrays, wherein the pair of arrays is arranged in a quasi-side scan configuration with the axis of each array rotated relative to a fore-aft axis of a platform, wherein a signal associated with a given transducer is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer such that the phase shifted signals form an acoustic beam having a direction; and 
 controlling the directionality of the acoustic beam by manipulating the frequency component of the signals. 
 
     
     
       19. The method of  claim 18 , wherein associating the signals to the transducers transducer arrays comprises associating the transducers with signals with a frequency f such that a formula cos θ=(Δφ/2π)(c/fd) represents a relationship between the direction of the acoustic beam and the frequency, where θ represents a direction angle relative to a plane defined by the transducers, Δφ represents the selected fixed phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, and d represents spacing between the adjacent transducers. 
     
     
       20. The method of  claim 19 , wherein associating the signals to the transducers transducer arrays comprises selecting the phase shift Δφ such that a signal associated with a given transducer is a simple linear combination of signals proportional to cos ωt and sin ωt, where ω=2πf and t represents time. 
     
     
       21. The method of  claim 20 , wherein the phase shift Δφ between the adjacent acoustic transducers is selected to be approximately π/2 radian such that repeating sets of four acoustic transducers can be associated by a sequence of signals proportional to cos ωt, sin ωt, −cos ωt, and −sin ωt. 
     
     
       22. The method of  claim 20  A method of using an underwater sonar system having an array of acoustic transducers, the method comprising:
 associating signals having a frequency component to the transducers wherein a signal associated with a given transducer is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer such that the phase shifted signals form an acoustic beam having a direction; and 
 controlling the directionality of the acoustic beam by manipulating the frequency component of the signals, wherein associating the signals with the transducers comprises associating a broadband signal having a range of frequencies with the transducers such that corresponding acoustic beams have a range of directions, and wherein the array of transducers comprises a first line array and a second line array oriented to form a cross shape so as to allow scanning in two dimensions, wherein associating the signals with the transducers comprises associating the transducers with signals with a frequency f, where Δφ represents the selected fixed phase shift between adjacent acoustic transducers, where ω=2πf and t represents time, wherein the phase shift Δφ between the adjacent acoustic transducers is selected to be approximately 3π/4 radian such that repeating sets of eight acoustic transducers can be associated by a sequence of signals proportional to cos ωt, −1/√{square root over (2)} cos ωt+1/√{square root over (2)} sin ωt, −sin ωt, 1/√{square root over (2)} cos ωt+1/√{square root over (2)} sin ωt, −cos ωt, 1/√{square root over (2)} cos ωt−1/√{square root over (2)} sin ωt, sin ωt, and −1/√{square root over (2)} cos ωt−1/√{square root over (2)} sin ωt. 
 
     
     
       23. The method of  claim 19 , wherein associating the signals with the transducers comprises associating a narrowband signal with the transducers and varying the frequency of the narrowband signal to change the direction of the acoustic beam. 
     
     
       24. The method of  claim 23 , wherein associating the narrowband signal with the transducers comprises supplying the narrowband signal to the transducers wherein the signal applied to the transducers results in an outgoing acoustic beam. 
     
     
       25. The method of  claim 23 , wherein associating the narrowband signal with the transducers comprises receiving an echo signal from the transducers wherein the echo signal result from an echo that impinges on the transducers. 
     
     
       26. The method of  claim 23 , wherein associating the narrowband signal with the transducers comprises supplying the narrowband signal to the transducers to yield an outgoing acoustic beam, and receiving an echo signal from the transducers that result from an incoming echo. 
     
     
       27. The method of  claim 19 , wherein associating the signals with the transducers comprises associating a broadband signal having a range of frequencies with the transducers such that corresponding acoustic beams have a range of directions. 
     
     
       28. The method of claim  27  19, wherein associating the broadband signal signals with the transducers transducer arrays comprises simultaneously providing a broadband signal to the transducers so as to yield a plurality of outgoing acoustic beams having a the range of directions. 
     
     
       29. The method of claim  27  19, wherein associating the broadband signal signals with the transducers transducer arrays comprises simultaneously receiving a broadband echo signal from the transducers that result from a plurality of incoming echoes. 
     
     
       30. The method of claim  27  19, wherein associating the broadband signal signals with the transducers transducer arrays comprises simultaneously proving providing a broadband signal to the transducers to yield a plurality of outgoing acoustic beams having a the range of directions, and simultaneously receiving a broadband echo signal from the transducers that result from a plurality of incoming echoes. 
     
     
       31. A method of scanning an angular sector underwater using an array a pair of acoustic transducers transducer arrays, the method comprising:
 forming a plurality of acoustic beams wherein each acoustic beam is formed by associating signals to the array of acoustic transducers transducer arrays such that a signal associated with a given transducer is phase shifted by a selected fixed amount relative to a signal assigned to the adjacent transducer and wherein the direction of each acoustic beam depends on the frequency of the signals; and 
 varying the frequency of signals corresponding to each acoustic beam so as to vary the direction of the acoustic beam, thereby allowing the acoustic beam to sweep a range of direction angles and wherein the frequency is selected for each acoustic beam such that resulting ranges of direction angles cover the angular sector, and wherein the pair of arrays is arranged in a quasi-side scan configuration with the axis of each array rotated relative to a fore-aft axis of a platform. 
 
     
     
       32. The method of  claim 31 , wherein a formula cos θ=(Δφ/2π)(c/fd) represents a relationship between the direction of the acoustic beam and the frequency f, where θ represents a direction angle relative to a plane defined by the transducers, Δφ represents the selected fixed phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, and d represents spacing between the adjacent transducers. 
     
     
       33. The method of  claim 32 , wherein the phase shift Δφ is selected such that a signal associated with a given transducer is a simple linear combination of signals proportional to cos ωt and sin ωt, where ω=2πf and t represents time. 
     
     
       34. The method of  claim 33 , wherein the phase shift Δφ between the adjacent acoustic transducers is selected to be approximately π/2 radian such that repeating sets of four acoustic transducers can be associated by a sequence of signals proportional to cos ωt, sin ωt, −cos ωt, and −sin ωt. 
     
     
       35. The method of  claim 33 , wherein the phase shift Δφ between the adjacent acoustic transducers is selected to be approximately 3π/4 radian such that repeating sets of eight acoustic transducers can be associated by a sequence of signals proportional to cos ωt, −1/√{square root over (2)} cos ωt+1/√{square root over (2)} sin ωt, −sin ωt, 1/√{square root over (2)} cos ωt+1/√{square root over (2)} sin ωt, −cos ωt, 1/√{square root over (2)} cos ωt−1/√{square root over (2)} sin ωt, sin ωt, and −1/√{square root over (2)} cos ωt−1/√{square root over (2)} sin ωt. 
     
     
       36. The method of  claim 32 , wherein associating the signals with the transducers comprises associating a narrowband signal with the transducers and varying the frequency of the narrowband signal to sweep the acoustic beam within the range of direction angles. 
     
     
       37. The method of  claim 36 , wherein associating the narrowband signal with the transducers comprises supplying the narrowband signal to the transducers wherein the signal applied to the transducers results in an outgoing acoustic beam. 
     
     
       38. The method of  claim 36 , wherein associating the narrowband signal with the transducers comprises receiving an echo signal from the transducers wherein the echo signal result from an echo that impinges on the transducers. 
     
     
       39. The method of  claim 36 , wherein associating the narrowband signal with the transducers comprises supplying the narrowband signal to the transducers to yield an outgoing acoustic beam, and receiving an echo signal from the transducers that result from an incoming echo. 
     
     
       40. A sonar system for forming a steerable underwater acoustic beams beam, the system comprising:
 an array of acoustic transducers; and 
 a beamforming system that associates a signal to each of the transducers to form an acoustic beam with a direction wherein the signal is phase-shifted by a selected phase relative to a signal assigned to the adjacent transducer and wherein the direction of the acoustic beam is determined by a combination of the phase and the frequency of the signals, wherein the beamforming system is adapted to vary the frequency of the signals for a given phase so as to permit steering of the acoustic beam, wherein a formula cosθ=(Δφ/2π)(c/fd) represents a relationship of the direction of the acoustic beam to phase and frequency, where θ represents a direction angle relative to a plane defined by the transducers, Δφ represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers, wherein the phase Δφ is selected to direct the beam in a general desired first direction, and the frequency f is varied to vary the direction of the beam about the first direction, wherein the spacing d is selected to be approximately half of the wavelength, and the phase Δφ is selected as 0, π/8 and π/4 radians progressively so as to allow progressive scanning about the different first directions as determined by the selected phases. 
 
     
     
       41. The sonar system of  claim 40 , wherein a formula cos θ=(Δφ/2π)(c/fd) represents a relationship of the direction of the acoustic beam to phase and frequency, where θ represents a direction angle relative to a plane defined by the transducers, Δφ represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers, wherein the phase Δφ is selected to direct the beam in a general desired first direction, and the frequency f is varied to vary the direction of the beam about the first direction. 
     
     
       42. The sonar system of claim  41  40, wherein the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam. 
     
     
       43. The sonar system of claim  41  40, wherein the beamforming system comprises a receiver that receives signals from the array that results from a received acoustic beam. 
     
     
       44. The sonar system of claim  41  40, wherein the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam, and a receiver that receives signals from the array that results from a received acoustic beam. 
     
     
       45. A method of using an underwater sonar system having an array of acoustic transducers, the method comprising:
 associating signals having a frequency component and a phase component to the transducers wherein a signal associated with a given transducer is phase-shifted by a selected phase relative to a signal assigned to the adjacent transducer; and   controlling the directionality of the acoustic signal by selecting a first direction of the acoustic signal as determined by the selected phase and varying the direction of the acoustic beam about the first direction by manipulating the frequency component of the signals.   
     
     
       46. The method of  claim 45 , wherein associating the signals to the transducers comprises associating the transducers with signals with a frequency f and a the phase Δφ such that a formula cos θ=(Δφ/2π)(c/fd) represents a relationship of the direction of the acoustic signal to the phase and frequency, where θ represents a direction angle relative to a plane defined by the transducers, Δφ represents the selected phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, and d represents spacing between the adjacent transducers. 
     
     
       47. A sonar system for forming a steerable underwater acoustic beams beam, the system comprising:
 an array of acoustic transducers; 
 a beamformning beamforming system having a set of beamformers wherein the beamformers associate a plurality of signals to the transducers to form an acoustic beam with a direction wherein each of the signals is phase-shifted by a selected phase relative to a signal assigned to the adjacent transducer and wherein the direction of the acoustic beam is determined by a combination of the phase and the frequency of the signals, wherein the beamforming system is adapted to vary the frequency of the signals for a given phase so as to permit steering of the acoustic beam; and 
 wherein a subset of the beamformers is connected to more than one repeating subsets of the transducers such that each beamformer associates a signal having an assigned phase and frequency to more than one transducer, thereby allowing the total number of beamformers to be less than the number of transducers in the array, wherein a formula sin α=(Δφ/2π)(c/fd) represents a relationship of the direction of the acoustic beam to phase and frequency, where α represents a direction angle relative to a normal to a plane defined by the transducers, Δφ represents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers, wherein the phase Δφ is selected to direct the beam in a general desired first direction, and the frequency f is varied to vary the direction of the beam about the first direction, wherein the phase Δφ is selected to be an integral fraction of 2π radians to allow repeated duplication of signal assignments of the subset of the beamformers to the more than one subsets of the transducers, and wherein the spacing d is selected to be approximately half of the wavelength, and the phase Δφ is selected as 0, π/8 and π/4 radians progressively so as to allow progressive scanning about the different first directions as determined by the selected phases. 
 
     
     
       48. The sonar system of  claim 47 , wherein a formula cos α=(Δφ/2π)(c/fd) represents a relationship of the direction of the acoustic beam to phase and frequency, where α represents a direction angle relative to a normal to a plane defined by the transducers, Δφrepresents a phase shift between adjacent acoustic transducers, c represents velocity of the acoustic beam, f represents the frequency of the signals, and d represents spacing between the adjacent transducers, wherein the phase Δφ is selected to direct the beam in a general desired first direction, and the frequency f is varied to vary the direction of the beam about the first direction. 
     
     
       49. The sonar system of  claim 48 , wherein the phase Δφ is selected to be an integral fraction of 2π radians to allow repeated duplication of signal assignments of the subset of the beamformers to the more than one subsets of the transducers. 
     
     
       50. The sonar system of  claim 49 , wherein the array of transducers comprises a first line array. 
     
     
       51. The sonar system of  claim 50 , wherein the spacing d is selected to be approximately half of the wavelength, and the phase Δφ is selected as 0, π/8, π/4, and π/8 radians progressively so as to allow progressive scanning about the different first directions as determined by the selected phases. 
     
     
       52. The sonar system of claim  51  47, wherein the frequency is varied at each of the selected phases by approximately 67% of bandwidth about a center frequency such that the resulting sweepings of the beam about the first directions yield a generally seamless coverage of scanning that has a range of approximately 0 to 41.8 degrees with respect to the normal. 
     
     
       53. The sonar system of  claim 50 , further comprising a second line array oriented perpendicularly to the first line array so as to form a cross shape to allow scanning in two dimensions. 
     
     
       54. The sonar system of claim  49  47, wherein the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam. 
     
     
       55. The sonar system of claim  49  47, wherein the beamforming system comprises a receiver that receives signals from the array that results from a received acoustic beam. 
     
     
       56. The sonar system of claim  49  47, wherein the beamforming system comprises a transmitter that supplies signals to the array so as to form a transmitted acoustic beam, and a receiver that receives signals from the array that results from a received acoustic beam.