underwater glider

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      An underwater glider is a type of autonomous underwater vehicle (AUV) that employs variable-buoyancy propulsion instead of traditional propellers or thrusters. It employs variable buoyancy in a similar way to a profiling float, but unlike a float, which can move only up and down, an underwater glider is fitted with hydrofoils (underwater wings) that allow it to glide forward while descending through the water. At a certain depth, the glider switches to positive buoyancy to climb back up and forward, and the cycle is then repeated.
      While not as fast as conventional AUVs, gliders offer significantly greater range and endurance compared to traditional AUVs, extending ocean sampling missions from hours to weeks or months, and to thousands of kilometers of range. The typical up-and-down, sawtooth-like profile followed by a glider can provide data on temporal and spatial scales unattainable by powered AUVs and much more costly to sample using traditional shipboard techniques. A wide variety of glider designs are in use by navies and ocean research organizations, with gliders typically costing around US$100,000.


      History



      The concept of an underwater glider was first explored in the early 1960s with a prototype swimmer delivery vehicle named Concept Whisper. The sawtooth glide pattern, stealth properties and the idea of a buoyancy engine powered by the swimmer-passenger was described by Ewan Fallon in his Hydroglider patent submitted in 1960. In 1992, the University of Tokyo conducted tests on ALBAC, a drop weight glider with no buoyancy control and only one glide cycle. The DARPA SBIR program received a proposal for a temperature gradient glider in 1988. DARPA was aware at that time of similar research projects underway in the USSR.
      This idea, a glider with a buoyancy engine powered by a heat exchanger, was introduced to the oceanographic community by Henry Stommel in a 1989 article in Oceanography, when he proposed a glider concept called Slocum, developed with research engineer Doug Webb. They named the glider after Joshua Slocum, who made the first solo circumnavigation of the globe by sailboat. They proposed harnessing energy from the thermal gradient between deep ocean water (2-4 °C) and surface water (near atmospheric temperature) to achieve globe-circling range, constrained only by battery power on board for communications, sensors, and navigational computers.
      By 2003, not only had a working thermal-powered glider (Slocum Thermal) been demonstrated by Webb Research (founded by Doug Webb), but they and other institutions had introduced battery-powered gliders with impressive duration and efficiency, far exceeding that of traditional survey-class AUVs. These vehicles have been widely deployed in the years since then. The University of Washington Seaglider, Scripps Institution of Oceanography Spray, and Teledyne Webb Research Slocum vehicles have performed feats such as completing a transatlantic journey and conducting sustained, multi-vehicle collaborative monitoring of oceanographic variables. In 2011, the first wingless glider, SeaExplorer, was released by a collaboration of French institutions and companies.
      In 2020, NOAA was using "hurricane gliders" to monitor the temperature of the water around the Gulf Stream, for the agency to better understand how warm waters affect hurricanes and storms.


      Functional description



      Gliders typically make measurements such as temperature, conductivity (to calculate salinity), currents, chlorophyll fluorescence, optical backscatter, bottom depth, and sometimes acoustic backscatter or ambient sound. They navigate with the help of periodic surface GPS fixes, pressure sensors, tilt sensors, and magnetic compasses. Vehicle pitch is controllable by movable internal ballast (usually battery packs), and steering is accomplished either with a rudder (as in Slocum) or by moving internal ballast to control roll (as in SeaExplorer, Spray and Seaglider). Buoyancy is adjusted either by using a piston to flood/evacuate a compartment with seawater (Slocum) or by moving oil in/out of an external bladder (SeaExplorer, Seaglider, Spray, and Slocum Thermal). Because buoyancy adjustments are relatively small, a glider's ballast must typically be adjusted before the start of a mission to achieve an overall vehicle density close to that of the water it will be deployed in. Commands and data are relayed between gliders and shore by satellite.
      Gliders vary in the pressure they are able to withstand. The Slocum model is rated for 200 meter or 1000 meter depths. Spray can operate to 1500 meters, Seaglider to 1000 meters, SeaExplorer to 700, and Slocum Thermal to 1200. In August 2010, a Deep Glider variant of the Seaglider achieved a repeated 6000-meter operating depth. Similar depths have been reached by a Chinese glider in 2016.


      Liberdade class flying wings


      In 2004, the US Navy Office of Naval Research began developing the world's largest glider, the Liberdade class flying wing glider, which uses a blended wing body hullform to achieve hydrodynamic efficiency. They were initially designed to quietly track diesel electric submarines in littoral waters, remaining on station for up to six months. By 2012, a newer model, known as the ZRay, was designed to track and identify marine mammals for extended periods of time. It uses water jets for fine attitude control as well as propulsion on the surface.


      Payloads


      Gliders were designed to carry oceanographic instrumentation. Initially simple conductivity, temperature and depth sensors were equipped. Since they are propelled by a buoyancy engine, gliders have moving parts that are only active occasionally, so there are minimal mechanical vibrations and noise, making them excellent vehicles for sensitive instrumentation including microstructure probes and acoustic sensors.
      Many existing oceanographic sensors have been modified to fit into a glider, or designed specifically for gliders. These include:

      Fluorometers
      Photosynthetically Available Radiation (PAR) sensors
      Dissolved Oxygen optodes
      Acoustic Doppler Current Profilers (ADCP)
      Laser In-Situ Scattering and Transmissometry (LISST) instruments
      Nitrate sensors
      Active acoustics
      Passive acoustic monitoring (PAM)
      Shadowgraph cameras
      Microstructure (turbulence) probes
      Hydrocarbon sensors
      The number of sensors a glider can be equipped with depends on how much space there is for sensors in its hull. Slocum gliders have modular hulls and can be extended to allow for new sensors to be added, other types of gliders only have their initial surface area that may be instrumented. For data reasons instruments may require special positioning, such as on the top of the vehicle to capture light penetration from the surface, or at the very front of the vehicle, outside of the area where the vehicle influences the water’s flow for microstructure probes. The number of sensors may also be restricted by the power required to run them.


      See also


      Autonomous underwater vehicle – Uncrewed underwater vehicle with autonomous guidance system
      Argo floats – International oceanographic observation program
      Liquid Robotics – American marine robotics corporation, developers of the Wave Glider
      Paravane (weapon) – Minesweeping device
      Paravane (water kite) – Towed hydrofoiled underwater object
      DeepFlight Super Falcon – Winged personal submarine
      Autonomous Robotics Ltd – UK company developing an autonomous underwater vehicle


      References




      External links



      GROOM - Gliders for Research, Ocean Observation and Management
      COST Action ES0904
      EGO network - glider user group
      Seaexplorer page at ALSEAMAR-ALCEN
      Oceanic Platform of the Canary Islands -PLOCAN-
      Spray page at Scripps Institution of Oceanography
      Spray underwater glider database
      Seaglider page at Applied Physics Laboratory - University of Washington
      Seaglider Operations page at APL-UW
      Rutgers University Coastal Ocean Observations Lab -- Glider Operations
      Slocum page at Webb Research Corp.
      Underwater glider configurations and details - AUVAC.org
      Underwater Gliders for Ocean Research
      Robot glider harvests ocean heat
      National Oceanography Centre, UK. Glider Home Page

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    Underwater glider - Wikipedia

    An underwater glider is a type of autonomous underwater vehicle (AUV) that employs variable-buoyancy propulsion instead of traditional propellers or thrusters.

    What is an ocean glider? - NOAA's National Ocean Service

    An ocean glider is an autonomous, unmanned underwater vehicle used for ocean science. Since gliders require little or no human assistance while traveling, these little robots are uniquely suited for collecting data in remote locations, safely and at relatively low cost.

    Underwater Glider User Group (UG2)

    Underwater gliders are a unique and important observing system used to serve a variety of subsurface observing missions. Gliders can monitor water currents, temperature, tagged animals and oceanographic conditions that reveal effects from …

    Slocum Glider - Woods Hole Oceanographic Institution

    The Slocum Glider is a uniquely mobile network component capable of moving to specific locations and depths and occupying controlled spatial and temporal grids. Driven in a sawtooth vertical profile by variable buoyancy, the glider moves both horizontally and vertically.

    SeaFlight Underwater Glider

    The SeaFlight Glider is our open source underwater AUV designed for greater durability, operability, repairability and lower cost. The SeaFlight Glider was the winning product at the 2019 WHOI Pitchathon and awarded a 2-year $100k Technical Innovation Award in 2021.

    Slocum Glider (Autonomous Underwater Glider) by Webb Research

    The Slocum glider is buoyancy driven to enable long range and duration remote water column observation for academic, military, and commercial applications. The Slocum Glider can be deployed and recovered from any size vessel with minimal time on station.

    Liberdade class underwater glider - Wikipedia

    The "Liberdade" class are the world's largest known underwater gliders and were developed as part of the US Navy's Persistent Littoral Undersea Surveillance Network (PlusNet) system of unmanned surveillance vehicles.

    Autonomous Underwater Glider: A Comprehensive Review - MDPI

    Underwater gliders are versatile and efficient tools for marine research and monitoring. Their unique propulsion method, based on buoyancy changes and hydrodynamic lift, allows them to operate for extended periods with minimal energy consumption.

    Seaglider - Wikipedia

    The Seaglider is a deep-diving Autonomous Underwater Vehicle (AUV) designed for missions lasting many months and covering thousands of miles. In military applications the Seaglider is more commonly referred to as an Unmanned Underwater Vehicle (UUV).

    Underwater Gliders – RUCOOL | Rutgers Center for Ocean …

    Whether the mission is a grad student project, hurricane forecasting model feedback, or entire ocean spanning sampling, the glider has become the instrument of choice for many ocean monitoring endeavors.