John H. Dunlap

An Acoustic Doppler and Electromagnetic Velocity Profiler

Abstract A freely failing current meter called the Absolute Velocity Profiler (AVP) is described. This profiler is an expansion of a previously developed instrument, the Electro-Magnetic Velocity Profiler (EMVP), with the additional capability of acoustic Doppler (AD) measurements to determine the reference velocity for the EM profiles. The AVP measures the motional electric currents in the sea and the Doppler frequency shin of bottom-scattered echoes. The EM measurements yield a profile of the horizontal components of velocity relative to a depth-independent reference velocity; the AD measurements determine the absolute velocity of the AVP with respect to the seafloor. The EM profile is obtained from the sea surface to the bottom, and the AD measurements are obtained within about 60–300 m of the seafloor. The combination of the EM and AD measurements yields an absolute velocity profile throughout the water column. Performance analyses show the method is accurate to within 1–2 cm s−1 rms. The profiler als...

Autonomous velocity and density profiler: EM-APEX

We developed an autonomous ocean profiling velocity and density float that provides exceptional vertical coverage and temporal resolution to depths of 2000 m for deployments of many years. Electrodes were added to the exterior of standard WRC APEX floats, and electronics were added inside. The electrode voltages result from the motion of seawater and the instrument through the Earths magnetic field. Other systems included magnetic compass, tilt, CTD, GPS, and Iridium (providing sampling/mission changes). Three EM-APEX floats were deployed from a C-130 aircraft ahead of Hurricane Frances. The floats profiled for 10 hr from the surface to 200 m, then continued profiling between 30 m and 200 m with excursions to 500 m every half inertial period. The velocity computations were performed onboard and saved for later transmission. After five days, the floats surfaced and transmitted the accumulated processed observations, then the floats profiled from 500 m every half inertial period until recovered early in October located by GPS and Iridium.

AN ELECTROMAGNETIC VORTICITY AND VELOCITY SENSOR FOR OBSERVING FINESCALE KINETIC FLUCTUATIONS IN THE OCEAN

Abstract An instrument has been developed that measures finescale velocity and vorticity in seawater based on the principles of motional induction. This instrument, the electromagnetic vorticity meter (EMVM), measures components of the gradient and Laplacian of the electrostatic potential field induced by the motion of seawater through an applied magnetic field. The principal innovation described here is the development of a sensor for measuring small-scale vorticity. The sensor head consists of a strong NdFeB magnet, a five-electrode array, low-noise preamplifiers, and 20-Hz digitizers. The main electronics includes attitude sensors, batteries, a microprocessor, and a hard disk. The vorticity sensors are usually carried on a heavy towed vehicle capable of vertically profiling to 200 m and at tow speeds of several knots. The theoretical response functions of the EMVM are evaluated for velocity and vorticity. Extensive measurements were obtained in Pickering Passage, Washington, as the sensor vertically pr...

Experience with an Expendable Temperature and Velocity Profiler

The performance of an expendable temperature and velocity profiler (XTVP) against several independent velocity profilers is discussed. The XTVP operates on the principles of motional induction resulting from the movement of sea water through the earths magnetic field. The induced electric currents are interpreted in terms of the generating velocity distribution. Profile intercomparisons have shown the XTVP capable of measuring ocean velocity (relative to an unknown, depth independent offset velocity) to within an uncertainty of 1 cm/s with 6 m vertical averaging for both the east and north velocity components. White noise density of about 3.5 (cm/s)2/cpm limits vertical resolution to about 10 m.The probe falls at about 4.5 m/s, requiring about 200 s to complete a profile to 900 m (3000 ft), and has a dynamic range of \pm400 cm/s. Profiles and comparison results are presented demonstrating the achievement of the development goal of velocity uncertainties of 1 cm/s rms at a vertical resolution of 10 m.

Barotropic flows observed by the towed transport meter

The structure of the meridional transport east of Abaco, the Bahamas, was simultaneously observed with moored current meters and a towed electric field sensor. Important currents in this region include the northward flowing Antilles Current near the surface and the Deep Western Boundary Current flowing southward at depth. The towed measurements of vertically averaged velocity were obtained with the Towed Transport Meter version 2 (TTM2) based on the observation of motionally induced electric fields. The electric field component parallel to the track of a ship is equal to the vertical component of the Earths magnetic field times the difference in the cross-track velocity component of the ship and the cross-track velocity component of the electrical conductivity-weighted, vertically averaged horizontal velocity (denoted as v~*) of the ocean. The velocity of the ship normal to its track was determined from radio navigation methods (e.g. LORAN-C) and the ships gyrocompass. The difference between the ships velocity normal to its track and the TTM2 observed velocity was v~*. The differences between v~* and the conventional v~ indicated that the conductivity weighting and the effect of bottom electrical conductances were small, of the order of 5%. The comparison with the moored current meter array revealed gyrocompass errors of up to 2.5/spl deg/. Such errors will be reduced with the application of GPS heading sensors in the near future. A change in the vertically averaged north velocity component of more than -0.25 m s/sup -1/ over less than 20 km was observed in both methods. The cumulative northward transport reached about 10 Sv 70 km east of Abaco, and by 130 km offshore it had decreased to -8 Sv. The velocity and transport structure measured by the two techniques was very similar, supporting the accuracy of the towed measurements as well as the transport estimates from the moored array.

Barotropic ocean velocity observations from an electric field float, a modified RAFOS float

A seldom observed characteristic of ocean motion is depth-averaged velocity, or transport per unit width. One approach to observing this quantity is to measure the ambient horizontal electric field of the ocean with electric field sensors added to a RAFOS float. The Electric Field Float (EFF) is a RAFOS float modified by the addition of a compass, electrode arms, amplifiers, and a microprocessor. The floats velocity is determined by the acoustic ranging of the RAFOS subsystem. The velocity determined by the electrode system is the electric field (EF) observed by the electrode array divided by the vertical component of the Earths magnetic field, F/sub 2/. The difference between the RAFOS and EF velocities is v~*, a conductivity weighted, vertically averaged velocity which is generally equal to the vertically averaged velocity (v~) to within 10%. An EFF launched off Monterey Bay, California, observed a velocity at 800-m depth of about 33 cm s/sup -1/ to the NW of which about 2 cm s/sup -1/ was contributed by the barotropic flow. As expected, the barotropic velocity closely paralleled the depth contours. The 2-h average EF velocities exhibited the expected CW/ACW asymmetry in the internal wave band.

Deep ocean profiles from electromagnetic and acoustic Doppler measurements

An instrument is described for measuring profiles of horizontal velocity as a function of depth in the deep ocean. The method is a hybrid technique based on the principles of electromagnetic induction and acoustic Doppler and is mobile since not dependent on bottom-installed equipment. The EM method measures weak electric currents in the sea induced by the motion of the water through the earths magnetic field. The resulting velocity profile reveals the velocity shears but is relative to an unknown, depth-independent reference velocity. The reference velocity is determined by acoustic Doppler measurements of the absolute velocity of the instrument as it nears the sea floor. The two methods are incorporated into a single freely-falling probe which measures and internally records the electric and acoustic signals and other variables such as temperature and vehicle orientation. The method yields velocity determinations every 5-10 m with an uncertainty of about \pm1 cm/s. A round trip in 6000 m of water lasts about 3 hours. Data from this method have been used to study mid-ocean eddies, internal waves, and the Gulf Stream.

Performance of an absolute velocity profiler based on acoustic doppler and electromagnetic principles

Prepared for the National Science Foundation, Office of the International Decade of Ocean Exploration under Grant OCE76-24605.

Autonomous control of marine floats in the presence of dynamic, uncertain ocean currents

Abstract A methodology is described for control of vertically profiling floats that uses an imperfect predictive model of ocean currents. In this approach, the floats have control only over their depth. This control authority is combined with an imperfect model of ocean currents to attempt to force the floats to maintain position. First, the impact of model accuracy on the ability to station keep (e.g. maintain X–Y position) using simulated planning and nature (ground-truth in simulation) models is studied. In this study, the impact of batch versus continuous planning is examined. In batch planning the float depth plan is derived for an extended period of time and then executed open loop. In continuous planning the depth plan is updated with the actual position and the remainder of the plan re-planned based on the new information. In these simulation results are shown that (a) active control can significantly improve station keeping with even an imperfect predictive model and (b) continuous planning can mitigate the impact of model inaccuracy. Second, the effect of using heuristic path completion estimators in search are studied. In general, using a more conservative estimator increases search quality but commensurately increases the amount of search and therefore computation time. Third are presented results from an April 2015 deployment in the Pacific Ocean that show that even with an imperfect model of ocean currents, model-based control can enhance float control performance.

Accurate Determination of Ship's Velocity Using Loran-C

Loran-C can be used to provide ships velocity with an rms uncertainty of several centimeters per second in 5-minute averaging intervals. Range rates obtained from linear least-squares fits to the ranges from several stations are used in a second spatial leastsquares fit to determine east and north velocity components. The combination of master independent range-range operation and cross-chain operation has greatly increased the usual Loran-C operational area by allowing operation between chains.