Coherent Doppler Sonar Research Articles

Velocity Measurement of Coherent Doppler Sonar Assisted by Frequency Shift, Kalman Filter and Linear Prediction

Velocity is vital information for navigation and oceanic engineering. Coherent Doppler sonar is an accurate tool for velocity measurement, but its use is limited due to velocity ambiguity. Velocity measured by frequency shift has no velocity ambiguity, yet its measurement error is larger than that of coherent Doppler sonar. Therefore, coherent Doppler sonar assisted by frequency shift is used to accurately measure velocity without velocity ambiguity. However, the velocity measured by coherent Doppler sonar assisted by frequency shift is affected by impulsive noise. To decrease the impulsive noise, Kalman filter and linear prediction are proposed to improve the velocity sensing accuracy. In this method, the Kalman filter is used to decrease measurement error of velocity measured by frequency shift, and linear prediction is used to remove the impulsive noise generated by a wrong estimate of the integer ambiguity. Lab-based experiments were carried, and the results have shown that coherent Doppler sonar assisted by frequency shift, Kalman filter and linear prediction can provide accurate and precise velocity with short time delay in a large range of signal to noise ratio.

SwathDop: Multibeam Pulse-Coherent Doppler Sonar for Scanning 2D Velocity Sections near the Sediment–Water Interface

AbstractThis paper considers the problem of accurately measuring the sediment transport over bedforms where flow evolves continuously both in time and space. For this purpose, we have developed a pulse-to-pulse coherent Doppler sonar system designed in bistatic geometry with two fan-beam transmitters symmetrically positioned on each side of a multielement receive array. The system resolves 2D velocity components within a ±20° (~0.5 m by ~0.5 m) swath. The software-defined radio data acquisition and control system limited us at present to eight independent receiver channels, and consequently the azimuthal resolution of the system is 4°. As a preliminary test of the sonar system, the system operation was simulated using a model developed to predict coherent sonar performance. The uncertainties with respect to the prescribed values and mean measurements in the model results were confined to 0.35 and 0.23 cm s−1, respectively, in the presence of strong shear (~150 s−1) and 50 cm s−1 horizontal flow. An important thing is that the model allowed us to test and develop the signal processing algorithms necessary to invert the multibeam sonar data. Using sand of 0.4-mm median diameter, the laboratory trials were carried out in active sediment transport conditions over dunes with 2-m wavelength and ~0.90 m s−1 unidirectional flow velocities. The results presented here focus mainly on 2D velocity field and indicate an average 4% deviation from the wake law and 8% from independent observations made with the wide-band multifrequency coherent Doppler profiler (MFDop) instrument under similar flow conditions.

Robust Resolution of Velocity Ambiguity for Multifrequency Pulse-to-Pulse Coherent Doppler Sonars

Pulse-to-pulse coherent Doppler sonars are widely used to explore boundary layer characteristics of oceans. Multifrequency pulse-to-pulse coherent Doppler sonars have been proposed in the literature to tackle the velocity ambiguity problem faced by single-frequency systems. A robust algorithm for resolving the velocity ambiguity is crucial in such systems. This paper proposes a method based on the robust Chinese remainder theorem to resolve the velocity ambiguity for multifrequency pulse-to-pulse coherent Doppler sonars. We evaluate the proposed method for resolving the velocity ambiguity in terms of the trial fail rate. The simulations show that the proposed method achieves 1/8 trial failure rate as the reference method. A theoretical criterion is also derived to support the observation that in most cases our proposed method is more robust to estimation errors than the reference method.

Observations of turbulent kinetic energy dissipation at the Labrador Sea surface layer

We present an analysis of turbulent kinetic energy dissipation rates in the upper ocean using in situ measurements collected by a coherent Doppler sonar in the Labrador Sea during summer 2004. The sonar recorded horizontal velocity fluctuations of the upper 2 m with an uncommonly small spatial resolution of 0.8 cm, allowing direct calculations of wavenumber spectra and the application of Kolmogorov theory to obtain turbulent kinetic energy dissipation rates for the first time in this area. The project presented a unique opportunity for the study of air–sea exchange during a phytoplankton bloom, being the first time a specialized air–sea interaction spar buoy was deployed during such particular event. An additional uniqueness of this experiment resulted from being the first turbulent kinetic energy dissipation rate observations obtained at higher latitudes, coincidentally in a well-known region of dense water formation, with a fundamental role in both global circulation and forecasting studies of global climate change. Focusing on the relationship between turbulent kinetic energy dissipation rates and wave phase in the upper 2 m, we estimated O[Formula: see text] turbulent kinetic energy dissipation rates, consistent with previous estimates obtained through similar devices and methods. A T-test between dissipation rates calculated at the crest and at the trough of waves showed no dependency of turbulent kinetic energy dissipation rates on the wave phase at 2 m depth, coinciding with many of the earlier findings available. a comparison with previous research showing conflicting results with our values is also discussed here linking them to the relative roles of experimental design variations, diverse dynamical frames, and particular environmental conditions.

Combined Method of Conventional and Coherent Doppler Sonar to Avoid Velocity Ambiguity

Combined Method of Conventional and Coherent Doppler Sonar to Avoid Velocity Ambiguity

On the Distribution of Velocity Measurements From Pulse-to-Pulse Coherent Doppler Sonar

Knowledge of the velocity measurement distribution is useful for designing new coherent Doppler systems, for resolving velocity ambiguity, and for optimally combining measurements from multiple receivers and multiple carrier frequencies. In this paper, the effects of pulse-pair averaging and pulse-to-pulse autocorrelation are analyzed for ensembles consisting of up to ten pulse pairs. A formula is presented for the probability distribution of velocity measurements from a single pulse pair. This distribution is nonnormal for all values of the autocorrelation coefficient. In the limit of perfect pulse-to-pulse correlation, single pulse-pair measurements obey a Pearson Type VII distribution which has a higher peak and broader tails than a normal distribution. Second and fourth moments of the multiple pulse-pair measurement distributions are evaluated using a Monte Carlo method. The applicability of perturbation analysis for predicting velocity standard deviation is investigated. In the analysis of Zrnić (1977), it is assumed that perturbations in autocorrelation phase are small compared to the mean phase. Three prediction failure mechanisms are demonstrated through changes in the distribution of the complex autocorrelation coefficient; these occur for both high and low autocorrelation magnitude, and for short ensemble lengths. Measurement distributions from the Monte Carlo method are also used to determine the autocorrelation magnitude necessary to resolve velocity ambiguity using a dual-frequency coherent Doppler sonar. Simulations with a high-fidelity coherent Doppler sonar model show how the distribution of velocity measurements is affected by finite particle concentrations. While velocity standard deviation is insensitive to concentration above a threshold of approximately 1 particle per sample volume, kurtosis exhibits a strong dependence on concentration in the range of 0.1-5 particles per sample volume.

Simultaneous Velocity Ambiguity Resolution and Noise Suppression for Multifrequency Coherent Doppler Sonar

Abstract Coherent Doppler sonar is a useful tool for noninvasive measurement of ocean currents, sediment transport, and turbulence in coastal environments. Various methods have been proposed to separately address two of its inherent limitations: velocity ambiguity and measurement noise. However, in energetic turbulent flows, both factors may be present simultaneously. The presence of measurement noise complicates velocity ambiguity resolution, and conversely velocity ambiguity presents a challenge for existing noise suppression methods. A velocity estimator based on maximum a posteriori (MAP) estimation has been developed to resolve velocity ambiguity and suppress measurement noise simultaneously rather than separately. The estimator optimally combines measurements from multiple acoustic carrier frequencies and multiple transducers. Data fusion is achieved using a probabilistic approach, whereby measurements are combined numerically to derive a velocity likelihood function. The MAP velocity estimator is evaluated using a high-fidelity coherent Doppler sonar simulation of oscillating flow and data from a towing tank grid turbulence experiment where both velocity ambiguity and backscatter decorrelation were present. Time series and spectra from MAP velocity estimation are compared to those obtained with conventional Doppler signal processing. In addition to robustly resolving velocity ambiguity, the MAP velocity estimator is shown to reduce high-frequency noise in turbulence spectra.

Asymptotic Properties of an Autocorrelation Coefficient for Coherent Doppler Sonar

Abstract A new formula is derived for the asymptotic form of the magnitude of an autocorrelation coefficient for coherent Doppler sonar. The autocorrelation magnitude is shown to be a biased estimator in the limit of infinite ensemble length. Numerical simulation of a Gaussian random process is used to verify the asymptotic formula and to show that a bias persists for finite pulse-pair averages. Validity of the asymptotic formula is also confirmed using a high-fidelity coherent Doppler sonar simulation, and from sonar measurements in a towing tank. It is shown that the distribution of observed autocorrelation coefficients is well predicted by a Gaussian random process once the autocorrelation bias has been removed.

Resolving Velocity Ambiguity in Multifrequency, Pulse-to-Pulse Coherent Doppler Sonar

Coherent Doppler sonar allows noninvasive velocity measurements and is suitable for both laboratory and field applications. The approach is particularly attractive in those environments where optical techniques are not suitable either because of power requirements or more critically, water turbidity. Notably, the technique has been employed successfully in oceanic and river boundary layer studies. However, the occurrence of range and velocity ambiguities limit the more general application of the technique. This paper introduces a method to overcome speed ambiguities by acquiring acoustic backscatter at two (or more) frequencies simultaneously with a broadband transmit pulse. The different frequencies have distinct velocity ambiguities allowing disambiguation of the velocity measurements. The approach is conceptually similar to the use of multiple transmit pulse rates but has the advantage that the data can be acquired simultaneously and so there is no loss in data rate. In addition, system geometry often restricts the allowed pulse repetition rate so that disambiguation using frequency is more flexible and more generally applicable. Theoretically, the effective ambiguity velocity of a dual-frequency system can be extended arbitrarily but phase noise in a practical system restricts the method to about a fivefold increase in ambiguity velocity.

Modeling coherent Doppler sonar in fisheries acoustics

A computer model was developed to simulate the operation of a 250-kHz coherent Doppler sonar used to detect fish movements. The backscattered signal was constructed by summing contributions from many point targets. That signal was then detected and analyzed using the same method as used in the actual Doppler system. The model results reproduce predictions for the standard deviation of Doppler velocity estimates from volume backscatter based on the standard theoretical model of coherent pulse processing [Zrnic, IEEE Trans. Aerosp. Electron. Syst. AES-13, 344–354 (1977)]. However when the signal is modified to simulate the backscatter from a swimming fish (a single strong target among many weaker targets) the modeled standard deviation is 2 to 4 times lower than predicted by theory. Furthermore, the unusually low modeled standard deviations agree with laboratory and field observations. The model results confirm that the theoretical treatment used to predict the performance of Doppler measurements of current flow, blood flow, and atmospheric phenomena does not apply to the backscatter resulting from a single strong target such as a fish.

Sources of uncertainty in Doppler sonar measurements of fish speed

A 250-kHz, 30-kHz bandwidth coherent Doppler sonar was evaluated to determine sources of uncertainty in fish speed measurements. Three separate tests were undertaken: (1) towtank tests using styrofoam balls to simulate fish, (2) tank tests with live free-swimming fish, and (3) field tests with wild free-swimming fish. The standard deviation in a single speed estimate was 9 cm<th>s−1 for styrofoam balls, 10–11 cm<th>s−1 for swimming fish observed from a dorsal aspect, and 19 cm<th>s−1 for swimming fish observed from a caudal aspect. The variation in precision was primarily due to the different signal-to-noise ratio (SNR) in each test: a larger SNR resulted in a smaller standard deviation. Doppler speed estimates were compared with independent estimates of target speed where possible. An accuracy of ±4 cm<th>s−1 was typical of Doppler speed estimates in all the experiments.

Theoretical Accuracy of Doppler Navigation Sonars and Acoustic Doppler Current Profilers

This paper addresses the problem of Doppler shift estimation in Doppler sonar systems. The analysis focuses on the single-beam geometry formed by a circular planar array and considers both narrow-band (or so-called incoherent) and wide-band (or coherent) Doppler sonars, transmitting, respectively, one long continuous-wave pulse and a train of short continuous-wave pulses. The correlation function of the reverberation signal at the beam output is derived for volume reverberation. Directive transmission or reception and a combination of both is considered. Estimation theory is applied to derive the Cramer-Rao bound of the Doppler parameter estimate. The effect of pulse duration, sonar geometry, beamwidth, and signal-to-noise ratio are discussed. The accuracy of coherent and incoherent systems is compared for a specific case.

Structure of Turbulent Flow in a Shallow Tidal Estuary

A high-resolution current profiler (HRCP), which belongs to a pulse-to-pulse coherent Doppler sonar, has been used to measure vertical profiles of turbulence parameters, such as the Reynolds stresses, eddy viscosity, production and dissipation rates, etc., and to test the parametrization of dissipation rate and eddy viscosity. The HRCP and automatic ascending/descending CTD are deployed during the autumn of 2001 for 24 h in a tidal estuary. Reliable velocities along the beams with HRCP are collected with 3 s intervals and a vertical resolution as fine as 0.03 m in the range 0.02–0.98 m above the bottom. Density profiles with the CTD are taken nominally every 30 sec. The turbulent velocity variables depend largely on the tidal phase; the variables during the ebb deviate from those in neutral equilibrium boundary layer. This deviation during the ebb presumably arises from the “inactive motion.” The stability function SM in the Mellor–Yamada (M–Y) model is smaller than 0.39 even when the stratification is negligible during the flood. The constant of proportionality B1 in the dissipation model is larger than 16.6 used in M–Y model. There is room for improving some of the mixing parametrizations in estuarine tidal flows.

Field trial of a Doppler sonar system for fisheries applications

Various deployments of commercial Doppler current profiling systems have demonstrated that these instruments can detect fish and measure their swimming speeds. However, research into the possible application of Doppler sonar to fisheries problems is limited and has not taken advantage of coherent signal processing schemes. A field trial was undertaken in August 2002 to explore the capabilities of a coherent Doppler sonar when applied to detecting discrete targets. The passage of migrating salmon on the Fraser River in British Columbia provided an ideal test opportunity with fish of well-defined swimming behavior and allowed for comparisons with conventional fisheries acoustics techniques. The instrument tested was a 250-kHz sonar which provided for phase coding of transmit pulses and coherent sampling of successive acoustic returns. The field trial resulted in 11 consecutive days of Doppler sonar data acquired during the peak of the sockeye salmon (Oncorhynchus nerka) migration. A total of 7425 individual fish were identified and their swimming speed was measured with an accuracy of between 10 cm<th>s−1 and 20 cm<th>s−1, which depended on pulse length, pulse spacing, and target range. By comparison, water velocity measurements made with the same instrument can only achieve a theoretical accuracy of 60 cm<th>s−1.

A three-component bistatic coherent Doppler velocity profiler: error sensitivity and system accuracy

We report on a 1.7-MHz coherent Doppler sonar system designed for near-shore environments that provides three-component velocity profiles over an O(1-m-depth) range. The use of a bistatic geometry allows three independent components of velocity to be measured simultaneously in time and coincident in space. The system was calibrated in a tow-tank facility for horizontal velocities up to 2 m/s. For multiple pulse-pair ensemble-averaged velocity estimates generated at a rate of 30 profiles/second in 0.7 cm depth bins, the standard deviation in measured vertical velocity is about 1% of the total flow speed. For the horizontal components (based on bistatic measurements), the standard deviation of velocity estimates is about 5%. Absolute accuracies are also about 5% of the speed but a portion of this bias is caused by flow disturbance around the instrument package: this disturbance is modeled using potential flow past a cylinder.

Measurements of turbulence in the wave boundary layer using high frequency acoustics

The properties of the turbulent boundary layer at the air–sea interface exhibits departures from classical wall bounded shear flows as a result of the free surface characterized by surface waves. As surface waves grow in the presence of wind, they become unstable and break, which leads to the injection of turbulence and bubbles into the water column. While an understanding of these physical processes is critical to fields of oceanography and mid-/high-frequency ocean acoustics, there remains much to be learned due in part to the difficulty of obtaining turbulence measurements at the air–sea interface as well as signal processing issues associated with separating turbulent water motions from wave orbital motions. One maturing solution to this measurement problem is the use of O(1) MHz pulse to pulse coherent Doppler sonars that allow direct wave number measurements in the inertial subrange of the turbulence. Recent field measurements of near-surface turbulence have been conducted by our group with this sonar technology both offshore Pt. Conception California and Hawaii. Measurement results will be discussed, including observations regarding the relationship between the intermittancy of the turbulent dissipation to the overlying surface wave field. [Work supported by ONR and NSF.]

A three-dimensional coherent Doppler profiler

A 1.7-MHz coherent Doppler sonar system has been designed for near-shore environments that provides three-dimensional velocity profiles over an O (0.5-m) depth range. The use of a bistatic geometry allows three independent components of velocity to be measured simultaneously in time and coincident in space. The system was calibrated in a tow-tank facility for velocities up to 2 m/s. For multiple pulse-pair ensemble-averaged velocity estimates generated at a rate of 30 profiles/s in 0.7-cm-depth bins, vertical velocity accuracies of the order 1 cm/s are achieved. For the horizontal components (based on bistatic measurements), standard deviation in velocity estimates of about 5% is realized with absolute accuracies of about 2%. In the configuration as tested, flow disturbance around the instrument introduces a bias of about 5%: this disturbance is modeled using potential flow past a cylinder. The disturbance errors could be corrected to the free-stream velocities using the present data set. Such a correction would not, however, reflect the actual water speed at the point of measurement. [This research was funded by the Office of Naval Research Coastal Sciences Program, and the Natural Sciences and Engineering Research Council of Canada.]

Ultrasonic scattering in highly turbulent clear water flow

Abstract Experiments have been made to investigate the source of ultrasonic scattering in clear turbulent water. Microstructure scattering in open-channel is related to turbulence parameters by adapting the Batchelor spectrum and theory valid in a steady uniform flow. Quantitative verification of this relationship is carried out by experiments with a coherent Doppler sonar, from which the profiles of the turbulence parameters are determined. Results from both multi-beam monostatic sonar and bistatic sonar show that the theoretically predicted scattering cross section profile coincides with the observed one under the assumption that fine air bubble microstructures dominate the scattering process. The scattering from temperature microstructures is too small to account for the observed scattering. Experiments made in a grid-stirred tank filled with normal tap water and with degassed water further confirm the hypothesis that the concentration variation of fine air bubbles is responsible for the scattering observed by high-frequency sonar in clear water open-channel flow.

Performance of a single-beam pulse-to-pulse coherent Doppler profiler

We report on the testing of a single-beam 1.7-MHz coherent Doppler sonar. The system is PC-controlled, using a digital signal processor (DSP) to acquire and extract the velocity and backscatter amplitude data. Results from a series of tow-tank calibration tests demonstrate an accuracy in the order of 5 mm s/sup -1/ for data rates of 10 profiles/second over a 1-2 m range with 1.5-cm range bins. An expression for system accuracy is developed which allows generalization to other pulse-to-pulse coherent Doppler systems. We present data showing the systematic decorrelation of backscatter signals due to particle advection: increased decorrelation is seen in the transducer near-field. Example observations of velocity profiles in laboratory-generated waves are presented.

Acoustic Doppler current meter

A current meter designed to measure velocity profiles in the water column above the bed is described. The solution proposed is a coherent backscatter Doppler sonar system using a single transducer. A range-velocity product of 20 m/sup 2//s can be achieved by combining state of the art sonar techniques with new processing methods and careful, highly flexible design. Large range-velocity products and very short range resolution together with the possibility of acquiring sensible results without averaging over long time periods are features of this current meter. >