Fleet Of Gliders Research Articles

Estimating the Energy Flux of Internal Tides in the Northern South China Sea Using Underwater Gliders

AbstractThe characteristics of the energy flux of internal tides in the northern South China Sea (SCS) were explored by analyzing a fleet of underwater gliders. It was found that the low‐mode diurnal internal tides with ∼300 km wavelength in the middle basin are generated predominantly in the Luzon Strait (LS) and propagate over 1,000 km to the western SCS. However, the semidiurnal internal tides are generated in multiple regions, including the LS in the east part, the continental shelf, and the islands in the west. The energy flux of the mode‐1 diurnal internal tides attenuated rapidly within 450 km of the LS and was less pronounced after that. The estimated dissipation rate based on the mode‐1 energy flux is about 10−8 W/kg, underling the significant role of mode‐1 diurnal internal tides in bolstering far‐field mixing. This study provides a unique view of the spatial pattern, energy flux, and energy sink of the internal tides in the northern SCS, which could supplement the altimetry‐based results and improve the parameterization in ocean models.

An Underwater Glider Fleet Coordinate Control Architecture for Ocean of Things to Enable Transparent Ocean

An Underwater Glider Fleet Coordinate Control Architecture for Ocean of Things to Enable Transparent Ocean

High-resolution study of the air-sea CO2 flux and net community oxygen production in the Ligurian Sea by a fleet of gliders

Intense glider monitoring was conducted in the Ligurian Sea for five months to capture the Net Community Production (NCP) variability in one of the most dynamic and productive regions of the Mediterranean Sea. Using the SeaExplorer glider technology, we were able to observe continuously from January to the end of May 2018 the physical and biogeochemical variables during the last period of intense convection observed in this region. High-frequency measurements from these gliders provided valuable information for determining dissolved O2 (DO) concentrations between coastal and open sea waters. Our DO balance approach provided an estimate of NCP fluxes complemented by the prediction of air-sea CO2 fluxes based on a neural network adapted to the Mediterranean Sea (CANYON-MED). Based on our NCP calculation method, our results show that the air-sea O2 flux and DO inventory have contributed largely to the NCP variability. The NCP values also suggest that heterotrophic conditions were predominant in winter and became autotrophic in spring, with strong variability in coastal waters due to the occurrence of sub-mesoscale structures. Finally, CO2 fluxes at the air-sea interface reveal that during the convection period, the central zone of the Ligurian Sea acted as a CO2 sink from January to March with little impact on NCP fluxes counterbalanced by a thermal effect of seawater.

Energy management and balance strategy for underwater glider fleet with VMD-SSA-LSSVM model and central type double-level framework

The management of energy has gained enormous attention as an efficient way to prolong the endurance of unmanned vehicle fleet. During ocean observation and exploration, the variation of the equipped sensor will cause uneven energy consumption for different underwater gliders (UGs), which will lead to the premature accomplishment of the planned mission. Therefore, a novel energy management and balance strategy for the UG fleet is proposed in this paper. Firstly, a new VMD-SSA-LSSVM model is presented to predict the energy consumption of UG based on sea trial data acquired in the northern South China Sea. Besides, the prediction results are compared with those of other benchmark methods and proved their effectiveness. Afterwards, a novel central type double-level framework is established based on low energy adaptive clustering hierarchy protocol to balance the energy of UG fleet, and double-level managing strategies are provided for minor or major energy adjustment based on the energy consumption model. Then the simulation is carried out to verify the effect on different sizes of UG fleet with energy balancing strategy, and the observation efficiency is defined and quantified by many indexes comprehensively. Finally, the simulated results with real data of UG fleet are given, verifying that the energy balance strategy could prolong the endurance by about 4.6% and improve the observation efficiency by about 7%.

Discrete-time formation control of multiple heterogeneous underwater gliders

The formation control of underwater gliders has been widely concerned due to its practical potential in various applications of ocean observation. This paper investigates the problems of formation control of multiple heterogeneous gliders based on consensus algorithm with discrete information. A nonlinear second-order dynamic model is obtained for the complex nonlinear underactuated gliders. By combining the consensus of speed control in forward direction and path following in lateral direction, formation control protocol with unavailable velocity information and asynchronous communication is proposed for multiple gliders. The glider formation control system (GFCS) is designed to drive a fleet of gliders following a set of explicitly specified trajectories. Two cases of simulation with six heterogeneous gliders and six homogeneous gliders respectively are performed to demonstrate the methodology and verify the effectiveness and reliability of the formation control protocol for discrete-time multi-glider system, which is proved to be applicable to formation scaling and transformation of multiple gliders.

A Four-Dimensional Survey of the Almeria–Oran Front by Underwater Gliders: Tracers and Circulation

Abstract A four-dimensional survey by a fleet of seven underwater gliders was used to identify pathways of subduction at the Almeria–Oran Front in the western Mediterranean Sea. The combined glider fleet covered nearly 9000 km over ground while doing over 2500 dives to as deep as 700 m. The gliders had sensors to measure temperature, salinity, velocity, chlorophyll fluorescence, and acoustic backscatter. Data from the gliders were analyzed through objective maps that were functions of across-front distance, alongfront distance, and time on vertical levels separated by 10 m. Geostrophic velocity was inferred using a variational approach, and the quasigeostrophic omega equation was solved for vertical and ageostrophic horizontal velocities. Peak downward vertical velocities were near 25 m day−1 in an event that propagated in the direction of the frontal jet. An examination of an isopycnal surface that outcropped as the front formed showed consistency between the movement of the tracers and the inferred vertical velocity. The vertical velocity tended to be downward on the dense side of the front and upward on the light side so as to flatten the front in the manner of a baroclinic instability. The resulting heat flux approached 80 W m−2 near 100-m depth with a structure that would cause restratification of the front. One glider was used to track an isotherm over a day for a direct measure of vertical velocity as large as 50 m day−1, with a net downward displacement of 15 m over the day.

Unsupervised anomaly detection for underwater gliders using generative adversarial networks

An effective anomaly detection system is critical for marine autonomous systems operating in complex and dynamic marine environments to reduce operational costs and achieve concurrent large-scale fleet deployments. However, developing an automated fault detection system remains challenging for several reasons including limited data transmission via satellite services. Currently, most anomaly detection for marine autonomous systems, such as underwater gliders, rely on intensive analysis by pilots. This study proposes an unsupervised anomaly detection system using bidirectional generative adversarial networks guided by assistive hints for marine autonomous systems with time series data collected by multiple sensors. In this study, the anomaly detection system for a fleet of underwater gliders is trained on two healthy deployment datasets and tested on other nine deployment datasets collected by a selection of vehicles operating in a range of locations and environmental conditions. The system is successfully applied to detect anomalies in the nine test deployments, which include several different types of anomalies as well as healthy behaviour. Also, a sensitivity study of the data decimation settings suggests the proposed system is robust for Near Real-Time anomaly detection for underwater gliders.

Net community oxygen production derived from Seaglider deployments at the Porcupine Abyssal Plain site (PAP; northeast Atlantic) in 2012–13

As part of the OSMOSIS project, a fleet of gliders surveyed the Porcupine Abyssal Plain site (Northeast Atlantic) from September 2012 to September 2013. Salinity, temperature, dissolved oxygen concentration and chlorophyll fluorescence were measured in the top 1000 m of the water column. Net community production (N) over an annual cycle using an oxygen-budget approach was compared to variations of several parameters (wind speed, mixing layer depth relative to euphotic depth, temperature, density, net heat flux) showing that the main theories (Critical Depth Hypothesis, Critical Turbulence Hypothesis, Heat-flux Hypothesis) can explain the switch between net heterotrophy to net autotrophy in different times of the year, The dynamics leading to an increase in productivity were related to shifts in regimes, such as the possible differences in nutrient concentration. The oxygen concentration profiles used for this study constitute a unique dataset spanning the entire productive season resulting in a data series longer than in previous studies. Net autotrophy was found at the site with a net production of (6.4 ± 1.9) mol m−2 in oxygen equivalents (or (4.3 ± 1.3) mol m−2 in carbon equivalents). The period exhibiting a deep chlorophyll maximum between 10 m and 40 m of depth contributed (1.5 ± 0.5) mol m−2 in oxygen equivalent to the total N. These results are greater than most previously published estimates.

Glider Routing and Trajectory Optimisation in disaster assessment

In this paper, we introduce the Glider Routing and Trajectory Optimisation Problem (GRTOP), the problem of simultaneously finding optimal routes and trajectories for a fleet of gliders with the aim of surveying a set of locations. We propose a novel Mixed-Integer Nonlinear Programming (MINLP) formulation for the GRTOP, which optimises the routes as well as the trajectories along these routes, while flight dynamics is modelled as constraints. We avoid solving a non-convex problem by linearising the gliders’ flight dynamics, converting the proposed MINLP into a Mixed-Integer Second-order Cone Programming (MISOCP) problem. To allow for a more tractable formulation, the dynamical constraints are relaxed and a penalisation is added to the objective function. Several different discretisation techniques are compared. The formulation is tested on instances inspired by risk maps of flooding-prone cities across the UK and on 180 randomly generated instances.

Dense CTD survey versus glider fleet sampling: comparing data assimilation performance in a regional ocean model west of Sardinia

Abstract. The REP14-MED sea trial carried out off the west coast of Sardinia in June 2014 provided a rich set of observations from both ship-based conductivity–temperature–depth (CTD) probes and a fleet of underwater gliders. We present the results of several simulations assimilating data either from CTDs or from different subsets of glider data, including up to eight vehicles, in addition to satellite sea level anomalies, surface temperature and Argo profiles. The Western Mediterranean OPerational forcasting system (WMOP) regional ocean model is used with a local multi-model ensemble optimal interpolation scheme to recursively ingest both lower-resolution large-scale and dense local observations over the whole sea trial duration. Results show the capacity of the system to ingest both types of data, leading to improvements in the representation of all assimilated variables. These improvements persist during the 3-day periods separating two analyses. At the same time, the system presents some limitations in properly representing the smaller-scale structures, which are smoothed out by the model error covariances provided by the ensemble. An evaluation of the forecasts using independent measurements from shipborne CTDs and a towed ScanFish deployed at the end of the sea trial shows that the simulations assimilating initial CTD data reduce the error by 39 % on average with respect to the simulation without data assimilation. In the glider-data-assimilative experiments, the forecast error is reduced as the number of vehicles increases. The simulation assimilating CTDs outperforms the simulations assimilating data from one to four gliders. A fleet of eight gliders provides similar performance to the 10 km spaced CTD initialization survey in these experiments, with an overall 40 % model error reduction capacity with respect to the simulation without data assimilation when comparing against independent campaign observations.

Underwater source localization using a fleet of three gliders

The NATO Centre for Maritime Research and Experimentation has been exploring the potentials of passive acoustic payload equipped underwater gliders for maritime intelligence, surveillance and reconnaissance missions. During the CMRE Glider Sensors and Payloads for Tactical Characterization of the Environment 2015 sea trial, it was successfully demonstrated that a fleet of three passive acoustic payload equipped gliders simultaneously detected the ‘anomalous’ sound in shallow water and reported back to the glider control center. This work studies the feasibility of localizing the source (the coordinates and the depth) using the information provided by the glider fleet. The results of three different source localization approaches are compared. The impact of the errors in conventional glider time and location data, as well as insufficient/incorrect knowledge of the ocean environment on underwater source localization is discussed. (Work funded by NATO-Allied Command Transformation.)

Lessons Learned in Developing a Canadian Operational Glider Fleet

AbstractCanada's expanding “Blue Economy” requires a major expansion of existing ocean monitoring if developments are to be sustainably managed. Dalhousie University, the Ocean Tracking Network, and the Marine Environmental Observation Prediction and Response Network of Centers of Excellence have operated a mixed fleet of gliders for 7 years on missions covering >50,000 km. The data from these missions are used by research programs, nongovernmental organizations, and government agencies. The gliders have proven to be reliable platforms for ocean observation, collecting data in inclement weather, and times of the year when it is difficult to get ships at sea. However, glider operations have a steep learning curve, and much of the expertise that resides within an operational glider group is gleaned through experience. Managing glider data also poses significant challenges. Planning, risk management, rapid adaptation to the unexpected, and dedicated highly qualified personnel are the keys to sustaining successful glider operations.

Coordinate Control, Motion Optimization and Sea Experiment of a Fleet of Petrel-II Gliders

The formation of hybrid underwater gliders has advantages in sustained ocean observation with high resolution and more adaptation for complicated ocean tasks. However, the current work mostly focused on the traditional gliders and AUVs. The research on control strategy and energy consumption minimization for the hybrid gliders is necessary both in methodology and experiment. A multi-layer coordinate control strategy is developed for the fleet of hybrid underwater gliders to control the gliders’ motion and formation geometry with optimized energy consumption. The inner layer integrated in the onboard controller and the outer layer integrated in the ground control center or the deck controller are designed. A coordinate control model is proposed based on multibody theory through adoption of artificial potential fields. Considering the existence of ocean flow, a hybrid motion energy consumption model is constructed and an optimization method is designed to obtain the heading angle, net buoyancy, gliding angle and the rotate speed of screw propeller to minimize the motion energy with consideration of the ocean flow. The feasibility of the coordinate control system and motion optimization method has been verified both by simulation and sea trials. Simulation results show the regularity of energy consumption with the control variables. The fleet of three Petrel-II gliders developed by Tianjin University is deployed in the South China Sea. The trajectory error of each glider is less than 2.5 km, the formation shape error between each glider is less than 2 km, and the difference between actual energy consumption and the simulated energy consumption is less than 24% actual energy. The results of simulation and the sea trial prove the feasibility of the proposed coordinate control strategy and energy optimization method. In conclusion, a coordinate control system and a motion optimization method is studied, which can be used for reference in theoretical research and practical fleet operation for both the traditional gliders and hybrid gliders.

Forecast skill score assessment of a relocatable ocean prediction system, using a simplified objective analysis method

Abstract. A relocatable ocean prediction system (ROPS) was employed to an observational data set which was collected in June 2014 in the waters to the west of Sardinia (western Mediterranean) in the framework of the REP14-MED experiment. The observational data, comprising more than 6000 temperature and salinity profiles from a fleet of underwater gliders and shipborne probes, were assimilated in the Regional Ocean Modeling System (ROMS), which is the heart of ROPS, and verified against independent observations from ScanFish tows by means of the forecast skill score as defined by Murphy(1993). A simplified objective analysis (OA) method was utilised for assimilation, taking account of only those profiles which were located within a predetermined time window W. As a result of a sensitivity study, the highest skill score was obtained for a correlation length scale C = 12.5 km, W = 24 h, and r = 1, where r is the ratio between the error of the observations and the background error, both for temperature and salinity. Additional ROPS runs showed that (i) the skill score of assimilation runs was mostly higher than the score of a control run without assimilation, (i) the skill score increased with increasing forecast range, and (iii) the skill score for temperature was higher than the score for salinity in the majority of cases. Further on, it is demonstrated that the vast number of observations can be managed by the applied OA method without data reduction, enabling timely operational forecasts even on a commercially available personal computer or a laptop.

Underwater Glider Observations in the Oxygen Minimum Zone off Central Chile

Abstract Gliders have become an efficient and reliable oceanographic platform for measuring physical and biogeochemical properties of the seawater, and the global glider fleet is rapidly expanding. In Chile, glider observations have been carried out in very different oceanographic environments, from the mild upwelling region of subtropical northern Chile to the channels of southern Patagonia. Herein, we briefly present observations and results obtained in the oxygen minimum zone off Concepcion (∼36°30′S). Many new features have been observed in this region thanks to the relatively high resolution of the glider measurements. Future plans for the glider program include an oceanic time series off central Chile that will contribute to the regional observing system of the ocean and allow evaluations of low-frequency changes like those associated with El Niño and La Niña events.

Mission Planning and Decision Support for Underwater Glider Networks: A Sampling on-Demand Approach.

This paper describes an optimal sampling approach to support glider fleet operators and marine scientists during the complex task of planning the missions of fleets of underwater gliders. Optimal sampling, which has gained considerable attention in the last decade, consists in planning the paths of gliders to minimize a specific criterion pertinent to the phenomenon under investigation. Different criteria (e.g., A, G, or E optimality), used in geosciences to obtain an optimum design, lead to different sampling strategies. In particular, the A criterion produces paths for the gliders that minimize the overall level of uncertainty over the area of interest. However, there are commonly operative situations in which the marine scientists may prefer not to minimize the overall uncertainty of a certain area, but instead they may be interested in achieving an acceptable uncertainty sufficient for the scientific or operational needs of the mission. We propose and discuss here an approach named sampling on-demand that explicitly addresses this need. In our approach the user provides an objective map, setting both the amount and the geographic distribution of the uncertainty to be achieved after assimilating the information gathered by the fleet. A novel optimality criterion, called , is proposed and the resulting minimization problem is solved by using a Simulated Annealing based optimizer that takes into account the constraints imposed by the glider navigation features, the desired geometry of the paths and the problems of reachability caused by ocean currents. This planning strategy has been implemented in a Matlab toolbox called SoDDS (Sampling on-Demand and Decision Support). The tool is able to automatically download the ocean fields data from MyOcean repository and also provides graphical user interfaces to ease the input process of mission parameters and targets. The results obtained by running SoDDS on three different scenarios are provided and show that SoDDS, which is currently used at NATO STO Centre for Maritime Research and Experimentation (CMRE), can represent a step forward towards a systematic mission planning of glider fleets, dramatically reducing the efforts of glider operators.

Developing Coordinated Communities of Autonomous Gliders for Sampling Coastal Ecosystems

AbstractUnderwater autonomous gliders have transitioned from exotic experimental systems to becoming a standard platform capable of collecting data over a critical range of spatial and temporal scales in the ocean. The data are proving to be extremely valuable for addressing a wide range of basic and applied research questions. This evolution has led to a growing international community of glider laboratories and offers the potential of increased opportunities to leverage efforts across this “young” growing community. To that end, we review the evolution of the glider communities and share our opinions of opportunities and hurdles. These communities grew from distributed research and/or education groups. It is crucial as systems continue to evolve that there is an effort to “harmonize” data products while preserving the diversity of approaches/science/experimentation. As the gliders have matured and new battery solutions provide additional energy, there is an increased focus on the integration of a wider range of sensors to be incorporated into gliders. Many of these new classes of sensors will be particularly effective for characterizing biological processes in the coastal ocean. As biological sensors generally provide proxy estimates of a parameter, developing robust quality control and assurance procedures is critical. These new sensors will be more power intensive, thus requiring the development of planning tools for increasing energy efficiency during missions. Given the significant growth in the highly distributed glider community, efforts are now focusing on the development mission planning tools to allow for efficient operation of glider fleets.

Cooperation or Coordination of Underwater Glider Networks? An Assessment from Observing System Simulation Experiments in the Ligurian Sea

Abstract The coordinated and cooperative-unaware networking of glider fleets have been proposed to obtain a performance gain in ocean sampling over naïve collective behavior. Whether one of these implementations results in a more efficient sampling of the ocean variability remains an open question. This article aims at a performance evaluation of cooperative-unaware and coordinated networks of gliders to reduce the uncertainty in operational temperature model predictions. The evaluation is based on an observing system simulation experiment (OSSE) implemented in the northern Ligurian Sea (western Mediterranean) from 21 August to 1 September 2010. The OSSE confronts the forecast skills obtained by the Regional Ocean Modeling System (ROMS) when assimilating data gathered from a cooperative and unaware network of three gliders with the prediction skill obtained when data comes from a coordinated configuration. An asynchronous formulation of the ensemble Kalman filter with a 48-h window is used to assimilate simulated temperature observations. Optimum sampling strategies of the glider networks, based on a pattern search optimization algorithm, are computed for each 48-h forecasting period using a covariance integrated in time and in the vertical direction to reduce the dimensionality of the problem and to enable a rapid resolution. Perturbations of the depth-averaged current field in glider motions are neglected. Results indicate a better performance of the coordinated network configuration due to an enhanced capacity to capture an eddy structure that is responsible for the largest forecast error in the experimental domain.

A Glider Network Design Study for a Synoptic View of the Oceanic Mesoscale Variability

Abstract This study presents an Observing System Simulation Experiment (OSSE) with a network of gliders in a realistic mesoscale field of eddies and filaments. The main objective is to demonstrate that the analysis skill evaluation, performed with different statistics, determines the optimal number of gliders needed to survey a “glider observatory” with a given simple topology of the glider array, in the shape of a “double comb.” Metrics, based on a spatial interpolation of the sampled data with a multiscale objective analysis method, are elaborated to evaluate the reconstruction of the three-dimensional temperature field with several glider networks, at a weekly time scale. The mesoscale structures obtained by the optimal network (front, eddies, eddies detachment) are also compared with the structures of the original simulation. This comparison demonstrates the efficiency of a glider fleet to sample a well-defined area at a given spatiotemporal scale. In this particular situation (midlatitude region, domain of 400 km × 600 km, reconstruction of weekly snapshots), the optimum network is composed of 10 gliders. A relationship is highlighted between the spatial scales of the sampled area, the physical characteristics of the studied region, the reconstruction time scale, and the optimum number of gliders. The results presented here can be applied to design an actual in situ experiment.

Mapping sub-surface geostrophic currents from altimetry and a fleet of gliders

Integrating the observations gathered by different platforms into a unique physical picture of the environment is a fundamental aspect of networked ocean observing systems. These are constituted by a spatially distributed set of sensors and platforms that simultaneously monitor a given ocean region. Remote sensing from satellites is an integral part of present ocean observing systems. Due to their autonomy, mobility and controllability, underwater gliders are envisioned to play a significant role in the development of networked ocean observatories. Exploiting synergism between remote sensing and underwater gliders is expected to result on a better characterization of the marine environment than using these observational sources individually.This study investigates a methodology to estimate the three dimensional distribution of geostrophic currents resulting from merging satellite altimetry and in situ samples gathered by a fleet of Slocum gliders. Specifically, the approach computes the volumetric or three dimensional distribution of absolute dynamic height (ADH) that minimizes the total energy of the system while being close to in situ observations and matching the absolute dynamic topography (ADT) observed from satellite at the sea surface. A three dimensional finite element technique is employed to solve the minimization problem. The methodology is validated making use of the dataset collected during the field experiment called Rapid Environmental Picture-2010 (REP-10) carried out by the NATO Undersea Research Center-NURC during August 2010. A marine region off-shore La Spezia (northwest coast of Italy) was sampled by a fleet of three coastal Slocum gliders. Results indicate that the geostrophic current field estimated from gliders and altimetry significantly improves the estimates obtained using only the data gathered by the glider fleet.