Patent Publication Number: US-2013235699-A1

Title: System and method of range estimation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/076,168, filed Mar. 30, 2011, which is a continuation of U.S. patent application Ser. No. 12/191,196, filed Aug. 13, 2008, each of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     This application generally relates to acoustic range estimation, and in particular to sonar range estimation using multi-beam devices. 
     2. Description of the Related Art 
     A current profiler is a type of sonar system that is used to remotely measure water velocity over varying ranges. Current profiles are used in freshwater environments such as rivers, lakes, and estuaries, as well as in saltwater environments such as the ocean, for studying the effects of current velocities. The measurement of accurate current velocities is important in such diverse fields as weather prediction, biological studies of nutrients, environmental studies of sewage dispersion, and commercial exploration for natural resources, including oil. 
     Typically, current profilers are used to measure current velocities in a vertical column of water for each depth “cell” of water up to a maximum range, thus producing a “profile” of water velocities. The general profiler system includes a transducer to generate pulses of sound (which when down-converted to human hearing frequencies sound like “pings”) that backscatter as echoes from plankton, small particles, and small-scale inhomogeneities in the water. The received sound has a Doppler frequency shift proportionate to the relative velocity between the scatters and the transducer. 
     The physics for determining a single velocity vector component (v x ) from such a Doppler frequency shift may be concisely stated by the following equation: 
     
       
         
           
             
               
                 
                   
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     In equation (1), c is the velocity of sound in water, about 1500 meters/second. Thus, by knowing the transmitted sound frequency, f T , and declination angle of the transmitter transducer, θ, and measuring the received frequency from a single, narrowband pulse, the Doppler frequency shift, f D , determines one velocity vector component. Relative velocity of the measured horizontal “slice”, or depth cell, may be further determined by subtracting out a measurement of vessel earth reference velocity, v e . Earth reference velocity can be measured by pinging the ocean bottom whenever it comes within sonar range or by a navigation system such as LORAN or GPS. 
     Commercial current profilers are typically configured as an assembly of four diverging transducers, spaced at 90° azimuth intervals from one another around the electronics housing. This transducer arrangement is known in the technology as the Janus configuration. A three-beam system permits measurements of three velocity components, v x , v y  and v z  (sometimes identified respectively as u, v, w in oceanographic literature) under the assumption that currents are uniform in the plane perpendicular to the transducers mutual axis. However, four beams are often used for redundancy and reliability. The current profiler system may be attached to the hull of a vessel, remain on stationary buoys, or be moored to the ocean floor. 
     Of particular importance to the vessel-mounted current profiler is the accurate determination of vessel velocity. The earth reference water velocities can then be calculated by subtracting out the vessel velocity. As is well-known, the movement of the vessel with respect to the earth is based on establishing at least two fixed reference points over a period of time. In a current profiler, one common technique to find the bottom is to interleave a bottom range pulse with the current velocity pulses. The bottom range pulse is generally of a longer duration than other pulses so as to fully ensonify the bottom. The length of the pulse may be chosen according to the assumed maximum depth and the angle subtended by the transducer. 
     In some existing current profilers the decision-making for bottom detection has been based on a simple comparison between received signal amplitude and a threshold value. While performing reasonably well, these systems may produce “false bottoms” as a result of strong inhomogeneities or life layers, such as plankton or schooling fish, which offer alternative sources of acoustic reflection. Thus, it will be readily appreciated that false bottoms, located at ranges from the transducer that are less than the range to the actual bottom, can lead to inaccurate range and velocity measurements. 
     Accordingly, more accurate sonar systems to detect the bottom of a body of water are desired. In particular, a sonar system that minimizes the detection of false bottoms will improve the quality of vessel and water velocities. It would be a further improvement if the sonar system could compensate for signal losses due to water absorption and spreading. 
     SUMMARY OF THE CERTAIN INVENTIVE ASPECTS 
     One aspect of the invention is a method of estimating range to a surface of a body of water, the method comprising transmitting at least one acoustic signal through the body of water, receiving a plurality of signals reflected from the surface, and estimating a distance to the surface based on data indicative of the received signals and an estimate of sidelobe coupling. 
     Another aspect of the invention is a range estimation system relating to transmission and reception of acoustic signals in a fluid medium, comprising a sonar system having a plurality of transducers configured to generate respective acoustic beams and to receive echoes from the beams, and a signal processor configured to estimate a distance based on data indicative of the received echoes and an estimate of sidelobe coupling of a selected one of the beams with at least one other of the beams. 
     Another aspect of the invention is a computer-program product for use in a sonar system, the product comprising a computer-readable medium having stored thereon codes executable by at least one processor to receive data indicative of signals reflected from a surface of a body of water in response to at least one transmitted acoustic pulse, and estimate a distance to the surface based on the data indicative of the reflected signals and an estimate of sidelobe coupling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is perspective view of a download-looking current profiler having a Janus configuration of transducers, wherein the current profiler is attached to the hull of vessel. 
         FIG. 2  is functional block diagram of a range estimation system. 
         FIG. 3  is a flowchart illustrating a method of detecting a range. 
         FIG. 4  is a flowchart illustrating a method of receiving a reflected signal. 
         FIG. 5  is a flowchart illustrating a method of applying a linear transform. 
         FIG. 6  is a flowchart illustrating a method of bottom detection. 
         FIGS. 7A and 7B  are diagrams illustrating the transmitted waveforms from two beams. 
         FIGS. 8A and 8B  are diagrams illustrating the received waveforms after the transmitting beams of  FIGS. 7A and 7B  have reflected off a surface. 
         FIGS. 9A and 9B  are diagrams illustrating waveforms based on linear transforms of the waveforms of  FIGS. 8A and 8B . 
         FIGS. 10A-10D  are diagrams of waveforms based on various methods of linear transformation. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
     Reference is now made to the drawings wherein like numerals refer to like parts throughout. 
     In other sonar systems including, for instance, depth sounders, bottom mapping sonars, sidescan sonars, speed logs and correlation logs, matched filtering techniques (or equivalent correlation techniques) have been used to minimize the number of false bottoms. Matched filtering is a technique that applies a signal to a linear filter so as to statistically determine the existence of a signal of interest. Prior approaches compare the output of the standard matched filter with a predetermined threshold value and thus they may still detect false bottoms. In addition, other systems generally do not account for the principal sources of signal loss in water. 
     Several sources of signal loss may produce errors whenever a sonar echo is compared with an absolute reference value or threshold. For example, unlike electronic emissions propagating through air, sound waves traveling in water are subject to water absorption losses due to thermal effects. Further, due to signal spreading (intuitively akin to the spreading ripples which emanate from a rock thrown into a pond), the strength of the transmitted signal is inversely proportional to the square of the range. Hence, these sources of signal loss will also affect any comparison of a filtered signal with a threshold value. 
     Sidelobe interference can cause bottom detection to be more difficult because bottom returns from other beams are mixed in with, or coupled with, the return from a beam oriented to receive a bottom return (e.g., the beam of interest for bottom detecting). Particularly, when a transducer is not perpendicular to the bottom, interfering returns may be at ranges other than the range of the beam of interest and thus may result in poor estimates being made by a detection filter. In applications, such as Doppler velocity logs, incorrect measurement of bottom range can result in incorrect velocity measurements. 
     In one embodiment, a method of range estimation comprises transmitting at least one acoustic signal through a body of water, receiving at least two reflected signals from a surface, generating data based on the reflected signals and an estimate of sidelobe coupling, and estimating the distance to the surface based on the generated data and sidelobe estimate. In particular, in one embodiment, the surface is a surface at the bottom of the body of water. 
       FIG. 1  illustrates a current profiler  100  which is attached to the hull of a moving vessel  102 . The current profiler  100 , as shown in  FIG. 1 , generates a set of acoustic beams  104   a ,  104   b ,  104   c ,  104   d  which emanate from one or more transducers. An exemplary current profiler is disclosed in U.S. Pat. No. 5,208,785, which is hereby incorporated by reference. In the illustrated embodiment, the current profiler  100  is downward-looking, that is, the acoustic beams  104  are directed in a generally vertical orientation towards an acoustically reflective surface such as the ocean bottom  106  and the beams  104  are in a Janus configuration. Each beam  104  “illuminates” a water column which can be decomposed into horizontal slices known as range, or depth, cells such as the cell indicated at  107 . By suitable transmission of acoustic beams and reception of resulting echoes received from the cells, the echo data can be transformed into a Doppler frequency, a velocity along the beam  104 , and then one or more orthogonal current velocity components such as those indicated at  108 . 
     Since the vessel  102  is moving in the illustrated embodiment, the measured velocity of the range cell  107  is relative to the velocity of the vessel  102 . Therefore, a bottom range pulse is periodically interleaved in the beams  104  to determine the orthogonal velocity components of the vessel such as those indicated at  110 . The earth reference velocity of the range cell  107  is then obtained by subtracting the velocity of the vessel  102  from the measured vessel reference velocity of the range cell  107 . 
     Although bottom tracking using a downward looking current profiler  100  is described herein, it is to be recognized that other uses may be made of the methods and systems described herein. For instance, embodiments may include, for example, an upward looking configuration to measure the movement of sheets of ice in one of the polar regions. 
     It is important to note that in measuring the range (R) between the vessel  102  and the bottom  106 , a life layer  112  such as a layer of plankton or schooling fish may also reflect the transmitted pulse with a relatively high signal strength causing detection of a “false bottom.” Also important to note is the possible presence of sidelobes in the received signals. When a signal, such as a bottom range pulse is transmitted in a particular beam oriented to detect the bottom, the reflected signal is most strongly received in the corresponding received beam, but also weakly received in the other received beams. When multiple signals are transmitted from each of the beams, the received signals from each beam may comprise a strong component corresponding to the reflected signal transmitted from the beam combined (coupled) with weaker components corresponding to reflected signals transmitted from the other beams. The weaker sidelobe components can constructively or destructively interfere with the strong, main component resulting in less accurate measurements. 
     Typically, the sidelobe components are, but not always, at least 20 dB weaker than the main component, however, this number can vary substantially with the configuration of the system. Typical piston transducers exhibit approximately 45 dB of sidelobe rejection, while a phased array transducers may exhibit from 30 dB to 33 dB of sidelobe rejection. 
       FIG. 2  is functional block diagram of a range estimation system  200  designed to compensate for the coupling with sidelobes in the various beam measurements. In one embodiment, the system  200  is integrated with the current profiler  100 . The range estimation system  200  comprises a number of transducers  210   a ,  210   b ,  210   c ,  210   d , each corresponding to a particular beam. A side lobe compensation module  220  receives return data from the transducers  210  and provides compensated data to one or more range estimation modules  230 . The transducers  210  generally transmit bursts of sound waves called pings, and/or receive waveforms such as echoes in response to the transmitted pings. In one embodiment, the received waveforms are fed into a sidelobe compensation module  220  which performs at least a linear transformation of the input waveforms to produce at least one output waveform. The sidelobe compensation module  220  may, for example, comprise a number of amplifiers incorporated into adders  222   a ,  222   b ,  222   c ,  222   d  in order to perform a linear transformation. The sidelobe compensation module  220  may be configured to perform other related functions, and therefore may include linear functional blocks such as delays and non-linear functional blocks such as a thresholding block. The one or more output waveforms are input into one or more range estimation modules  230   a ,  230   b ,  230   c ,  230   d  that produce an output indicative of the range, which is derived from the waveforms output from the sidelobe compensation module  220 . The output may be displayed, printed, stored, transmitted, or otherwise communicated via an output module  240 . 
     The sidelobe compensation module  220 , the range estimation module(s)  230 , the output module  240 , and/or other illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any suitable combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in any suitable computer readable medium such as a volatile or non volatile memory such as a DRAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of suitable storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC or in any suitable commercially available chipset. 
       FIG. 3  is a flowchart illustrating such a method of detecting a range. The method  300  begins, in block  400 , in which the transducers  210  receive reflected signals from two or more beams. Next at a block  500 , a processor such as the sidelobe compensation module  220  performs a linear transformation on data derived from the reflected signal. Moving to block  600 , the range estimation modules  230  perform range estimation on the transformed data. Further details of each of the blocks  400 ,  500 , and  600  is set forth below with reference to  FIGS. 4 ,  5 , and  6 . 
       FIG. 4  is a flowchart illustrating in more detail one embodiment of the block  400  of  FIG. 3  of receiving a reflected signal. The method  400  begins, in block  410 , by transmitting at least one pulse. In some embodiments, four pulses are simultaneously transmitted from four transducers in a Janus beam configuration. In block  420 , the transducers  220  receive signals from two or more beams, which are then converted into amplitude data. In some embodiments, the amplitude data is received in four beams by the above-described four transducers in a Janus beam configuration. In other embodiments, a pulse is transmitted from one transducer and received on two transducers. The transducers involved in transmitting the pulse in block  410  are not necessarily the same transducers that receive amplitude data in block  420 . In block  430 , the amplitude data is digitized into equivalent dB counts. This may involve a logarithmic transformation and quantization. In other embodiments of the invention, the method  400  lacks blocks  410  and/or  430 . 
       FIG. 5  is a flowchart illustrating in more detail one embodiment of the block  500  of  FIG. 3  of applying a linear transformation such as using the sidelobe compensation module  220 . In general, a linear transformation of waveforms involves summing weighted versions of the waveforms to produce one or more transformed waveforms. This can be performed in software executed by a processor, or electronic hardware, using well-know matrix-vector multiplication methods. The method  500  illustrated in  FIG. 5  begins, in block  510 , by converting counts (logarithmic) to power (linear) using known hardware scale factors. In block  520 , interfering beams&#39; power is subtracted based on an estimate of sidelobe coupling between the beams. Subtracting the interfering beams power is a form of linear transformation of the beams. In one embodiment, the estimate of sidelobe coupling comprises an a priori estimate of sidelobe coupling. The a priori estimate of sidelobe coupling may be measured in situ or pre-programmed into the system. For example, a pulse may be sent from a single beam at a time the other beams are kept silent. The reflected signal should be strongest on the beam which sent the signal, but the other beams will record weaker versions of the reflected signal corresponding to the sidelobe coupling. This measurement could be performed in a round-robin fashion to measure the full sidelobe coupling matrix. In other embodiments, it is assumed that certain sidelobe coupling factors are identical based on the geometry of the system. For example, in a Janus beam configuration, it may be assumed that the coupling between a beam and the two closest beams is identical. In other embodiments, the sidelobe coupling is not known a priori, but based, at least in part, on the reflected signal. For example, principal component analysis (PCA), or independent component analysis (ICA), could be used to determine the sidelobe coupling based on the reflected signal. The use of PCA or ICA could directly perform the linear transformation without determining a sidelobe coupling. 
     In some circumstances, such as when the sidelobe coupling is inaccurate, the linear transformation can result in values for the transformed waveform which are physically unlikely. As mentioned, a received waveform may comprise a main component and sidelobe components. Additionally, the received waveform may comprise a noise component, which is generally of constant power. A linear transformation to remove the sidelobe components should not result in a waveform lacking the noise component. Thus, any values less than the constant power of the noise, the so-called noise floor, such be replaced with the noise floor or some other appropriate value, which is described in block  530 . 
       FIG. 5  is a flowchart illustrating in more detail one embodiment of the block  600  of estimating range such as using one or more of the range estimates 230. In block  610 , the method begins by converting power (linear) back to counts (logarithmic) for input into a detection filter. In block  620 , the transformed waveforms are convolved with a detection filter. The detection filter may be, in some embodiments, a matched filter, such as a filter matched to transmitted ping. The detection filter may also be a modified version of the transmitted ping, such as a version of the ping of slightly longer duration, or a derivate of the transmitted ping. One embodiment of a suitable detection filter is described in U.S. Pat. No. 5,112,990, entitled “Bottom Tracking System,” which is hereby incorporated by reference in its entirety. In block  630 , a search is performed for the highest peak of the result of convolving the transformed waveform with the detection filter. In block  640 , the value of this peak is compared to a threshold. If the value of this peak is below the threshold, the method  600  moves to block  650  where it is indicated that the bottom was not found. If the value of this peak is above a threshold, the bottom has been detected and the method  600  moves to block  660  where the range is estimated. The range may be estimated, for example, based on the position of the peak. In general, the travel time of the ping from the transducer to the bottom back to the transducer, the declination angle of the transmitter/receiver, the speed of sound in the medium (corrected for refraction), etc. may be used to estimate the range from the transducer to the bottom. 
     An embodiment is now described with respect to exemplary waveforms generated using, for example, the system  200 . The waveforms shown in  FIGS. 7-10  are greatly exaggerated for instructive purposes, and are not to scale. Each waveform is a plot of power versus time in a logarithmic scale.  FIGS. 7A and 7B  are diagrams illustrating the transmitted waveforms from two interfering beams. The waveform  700  transmitted by Beam 1 comprises a pulse  702  of duration T 0  seconds. The waveform  750  transmitted by Beam 2 comprises a similar pulse  750  of the same duration. Each waveform  700 , 750  may be transmitted in slightly different directions, such as in the case in two beams in a Janus configuration. After propagating through a medium, such as a body of water, and a beam is reflected off a surface, such as a bottom surface of an ocean or other body of water, a waveform is received by each beam. 
       FIGS. 8A and 8B  are diagrams illustrating the received waveforms after the transmitting beams of  FIGS. 7A and 7B  have reflected off a surface in a body of water. The waveform  800  received on Beam 1 comprises a main component and a sidelobe component from Beam 2, and shows a general exponential decay due to the lossy propagation in the body of water. The main component is first received at time t 1 , and is of duration T 1 . The sidelobe component is first received at time t 2  and is of duration T 2 . Thus, the received waveform between t 1  and t 2    802  consists only of the main component. The received waveform between t 2  and t 1 +T 1    804  consists of both the main component and the sidelobe component. Between t 1 +T 1  and t 2 +T 2    806 , the waveform consists of only the sidelobe component. After a certain amount of time, the received waveform has decayed into the noise floor  808 . Similarly, the waveform  850  received on Beam 2 comprises both a main component and a sidelobe component. Between t 1  and t 2    851 , only the sidelobe component from Beam 1 is received. Between t 2  and t 1 +T 1    854 , both components interfere (in this case, constructively). Between t 1 +T 1  and t 2 +T 2    856 , the waveform consists on only the main component. Eventually, the received waveform decays into the noise floor  858 . 
     As discussed above with reference to block  300  of  FIG. 3 , a linear transformation is performed on the waveforms such as illustrated by traces  800 , 850  of  FIGS. 8A and 8B  to generate the waveforms such as illustrated by traces  900 , 950  of  FIGS. 9A and 9B . Mathematically, the received waveforms  800 , 850 , denoted y 1  and y 2  respectively can be expressed by equations 2 and 3: 
         y   1   =A+αB,   (2)
 
         y   2   =αA+B,   (3)
 
     where A and B are the returns due to the transmitted waveforms  700 , 750  of Beam 1 and Beam 2, respectively, and α is a sidelobe coupling. Transformed waveforms, A and B can be expressed respectively, by weighting and adding y 1  and y 2  appropriately, as shown below in equations 4 and 5. 
     
       
         
           
             
               
                 
                   
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     Equations 2 and 3 shown above neglect the noise floor. 
       FIGS. 10A-10D  are diagrams illustrated waveforms based on various linear transformations with noise considered. The waveform  1000  shown in  FIG. 10A  is a transformed waveform after an ideal transformation showing only the return due to the transmitted waveform  700  of Beam 1. In particular, such a result is given by the use of the constants shown in Equations 4 and 5, assuming the sidelobe coupling given in Equations 2 and 3 is accurate. 
       FIG. 10B  illustrates a transformed waveform  1010  that may result if the estimate of sidelobe coupling or the transformation is inaccurate. In particular, the waveform  1010  is the result of inaccurately transforming the waveforms  800 , 850  shown in  FIGS. 8A and 8B . Between times t 1  and t 2    1012 , only the main component is present. Between times t 2  and t 1 +T 1    1014 , both the main component and the sidelobe component is present. This portion of the waveform  1014  differs from the portion of the waveform  804  shown in  FIG. 8A  in that the sidelobe component is somewhat reduced, but not entirely removed as in the waveforms of  FIG. 9A  or  10 A. Similarly, between times t 1 +T 1  and t 2 +T 2    1016 , the waveform consists on only the sidelobe component, which has been reduced but not eliminated. 
       FIG. 10C  illustrates another transformed waveform  1020  that may result if the estimated sidelobe coupling or the transformation is inaccurate. As with the waveform  1010  shown in  FIG. 10B , the waveform  1020  shown in  FIG. 10C  is the result of inaccurately transforming the waveforms  800 , 850  shown in  FIGS. 8A and 8B . Between times t 1  and t 2    1022 , only the main component is present. Between times t 2  and t 1 +T 11024 , both the main component and the sidelobe component is present. This portion of the waveform  1024  differs from the portion of the waveform  804  shown in  FIG. 8A  in that the sidelobe component is so reduced as to become a negative component, instead of being a zero component as in the waveforms of  FIG. 9A  or  10 A. Similarly, between times t 1 +T 1  and t 2 +T 2    1026 , the waveform consists on only the sidelobe component, which has been so reduced as to be a negative component. The sidelobe component has been so reduced as to be below the noise floor  1028 . As it is physically unlikely that a proper transformation would result in values below the noise floor, such values may be replaced with the noise floor, as shown in  FIG. 10D . The waveform  1030  shown in  FIG. 10D  is the result of overcompensating for sidelobe interference and replacing values below the noise floor with the noise floor. 
     As can be seen in the  FIG. 10A ,  10 B, and  10 C, even slightly inaccurate estimates of sidelobe coupling or transformations such as provided by embodiments described herein can have beneficial results, as the undercompensated and overcompensated (with or without noise floor replacement) transformed waveforms are closer to the ideal waveform than the received waveforms without transformation. 
     While the above description has pointed out novel features of the invention as applied to various embodiments, the skilled person will understand that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made without departing from the scope of the invention. Therefore, the scope of the invention is defined by the appended claims rather than by the foregoing description. All variations coming within the meaning and range of equivalency of the claims are embraced within their scope.