Broadband acoustic doppler current profiler

Abstract

A system and method for measuring current velocities using coded-pulse broadband acoustic signals. Autocorrelation of two phase coded pulses which are in the water during a single transmission cycle is used to calculate a Doppler frequency. The effective result is current profilers having improved profiling range and spatio-temporal resolution.

What is claimed is: 
     
       1. A velocity measuring system, comprising: a transmitting transducer for producing a beam pointing in a direction along which a phase change is measured;   pulse transmission means for providing a pulse train to the transmitting transducer, the pulse train comprising at least first and second emitted pulses having a predetermined pulse separation;   mean for complex sampling in echo return of the emitted pulses received by a receiving transducer so as to provide a first set of complex samples;   means for displaying the first set of complex samples by a selected time lag thereby producing a second set of complex samples;   means for computing a measured value of complex correlation using at least a portion of the first set of complex samples and at least a portion of the second set of samples; and   means for obtaining a velocity measurement based on a Doppler frequency calculated from the complex correlation.   
     
     
       2. The system defined in claim 1, wherein each pulse is coded. 
     
     
       3. The system defined in claim 2, wherein the code comprises phase coding. 
     
     
       4. The system deemed in claim 3, wherein the phase coding includes 0°and 180°phase codes. 
     
     
       5. The system defined in claim 1, wherein the transmitting transducer and the receiving transducer are together a unitary transducer. 
     
     
       6. The system defined in claim 1, wherein the transmitting transducer and the receiving transducer are separate structures. 
     
     
       7. The system defined in claim 1, wherein the beam emits over a water column so that a plurality of velocity measurements are made at varying depths. 
     
     
       8. The system defined in claim 1, wherein the beam is an acoustic signal. 
     
     
       9. The system defined in claim 1, wherein the lag time is equal to the length of a selected pulse. 
     
     
       10. The system defined in claim 1, wherein the pulses are the same length. 
     
     
       11. The system defined in claim 1, wherein the lag time includes a time interval when the transducer is not transmitting. .[.12. A method of deriving a measure of the relative velocity of a signal source-sensor combination and a field of scatterers separated therefrom by a medium through which the signal is propagated, wherein the source-sensor combination includes a transducer, the method comprising the steps of: energizing the transducer to emit into the medium towards the field of scatterers a signal comprising a plurality of pulses having a predetermined separation, wherein the pulses include at least a first and second pulse;   sensing a signal comprising a set of echo returns of the pulses reflected from the field of scatterers;   complex sampling the echo return signal at preselected sampling intervals so as to provide a first set of complex samples;   delaying the first set of complex samples by a predetermined lag time so as to provide a second set of complex samples;   presenting to a complex correlator the first and second sets of samples wherein the fast set consists primarily of echoes from the first pulse and the second set consisting primarily of echoes from the second pulse, and forming as outputs the complex products of members of the first set with complex conjugates of members of the second set, to produce a complex correlation value; and   obtaining a Doppler frequency based on the phase change calculated from the complex correlation value..]..[.13. The method of claim 12, additionally comprising the step of calculating a velocity component of the scatterers using the Doppler frequency..]..[.14. The method of claim 12, wherein the pulses are coded..]..[.15. The method of claim 12, wherein the step of delayed samples comprises storing the first set of samples in a memory..]..[.16. A current profiler, comprising:   a transducer for transmitting and a transducer for receiving an acoustic signal having fast and second coded pulses separated by a preselected lag time wherein the coded pulses are in water together for at least a portion of a selected time interval;   a sampler for sampling quadrature components of a received signal over a time interval which depends on the selected range cell, wherein the received signal comprises echoes of the coded pulses;   means for calculating the autocorrelation of the sampled quadrature components;   means for determining the Doppler frequency of the received signal comprising means for calculating the phase change of the acoustic signal from the autocorrelation result; and   
     
     
       means for calculating a velocity using the Doppler frequency..].17. .[.The.]. .Iadd.A .Iaddend.current profiler.Iadd., .Iaddend..[.defined in claim 16.]. .Iadd.comprising: a transducer for transmitting and a transducer for receiving an acoustic signal having first and second coded pulses separated by a preselected lag time wherein the coded pulses are in water together for at least a portion of a selected time interval;   a sampler for sampling quadrature components of a received signal over a time interval which depends on the selected range cell, wherein the received signal comprises echoes of the coded pulses;   means for calculating the autocorrelation of the sampled quadrature components;   means for determining the Doppler frequency of the received signal comprising means for calculating the phase change of the acoustic signal from the autocorrelation result; and   means for calculating a velocity using the Doppler frequency.Iaddend., wherein a plurality of transducers are configured such that a plurality of   
     
     
        orthogonal velocity components can be obtained. 18. The current profiler defined in claim 17, wherein the transducers are arranged in a Janus 
     
     
        configuration. 19. The current profiler defined in claim .[.16.]. .Iadd.17.Iaddend., wherein the sampler comprises control means for 
     
     
        limiting voltage offsets. 20. The current profiler defined in claim .[.16.]. .Iadd.17.Iaddend., wherein the velocity calculating means comprises means for normalizing the velocity to fixed earth reference 
     
     
        coordinates. 21. The current profiler defined in claim .[.16.]. .Iadd.17.Iaddend., wherein the selected time interval depends on a round-trip time between scatterers at a selected range and the current 
     
     
        profiler. 22. The current profiler defined in claim .[.16.]. .Iadd.17.Iaddend., wherein the transmitting and receiving transducers are 
     
     
        together a unitary transducer. 23. The current profiler defined in claim .[.16.]. .Iadd.17.Iaddend., wherein the transmitting and receiving 
     
     
        transducers are separate structures. .[.24.  A Doppler sonar system for providing velocity measurements, comprising: a transducer;   a pulse generator communicating to the transducer two or more coded pulses of preselected length separated by a preselected lag time;   a complex sampling circuit connected to the transducer so as to provide quadrature components of a received echo signal; and   a processor including autocorrelation means for generating the autocorrelation between a first set of quadrature samples and a second set of quadrature samples delayed by the lag time, the processor also including means for obtaining a velocity measurement based on a Doppler   
     
     
        frequency calculated from the autocorrelation..]..[.25.  The system defined in claim 24, wherein the autocorrelation means comprises a digital 
     
     
        signal processor..].26. .[.The.]. .Iadd.A Doppler sonar .Iaddend.system .[.defined in claim 24.]. .Iadd.for providing velocity measurements, comprising: a transducer;   a pulse generator communicating to the transducer two or more coded pulses of preselected length separated by a preselected lag time;   a complex sampling circuit connected to the transducer so as to provide quadrature components of a received echo signal; and   a processor including autocorrelation means for generating the autocorrelation between a first set of quadrature samples and a second set of quadrature samples delayed by the lag time, the processor also including means for obtaining a velocity measurement based on a Doppler frequency calculated from the autocorrelation.Iaddend., wherein a plurality of transducers receive independently transmitted signals so as   
     
     
        to measure a plurality of orthogonal velocity components. 27. The system defined in claim 26, wherein the transducers are configured to generate 
     
     
        non-interfering signals. 28. The system defined in claim .[.24.]. .Iadd.26.Iaddend., wherein the coded pulses include one or more code elements, each code element defined by a predetermined portion of a carrier signal which is phase modulated according to one of a preselected 
     
     
        set of phase codings. 29. The system defined in claim .[.24.]. .Iadd.26.Iaddend., wherein the lag time is selected according to a 
     
     
        predetermined accuracy of range-velocity resolution. 30. The system defined in claim .[.24.]. .Iadd.26.Iaddend., wherein the coded pulse length is selected according to a predetermined accuracy of range 
     
     
        resolution. 31. The system defined in claim .[.24.]. .Iadd.26.Iaddend., 
     
     
        additionally comprising means for measuring amplitude. 32. The system defined in claim 31, wherein the amplitude measuring means comprises means 
     
     
        for determining backscatter strength. 33. The system defined in claim 31, wherein the amplitude measuring means comprises means for determining 
     
     
        particle concentration. 34. The system defined in claim 31, wherein the 
     
     
        amplitude measuring means comprise means for measuring particle flux. 35. The system defined in claim 28, wherein the code elements of a second 
     
     
        coded pulse are inverted with respect to a first coded pulse. 36. The system defined in claim 35, wherein the code elements are selected so that: (1) the code has zero autocorrelation at one lag time to each side of the sidepeak;   (2) the code has less than a preselected number of sidelobes near the sidepeak; and   
     
     
       (3) pairs of Golay codes are used in successive coded pulses. .Iadd.37. The system defined in claim 26, wherein the autocorrelation means comprises a digital signal processor. .Iaddend.