New Oceanographic Uses of Autonomous UnderwaterVehicles

PAPER

James G. BellinghamMassachusetts Institute of

Technology. Sea GrantCollege Program

Cambridge. MA

ABSTRACT

I'-?tile the potelllial utility (if Autol/()mollS(!nde-l"lcaler v'phicles (AUV,.,) has been l"ecog-ni::ed Jor decades, OIU,!J no'iv ar(~ the..,e syslernsreaching thresholds of co..,l and demonstratedcapability to f!lIcoumge ,vide use. Small, highperjol7nllnce rehicles lIre under det'eloptnenlin academic alIa industrilll utborat()I"ies. TheOdysse'lJ ,~er"ie,s ofAWs have been used innumerou.s}ield opemtion.s, includill./) under-ice(uld in the deep-()cean, and I':1.ppriellce luilhthese robotic Slj..,terns is !JrlJlvin./) rapidl.!J. Totake lId vantage (if their UniIJ1le ('llpllbilities,nt'lV operatiollllltechniq1le,s (Ire bei n!l devel-oped, includi II./) the u..,e of multiple vehicles awlinte.l}l'Ution (if the AUV,., ,vith (Icoustic tomo!J1'a-phy alld ocean p'redictioll ,~.!J,5tem,.,.

sanlpled, limiting our scientific underst:indinof the earth's enltironment, This defines a Criti~scientific need, which AL"Vs ,Ire \vell suited toaddress, However, \vithin the context of a 1argoceanographic field progran1, AL"Vs are one ofmany remote sensing assets. The central chal-lenge is how can AL"Vs be integrated into thelarger family of oceanographic capabilities.?

Autonomous Ocean Sampling :o."et\vorks(AOSNs) prollide an integrated approach to usof,AL"vs (Curtin et aI., 100;3). The .\OSN is anobsef\'ational paradigm in which mobile autonomOllS sulVey platforms are (:oupled synergist]cally \vith moorings and existing remote sens.ing assets to prollide a long-term, reactive mantoring capability. Components can include: fasSUf\'ey AL"Vs, buoyanl'y (jriven gliders, moor-ings, and the software en\ironment binding th'system elements together. From an operationsperspective, AOSN pro\ides a fran1ework sup-portin~ extended AL"V dpployments llis-a.ltispo\vpr. (:ommunications, and integrated mis-sion planning. The paradigm creates a long-ternreacti'.e presence in the oce~ nearly indepen-dent of ships. .\lthough the full AOSN capabili,has yet to be proven. significant elements havealready been demonstr.lted lli1d are discussed i:this paper.

The obsef"\('ational techniques employ-ing AL"Vs discussed belo\v ;lre primarily moti-vated by two oceanographic programs. The fir:;is the Frontal Dynamic Primer Experiment inHaro Strait. The Haro Strait field progran1 occurred in .Jllne ~d .July 1091i. The second progran~is the Labrador Sea AOSN Deployment, in \vhiciAL "\'s \vill be deployed in early 1998 as part ofa larger progr~1 to study deep-oceanic convection. Both experiments are concerned withocean circulation and mixing processes, but at

very ditTerent scales.

Haro Strait Tidal Frontal Dynamics

ExperimentThe need to better understand tidal

mixing processes brought together an interdisciplinary team of researchers to study tidal mi'rin~in Haro Strait (Schmidt, 1996). Participantsincluded the Massachusetts Institute of Technoiogy, the Woods Hole Oceanographic Institu-tion. The Institute for Oct'lli1ogruphic Science(Sidney, British Columbia), HalVard Universitylli1d the L'niversity of \'ictoria. Tllese groupSbrought expt'rtL,;e in enltironmental acoustics,Al"\'s. acoustical ocean()~raphy, acoustic~

INTRODUCTION

T his paper ctescribes the cllrrent state of AL"\"technology and provides ex.mlples of

emerging operational techniqlles that exploittheir capabilities. Small, high pert'ormance vehi-cle capabilities are outlined, particularly withrespect to oceanographic sensors, na\;gation,and autonomous control logic. Discussion ofapplications centers on complimentary sensingstrategies, acoustically focused sronpling, andcoupled obsel"\'ation/modeling systems, Thereview is not comprehensive, instead the objec-tive is to present technologies and applicationsrelating to two field programs employing Au-Vs,the Frontal MLxing Primer Experiment and theLabrador Oceanic Deep-Convection Experiment,

Part of the research and developmentcycle for Autonomous Underwater Vehicles(AL-Vs) is the creation of operational techniquesto use these vehicles most effectively. There isa temptation to fit new technologies into oldroles. Attempts to use AUVs as Remotely Oper-ated Vehicles (ROVs) without tethers,or ascrewless oceanographic vessels, miss some ofthe most promising applications, The economicopportunities and technical challenges shapingthis emerging clas... of vehicles are fundamen-tally different from those of the existingplatforms.

Oceanography is an observationallylinlited science. Characterizing the oceanreqllires introducing sensors to its interiordepths at great expense. Virtllally all currentmea...urement techniques require an oceano-graphic vessel to support a single undersea plat-foml. Consequently, the ocean is very poorly

.:14 . ,W7:s' .Journal . Vol. .11 V..

Existing survey tools are not well suited to mak.ing measurements where spatial variabilitY andtemporal evolution can be Iffialubiguously sepa-rated. These requirements are driving the devel.opment of an infr.1Structure to support remote,extended deployments of ALNs. includingdocking, in-situ battery charging, and satellitecommunication.

tOmography, and ocean circulation modeling.frOm a technical perspective, the frontal dynan1-ics experiment focllSed on the integration ofseveral classes of observational ;lSsets and mod-eling systems to provide a real-time observa-

tion capability.Haro Strait is a region of intense tidal

mL,cjng between Vancouver Island and the SanJuan Islands IF,trmer et aI., 1995). A surface layerof fresher water from the north. primarily ofriverine origin, is mLxed with lmderlying waterfrom the Pacific. This mLxing is associated withstIong trontal structures that form throughoutthe area. Locating and characterizing thesefronts was an important objective of the experi-ment. AL -V operations were complicated byvery strong currents, characteristic of the region,on the order of I. 5 In/so Acoustic conditionswere poor since the bathymetry of the regioncontains sound, and heavy shipping traffic cre-ates high an1bient noise levels. Despite thesechallenges, more than (50 AlTV survey runs were

successfully completed.

Labrador Sea AOSN Deployment

The AOSN deployn1ent in LabradorSea. scheduled for early 1998, \vill allow observa-tions of convection events in ;i responsive, repet-itive manner by pro\iding ,I long-teml unat-tended deploynlent capability, Deep-oceanicconvection occurs primarily in the north Atlantic,regions ;lround .\ntarctic;i. ,uld the Mediterra-nean (D'.~aro, IO9:!.). Deep convection in highlatitudes, esperially in the L.lbr;idor and Green-land Seas, is the origin of much of the world'sdeep \vater. Circulation of this deep waterthrough the worlll Ol:e,U1. which e\"entu;illyreturns to the surface in tht' tropics, is an impor-tant part of the ~Iobal heat tr;insport betweenlow and high latitudes. \V1tile deep convectionplays an importtu1t role in ocean mixing, it isnot well understood. in part because it h;lS beenpoorly obsel"l.'ed. Deep ocean convection hasbeen largely studied by obsel'\;ng its effectsrather than by studying the t1ctual process,other thtm in simult1tion. TIle small plumes (onekilometer sc;ile) that ;ire predicted to t1CCom-plish the actual ntixing have not been I:le;lrlydetected or characterized.

Challenges to metlSuremt'nt of deep-oceanic convection stem partly from the hostileconditions of the North Atlt1ntic during Februarythrough April. the periotl when convectionreacht's its pt'ak. The t'pisodit. nature of the con-vection process ;uso ('r{'atps (lifficulties. Thetime of year, 1IIId tht' approximtltP ptlrt of theOcptm wherp it is likply to o('cur also dependsUpon the wt'athpr an(! uth('r fact()!'S SItch ;IS icecovpra~e. .-\ firuu problt'm stt'IUS frolu thedY1ltunic ntlttlre of t'onvt,(,ti()!1 ,md thl' small-sptl-tial scali- ()f tilt' a('tutu ('onvt'(.ti()1\ plwnes.

VEHICLES Al'1D THEIRCAPABILITIEST he highly demanding, yet cost constrained

nature of the oceanographic sciences has

given rise to a family of small, high performanceAL"Vs, Vehicles that ;lEe presently operationalinclude the Odyssey series of AL'Vs (Bellinghamet ai" 1992; Bellinghanl et ai" 1994; .\ltshuleret aI., 1996), the Autonomous Benthic Explorer(Yoerger et aI., 1991), Ocean Voyager II (Smith~U\d Dunn, 1994), Ocean Explorer (Smith andDunn, 1995), REMl'S (von .-\It et ai, 1994) andFetch tSias Patterson, Gloucester Point, Vir-ginia), \Vhile these systems represent signifi-cant achievements, small Al'V c;apabilities con-tinue to improve, ~U1d the limits of pert'ornla1lCehave not yet been reached, -

TIle developmellt of small vehicles ismotivated by a v~uiety of considerotions, Cost isone factor, both \\ith respect to manufactureand IJperotions, However, small vehicles havemany other stn'ngths, Their higher maneuver-ability allows operotions in <:(Jnfined environ-ments, for exanlple, in vPry sh~ulo\v water ornear rough bottom topography, Their ability tobe opproted with less SUPPOl1 equipment andfrom smaller ships is extremely attractive, andis an important factor in keeping costs low, Inprinciple, they Call be deployed and recoveredin rougher surtal'e conditions because of simpli-tied handling, Perhaps most important, small,lo\v-cost .\l"Vs offer the prospect of multiplevehicle operations, the advantages of which aredemonstrated in subsequent sections.

Field l'xpelil'n<:e with small vehiclesh~\.., proven that they can be highly robust, TheO~lyssey series of Al\'s (Figure 1) are (jOOO mroted, and ne\ver Odyssey AL"Vs have sufferednwnerOllS impacts, \vith minimal or no danlage,including being Ilropped, nm over by boats, andmishandled during shipping (a fork lift skeweredone Al"V's shipping container), The mpchanicalrobustness of the vphicles owes much to theircompliant polyetheleyne <:onstruction-amaterial that can only l)e ust'li for structurollysm~tll<'r systems, Despite thirteen tield deploy-ments, in<:luliing operations in the ,\lltal'cti<:,unclpr ic(' in the .\rctic, in the deep ocean ofth(' ,!tl~m (Ie FI1C~1 rid~l'. and in the high tidal('llm'nts Ilf Ilaro Stroit, no O(ryssey vehicle has('\'I'r lJP('n U)st, Pffhaps most te\lin~, for the last

,~[TS

11. Picture 01 an Odyssey AUV being recovered alter operations In Buzzards Bay,Ichusetts.

two science field deployments, to Haro Strait,British Colun1bia, and to Kaikoura Canyon, ~ewZealand, not a single day of operations was lostdue to vehicle failure. This is despite a total ofapproximately ninety missions run in the twodeployments. including routine operations todepths of 750 meters in Kaikoura Canyon.

Endurance is a central concern formany prospective At'V uses, and the design ofsmall vehicles is heal.ily driven by considera-tions of power consumption and energy capac-ity (Bradley, 1992). As vehicle size is reduced,the overall range of the vehicle diminishes, pri-marily because battery capacity shrinks morerapidly than propulsion power. ~onetheless,significant endurance can be achieved. .\n Odys-sey vehicle presently being prepared for theLabrador Sea experiment will be capable of 50hour missions, cruising at three knots (Bales,1997). Demonstrated docking and power trans-fer capabilities, discussed later, offer a meansof extending the deployments of a small vehicleby prol.iding the ability to recharge batteries.

Oceanographic Sensors

The variety of sensors available forsmall AlNs is substantial and growing rapidlyas manufacturers develop smaller, more powerefficient ocean instrumentation. High qualityCTD (conductil.ity, temperature, and depth)instrumentation is routinely installed on Odysseyvehicles (Bales, 1996). The Odyssey has alsobeen equipped with side-scan sonar (Oceane-tics, Sidney, British Columbia), Acoustic Dopp-ler Current Profilers (ADCP; RD Instruments,San Diego California), sector scanning sonar,tomography sources, optical backscatter

instruments (Sea Tech. Cor\'allis. OR). and Vaous camera and lighting systems. including thNational Geographic CritterCam system.

Part of the integration of a new instrment into an AL"V is an interface to the mainvehicle computer for control and data loggin~While many instruments have internal datarecording capabilitY. it is much more attracti,to use the main vehicle computer to log sens,data. This assures that time synchronization i:;achieved, and simplifies data analysis. Somesensors, for example \ideo systenlS or side-sc:sonar, generate sufficient data to wacrant thei:own data storage. For these ~ystems. routinesfor synchronizing vehicle and sensor data sec.,are critical for later data interpretation.

The use of a free-flooded fairing great!simplifies integration of new instruments, sinc.most off-the-shelf sensors have their own pres.sure housings. Retaining systems .lS near totheir origin." configuration as possible mini-mizes the need to adapt electronics built byanother organization into an alrea£ly complexvehicle. For example. it is unlikely that a high!sensitive acoustic system's sppcilicationsincluded the requirement that it be able to opelate in the same enclosure as a motor controll!'or other electrically active systems. Such sys-tems can be integrated into a vehicle's dry vol-ume successflllly. For exan\ple. a side-scansonar is integrated into the same housing as thothruster controller and vehicle power systemsin one of the existing Odysseys. However, thelevel of effort required is typically substantiallyhigher than a wet volun\e integration effort. Nellwet systems are routinely integrated into Odys-sey in a few days to a week. and the fastestintegration (the National Geographic Critter-Cam) occurred in a few hours.

Navigation and TrackingLong-base-line (LBL) navigation is

widely used for undersea vehicles, and employsan array of transponders or synchronized ping-ers which are interrogated or detected by thevehicle. The array provides position referencesfor the vehicle to compute its position. oncethe ranges from the vehicle to the array elementsare known. These ranges are detennined fromacoustic time-of-flight measurements. The accu-racy of the position !Lx depends on a varietY offactors such as array size, signal processing tech-niques, acoustic frequency employed, soundpropagation characteristics of the environment,and the accuracy with which the positions ofarray elements are known. Depending on thesefactors, accuracy's can range from centimetersto tens of meters.

To the degree possiblp, Odyssey opera-tions have used standard deep-ocean transpon-ders with rrpquencies between eight and fifteen

.!ountal . Vol. .11. ,Vo. .J

can be achieved through the effective utilizationof multipath for acoustic navigation (Deffen-baugh, 1997). Feature-relative navigation tech-niques, in which sonar targets are used ;JS posi-tion references, have been demonstrated in testenvironments (Davis, 1996). Integrated Acous-tic Doppler sonar/Inertial Navigation Systemsprovide a near-bottom navigation capability forlarge militarY AtJ"Vs, and ongoing work is transi-tioning this technology to smaller vehicles.

Autonomy and Control

A nun\ber of institutions have devel-oped and implemented high-level-control archi-tectures for ,\L'Vs. M:lI\Y share in common athree-layer approach in which control is sepa-rated into hierarchical layers which operate atincre;JSing levels of abstraction (Bellinghan\and Consi, 1991; Bymes et al.. 1993; Bizingre etal.. 1994). Layered control is one of a varietyof architectures which have been employed formission-level control of mubile robots, and hasbeen adopted for Odyssey operations. Originallyimplemented on land robuts, it was llSed tosolve relatively low-level problems such ;JS coor-dination of limbs 011 a walking robot (Brooks,1986). In subsequent work, it \vas llSed to embedmission-level control in mission-dedicated vehi-cles (Loch et al.. 1989; Connell, 1990). Extensionof the architectlu'e to mission reconfigurableapplications highlighted the subtlety of program-ming layered control and led to the develop-ment of hybrid architectures such as state-con-figured layered-control (Bellingham .md Col\Si,1991) and Ssg (Connell, 199:?).

Layered control explicitly modularizesGontrollaws in units called "beha"iors." behavioris the elementary unit of layered control. andrepresents the lowest level of mission-relatedexpertise. A beha\ior receives sensory input andgenerates comn\ands to the vehicle. For exam-ple. the objective of an obstacle avoidancebehavior is to prevent the vehicle from hittingobjects. Much of the strength of layered controlcomes from the ability to run multiple controllaws (i. e. behaviors) simultaneously. A layered-control command-structure consists of a num-ber of behaviors. of which the outputs areresolved into the final command that is senU2--

the vehicle.By creating libraries of configurable

behaviors from which missions can be con-structed, highly flexible mission-level controlcan be obtained. Odyssey operations routinelyrun surveys that can include bottom following,profiling the w.\ter colunm, or level flight,depending on which behaviors are enabled.Horizontal trajectories and vertical flight-pro-

~fires can be mi..xed, providing a wide range ofmissions. A survey might be proceeded by orfollow other trajectories, for example, a spiral

kilohertZ. In operations over the Juan de Fucaridge (approXImate water depth of :!300 m), anOdyssey \vas able to detect beacons sLx kilome-ters away. The LBL can also be used for vehicletracking, pro'-ided the AI-IV can be configuredfor 'fish mode.' This technique, widely used fortrackirlg lmdersea vehicles, uses a transponderon the vehicle that replies at a frequency thatinterrogates the na'-igation array. The shipinterrogates the vehicle. which in replying alsointerrogates the array. The vehicle, by detectingthe array replies, can determine its own position.The ship, by detecting the replies of both thevehicle and the array, and by knowing its ownposition, can also compute the vehicle position.This is a commonly llSed tracking technique,particularly for deep-ocean operations.

~en a surface vessel is in constantattendance of an AL'V', ultrashort baseline (uSBL)acoustic systems pro\ide a means of trackingvellicles \vithout reqturing the deployment of a1.BL array. L"SBL systems measure the directionof propagation of an acoustic wavefront, and,therefore. pro'-ide a means to obtain bearing anddepression angles to a vehicle. By combiningthis with a transponder raIlging capability, theposition of a vehicle relative to the ship can be

detem1ined. Typically, aI1 integr.lted na..igationsystem on the support vessel (for exan1ple: Win-

phrog, by Pelagos, San Diego) accepts inputfrom the tracking system and merges the mea-surement \vith ship molU1ted global positioningsystem and compass output to compute theposition of the vehicle. The position of the Al;"Vand other targets can be displayed in real timeas well as logged. \\l1en it is possible to sendcommands to the vehicle, the tracking system.sability to track multiple targets can be used tomaneuver a vehicle relative to another target.For exan1ple, such methods were used to maneu-ver an Odyssey rel.\tive to a drifting sonar plat-form during the Haro Strait experiment (Belling-ham et al.. 1996).

In shallow water, the performance of

both LBL and USBL systems frequently degradedue to the more complex acoustic environment.In effect, the d~amic oceanography whichmakes many regions scientifically interestingcan cause multipath, fading, and reverberation.This is a problem for na'-igation systems, espe-cially those that rely on the first detected arrivalfrom na'-igation beacons being a direct path.While recent Odyssey operations in Massachu-setts Bay (25 m ..vater depths) used deep-oceantransponders with great success out to dis-tances of t,vo kilometers, LBL na'-igation ofOdyssey in the :!OO m waters of Haro Straitencountered significant difficulties due to highshipping noise .md reverberation.

Na'-igation is a particularly active areaof research. Improvements in LBL performance

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descent to the bottom. Beha~ors that are moresingle-purpose in nature include dockiIlg andrecovery routines. Docking beha~ors, of whicha variety have been written, are capable ofdetecting missed approaches, iU\d subsequentlyreacquiring iU\d reapproaching the dock. Toenable remote commands ";a an acoustic link, abeha";or was created to translate LBL sequencesto heading commands. Other beha,,;ors areused to force transitions between mission states.For example, Odyssey's homing beha~or \vastemtinated when the collision and capture \~ththe recovery net was detected. Over the lastfive years of Odyssey software development, thenumber of beha,,;ors increased to a peak ofroughly fifty, but has subsequently been consoli-dated to twenty three in the present vehicl~s.Initially, a large number of special purposebeha,,;ors were created. However, as the behav-ior library became larger, common threads tothe beha,,;ors began to emerge, ;md a fewernumber of more versatile beha,,;ors began toemerge.

cle state :md sensor readings as :l result of COrti-m:mds from the control software. This modulecontains the equations of motion. models of thepert.ormance of the vehicle thruster and controlsurface actuators. models of sensor system per.formance. including noise. :md a model of thevehicle en..ironment. Development of a highfidelity simulator is a major investment, but pro.vides the only way to test m:my aspects of thecode before field trials.

The autonomous control software,while central to operation of the AL"V, may Com.prise a relatively small fraction of the requiredcode. Software elements necessary to suPPOrtAl"V operations include mission configuration,vehicle self-check, autonomous control logic.payload management, data logging, navigation.data :malysis, :md simulation. If acoustic com-munications are in place. then :m interface forreal-time hW1\:m control is also required. Toease development of new soft\vare capabilities.specialized software development tools may becreated. Supporting development tools allo\vprogrammers to add capabilities to one part ofthe code without hanng to explicitly managefunctions in other ai1ected portions. ror eX0UI1-pie. Odyssey software development is supportedby the data structure editor. which allo\vs thecreation of new variables for access by the n1is-sion, :md which .1re automatically logged OUIdparsed for data re\iew. A minimal software con-figuration illustrating interactions bet\veen sub-components is depicted in rigure :!.

Configtlring a mission within laypredcontrol t:}'Pically involves three steps: decidingwhich behall;ors are required, assigning themappropriate priorities, .md ,lSSigning values tothe arguments of the beha\;ors. These steps canbe pert'ormed by editing a simple text-basedmission-initialization file. The initialization files,u-e short and relatively simple to understand.

Simulation is a necessary tool for effi.cient vehicle software developm~nt. Key to thesimulation is the module that calculates the vehi-

,d.t',)I"JI,.",)

EFFECTIVE USE OF AUVs'" UVs differ substantially from other sensing

~latfomls used for oceanography. The desireto make the most effective use of these instru-nlents is prompting the development of newoperational metholis.

Time and Energy Constrained Surveys

Even \vith the introduction of these rel-atively inexpensive mobile platfomlS. mostoceanographic field programs will remain plat-form linlited. Synoptic characterization of theocean remains lmattainable for all but excep-tional cases. To state the problem simply, theocean changes faster than we can measure it.Equivalent characterization of the atmosphereare accomplished routinely, ;).., demonstratedeach evening during the weather report on thelocal news. However, the spatial variability ofthe ocean is greater than in the atmosphere,and the ocean is much less amenable to beingprobed.

For an untethered. lmderwater surveysystem. the an1ount of energy stored proviqes afundanlentallimitation on t:apabilities. Conse-quently. an important Ii~ure of merit is theenergy required to accomplish a given survey.In a pr.lctical sense. the energy required is mini-mized by reducing the distances vehicles musttravel. For surveys that are not tinle cons-trained. the energy reduction is proportional tothe reduction of distance traveled. For time-constrained SU1"\'eys. the energy sa\ingsachieved hy distance-reducing strategies can beeven greater. .-\ri analysis of surveys constrainedby the dynamics of oceano~raphic processes isgiven in Bellinghanl anI! Willcox (1996).

The objective of adaptive sampling isto obtain the best understallding of the phenom-ena under study for the least an1ount of effort.To achieve these efficient surveys. a range ofadaptive sampling techniques are under develop-ment. They range in sophistication from appro-priately sized grid surveys to coupled observa-tion-modeling systems. Between these twoextremes lie methods that concentrate measure-ments in the regions of greatest interest, usuallyregions with high spatial gradients. For exanlple.to map an ocean front. an AL'V might first runa coarse survey to localize the front. then con-centrate operations in the vicinity of the identi-fied front.

sive vehicles is that multiple vehicle operationsC:ln be sustained at reasonable costs. Coordina-tion of multiple vehicles can be achieved via avariety of methods. Perhaps the simplest is topreprogram the vehicle paths so that coordi-nated activity is predetennined. However, thismay impose substantial dem:lnds on navigation:lnd control, particularly if closely coordinatedmMeuvers are required, and it effectively pre-cludes MY reactive operations.

The activity of multiple AL"\'s C:ln becoordinated by a central controller given M abil-ity to monitor :lnd communicate with the vehi-cles. The controller could be a hum:ln, a mastervehicle, or a computer located on a sep.u-ate sitesuch as a mooring. This method has beenemployed for Odyssey operations in Haro Strait.Figure ;3 shows vehicle tracks for two OdysseyAL'Vs under acoustic control for SUf\'ey opera-tions near a tidal front. The objective was touse one vehicle to profile the water columndirectly under the second vehicle. Vehicle posi-tions were monitored from a surface vessel using:In USBL tracking system (Trackpoint II, ORE,Falmouth, :'Ylassachusetts). A simple one-waycommunication system \vas created by using theLBL system to send heading comnl:lnds to thevehicles. The method imposes a substMtiai work10.ld on the ()perators and [he communicationlinks. This example \V.lS particularly demMdingsince operations were in a tidal front region :lndsubst:lntial I:urrent shears \vere experienced,especially in depth.

The flight of a group of AL v's in fonna-tion becomes sinlpler \vhen vehicles C3Il senseeach others positions. Tlus capability enablestechniques such as virtual chains (Tri3IltafyllouMd Streitlien, 1991). .~ acoustic l'SBL systemprovides the ability to detennine the directionto M acoustic source. By integrating a CSBLsystem onto an Odyssey, and using it to tracka source mounted on a second Odyssey, fonna-tion flight of AL'Vs has been demonstrated(Singh et al.. 1996). The lead vehicle executeda grid-survey pattern, while the second vehiclefollovved the first, resulting in the dead-reckonedpaths shown in Figure -t. In contrast to centrallycontrolled operations, fornlation flight \vasmuch simpler to manage as the two-vehicletealIlS nmctioned as a single independent entity.

Nlultiple vehicle operations C3Il stressacoustic navigation 3Ild tracking capabilitieswithout careful pl3Ilning. Such systems have rel-atively little working b3Ildwidth (for example8-15 kHz for deep-oce3Il navigation systems).With traditional transponder-based navigation,update rates of positions will typically drop inpropoltion to the number of vehicles. The sameis true of tracking; One solution is to use hyper-bolic-b.lSed systems, whil~h C3Il be config-uredto provide navigation for an arbitrary number of

Multiple Vehicles Operations

Multiple vehicle survey operations canbe motivated for a variety of re:1Sons, from mak-ing most effective use of :111 expensive oceano-~raphic vessel, to mapping ;m oceanographicprocess as rapidly a/l!1 densely ;IS possible. Onec\dvantage of hailing sm:lll, relatively inexpen-

lifTS .Jou/7!al . Vol. . ,]9

Figure 3. Coordinated multiple-AUV survey. AUVs Xanthos (XJu104 01) and Aphrodite(AJu104. 05) both under remote command from the FX Ross Bay. Equipped with downwardfacing ADCP, Xanthos maintains a constant depth of 10 m while Aphrodite. equipped withupward facing side-scan sonar, yo-yo's between the depth limits of 10 and 90 m. The twoplots in the top right indicate the remote heading commands as received by the sparechannel in the LBL transceiver The time axis in the three plots to the right originate at thestar1 of the Xanthos mission, 00:47:54 UTC. July 4. 1996.

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Figure 4. Formation-flight AUV surley in Buzzards Bay. AUV Wodyssey (WaprOS.07. solidline) follows Xanthos (AprOS.11. x'S) through a tight surley pattern. The start pOSitionswere (0.11) and (0.0) respectively. To prevent collisions. the two I/ehicles wereprogrammed to operate at different depths. The acoustic system used for vehiclecoordination interlered with the surlace tracking system. consequently the tracks shown are

deadreckoned.

so,

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Following AW

//'/ """".,,,"/""",," "- L..dir19 AW.10

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recharging or replacement. data processing,and general maintenance. The t~uipmentrequired to support a single vehicle is typicallyunderutilized, consequently multiple vehicleoperations do not require a rlranlatic increasein support equipment. Odyssey deploynlentshave fielded as many as four vehicles. and sup-ported t\VO vehicles simultaneous operations.

Distributed ComplementaryInstruments

Under some circumstances, the com-plementary nature of sensor instrumentation isonly realized by separating elements of the sys-tem spatially. For example, camera-ship/light-ship operations take advantage of large separa-tions bet\veen t\VO platforms equipped respec-tively with a C:ln\era :lnd light to obtain imagesless effected by backscatter. This technique hasbeen used to obtain dramatic underwater images\vith manned vehicles. notably the Titanic foot-age obtained with the two Russian MIR submers-ibles. and has been proposed for Al"Vs (Turneret al., 1091).

.\ distributed sensing strategy employ-ing complementary instruments was used toobtain ;l vertical section of current. temperature..md salinity in the \/icinity of a titlally generatedfront (Figure ;j). Temperattlre and salinity \vereme:lSureli by a Sea-Bird CTD (Sea-l3ird Elec-tronics, Inc.. Bellevue. Washington). :lnd currentwas measured using :In RD Instruments .-\DCP.In contrast to the CTD measurement. whichcharacterizes the water in its immediate \icin-ity, the .-\DCP profiles current at a dist:lnce, inthis case from 4 to 102 meters from the vehicle.Thus to use these two systems to characterizethe same vertical section. they \vere placed onseparate vehicles.

To obtain the joint .\DCP/CTD section.the .-\DCP vehicle was operated at a depth of10 m. \vith the .-\DCP profiling the water columnbeneath it, \vhile the CTD vehicle fle\v a yo-yopath between 10 and 90 meters. The vehicle mis-sions are the same as described in multiplevehicle dis(~ussion above. Ide.uly, the CTD plat-form would have luways been directly underADCP vehicle, however, strong currents andshears provided sut"ficient perturhation that this\vas difficult. TIle path actually followed by thevehicles is sho\vn in Figure :3.

In contrast to survey schemes, wherea single vehicle carries all the sensors requiredin a particular operation, distlibutetl comple-mentary Al'V str.ttegies strongly motivate useof smaller vphides. Not only (10 individual velu-

des cany only pmt of the ovt'rall instrumenta-tion. hut the <li\ision of labor can t'nable theacquisition of otherwise unohuunable data sets.However, lIS the ex~m\ple above (lelnonstrates.cotJrlliruttion of \"ehil:les is m)t al\vays easy.

vehicles (Bellingham et al., 1992). The acouSticbeacons in such a system are not interrogatedby the vehicle, rather they are slaved to a com-mon time-base similar to Loran. However, hyper-holic systems are not commonly llSed in theoceanographic community. In addition-to track-ing and navigation, other elements of vehiclesupport include mission configuration, battery

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region. while an in-situ suC',.ey must explore thearea encompassed by the region, Since theperimeter scales only as the square root of thearea, moving source tomo~phy requires sub-stantially shorter vehicle travel paths. espe-cially as the size of the region grows. This trans-lates into faster sun-eys and lower energy

expenditures.11\e necessalY ingredients of acousti-

cally-focused sanlplin~ were (!emoltstrated in thetidal-mi.\:ing experiment conducted in summerof 10!)6 in H:'lro Strait, .\11 important part of theexperiment wa." to !tJcalize frontal acti"ity,\vhere much of the mixing appeared to occur,.\11 acollStic tomography array \vith radio linksto shore was ueployed in the expected frontalregions, A source \vas installed on an Odysseyvehicle, and data for mo"ing-source tomogra-phy \vas acquired. \Vl1ile extremely high ambientnoise levels greatly complicated tomographywith the Al 'Y', acou~tic inversions were acquiredfor light-bulb sources (Figure 6).

Coupled Observation/ModelingSystems

As sensing platfonns proliferate, theneed for management tools to optimize their usebecomes incre:'1Singly important. This hM leadto efforts to couple obseC','ational systems \vith

concurrently running oceanographic models(Figure 7). From the observational perspectivea variety of benefits are realized. Perhaps mostimportant, assinulation of observations into amode! offer the potential to employ temporallydecorrelated mea..,urements in estimatingoceanographic fields, In addition, error fieldsgenerated by the modeling and assimilationprocesses are of significant llSe in uesigning sub-sequent surveys, The prror fields provide guid-ance for designing sampling strategies by indi-cating locations which have bpen poorly cons-trained by previous measurements, Thus, notoQ~y is the time constraint on survey operationsrelaxed, but mort' infom\ed sampling strategiescan be developed,

Acoustically-Focused Sampling

Acoustic tomography pro\ides anextremely po\verful method for characterizingthe ocean en\ironment remotely. The transmis-sion characteristics of acoustic signals, mostsignificantly the travel time, are inverted toobtain the average sound speed bet\veen asource and a receiver. Typically, sources andreceivers are moored in arrays. the number and,lfrlilgement of which deternlines the horizontalspatial-resolution that "lil be obtained. Forpxample. a squ.lre arr.1Y of four source/receiverpairs pro\ides a total of six distillct holizontalpaths that Call he llSed to creare a rough horizon-tal set.tion. If receiver data C:Ul be recoveredin real time. then the ;lpproach pro\ides a fast,although low resolurion, mer hod to character-ize the oceal! (Schmidt et al., l!J97).

The resolution of the technique can beimproved by increasing the number of ray pathsthrough the \vater t:olumn. This is t}-pically.lccomplished by mo\ing sources or receivers.AL"Vs can be equipped .L'i mo\ing sources andcommanded to move in patterns pro\idingdetailed acoustic coverage in regions of interest.Given a real-time inversion t:apability, and com-munication with the AL "V(s), a techniquereferred to as 'acoustically-focused sampling'is possible. In this method the SUf\'ey patterncan be refined in real time to pro\ide bettercoverage in regions of hi~h spati.u structure ortemporal evolution.

Benefits of acousticaily-focllSed sam-pling, as opposed to in-situ measurement ofocean properties, are two-fold. First, mooring-based tomography results can be llSed defineregions of interest, for exanlple, providing initialestimates of the location of a front. This allowsone to 'focus' AI-TV survey acti\ity more rapidlyon the region of greatest interest. Second, mov-

ing-source tomography substantially reduces t.hedistance a vehicle must travel to map a particu-lar volmne. This is Sl~en by obsef\ing that for avphicle to assi~t in tlmlogr.lphil:ally mapping art'~ion it mu:;t only tr.lvel the perimeter of the

.J.fTS .faun/fll . Vol..]]. .41

Figure 6. Front localization by acoustic tomography in Haro Strait (from Schmidt et al.1997) This data set was obtained by deploying lightbulbs. which implode at depth toprovide an Impulsive source. from a ship (Chapman et al., 1997).

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Figure 7. Coupled modeling/observation system. Data from AUVs and other observationalassets. for example satellite measured sea-surface temperature. are assimilated into acirculation model. The model provides a realization of the volume under study. as well asan error field estimate. The error estimate is used to plan new vehicle observations. whichonce made. can be assimilated into the model. thus closing the mission control loop for

the obserJatlonal assets.

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A coupled obse!""'ation-modeling sys-tem employing AL"Vs as part of the sampling Com-ponent was demonstrated in Haro Strait in June1996. During the experiment. the HarvardOce:m Prediction group provided operationalforecasts (Robinson. 1997) llSing the HarvardOcean Prediction System (HOPS; Robinson1996). Advection of the tracer field was dri~enby the Institute for Oce:m Science (IOS) tidalmodel (Foreman et aI.. 1995). HOPS modeledthe mixing process par3IT\etrically, ;JSSumingthat mixing was dependent on the square of thespeed :md inversely proportional to depth. Theoperational product was predictions of salinityat a 10 m depth.

The mT Sea Grant AL"V Lilb operatedtwn Odyssey vehicles daily from a -12' long \vorkboat throughout the experiment. Forecasts gen-erated by HOPS were prnvided to the AL"V teameach morning. Hourly predictions were gener-ated for the period around ma.ximun\ ebb. Vehi.cle operations typic:1l started at sL'C in the mom.in~ :md ran through early afternoon. Predic-tions of front:1llocation and currents \vere usedto plan d~y vehicle operations. Currents inexcess of the Al"V cruising speed \vere routinelyobse!"".ed in Haro Strait. cnnsequently Cllrrentswere :m important factor in Slu-..'ey planning.Figure Sa and Sb sho\v predictions of frontallocation and currents for Jlme ~5, 1996.

Typic:1lly, several vehicle runs \veremade in the vicinity of the predicted front. Trajec-tories designed to intersect the front \vereemployed. with the vehicle yo-yoing bet\veen20 m depth and the surface. The motion in thevertic:1l plane was rlesigned to pro\ide :mexcursion around the prediction depth of 10 m..-\5 data was acquired and re"iewed. successivevehicle runs \vere adjusted to char.1cterize thefront. Figure 8c shows a vertic:1l section ofs:1linity measllfed by the vehicle through the pre-dicted frnnt illustrated in Figllfe 8a and 8b. Onthis particular run. the vehicle reversed coursetwentY minutes into the mission. and thuscrossed the frnnt twice.

Data acquired by the vehicle \vas pro-cessed on the vessel tendin~ the AL"V and. uponreturning to shore. it was delivered to the model-ing team. Vehicle data. :1long with CTD caststaken from a charter boat, were assimilated intothe model. Fi~ure Sd shows a hindcast for June25 incorporating the data acquired by four AL'Vruns. Since the vehicles did not have an acousticuplink for the Haro Strait experiment. recoveryof the vehicle was the only way that dataacquired by the vehicle could be examined. Witha fully operatinn:1l acoLL.,tic link. recovery willbe unnecessary. and substantially more efficient

vehicle operations will be possible..A.s with acoustically focused sampling,

coupling observation and modeling systems

! 8. Predictions, measurements. and hindcast with assimilated data for Haro Strait experiment. a) and b) Predictions of Harvard Ocean Predictionm for June 25. 1996 for 10 am and noon. local time. C) Paths followed by four separate vehicle missions. as determined by the long-baseline;tic navigation system. d) Temperature section measured by AUV in the vicinity of the predicted front position. This mission. X;un25.14. the secondthe bottom in Figure Sc. was composed of an outbound leg and a return leg. The return leg starts 1200 seconds into the mission. The vehiclees the front on both legs. e) Hindcast for June 25. 1996, noon. using data from vehicle runs. The predictions and assimilations shown were given touthor by N. Q. Sloan III during the Haro Strait experiment.

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The moorin~ that carry the dockingstations provide substantial technical chal-lenges in their own right. For the Labrador Seadeployment, maintaining a surface expressionthrough violent winter conditions of the :-.iorthAtlantic is extremely difficult. However. the Sur-face expression is necessary to provide satelliteor other radio frequency communications.Without such a communications lillk. the abilityto recover data and download new instructionsto the AOSN can be difficult or inlpossible. andthe ability to use the system reactively greatlyreduced. Consequently. substantial effort isrequired on the moorings and their attendantcommunications systems for AOSN applications.

CONCLUSIONSA !J"VS are rapidly pro..ing themselves :JS useful

rl..tools for oceanography. .\5 they are notdirectly analogous to existing suf\-ey platfol111s,unique operational methorls are being createdto exploit their capabilities. Exan1ples whichhave been presented include: multiple-vehicleoperations. acoustically focused sampling, andcoupled observation/modeling systems- A <:om-mon theme is the LISe of :\l"\'s ,IS ('omponentsof larger observation,li net\vorks. :I

Future ocean obser\-ation systems \\illbe required for obsef\ing the ocean across arange of spatial and temporal scales. Nestedapproaches. similar to those LlSed by the model- :ing community are likely to be used. For exan1-pie, at the largest scale. a b:JSin level obser\-a-tion system may be required. Spatial resolution I

can be sacrificed, hut large area coverage iscritical. The next lower level operates over asmaller area. but at a resolution capable ofresol..ing mesoscale processes. Finally, a highresolution, rapid mapping capability comprisesthe finest spatial scale element of the overallobservation system- The mapping systemwOLud not operate continuously. but remains onstandby, ready to operate \\-hen required. ALvswill have roles at each level of the system.although the types of vehicles used might bevery different in design- This multi-level config-uration is a natural application of the Autono-mous Ocean Sampling Net\vork paradigm-

En\ironmental monitoring systemslike those described above have applicationsbeyond oceanographic science. Climate predic-tion, fisheries, aquaculture, pollution monitor-ing, defense ,md nunernl extraction irnJustries\vill all benefit from the ahility to characterizeand predict ocean processes- \Vhile the precisenature of these application:; remains 10 emerge.the general need for more in!()mlation about thehighly variable ocean en..ironment promi:;esincreasingly aggressive effol1s to u:;e Al:Vs.

requires that the data be processed in near real-time. This mode of operation is in some sensesmore akin to \veather forecasting than an ocean-ographic field experiment. For the effort inHaro Strait, data was manually transferred fromthe AL"V operations boat to the modeling cen-ter. Routines for importing data were \vritten ;ISneeded. However, the process was highly laborintensive, and future experiments will benefitfrom a more seanlless integration of the various

components.

Long-Duration Deployments

The ability to dock vehicles to moor-ings and solar cells is central to the utility ofAL"Vs for extended deployments. TIlis capabilitywill allow vehicles to be operated autono-mously for extended periods, using moorings ;ISsites to recharge batteries, download data,receive new commands, and be secured to dur-ing periods of inactivity. Successful dockingconsists of homing, latching, ;md establishingpower ;md communication links. To optimallysupport Al;"v operations, the dock will be ableto communicate with human operators. per-haps on shore or On a ship, and have sufficientenergy capacity for multiple recharges of theAl;"v batteries.

Docks \vhich have been tested forOdyssey operations c;m be characterized aseither cones or poles. Vehicles approach ;mdenter cones horizontally. A cone is attractivein that once the vehicle is engaged. establishingconnections is relatively straight-for\vard. Thesecond technique employs a vertical pole ontowhich the vehicle latches. A pole is attractivein that it is insensitive to approach direction andsimplifies homing. A variety of homing systemshave been successfully demonstrated on Odys-sey, including acoustic (Bellinghaln, et al., 1992;Singh et al., 1996), electromagnetic (Feezor etal., 1997), and optical systems (Cowen et al.,

unpublished).Establishing a power link between a

docked vehicle and its docking station allowsrecharging of vehicle batteries. A communica-tions link is equally important. even if the dockitself cannot communicate to the outside world,as it allows vehicle data to be transferred tothe dock, providing a backup against possiblevehicle loss during subsequent survey opera-tions. An inductive coupling technique whichprovides both power transfer and ethemet com-munications is being developed by ElectronicDesign Consultants (Chapel Hill, North Caro-lina). This system demonstrated power tr;msferin excess of ~OO W at i9% efficiency in recentdocking tests in Cape Cod Bay. The dol-:k usedfor this test, a pole configuration, is describedby Singh et al. (1997) and illustratpd in Figure 9.

ACKNOWLEDGMENTSTh~ paper, being ;m overview in

nature, Ilescribes work to which m;my collabo-rators have contributed ;md many sponsors sup-ported, ;md without whom this paper wouldnot have been possible. The author would partic-ularly like to th;mk Dr. Bradley.-\. :'fIoran and~Ir. Yanwu Zh;mg for assistance processing HaroStr.lit experimental results, and fellow investi-gators of the Labrador Sea URI for m;my stimu-lating conversations. The Haro Strait AL -V effortwas supported by the Office of Naval Researchunder contracts NOOOI-l-95-1-1316 and ~OOOl-l-95-1-0495. The Labrador Sea AOSN deploymentis supported by the Office of Naval Researchunder contract ~OOOl-l-95-1-1316. Adaptive sam-pling research has been supported by the :'flITSea Grant College Program under grant ~0001-l-95-1-099-l. Early development of Orlyssey wassupported by the Sea Grant College ProgramwIder grant ~A-l6RG-04;3-l, the ~ational l"nderseaResearch Pro~ranl under grant ~OOOI-l-95-1-0670, and the ~Iassachusetts Institute ofTechnology.

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