Reprinted from IEEE Oceans '93 Proceedings

Automated Ice-Ocean Environmental Buoys (lOEBs)for the Telemetry of Air, Ice and Ocean

Data from the Polar Oceans

R. Krishfield and S. HonjoWoods Hole Oceanographic Institution,

Woods Hole, MA 02543

W. B. Tucker IIICold Regions Research and Engineering Lab,

Hanover, NH 03755

Abstract -The Ice-OceaD EDvlroDmeDtal Buoy (lOEB) was

developed to acquire aDd telemef;er In Dear real.tlme Inter-relatable time-series data ODatmospheric, oceaDographlc aDdIce physics In Ice-covered oceans during aU seasons.MechaDlcally, the IOEB consists of an extremely durablesurface notatloD package aDd aD uDderwater mooring line ofInstrumeDts aDd sensors. The apex cODtalns data loggers formeteorological, Ice physics aDd eDgineering measuremeDts,mlcrocoDtroller modules for accumulating the data, aDdARGOS platform transmit terminals (P1Ts) for broadcastlDgthe data. The oceaD sensors Include cODductlvlty/temperaturerecorders, aD Acoustic Doppler CurreDt Promer (ADCP), adissolved 0xYgeD sensor, a transmlssometer aDd twonuorometers. .Furthermore, a suspeDded particle collector aDdsedlmeDt trap collect biogeochemical samples at the bottom ofthe 110 m suspeDded mooring. ID April 1992, two IOEBs weresuccessfully deployed at two separate Ice camps In the ArcticOceaD with battery power adequate to sustain the systems forover two years.

I. INTRODUCfION

The Arctic is a fascinating area to study, not onlybecause it is one of the last, and least understood, regions ofthe Earth that has been explored by man, but also becausemany of its unique properties are so important to globalclimate change. As a system, it is composed of genuinelyinteractive elements and processes that cannot be adequatelydescribed by confined individual disciplines. Complexity isfurther increased by the large amplitudes of seasonalvariability of the polar environment, which preventsextrapolating knowledge over even relatively short timeperiods. Consequently, in order to effectively examineprocesses in this region, what is needed primarily arecontinuous concurrent observations of the atmosphere, iceand ocean behavior in all seasons. Due to the extreme

severity of the Arctic winter, however, manned observationstations are not only costly, but also very dangerous. Whilethere are some particular advantages to the remote sensingof the polar regions by satellites and single-sensored buoys,

T. Nakanishi and T. TakizawaJapan Marine Science and Technology Center,

Yokosuka 237, JAPAN

in situ measurements of a large array of sensors with greateraccuracy and intercomparability can be made by unattendedautomated telemetry stations. Since 1987, we have beendeveloping such a buoy system, called the Ice-OceanEnvironmental Buoy (IOEB) which includes extensiveatmospheric, ice physics, and oceanographic sensors. Interms of the number of sensors, volume of data, andsophistication of the telemetry system, the IOEB representsan exceptional achievement in the area of ocean remotesensing technology. In particular, emphasis is on thecollection of biogenic flux by the buoy mooring system; atechnology unprecedented under the ice-covered oceans.Combination of the organic carbon flux with the othercritical data will not only aid in understanding thebiogeochemical cycles and processes of the Arctic Ocean,but also the associated oceanic interactions with the air andthe ice, and their relation to global changes.

II. BACKGROUND

A Arctic Environmental Drifting Buoy

In 1986 the Arctic Environmental Drifting Buoy(AEDB) was developed to suspend a time-series particlecollecting sediment trap underneath the icefloes in theTranspolar Drift, primarily to collect biogeochemicalsamples in this area of sparse measurements. Furthermore,this mooring was outfitted with an acoustic Doppler currentprofiler (ADCP) and ice thermistors, to measure respectivephysical characteristics of the sea and ice. The followingyear, this prototype of an IOEB was deployed from the FISPolarstern at the northernmost point, 86. N latitude,obtained during a 61-day cruise [1]. The AEDB wastransported over 3,900 km by the Transpolar Drift out fromthe Nansen Basin, through the Fram Strait, along the EastGreenland Current, and was recovered west of Iceland after

. 255 days in the pack ice and marginal ice zone (MIZ). TheAEDB did not have telemetering capability, but eachinstrument stored the data internally. Upon recovery,significant data were retrieved from a number of instruments

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on the mooring system. For example, the first independentlyobserved, long-term acoustic Doppler profiler (ADCP) dataprovided observations on internal waves [2] and diurnaltides [3]. For another example, ice thermistor stringsprovided data showing the evolution of the thermal profileof sea-ice during the summer in the Nansen Basin, andallowed calculation of the oceanic heat flux in the Fram

Strait [4].Besides the importance of the recovered scientific

data, the AEDB demonstrated that it was possible to trackand recover a durable suspended mooring system in theArctic where conventional mooring systems are not alwayslogistically or technically feasible. However, severallimitations of the AEDB needed to be addressed to make this

system more durable and more valuable to the science. Forinstance, the 5 foot diameter steel surface package whichhoused the transmitters was severely dented on all sides byice pressure north of the Fram Strait and in the EastGreenland MIZ. Since this buoy was recoverable, thenfuture systems would need reusable surface packages.Furthermore, because the sediment trap package at thebottom of the mooring parted from the buoy beforerecovery, a stiffer but stronger steel-based mooring segmentwould be used to suspend the package instead of elasticnylon rope. However, it is nearly impossible to design anydevice that can withstand the most enormous ice pressuresand environmental characteristics that can occur in theArctic, and even the most durable system could bedestroyed. Consequently, the mooring cables required theaddition of conductors to allow transmission of the scientificdata from all the instruments, in case of destruction. Withthese concerns in mind, a Preliminary IOEB (P-IOEB)mechanical package was constructed to test severalimprovements in the mechanical and electronic design.

B. PrellmlnDry lOEB'

One of the major improvements of the P-IOEB overthe AEDB was the use of Dupont SurlynRionomer foam forthe outer protective shell of the buoy. This flotation collarsurrounds an aluminum chamber housing the surfaceelectronics and is sandwiched between an aiuminum topplate and steel bellmouth weldment. The semi-conical collarprovides nearly twice the buoyancy required to sustain the1800 Ibs. of surface package and mooring system. Toincrease abrasion resistance, the foam density used was 10PCFj double what is commonly used for open ocean floats.The outer skin is further densified by heat and pressure toform an extremely tough outer skin. Furthermore, electricalconductors were added to the 10 m strength cable that was topenetrate through the ice and suspend the underwatermooring system. Only one instrument was attached at thebottom of the ice strengthened ElM cable which comprisedthe whole mooring system. Below this, a 600 Ib anchor was

fashioned in the field to simulate a heavier mooring system.Inside the electronics tube, two Synergetics PITs

transmitted ARGOS data via patch antennas mountedexternally on the surface of the top plate. One OnsetComputers TattletaleR 3 (Tf3) data logger was used to feeddata to both PITs via an RS-232 serial port connection. Thedata consisted of information from a tensiometer mounted inthe bellmouth flange, and seawater conductivity andtemperatures from the standard SeaBird SeaCatR at the endof the mooring.

This test package was deployed in sea-ice in theNorthwest Passage from the USCGC Polar Sea during mid-September 1990 for 20 hours, while the operation wasmonitored and engineering measurements were taken. Theobjectives of the Northwest Passage experiment were to testthe strength of a surface buoy and the reliability of thethrough-ice ElM cable in broken sea ice. The icebreakerwas used to induce ice pressure on the float, whileobservations on the mechanical effects were performedvisually, and via ARGOS data transmission. After manysuccessful collisions of the buoy between icefloes, andextended periods of gradual pressure, the buoy was retrievedundamaged from the Canadian waters.

The visual results from this test confirmed the

slipperiness of the outer SurlynR foam shell by escapingvertically between 1 m icefloes as they came togetheraround the buoy. Conductivity, temperature, and depthreadings were reliably transmitted at all times by theSeaCat'sR serial RS-232 port up the ice-penetrator cable tothe Tr3. Tension readings from the load cell mounted in-line with the mooring system showed remarkable sensitivityto variations as the surface float bobbed, or was pushed upby icefloes. '

The results of the P-IOEB test confirmed that

ionomer foam at 10 pcf density could endure Arctic iceconditions and could be reused without major repair anumber of times. In addition, although the ice strengthenedElM link performed reliably in marginal ice conditions, laterthis was changed from an assembly based on wire rope toone which incorporated chain for extra robustness.

10. DESCRIPTION OF IOED

A. Mechanical Design

Two IOEBs were developed in 1990-91incorporating the combined improvements over theprevious systems, and with additional environmentalsensors. The new surface package was constructed using theSurlynRionomer foam shell, and most importantly, all of the

. instruments on the IOEB were networked to communicatewith independent microcontrollers (MCUs) using an EIA48Sstandard network configuration in order to provide thetelemetry capability. Furthermore, new meteorological and

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J

Ice-OceanEnvironmentalBuoy (IOEB)

Depth below buor a m ->

14 m ->

1. Air ..nlor.2. Top ploto ond ondcop3. Eloclronlcs tubo4. room shon5. aon-mouth flongo6. Rey.r.. echo sounder7. Stroln/optlcol IOnsoro8. Ico thormlstor.9. CTDwith DO, fluoromet.r1O. AOCP11. CT recordor12. CT recorder13. Sodlmont trop14. Wat.r Iransf., syslem15. rluoromoter16. Tronlmluom.I.,17. Curronl motor with loggor18. Anchor

45 m ->

76 m ->

107 m ->

110 m ->

111 m ->

Fig.t. Schematic drawing of the IOEB mooring system withinstrumentation.

ice sensors were added to the array of scientific sensors.Each IOEB consists of a surface flotation package

and a 110 m long mooring system of oceanographic sensors(Fig.1). The surface buoy, or apex, supplies the buoyancyfor the suspended instrumentation, and serves as theplatform for the satellite transmitters, network MCVs, andmeteorological and ice sensors. Deployed through aImhole in an icefloe, the underwater mooring system includesan ADCP, err recorders and a complete biosensor package.A 500 lb. anchor serves to keep the mooring taught at thebottom of the system.

The 124 cm diameter apex of the IOEB protectivelyhouses the electronic controller, satellite communicationsystem and battery array for the surface devices, andsupplies buoyancy for the drifting mooring. The length ofthe apex alone is 200 cm. The' electronics pressure housingis centrally located in the surface buoy and measures 25 cm

I.D. by 125 cm in length. The top end cap accommodates 11pressure resistant electrical penetrators through which sensor

. informationis conveyedinto the tube, and subsequentlyrouted out to the antennas. Inside the electronics tube aredual Synergetics PTTs which provide the uplink signal tothe ARGOS satellite. Also inside are dual MCV units, basedon TI'3 data loggers. These units acquire the data from allthe individual instruments on the network, and each suppliesthe resulting datastream to one of the PTTs. In addition,several mechanical and meteorological sensors, and the dataloggers to acquire data from all the air and the ice sensorsare also mounted in this tube. The remaining space isoccupied by the battery packs to power the surfaceelectronics. This is provided by fused and diode protectedpacks of Electrochem lithium "DD" oxyhalide batteriesassembled in plastic molds. These batteries were chosen fortheir excellent power density and extreme low temperaturecharacteristics. Nearly 200 cells are required to power theIOEB surface electronics for 3 years at Polar atmospherictemperatures. To create modularity in the design of theelectronic network, each PTT, MCV, and data logger waswired to a separate battery supply to provide powerindependent of the other electronics.

Covering the pressure housing and flotationpackage, an aluminum top plate protects the cables attachedto the penetrators on the pressure housing, provides aplatform for mounting the transmitting antennas andmeteorological mast assembly, and transmits the mooringline tension through a tie rod assembly. The shape of thefIat patch antennas used to convey the transmission from thePTT to the ARGOS satellite allow them to be mounted flushalong the top surface of the buoy to prevent damage, andstill provide a nearly 180 degree radiation pattern.Furthermore, these antennas may be submerged to a fewmeters depth without being harmed. In the AEDB, similaroil-filled patch antennas were successfully used, althoughthe 401.650 Mhz ARGOS carrier frequency would shift ifoil leaked from the housing. As a result, on the IOEBs, theradiating and reflecting surfaces are now encased in a hardepoxy mold, with a two-wire coaxial pressure resistant cableprotruding out from the underside.

Located at the bottom of the surface buoy, agalvanized steel bell-mouth flange reduces the strain on thearmored conducting cable which attaches to the mooringsystem. The ElM link connecting the surface buoy to theElM mooring cable consists of 3/8" trawler chain with threeconductor jacketed cable terminated with steel clevises, andpolyurethane potted to form a semi-rigid, water-tight link6.35 cm in diameter and over 7 m long. The otherunderwater mooring segments are constructed from jacketedsteel armored three-conductor cables. Each end of the cableis fitted with an ElM termination and a unique underwaterT-splice connector for electrical access to the EIA485network. These cable assemblies have a breaking strength

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of 4700 kg. Between terminations, stainless steel cagescontain most of the IOEB instruments; positioning them in-line with the mooring system.

B. Sensors and InstrIInWIts

The scientific sensors on the IOEB can be dividedinto three main categories: meteorological sensors, icesensors, and ocean sensors. Simultaneously, data is acquiredby all the sensors throughout the lifetime of the buoy. Whileindividual groups of sensors are included with specificscientific objectives in mind, intercomparison betweensensors from different fields may lead to the most innovativeresults. All sensors that are located in the air, in the ice, orin the surface float are rated to operate to temperatures atleast as low -40 .C, in order to function during the severecold Arctic winter. Underwater instruments are rated to atleast -5 .C operation.

The IOEB meteorological sensors consist of aParoscientific DigiquartzR barometric pressure sensormounted inside of the electronics tube, and ported to thesurface atmosphere through a labyrinth water trap, and a 6foot long aluminum mast protruding above the top plate andsupporting a R.M. Young air temperature sensor withradiation shield, and a R.M. Young wind monitor.Furthermore, the meteorological data logger obtains a smallamount of mechanical information: an Aanderaa magneticcompass and Spectron electrolytic XIY tilt sensor detect therotation and incline of the top package, while a Metrox loadcell measures strain at the mooring cable junction.

Nearby the apex are installed ice sensors which areinterrogated by a Campbell ice data logger inside theelectronics housing. Two ice thermistor chains, comprisinga total of 33 thermistors profile temperatures from the icesurface down into the surface seawater, similar to theAEDB. Improving on the previous buoy, however, theIOEB obtains a more precise ice thickness measurementfrom a Simrad echo sounder module pointed upward from amount situated below the icefloe, and Geokon stress sensorsmeasure internal ice stress in two direction at three depths inthe icefloe.

In the seawater below the icefloe, oceanicmeasurements are made by an ADCP, an electromagneticcurrent meter, 3 err recorders, a dissolved oxygen sensor, 2fluorometers, a transmissometer, a time-series microfilteringsuspended particle collector, and a time-series sediment trap.

Two units on the mooring line measure currents.The first is an RD Instruments 150 kHz ADCP which

collects ocean current data in a profile underneath the sea-ice. Using the Doppler principle, the ADCP determinescurrents in a series of depth bins below its sensor faces, andcollects temperature, heading and tilt information at thedevice's depth. The ADCP produces (and internally records)the largest amount of data each transmission cycle,

compared to the other instruments. As a result, a dataprocessing module (DPM) is required to interface betweenthe ADCP and the buoy telemetry system by processing andreducing the information so that a representative subset ofthe data can be broadcast via the ARGOS system. The DPMwas developed by Pleuddemann et al. [5] primarily forimplementation on the IOEB. Due to the maximumallowable data rate through ARGOS, the ADCP data isreduced by about a factor of 170 by the IOEB DPMs.

The other current measuring unit is an InterOceansS4R current meter which determines current

electromagnetically. In order to communicate with thesurface electronics, a separate underwater data buffermodule is required to interface between the units. Outfittingthe IOEB S4sR with a temperature probe and an inductiveconductivity sensor also provides a deep watermass datapoint along the IOEB mooring line.

SeaBird SeaCatR conductivity and temperaturerecorders are used at 3 locations along the mooring line toprovide points above, below, and in the holocline. One unitis outfitted with a dissolved oxygen probe and a fluorometerspecifically to detect primary productivity. SeaBirdElectronics cooperated extensively by modifying both thefirmware and communications hardware to converse with

the data acquisition electronics in the surface buoy, using theappropriate protocol. A second Sea Tech fluorometer,paired with a Sea Tech transmissometer, have a separateTT485 data logger for acquiring data and communicatingwith the surface electronics.

Mclane Research water transfer systems with 18NucleoporeR filters, are deployed at different locations onthe separate IOEB mooring lines. One water transfer systemis independently located at a shallow depth, while at the verybottom of the conducting cable, the other pumping system ispaired with a Mclane Research 21-sample sediment trap,providing information on both suspended and fallingparticles at that depth.

Except for the suspended particle collector andsediment trap, most of the information from all of the aboveinstruments are digitized electronically, collected by eachMCU, and transmitted via the ARGOS satellite back to thelaboratory in near-real time. The buoy systems are intendedto be recovered and most instruments record their completedatasets internally, but only the particle collectors requireretrieval so that laboratory analysis can be performed on theacquired samples upon completion of each buoy's journey.

C. Electronk Architecture

The IOEB uses the IEEE ElA485 standard [6] for. network communications from individual instruments to twoMCUs, which in turn transfer data to PTTs for real-timesatellite transmission. The ElA485 standard specifies theelectrical characteristics of generators and receivers for the

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PTTa

PTTb

Sehedu(e HOUT 1 Hour 2 Hour 3 Hour -I Hour 5 Hour 6

Fig.2 Schematic of IOEB transmission scheme. Numbers in parentheses

indicate the number of bils of data transmitted by that particular instrument.

interchange of binary signals in multipoint interconnectionof digital equipment. Implementing this standard means thatmultiple instruments may be networked using only twoindividual conducting cables. This is mechanicallyadvantageous because it eliminates the bulkiness of multi-cable assemblies that would be required using otherdedicated single-point standards. On the other hand,complexity in the system is increased, since care must betaken to ensure that only one driver is active at a time, ormeaningless overlapping communications will occur. Bysequential addressing, each controller allows only one of thedata buffers to be active on the network at any time. Eachindividual data logger, data buffer, controller, andtransmitter module is provided with separate battery powersupply to ensure isolation from faults passing between units.

The Serial ASCII Instrumentation Loop (SAIL)data communications sta'ndard [7] was adapted to define thesoftware protocol on the instrument network. On theIDEBs, the network MCUs begin each sequence by sendingan attention character ("#") over the network, which wakensall of the instruments on the mooring system. Each of thedevices is individually interrogated serially at 9600 baud, bysending the attention character first, followed by the uniquetwo character address and the character "R". When

addressed, an instrument responds by repeating its addressfollowed by ASCII hexadecimal data, and the messageterminator character ETX.

Because more than 256 bits of data is beingacquired by the 10EB sensors every measurement interval, acomplicated transmission scheme is used to compress all ofthe information for ARGOS broadcasting (Fig.2).According to the particular sequence, each MCUinterrogates a subset of the 10EB instrumentation, compilesthe data and then passes it to a P1T via an auxiliary UARTcircuit. Then, after hibernating in micropower mode forapproximately 57.s minutes, the MCU implements the nextsequence and repeats the process. There are a total of sixsequencesthat each MCUutilizes,and for simplicityin themodular design, each MCU exclusively controls the flow ofall sensor data to only one P1T.

The first three bits in every 10EB transmissionindicate the sequence number that is being broadcast. Sinceeach sequence has its data broadcast for a period of justunder one hour, after nearly six hours, all of the sequenceshave had all of their data transmitted, and this processrepeats from the beginning. According to this scheme,instrument data that is acquired infrequently may only bebroadcast once during the six sequences, while the morefrequent data may be updated every other sequence. Thesame sequence formats are transmitted by each P1T/MCUpair, only during operation their timing is offset by 3 and ahalf hours to maximize the data throughput of the combinedunits. In effect, when both pairs are operating properly, thefrequency of the data transmission is doubled, which allowssome of the broadcast sensor data to be updated nearly everyhour. '

During operation, each IOEB transmits 512 bits ofnew sensor information every 60 minutes via ARGOS. Thisis equivalent to a throughput of 12k bits/day of new data perbuoy. Currently this data is being collected and processeddaily in both the United States and Japan.

IV. FIElD OPERATION AND PRELIMINARY RESULTS

Two 10EB systems were deployed in April 1992from separate air-supported ice camps. Simultaneously, areduced version of the 10EB (based on the P-IOEB), wasdeployed by the Scott Polar Research Institute in theWeddell Sea, Antarctica, but that system will not beincluded here. One 10EB was deployed on April 15, 1992at 88" N, 57" E, and the second at 73" N, 142" W, nine dayslater. The first was situated in the center of the TranspolarDriftstream ice current, from the northernmost ice camp,Camp Crystal of the Arctic Regional EnvironmentalActivity, 1992 (AREA 92) of the Space and Naval WarfareCommand. The other was located along the edge of theBeaufort (Canadian) Gyre current, being installed from theONR maintained Lead Experiment (LEADEX) Camp,

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Fia.3. Deployment locations and first year drift tracks of 199210EBs.

approximately 150 miles north of Alaska. In terms ofefficiency and flexibility, these ice camp expeditions provedto be better platforms for deploying the intricate IOEBsystems than conventional icebreakers. Fig.3 indicates thedeployment location and drifts of each buoy for the firstyear.

Sixteen months after deployment, the BeaufortIOEB is located at approximately 78" N 158" Wand isproceeding north. The ice sensors onboard this IOEB werelost when the icefloe cracked and the package wastemporarily dropped into a small lead on July 9, 1992. Thecrr recorders stopped functioning after only 2 months;presumably because of power problems. At present most ofthe meteorological sensors, ADCP, fluorometer, andtransmissometer are still transmitting credible data. Thesediment trap and suspended particle collectors should befunctioning as well; however, the real status of the longawaited biogeochemical samples will not be known until thebuoy is recovered. We expect the Beaufort IOEB to beactive for another two years, based on power consumptioncalculations.

The IOEB deployed at Camp Crystal stoppedtransmitting on July 9, 1992 at approximately 85" N 37" E,after having transmitted intermittently since May. Wesuspect PIT malfunction to have caused this failure.However, most data gathered by this IOEB are beingrecorded in memory, so if this system is able to berecovered, we should be able to retrieve this information.

Currently we are still acquiring and interpreting thenear real-time data from the Beaufort IOEB. The full resultswill be published elsewhere.

V. SUMMARY

In general, the concept of an automated polartelemetering station with many atmospheric, ice profilingand oceanographic sensors is feasible for gatheringfundamental data to aid in the understanding of long-termenvironmental alterations in the Arctic Basin. In particular,the use of a SurlynR ionomer foam rather than aconventional steel surface -buoyancy package promotessurvivability, as well as reusability of the buoy package.Furthermore, applying the EIA485 standard to two-conductor multipoint mooring cable usage enables thetelemetry of most of the data from tbe scientific sensors.The related development of the DPM for the compression ofADCP datademonstratesbow a representativesubsetof amassive volume of data can be compressed to be easilytelemetered using via ARGOS. By incorporating a modulardesign, the number and type of sensors which are employedon the IOEB may vary as required, but in order to succeed asan automated station, each system should include sensors inthe air, ice and ocean to assist interdisciplinary study.

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ACKNOWLEDGMENTS

We thank K. Doherty for designing the mechanicalaspects of the IOEB and J. Kemp for directing thedeployment of both IOEBs. H. Bosworth ably assisted thedeployment by operation at Camp Crystal. We thank theUSCGC Polar Sea for providing the platform for the p.IOEB experiment. This program has been supported by theOffice of Naval Research (Contract No. N00014-89-J-1288)and the Japan Marine Science and Technology Center.

REFERENCES

[1] Honjo, S., R. Krishfieldand A. Pleuddemann, The ArcticEnvironmental Drifting Buoy (AEDB): Report of Field Operationsand Resulrs, Woods Hole Oceanographic Institution, Woods Hole,MA, Technical Report WHOI-90-2, 1990.

(2) Plueddemann, A. I., Internal Wave Observations From the ArcticEnvironmental Drifting Buoy, J. Geophys. Res., 97(CS), 12,619-12,638, 199:2-

(3) Padman, L., A.1. Pleuddemann, R.D. Muench and R. Pinkel, DiurnalTides Near the Yermak Plateau, J. Geophys. Res., 97(C8), 12,639-12,652, 1992.

[4] Perovich, D.K., W.B. Tucker III and R.A. Krisbfield, Oceanic heat fluxin the Fram Strait measured by a drifting buoy, Geophys. Res. Let., 16,995-998, 1989.

[5] Plueddemann, A. I., A. L. Oien, R. C. Singer and S. P. Smith, A DataProcessing Module for Acoustic Doppler Current Meters, WoodsHole Oceanographic Institution, Woods Hole, MA, Technical ReportWHOI-92-05, 1992.

[6] Electronic Industries Association, ErA 485 StandGrd for ElectricalCharacteristics of Generator and Receivers for use in BalancedDigital Mulitpoint Systems, Electronic Industries Association,Washington, D.C., 1983.

[7] IEEE Computer Society, IEEE StandGrdSeriJJlASCllInstrumentationLoop (SAIL) Shipboard Data Communication, ANSIlIEEE, Std 997.1985,1985.

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