Identifying and Screening Filter Skew and Noise Bias in Acoustic Doppler Current Profiler Measurements

Chereskin, Teresa K.; Firing, Eric; Gast, J.

doi:10.1175/1520-0426(1989)006<1040:iasfsa>2.0.co;2

Cited by 32 publications

(9 citation statements)

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“…The percentage of good data on the right-hand side of the graph indicates the data with signal-to-noise (S/N) ratio exceeding 3 dB (the default value) in all four beams simultaneously. This would suggest about 300-m range, but caution has to be applied : at low SIN, the bin to bin tracking can be biased, resulting in biased current shears [Chereskin et al, 1989]. Hence the cutoff of good data may actually occur at a lesser distance from the transducer, but since we have no precise way of determining this range, we show the data over the 300-m range.…”

Section: Adcp Peformance and Profile Evaluationmentioning

confidence: 97%

Observations of vertical currents and convection in the central Greenland Sea during the winter of 1988–1989

Schott¹,

Visbeck²,

Fischer³

1993

J. Geophys. Res.

138

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During the winter of 1988--1989 five acoustic Doppler current profilers (ADCPs) were moored in the central Greenland Sea to measure vertical currents that might occur in conjunction with deep mixing and convection. Two ADCPs were looking up from about 300 m and combined with thermistor strings in the depth range 60--260 m, two were looking downward from 200 m, and one was looking upward from.1400 m. First maxima of vertical velocity variance occurred at two events of strong cold winds in October and November when cooling and turbulence in the shallow mixed layer generated internal wav!!S in the thermocline. Beginning in late November the marginal ice zone expanded eaktward over the central Greenland Sea, reaching its maximum extent in late December. In mid-January a bay of ice-free water opened over the central Greenland Sea, leaving a wedge of ice, the "is odden," curled around it along the axis of the Jan Mayen Current and then northeastward and existing well into April 1989. Below the ice a mixed layer at freezing temperatures developed that increased in thickness from 60 to 120 m during the period of ice cover, corresponding to an average heat loss of about 40 W m-2 . Through brine rejection, mixed-layer salinity increased steadily, reducing stability to underlying weakly stratifi ed layers (Roach et al., 1993). During the ice cover period, vertical currents were at a minimum. After the opening of the ice-free bay, successive mixed-layer deepening to > 350 m occurred in conjunction with cooling events around February 1 and 15, accompanied by strong small-scale vertical velocity variations. Upward mixing of more saline waters of Atlantic origin during this phase reduced the stability further, generating a pool of homogeneous water of> 50 km horizontal extent in the central Greenland Sea, preconditioned for subsequent convection to greater depths. Individual convection events were observed during March 6--16, associated with downward velocities at the 1400-m level of about 3 cm s -1 • One event was identified as a plume of about 300-m horizontal scale, in agreement with recently advanced scaling arguments and model results, and with earlier similar observations in the Gulf of Lions, western Mediterranean. The deep convection occurred in the center of the ice-free bay; hence brine rejection did not seem necessary for its generation. Plume temperatures at 1400 m were generally higher than that of the homogeneous surface pool, suggesting entrainment of surrounding warmer waters on the way down. Mean vertical velocity over a period of convection events was indistinguishable from zero, suggesting that plumes served as a mixing agent rather than causing mean downward transport of water masses. However, different from the surface pool that was governed by mixed-layer physics, the water between 400 and 1400 m was not horizontally homogenized in a large patch by the sporadic plumes. Overall, and compared to results from the Gulf of Lions, convection activity in the central Greenland Sea was weak and limited to intermed...

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Section: Adcp Peformance and Profile Evaluationmentioning

confidence: 97%

Observations of vertical currents and convection in the central Greenland Sea during the winter of 1988–1989

Schott¹,

Visbeck²,

Fischer³

1993

J. Geophys. Res.

138

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“…m was used with a pulse length of 4 m (6.3 ms) and a bin width of 4 m (6.3 ms). For these parameters and for typical current shears, the filter skew error in the deep data can be several (0-10) centimeters per second, highest at large ship speeds and current shears [Chereskin et al, 1989]. On both ships, profiles typically extended below 200 m, except when steaming into rough seas.…”

Section: Introductionmentioning

confidence: 99%

Currents and water masses of the Coastal Transition Zone off northern California, June to August 1988

Huyer¹,

Kosro²,

Fleischbein³

et al. 1991

J. Geophys. Res.

101

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In summer 1988, we made repeated mesoscale surveys of a grid extending 200 km offshore between 37°N and 39°N in the coastal transition zone off northern California, obtaining continuous acoustic Doppler current profiler data and conductivity‐temperature‐depth data at standard stations 25 km apart on alongshore sections 40 km apart. All surveys showed a baroclinic equatorward jet, with core velocities of >50 cm s−1 at the surface decreasing to about 10 cm s−1 at 200 m, a width of 50–75 km, and a baroclinic transport of about 4 Sv. The core of the jet lay between the 8.6 and 9.4 m2 s−2 contours of geopotential anomaly (relative to 500 dbar). Three current meter moorings, deployed at 25‐km separation across the jet at the beginning of the survey sequence, provided time‐series of the velocity; throughout the 37‐day deployment, at least one mooring was within the core defined by the 8.6 and 9.4 m2 s−2 contours. The jet flowed southwestward across the grid from late June until mid‐July 1988, when the jet axis moved offshore in the north and onshore in the southern portion of the grid. Temperature‐salinity analysis shows that jet waters can be distinguished from both the freshly upwelled coastal waters and the offshore waters. Isopycnal maps indicate alongshore advection of relatively fresh, cool water from farther north, as well as small‐scale patchiness not resolved by our survey grid. The baroclinic jet observed here may be continuous with the core of the California Current off central California. The later surveys clearly showed a poleward‐flowing undercurrent adjacent to the continental slope, with core velocities up to 20 cm s−1 at depths of 150–250 m. Its baroclinic transport (relative to 500 dbar) increased from <0.5 Sv to >1.0 Sv between late June and early August 1988. Within the survey grid, there was a definite onshore gradient in the characteristics of North Pacific Intermediate Water. The subsurface waters adjacent to the continental margin were warmer and more saline than those offshore, indicating net northward advection by the California Undercurrent over the inshore 100 km and equatorward advection farther from shore.

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“…Model and ADCP errors agree well. Since these simulations were made at high signal-to-noise ratio, the primary source of error in the velocity estimate is due to filter skew ( [5], [9]), i.e., a bias due to the low pass filter not being centered on signal spectrum (non zero error frequency). Table 1 results indicate that errors increase with increasing velocity shear, due to increased misposition of the tracker (increased error frequency).…”

Section: Resultsmentioning

confidence: 99%

“…As the tracker frequency reaches switch points defined by a decrease in step size by a factor of two from the previous switch point, the gain is increased by a factor of two ta compensate. A previous paper looked at errors that resulted when the tracking loop was disabled [5]. If the loop is disabled and the Doppler frequency is significantly non zero, the lowpass filtered signal frequency will be underestimated since the signal peak will be positioned near the cutoff of the filter rather than near the center.…”

Section: Freauencv Trackingmentioning

confidence: 99%

“…The model is validated by signal simulation experiments where statistically identical signals are analyzed by both model and an actual ADCP. A model approach was used in an earlier study to identify and estimate measurement bias in RD Instruments (RDI) ADCP velocity estimates, due to disabling of the system feedback control loop (frequency tracker) [5]. One result of that study was that frequency tracking is now continuously enabled in RDI profilers.…”

Section: Introductionmentioning

confidence: 99%

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A Model Approach To Predicting Errors In Acoustic Dopler Current Profiles

Chereskin

Harding²

OCEANS 92 Proceedings@m_Mastering the Oceans Through Technology

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A modeling approach is applied to predicting errors in velocity estimates made by pulse to pulse incoherent acoustic Doppler current profilers (ADCPs). A system model is described that provides a direct simulation of ADCP signal processing, including a tracking feedback loop, sometimes referred to as a frequency tracker. Although the model follows general principles of incoherent sonar signal processing (autocovariance estimation of the Doppler shift frequency) and phaselocked loops, we present model predictions for a particular system, an RD Instruments 153.6 kHz ADCP, a system in widespread use on oceanographic research vessels and current meter moorings. Model validation is achieved by comparison with ADCP measurements of the same signals, coupled into the ADCP using a signal simulator.By assuming a statistical model of the backscatter, the model can be used to predict errors in velocity estimates determined by Janus beam pairs, measuring a velocity shear in a medium of randomly distributed scatterers. In addition to error prediction, the model is used to explore the dependence between velocity errors and system parameters, with an emphasis on optimizing those parameters that are accessible to users of commercial instruments.

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Identifying and Screening Filter Skew and Noise Bias in Acoustic Doppler Current Profiler Measurements

Cited by 32 publications

References 0 publications

Observations of vertical currents and convection in the central Greenland Sea during the winter of 1988–1989

Observations of vertical currents and convection in the central Greenland Sea during the winter of 1988–1989

Currents and water masses of the Coastal Transition Zone off northern California, June to August 1988

A Model Approach To Predicting Errors In Acoustic Dopler Current Profiles

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