Abstract:
An acoustic ranging system and method of use is provided that measures the magnitude of the separation of a pair of towed acoustic line arrays at a discrete point along the length of each array. One array acts as the measurement source, while the other array acts as a frequency-shifted echo repeater. The source array incorporates one sonar transmitter and two sonar receivers. The system further includes a configurable frequency shifter that enables one measurement source to make measurements with multiple repeater arrays.

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 REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to an acoustic ranging system for multi-line towed acoustic arrays. More particularly, the present invention provides an acoustic ranging system that measures the magnitude of the separation of a pair of towed acoustic line arrays at a discrete point of the array in such a manner as to eliminate a multitude of measuring errors. 
     (2) Description of the Prior Art 
     Towed underwater acoustic sonar arrays are employed onboard surface ships, submarines and unmanned undersea vehicles to detect ships, marine life, marine geology, and other underwater sound sources. The towed sonar array comprises a long cable that trails the employed vehicle when the array is deployed. 
     Acoustic sensing elements, called hydrophones, are placed throughout the cable. The hydrophones of the array can be used individually to detect sound sources, but the real value of the hydrophones servicing the array is in the signal processing technique of sonar beamforming. 
     Sonar beamforming is a signal processing technique used in acoustic arrays for directional signal reception. The beamforming technique involves combining delayed signals from each hydrophone of the acoustic array at slightly different times, so that every signal reaches the output of the array at exactly the same time, making one loud signal, as if the signal came from a single sensitive hydrophone. By properly selecting the delayed signals, the array under consideration can effectively be steered to enhance the gain in one direction while decreasing gain in other directions. However, the relative position of each individual sensing element, such as the hydrophone, should be precisely known in order to properly select each time delay. 
     In a towed acoustic array that is lying perfectly straight, the positions are fairly straightforward to measure. However, various hydrodynamic forces acting on a towed array as the array travels through the water induce enough movement in the individual sensing elements that a straight-line approximation is no longer valid. Some techniques should be used to estimate the actual positions of the sensing elements and known so-called shape estimators perform this function. 
     Typical towed arrays shape estimators perform an integration operation over a combination of parameters, such as pressure (depth) and heading sensors positions located throughout the array, along with tow ship and array physical parameters so as to calculate the positions of the sensing elements. Current techniques are able to provide acceptable error margins for the determination of these positions in most applications for towed arrays. 
     In addition to the consideration of the difficulties of the estimation of the actual position of the sensing elements, one of the problems with towed arrays is their so-called left/right ambiguity. Without requesting the tow ship to perform maneuvers, it is difficult if not impossible to know if a sound source is coming from the left or right side of the array being utilized. More particularly, the beams that are generated by beamforming for an approximately linear array are conical in nature, leading to an ambiguity that rotates a full 360 degrees around the array. 
     To combat the ambiguity problem, some modern towed systems employ two or more arrays that are towed alongside each other. In this case, proper beamforming can estimate the relative depth/elevation and unambiguous direction of the sound source. In addition, utilization of the two arrays makes the whole system able to identify a quiet source on one side of the array in the presence of a loud source on the other side of the array. The difficulty with this technique is that the shape of the array estimation becomes more critical. 
     Errors that could previously be tolerated in a single array may no longer be acceptable in a multiline system that employs multiple arrays. This non-tolerance is more fully described by authors Cox, H., Lai, H., Heaney, K., &amp; Murray, J. (2003) in the technical article entitled “Hybrid Adaptive Beamforming for Multi-line Arrays” discussed in  Signals, Systems and Computers , (2003), and included in the  Conference Record of the Thirty - Seventh Asilomar Conference on pages  1858-1862. As an example for this non-tolerance, a two degree measurement error in heading may not be significant in a single-line system, but in a multi-line system (if the two degree measurement error occurs) it may cause arrays to cross over each other which can be highly detrimental to the calculations being performed for the multi-line system. 
     Accuracy requirements for a multi-line towed acoustic array may be achieved by adding in a system for measuring line-to-line separation at discrete points along the arrays. Current systems do this acoustically, using one array in one line as the transmitter and the other array in the other line as the receiver. Raw data is sent from the measurement station in each line of the array back to the tow vehicle, where the two data sources are compared by means of an envelope correlator contained in the tow vehicle and used with the speed of sound in water to create a range estimate. 
     Measurement resolution is based on integration time and signal center frequency. As either of these two parameters increases, resolution improves. Integration time is fixed due to the motion of the array in the water. As the signal center frequency increases, the required data bandwidth servicing a hydrophone increases as well. To make useful measurements, a single range measurement system in a modern multi-line array may require sixteen or more times the bandwidth of a single hydrophone channel. 
     Since a number of stations are needed, this required bandwidth puts an extreme load on the array data telemetry system. Depending on the particular array, range measurement may require tens of kilo-samples per second using conventional techniques. It is therefore desired that a ranging system be provided that uses the equivalent of only a few samples per second. 
     To avoid the problems associated with range measurement systems having high bandwidths; a lower-bandwidth alternative to sending raw data can be achieved if the measurement stations in all arrays are synchronized in time. In this case, a unidirectional acoustic signal path is used. One array is the transmitter in one line and another is the receiver in the other line. 
     Since the receiver detects when the signal was transmitted because of the synchronization; the receiver is able to calculate transmit time internally and send only the correct result by way of the array telemetry to the tow ship. This solution is not viable in many towed array systems for the reason that synchronization among the engineering sensors (heading, depth, range) is not always guaranteed or achievable. It is desired that a ranging system be provided for multi-line towed acoustic arrays that does not require synchronization between its arrays, while still achieving accurate measurements. 
     A further additional consideration for ranging systems for multi-line towed acoustic arrays, is the ability for the receivers in the source array of the towed arrays to be able to distinguish the arriving signals from; 1) the transmitted signal emanating from the same hose of the multi-array having provisions for carrying both arriving and transmitting signals; and 2) the echo from different arrays when there is more than one repeater array present in the system. It is desired that a ranging system be provided for multi-line towed arrays that correctly interprets arriving and transmitted signals carried by the same hose and also correctly interprets echo signals from repeaters in the array. 
     Another parameter of interest is the parameter of dissimilar stretches in the towed arrays. Towed arrays stretch when under tension, and imperfect manufacturing tolerances may cause two towed arrays to stretch unevenly. This error, called array skew, increases toward the rear of the array being utilized. The amount of array skew may be calculated from the data that is collected for measuring separation of the elements of the array being utilized. It is desired that a ranging system be provided for multi-line towed acoustic arrays that accommodates for array skew in its measurement technique. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general purpose and primary object of the present invention to provide a ranging system for multi-line towed acoustic arrays having a low bandwidth; thereby yielding a relatively low sampling rate for the data being utilized by the ranging system. 
     It is a further object of the present invention to provide a ranging system for multi-line towed acoustic arrays that is free of the need for synchronization among sensing and measuring elements. 
     It is a still further object of the present invention to provide an acoustic ranging system for multi-line towed acoustic arrays that yields data corresponding to the accurate locations of the sensors being utilized by the ranging system. 
     It is a still further object of the present invention to provide an acoustic ranging system for multi-line towed acoustic arrays that accommodates for array skew, so as to yield accurate range measurements. 
     In accordance with the present invention, an arrangement comprising a source array and an echo repeater array is provided. Each of the source and echo repeater arrays has a centerline. The source and repeater arrays are separated from each other relative to their centerlines by a distance and by an array skew. 
     The source array comprises a signal generator, a transmitter providing an output, and first and second receivers each receiving a signal. The echo repeater array comprises a receiver for receiving the output of the transmitter of the source array, a configurable frequency shifter, and a transmitter for transmitting a signal that is received by the first and second receivers of the source array. 
     The arrangement provides a system which measures the magnitude of the separation between the source array and the echo repeater array at a discrete point along the length of each array. The source array acts as the measurement source, while the echo repeater array acts as a configurable frequency-shifted echo repeater. The source array incorporates one sonar transmitter and two sonar receivers. 
     The configurable frequency shifter of the echo repeater array enables one measurement source to make measurements, so as to cooperatively operate with multiple repeater arrays, as in a multi-line towed acoustic array system. The arrangement of the present invention, among other benefits, uses several orders of magnitude less digital data bandwidth than prior art systems, while at the same time requiring no synchronization between the source array and the echo repeater array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
         FIG. 1  illustrates the arrangement and locations of the components comprising the acoustic ranging system of the present invention; and 
         FIG. 2  is a block diagram of the acoustic ranging system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and more particularly to  FIG. 1 , there is shown an acoustic ranging system  10  that determines the distance between two towed arrays, that is, a source array  12  and an echo repeater array  14 . This determined distance is required, as will be further described hereinafter, to accurately perform adaptive beamforming used, among other things, to differentiate signals as arriving from the left or from the right of the pair of arrays,  12  and  14 . 
     The source array  12  has a centerline  16 , while the echo repeater array  14  has a centerline  18 . The source array  12  and echo repeater array  14  are separated from each other, relative to their centerlines  16  and  18 , as shown in  FIG. 1 , by a distance  20  and by an array skew  22 . The source array  12  has a signal generator and transmitter  24 , first and second receivers  26  and  28 , whereas the echo repeater array  14  has a receiver  30  that has a configurable frequency shifter to be described with reference to  FIG. 2 . As seen in  FIG. 1 , the signal generator and transmitter  24  transmit an acoustic signal  32  to the receiver  30 , and similarly, the receiver  30  transmits acoustic signals  34  and  36  respectively to receivers  26  and  28 . 
     The acoustic ranging system  10 , as will be further described hereinafter, performs an operation without the need of high bandwidth data and without the need of synchronization between the source array  12  and the echo repeater array  14 , while at the same time measuring separation and array skew both measured between the source array and the echo repeater array. 
     As seen in  FIG. 2 , the signal generator and transmitter  24  is comprised of a signal generator  38  and transmitter  40  which in actuality is a transducer. The signal generator  38  provides first and second outputs  42  and  44 , wherein the output  42  is routed to a power amplifier  46  which, in turn, provides an output  47  that is routed to the transmitter  40 . 
     The second output  44  of signal generator  38  is routed to a first timer  48  and a second timer  50 . The first and second timers  48  and  50  respectively receive input signals  52  and  54 , to be further described hereinafter, that respectively provide output signals  56  and  58  that are both routed to a microprocessor  60 . 
     As further seen in  FIG. 2 , the signal generator and transmitter  24  provides an output signal  32  (also shown in  FIG. 1 ), via a transmitter  40 , that is an acoustic signal and is received by the receiver  30  (also shown in  FIG. 1 ), and in particular receiver  62  (shown in  FIG. 2 ), which in actuality is a second transducer. The receiver (transducer)  62  provides an output signal  64  that is routed to a first bandpass filter  66  which, in turn, provides an output signal  68  that is routed to a configurable frequency shifter  70 . 
     The configurable frequency shifter  70  is composed of an oscillator  72  having a selectable frequency f n , and a mixer  74 . The terminology for the configurable frequency shifter  70  is used herein to represent that the frequency f n  may be selected (configured) to meet the operational parameters of the system. The oscillator  72  provides an output signal  76  that is routed to the mixer  74 . 
     The output of the configurable frequency shifter  70 , in particular, the mixer  74  provides an output signal  78  that is routed to a second bandpass filter  80  which, in turn, provides an output signal  82  which, in turn, is routed to a second power amplifier  84 . The power amplifier  84  provides an output signal  86  that is routed to a transmitter  88  (which in actuality is a transducer) and which provides the acoustical signals  34  and  36  (also shown in  FIG. 1 ). 
     The receiver  26  shown in  FIG. 2 , in particular, a third transducer  90  receives the acoustical signal  34 . The transducer  90  produces an output signal  92  that is routed to a third bandpass filter  94  which, in turn, provides a signal  96  that is routed to replica correlator  98  which, in turn, provides the input signal  52  to the timer  48 —as previously discussed. 
     The receiver  28 , in particular, the transducer  100  receives the acoustical signal  36  and provides an output signal  102  that is routed to a fourth bandpass filter  104 . The bandpass filter  104  provides an output signal  106  that is routed to replica correlator  108  which, in turn, provides the input signal  54  to the timer  50 —as previously discussed. 
     In Operation 
     In general, and with reference to  FIG. 2 , two arrays, such as  12  and  14 , from a multi-line system are selected for measurement purposes. Once the arrays  12  and  14  are identified, the signal generator and transmitter  24  projects an acoustic signal  32  by way of the transmitter  40  (transducer). The transmitted signal  32  is received by receiver  30 , processed and rebroadcast back to be received by the first receiver  26  and the second receiver  28  by way of the acoustic signals  34  and  36  respectively. Calculations are performed by the microprocessor  60  that generate the quantities array separation  20  and array skew  22  (shown in  FIG. 1 ). Someone versed in the art can generate these equations using simple geometry. The quantities array separation  20  and array skew  22  are transmitted, via a low bandwidth connection, provided by a conventional array telemetry system back to a signal processor (not shown) onboard the tow ship (not shown). 
     More particularly, and again with reference to  FIG. 2 , in the source array  12 , an electronic signal is generated by the signal generator  38  amplified by the power amplifier  46 , converted to acoustic energy by the transmitter (transducer)  40  and transmitted through the water. At the same time the electronic signal generated by the signal generator  38 , by means of the operation of either timer  48 , the receiver  26  and the microprocessor  60  or the timer  50 , the receiver  28  and microprocessor  60 , is put into an replica correlator, contained in the signal processor onboard the tow ship, that is matched to the frequency spectrum of the transmitted signal  32 . In the receiver  30 , the transmitted signal  32  is received, filtered, frequency-shifted, bandpass-filtered, amplified, converted back to acoustic energy and transmitted as signals  34  and  36  and accepted by receivers  26  and  28 , respectively. 
     The receivers  26  and  28  convert the acoustic signals  34  and  36  back to electronic signals, filter, and run the electronic signals through the replica correlators  98  and  108  respectively, with parameters matched to the expected receiver  30  frequency shifted signal created by the operation of the oscillator  72  and mixer  74  comprising the configurable frequency shifter  70 . 
     The outputs from the now received quantities, created by the configurable frequency shifter  70  present at the replica correlator  98  and  108  are compared to those derived from the already existing quantities previously formed by the receivers  26  and  28 , the replica correlators  98 ,  108  and timers  48  and  50  and calculated with the assistance of microprocessor  60 . 
     The microprocessor  60  is used to remove all fixed time delays and leave only the acoustic propagation time. This value is then transmitted by way of the array telemetry (not shown) to be interpreted onboard the tow ship (not shown) in the signal processing equipment (not shown) therein. The signal processing equipment, via techniques known in the art, calculates the distance  20  between arrays  12  and  14 , and also the array skew  22 . 
     It should now be appreciated that the practice of the present invention provides an acoustic ranging system for multi-line towed acoustic arrays that accurately measures the magnitude of the separation of a pair of towed acoustic line arrays at a discrete point along the length of each array, while at the same time producing a measurement of the distance  20  separating the arrays  12  and  14  and array skew  22 . 
     It should be further appreciated that the practice of the present invention provides an acoustic ranging system for multi-line towed acoustic arrays that can operate regardless of synchronization of towed arrays engineering sensors (known in the art) with respect to each other. 
     It should also be appreciated that the practice of the present invention provides an acoustic ranging system for multi-line towed acoustic arrays that may incorporate the feature that by calculating round trip acoustic propagation delay internal to the array comprised of the source array  12  and the echo repeater array  14 , bandwidth requirements are decreased by several orders of magnitude regardless of signal center frequency being utilized 
     It should now be still further appreciated that the practice of the present invention provides an acoustic ranging system for multi-line towed acoustic arrays that allows for the capability to select high transmit frequencies, so as to permit increased measurement accuracy. 
     Furthermore, it should now be appreciated that the practice of the present invention provides an acoustic ranging system for multi-line towed acoustic arrays that allows for the capability to select high transmit frequencies, so as to allow shorter integration times and, thus, more range measurement samples per second. 
     In addition, it should now be appreciated that the practice of the present invention provides an acoustic ranging system for multi-line towed acoustic arrays that allows for the use of a configurable frequency shifter that permits tailoring of the principles of the invention to service several arrays at once as occurring in multi-line configuration systems. 
     Moreover, in addition to above described embodiments, there are a few alternate configurations which can be included as part of the practice of the present invention such as; 1) the horizontal component of array separation can be calculated from the absolute separation between arrays by incorporating values from nearby pressure sensors; 2) additional receiver stations can be added to the source array  12  to improve measurement accuracy; 3) the signal generator  38  can be reprogrammed remotely to upload new transmit signatures, so as to better adapt the practice of the present invention to different environments; 4) instead of transmitting back a frequency-shifted version of what it receives, the echo repeater array  14  may have its own replica correlator and signal generator so that it can reply with any preprogrammed signature; and 5) if there are only two arrays in the system, such as arrays  12  and  14 , the mixer  74  and oscillator  72  can be removed from the receiver  30  to simplify the hardware and software implementation. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of that expressed in the appended claims.