Thursday, March 15, 2018

Submarine Cables: UNDERSEA CABLES AND THE CHALLENGES OF PROTECTING SEABED LINES OF COMMUNICATION MARCH 15, 2018 GUEST AUTHOR LEAVE A COMMENT Seabed Warfare Week Center for International Maritime Security By Pete Barker


Center for International Maritime Security
By Pete Barker
For centuries, the sea has enabled trade between nations. Shipping continues to underpin international commerce today. But there is another unseen contribution that the oceans make to the current global order. Deep below the waters, travelling at millions of miles per hour, flickers of light relay incredible quantities of information across the world, powering the exchange of data that forms the internet. From urgent stock market transactions to endless videos of cats, undersea cables support many aspects of twenty first century life that we take for granted. A moment’s thought is sufficient to appreciate the strategic importance of this fact. As a result, any discussion of future seabed warfare would be incomplete without a consideration of the challenges presented by ensuring the security of this vital infrastructure.
Strategists have neglected submarine cables. Whilst topics such as piracy and cyber attacks on ports frequently arise in discussions on maritime threats, cables have not always been as prominent. Some authors have identified the potential risks (such as this 2009 reportfor the UN Environment World Conservation Monitoring Centre), but these works have not always received the attention they deserve.
There are signs that this is changing. A recent report for the Policy Exchange by Rishi Sunak, a member of the UK Parliament, gained significant media coverage. It was not ignored by senior military figures. A few weeks later, the United Kingdom Chief of Defence Staff, Air Chief Marshall Sir Stuart Peach, gave a speech to RUSI, where he said “there is a new risk to our way of life that is the vulnerability of the cables that crisscross the seabed.” The same month, Mark Sedwill, the UK National Security Advisor, gave evidence that “you can achieve the same effect as used to be achieved in, say, World War Two by bombing the London docks or taking out a power station by going after the physical infrastructure of cyberspace in the form of internet undersea cables.”
This is a present threat, not just a hypothetical one. In late 2017, the NATO Submarine Commander Rear Admiral Lennon of the United States Navy revealed “We are now seeing Russian underwater activity in the vicinity of undersea cables that I don’t believe we have ever seen. Russia is clearly taking an interest in NATO and NATO nations’ undersea infrastructure.” The challenge is to maintain this focus and turn a passing spotlight into seriously considered policy.
Understanding Submarine Cables
Vast technical expertise is not necessary to understand why submarine cables are so important. A basic awareness of their construction and use is sufficient. The internet is, at its most basic level, a transfer of information. With the advent of cloud computing, the simple act of storing a file means that data travels from a user on one continent to a server halfway around the world. Although popular imagination sees this happening by satellite relay, in over ninety five percent of cases the physical means for moving this information is a series of light pulses, travelling along a fiber optic cable laid over land and under the sea. These cables are thin silica tubes embedded in a protective cladding, approximately the size of a garden hosepipe. The capacity of these cables to transmit data is ever-increasing. Recent experimental cables have been reported as being capable of transmitting up to one petabyte of data per second. To add some perspective, a petabyte of storage would allow you to store enough music that you could play it continuously for two thousand years.
Submarine cables are mainly private assets. Although expensive (an intercontinental cable is cited as costing between $100 million to $500 million), they are significantly cheaper than the satellite alternatives. In addition to the ownership by telecommunications companies, internet companies, including Facebook and Google, now heavily invest in submarine cables. These cables are laid by specialized ships, capable of carrying up to 2000km of cable, which can be laid at a rate of up to 200km per day. In offshore areas, the cable is laid directly onto the seabed. On the continental shelf, a plough is used to bury the cables and provide some protection from accidental damage, usually caused by anchors.
Attacks on Submarine Cables
These cables are vulnerable to deliberate attack in many ways. The most basic method of attack is simply to break the cable. Their construction means that this task presents little difficulty either mechanically or through the use of small explosive charges. Finding these cables is equally simple. The location of the cables is widely promulgated in order to prevent accidental damage but there is little to stop adversaries from exploiting this information for nefarious ends. Whilst there are a network of repair ships around the world, it is obvious that any service denial cannot be instantly fixed. Multiple attacks, particularly on alternative cable routes, would quickly exacerbate problems and could be organized relatively easily. As the Policy Exchange report highlighted, there is no need to actually proceed to sea to attack the cable network. The landing stations, locations where the submarine cables come ashore, are both well-known and lightly protected. This is a potent combination, particularly when cables are located in fragile states and presents additional challenges when assessing the security of the network.
Cables can also be attacked in non-physical ways. Although shrouded in classification, intelligence analysts have openly stated in national newspapers that the U.S. submarine, USS Jimmy Carter, may have the capability to “tap” undersea cables and obtain the data being transferred without breaching the cable. There are concerns that theRussian Yantar vessels share similar capabilities and these are explored in depth in a recent post by Garrett Hinck. Military planners must understand that defending the submarine cable network might not mean simply preventing physical attack but also ensuring the integrity of the data being transmitted.
Legal Protections
Legally, the status of undersea cables have little protection, particularly when they are outside the jurisdiction of any state and lie on the seabed of the high seas. This is certainly the conclusion of the two major legal studies that have addressed the problem. Professor Heintschel von Heinegg considered submarine cyber infrastructure in a chapter of a NATO Cooperative Cyber Defence Centre of Excellence publication in 2013 and concluded that “the current legal regime has gaps and loopholes and that it no longer adequately protects submarine cables.” Similarly in 2015, Tara Davenport of Yale Law School examined the same topic and stated “the present legal regime is deficient in ensuring the security of cables.” The peacetime protection of submarine cables is a grey area in the law and this provides an additional challenge when assessing how cables should be protected.
The legal status of submarine cables in times of war is equally unclear as observed recently in a post for the Cambridge International Law Journal and another post on Lawfare. There is no authoritative work examining the status of submarine cables in armed conflicts, but even a brief overview is sufficient to highlight the problem. The first question is whether an attack on a submarine cable (outside of a state’s jurisdiction) qualifies as an “armed attack” for the purposes ofarticle 51 of the UN Charter, permitting the use of force by a state in self-defense. The Tallinn Manual on the Law Applicable to Cyber Operations takes the position that the effects of a cyber operation must be analogous to those resulting from a “standard” kinetic armed attack. Simultaneously, it acknowledges that the law is unclear as to when a cyber operation qualifies as an armed attack. Would the consequences of a submarine cable breach be sufficiently serious to raise it to the level of an armed attack? It is difficult to provide a definitive answer but if the answer is ‘no’, then states would not be entitled to use military force to defend submarine cables in the absence of an existing armed conflict. With regard to illicit surveillance of cables, the Tallinn Manual clearly concludes that intelligence gathering from submarine cables would not amount to an armed attack.
The ability of States to target submarine cables during times of war is also open to discussion. Objects may be targeted under international humanitarian law if they make an effective contribution to military action due to their nature, location, purpose, or use and if their total or partial destruction, capture or neutralization offers a definite military advantage. The best example of the extent of military reliance on civilian owned and operated undersea cables is contained in a 2010Belfer Center paper. This records that three of the largest cables between Italy and Egypt were severed in late 2008. As a result, U.S. UAV operations in Iraq were significantly reduced. Submarine cables simultaneously transmit critical military and civilian data. Whilst the presence of the former means that they may be targeted, this is always subject to the principles of proportionality and precautions in attack, designed to minimize the harm to the civilian population. Due to the range of data carried by cables and the number of services that are likely to be affected, these assessments may be very difficult to carry out. An understanding of when cables can be targeted is likely to be highly fact sensitive and it is entirely possible that states will take different views on when this is permissible.
Strategies for the Undersea Cable Problem
Clearly, a protection strategy for undersea cables cannot depend solely on military action. It is impossible to protect the entire cable network given its global expanse. The geographic area requiring protection is simply too large, even for the most powerful of navies. The natural consequence of this conclusion is to focus on identifying and intercepting ships and submarines capable of interfering with the cable network. However, the practicalities of this option are not promising. The technology required to tamper with cables is not overly sophisticated. It can be hosted in a wide range of vessels and easily transferred between them. Submarines present additional challenges in monitoring, tracking and interception, requiring the use of satellites, intelligence, and underwater sensors. For a military commander, the task of protecting seabed submarine cables from attack can seem almost impossible.

Global map of submarine cables [click to expand] (Ben Pollock/Visual Capitalist)
Given this conclusion, national strategies may need to focus on alternative methods of safeguarding the exchange of information. One method would be to increase the level of redundancy within the system by laying additional cables. As cables are expensive and most cables are privately owned, additional routes have to be assured of sufficient funding to make them viable. Somewhat ominously, the International Cable Protection Committee (which represents cable owners) states that “most cable owners feel that there is enough diversity in the international submarine cable network.” This might be true if the only threat is from accidental damage. However, this analysis might change with the realistic prospect of deliberate targeting.
The ideal solution would be the existence of a globally accepted international treaty giving protection to submarine cables by prohibiting interference and clarifying the status and protections of cables. It is a solution advocated by a number of the sources previously cited. Given the shared interests of many, if not all states, in securing the submarine cable network, this may not be unattainable. Regulation of these cables outside the territories of states would not involve any restriction on national territorial sovereignty, increasing the chance of multilateral agreement. Unfortunately this opportunity has not been seized by a distracted international community.
Arguably the most important strategic asset on the seabed is the submarine cable network. They present a unique vulnerability that is challenging to protect and subject to an uncertain legal regime. Any analysis of seabed warfare must concern itself with cable protection. The best way to achieve this is the adoption and acceptance of a treaty regime that acknowledges their importance to the modern world. Until this is achieved, military commanders must factor the exceptional challenges of defending these cables into their plans for seabed warfare.
Lieutenant Commander Peter Barker is a serving Royal Navy officer and barrister. He is currently the Associate Director for the Law of Coalition Warfare at the Stockton Center for the Study of International Law (@StocktonCenter), part of the U.S. Naval War College.  He can be contacted at
This post is written in a personal capacity and the views expressed are the author’s own and do not necessarily represent those of the UK Ministry of Defence or the UK government.
Featured Image: The submersible Alvin investigates the Cayman Trough, a transform boundary on the floor of the western Caribbean Sea. (Emory Kristof, National Geographic)

Tuesday, March 13, 2018

Underwater glider

Underwater glider

From Wikipedia, the free encyclopedia
NOAA personnel launch a Slocumglider off Florida
Rutgers Slocum glider RU02 deployed in Sargasso Sea
Dr. Bruce Howe and Bill Felton of the University of Washington prepare a Seaglider for deployment
University of Washington's Seaglider at the surface between dives
An underwater glider is a type of autonomous underwater vehicle (AUV) that uses small changes in its buoyancy in order to move up and down in the ocean like a profiling float. Unlike a float, a glider uses wings to convert that vertical motion to horizontal, propelling itself forward with very low power consumption. While not as fast as conventional AUVs, gliders using buoyancy-based propulsion represent a significant increase in range and duration compared to vehicles propelled by electric motor-driven propellers, extending ocean sampling missions from hours to weeks or months, and to thousands of kilometers of range. Gliders follow an up-and-down, sawtooth-like profile through the water, providing data on temporal and spatial scales unavailable to previous AUVs, and much more costly to sample using traditional shipboard techniques.[1] A wide variety of glider designs are in use by Navy and ocean research organizations and typically cost US$100,000.[2]


The concept of an underwater glider was first explored in the early 1960s with a prototype swimmer delivery vehicle named Concept Whisper.[3] 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.[4] In 1992, the University of Tokyoconducted tests on ALBAC, a drop weight glider with no buoyancy control and only one glide cycle.[1] 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.[5] 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.[3]
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.[6] These vehicles have been widely deployed in the years since then. The University of Washington SeagliderScripps Institution of Oceanography Spray, and Teledyne Webb Research Slocum vehicles have performed feats such as completing a transatlantic journey[7] and conducting sustained, multi-vehicle collaborative monitoring of oceanographic variables.[1]
In 2011, the first wingless glider, SeaExplorer was released with a large payload capacity, dedicating the first third of the vehicle to interchangeable payloads, in addition to typical CTD sensors. The vehicle achieves 1 knot speeds, is equipped with externally rechargeable Li-Ion batteries and its torpedo shape is able to glide relying on two pairs of small static rear fins for stability.[citation needed]

Functional description[edit]

Gliders typically make measurements such as temperatureconductivity (to calculate salinity), currents, chlorophyll fluorescence, optical backscatter, bottom depth, and (occasionally) acoustic backscatter. 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 SeaExplorerSpray 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 (SeaExplorerSeagliderSpray, and Slocum Thermal). Commands and data are relayed between gliders and shore by satellite.[3]
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.[1] Similar depths have been reached by a Chinese glider in 2016. [8]

Liberdade class flying wings[edit]

In 2004, the US Navy Office of Naval Research began developing the world's largest gliders, the Liberdade class flying wing gliders, which uses a blended wing bodyhullform to achieve hydrodynamic efficiency. They were initially designed to quietly track diesel electric submarines in littoral waters, remaining on station for up to 6 months. The current model is known as the ZRay and is designed to track and identify marine mammals for extended periods of time.[9] It uses water jets for fine attitude control as well as propulsion on the surface.[9][10]

Autonomous underwater vehicle. AUV.Unmanned underwater vehicle. UUV. ROV

Autonomous underwater vehicle

From Wikipedia, the free encyclopedia
Picture taken of the Battlespace Preparation Autonomous Underwater Vehicle (BPAUV) by an employee of Bluefin Robotics Corporation during a US Navy exercise.
The Blackghost AUV is designed to undertake an underwater assault course autonomously with no outside control.
Pluto Plus AUV for underwater mine identification and destruction. From Norwegian minehunter KNM Hinnøy
An autonomous underwater vehicle (AUV) is a robot that travels underwater without requiring input from an operator. AUVs constitute part of a larger group of undersea systems known as unmanned underwater vehicles, a classification that includes non-autonomous remotely operated underwater vehicles (ROVs) – controlled and powered from the surface by an operator/pilot via an umbilical or using remote control. In military applications a AUV is more often referred to as unmanned undersea vehicle (UUV). Underwater gliders are a subclass of AUVs.


The first AUV was developed at the Applied Physics Laboratory at the University of Washington as early as 1957 by Stan Murphy, Bob Francois and later on, Terry Ewart. The "Special Purpose Underwater Research Vehicle", or SPURV, was used to study diffusion, acoustic transmission, and submarine wakes.
Other early AUVs were developed at the Massachusetts Institute of Technology in the 1970s. One of these is on display in the Hart Nautical Gallery in MIT. At the same time, AUVs were also developed in the Soviet Union[1] (although this was not commonly known until much later).


Until relatively recently, AUVs have been used for a limited number of tasks dictated by the technology available. With the development of more advanced processing capabilities and high yield power supplies, AUVs are now being used for more and more tasks with roles and missions constantly evolving.


The oil and gas industry uses AUVs to make detailed maps of the seafloor before they start building subsea infrastructure; pipelines and sub sea completions can be installed in the most cost effective manner with minimum disruption to the environment. The AUV allows survey companies to conduct precise surveys of areas where traditional bathymetric surveys would be less effective or too costly. Also, post-lay pipe surveys are now possible, which includes pipeline inspection. The use of AUVs for pipeline inspection and inspection of underwater man-made structures is becoming more common.


University of South Floridaresearcher deploys Tavros02, a solar-powered "tweeting" AUV (SAUV)
Scientists use AUVs to study lakes, the ocean, and the ocean floor. A variety of sensors can be affixed to AUVs to measure the concentration of various elements or compounds, the absorption or reflection of light, and the presence of microscopic life. Examples include conductivity-temperature-depth sensors (CTDs), fluorometers, and pH sensors. Additionally, AUVs can be configured as tow-vehicles to deliver customized sensor packages to specific locations.


Many roboticists construct AUVs as a hobby. Several competitions exist which allow these homemade AUVs to compete against each other while accomplishing objectives.[2][3][4] Like their commercial brethren, these AUVs can be fitted with cameras, lights, or sonar. As a consequence of limited resources and inexperience, hobbyist AUVs can rarely compete with commercial models on operational depth, durability, or sophistication. Finally, these hobby AUVs are usually not oceangoing, being operated most of the time in pools or lake beds. A simple AUV can be constructed from a microcontroller, PVC pressure housing, automatic door lock actuator, syringes, and a DPDT relay.[5] Some participants in competitions create open-source designs.[6]

Illegal drug traffic[edit]

Submarines that travel autonomously to a destination by means of GPS navigation have been made by illegal drug traffickers.[7][8][9][10]

Air crash investigations[edit]

Autonomous underwater vehicles, for example AUV ABYSS, have been used to find wreckages of missing airplanes, e.g. Air France Flight 447,[11] and the Bluefin-21 AUV was used in the search for Malaysia Airlines Flight 370.[12]

Military applications[edit]

MK 18 MOD 1 Swordfish UUV
Mk 18 Mod 2 Kingfish UUV
Kingfish UUV launch
The U.S. Navy Unmanned Undersea Vehicle (UUV) Master Plan[13] identified the following UUV's missions:
  • Intelligence, surveillance, and reconnaissance
  • Mine countermeasures
  • Anti-submarine warfare
  • Inspection/identification
  • Oceanography
  • Communication/navigation network nodes
  • Payload delivery
  • Information operations
  • Time-critical strike
The Navy Master Plan divided all UUVs into four classes:[14]
  • Man-portable vehicle class: 25–100 lb displacement; 10–20 hours endurance; launched from small water craft manually (i.e., Mk 18 Mod 1 Swordfish UUV)
  • Lightweight vehicle class: up to 500 lb displacement, 20–40 hours endurance; launched from RHIB using launch/retriever system or by cranes from surface ships (i.e., Mk 18 Mod 2 Kingfish UUV)
  • Heavyweight vehicle class: up to 3000 lb displacement, 40–80 hours endurance, launched from submarines
  • Large vehicle class: up to 10 long tons displacement; launched from surface ships and submarines

Vehicle designs[edit]

Hundreds of different AUVs have been designed over the past 50 or so years,[15] but only a few companies sell vehicles in any significant numbers. There are around 10 companies that sell AUVs on the international market, including Kongsberg Maritime, Hydroid (now a wholly owned subsidiary of Kongsberg Maritime[16]), Bluefin RoboticsTeledyne Gavia (previously known as Hafmynd), International Submarine Engineering (ISE) Ltd, Atlas Elektronik, and OceanScan.[17]
Vehicles range in size from man portable lightweight AUVs to large diameter vehicles of over 10 metres length. Large vehicles have advantages in terms of endurance and sensor payload capacity; smaller vehicles benefit significantly from lower logistics (for example: support vessel footprint; launch and recovery systems).
Some manufacturers have benefited from domestic government sponsorship including Bluefin and Kongsberg. The market is effectively split into three areas: scientific (including universities and research agencies), commercial offshore (oil and gas, etc.) and military application (mine countermeasures, battle space preparation). The majority of these roles utilize a similar design and operate in a cruise (torpedo-type) mode. They collect data while following a preplanned route at speeds between 1 and 4 knots.
Commercially available AUVs include various designs, such as the small REMUS 100 AUV originally developed by Woods Hole Oceanographic Institution in the US and now produced commercially by Hydroid, Inc. (a wholly owned subsidiary of Kongsberg Maritime[16]); the larger HUGIN 1000 and 3000 AUVs developed by Kongsberg Maritime and Norwegian Defence Research Establishment; the Bluefin Robotics 12-and-21-inch-diameter (300 and 530 mm) vehicles and the International Submarine Engineering Ltd. Most AUVs follow the traditional torpedo shape as this is seen as the best compromise between size, usable volume, hydrodynamic efficiency and ease of handling. There are some vehicles that make use of a modular design, enabling components to be changed easily by the operators.
The market is evolving and designs are now following commercial requirements rather than being purely developmental. Upcoming designs include hover-capable AUVs for inspection and light-intervention (primarily for the offshore energy applications), and hybrid AUV/ROV designs that switch between roles as part of their mission profile. Again, the market will be driven by financial requirements and the aim to save money and expensive ship time.
Today, while most AUVs are capable of unsupervised missions, most operators remain within range of acoustic telemetry systems in order to maintain a close watch on their investment. This is not always possible. For example, Canada has recently taken delivery of two AUVs (ISE Explorers) to survey the sea floor underneath the Arctic ice in support of their claim under Article 76 of the United Nations Convention of the Law of the Sea. Also, ultra-low-power, long-range variants such as underwater glidersare becoming capable of operating unattended for weeks or months in littoral and open ocean areas, periodically relaying data by satellite to shore, before returning to be picked up.
As of 2008, a new class of AUVs are being developed, which mimic designs found in nature. Although most are currently in their experimental stages, these biomimetic(or bionic) vehicles are able to achieve higher degrees of efficiency in propulsion and maneuverability by copying successful designs in nature. Two such vehicles are Festo's AquaJelly (AUV)[18] and Evologics' Bionic Manta (AUV).[19]


AUVs carry sensors to navigate autonomously and map features of the ocean. Typical sensors include compasses, depth sensors, sidescan and other sonarsmagnetometersthermistors and conductivity probes. Some AUVs are outfitted with biological sensors including fluorometers (also known as Chlorophyll sensors), turbidity sensors, and sensors to measure pH, and amounts of dissolved oxygen.
A demonstration at Monterey Bay in California in September 2006 showed that a 21-inch (530 mm) diameter AUV can tow a 400 feet (120 m) long hydrophone array while maintaining a 6-knot (11 km/h) cruising speed.[citation needed]


Radio waves cannot penetrate water very far, so as soon as an AUV dives it loses its GPS signal. Therefore, a standard way for AUVs to navigate underwater is through dead reckoning. Navigation can however be improved by using an underwater acoustic positioning system. When operating within a net of sea floor deployed baseline transponders this is known as LBL navigation. When a surface reference such as a support ship is available, ultra-short baseline (USBL) or short-baseline (SBL)positioning is used to calculate where the sub-sea vehicle is relative to the known (GPS) position of the surface craft by means of acoustic range and bearing measurements. To improve estimation of its position, and reduce errors in dead reckoning (which grow over time), the AUV can also surface and take its own GPS fix. Between position fixes and for precise maneuvering, an Inertial Navigation System on board the AUV calculates through dead reckoning the AUV position, acceleration, and velocity. Estimates can be made using data from a Inertial Measurement Unit, and can be improved by adding a Doppler Velocity Log (DVL), which measures the rate of travel over the sea/lake floor. Typically, a pressure sensor measures the vertical position (vehicle depth), although depth and altitude can also be obtained from DVL measurements. These observations are filtered to determine a final navigation solution.


There are a couple of propulsion techniques for AUVs. Some of them use a brushed or brush-less electric motor, gearbox, Lip seal, and a propeller which may be surrounded by a nozzle or not. All of these parts embedded in the AUV construction are involved in propulsion. Other vehicles use a thruster unit to maintain the modularity. Depending on the need, the thruster may be equipped with a nozzle for propeller collision protection or to reduce noise submission, or it may be equipped with a direct drive thruster to keep the efficiency at the highest level and the noises at the lowest level. Advanced AUV thrusters have a redundant shaft sealing system to guarantee a proper seal of the robot even if one of the seals fails during the mission.
Underwater gliders do not directly propel themselves. By changing their buoyancy and trim, they repeatedly sink and ascend; airfoil "wings" convert this up-and-down motion to forward motion. The change of buoyancy is typically done through the use of a pump that can take in or push out water. The vehicle's pitch can be controlled by changing the center of mass of the vehicle. For Slocum gliders this is done internally by moving the batteries, which are mounted on a screw. Because of their low speed and low-power electronics, the energy required to cycle trim states is far less than for regular AUVs, and gliders can have endurances of months and transoceanic ranges.


Most AUVs in use today are powered by rechargeable batteries (lithium ionlithium polymernickel metal hydride etc.), and are implemented with some form of Battery Management System. Some vehicles use primary batteries which provide perhaps twice the endurance—at a substantial extra cost per mission. A few of the larger vehicles are powered by aluminum based semi-fuel cells, but these require substantial maintenance, require expensive refills and produce waste product that must be handled safely. An emerging trend is to combine different battery and power systems with supercapacitors.