a moored proﬁling instrument - whoi

1816 VOLUME 16J O U R N A L O F A T M O S P H E R I C A N D O C E A N I C T E C H N O L O G Y

q 1999 American Meteorological Society

A Moored Profiling Instrument*

K. W. DOHERTY, D. E. FRYE, S. P. LIBERATORE, AND J. M. TOOLE

Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

(Manuscript received 13 March 1998, in final form 3 December 1998)

ABSTRACT

The specifications and performance of a moored vertical profiling instrument, designed to acquire near-full-ocean-depth profile time series data at high vertical resolution, are described. The 0.8-m-diameter by 0.4-m-wide device utilizes a traction drive to propel itself along a standard mooring wire at a speed of ;0.3 m s21.The average power required to profile at this speed is 1–2 W; the present sensor suite and controller draw about1.5 W. Based on these figures, the instrument’s battery capacity will support approximately 1 million meters ofprofiling. Instrument actions are regulated by an onboard microcontroller, allowing complex dive programs tobe carried out. Oceanographic and engineering data are recorded internally on a hard disk interfaced to thecontroller. The measurement suite thus far deployed includes a CTD for deriving ocean temperature and salinityprofiles, and an acoustic current meter that returns ocean velocity profile data. Addition of other oceanographicsensors is anticipated. Results from several trial deployments in the open ocean are reported.

1. Introduction

Society is increasingly concerned with global climatevariability. Yet, very few long records of ocean vari-ability that might document secular change currentlyexist. Time series were once routinely acquired from anetwork of ocean weather ships, and these data providea tantalizing glimpse of ocean variability on seasonalto decadal timescales (e.g., Lazier 1980; Østerhus et al.1996; Talley and Raymer 1982; Talley 1996; Joyce andRobbins 1996; Karl and Michaels 1996; Denman et al.1992; Freeland et al. 1997). However, the logistics andcosts of using ships to maintain such stations has be-come prohibitive. Only a handful of sites are presentlysampled, some at rather long and irregular time inter-vals. Here a moored instrument (termed the MooredProfiler) capable of autonomous, near-full-water-columnprofiling is described. This device, when deployed incombination with drifting, gliding, and/or self-propelledprofiling vehicles, shows promise for effectively mon-itoring future climatic ocean change. The new instru-ment is also well suited for short-term, process-orientedexperiments that require information at high-verticaland temporal resolution.

Ocean time series data are routinely collected using

* Woods Hole Oceanographic Institution Contribution Number9696.

Corresponding author address: Dr. J. M. Toole, WHOI, Clark MS#21, Woods Hole, MA 02543.E-mail: [email protected]

moorings with discrete instrumentation. The ATLASbuoys presently forming an array in the equatorial Pa-cific Ocean, for example, utilize a set of individual tem-perature sensors spanning the upper 500 m of the watercolumn (Hayes et al. 1991). In similar fashion, surfacemoorings with discrete physical and biological ocean-ographic sensors are being maintained at stations off-shore from Bermuda and Hawaii, respectively, supple-menting ship-based observation programs (Dickey et al.1997; R. Lukas 1997, personal communication). A ma-jor difficulty with this multiple-sensor approach, beyondthe expense, is the determination of relative sensor cal-ibrations. For example, discrimination between a trulywell mixed layer, and one with residual stratificationcan be ambiguous if individual sensors have unknownoffsets. This is a particular problem for conductivitysensors that are prone to calibration drift. Profiling in-strumentation in which one instrument package movesvertically through the water column significantly re-duces this uncertainty.

Moored profiling systems have been utilized in thepast to obtain upper-ocean time series information. TheCyclesonde (van Leer et al. 1974) and the ProfilingCurrent Meter (PCM; Eriksen et al. 1982) employ var-iable buoyancy to move vertically over a limited depthrange. More recently, Provost and du Chaffaut (1996)have developed a buoyancy-driven profiler capable ofcycling to 1000-m depth. In contrast, Eckert et al. (1989)describe a profiling instrument platform that derives ver-tical lift from ambient currents, and Fowler et al. (1997)have devised a method to extract energy for profilingfrom surface-wave-induced mooring motion. Distinct

NOVEMBER 1999 1817D O H E R T Y E T A L .

FIG. 1. A schematic drawing of the Moored Profiler equipped witha CTD and acoustic current meter.

from these techniques, the system described here usesa motorized traction drive to propel itself vertically. Thegeneral specifications for the new instrument are dis-cussed below. Section 2 details the mechanical designof the profiler and its control system. Results from trialdeployments in the open ocean are given in section 3.

The basic design of the new system was definedthrough specification of its scientific requirements. Theability to profile over the full water column, from withinthe surface mixed layer to within about 100 m of theocean bottom, was stipulated. This sampling range al-lows the exploration of relationships between upper-ocean changes and those at depth. Furthermore, decom-position of ocean variability in terms of vertical modesis facilitated with full-depth information (e.g., Hayes etal. 1985). From a practical standpoint, deep profiles canprovide in situ checks of sensor calibrations by samplingwater properties at levels known to be stable on annualtimescales. Cycling to great depth (and positioning theinstrument at depth between cycles) additionally im-proves data quality (sensor stability) by inhibiting bio-fouling. To minimize mooring motion and thereby fa-cilitate deployments of a year or longer, a subsurfacemooring configuration was chosen. With care, subsur-face moorings can be deployed to within 25 m of thesurface. It is recognized, however, that mooring blow-down by strong currents might at times limit access tovery shallow mixed layers.

Flexible temporal sampling was specified to addressdiverse scientific problems. For climate monitoringstudies, our goal was to obtain better than monthly res-olution in analyzed data from a 1-yr deployment, there-by resolving the seasonal cycle, including any embed-ded changes on 1–2-week periods. For design purposes,a mission was defined as 100 round-trips to 5000 m,yielding 200 full-depth vertical profiles, or about 1 mil-lion meters of vertical travel, per deployment. It wasrecognized that users of the new instrument might wishto highlight one depth interval over another, or to focusattention on one or more time periods in the year. To

accommodate this, the profiler was designed to allowvariable depth sampling based on a user-defined sam-pling schedule. For example, deployments focusing oninternal wave variability have been made where instru-ments profiled every 1 to 2 h over about a 1-km segmentof the water column for 1-month intervals. Alternative-ly, an instrument set to observe winter convection mightbe programmed to acquire several profiles a day in mid-winter, dropping back to only a few samples per weekin the other seasons.

Energy considerations, and concern with aliasing, setbounds on profiling speed. High speed incurs excessiveenergy dissipation through hydrodynamic drag. Slowspeed can introduce spurious vertical structure in pro-files when depth segments are sampled at different tidalphases. The compromise speed of 0.3 m s21 was chosen(which is near optimal from an energetic standpoint, seebelow); at this rate a 5000-m profile is collected overa time span of approximately 4.6 h. An individual, full-depth profile is thus subject to some tidal aliasing. Burstsampling (e.g., collecting five profiles within a one-dayperiod every fifth day as opposed to doing one profileeach day) is one technique to combat these errors.

The chief design issue for the new instrument wasthe mechanism to propel it vertically. As noted above,several ocean profilers developed previously employedvariable displacement devices; others relied on ambientflow or mooring motion. In contrast, the new instrumentutilizes an electrical motor, gear train, and traction wheelto move itself along the mooring wire. Conceptuallythis approach was attractive since it takes advantage ofthe existence of the mooring wire, allows maximumprofiling flexibility, and is relatively inexpensive to im-plement. Reliance on lift from ambient currents or moor-ing motion over the full water column was deemed un-certain, especially in the deep ocean where currents maybe low for long periods of time and mooring motion isreduced. From an energy standpoint, there is little toseparate buoyancy-change devices from traction sys-tems in a moored configuration. Both expend batteryenergy in doing work to move verticlly, and both sufferinefficiencies through electromechanical system lossesand hydrodynamic drag. Our design study found thatthe requirement for a full-ocean-depth operating rangepresented complications in using a variable displace-ment system because of the large pressure forces thatare incurred at great depth. Furthermore, a traction driveappeared capable of carrying out complex sampling sce-narios (repeated shallow cycles interspersed with full-depth sampling for instance) and was able to apply ad-ditional force to pass obstructions on the wire in a moreenergy efficient manner than could a variable-buoyancysystem.

Last, the vehicle was envisaged supporting a diverseset of oceanographic instrumentation. It was acceptedthat all systems would include a conductivity–temper-ature–depth (CTD) instrument for observing finescaletemperature and salinity information as a function of


FIG. 2. A representative mooring diagram for a profiler equippedwith a real-time telemetry link.

FIG. 3. Instantaneous estimates (300-ms averages recorded every11 s) of motor current as a function of profiler depth from (a) thefall 1997 profiler deployment on the continental slope south of NewEngland and (b) offshore from Bermuda in spring 1997. Up-goingprofiles are displayed with positive motor current values, down-goingwith negative values. Large-magnitude motor currents are observedwhen the instrument impacts the mooring stops at the top and bottom.pressure. Provision for easy addition of other sensors

constituted a major design element for the system elec-tronics. Our design assumes that each sensor maintainsan accurate time base during a profile so that pressuredata from the CTD may be used to register these otherobservations in depth.

2. Instrument design

A prototype of the Moored Profiler was constructedand tested in the Woods Hole Oceanographic Institution(WHOI) tow-tank facility, and at the WHOI dock. Basedon this work, ocean-going systems were developed anddeployed for testing on deep-sea moorings. Second-gen-eration instruments were subsequently developed to ad-dress shortcomings in the initial design revealed duringsea trials. The current version of the instrument systemis described here.

The Moored Profiler consists of a propulsion system,controller, sensor suite, batteries, and buoyancy ele-ments, housed within a segmented oblate–spheroidalcowling of vacuum-bagged polyester fiberglass (Fig. 1).The instrument is deployed on a conventional plastic-jacketed wire rope in an oceanographic mooring (Fig.2) and repeatedly traverses that length based on a user-defined operation program. The profiler’s cowling hasa 2:1 aspect ratio with a major-axis diameter of 0.8 m.This basic shape was selected for low drag (Cd is ap-proximately 0.16, Savage and Hersey 1968), and sym-metry with respect to horizontal and vertical flows. Themooring wire threads vertically through the leading edgeof the instrument, guided by sheaves at the upper andlower ends of the front surface. The sheaves, whose


axles are oriented fore–aft, have a broad V-shaped bear-ing surface with diameter decreasing from 7.62 cm atthe ends to 5.08 cm in the center. Torlont ball bearingsare used with bearing races and wheels manufacturedfrom acetront NS, a filled acetal plastic having excellentwear characteristics. Location of the mooring wire nearthe leading edge of the instrument allows the body toalign with the horizontal flow. This minimizes drag forc-es caused by horizontal currents and positions the ocean-ographic sensors in undisturbed flow. Retaining bracketsat each sheave, used to constrain the instrument on themooring wire, support the weight of the profiler on de-ployment and recovery when the mooring wire is hor-izontal. These have a cross-sectional clearance of 19.6cm2, allowing the instrument to pass over obstructionson the mooring wire.

The propulsion unit is mounted on a hinge that allowsthe drive wheel to pass over obstructions on the mooringwire. The drive wheel is a 4.4-cm-diameter sheave po-sitioned between the guide sheaves in the same fore–aft orientation. This arrangement negates the need fora right-angle gear in the drive train (with its attendantlosses). A high-friction coating is applied to the surfaceof the wheel to minimize slippage. The wheel is heldagainst the mooring wire by a spring that exerts a tensionof 35.6 N in normal operation. This tension in turn actsto center the wire on the drive and guide sheaves. Theheart of the propulsion system is a brush-type DC motorfrom Maxon Precision Motors, Inc., running in air atapproximately 3500 rpm through a 27-to-1 gear. Themotor is specified by the manufacturer to be 84% ef-ficient, while the gear train is rated at 75%, giving atotal electrical to mechanical conversion efficiency of63%. A magnetic coupler is used to transfer motortorque across the titanium motor housing to the drivewheel. The coupler is capable of delivering 0.5 N m oftorque. Thus, the maximum driving force that can beapplied to the profiler is 22.7 N. In normal operation,the motor turns the drive wheel at approximately 130rpm to give the profiler ascent/descent rates of approx-imately 0.3 m s21. The direction of travel is determinedby the sense of the voltage applied to the motor. Adynamic brake was created using a switch that shuntsthe motor leads to ground. The induced electromotiveforce (emf) developed if the drive wheel is rotated whenthe switch is closed, acts to resist profiler motion.

Profile speed was chosen in light of the efficiencycurve for the Moored Profiler. Mathematically, this be-gins with an energy equation of the form ED21 5 FDe21

1 Hw21, where E is the energy expended on one profileof length D, FD represents the drag force (typically mod-eled as a squared velocity law), e is the efficiency ofthe motor and drive train, H is the (constant) electricalpower drawn by the instrumentation and controller (the‘‘hotel’’ load), and w is the profile speed. This relationmay be used to predict the optimal profile speed betweenfast travel, where drag losses become prohibitive, andslow travel, where the hotel load is limiting. With FD

5 rCdAw2 and ignoring variations in e, the optimalspeed is given as woptimum 5 [eH/(2rCdA)]1/3. For theMoored Profiler fitted with a CTD and ACM, the hotelload is ;1.5 W giving woptimum ; 0.2 m s21. But tidalaliasing concerns become more severe at slow profilespeed. As the efficiency curve is quite flat about theoptimum speed, we opted for a somewhat faster-than-optimal profile speed of 0.3 m s21 at the cost of using;15% more energy per profile.

The internal frame holding the various profiler com-ponents is machined from ultrahigh molecular weightpolyethylene, which is slightly buoyant in seawater. Theinternal components include two 0.30-m-diameter glassballs that provide pressure vessels for the instrumentelectronics and batteries, as well as buoyancy for thedevice. Power for the various systems is supplied bytwo lithium battery packs (combined mass of 7.3 kg)assembled from double-D-format cells that are mountedin the lower of the two glass balls. The pack supplyingthe drive system has an open-circuit voltage of 14.4 Vand a 150 A h capacity, that supporting the scientificinstrumentation is specified at 10.8 V and 120 A h.Underwater cables between the various pressure vesselsdistribute electrical power, and support system-controlcommunications and data transfers. A titanium strain-gauge pressure sensor mounted on the electronics sphereand sampled by the instrument controller is used to mon-itor instrument depth. (A dedicated sensor was em-ployed to simplify development; in future, pressure in-formation will be obtained from the CTD instrument.)

Profiler operations are regulated by a low-power(,0.02 W average consumption during profiling) mi-crocontroller manufactured by Onset Computers, Inc.(This value and subsequent power estimates for the in-strumentation are based on a supply voltage of 10.5 V.)Operationally, the profiler travels between a series ofuser-defined depth stations, with variable wait-periodsbetween each cycle. For example, an instrument mightbe programmed to rapidly cycle 10 times over the upper1 km of the water column, perform a full-depth profile,then wait three days before repeating the pattern. Lim-ited problem solving has also been implemented to over-come obstructions on the wire. If vertical travel isblocked, the profiler can be instructed to back up andtry again to move through the problem spot. If afterseveral tries it cannot pass the obstruction, the instru-ment reverses direction and proceeds to its next depthstation. The microcontroller also monitors the batterylevels and terminates operations if the voltage of eitherpack falls below user-specified levels (typically three-quarters of their initial open-circuit voltages). This ac-tion reduces the chance of venting that can occur whenbatteries are fully drained. The voltage test is based onaverage values over several hours of profiler operation.Thus, single spurious voltage values will not prema-turely terminate operations.

Moored Profilers constructed to date have beenequipped with CTD instruments manufactured by Fal-


mouth Scientific, Inc. (FSI, their Micro-CTDy). Thisinstrument was chosen for its small size (36 cm inlength, 5 cm in diameter), relatively low power con-sumption (1.2 W; a new version of the instrument is indevelopment that will consume 10% of the power ofthe present version), and freely flushing sensors. Themanufacturer’s specifications for the accuracy of thepressure, temperature, and conductivity sensors of thisinstrument are 65 db, 60.0058C, and 60.0005 S m21,respectively. The CTD is mounted through the forwardplate of the profiler body with the sensors extending outapproximately 10 cm. The instrument is supplied with1 Mbyte of internal RAM for temporary data storageduring profile operations. The CTDs deployed to datehave sampled at 1.65 Hz. Several Moored Profilers havealso been equipped with an FSI 3D-ACMy acousticphase-shift current meter with a customized, remotelymounted sensor head. The ACM consumes less than 0.3W of power. The remote mounting allows for the elec-tronics pressure case to be housed fully inside the pro-filer cowling. The horizontal arrangement of the caserequired a special mount for the standard tilt sensors;no change was required to the three-axis compass. Thestandard ACM ‘‘sting’’ was modified to the 458 pyramidarrangement seen in Fig. 1 to minimize measurementerrors caused by wakes off the struts supporting theacoustic transducers. Tow-tank tests were conducted toverify the performance of the modified design. As theprofiler is free to align with the horizontal flow, themodified arrangement presents two orthogonal horizon-tal acoustic paths and one vertically angled path to theincident flow that are upstream of any wake-generatingsupport struts. As with the CTD, the raw data from theACM (four path–velocity values, three components ofmagnetic compass and two of tilt, all logged at 2 Hz)are temporarily recorded on internal RAM during eachprofile. Communications between the instrument con-troller and external oceanographic sensors follow theRS-485 standard.

The mass of an assembled profiler, depending on in-strumentation, is 42–49 kg. In operation, the instrumentsare typically ballasted to be neutrally buoyant at mid-depth. The use of plastics and glass enhance the system’scompressibility, thereby minimizing buoyancy forcesincurred during profiling. Mechanical adjustment of theprofiler displacement during a depth cycle is not re-quired as the maximum buoyancy forces experiencedduring a 5000-m profile are less than 2.5 N. Further-more, this buoyancy change causes minimal energy losssince work against gravity in one direction of profilingis returned in the other direction. Ballasting (carried outshoreside following standard procedures) involvesweighing the assembled instrument in air and in a waterbath of known temperature and density. The mass ad-justment to achieve neutral buoyancy at midocean depthis derived using estimates of the instrument’s effectivecompressibility and thermal expansion coefficient anda representative in situ ocean temperature and density

profile from the deployment site. Based on the efficiencycurve of the drive motor, we estimate that the overallsystem efficiency is little changed by ballasting errorsas large as 0.4 kg because the added power required tomove in one direction is largely compensated by reducedpower in the other. However, drive wheel traction mustalso be considered; gross misballasting will result inwheel slip. Increasing the drive-wheel spring tensioncan reduce the chance of slipping, but at the expenseof more rolling friction and wear. As presently config-ured, we judge the instrument able to tolerate ballastingerrors up to about 0.15 kg without significant changein performance.

Vertical travel of the instrument along the mooringline is regulated by the controller, based on the observedpressure. At the start of a profile, the CTD and ACMare powered up and instructed to acquire and store theirrespective raw data. Engineering data (pressure, batteryvoltages, and electrical current to the drive motor) areacquired independently by the controller at 0.091 Hzand are also temporarily stored. A profile continues untilthe instrument achieves a scheduled pressure level, oran obstruction blocks travel along the cable. Rate oftravel is estimated over a user-specified time interval(typically 1–3 min) from the time-rate-of-change of ob-served pressure. The system concludes that it is stalledwhen the average travel speed falls below a preset value,typically 10 cm s21.

Upon completion of a vertical profile, the CTD andACM sampling is halted. The CTD data are then down-loaded to the controller’s RAM, combined with the en-gineering data, and archived to the hard disk interfacedto the controller. Data are written to disk in ½-Mbyteblocks. (Fixed block size is a limitation of the Onsetcontroller.) In normal operation, the CTD and engi-neering data acquired during a 5000-m profile fill ;60%of a block. ACM data are subsequently offloaded andarchived, again in ½-Mbyte blocks. (Two to three blocksare required for a 5000-m profile.) It takes approxi-mately 10 min to transfer each block of data to thecontroller and about 10 s to write these data to disk.(Each disk write, including the 3-s period when the diskis accelerated to speed, consumes ;34 J, a negligiblepart of the overall energy budget.) Then, depending onthe operation program, the instrument either proceedsto its next depth station or enters a ‘‘sleep’’ mode fora user-specified period. In the case of short profiles,multiple data segments are logged in the FSI instrumentRAMs prior to disk archiving (thereby optimizing diskstorage space). Profile segments are separated in the datafiles by two full data scans holding null values for eachvariable. Time is recorded at the initiation of each datacollection cycle. These times, plus the respective samplerates of the CTD, ACM, and engineering data, are usedto derive the time of each datum. Pressure data fromthe CTD are then used to register each observation indepth.

To facilitate, monitoring instrument performance


from a support vessel (e.g., to verify operation afterdeployment), the profiler has been fitted with an acoustictransponder. Operationally, the vessel holds positionover the mooring and a series of acoustic ranges aretaken over a 5–10-min interval (using a standard acous-tic release deck unit). A single range value is usuallysufficient to roughly determine where the profiler isalong the mooring wire. Variation of range with timedenotes profiler motion (assuming station keeping is rea-sonably good). In addition to this simple system, a real-time data telemetry link has been designed and docktested but not as yet demonstrated at sea.

System installations to date have followed standard‘‘anchor-last’’ deployment procedures. Typically, theprofiler is attached to the mooring wire after the mainbuoyancy and a few hundred meters of mooring wirehave been trailed overboard. A slip line is used to easethe instrument into the water. Drag on the profiler duringthe subsequent pay-out of the remaining wire pushesthe instrument up against the top mooring stop (placedto prevent profiler impact with the wire termination) sono major impact shock occurs when the anchor is re-leased. Overvoltage circuitry protects the profiler elec-tronics from induced emf caused by drive-wheel-in-duced motor turning during deployment. Standard re-covery procedures are also followed, though some skillis required with short (;1000 m) moorings to quicklyinitiate the tow on the mooring line so that the backupbuoyancy stays clear of the profiler after release. Serialfiring of two releases spanning the backup buoyancy,or doing without back-up buoyancy, has also proveneffective for short moorings.

3. Instrument performance

Observations gathered during several ocean deploy-ments will be used to highlight the performance of theMoored Profiler. A coastal deployment of an instrumentequipped with both a CTD and an ACM was made nearthe 1500-m isobath south of Woods Hole in September–October 1997. The mooring consisted of a 1.22-m-di-ameter steel buoyancy sphere positioned at ;90-mdepth (providing 5938 N of buoyancy), a single 1300-mlength of 0.64-cm-diameter jacketed wire rope, dual re-leases in series spanning backup buoyancy (glass balls),20 m of 2.5-cm plaited nylon rope and a 1360-kg cast-iron anchor. The Moored Profiler was programmed tocycle continuously in time between stops on the wireat ;100- and ;1400-m depth, pausing only as long asit took to archive data from the CTD and ACM to harddisk. The instrument completed 501 vertical profiles be-tween 100- and 1400-m depth during the 36-day de-ployment and was attempting profile number 502 whenthe release was fired and the mooring recovered. Totalvertical distance profiled on this trial was 652 000 m;analysis of the drive-system battery after recovery sug-gested that it was on schedule to meet the design goalof one million meters. The only irregularity in the pro-

filing was a 4-h interruption caused by a communicationerror between the controller and the oceanographic sen-sors. The controller software failed to deal with thiserror in a timely fashion but was eventually able toresume regular profiling. (Controller software has sincebeen revised to increase the profiler’s reliability.)

Deep-ocean performance is described using data froman older-style profiler fitted with a CTD deployed in4500 m of water southeast of Bermuda in June–July1997. This version of the profiler had the guide anddrive wheels enclosed inside the instrument cowling,and had minimal clearance for the mooring wire at theguide wheels. It was, consequently, prone to fouling.The mooring consisted of a 1.52-m-diameter syntacticfoam buoyancy sphere (providing 3923 N of buoyancy),a single 4400-m length of 0.64-cm-diameter jacketedwire rope, back-up buoyancy, a release, and a 1800-kgcast-iron anchor. Once deployed, the top wire stop waslocated at 60-m depth. However, an unknown agentfouled this instrument during the deployment operation.The profiler eventually freed itself, but the fouling ma-terial effectively acted as a top stop at ;1300 m duringthis trial. This instrument was programmed to pause for24 h at the bottom stop between profiles, but only longenough to archive data at the top stop. This instrumentcompleted 60 profiles between 4300 and ;1300 m be-fore it was recovered.

a. Mechanical system

Our principal diagnostic for the propulsion system isthe motor current, sampled every 11 s during profiling.During the fall 1997 coastal deployment, motor currentvaried between about 80 and 120 mA from the unreg-ulated 14-V supply, which translates to a power expen-diture of 1.1–1.7 W (Fig. 3a). The hydrodynamic drag(based on a speed of 0.3 m s21, 0.25 m2 cross-sectionalarea, and drag coefficient of 0.16) accounts for roughlyhalf of the power expended, as expected from the man-ufacturer’s quoted motor and drive-train efficiency.Somewhat greater current was drawn during these up-profiles than down-profiles (Fig. 3a), suggesting that theprofiler had been ballasted heavy. Based on the up–down motor current difference and the relationship be-tween motor current and torque, we estimate that thisinstrument carried 70 gm of excess ballast weight. Pro-file-averaged motor current varied during the deploy-ment (Fig. 4a). Initially, both up- and down-profile av-erage current fell with time, possibly because frictionallosses were reduced as drive-train components andguide wheels were broken in. Subsequent variations ap-pear related to profile speed (Fig. 4b) which, in turn, isa function of the incident currents (Fig. 4d). This as-sociation with ambient currents is seen more clearly intotal energy expended on each profile (Fig. 4c). Wesurmise that stronger horizontal currents cause higherdrag forces on the profiler body that increase the loadson the guide and drive wheel bearings and ultimately,


FIG. 4. Engineering data from the fall 1997 profiler deployment on the continental slope southof New England. (a) Profile-averaged motor current versus time for the up-going (light line) anddown-going (bold line) profiles. (b) Profile-averaged speed versus time for the up-going and down-going profiles. Down-profiles were faster than up-profiles. (c) Total energy expended by the drivesystem per vertical profile. Down-profiles took less energy than up-profiles. (d) Depth-averaged(100–300 m) horizontal ocean current speed vs time derived from the data acquired by the acousticcurrent meter fitted to the Moored Profiler.

their frictional losses. Additional energy is thus requiredto profile, and profile speed is somewhat diminished.But slower profile speed reduces hydrodynamic draglosses, which partially compensates in the total energyexpenditure per profile. The deployment-averaged en-ergy expenditure for vertical profiling during this trialwas 5.7 J m21. This figure extrapolates to 1583 W h toprofile 1 million meters; the battery for the propulsionsystem is rated at about 2100 W h.

Comparable drive system performance was seen inthe deep-ocean deployments off Bermuda. Motor cur-rent ranged between 100 and 200 mA with somewhatgreater scatter than seen during the coastal deployment(Fig. 3b). Motor current noise is caused by fluctuationsin drive-wheel speed. The enhanced variability suggeststhe profiler experienced larger-amplitude vertical wiremotions on the 4500-m Bermuda mooring than on the1500-m coastal mooring, as might be expected fromconsideration of wire stretch. Up- and down-profile mo-tor current values were comparable at about 3500-m

depth during the Bermuda deployment, suggesting thatthis instrument had been ballasted well. The greater spanof ocean in situ density and incurred buoyancy forceson this deployment, as compared to the coastal deploy-ment, is reflected in a slightly larger top-to-bottom rangein motor current. The top-to-bottom range is not large,however (&50 mA); this behavior convinced us thatneither active nor dedicated passive ballast control (be-yond choice of construction materials) was necessary.Indeed, the average energy expended to profile duringthis deployment was 7.1 J m21, which is close to thefigure obtained on the coastal deployment.

Compass and velocity data from the ACM acquiredduring the coastal trial shed light on the azimuthal be-havior of the profiler. At incident horizontal-currentspeeds less than about 10 cm s21, the profiler appearsto align closely with the incident horizontal flow. Theorientation of the instrument in geographic coordinatesthus varies slowly with time (depth) depending on thestructure of the horizontal flow (Fig. 5a). In stronger


FIG. 5. Time series (at 2-Hz sampling) of the profiler compassheading (black dots) and the relative direction (gray dots) and speed(thin line) of the measured horizontal currents relative to the instru-ment for two down-going profiles. (a) Time of weak horizontal oceanflow and (b) during stronger flow. Alignment of the incident hori-zontal flow with the profiler corresponds to a relative direction of 08.A profiler compass heading of 08 indicates the body pointing east-ward. Note the wagging seen in the instrument heading and its sig-nature in the measured horizontal current in (b) after stop impact atscan ;9200.

FIG. 6. Variation of observed conductivity averaged between po-tential temperature 2.258 and 2.758C as a function of profile numberfor the 1997 Bermuda deployment (3). Also shown is the variationof salinity averaged between potential temperature 5.08 and 5.58C inthe main thermocline after a profile-dependent conductivity bias wasapplied to match the historical deep-water potential temperature–sa-linity relationship (V). Discounting the handful of profiles where theconductivity cell was obviously fouled, the residual main-thermoclinewater mass variations of 60.01 pss are comparable to what has beenseen by the Bermuda shipboard sampling program.

currents, the instrument appears to adopt a persistentangle to the incident flow of about 208, independent ofprofile direction, possibly due to asymmetries in thecowling and/or sensor arrangement (Fig. 5b). This at-titude presents greater surface area to the horizontal flowand likely increases drag somewhat. The incident angleof the relative flow at these times does remain withinthe valid measurement sector of the ACM, however.After impacting the mooring stops, the profiler fre-quently oscillates through 6158 at a period of 10–20 s;signature of this wagging is evident in the velocity dataafter about scan 9500 in Fig. 5b. We may be seeing theeffects of vortex shedding off the instrument cowling,though it is unclear why this happens only when theinstrument is at rest. A tail fin to improve the profiler’salignment with the flow and reduce its wagging has beenconsidered but not yet implemented.

A two-axis tilt sensor integrated with the ACM andpositioned within its electronics housing may be usedto monitor mooring wire angles since the guide wheelsforce the profiler to align with the wire. Maximum tiltsobserved during the coastal deployment were &88 andwere typically ,28; the largest tilts were at the bottomduring episodes of strong horizontal flow, as predictedby mooring-response programs. The current meter mea-sures orthogonal components of the flow, and so byusing the tilt data, the relative velocity data may berotated into true vertical and horizontal components.Also significant are the horizontal profiler motions rel-ative to the ground as the instrument moves along thetilted mooring wire. Analysis for the flow in body co-ordinates (a coordinate system rotated into the plane ofthe local mooring wire slope) relates the ocean’s hori-zontal velocity (uo) to the measured relative velocitiesin body coordinates (ur, wr): uo 5 ur cosu 2 (wr 1dl /dt) sinu. Here, u is the angle of the mooring wirefrom vertical and dl /dt is the speed of the profiler alongthe wire. For small wire angles and weak ocean verticalvelocity, the second and third terms in the expression(both of which depend strongly on the speed and di-rection of travel along the wire) tend to cancel, so thatthe measured velocity perpendicular to the mooring wireis a good estimate of the ocean’s horizontal velocity.Insensitivity of the derived ocean current to directionof travel was confirmed in an analysis of 226 down–upprofile pairs from the coastal deployment. For eachavailable profile, the estimated speed of the horizontalocean current (with no tilt corrections) between 1300-and 1400-m depth were averaged. The speeds sampled


FIG. 7. Potential temperature–salinity curves from selected profilesfrom the beginning (black) and end (gray) of the 1997 Bermudadeployment. A bias correction was applied to conductivity on eachprofile to match the historical deep-water potential temperature–sa-linity relationship. Also shown is an example of a profile where theconductivity sensor was fouled, probably by seaweed (notable byanomalously low-salinity values at potential temperatures warmerthan about 3.58C).

on down-profiles were different from those sampled onthe subsequent up-profiles by only 0.5 cm s21 on average(up-profiles larger) with a standard deviation of 0.9 cms21. This small difference between up- and down-goingvelocity estimates could be explained by a 18 misalign-ment of the current meter sensor sting relative to thevertical axis of the profiler body/guide wheels.

A similar analysis for the ocean’s vertical velocity,wo, yields wo 5 (wr 1 dl /dt) cosu 1 ur sinu. Additionof a device to measure the instrument translation speedalong the wire would facilitate estimation of the ocean’svertical velocity. However, assuming the pressure fieldis dominantly hydrostatic, the time rate of change ofmeasured pressure provides an independent estimate ofthe profiler’s vertical velocity, and in turn, an alternateestimate of wo: wo 5 wr cosu 1 ur sinu 1 dZ(P)/dt.

b. Oceanographic sensors

High-frequency noise levels of the MicroCTD in-struments deployed on Moored Profilers appear consis-tent with the manufacturer’s specifications. The standarddeviations of high-passed (15-s cutoff period) temper-ature, conductivity, and pressure data from the coastal(Bermuda) deployment were 0.7 (0.5) m8C, 0.001(0.0015) mmho, and 0.1 (0.2) db, respectively. On oc-casion, seaweed or some other material appeared to im-pact the conductivity sensors and greatly increase high-frequency noise (as well as temporarily shift the sensorcalibration, see below).

Laboratory calibration data obtained before and afterprofiler deployments shed light on the stability of thetemperature and pressure sensors. Owing to frequentmodifications and upgrades to the instrumentation be-tween deployments, we do not as yet have a long time-history of any one sensor. Short-term stability appearsgood. For example, the postrecovery temperature sensorcalibration of the CTD instrument used on the fall 1997coastal deployment was within 3 m8C21 of that obtainedprior to the deployment over the temperature range 18–308C. Pre- and postdeployment laboratory pressure cal-ibrations typically agree to better than 5 db over therange 0–6000 db with the difference largely describedby a bias change. It is our intent to monitor the sensorsthrough repeated laboratory calibrations over extendedtime periods once the instrument design is stabilized.

Stability of the CTD pressure sensors were also as-sessed using in situ observations. On both the coastaland Bermuda deployments, the instruments were in-structed to profile to the bottom stop (which was po-sitioned within 100 m of the anchor). Thus, change inthe observed maximum pressure sampled on each profilemay be interpreted as sensor drift, as any mooring blow-down in strong flow causes little depth change of thebottom stop. Maximum observed pressure decreasedover the first 10–20 profiling cycles in these two de-ployments, then remained stable (61 db) for the balanceof the deployments. During the coastal deployment

(maximum pressure 5 1430 db), the pressure drifted atotal of 6 db; the Bermuda data (maximum pressure 54370 db) show a 6-db drift in the first two cycles, fol-lowed by an additional 5–6-db drift over the next 10cycles. A comparable analysis of sensor drift using dataat the top mooring stop is precluded by real top-stopdepth changes associated with mooring motion. How-ever, comparison of the engineering pressure sensor ob-servations with those from the CTD suggest similarCTD pressure drift behavior with time at the lower pres-sures of the top stop. (Thus, the MicroCTD pressuresensor drift appears to be basically a bias error, cor-rectable by monitoring pressure at the bottom-stop.)Analysis of more recent deployments of CTDs with up-graded pressure sensors show bottom-stop pressuredrifts of less than 2 db.

Conductivity sensor behavior may be monitored within situ data if the instrument profiles a segment of thewater column having a stable deep water potential tem-perature–salinity relationship. The deep waters off Ber-muda satisfy this requirement. Raw conductivity ob-servations on deep isotherms fluctuated by 60.01 mmhoduring the Bermuda deployment (Fig. 6) superimposedon a drift of comparable magnitude towards larger val-ues with time. Time-dependent adjustment of the cellcalibration to force the deep temperature–salinity datato agree with observations from the Bermuda AtlanticTime Series Station (BATS: Michaels and Knap 1996)is reasonably successful in producing valid salinity dataat all depths (Fig. 7). Those profiles exhibiting highlyanomalous salinity data at thermocline depth (Fig. 6)are obviously affected by cell fouling (e.g., Fig. 7). Be-cause the cell appears to recover substantially after these


FIG. 8. (a) East and north components of ocean velocity observedon one vertical traverse by a Moored Profiler in May–June 1998(black lines) with a nearly simultaneous profile obtained with anExpendable Current Profiler (XCP, gray line) deployed within 1 kmof the mooring. Both profiles have been binned at 2-m vertical res-olution. The (absolute) Moored Profiler components have been offsetby 225 (east) and 125 (north) cm s21 for clarity. The relative XCPcomponents were referenced to the absolute profiler components bymatching depth-averaged values and are plotted offset by 110 cms21 from their respective Moored Profiler components. (b) East andnorth components of ocean velocity simultaneously observed by threeMoored Profilers spaced horizontally by approximately 500 m. Theblack curve is the same velocity profile shown in (a), similarly offset.The two companion velocity profiles (gray curves) have been offsetfrom the first by 210 and 110 cm s21 for clarity.

events, we believe seaweed or other material occasion-ally drapes over the cell (as opposed to something at-taching and possibly growing on the cell; the cell was,in fact, visually clean on recovery.) Being in shallowerwater, an in situ check of the coastal deployment con-ductivity data is more problematic as real temperature–salinity variability cannot be ruled out. With a constantcell calibration derived to match reference salinity dataobtained on mooring recovery, salinity on potential iso-therms of about 800–1100-m depth exhibited variationsranging over 0.03 pss (discounting those handful of pro-

files where the cell was obviously fouled). These var-iations are slow in time (spanning multiple profiles), asmight be expected from real ocean water mass changeon the continental slope. The differences in salinity onisotherms between successive profiles have a standarddeviation of only 0.002 pss. Ongoing trials offshorefrom Bermuda will further investigate longer-term con-ductivity sensor stability, exploiting the stable deep wa-ter properties there and the BATS observations as ref-erence information.

Noise in the raw path-velocities of the ACM wereestimated from high-pass-filtered time series (cutoff fil-ter of 33-s period, where frequency spectra of the path-velocities flatten). When incident horizontal currentswere less than about 10 cm s21, the standard deviationof the high-passed raw velocity data was 0.6–0.9 cms21. This increased to 1.3–1.5 cm s21 when incidentcurrents approached 20 cm s21. The velocity noise ischiefly due to instrument vibrations associated with pro-filing along the irregular mooring wire; the high-fre-quency standard deviations drop to around 0.1 cm s21

after the drive motor is shut down at the bottom stop.(The current meter collects data for a few minutes afterthe profiler hits the mooring stops.) Assuming the noisein each 2-Hz sample is independent and normally dis-tributed, the standard error in 2-m-averaged velocityestimates is &0.6 cm s21 and in 10-m averages is &0.3cm s21. Bias error in the raw path-velocity data wasquantified in the WHOI tow-tank facility and investi-gated using the in situ observations. (Unlike conven-tional instruments that vector average data in time, theprofiler records raw data, allowing bias corrections dur-ing postrecovery processing.) These latter observationsfrom the bottom stop (when all four channels shouldread the same velocity value if the instrument is alignedwith the incident flow) hinted that one of the ACMchannels had a bias of approximately 3 cm s21, a factorof 2 larger than observed in the tow-tank. This is cur-rently under investigation.

An experiment conducted in May–June 1998 on thecontinental slope east of Virginia provided an oppor-tunity to compare velocity profiles from the MooredProfiler and an Expendable Current Profiler (XCP).Three profiler moorings were deployed in this experi-ment, configured much like the 1997 coastal trial de-scribed above. The profilers were synchronized to ini-tiate a cycle from the bottom stop (1100–1200 m) tothe top stop (;100 m) and back every 3 h. The XCPwas launched within 1 km of these moorings when theprofilers were midway between their top and bottomstops. Good correspondence is seen between the twoindependent measurements of ocean velocity (Fig. 8a).Depth offsets of individual velocity features may be dueto error in the XCP fall-rate prescription. Comparisonof the simultaneous Moored Profiler velocity data fromthe three instruments (deployed in a triangular arraywith about 500 m between moorings) (Fig. 8b) suggeststhat much of the remaining profiler–XCP differences


FIG. 9. Depth–time contoured data from the Moored Profiler deployed on the New England continental slope. (a)Temperature and salinity data and (b) the east and north velocity components. For this display, the data were regriddedto 20 db and 4 h prior to contouring.


FIG. 9. (Continued)

may be real. A detailed analysis of the spring 1998 datais under way.

The 1997 time series data from the continental slopeevince the exciting science that is possible with this newinstrument. The general stratification at this time had

temperature and salinity both decreasing with depth. Onlong timescales, the upper waters appeared to warm andincrease in salinity (Fig. 9a), probably the result of on-shore advection of the shelf-break front. Two mesoscaleevents were sampled by the mooring that caused sig-


FIG. 10. Vector time series plot of the vertically averaged oceancurrent between 200- and 1300-m depth. A vector for each profileoriginates on the abscissa at the time that profile was initiated. Astick pointing along the time axes corresponds to eastward flow, oneparalleling the ordinate denotes northward flow. Amplitude infor-mation is given by the ordinate label.

nificant vertical displacement of deep isotherms and iso-halines (centered on days 12 and 27 in Fig. 9). High-frequency vertical displacements by the internal tide arealso evident; some but not all exhibit high vertical co-herence, suggesting a low-vertical mode. These ageo-strophic motions potentially alias traditional hydro-graphic data and geostrophic transport estimates (Chi-swell 1994). The accompanying horizontal velocity dataare equally rich in temporal variability. The depth-av-eraged current was directed to the northwest during thisdeployment (Fig. 10), the northward component beingconsistent with the warming trend seen through the rec-ord. The mesoscale events seen in the temperature andsalinity data are also evident in the depth-averaged ve-locity record. As seen in Fig. 9b, these flow featureswere bottom-intensified. At other times in the record,the flow was strongest at the surface (e.g., the first weekof the deployment). At higher frequency, tidal currentsappear well resolved by the 2-h sampling. In additionto a 2–3 cm s21 peak-to-peak amplitude barotropic tide(see Fig. 4d), the profiler also documented highly var-iable baroclinic structures. Individual profiles frequentlyshow significant coherence between components andturning of the velocity vector with depth typical of low-frequency internal waves. Ongoing analysis will inves-tigate relationships between the barotropic and baro-clinic tides and other internal waves, and the nature ofthese baroclinic mesoscale flow features.

4. Discussion and future plans

The Moored Profiler has thus far been equipped witha CTD and an ACM. Given its flexible control system,

other oceanographic sensors could be easily added tothe sensor suite. These might include a dissolved oxygenprobe, nutrient sensors, and optical devices. The chiefconstraints are the size and weight of the added sensorsand their energy budgets. Intermittent sampling may berequired for ‘‘power-hungry’’ sensors, and additionalbuoyancy may be need to balance a substantially moremassive instrumentation payload. Neither is seen as lim-iting.

Preliminary design and partial testing of a real-timedata telemetry system has been carried out. The conceptutilizes an inductive modem (Frye and Owens 1991) torelay information from the profiler along the jacketed(hence insulated) mooring wire and a slack surface teth-er to a small surface buoy that supports the satellitecommunication hardware. The modem utilizes a ferritecore fitted to the profiler that surrounds the mooringwire. Seawater provides the electrical return path. Anacoustic link between the profiler and surface buoy ap-pears to be a viable alternative. A second round of de-sign work is needed to explore these options followedby field trials.

The present Moored Profiler design that has increasedclearance around the mooring wire and places the driveand guide wheels out in the flow has proven far lesssusceptible to instrument arrest by natural biologicalfouling than our earlier designs. Although the new de-sign has not suffered such fouling, concern remainsabout entanglement by fishing line or other human in-tervention. We may in the future explore systems toclear these types of obstructions from the mooring wire.Profiling free vehicles may ultimately prove more re-liable than moored systems for monitoring water prop-erties in heavily fished regions.

The Moored Profiler testing program is beginning todemonstrate the promise of this new technology foroceanographic research. The ability to collect rapidlysampled time series data at high-vertical-resolutionnearly spanning the full water column is proven. Thus,limited-duration process studies that exploit this capa-bility are immediately possible. The experiment in May–June 1998 investigating internal waves on the conti-nental slope successfully deployed an array of three pro-filers in this mode. Testing of the profiler’s long-termbehavior, a capability required for monitoring studies inremote, inhospitable regions of the world, is under way.We are also working to make Moored Profilers availableto the community. A nascent shared-use facility has beencreated at the Woods Hole Oceanographic Institution.Moreover, the Moored Profiler technology has been li-censed to McLane Research Laboratories, Inc., of Fal-mouth, Massachusetts, which anticipates commercial re-lease of the system when development is complete.

Acknowledgments. Initial development and testing ofthe Moored Profiler was supported by the Ocean Tech-nology Program of the National Science Foundation andthe WHOI Director’s discretionary fund. Additional de-


velopment and testing have been funded by the Officeof Naval Research, and the NOAA–University Consor-tium program. We greatly appreciate this support andthe sustained confidence of our program managersthrough our lengthy and at times trying developmentprogram. We wish to also acknowledge the technicalsupport to the project provided by M. Cook, K. Fair-hurst, T. Hammar, A. Hinton, E. Hobart, J. Kemp, N.McPhee, E. Montgomery, and G. Packard, and thanksto E. Kunze for providing the XCP data.

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