TOWARDS NEW PLATFORM TECHNOLOGY FOR SUSTAINED OBSERVATIONS

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Towards New Platform Technology for Sustained Observations

Gwyn Griffiths,1 Russ Davis,2 Charles Eriksen,3 Daniel Frye,4 Phillippe Marchand,5Tommy Dickey6 and Robert Weller4

1Southampton Oceanography Centre, Empress Dock, Southampton, UK, 2Scripps Institution ofOceanography, San Diego, CA 92093-0230, USA, 3School of Oceanography, University of Washington, Box 357940, Seattle WA 98195-7940 USA, 4Woods Hole Oceanographic Institution, Woods Hole, MA, USA,5IFREMER, BP70, 29280 Plouzane, France, 6University of California Santa Barbara, 6487 Calle Real Suite A, Goleta, CA 93117, USA

ABSTRACT – Affordable platforms underpin our ability to make sustained in situ observa-tions of the ocean. Autonomous vehicles such as drifting and profiling floats alreadycomplement research, survey and voluntary ships. Floats are but one, more mature, memberof what can be considered a family of autonomous platforms. The more recent additions to thefamily include moored profilers, autonomous surface craft, gliders and propelled autonomousunderwater vehicles. Research communities of scientists and technologists are rapidlygaining experience of these newer vehicles, building on the proven attributes of today'stechnology. This experience will provide a firm foundation for the introduction of newplatforms into the arena of sustained observations.

IntroductionOcean observations rely on platforms and sensors.Programs of sustained observations naturally placea greater emphasis on affordability and reliabilitythan on pure technical innovation. Historically,innovative platforms and sensors have been provenduring short-duration process studies or otherlimited experiments. But we are in a period ofchange. Platforms are now being built specificallywith sustained observations as a design goal andwith cost per profile or per kilometre as a keyspecification. Prior to the sustained use ofadvanced technology platforms, it is imperativethat their reliability in an operational systemshould be proven. In addition, in situ observa-tions, in themselves, are but one facet of anobserving system; their systematic integrationwith remote sensing data and numericalsimulation models is becoming the norm. Paralleldevelopments in data-handling infrastructure willbe required if the integrity and timeliness of theinformation from advanced technology platformsis to be assured.

The purpose of this paper is threefold: (1) toreview the current thinking on how science needsinfluence and guide new platform development;(2) to couple platform development to the separateyet symbiotic developments in sensor technology;

(3) to discuss key developments in autonomousvehicles (profilers, gliders, powered vehicles).

The developments in the third and fourthsections of this paper are those most likely tocontribute to sustained ocean observations forclimate by 2010. Some of the ideas in the fifthsection may come to fruition after 2010, whilesome may founder. Over the coming decades,improved energy storage methods will drive downcosts, as will new generations of satellitecommunication systems. The life expectancy andmultidisciplinary capabilities of moorings are bothexpected to increase. For some vehicles theemerging needs of offshore oil and gas companiesmay make for a larger market and may help reducecosts.

Docking systems, already demonstrated inshallow water, will enable autonomous datagathering between moorings. Those mooringsmay utilize wire-guided profiling vehicles withmegametre endurance for gathering physical,biological and chemical variables. In addition, themoorings could provide data-transmission andbattery recharging facilities for the autonomousvehicles.

Progress is not limited to oceanographicautonomous vehicles. Autonomous aircraft (for

Observing the Oceans in the 21st Century, C.J. Koblinsky and N.R. Smith (Eds), GODAE Project Office and Bureau of Meteorology, Melbourne.

example the Aerosonde) are already capable ofoperating over ranges of 2500 km, a range that isset to double each year over the next 2–3 years.With sensor suites measuring temperature,pressure, humidity and wind, autonomous aircrafthave a clear role to play in marine meteorology.

The paper concludes with a brief note on thelegal aspects of autonomous vehicles. We live in anera where technical innovation far outpacesprogress in matters of international law. While thisshould not be seen as restrictive, an awareness ofthe issues is important, especially for a globalintegrated observing system.

The need for autonomous observations of climateFifty years ago the interior ocean circulation wasperceived as sluggish and essentially steady. Asaccurate direct velocity observations becameavailable from early moorings and neutrallybuoyant Swallow floats, it became clear that muchof the ocean was populated by energetic mesoscaleeddies with scales of O(30 days) and O(100 km).At about the same time, Jerome Namias andcolleagues noticed that large-scale variations ofroutinely observed sea surface temperature (SST)in the North Pacific persisted for months andseemed to be associated with changes in theseasonal climate of North America. These twoobservational discoveries were the origins of boththe motivation for global ocean observations forclimate and the understanding that observingocean climate variability would require substantialsampling to separate climate variability from othervariability with smaller space and timescales.

The 1980s brought an understanding of theprocesses involved in the coupled ocean–atmosphere El Niño–Southern Oscillation(ENSO), an awareness that this phenomenonaffected climate over much of the globe, establish-ment of an observing system including theambitious TAO array to monitor the equatorialevolution of ENSO, and the development ofclimate forecasts with significant skill at rangesnear one year. At the same time there was anincreasing awareness that anthropogenic climateforcing by changes in the composition of theatmosphere resulting from energy consumption,industrial production and changes in land usemight lead to significant climate change. The longtimescales of the phenomena involved in anthropogenic climate forcing and the fact that it

has no parallel in the paleoclimatic record makesan empirical approach impractical and forces aheavy reliance on dynamical models. This reliancerequires us to determine how to verify the abilityof these models to predict a response that hasnever been observed.

One strategy for verifying models of anthro-pogenic climate change is to use quantitativeobservations of both the state of today’s climateand the processes that maintain that climate asbenchmarks for models and to simultaneouslyinsist that the models simulate the major modes of observed interannual-to-decadal climatevariability.

The diversity of climate phenomena, fromseasonal-to-interannual predictability of ENSOthrough decadal variability of the North AtlanticOscillation and the Pacific Decadal Oscillation,and on to rapid climate change and anthropogenicforcing means that the ocean observing system forclimate must be designed to detect a wide range ofsignals in the face of competing high-frequencynoise. Even if one sets aside the critical short-termexploratory and process-oriented studies needed toclarify specific phenomena to be included inmodels, the observing system must deal with abroad range of processes shaping the ocean’s rolein climate and must do this over the globe and thefull oceanic water column. Several oceanicparameters affect the atmosphere, thus it isimportant to understand a variety of interdiscipli-nary oceanic processes that affect SST on a broadrange of timescales.

When considering climatic responses to anthro-pogenic forcing, and the potential for rapidclimate shifts, the very long timescales ofimportance bring the entire wind-driven andthermohaline circulation into play. This means thatthe climate-observing system must include ways tomonitor processes in the harsh, high-latitudewinters as well as interactions with sea ice andtransport processes at great depth. At the sametime, other models suggest that substantial climateshifts can be caused by changes in the tropicsmodulating the evolution of ENSO-like cycles.Thus, to address response to anthropogenicforcing all latitudes and depths must be observed,albeit perhaps at a slower rate than required for thetropical upper ocean.

Even the assertion that slow climate drifts canbe understood by observing only low-frequency

variability may be an oversimplification. The roleof eddies in modulating the general circulation haslong been an issue, and recent work in thesubtropical North Pacific suggests that slowvariability of the aggregate effect of mesoscaleeddies or Rossby waves may be important inunderstanding climate variability in the ocean.

Data-assimilating models will be essential tointegrating the necessarily sparse satellite and insitu observations needed to integrate observationsfrom the future observing system. Nevertheless, itwill be some time before these models and ourknowledge of which ocean processes are central toclimate are adequate to design the ultimate oceanclimate observing system. This design must relyon our best estimates today and the flexibility toadapt to new knowledge. There are, however, atleast three things that are certain about the design:

1. Global observations from all depths at a densityhigher than today's and sustained for manyyears will be required to separate climateprocesses from each other and from the noise ofregional phenomena and mesoscale variabilityin order to diagnose the climate's dynamics.Simply marking the climatic changes of a fewkey parameters will not do. This means massivenear-real time observations from around theglobal ocean assimilated within the mostsophisticated numerical models. Satelliteremote sensing naturally provides the neededtemporal and spatial coverage, but numerous insitu observations will be needed if the internalstructure of the ocean is to be observed.

2. Implementing such a system will be expensiveand unless every economy afforded by newtechnology is used it will be too expensive tohappen. Sophisticated measurements will stillneed to be taken from research ships, but thevast volume of data must come from remotesensing and in situ systems designed to delivervoluminous accurate measurements at minimaloperational cost.

3. Unless the flexibility to respond to newunderstanding of climate dynamics is built intothe observing system it will soon be obsolete.Similarly, if the observing system is too finelytuned to today's conventional wisdom it mayfail to discover anything new.

It is within this framework that we feelautonomous vehicles present great promise forevolution of a global ocean observing system for

climate. Together with strategically sited long-term moorings and bottom observatories, driftingand mobile platforms offer the means to greatlyimprove upon our present sparse samplingsystems. Recent developments in digital signalprocessing, mooring designs and materials,satellite communication, miniaturization ofmechanical components and sensors are makingpossible a new approach to gathering climate datafrom autonomous platforms. A new generation ofstable, low-power sensors to measure not onlyparameters of physical interest, but propertiesdescribing chemical and biological oceanprocesses, promises to make these platforms usefulto the broad oceanographic community concernedwith all aspects of climate phenomena.

Existing technology for sustainedocean observationTraditional methods of making sustained observa-tions of the ocean’s interior include shipboardprofiling and sample collection, deployment ofmoored arrays of internally recording instruments,use of cabled (to shore) sensor arrays, anddeployment of various types of drifting buoys.These methods are generally viewed as inadequateand/or too costly to meet the future societal needsfor continuous real-time observations at manylocations throughout the world’s oceans.Shipboard measurements will continue to play acritical role in oceanographic science, but the costof oceanographic research vessels preclude theiruse for continuous observations at fixed sites.However, commercial ships that routinely transitfixed routes do offer a cost-effective means ofdoing repeated surveys across ocean basins. Todate, oceanic ship-of-opportunity programs havebeen limited to XBT and ADCP profiling from alimited number of vessels. An opportunity existsin the oceanographic instrumentation communityto develop highly automated atmospheric, surfacemeteorological and oceanographic shipboard, insitu, remote sensing and expendable profilingsystems for use on ships of opportunity toroutinely and continuously sample broad reachesof the oceans. The critical elements needed in sucha development program are improvements in thesensors that can be used in this kind ofenvironment and fully automated systems that donot impact normal vessel operations. Such stand-alone sensor systems should also be capable ofoperating on many different types of vessels.

3.6 G. Griffiths et al.

Moored arrays of internally recordinginstruments have been the mainstay of efforts overthe past 30 years to observe ocean variability andto gain insight into the forces driving oceancirculation. However, the cost of traditionalmoored arrays in the deep ocean including instru-mentation, supplies, labour and ship time islimiting their application to specific high interestlocations, often for relatively short time periods (1 or 2 years). To play a significant role in sustainedglobal observational programs, moored tech-nology needs to be enhanced. First, they need to be engineered to operate for extended periods(perhaps 5 years or longer) without maintenance.Present systems are typically maintained at 6 month(surface moorings) to 1 or 2 year intervals(subsurface moorings). Second, they need toautomatically transfer data to shore so that thedata is available on a reasonable schedule. Inaddition, they need to be developed as moregeneral-purpose facilities, capable of carryingdiverse instrumentation and possibly linked tobottom observatories and autonomous vehicles. Ifthese goals can be achieved at a reasonable cost,then an extensive grid of moored arrays would bea feasible and cost-effective part of a globalobservational system. Researchers are presentlydesigning a prototype mooring system for makinglow-frequency current and temperature measure-ments with the requisite features, i.e. 5-year lifeand data telemetry (Frye, 2000). The success ofthe TAO array has led to plans for its expansion(McPhaden et al., this volume). Already mooringsare in place in the equatorial Atlantic (Garzolipers. comm., 1999) and plans are developing toinstall an equatorial array of moorings in theIndian Ocean (Meyers et al., this volume). Plansare also being put in place to deploy an extra-tropical network of surface moorings to provide aglobal network of surface-flux reference sites(Taylor et al., this volume) and deep oceanobservations (Send et al., this volume).

Cabled sensor systems are an ideal way tocontinuously observe the ocean. They are reliablefor many years, offer high bandwidth telemetry,provide almost unlimited power for sensors, andare inexpensive to operate once installed.Unfortunately, most oceanic sites are so far fromshore that cables are prohibitively expensive toinstall. In areas where cables already exist, such asalong abandoned telephone-cable routes, there isan opportunity to utilize their bandwidth andpower delivery for reasonable cost. Prototype

systems that take advantage of abandonedtelephone cables are being developed (Chave,1997). This approach is of course limited by theavailability and location of existing cables.

Drifting buoys and floats have always been acost-effective means of collecting data over broadoceanic areas, and the new generation of profilingfloats is enhancing the quality and quantity of data that can be acquired with this technology. As low-power satellite telemetry options improve,the quantity of data available should increasesubstantially. The limiting technology for profilingfloats is probably sensor technology (see latersection).

Emerging technologiesProfiling floats

Planned global coupled ocean–atmosphere modelswill be able to improve their forecasting skill byassimilating 3D ocean in situ observations. Animportant source of these 3D in situ observationswill be profiling floats. Historically, floats havebeen deployed as Lagrangian drifters and the Argoproposal (Argo Science Team, this volume) callsfor a significant increase in the number of floatsdeployed in this mode. A complementary methodof deploying floats will enable quasi-Eulerianprofiling at fixed locations.

Lagrangian systems

Subsurface drifters were deployed in significantnumbers during the WOCE experiment. ALACEand MARVOR floats have contributed (andcontinue to contribute) to our knowledge of oceancirculation and processes. They deliver mainlyundersea trajectories—precise trajectory of watermasses for the acoustically tracked MARVOR, andmean trajectory for the ALACE positioned byARGOS when surfacing periodically. Profilerswere derived from those drifters: the P-ALACE(Davis, 1998) and the PROVOR (Loaec et al.,1998). However, these instruments suffer fromsome deficiencies that need to be overcome beforewe have profilers usable on a routine basis inglobal observation networks.

• They are presently deployed by research vesselsand scientists. They will have to be deployed bynon-specialists from ships of opportunity andaircraft as it is presently done for meteorologi-cal drifters. Design and initial experiments arenow in progress.

• The commonly used one-way satellite com-munication system limits transmission of datawhen selecting 80–100 (temperature, salinity,depth) points from a typical 2000 m profile. In addition, the transmission time requires the float to be on the surface for 6–20 hours.Two-way communication systems offering alarge increase in data throughput are becomingavailable.

• The salinity sensor has to be improved in orderto give accurate data during the 4-year lifetimeof the float (see section on Sensors and Tele-metry for autonomous platforms). However,long-term moorings at select sites can provideT–S records from the deep ocean; and therelative stability of the deep TS properties couldthen be used to calibrate salinity sensors on thefloats.

• Obviously, it is desirable to decrease the unitcost of profilers. The needs of Argo (3000 floatsat sea) will spur several manufacturers tocompete on price and performance.

Quasi-Eulerian systems

Eulerian measurements are an importantcomplement to the Lagrangian Argo network. Inaddition to the traditional fixed moorings, severaltechniques can be employed to provide quasi-Eulerian measurements. This could be done byusing gliders as virtual moorings, or mooredprofilers. One can also imagine using theLagrangian multicycle profiler in a pseudo-Eulerian way: the float lying on the bottombetween an up and down vertical profile. Pop-upexpendable instruments are also good candidates.One can observe that the meteorologicalobservation network is mainly constituted ofexpendable radiosondes profiling the atmospheretwice a day in 700 places all over the globe.

As an example, the expendable pop-up system(EMMA) has the following characteristics(Conogan and Guinard, 1998):

• Expendable lighter-than-water probes,equipped with CTD and other sensors profilingthe water column from the bottom (5000 m) tothe surface and, after surfacing, transmitting thedata recorded during ascent through theARGOS link;

• Probes may be dropped from vessels or aircraftand stored on the sea bottom until released by a

timer. During storage the probe sensors areencapsulated as protection against fouling and immersed in a standard solution for recalibration before popping up. A very highquality of measurement is expected.

• A platform operating a number of such probesmoored at the same location, with probesreleased sequentially, can provide a pre-programmed time series of profiles atpredefined geographical co-ordinates.

Gliders

Inadequate sampling is the fundamental problemfor ocean observations, be they physical, chemical,or biological. Oceanographers continue to befrustrated in trying to distinguish betweentemporal and advective changes in the oceaninterior because they are unable to measuredensely enough in space and time, being limitedby the cost or depth range of conventionalmethods (ships, moorings, and satellites). Glidertechnology is intended to answer the need toobserve the ocean at more places, more often, andfor longer periods than is affordable usingtraditional platforms. Gliders embody thephilosophy that a distributed network of smaller,smarter, cheaper platforms is the most cost-effective way to observe vast regions of the ocean.

Henry Stommel (1989) dreamt of a global fleetof autonomous vehicles travelling long distanceswhile telemetering temperature and salinityprofiles via satellite to ground control stations.Davis et al. (1992) took a large step in realizingthis dream by developing ALACE floats, vehiclesthat profile vertically under buoyancy controlwhile drifting horizontally. Gliders use wings andhydrodynamic shape to induce horizontal travelfrom buoyancy control. The key elements makingpossible long-range gliders are efficient buoyancycontrol, modest hydrodynamic performance, low-power electronics, on-board computing, satellitenavigation and two-way communication for near-real time data transmission and remote control.

Unlike gliders in the atmosphere, theirunderwater counterparts glide both down and upby making themselves alternately more andless dense than the fluid through which they travel.By assimilating Global Positioning System (GPS)navigational fixes obtained at the sea surface, they set an underwater course to reach an intended target. By gliding to a sequence of targets, they may be controlled to execute a survey. By homing

on a target, they can profile vertically at a selectedgeographic location, imitating the sampling of amoored profiler. Through two-way communica-tion at the sea surface, they can be controlledremotely to observe at arbitrary locations in near-real time. By virtue of remote control, they can berecovered and reused. Those now underdevelopment are small enough to be launched andrecovered from small boats, largely obviatingreliance on ships. Battery-operated gliders nowbeing tested have design ranges measured inthousands of kilometres and operational durationmeasured in months or years. In regions where itis sufficiently large, thermal stratification may beused to greatly augment glider range (Webb andSimonetti, 1999).

Gliders are inexpensive enough that loss ofindividual units is not catastrophic. That is, anarray of gliders is reasonably fault-tolerant, bothbecause of comparatively low unit cost and theability to adjust a sampling plan in response tofailures.

Traditional oceanographic surveys follow fixedpre-selected transects and are seldom modifiedduring their executions because of constraints oncruise duration. Moored data are perforcecollected where the moorings are set. Data-adaptive sampling is relatively new in oceanogra-phy and has been associated generally with shortduration, limited extent surveys because conven-tional autonomous underwater vehicles (AUVs)have ranges and duration measured in tens ofkilometres and hours (Zhang et al., 1999).Because of their modest cost and long range,gliders can perform data-adaptive sampling overmuch more extended durations than otherwisefeasible.

Gliders are now under development by anumber of groups. Those now being tested are~50 kg devices 1–2 m long, designed to operatefrom the ocean surface to ~2 km depth at speeds~0.25 m/s with power consumption of ~0.5 W.They use ~1 N buoyancy force to alternately diveand climb, requiring volume changes of ~0.2 L.Gliders dead reckon underwater, controlling pitchand roll trim (used to turn) by shifting masswithin the vehicle. In battery-powered vehicles,volume change at depth is accomplished bypumping oil from an internal reservoir to abladder external to the vehicle pressure case.Roughly three-quarters or more of the energy usedby battery-powered gliders goes to run the pump.

The technology is maturing rapidly; onecompany, Webb Research Inc. (see http://www.webbresearch.com) started to ship limitednumbers of gliders to customers in 2001. Severalof Webb Research’s environmentally poweredgliders were used successfully in the SlocumBermuda Pilot Experiment (Fratantoni et al.,2001).

Low hydrodynamic drag together withsubstantial lift from wings is crucial to gliderperformance. Gliders now being tested are capableof gliding along slopes from as steep as 2:1 to asgentle as 1:5. A range of possible slopes allows aglider a range of possible horizontal speeds and arange of power usage for a given vertical speed.The gentle glide slopes attained by vehicles nowbeing tested imply horizontal ranges of severalthousand kilometres. All of the ocean is within2700 km of land, a small percentage being fartherthan 1500 km away, so that gliders typically mayneed only to devote a fraction of their energyreserves to transit from land to a region of interestand back.

Global two-way communication using hydrody-namically unobtrusive antennas and low-powertransmitters is crucial to glider operation.Commercial systems promise faster (~2 kbps) andless expensive two-way communication. Fast datatransfer is necessary for gliders to stay at thesurface only for a few minutes.

Instrumentation carried by gliders is limited bypower, hydrodynamic drag, and cost. Gliders nowcarry CTDs whose accuracy is comparable to thoselowered from ships. The additions of dissolvedoxygen and optical sensors are underdevelopment.

Gliders as virtual moorings

Gliders are constrained by energy considerationsto move through the ocean at speeds comparableto ocean currents. Transects made by them arenecessarily more prone to aliasing by temporalvariability than are conventional ship-basedsections. One way to use gliders is to control themto maintain their geographic position bystemming the ambient currents. As long as thedepth-averaged current is weaker than themaximum horizontal speed a glider is controlledto attain, a glider can maintain its position while profiling vertically. In this mode, gliders can collecttime series of profiles much as a moored profilerdoes, yet with two orders of magnitude less total

mass and without the need for a ship to deployand recover it. The ability of gliders to transit toarbitrarily chosen locations makes possibleintercomparison to monitor sensor drift.

Virtually moored gliders can be expected tomaintain their positions as well as or better than asurface mooring does. They use the differencebetween successive GPS fixes and the displacementpredicted by dead reckoning to estimate horizontalcurrents averaged over the depth range of gliderprofiles. In addition to temperature and salinityprofiles, then, gliders can be used to collecttransport time series. In principle, pairs of virtuallymoored gliders can be used to estimate absolutegeostrophic currents by adjusting geostrophicshear to fit depth-averaged current. A network ofgliders could be used to solve the geostrophicreference problem regionally or globally,depending on the scale of the array.

Establishing a relatively sparse global network(~1000 km separation) of virtually mooredgliders is likely to be a cost-effective solution to theproblem of monitoring upper ocean climate. Theapproach is opposite to that used by hydrogra-phers to estimate the large-scale slopes of propertysurfaces. Instead of resolving mesoscale eddies andfronts with closely spaced depth profiles alonglong transects, a network of virtually mooredgliders would resolve variability temporally,including tides and internal waves, to estimatelateral gradients by differences over comparativelylarge distances. Variance in such quantities asdynamic height contributed by higher frequencyfluctuations (e.g. tides, internal waves) iscomparable to that contributed by mesoscalefluctuations in many oceanic regions.

Repeated glider sections

A majority of what we know about oceancirculation has come from hydrographic sectionstaken from research vessel, such as those carriedout during WOCE. The repeated hydrographicsections from which it would be possible to studyclimate variability are extremely rare. Recentdevelopments allow Voluntary Observing Ships(VOS) to deploy XBT and XCTD probes at highenough density that transports can be accuratelycalculated over the upper 2 km, and it is feasible torepeat such VOS sections several times per year ifthere is sufficient shipping.

Although gliders move more slowly than shipsthey provide an opportunity for repeated section

measurements to observe both variability ofproperty distributions and of the lateral heat andwater property transports that drive the oceanclimate engine. It is relatively simple to outfitgliders with CTD sensors that have precision andaccuracy equal to (in the case of temperature) orsuperior to (for conductivity and depth) those ofexpendable probes. Gliders are most efficientwhen used in the upper 2 km, like VOS probes,but the section location and timing need not betied to shipping and could be adapted to theconditions observed. Additional sensors suitablefor glider installations and capable of observingchemical and biological variability along a sectionare also available.

Although long-range performance has not yetbeen demonstrated, it is feasible for a glider tooperate at a speed near 25 cm/s over a range of theorder 5000 km; longer ranges are feasible atreduced speed. This is much too slow to considera glider section to be synoptic but is, in manyplaces, sufficient to adequately measure the along-track gradients needed to compute transports. Byusing altimetric and surface temperature measure-ments inside a model framework, it may bepossible to extend this capability to relativelystrong currents in which the propagation ofvariability features is relatively simple.

The most straightforward use for glidersoperating in section mode may be within 1000 kmof coast locations to observe the physical andbiological manifestations of climate variability.Here, gliders could be repeatedly launched andretrieved without use of ships, allowing accumula-tion of long time series of repeated sections at verylow cost.

In the final analysis, the balance of autonomoussampling from floats that are carried by currents,gliders in virtual-mooring mode which holdposition against the currents and gliders whichoperate along sections will depend on thephenomena to be measured, the overall density ofobservation that can be afforded and the nature of the noise which obscures the signals of interest. Ifthe observational density is high enough or thesignal has large enough scale that it is fullyresolved, then all methods are equal. In lowdensity, the ease of interpretation might favour avirtual mooring where spatial variability does not confuse interpretation. In a low-noise environment it may be efficient to sample severallocations with one glider (sections) while in a

high-noise environment sampling may need to beconfined to a single location (virtual mooring) tokeep the signal to noise ratio high. In a regionwith large spatial variation a virtual mooring maybe aliased by spatial variability, whereas a glidersection will be aliased by temporal variability if itis large.

Moored instrumentation

Great advances have been made in moorings andmoored instrumentation over the past few yearsand continuing development is anticipated.Instrumentation is becoming smaller, more powerefficient and more capable. Mooring technologyitself has extended our reach to more severeenvironments and allowed longer deployments.

Surface moorings continue to offer the uniquecapability of sampling from the surface of theocean down to the sea floor. Improved meteoro-logical sensors now record all the variables neededto estimate the air–sea fluxes of heat, moisture,and momentum. New sensors are under test thatsample and chemically analyse aerosols, andremote sensors of the atmosphere are beingconsidered for deployment on buoys. Along themooring line, and for use on subsurface as well assurface moorings, one of the greatest advances inthe recent past has been the development of stablemoored salinity sensors, capable of remainingstable to ~0.05 for one year. These are now beingfollowed by in situ chemical analysers and bio-optical instruments. In the near future it is possiblethat sensors based on genetic material may be usedto assess the presence of specific species. Workalso continues on acoustic sensors: passive onesfor wind speed and rain rate, and active ones,including sensors for measuring long-rangeintegrated temperature change (acoustic tomo-graphy) and identifying specific species of fish.Some new subsurface mooring designs looktoward AUVs that dock to unload data periodicallyor to buoyant data capsules that float to the surfaceperiodically and telemeter the mooring's data backvia satellite. If all the data is recovered, the costand necessity of recovering these moorings couldbe avoided.

Less revolutionary than virtual moorings,moored profilers have great potential forproviding cost-effective and very detailed oceanobservations over extended time periods.Essentially, a moored profiler is an AUV that istethered to a mooring line so as to restrict its range

of motion to two degrees of freedom: verticaltravel along the wire and rotation about it. Thus incomparison to AUVs and gliders, far simplercontrol systems may be employed. While mooredprofilers do require the added expense of conven-tional mooring components (i.e. anchors, wirerope, buoyancy and a ship to deploy, service andrecover), these moorings are very simple in designand do not add a great deal to the overall systemcost. Unlike virtual moorings, moored profilersare able to operate in most current regimes and inareas where surfacing is not an option (such as inice-covered seas). The key to the effectiveness ofmoored profilers is their ability to carry a singlesuite of sensors as they make repeated profiles ofthe water column. Thus, moored profilers areideally suited for ‘time-series stations’ withinsustained ocean observations programs, but arealso well matched to short-term, process-orientedexperiments that require information at highvertical and temporal resolution.

Moored profilers were developed by severalinvestigators in the past and new designs are beingactively pursued by several groups at this time. Thebest known examples of the previous generation ofprofilers include the Cyclesonde developed by vanLeer et al. (1974), the Profiling Current Meter(PCM) developed at Draper Laboratory by Eriksenand Dahlen (Eriksen et al., 1982), and the current-driven profiler developed by Eckert and Morrison(Eckert et al., 1989). Although each of theseinstruments provided unique views of oceanvariability, none of these earlier systems had thegeneral utility of the more recent designs, and noneare being produced commercially at this time.Groups actively working on the next generation ofmoored profilers include Brooke OceanTechnology (Fowler et al., 1997), who are testinga wave-driven profiler for coastal waters; theUniversite Pierre et Marie Curie, Paris, who have aprototype buoyancy-driven device capable ofcycling to 1000 m (Provost and du Chaffaut,1996), and McLane Research Laboratory, who are licensed commercial builder of the WHOItraction-drive profiler. It should be noted that theMcLane profiler, while similar to the WHOI-developed system described below, has significantmechanical differences from the WHOI unit interms of improved payload and lower hydrody-namic drag characteristics.

To introduce the capabilities and limitations ofthis class of oceanographic instruments, wedescribe here the WHOI Moored Profiler(Doherty et al., 1999), which is in the advancedprototype stage. The profiler is an oblate spheroid0.8 m in diameter and 0.4 m thick. To move alongthe mooring wire, the profiler utilizes a small DCmotor and traction wheel. Profiling at about 30cm/s, the drive system draws about 1.7 W fromthe battery. About 50% of this energy is used toovercome hydrodynamic drag. One million metresof profiling is possible over deployments of a yearor more, by using a lithium battery.

The present sensor payload of this profilerincludes an FSI Micro-CTD and 3-axis acousticcurrent meter (3D-ACM). A new generation CTDbeing developed at FSI will reduce powerconsumption from 1.5 W to less than 0.5 W,which should increase profiling range to morethan 1.5 million metres. While not yet a standardimplementation, real-time telemetry methods havebeen developed which allow moored profilers totransfer their data to shore in near-real time. Thesemethods include inductive data transfer via themechanical mooring wire and acoustic datatransfer from an acoustic transmitter in the profilerto an acoustic receiver in the surface buoy.

Future development ideas include extractingenergy from wave-induced mooring motion,extending the operating window of profilers to thesurface, and increasing the sensor payloads toinclude biological, chemical and optical sensors.As profiler payloads are not strongly limited, thislatter task is not particularly daunting, althoughpower consumption by the sensor payload remainsa major issue, as does biofouling for some types of sensors.

The primary advantage that moored profilershave over traditional moorings with discreteinstruments is that they are capable of collectinghigher quality data with higher vertical resolutionat lower cost. A single suite of instruments on amoored profiler can replace 5, 10 or even 20individual instruments on a traditional mooring.This can lead to substantial cost savings in terms ofcapital costs for instruments and recurring costsfor instrument maintenance and calibration. Dataquality may be improved because multiple calibra-tions are not needed to compare data fromdifferent instruments. The primary disadvantagesof moored profilers relative to conventionalcurrent meter moorings include non-synopticity of

the measurements and limited temporal resolution.These disadvantages are a consequence of the timeit takes for a profiler to traverse the water column(typically about 4 or 5 hours for a 5000 m deepocean profile by the WHOI instrument.

In this regard, moored profilers and gliders sharemany of the same characteristics. Their activenature brings with it more potential failure modesthan are typical for passive systems. Solving these‘active’ problems is an interesting challenge for thedesigners of moored profilers, and of gliders andother AUVs. The ultimate success of these systemsas oceanographic tools may rest on the quality ofthe solutions to this new problem set.

Autonomous surface vehicles

Autonomous surface vehicles (ASVs) havereceived less attention from developers than theirunderwater counterparts. However, a few projectsare making significant advances. ASVs haveseveral advantages compared to AUVs, including asimpler and cheaper energy supply, possibly usinginternal combustion engines and generators, easilyproviding ranges in excess of 1000 km; datacommunications and navigation are more straight-forward; construction costs are lower; and transitspeeds are higher. ASVs are limited to making near-surface observations unless they carry automaticwinches to emulate a standard shipboard hydro-graphic station. Current development projectsinclude:

• Caravella—a multi-national EU Eurekaprogram to develop a 7 m long, 2 m wide, 6 mhigh self-righting autonomous surface craftwith satellite data and control and a range inexcess of 1300 km at 4 kt. It includes anautonomous CTD winch. Commercial launch isexpected in 2002.

• SASS—from a UK consortium, is a 5 m longsemi-submersible design with a planned rangeof 600–1000 km at 12–15 kt and a 200 kgpayload space. A 1/3 scale prototype hascompleted its trials and a commercial version isexpected in 2001. There are no current plans toadd an autonomous winch (Young and Phillips,2000).

• MIMIR—an autonomous above-water vehicledeveloped by Qinetiq in the UK (formerly theDefence Evaluation and Research Agency),MIMIR was designed specifically for marine

environmental and oceanographic surveyanddata acquisition. While best suited forcoastal applications in its present form, thevehicle could be adapted for work in the deepocean (see http://www.dera.gov.uk).

• ENSR—a collaborative project between theport of Bordeaux, France and other partners. This autonomous surface craft, 5 m long and just under 1 tonne in weight, is currentlysuited to coastal waters, but offshore applica-tions have been considered (see http://www.bordeaux-port.fr/f/prod_f.html).

While recognizing that ASVs may face similarlegal issues to AUVs (see ‘Legal Issues’), this classof platform does have considerable potential tocontribute towards a sustainable observing systemin coastal and continental shelf waters or inoceanic waters served by island bases.

Autonomous underwater vehicles

Propelled autonomous underwater vehicles(AUVs) have been under development by themilitary and civilian research community since the1970s. Busby (1987) compiled a comprehensivereview of progress up to 1987, while over 60AUVs were listed in the 1999 edition of JanesUnderwater Technology (Funnell, 1999). Overthe last 5 years significant advances have beenmade; in particular, vehicles are now completingscience missions funded through competitiveprograms. AUVs have moved away from beingtest-beds for engineering evaluation, control andstrategy theories; they are no longer curiosities.

Major projects underway in several countriesare now at the stage of demonstratingautonomous missions of real scientific utility. But,perhaps naturally, at the present state ofdevelopment, AUV missions are focused onprocess studies rather than sustained operationalobservations. Key examples include:

• A survey of coastal fronts in Haro Strait, BritishColumbia, was completed in June 1996 by theOdyssey II vehicle (Nadis, 1997), and thevehicles have been used in the harshenvironment of the Labrador Sea in winter.Prior to the survey careful comparisons of thetemperature and salinity measurements fromOdyssey with those from a conventional CTDhad been made, Bales and Levine (1994)showed that with careful sensor placement it waspossible to achieve minimal contamination.

Such experience is an essential prerequisite toacceptance of autonomous vehicle technologyinto an operational system.

• Although not an ocean science application, the8.5 tonne Theseus completed a 350 km roundtrip mission in April 1996 to lay a fibre-opticcable under sea ice in the Canadian Arctic(McFarlane, 1997). This mission amplydemonstrated the ability of AUVs to operate inenvironments otherwise only accessible by navalsubmarines, and has implications for futuretrans-Arctic oceanography.

• Dhanak and Holappa (1996) have shown thatit is possible to reduce the self-noise andvibration of an AUV to such an extent thatmeaningful turbulence measurements can bemade within the seasonal thermocline.

• The Autosub-1 vehicle (Millard et al., 1998)completed a 53 h, 263 km mission surveyingthe upper 500 m of the ocean off Bermuda, inSeptember 1998. The mission was part of theAutonomous Vehicle Validation Experiment,which had an objective of demonstrating thatan AUV could leave near-shore Bermuda,transit to the Hydrostation ‘S’ site and carry outphysical, chemical and biological measure-ments and return to a near-shore location (Griffiths et al., 2000).

Following on from these achievements some ofthese groups have developed proposals to furtherdemonstrate the utility of AUVs to ocean science.These include:

• ALTEX—the Atlantic Layer TrackingExperiment. This is a US National OceanPartnership Programme project using a vehicledeveloped from Odyssey by the Monterey BayAquarium Research Institute and BluefinRobotics Inc. to track the Atlantic water inflowinto the Arctic Ocean (see http://www.bluefin-robotics.com). Novel features of the projectinclude the regular deployment of satellitetelemetry data capsules from the vehicle and apower supply based on a fuel cell. The vehiclewill be used in an experiment north ofSpitzbergen from the USS Healy in fall 2001.

• A trans-Atlantic transit by an AUV. Theseus isthe only powered AUV with the energycapacity to cross the Atlantic. An eight-partnerconsortium led by the Autonomous UnderseaSystems Institute proposed such a project in1998. While there was little doubt about the

capability of the vehicle to perform such amission, the cost of the lithium primary batterypack was too high to be practical. Nevertheless,such a mission remains technically achievable.

• Under ice shelves. Little is known of the oceancirculation under the ice shelves that occupy40% of the Antarctic continental shelf or of the processes that take place. AUVs offer theopportunity to make interdisciplinary observa-tions under warm water ice shelves in the southPacific (e.g. Pine Island Bay Glacier), and thecold water Filchner-Ronne Ice Shelf in theWeddell Sea, as well as under the glaciers ofnortheast Greenland (e.g. 79°N Glacier). Inaddition to measurements under ice shelves,AUVs would be capable of traversing frombeneath an ice shelf, under a coastal polynyaand out beneath sea ice regions measuring heat and salt flux and ice thickness. Eight multi-disciplinary projects will take place using theAutosub-2 AUV in these areas during 2002–2000 (see http://www.soc.soton. ac.uk/SOES/MSC/OC/CEO/aui/main.html). The AlfredWegner Institute is also considering AUVtechnology for multidisciplinary sub-ice shelfobservations (M. Klages, pers. comm.).

AUVs have been proposed as a key componentof multi-institution, multi-platform underwatersurveillance and monitoring systems, and initialfeasibility trials have taken place, which involvedmoored and free-swimming componentscommunicating with each other and with scientistsashore using acoustic and radio communication(Schmidt et al., 1996). AUVs shuttling betweenmoored docking stations is one possible scenariofor their early introduction as parts of a sustainableobserving system. Much of the technologyrequired for docking, recharging batteries,downloading data and communicating with shorehas already been proven. In the future, AUVs willbecome less dependent on support ships. They willalso become able to leave and return to harbourautonomously. Limited demonstrations of suchcapabilities have already been made.

Future directions

Some of the ideas on new platforms now at thestage of conceptual design or laboratory testingmay survive long enough to become practicallyuseful in 10–20 years. The same can be said forsensors, instrumentation and power sources.Candidate developments relevant to this paper

have been reviewed by Griffiths (2000) andinclude:

• energy-efficient platforms based on bio-mimetics, replicating the swimming behaviourof marine animals;

• very high energy density electrochemical cells,e.g. using hydrogen stored in novel materials(such as artificial mesoporous zeolites andoxygen extracted from seawater) enabling verylong duration deployments;

• increasing trend towards using commercialmass-market sub-assemblies to minimize cost(e.g. the fuel cell described above may havebeen developed for a laptop computer);

• platforms specifically designed for sustainedobservations, bypassing the proving ground ofprocess studies and driven by companiesrecognizing the market potential of operationaloceanography;

• increasing use of adaptive sampling madepossible through intercommunication betweencomponents of a system and between thecomponents and shore laboratories.

The key issues of price and ease of deployment,use and data recover, reliability and appropriateaccuracy of the data will also continue to test theingenuity of designers.

Sensors and telemetry forautonomous platformsDuring the past decade, there has been a majorthrust forward in sensors that are capable ofproviding important chemical, optical, biologicaland acoustical as well as physical data (Dickey1991, 2000a). Interdisciplinary sensor suites areimportant for studying problems such as carbondioxide cycling and variability, the role of biologyin upper ocean heating, phytoplankton productiv-ity, upper ocean ecology, population dynamics,and sediment re-suspension (Dickey andFalkowski, 2000). Many of the new sensors arerelatively small and have modest power require-ments. Thus, the deployment of an increasingnumber of these sensors from autonomousplatforms is becoming practical.

Already, new scientific insights into inter-disciplinary processes have resulted from con-current, multi-sensor measurements from moorings

(Dickey et al., 1998; Dickey and Falkowski,2000). Examples include the roles of seasonal andepisodic forcing and eddies in increasing upperocean nitrate and levels of primary productivity atmid- and high-latitudes; monsoonal atmosphericand eddy forcing of productivity in the ArabianSea; modulation of productivity in the equatorialPacific through tropical instability waves, Kelvinwaves, and El Niño/La Niña sequences; sedimentresuspension via internal solitary waves andhurricanes; and variability in upper ocean heatingcaused by phytoplankton. Moorings are also beingused to groundtruth ocean colour data collectedfrom satellites (Dickey, 2000b). Durations ofinterdisciplinary moorings have typically been afew months to a year. The major constraintremains biofouling; however, new anti-biofoulingmethods are being developed and tested, andencouraging results suggest that this impedimentwill be considerably less limiting in the future.

Drifters, and most recently AUVs, have alsobeen used to collect limited interdisciplinary datasets. Size and power are more constrainingparameters for drifters, AUVs, floats and glidersthan for moorings. Nonetheless, some optical andchemical sensors have been successfully deployedfrom drifters, and plans are underway for float andglider applications. AUVs have already carriedsimilar sensor suites as well as ADCPs andturbulence probes. Again, biofouling will beproblematic for long-term measurements fromthese various platforms.

The ‘Marine Technology Frontiers for Europe’workshop at Brest in April 2001 discussed the potential for importing new sensor de-velopments into marine science (see http://zest.esf.org/life/ac/Marine_Board/structure.htm).In the coming decades biosensors are expected tomake a major impact in medicine, food industriesetc. as a result of massive investment in this post-genome era. In particular, micro-arrays of bio-recognition elements (e.g. antibodies) willenable simultaneous measurement of a range ofbiochemical parameters. Other biosensortechnologies that may be applied in the marineenvironment include those based on enzymeinhibition, bioluminescence and nanoparticles.Molecularly imprinted polymers can now bedesigned to recognize and quantify biomolecules(e.g. algal toxins). Electronic and optical ‘tongues’are under development for environmental water-quality assessment. They can be used to detect

radiatively active gases such as dimethyl sulfide,methylamine or methane as encountered in thedeep-sea environment, or as biogenic gasses in thesurface ocean. Mass spectrometers have alreadybeen used in situ, but currently have limited depth,atomic-mass and detection thresholds.Improvements are planned to permit long-termoperation through improved sample injectionsystem, wider atomic-mass range and lowerdetection limits.

In the future, it is likely that continuedexpansion will occur in the areas of small, energy-efficient, interdisciplinary sensors. In particular,sensors will likely be capable of measuring a muchwider range of chemical compounds and traceelements (Tokar and Dickey, 2000), higherspectral resolution inherent and apparent opticalproperties and spectral fluorescence, and multi-frequency acoustical systems for better resolutionof zooplankton size classes. Cost per sensor is animportant issue and may be a major limitingfactor, especially for expendable platforms.Commercialization of key sensors will be essentialfor this reason.

Telemetry of data from the various platforms iscritical for many, if not most, new applications.While future low earth orbit (LEO) satellitesystems should greatly expand bandwidth andenable transmittal of much greater volumes ofdata, there are concerns over the commercialviability of several of the current generation ofsatellite communication companies. OrbcommGlobal LP provides a two-way alphanumericpackage system, similar to email, using a constellation of 35 satellites (see http://www.orbcomm.com). Intersatellite com-municationprovides an orbital packet router for messages,reducing latency times. The ARGOS systemremains the most reliable, benign satellitecommunication channel for oceanic data sources.Currently limited to one-way, low bandwidthcommunication, and with gaps of some hoursbetween satellite passes, plans are in hand to enable two-way communications (see http://www.argosinc.com/docs/sysover.htm).

Recently, the development of high-gain, multi-element patch antennas has improved the case forusing data links to high orbit and geostationarysatellites. The new antennas obviate the need forsteerable systems, but they still require higherpower transmitters than those for use with LEOsatellites. For example, Internet Protocol links are

available through ViaSat at up to 2 Mb/s (seehttp://www.viasat.com/commercial/mobile/).

Legal issuesThere are two main sets of legal issues surroundingthe use of autonomous platforms at sea (1) access,involving national and international public law,and (2) risk, primarily involving private law.

Access

There is no accepted framework in internationalmaritime law for the operation of autonomousocean measurement platforms. Indeed, there is nostandard internationally accepted definition forsuch vehicles. A definition could include driftingprofiling floats; it could certainly include glidersand ASVs as well as AUVs. Furthermore, onlyslow progress has been made over the past 40years on a draft international convention on the farmore mature technology of moored and driftingsurface buoys, classed as ODAS (Ocean DataAcquisition Systems). The need was recognized asearly as 1961. It is possible that in any futureredrafting of the convention autonomousplatforms may be considered as a class of ODAS.

The legal aspects of access have been well reviewedby Brown and Gaskell (2000). Current use ofautonomous research platforms would comeunder the general heading of Marine ScientificResearch, as detailed in Part XIII of theConvention (UN, 1983) and would seem to coverinter alia the transit and use of an AUV in aCoastal State’s declared maritime zones, inparticular its Territorial Sea. The articles detailingthe rules governing access in relation to theTerritorial Sea are set out in Part II of theConvention. However, operational use of AUVsmight not be considered as marine scientificresearch; what effect this change from research tooperation might have is a matter for debate. Thatdebate might possibly take place under the auspicesof the Advisory Body of Experts on the Law of theSea (ABE-LOS) of the Intergovernmental Ocean-ographic Commission.

Some autonomous platforms have the potential tocause damage to third parties. A small number ofusers take out insurance to cover such risks as wellas their own loss. The issues are no longer

‘academic’, as a collision between an AUV andanother vessel has already resulted in a court casein Canada. As these platforms come to be usedfrom a wider range of support vessels, includingaircraft, the issues of liability may become morecomplex. Within the UK these issues have beenrecognized as needing a coordinated approach,and the Society for Underwater Technology haspublished a code of practice for the safe andefficient operation of AUVs (Dering, 2000). Theprivate law issues of operating AUVs have beenreviewed by Brown and Gaskell (2000).

ConclusionsThis paper has attempted to provide a realisticview of how new platforms currently beingdeveloped or in early use by the researchcommunity might contribute to a sustainedobserving system. As such, it is not ‘future gazing’;it is rooted in the present and supportive of astepwise increase in capacity and technologicalcomplexity. By giving encouragement and broaddirection to the ocean engineering community inacademia, research institutes and companies, thescience community can help pull the technologyinto routine use.

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Towards New Platform Technology for Sustained Observations.

TOWARDS NEW PLATFORM TECHNOLOGY FOR SUSTAINED OBSERVATIONS

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