Abstract:
Systems and methods for detection and analysis of amplitude modulation of underwater sound employ a product of a time delayed first electrical signal with a second electrical signal to generate a summed-product signal. The time delayed first electrical signal and the second electrical signal have an amplitude modulation indicative of characteristics of a vessel propeller. The summed-product signal is analyzed to detect a vessel and to determine the characteristics of the vessel propeller. In some arrangements a plurality of summed-product signals are analyzed to also determine a depth of the detected vessel.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 60/894,317, filed on Mar. 12, 2007, which application is incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   This invention relates generally to sonar systems and methods and, more particularly, to sonar systems and methods used to analyzes an amplitude modulation of underwater sound resulting from propeller rotation of a water born vessel. 
   BACKGROUND OF THE INVENTION 
   Some passive sonar systems are designed to receive and process underwater sound signals emitted by an unknown water born vessel. The sound signals can be active sound pulses emitted by an active sonar system on board the unknown vessel, or vessel noise (e.g., engines, generators, and the like). The passive sonar systems can receive a combination of ambient ocean noise and the vessel-generated sound signals. The passive sonar systems can employ a variety of processing upon the received sound signals in order to detect, to localize, and to classify the unknown vessel. 
   Some sounds in the water tend to be amplitude modulated by the sound field emitted by the vessel&#39;s propellers. In particular, the sound received by the passive sonar system can be amplitude modulated in a manner related to characteristics of the propeller. 
   Some passive sonar systems have exploited the amplitude modulation of the received sound in order to identify characteristics of the propeller, for example, rotation speed and number of propeller blades. With this information, the passive sonar systems are often able classify the type of vessel, including, but not limited to, whether the vessel is a surface vessel or a submarine. The processing can be of a type referred to as “detection of envelope modulation on noise.” 
   Referring to  FIG. 1 , a conventional “detection of envelope modulation on noise” system  10  includes a hydrophone  14  adapted to receive underwater sound  12 . The hydrophone  14  is conventionally an omnidirectional hydrophone, which has substantially the same sensitivity to sound received from all spatial directions. The hydrophone  14  generates a signal in response to the sound signal  12 . The signal is preprocessed, for example, by an amplifier  16 . The amplifier  16  is coupled to an analog to digital (A/D) converter  18 , which generates a signal x(t), which is comprised of digital time samples of the preprocessed signal. 
   The signal x(t) can be processed to identify the above-described amplitude modulation of the received sound  12 . One of ordinary skill in the art will recognize a variety of circuits that can be used to identify the amplitude modulation of the received sound  12 . In one conventional arrangement, the signal x(t) can be processed by a “square law” detector, including a squaring module  20  and a low pass filter (LPF) module  22 . An output signal generated by the low pass filter  22  is representative of the envelope of (i.e., the amplitude modulation of) the received sound signal  12 . 
   The output signal generated by the low pass filter module  22  can be analyzed by a spectrum analyzer  24 , for example, a Discrete Fourier Transform (DFT). It will be understood that the spectrum analyzer  24  provides a frequency domain signal (e.g., one or more frequency spectra) representative of frequency content of the envelope of the received sound signal  12 . The frequency spectra generated by the spectrum analyzer  24  can be further processed and displayed by a detector/display module  26 . For example, the detector/display module  26  can display the frequency spectra in a waterfall type display (not shown). The detector/display module  26  can also detect and analyze spectral lines present in the frequency spectra. 
   It is possible to determine a propeller shaft rate (revolutions per second (rps)) and a number of propeller blades of a detected vessel by analyzing the frequency spectra. From the shaft rate and the number of propeller blades it is often possible to identify the type of vessel and whether the vessel is a surface vessel or a submarine. 
   In general, a fundamental frequency of the frequency domain signal (frequency spectra) generated by the spectrum analyzer  24  in Hz corresponds to the propeller shaft rate of the unknown vessel in revolutions per second. Furthermore, the number of propeller blades can be determined from frequencies and relative amplitudes of harmonic signal components in the frequency domain signal generated by the spectrum analyzer  24 . 
   The “detection of envelope modulation on noise” system and methods described above are often able to detect and to classify a vessel. However, in general, it is always desirable to improve detection performance, localization performance, and/or classification performance of a sonar system. 
   SUMMARY OF THE INVENTION 
   The present invention can provide improved detection performance, localization performance, and/or classification performance compared with a conventional “detection of envelope modulation on noise” arrangement. 
   In accordance with one aspect of the present invention, a method of processing sound includes receiving the sound with one or more sound sensors, converting the received sound to first and second electrical signals, correlating the first and second electrical signals to provide a correlation signal, identifying a peak in the correlation signal, identifying a time delay associated with the peak in the correlation signal, applying the time delay to the first electrical signal to provide a first time-delayed electrical signal, multiplying portions of the second electrical signal by respective portions of the first time-delayed electrical signal to provide respective pluralities of product values, calculating respective sums of each one of the pluralities of product values to provide a plurality of summed values, and converting the plurality of summed values to a frequency domain signal. 
   In accordance with another aspect of the present invention, apparatus for processing sound includes one or more sound sensors adapted to receive the sound signal. The apparatus further includes a converter coupled to the one or more sound sensors and adapted to convert the received sound to first and second electrical signals, a correlator adapted to correlate the first and second electrical signals to provide a correlation signal, a correlation peak detector adapted to identify a peak and the time delay associated with the peak in the correlation signal, at least one time delay module adapted to apply the time delay to the first electrical signal to provide a first time-delayed electrical signal, at least one multiplication/summing module adapted to multiply portions of the second electrical signal by respective portions of the first time-delayed electrical signal to provide respective pluralities of product values and adapted to calculate respective sums of each one of the pluralities of product values to provide a plurality of summed values; and at least one spectrum analyzer adapted to convert the plurality of summed values to a frequency domain signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing features of the invention, as well as the invention itself may be more fully understood from the following detailed description of the drawings, in which: 
       FIG. 1  is a block diagram showing a prior art system having one omnidirectional hydrophone, the system adapted to perform “detection of envelope modulation on noise” processing; 
       FIG. 2  is a block diagram showing a system having two omnidirectional hydrophones, the system adapted to perform “dual-channel modulation detection” (DCMD); 
       FIG. 2A  is a block diagram showing a system having two arrays, the system adapted to perform dual-channel modulation detection (DCMD); 
       FIG. 2B  is a block diagram showing a system having one array, the system adapted to perform dual-channel modulation detection (DCMD); 
       FIG. 3  is a block diagram showing a portion of a system adapted to perform dual-channel modulation detection (DCMD) and also having a feature detector and a multipath delay association processor; 
       FIG. 3A  is a block diagram showing further details of the feature detector of  FIG. 3 ; and 
       FIG. 4  is a series of graphs showing frequency spectra associated with the system of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Before describing the present invention, some introductory concepts and terminology are explained. As used herein, the term “spectrum analyzer” is used to describe a circuit or software algorithm, which receives a signal in the time domain and which generates an associated signal in the frequency domain. A spectrum analyzer can include a variety of continuous circuits or discrete circuits (e.g., digital circuits) or algorithms. For example, the spectrum analyzer can include a discrete Fourier transform (DFT) module, which can, in some arrangements, be a fast Fourier transform (FFT) module. It will be recognized that the DFT module can generate a frequency spectrum. In other arrangements, the spectrum analyzer can include one or more multiplication modules, each of which is adapted to multiply the time domain signal by a respective sinusoid signal, resulting in one or more respective product signals. In some particular arrangements, the spectrum analyzer can include at least two multiplication modules, a first one of which is adapted to multiply the time domain signal by a sine signal, resulting in a first product signal, and another one of which is adapted to multiply the time domain signal by a cosine signal, resulting in a second product signal. One of ordinary skill in the art will recognize that the first and second product signals can be combined to generate a magnitude and a phase of a frequency within the time domain signal, wherein the frequency is the frequency of the sine and cosine signals. By performing a plurality of such multiplications, a frequency spectrum can be generated. 
   Referring now to  FIG. 2 , an exemplary system  50  includes first and second sound sensors  52   a ,  52   b , respectively. The first and second sound sensors  52   a ,  52   b  can be omnidirectional hydrophones, each of which has substantially the same sensitivity to sound received from all spatial directions. The first and second sound sensors  52   a ,  52   b  can be physically separated by at least a correlation distance, which will be understood by one of ordinary skill in the art. However, in other arrangements, the sound sensors  52   a ,  52   b  can be separated by less than a correlation distance. 
   The first sound sensor  52   a  generates a signal  54   a , which is received by an amplifier  56   a . The amplifier  56   a  generates an amplified signal  58   a , which is received by an analog to digital (A/D) converter  60   a . The A/D converter  60   a  generates a first digital signal  62   a , which is comprised of digital time samples x 1 (t) (referred to herein as a first electrical signal) representative of a sound signal received by the first sound sensor  52   a.    
   The second sound sensor  52   b  generates a signal  54   b , which is received by an amplifier  56   b . The amplifier  56   b  generates an amplified signal  58   b , which is received by an analog to digital (A/D) converter  60   b . The A/D converter  60   b  generates a second digital signal  62   b , which is comprised of digital time samples x 2 (t) (referred to herein as a second electrical signal) representative of a sound signal received by the second sound sensor  52   b.    
   The first and second electrical signals  62   a ,  62   b , respectively, are received by a cross-correlation module  64 . The cross-correlation module  64  cross correlates the two signals  62   a ,  62   b  resulting in a correlation signal  66 . 
   Cross-correlation of time sampled signal x(t) and y(t) can be described by the following relationship:
 
XCorr(τ)=1 /N |Σ( x ( t )* y ( t −τ))|
 
   where:
         t=t 1  . . . t N =time sample times   τ=τ 1  . . . τ N =Correlation function times (correlation time delays)   N=number of time samples       

   From the above expression, it should be understood that the time samples 1 to N of the signals x(t) and y(t) are multiplied together and summed at each con-elation time, τ, resulting in one correlation value for each correlation time, τ. The correlation time is then changed and the multiplication and sum is repeated. A plurality of con-elation values are thus obtained, each correlation value associated with a corresponding correlation time. 
   The correlation signal  66  will be understood to have a time delay scale and an amplitude scale, when graphically represented. In particular, for certain relative time delays applied between the signals x 1 (t) and x 2 (t), the correlation signal  66  may have relatively high correlation magnitudes, also referred to herein as correlation peaks. 
   The correlation signal  66  is received by a peak detector module  68 , which is operable to identify correlation peaks. In some arrangements, the peak detector module  68  uses a threshold, and portions of the correlation signal  66  that are above the threshold are deemed to be correlation peaks. 
   The peak detector generates a time delay output signal  70  representative of a time delay used by the cross correlation module  64  that produces the highest magnitude correlation peak in the correlation signal  66 . The time delay output signal  70  is received by a time delay module  72 , which applies a time delay to the first electrical signal x 1 (t) corresponding to the time delay signal  70 , in order to generate a time delayed first electrical signal  74 , x 1 (t−T). 
   The time delayed first electrical signal  74  and the second electrical signal  62   b  are received by a cross-correlation module  76 . The cross-correlation module  76  operates with only one correlation time delay by the following expression:
 
XCorr( T )=1 /N |Σ( x ( t )* y ( t−T ))|
 
   where:
         t=t 1  . . . t N =time sample times   T=single time delay T   N=number of time samples       

   Therefore, the cross-correlation module  76  operates as a multiplication and summing (multiplication/summing) module  76 , which multiplies the two signals  74 ,  62   b , (e.g., time samples  1  to N), resulting in a plurality of product values, and which sums the plurality of product values, resulting in a summed-product value. The multiplication/summing module  76  repeats the multiplication and summation for other portions (e.g., time samples  10  to N+10, etc.) of the two signals  74 ,  62   b , resulting in a summed-product signal  78  having a plurality of summed-product values. 
   The summed-product signal  78  can be received by an optional low pass filter module  80 , which can generate a filtered signal  82 . The filtered signal, or in other arrangements, the summed-product signal  78 , can be received by a spectrum analyzer  84 . The spectrum analyzer  84  can generate a frequency domain signal  86  (or frequency spectrum). A detector/display  88  can receive the frequency domain signal, and can present the frequency domain signal in a display, for example, in a waterfall display. 
   It will be appreciated that the time delay, T, can be a positive or a negative time delay relative to the second electrical signal  62   b . It will also be appreciated that a negative time delay, T, applied to the first electrical signal  62   a  is equivalent to a positive time delay applied to the second electrical signal  62   b . The time delay, T, is shown to be applied to only the first electrical signal  62   a  for clarity. 
   Referring to  FIG. 2A , in which like elements of  FIG. 2  are shown having like reference designations, a system  100  includes a first array  102   a  and a second array  102   b , the array centers of which are physically separated by at least a correlation distance, which will be understood by one of ordinary skill in the art. 
   The arrays  102   a ,  102   b  can be any form of arrays formed by a plurality of array elements. For example, the arrays  102   a ,  102   b  can be line arrays, planar arrays, or volumetric arrays, each of which is capable of generating spatial receiving beams. The arrays  102   a ,  102   b  need not be the same form of array. The arrays  102   a ,  102   b  also need not have the same number of acoustic array elements. 
   Signals  104   aa - 104   a N from acoustic elements of the first array  102   a  are received and amplified by amplifiers  106   aa - 106   a N, respectively, resulting in amplified signals  108   aa - 108   a N. The amplified signals  108   aa - 108   a N are received by A/D converters  110   aa - 110   a N, respectively, resulting in intermediate signals  112   aa - 112   a N, respectively. The intermediate signals  112   aa - 112   a N are received by a first beam former  114   a . The first beamformer  114   a  combines the intermediate signals  112   aa - 112   a N so as to generate a first beamformed signal  116   a , which is comprised of digital time samples x 1 (t) (referred to herein as a first electrical signal) representative of a sound signal received by the first array  102   a  from a first (beam formed) spatial direction. 
   Similarly, signals  104   ba - 104   b M from acoustic elements of the second array  102   b  are received and amplified by amplifiers  106   ba - 106   b M, respectively, resulting in amplified signals  108   ba - 108   b M. The amplified signals  108   ba - 108   b M are received by A/D converters  110   ba - 110   b M, respectively, resulting in intermediate signals  112   ba - 112   b M, respectively. The intermediate signals  112   ba - 112   b M are received by a second beam former  114   b . The second beam former  114   b  combines the intermediate signals  112   ba - 112   b M so as to generate a second beamformed signal  116   b , which is comprised of digital time samples x 2 (t) (referred to herein as a second electrical signal) representative of a sound signal received by the second array  102   b  from a second (beamformed) spatial direction. 
   The first and second spatial directions can be the same spatial directions, or they can be different spatial directions. In some arrangements, the first and second spatial directions are changed from time to time, for example, during sequential processing cycles, so that the system  100  processes signals from a first pair of spatial directions, then from another pair of spatial directions, and so fourth. 
   It will be apparent that the first and second electrical signals  116   a ,  116   b  (x 1 (t) and x 2 (t)), respectively, are processed by the same elements  64 - 88  described above in conjunction with  FIG. 2 , and therefore, those elements are not discussed again. 
   It should be appreciate from the discussion of  FIGS. 2 and 2A  that a system can be constructed, which has one omnidirectional sound sensor, e.g.  52   a  or  52   b  of  FIG. 2 , in combination with one array, e.g.,  102   a , or  102   b . Therefore, in one arrangement, the array  102   a , the amplifiers  106   aa - 106   a N, the A/D converters  108   aa - 108   a N and the beamformer  114   a  can be replaced by the sound sensor  52   a , the amplifier  56   a , and the A/D converter  60   a  of  FIG. 2 . In another arrangement, the array  102   b , the amplifiers  106   ba - 106   b M, the A/D converters  108   ba - 108   b M and the beam former  114   b  can be replaced by the sound sensor  52   b , the amplifier  56   b , and the A/D converter  60   b  of  FIG. 2 . 
   Referring now to  FIG. 2B , in which like elements of  FIG. 2  are shown having like reference designations, a system  150  includes by one array  152 . The array  152  can be any form of array formed by a plurality of array elements. For example, the array  152   a  can be a line array, a planar array, or a volumetric array, each of which are capable of generating spatial receiving beams. 
   Signals  154   a - 154 N from acoustic elements of the first array  152  are received and amplified by amplifiers  156   a - 156 N, respectively, resulting in amplified signals  158   a - 158 N. The amplified signals  158   a - 158 N are received by A/D converters  160   a - 160 N, respectively, resulting in intermediate signals  162   a - 162 N, respectively. The intermediate signals  162   a - 162 N are received by a first beamformer  164   a . The first beamformer  164   a  combines the intermediate signals  162   a - 162 N so as to generate a first beam formed signal  166   a , which is comprised of digital time samples x 1 (t) (referred to herein as a first electrical signal) representative of a sound signal received by the array  152  from a first (beamformed) spatial direction. The intermediate signals  162   a - 162 N are also received by a second beamformer  164   b . The second beamformer  164   b  combines the intermediate signals  162   a - 162 N so as to generate a second beamformed signal  166   b , which is comprised of digital time samples x 2 (t) (referred to herein as a second electrical signal) representative of a sound signal received by the array  152  from a second (beamformed) spatial direction. 
   The first and second spatial directions can be the same spatial direction or different spatial directions. In some arrangements, the first and second spatial directions are changed from time to time, for example, during sequential processing cycles, so that the system  150  processes signals from a first pair of spatial directions, then from another pair of spatial directions, and so forth. 
   It will be apparent that the first and second electrical signals  166   a ,  166   b  (x 1 (t) and x 2 (t)), respectively, are processed by the same elements  64 - 88  described above in conjunction with  FIG. 2 , and therefore, those elements are not discussed again. 
   Referring now to  FIG. 3 , the first and second electrical signals, x 1 (t) and x 2 (t) of any of the above-described systems  50 ,  100 ,  150  of  FIGS. 2 ,  2 A, and  2 B, respectively, can be processed by the system portion  200 , instead of or in addition to the system portions shown in those figures. The signals x 1 (t) and x 2 (t) can be received by a cross-correlation module  204 , the same as or similar to the cross-correlation module  64  of  FIGS. 2-2B . The cross-correlation module  204  generates a correlation signal  206  accordingly. The correlation signal is received by a peak detector module  208 . The peak detector module is adapted to identify two or more peaks in the correlation signal  206 , unlike the peak detector modules  68  of  FIGS. 2-2B , which, some embodiments, identifies only a largest correlation peak. In some arrangements, the peak detector module  208  uses a threshold, and portions of the correlation signal  206  that are above the threshold are deemed to be con-elation peaks. 
   As will be understood, each identified correlation peak is associated with a time delay, here T 1 , T 2 , . . . TN. Time delay signals  211   a - 211 N generated by the peak detector module  208  are applied to time delay modules  210   a - 210 N, respectively, and the time delay modules  210   a - 210 N apply time delays T 1 -TN, respectively, to the first electric signal  202   a , resulting in time delayed first electrical signals  212   a - 212 N, respectively. 
   The time delayed first electrical signals  212   a - 212 N and the second electrical signal  202   b  are received by respective cross-correlation modules  214   a - 214 N. The cross-correlation modules  214   a ,  214 N each operate with only one correlation time delay. Therefore, the cross correlation modules  214   a - 214   n  operate as multiplication and summing (multiplication/summing) modules  214   a - 214 N, respectively, each one of which multiplies and sums respective values in a process the same as or similar to that described above for the multiplication/summing module  76  of  FIG. 2 , resulting in summed-product signals  216   a - 216 N, respectively, each having a respective plurality of summed-product values. 
   The summed-product signals  216   a - 216 N can be received by optional low pass filter modules  218   a - 218 N, respectively, which can generate filtered signals  220   a - 220 N, respectively. The filtered signals  220   a - 220 N, or in other arrangements, the summed-product signals  216   a - 216 N, can be received by spectrum analyzers  222   a - 222 N, respectively. The spectrum analyzers  222   a - 222 N can generate frequency domain signals  224   a - 224 N (or frequency spectra), respectively. The frequency domain signals  224   a - 224 N can be received by feature detectors  226   a - 226 N, respectively. Each one of the feature detectors  226   a - 226 N can identify one or more features (e.g., spectral lines) in a respective frequency domain signal  224   a - 224 N, resulting in feature signals  228   a - 228 N, respectively. A multipath delay association processor  230  can receive two or more of the feature signals  228   a - 228 N. 
   Operation of the multipath delay association processor  230  will be better understood from the discussion below in conjunction with  FIG. 4 . However, let it suffice here to say that the delay association processor  230  can identify similarities among the feature signal  228   a - 228 N, and therefore, can identify which of the frequency spectra  224   a - 224 N were likely to have originated from the same vessel. The multipath delay association processor can generate an association signal  232  accordingly, which can be used by further processing (not shown) in order to detect, localize, and classify the vessel. 
   The multipath delay association processor  230  can also apply Doppler corrections to the feature signals  228   a - 228 N. The Doppler corrections are discussed more fully below in conjunction with  FIG. 4 . 
   Referring now to  FIG. 3A , a feature detector  234  can be the same as or similar to one of the feature detectors  226   a - 226 N of  FIG. 3 . The feature detector  234  can include a threshold generator  238  coupled to receive a frequency domain signal  236 , which can be the same as or similar to one of the frequency domain signals  224   a - 224 N of  FIG. 3 . The threshold generator  238  can generate a threshold signal  240 . A threshold comparison module  240  can receive the threshold signal  240  and the frequency domain signal  236  and can compare the frequency domain signal  236  with the threshold signal  240 , resulting in a feature signal  244 , which can be the same as or similar to one of the feature signals  228   a - 228 N of  FIG. 3 . 
   The threshold generator  238  can select a threshold in a variety of ways. For example, the threshold generator  238  can select a signal threshold level based upon an average of the frequency domain signal  236 . In other arrangements, the threshold generator  238  can select a threshold to be a predetermined number of decibels above the frequency domain signal (excluding spectra line or features) across a frequency band. In yet other arrangements, the threshold generator  238  can select a threshold to be a predetermined number of decibels above the frequency domain signal (excluding spectra line or features) across a frequency band (e.g., one to ten Hz) and another predetermined number of decibels above the frequency domain signal in another frequency band (e.g., ten to twenty Hz). In some arrangements, the above-described predetermined numbers of decibels are statically defined and in other arrangements, the predetermined numbers of decibels are dynamically defined. For example, the predetermined numbers of decibels can be related to a variance across a band of the frequency domain signal  236 , such that a higher variance results in a higher predetermined number of decibels. 
   Referring now to  FIG. 4 , graphs  250 ,  260 ,  270  include horizontal scales in units of frequency in Hz and vertical scales in units of amplitude in arbitrary units. A curve  252  is indicative of a frequency domain signal, for example, the frequency domain signal  224   a  of  FIG. 3 . A curve  264  is indicative of a threshold generated by and used by the feature detector  226   a  of  FIG. 3 . Spectral lines  252   a ,  252   b ,  252   c  are indicative of features detected by the feature detector  226   a , which are above the threshold  254 . 
   A curve  262  is indicative of another frequency domain signal, for example, the frequency domain signal  224   b  of  FIG. 3 . A curve  264  is indicative of another threshold generated by and used by the feature detector  226   b  of  FIG. 3 . Spectral lines  262   a ,  262   b ,  262   c  are indicative of features detected by the feature detector  226   b , which are above the threshold  264 . 
   A curve  272  is indicative of another frequency domain signal, for example, the frequency domain signal  224 N of  FIG. 3 . A curve  274  is indicative of another threshold generated by and used by the feature detector  226 N of  FIG. 3 . Spectral lines  272   a ,  272   b ,  272   c  are indicative of features detected by the feature detector  226 N, which are above the threshold  274 . 
   It will be apparent that the features  252   a ,  252   b ,  252   c , which occur at frequencies f 1 , f 2 , and f 3 , have similarity to the features  272   a ,  272   b ,  272   c , which can also occur at (or near) the frequencies f 1 , f 2 , and f 3 . Therefore, the multipath delay association processor  280  of  FIG. 3  can identify that the two spectra  252  and  272  likely originated from the same vessel, whereas the spectrum  262 , which has spectral lines at different frequencies, did not. 
   The frequencies of the features  252   a ,  252   b ,  252   c  need not be at exactly the same frequency as the features  272   a ,  272   b ,  272   c  in order to identify that sound signal associated with the features  252   a ,  252   b ,  252   c  originated from the same vessel as the features  272   a ,  272   b ,  272   c . For example, in some arrangements, a calculated or predetermined frequency ratio threshold is used, so that the features  252   a ,  252   b ,  252   c  are deemed to have come from the same vessel as the features  272   a ,  272   b ,  272   c  if frequency ratios between corresponding features ( 252   a  and  272   a ,  252   b  and  272   b ,  252   c  and  272   c ) are less than the calculated or predetermined frequency ratio threshold. In some arrangements, more than one calculated or predetermined frequency ratio threshold is used, so that the frequency ratios between features  252   a  and  272   a ,  252   b  and  272   b ,  252   c  and  272   c  must meet different threshold criteria in order to deem that the spectra  252  and  272  originated from the same vessel. Use of calculated or predetermined frequency ratio thresholds is particularly useful in view of Doppler shifts and corrections thereof described more fully below. 
   It will be appreciated that each one of the spectra  252 ,  262 ,  272  can be associated with a particular respective time delay. For example, the spectrum  252  can be associated with the time delay T 1  of  FIG. 3 , the spectrum  262  can be associated with the time delay T 2 , and the spectrum  272  can be associated with the time delay TN. It will be further understood that each one of the time delays T 1 -TN of  FIG. 3  is associated with a particular propagation path of sound as it traverses from a vessel to one of the systems  50 ,  100 ,  150 ,  200  of  FIG. 2 ,  2 A,  2 B, or  3 . 
   As is known, sound travels in a variety of paths as it traverses through water. For example, on a direct path, D, the sound travels directly from a source to a receiver. On a surface reflected path (SR), the sound travels from the source to the ocean surface, where it generally reflects, traveling downward to the sound receiver. On a bottom reflected path, BR, the sound travels from the source to the ocean bottom, where it generally reflects, traveling upward to the sound receiver. On each path, the sound experiences a different time delay and possibly a phase shift. Knowledge of the relative time delays may be used to identify a depth of the sound source, i.e., and the vessel. Therefore, knowledge of the time delays, the associated propagation paths, and the associated receive angles of sound propagating from the vessel to the sound receiver can be used not only to distinguish a submarine from a surface vessel, but also to localize a depth, and in some cases, a range, to the vessel. 
   Some methods and systems that can be used to localize the vessel in range and/or in depth are described, for example in U.S. patent application Ser. No. 11/422,435, entitled Methods and Systems for Passive Range and Depth Localization, filed Jun. 6, 2006, which application is incorporated herein by reference in its entirety. 
   While the spectral lines at the frequencies f 1 , f 2 , and f 3  in the spectrum  252  are shown to be the same frequencies f 1 , f 2 , and f 3  in the spectrum  272 , it should be recognized that the frequencies, which arrive on different sound paths and therefore on different angles, may be differently affected by Doppler shift resulting from a relative speed between the detected vessel and the platform on which the systems  50 ,  100 ,  150 , or  200  are disposed. It will also be understood that an absolute frequency shift due to the Doppler effect is proportional to the frequency of a feature. However, because the multipath delay association processor  230  of  FIG. 303  has knowledge of the spectral feature time delays, the associated sound propagation paths, and therefore, the arrival angle of the sound on the sound paths, in some arrangements, the multipath delay association processor  230  operates to adjust the feature signals  228   a - 228 N according to one or more estimated relative speeds between the detected vessel and the platform on which the sonar system is disposed. For each estimated relative speed, the frequency of the adjusted spectral features can be compared. 
   All references cited herein are hereby incorporated herein by reference in their entirety. 
   Having described preferred embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may be used. It is felt therefore that these embodiments should not be limited to disclosed embodiments, but rather should be limited only by the spirit and scope of the appended claims.