This paper addresses the design of mobile sensor networks for optimal data collection. The development is strongly motivated by the application to adaptive ocean sampling for an autonomous ocean observing and prediction system. A performance metric, used to derive optimal paths for the network of mobile sensors, defines the optimal data set as one which minimizes error in a model estimate of the sampled field. Feedback control laws are presented that stably coordinate sensors on structured tracks that have been optimized over a minimal set of parameters. Optimal, closed-loop solutions are computed in a number of low-dimensional cases to illustrate the methodology. Robustness of the performance to the influence of a steady flow field on relatively slow-moving mobile sensors is also explored

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Collective Motion, Sensor Networks, and Ocean Sampling

Underwater gliders are autonomous vehicles that profile vertically by buoyancy control and move horizontally on wings. Gliders are reviewed, from their conception by Stommel as an extension of autonomous profiling floats, through their development in 3 models, and including their first deployments singly and in numbers. This paper discusses the basics of glider function as implemented by University of Washington, Seaglider, Scripps Institution of Oceanography, and Webb Research in Slocum. Preliminary results are presented from a recent demonstration project that used a network of gliders off Monterey. A wide range of sensors has already been deployed on gliders, with many under development, and a wider range of future possibilities. Glider networks appear to be among the best approaches to achieving subsurface spatial resolution necessary for ocean research.

Underwater gliders for ocean research

Measuring sea level change and understanding its causes has considerably improved in the recent years, essentially because new in situ and remote sensing observations have become available. Here we report on most recent results on contemporary sea level rise. We first present sea level observations from tide gauges over the twentieth century and from satellite altimetry since the early 1990s. We next discuss the most recent progress made in quantifying the processes causing sea level change on timescales ranging from years to decades, i.e., thermal expansion of the oceans, land ice mass loss, and land water–storage change. We show that for the 1993–2007 time span, the sum of climate-related contributions (2.85 ± 0.35 mm year −1 )i s only slightly less than altimetry-based sea level rise (3.3 ± 0.4 mm year −1 ): ∼30% of the observed rate of rise is due to ocean thermal expansion and ∼55% results from land ice melt. Recent acceleration in glacier melting and ice mass loss from the ice sheets increases the latter contribution up to 80% for the past five years. We also review the main causes of regional variability in sea level trends: The dominant contribution results from nonuniform changes in ocean thermal expansion.

Contemporary sea level rise.

Operations with multiple autonomous underwater vehicles (AUVs) have a variety of underwater applications. For example, a coordinated group of vehicles with environmental sensors can perform adaptive ocean sampling at the appropriate spatial and temporal scales. We describe a methodology for cooperative control of multiple vehicles based on virtual bodies and artificial potentials (VBAP). This methodology allows for adaptable formation control and can be used for missions such as gradient climbing and feature tracking in an uncertain environment. We discuss our implementation on a fleet of autonomous underwater gliders and present results from sea trials in Monterey Bay in August, 2003. These at-sea demonstrations were performed as part of the Autonomous Ocean Sampling Network (AOSN) II project

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Multi-AUV Control and Adaptive Sampling in Monterey Bay

Internal oceanic waves are subsurface gravity waves that can be enormous and travel thousands of kilometres before breaking but they are difficult to study; here observations of such waves in the South China Sea reveal their formation mechanism, extreme turbulence, relationship to the Kuroshio Current and energy budget. Internal waves are the underwater version of more familiar surface waves. They can be enormous and travel thousands of kilometres before breaking. The South China Sea is known to be home to the largest internal waves in the world's oceans, but their size, generation mechanisms and role in the regional energy budget are unknown. Matthew Alford and colleagues now present the results from the IWISE observational campaign and reveal that internal waves more than 200 metres high break in the South China Sea and create turbulence that is orders of magnitude larger than in the open ocean, and that wave formation is influenced by the Kuroshio current. These results now allow for a complete energy budget of the South China Sea, and for a more accurate incorporation of internal waves into climate models. Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis1, sediment and pollutant transport2 and acoustic transmission3; they also pose hazards for man-made structures in the ocean4. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking5, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects6,7. For over a decade, studies8,9,10,11 have targeted the South China Sea, where the oceans’ most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.

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The formation and fate of internal waves in the South China Sea

A small (50-kg, 2-m long) underwater vehicle with operating speeds of 20-30 cm/s and ranges up to 6000 km has been developed and field tested. The vehicle is essentially an autonomous profiling float that uses a buoyancy engine to cycle vertically and wings to glide horizontally while moving up and down. Operational control and data relay is provided by GPS navigation and two-way communication through ORBCOMM low-Earth-orbit satellites. Missions are envisioned with profile measurements repeated at a station or spaced along a transect. The initial instrument complement of temperature, conductivity, and pressure sensors was used to observe internal waves and tides in the Monterey underwater canyon.

The autonomous underwater glider "Spray"

: Phase 1 (9/99 - 9/02) and Phase 2 (7/00 - 6/02) of US Argo provided a total of 187 CTD profiling floats in the Pacific, Atlantic and Indian Oceans. Objectives were to demonstrate technological capabilities for fabrication and for deployment of float arrays in remote ocean locations (Phase 1) and to demonstrate the capability for manufacture and deployment of large float arrays (Phase 2). Recent technology developments in profiling floats were also implemented, including new generation salinity sensors, improved depth capability, and deployment techniques using fast ships and aircraft. Development of the U.S. Argo Data System was part of Phase 1, on a collaborative basis with international partners. Objectives are to make all Argo data publicly available within a day of collection, applying automated quality control procedures consistent with international Argo practices. Data appropriate for research applications and for comparison with climate change models are not available for several months since they need quality control by salinity experts and evaluation of data over many (10 day) float cycles. Phase 3 is a 5-year project (8/01 - 6/06) including full implementation of the US component of Argo. This report includes Phase 3/Year 2, which deployed 290 CTD profiling floats during CY2003 plus support for these deployments, data management activities and for national and international coordination of Argo. Float deployments targeted the Atlantic, Pacific, Indian, and Southern Oceans. Priorities for US float deployment are set by the US Argo Advisory Panel.

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The Argo Project: Global Ocean Observations for Understanding and Prediction of Climate Variability

: Argo is a global array of 3,000 free-drifting profiling floats that measure the temperature and salinity of the upper 2,000 meters of the ocean. This allows, for the first time, continuous monitoring of the temperature, salinity, and velocity of the upper ocean, with all data being relayed and made publicly available within hours after collection. The Argo array provides unprecedented views of the evolving physical state of the ocean. It reveals the physical processes that balance the large-scale heat and freshwater budgets of the ocean and provides a crucial data set for initialization of and assimilation in seasonal-to-decadal forecast models. Argo is central to an unprecedented capability for global assessment of the evolving climate state of the ocean. The thermal structure of the upper ocean controls the temperature of the lower atmosphere, and is the primary variable defining the physical environment of ocean ecosystems. Over 90% of the increased heat content due to global warming of the air/sea/ice climate system in the past 40 years occurred in the oceans. Climate stresses on ocean ecosystems have serious consequences, and sometimes dramatic ones, such as coral reef bleaching. In the future, the impacts of a varying climate on the health of the seas and coastal ecosystems will become an increasingly important aspect of resource management. The unique niche of the Argo array is to provide global broad-scale observations of the upper ocean. This report covers Year 4 of the 5-year project, and builds on progress made by previous awards (Phases 1 and 2) for pilot float arrays, data systems development, and international coordination. In 2005, 450 CTD profiling floats were deployed. Float deployments targeted the Atlantic, Pacific, Indian, and Southern Oceans. Developments in 2005 included deployment of the first operational floats employing Iridium telecommunications and deployment of 22 floats equipped with oxygen sensors.

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The ARGO Project: Global Ocean Observations for Understanding and Prediction of Climate Variability. Report for Calendar Year 2004

The Woods Hole Oceanographic Institution is developing techniques for telemetering oceanographic data from the deep ocean to the laboratory in near real time. Three general approaches that provide a link between subsurface instruments and surface buoys equipped with satellite transmitters are being pursued. These approaches are: cabled systems that use electromechanical cables to connect subsurface instruments to a central controller; high data rate acoustic modems to transfer information between multiple remote units and a central controller; and inductive modems that use standard mechanical mooring lines as the transmission medium between instruments deployed on the mooring and a central controller. These telemetry systems are targeted for general use by the oceanographic community and are designed to be power efficient, low in cost, and capable of integration with most oceanographic data collection systems. >

W.B. Owens

Papers

The autonomous underwater glider "Spray"

The Argo Project: Global Ocean Observations for Understanding and Prediction of Climate Variability

The ARGO Project: Global Ocean Observations for Understanding and Prediction of Climate Variability. Report for Calendar Year 2004

Recent developments in ocean data telemetry at Woods Hole Oceanographic Institution