CA1087291A - Antisubmarine warfare system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

It is known to detect submarines by using several spaced apart sonobuoys but launching the sonobuoys is slow and positioning errors can result in large bearing errors.
The present invention avoids these problems by providing a semi-rigid generally neutrally buoyant underwater platform made of a water inflated soft walled element on which is mounted an acoustic wavefront sensing array of at least three omnidirectional hydrophones in a preselected geometric arrangement. Circuitry connected to the hydrophones and associated array steering circuitry allow determination of the time relationship of the signals obtained from the hydro-phones.

Description

`",. , 10*7291 This invention relates to an antisubmarine warfare system~ particu-larly a ~onobuoy system for detecting the presence of a discrete source of acoustic vibrations and for determining the direction of said source.
It i8 known that gubmarineg produce acoustic vibrationg which can be detected by means of submerged hydrophones over very long distances, e.g. tens Or nautical miles due to the relative b high sound conductivity of water.
Thu3, detection and classification of submarines can be achieved by a series of sonobuoys spaced 20 to 30 nautical miles apart which transmit the infor-mation to an investigating shlp or aircraft. Thereafter, the position of the submarine must be determined.
In a known submarine localization system omnidirectional sonobuoys are used which are air or ship-launched in pairs, and the information produced by each sonobuoy of any pair or 80 called "plant" i9 telemetered to the investi-gatinB ship or aircraft. This information is processed, and the time or phase difference between the times of arrival of the two data signals provide~ an ambiguous target bearing line. To resolve the ambiguity and refine the "fix"
further "plants" are laid.
This method of submarine localization has a number of drawbac~s which con dderably reduce its efficiency partlcularly when dealing with fa~ter and more manoeuverable modern submarines. For instance the time that an inve~ti-gating aircraM requires to complete a sufficiently refined fix typically averages between 25 to 40 minutes and because of launching errors and uncer-tainties as to the exact tra~ectory of each sonobuoy, bearing errors can be a~ high a~ + 20 degrees.
Furthermore, this method is very costly as it often requires re than a dozen sonobuoys before a reasonably accurat0 fix can be defined.
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The ob~ect of this invention is therefore to improve the localization capabilities Or anti~ubmarine warfare equipment. More specifically an obJect of this invention i8 to provide a sonobuoy system capable of rapidly detecting the signals from and to a distant submarine using only the information received frcm a ~ingle buoy. To this effect the ~onobuoy must carry at least three hydrophones to provide an unambiguous bearing, and the signal to ambie~t noise ,.
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ratio of the acoustic information arriving at the hydrophone array must be surficiently high to allow a usable bearine accuracy to be obtained.
A first proposal suggests the use of a station of three hydrophones tied to one another by means of flexible cables or ropes. However, due to the relative motion of the hydrophones the geometry of the array would not be sufficiently stable for rapid bearing determination and difficulties would be encountered in properly layine and maintaining the array.
A second proposal involves the use of a rigid structure for support-ing the hydrophones. However, it is well known that underwater rigid struc-tures generate considerable self noise, due to turbulence flow, resonance, etc.,which would be im~ediately picked up by sensitive hydrophones and would render the data signals practically useless for beam steering processing.
In accordance with this invention we provide a sonobuoy for sens-ing acoustic vibrations emanating from a discrete source, and adapted to produce a signal suitable for remote reception and processing by beam forming and steering techniques, said sonobuoy comprising an array of at least three hydrophones maintained in a preselected geometric configuration one fro~ the other, a semirigid generally neutrally buoyant underwater platform made of a water inflated soft walled element and mounting said hydrophones, and circuit means for transferring the signals produced by said hydrophones to a remote location.
We also provide a system for determining the direction of an under-water discrete source of acoustic vibrations by sensing acoustic wavefronts produced thereby, comprising a semirigid generally neutrally buoyant under-water platform made of a water inflated soft walled element, an acoustic wave-front sensing array of at least three omnidirectional hydrophones mounted on said structure in a preselected geometric arrangement, circuit means connected to said hydrophones, and array steering means associated with said circuit means and adapted to allow determination of the time relationship of the signals obtained ~rom said hydrophones.
By this arrangement we have discovered that multi-sensor array technoloey well known in the field of radio could be successfully applied to 108729~

the art of submarine detection and bearing determination since a platform is provided which is relatively silent when submerged, and which presents many practical advantages when used as an underwater expandable structure.
Water inflated soft walled elements per se constitute the sub~ect matter of related Canadian Patent Application Serial No. ~O~D ~ iled on even date by John Mar et al and entitled "Water Inflatable Elements".
The following is a description by way of example of certain embodi-ments of the present invention, reference being had to the accompanying draw-ings in which:
Figure lA is a diagrammatic elevation view of a sonobuoy including a submerged sensor array;
Figure lB is a plan view of a hydrophone array in a sound field;
Figure 2 is a simplified block diagram of signal processing equipment, Figures 3, 4 and 5 are plan views of three different platforms; and Figure 6 (on the sheet of Figure 2) is a simplified plan view of an electro-mechanical analogue of an array in the sound field.
GENERAL DESCRIPTION OF SYSTEM
The sonobuoy shown in Figure lA comprises a submerged horizontal platform referred tc~ by numeral 10 on which a number of hydrophones are mounted, three being shown in Figures lB at 12, 14 and 16. The planar hydrophone array is suitably tethered to a surface float or buoy 20 which contains suitable telemetry equipment for transmitting the information picked up by the hydro-phones to an investigating ship or aircraft. The telemetry system can be of any type~ F-FM radio, and direct cable transmission are two examples.
The processing equipment of the investigating ship or plane can be as shown diagrammatically in Figure 2, wherein an antenna 30 is coupled to a receiver 32 which provides three separate output signals corresponding to the information picked up by the individual hydrophones. The output of receiver 32 is fed to a combining processor 34 which is controlled by a synchronization siÆnal produced by a relative direction control 36. The combined output of ; combining processor 34 is fed to an output detector 3~ which causes a greater A indication to appear at bearing display 40 whenever the array has been electric-ally "steered" in the direction of the wavefront being sensed by the hydro-108729~

phones 12, 14, 16. To maintain the bearing display 40 in synchronism with the combining processor 34, directional control 36 also provides a synchronisa-tion signal to bearing display 40.
In operation the background noise picked up by the three hydrophones 12, 14 and 16 is practically incoherent because in effect it emanates from all directions Howe~er, a wavefront such as at 50 will be sensed by the three hydrophones in sequence and thus their output signals will be slightly out of phase. By suitable transmission these signals will maintain the same phase relationship to one another as they arrive at the combining processor 34.
SONOBUOY
Referring to the sonobuoy illustrated in Figure lA, the important reatures for unambiguous and acceptable hydrophone signals are that there must be at least three hydrophones usually omnidirectional, and a "silent" platform (i.e. one which produces practically no noise when submerged) for maintaining the hydrophones in a known, fixed geometric configuration. Preferably the hydro-phones are evenly spaced on a circle to form, depending on the number of hydro-phones, a triangle or a regular polygon, as the processing of the data signals is generally simpler when the array has at least one axis of symmetry, usually the vertical axis.
The sonobuoy system can be dified in various forms to suit the particular requirements of different applications as will be apparent to those skilled in the field of antisubmarine warfare. For instance, the radio equip-ment, when required, does not have to be mounted in the surface float as at least some components of it can be mounted on the array, and the surface float is not always required. It is only essential that the structure be quickly ; deployed and remain stable and operative for a sufficient length of time to en-able detection of the submarine and determination of its bearing. Thereafter, the antenna could be allowed to sink to the bottom of the sea with the rest of the sonobuoy to prevent its falling into the hands of an enemy. A compass instrument can be mounted on the array platform for providing an indication of ! ~ the orientation of the array, but it is conceivable that the compass instrument could also be dispensed with in some cases, with an explosive charge used instead for determinine the orienta~ion o~ the array as i3 well known in the art. The n~ber o~ hydrophones can be increased to improve the signal to noise ratio and reduce the side lobe effect.
In accordance with this invention, the array platform is in the form of a water inflated soft walled element which is generally neutrally buoy-ant but which may sometimes be made slightly negatively buoyant to assist in maintainine constant depth. The water inflated soft walled element or platform lO can be tethered to a surface float 20 as shown in Figure lA, and a suitable compliant link 21 can be used to reduce vibrations caused by the surface float ~eps and transmitted by the tether to the platform, and thus ~n~ the resonant frequency of any part of the system outside the acoustic frequency range in use.
Various water inflated soft walled elements could be used and a few examples are shown in Figures 3, 1~ and 5.
In Figure 3 the element is made of three elongated tubular members 82, 84, ~6 radiating from a central header 88 to define a "wye" or "star"
shaped configuration. The hydrophones are mounted near the ends of the members, and an additional hydrophone could be mounted on the central header 88 to re-duce the side lobe level of the beam pattern, and further hydrophones could be mounted intermediate the ends of the members 82, 8~, 8~ to provide A better signal-to-noise ratio. The number of radiating members with hydrophones can be increased to improve the symmetry of the array about its central vertical axis and thereby provide a more uniform beam pattern at the expense of a more complicated system.
In Figure ll a mattress-like platform 90 is shown which consists of a single water inflated "bag" of flexible sheet material supporting three or more hydrophones. The shape of element 90 might be varied as desired to facili-tate mounting of the hydrophones and simplify manufacture and packaging. For ex-ample, the outer contour of the element could be generally circular or in the form of a regular polygon of three or more equal sides. In order to prevent ex-cessive bulging of the top and bottom sheets and thus keep a generally flat con-figuration in the inflated condition it may be necessary to provide internal webs A such as shown at 91, 92, 94. These webs which may be in the form of strips of ' , flexible material glued edgewise to the inner surfaces of the top and bottom sheets serve to maintain a fixed spacing between them and greatly împrove the rigidity Or the platform.
In ~igure 5 the elment 70 is delta-shaped and consists of three similar, interconnected tubular members 72, 74, 76 supporting hydrophones 12, 1 16 at their ends, and suspension means 78 such as wires or small cables for sup-el~,n~"~porting the e~m~t to the surface float and connecting the hydrophones to the transmitting end of the telementry link.
The material for the water inflated element must be substantially water impermeable and sufficiently flexible to provide a semirigid structure when water inflated that can yield slightly to environmental perturbations such as underwater currents and the like so as to keep the level of the noise gener-ated by the inflated 8trueture sufficiently low. Various types of materials may be used for the water inflated element, for example, certain plastic materials that can be extruded or formed as a closed surface with a heat seal or glued seam such as polyethylene, or polyethylene-Mylar laminates, produced by Deerfield Laminates Ltd., Toronto. (Mylar is a trademark). The material ;t,S
used should retain ~ flexibility at the temperature of the water in which it is used.
In order to maintain the water inPlated structure in its semirigid condition the internal pressure oP the inflated element should be kept at a pressure somewhat greater than the ambient water pressure, such as about 2 psig.
The sonobuoy system is particularly advantageous for aircraft opcra-tions, where smallness of size is important in the air dropped package. In fact, the water inflatable platform can be rolled up into a compact arrangement and deployed only once in the water, inflation of the deployed package taking place in the water by virtue of a small water pump and valves associated with the package and as described in greater detail in the above mentioned copending , Canadian Patent Application.
When towed behind a ship the water inflated structure may require certain modifications to improve the stability and rigidity of the array, such as stabilizing fins and the like.
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The spacing of the hydrophones will be rela~ed to the number of hydrophones used and the maximum permissible width Or the beam, and in practice it has been found for example that satisfactory results can be obtained in the frequency range of 100 Hz to 3000 E~z w;th delta, Y or star arrays with a 24 foot hydrophone spacing, i.e. in the delta array the tubular members are about 24 feet long whereas in the other cases the length of the tubular members is about 14 feet. The members can be constructed from plastic tubing of .003 inch wall thickness, 2 to 3 inches in diameter.
SIGNA~ BROCESSING SYSTEM
Combining processor 34 operates in accordance with known beam form-ing techniques which by introducing different phase shifts in the three hydro-phone signals creates a preferred direction of listening. In order to electrical-ly "steer" the array i.e. rotate this beam or preferred direction of listening throughout 360 degrees, the directional control 36 systematically changes the phase shifting effected by combining processor 34. It will be appreciated that as the three signals approach an in-phase condition the combined output of processor 34 approaches its maximum.
Various types of combining processors, directional control and bear-ing display arrangements can be used for processing the array signals. For 20 example, an analogue sygtem is illustrated in Figure 6 which causes the various data signals emanating from receiver 32 to be phase shiPted and combined by means of a delay line before reaching output detector 38. A multi-tap delay line 51 with a large number of p~rallel conductor strips arranged as a platen type commutator 52 simulate the time propagation characteristics of a sound field, and three rigidly interconnected contacts 62, 64, 66, each representing a different one Or the hydrophones 12, 14, 16 form an analogue of hydrophone array 10. By interconnecting the output channels of receiver 32 to the contacts 62, 64, 66 of model array 54, and by placing the contacts points 62, 64, 66 on conductor strips 52, each data signal passes through a section of delay line 51 30 and undergoes a corresponding phase shiPt. Therefore, for any position of the model array 54 a preferred direction of listening of the hydrophone array is defined, i.e. if a wavefront coming in this direction were sensed by the hydro-~087291 phones the phase differences between the data signals would be compensatedfor the combination of phase shifts produced by the delay line ~, and in effect these three phase shifted signals would be effectively in-phase as they reach output detector 40.
In order to electrically "steer" the array 10, i.e. change the pre-ferred direction of listening of the system, the model array 54 (or the com-mutator 52) can be rotated manually or by means of a suitable directional con-trol system 36.
Another method of scanning could be the use of multiple delay lines to pre-form at 360 pattern of overlapping "beams", the electrical "steering"
of the system being ef4ected by sequential sampling of the various beam forming circuits.
There can be other techniques for beam forming and steering using digital rather than analogue processing. For instance, the received hydrophone signals could be fed to analogue-to-digital converters and then to a network of shift registers to provide the required delays. Such methods would obvious-ly require more complex circuitry but would afford elimination of the mechanic-al switches or sliding contacts of the previously noted steering proposals.
In all these methods of scanning, the relative bearing information can be obtained by means of a pointer on a steering handwheel where steering is done manually, or by a suitable bearing display 40 which would be electrical-ly or mechanically synchronised by the directional control 36. By way of ex-ample, directional control 36 could operate two synchronous motors, the first controlling the phase shifting effected by the combining processor 34, the other operating the indicator means of a remote bearing display 40.
Bearing display 40 which, as mentioned before can be a pointer on the model array 54 (Figure 6), ca~l also be of different types depending upon the type of combining processor 40 used, and whether the information should be per-manently recorded or not. For example, a linear chart recorder could be pro-vided for recording the amplitude of the combining processor as a function ofthe orientation of the "beam", or the same information could be represented as A a radial display on a cathode ray tube as is used in radar with a sweeping ~':

1(~87291 beam rotating in synchronism with the electrical steering Or the hydrophone array 10.
The orientation of hydrophone array 10 can be related to the angular position of model srray 54 by means of suitable compass instrumentation (not shown), aboard the hydrophone array 10, producing a relative bearing correction signal which could be transmitted on a fourth channel of the telemetry system.
Alternatively, a charge could be used to determine the orientation of the hydrophone array 10 in accordance with well known techniques but this method has some disadvantages, both technical and tactical, for example, the orientation information is true only so long as the orientation of array 10 remains fixed after the detonation, and the noise produced by the charge will render any nearby submarine aware of the searching operation thus voiding the dvAntAg~s oi vh~t otherviae would be A CoApletely ACoUBtiCAly pA88iVe SysteA.

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Claims (39)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A system for determining the direction of an underwater discrete source of acoustic vibrations by sensing acoustic wavefronts produced thereby, comprising a semirigid generally neutrally buoyant underwater platform made of a water inflated soft walled element, an acoustic wavefront sensing array of at least three omnidirectional hydrophones mounted on said structure in a preselected geometric arrangement, circuit means connected to said hydrophones, and array steering means associated with said circuit means to allow determi-nation of the time relationship of the signals obtained from said hydrophones.

2. A system as defined in claim 1, wherein said circuit means includes transmitting means and receiving means forming a telemetry link between said acoustic wavefront sensing array and said array steering means.

3. A system as defined in claim 1, wherein said circuit means comprises a multichannel VHF telecommunication system.

4. A system as defined in claim 1 including bearing display means for displaying the output of the array steering means so as to provide an indi-cation of the bearing Or said source relative to the geometry of said array.

5. A system as defined in claim 4 additionally including compass means producing a signal representative of the orientation of said platform.

6. A system as defined in claim 5 including means for combining said indication of the bearing of said source relative to the geometry of said array and the compass means signal to provide an indication of the bearing of said source relative to the orientation of said array.

7. A system as defined in claim 1, wherein the preselected geometric arrangement of said hydrophones is planar and horizontal.

8. A system as defined in claim 7, wherein said hydrophones are evenly spaced on a circle.

9. A system as defined in claim 1, wherein the preselected geometric arrangement of said hydrophones is symmetrical.

10. A system as defined in claim 8, wherein said hydrophones define the apexes of an equilateral triangle.

11. A system as defined in claim 10 including a fourth hydrophone dis-posed in the centre of said equilateral triangle.

12. A system as defined in claim 1, wherein said water inflated sort walled element is Y-shaped and comprises three elongated arms outwardly radiat-ing from a center header, each arm carrying at its outer end one of said hydro-phones.

13. A system as defined in claim 1, wherein said water inflated soft walled element is delta-shaped and comprises three elongated tubular members of equal length and interconnected in an end-to-end relationship, each hydro-phone being mounted on a different one of the apexes of said delta-shaped element.

14. A system as defined in claim 1, wherein said water inflated sort walled element is a mattress-like member whose outer contour is generally circular.

15. A system as defined in claim 1, wherein said water inflated soft walled element is a mattress-like member whose outer contour is generally triangular.

16. A system as defined in claim 1, wherein said water inflated soft walled element is a mattress-like member whose outer contour is generally in the form of a regular polygon.

17. A system as defined in claim 1, wherein said array steering means comprises phase shifting means for simultaneously phase shifting the hydro-phone signals to thereby electrically define a preferred direction of listen-ing, and phase shifting control means to systematically vary the phase shift-ing of said hydrophone signals so as to thereby electrically steer said pre-ferred direction of listening.

18. A system as defined in claim 17, wherein said phase shifting means is a multi-tap delay line and said phase shifting control means is a set of array signal contacts modelling said array and cooperating with a commutator platen coupled to the taps of said delay line to simultaneously phase shift said array signals and enable systematic variation of the phase shifting there-of by relative rotation of said set of contacts and said commutator platen.

19. A system as defined in claim 1, wherein said array steering means comprises analogue-to-digital converters and a network of shift registers for processing the array signals.

20. A sonobuoy for sensing acoustic vibrations emanating from a discrete source, and adapted to produce a signal suitable for remote reception and pro-cessing by beam forming and steering techniques, said sonobuoy comprising an array of at least three hydrophones maintained in a preselected geometric con-figuration one from the other, a semirigid generally neutrally buoyant under-water platform made of a water inflated soft walled element and mounting said hydrophones, and circuit means for transferring the signals produced by said hydrophones to a remote location.

21. A sonobuoy as defined in claim 20, including a surface float to which said underwater platform is tethered.

22. A sonobuoy as defined in claim 21, wherein said platform is kept slightly negatively buoyant.

23. A sonobuoy as defined in claim 21 or 22, including a compliant link between said surface float and said platform to reduce transmission of vibra-tion of said surface float to said platform.

24. A sonobuoy as defined in claim 20, including compass means for pro-viding a signal representative of the orientation of said platform.

25. A sonobuoy as defined in claim 20, wherein said circuit means com-prises a multi-channel radio transmitter.

26. A sonobuoy as defined in claim 25, including a surface float and an antenna mounted on said surface float.

27. A sonobuoy as defined in claim 26, wherein said radio transmitter is mounted inside said surface float.

28. A sonobuoy as defined in claim 20, 21 or 25, including pump means and valve means adapted to water inflate said soft walled element and maintain a pressure inside said element slightly greater than the pressure of the ambient water.

29. A sonobuoy as defined in claim 20, wherein said platform is made of interconnected elongated tubular members.

30. A sonobuoy as defined in claim 29, wherein said platform is delta-shaped.

31. A sonobuoy as defined in claim 29, wherein said tubular members out-wardly radiate from a central header.

32. A sonobuoy as defined in claim 29, wherein said tubular members form a planar star-shaped array.

33. A sonobuoy as defined in claim 20, wherein said platform is a mattress-like structure.

34. A sonobuoy as defined in claim 33, wherein the outer contour of said structure is generally triangular.

35. A sonobuoy as defined in claim 33, wherein the outer contour of said structure is generally in the form of a regular polygon.

36. A sonobuoy as defined in claim 33, wherein the outer contour of said structure is generally circular.

37. A sonobuoy as defined in claim 20, 29 or 33, wherein the material of said soft walled element is a formable or extrudable plastic.

38. A sonobuoy as defined in claim 20, 29 or 33 wherein the material of said soft walled element is polyethylene or polyethylene-mylar laminate.

39. A sonobuoy as defined in claim 33, 34 or 35 including an internal member for maintaining the central regions of the top and bottom surfaces of said structure a predetermined distance apart.