UUVs—submersible unmanned vehicles—are divided into two categories: remotely operated underwater vehicles (ROVs) and autonomous underwater vehicles (AUVs). ROVs are controlled and powered by a person or crew on either land or neighboring craft via an umbilical or using remote control. The cables enable maneuverability of the ROV, allowing it to travel and perform per the control of a remote operator. It may include built-in sensors for video (camera and lights), thrusters, a flotation pack, sonar, and articulating arms. It may be coordinated to retrieve objects, cut lines, or assist in lifting objects. While a human diver could perform the same functions, an ROV may not only assist a diver, but also go where it would be unsafe for a diver to go.
The truly autonomous AUV systems are typically deployed from a research vessel, they are not tethered to the vessel and do not require direct human control while collecting data. It is often preprogrammed with waypoints and a designated task and when it finishes its mission, it returns to a preprogrammed location. Its data and information is then gathered and dowloaded for further analysis. Modern autonomous underwater vehicle (AUV) is an intelligent unmanned platform to perform a variety of military and civilian missions in complex marine environment, which can better meet the needs, such as scientific research, military operations and commercial applications.
The main reason for the development of this ROV, UUV and AUVs is that there are many military benefits to be gained from asymmetric forces like them. There is little chance of being detected by enemies and there are fewer acoustic and magnetic signals because all missions are carried out in underwater. The vessels can be made at a significantly lower cost than the manned submarines, and because it’s unmanned, the size of the vessels can be made significantly smaller as well. Additionally, UUVs can be sent to the hostile and dangerous missions without fear of losing human lives. It can also relieve human operators from monotonous and dull missions.
AUVs are used for underwater survey tasks: the discovery and mapping of submerged wrecks, as well as obstructions that might obstruct navigation for commercial and recreational vessels. In military applications an AUV is more often referred to as an unmanned undersea vehicle (UUV). Underwater gliders are a subclass of AUVs. AUV performs a variety of military and civilian missions in complex marine environment, which can better meet the needs, such as scientific research, military operations and commercial applications. Especially, in case of maritime environment, as the centerstage of combat has changed from ocean to coastal areas, it is difficult for the existing naval forces to effectively operate in shallow waters. Therefore, unmanned underwater vehicles (UUVs) are being required at an increasing pace.
Their autonomy allows AUVs to be used for missions where a surface vehicle or manned submersible would be at risk, such as mine countermeasure (MCM), under-ice operations or underwater survey missions such as detecting and mapping submerged wrecks, rocks, and obstructions. Their ability to operate autonomously of a host vessel makes them well suited to exploration of extreme environments, from the world’s deepest hydrothermal vents to beneath polar ice sheets. They have revolutionised our ability to image the seafloor, providing higher resolution seafloor mapping data than can be achieved from surface vessels, particularly in deep water. The AUVs are applied mainly in the military aspects such as the underwater search, surveillance, anti-submarine warfare, reconnaissance, and navigation.
William Courtney is a Chief Mate, also with the U.S. Navy’s Military Sealift Command and has worked with in maritime environments where ROVs and AUVs are used. He views ROVs and AUVs as allowing greater flexibility for subsea missions. “On the surface and underwater, unmanned vehicles can generally tolerate more adverse environmental conditions such as sea state, current, temperature, than divers can, which minimizes downtime due to poor conditions,” he says. Courtney says this equipment doesn’t require a lot of downtime aside from maintenance and recharging. With multiple vehicles, simultaneous missions are possible, as well as more and longer continuous missions.
The use of ROVs and AUVs, Courtney says, may coordinate with vessel equipped with dynamic positioning — which allows them to keep station over a specific point — for even greater flexibility to investigate targets of opportunity or reposition a ship or floating platform as needed without anchoring or mooring systems normally required for diving operations. “I anticipate that unmanned systems will continue to grow in reliability, endurance, range, battery power, and overall capability,” Courtney says. “The systems that handle them will likely continue to become smaller, more streamlined and portable.”
Courtney continues, “The potential of these vehicles is incredible. The technology seems to be growing exponentially, utilizing greater robotic abilities and possible inclusion of artificial intelligence these vehicles create the ability to open up and get a better understanding of what’s below the surface of our oceans. Ultimately, no matter how advanced we think our current vehicles are, they will only become smaller, more efficient, and more autonomous, creating the ability to launch teams of vehicles to perform mapping functions, exploration or even salvage.”
UUW Military impact
The AUVs are applied mainly in the military aspects such as the underwater search, surveillance, anti-submarine warfare, reconnaissance, and navigation. Their autonomy allows AUVs to be used for missions where a surface vehicle or manned submersible would be at risk, such as mine countermeasure (MCM), under-ice operations or underwater survey missions such as detecting and mapping submerged wrecks, rocks, and obstructions. Their ability to operate autonomously of a host vessel makes them well suited to exploration of extreme environments, from the world’s deepest hydrothermal vents to beneath polar ice sheets. They have revolutionized our ability to image the seafloor, providing higher resolution seafloor mapping data than can be achieved from surface vessels, particularly in deep water.
The US Navy announced recently that it had awarded contracts to Boeing and Lockheed Martin to develop extra-large diameter AUV systems. These systems will replace submarines in many of their missions, deploying from shore and travelling thousands of nautical miles to conduct intelligence gathering, surveys or inspections. Extra-large diameter AUV systems can be deployed and recovered from shore, so launch and recovery is simpler and they can operate in a much wider set of sea states. They do, however, need a large battery pack and very accurate navigation.
Another future use for these systems in the military domain may be anti-submarine warfare (ASW) operations. ASW traditionally relies on a limited number of costly manned platforms such as attack submarines (SSNs and SSKs), frigates and maritime patrol aircraft fitted with a variety of sensors. Today, there’s evidence of a move away from this model towards unmanned aerial vehicles (UAVs), unmanned surface vehicles (USVs), and unmanned underwater vehicles (UUVs) fitted with equivalent sensors, which are more expendable and are becoming cheaper to develop, produce, modify and deploy at scale.
The persistence of extra-large diameter AUV systems is also attractive for this purpose. They can monitor choke points or work together in the open ocean. Since ageing fleets of ASW vessels are nearing replacement and these AUV systems are seen as a cost-effective force multiplier, they may, in time, become the ASW force.
A prime example is the US Navy’s medium displacement USV, or MDUSV. The prototype launched in April 2016, Sea Hunter, was reported to have demonstrated autonomous SSK detection and tracking from the ocean surface from 3.2 kilometers away, requiring only sparse remote supervisory control for patrols of three months, using a combination of ‘advanced hydro-acoustics, pattern recognition and algorithms’. Since the range and resolution of acoustic sensors are highly variable according to oceanic conditions (such as depth, temperature and salinity), the range may well go further in favorable conditions; a Chinese estimate puts it at 18 kilometers. Since SSKs using air-independent propulsion or running on batteries are virtually silent, MDUSVs should theoretically be capable of pursuing SSNs and SSBNs (whose nuclear reactors continuously emit noise) at greater distances, and there are reports that they will be armed.
Whereas the new US FFG(X) frigate costs a sizeable $1 billion per ship, MDUSV platforms are reported to cost only $20 million each and so could conceivably be produced at scale to autonomously or semi-autonomously seek and trail submarines. Former US deputy defence secretary Robert Work has suggested as much: ‘These will be everywhere.’
An AUV is a underwater robot that operates in six degrees of freedom (6DOF) and can conduct planned missions by using its own propulsion system controlled by an on board computer. The payload of an AUV includes, beside the CPU and the electro-energetic system, different types of sensors and technology like navigation, system and communication.
Cylinder shaped AUV systems with a diameter of approximately 9in-12in have become a common sight in MCM operations. These systems are typically equipped with side-scan sonar systems and high-grade survey systems. They are launched from small vessels or rigid hull inflatable boats and survey rectangular areas of the seabed in search of objects of interest. The data quality and speed of survey make them the ideal tool for this purpose.
The UUV design should be evaluated according to each mission through a detailed comparative analysis. In addition, to improve operability, it is important to have the advanced onboard equipment and to standardize the interface technology and modularization among the equipment.
The UUVs can be categorized by the hull size, types of energy source and operation range. Those determine the endurance of the UUVs, which can be very important for the reconnaissance mission. The onboard sensors and mission equipment determine the type of missions that the UUVs can carry out. Since the UUVs are operating in deep waters where the constant connection between the remote ground operators can be highly difficult, the precise navigation capability becomes the critical issue. The autonomy level also determines the UUVs flexibility in terms of when the situations change too rapidly for the remotely situated human operators can handle.
Operating autonomously subsea is challenging: lack of communications, intense physical pressure, no ambient light and unchartered waters add up to make it one of the most difficult technical challenges for humanity to solve. Underwater operation presents additional challenges for researchers and developers. They share many of the same navigation, power and logistical challenges as their UGV (unmanned ground vehicles) and UAV (unmanned aerial vehicles) counterparts. However, unlike those platforms, underwater systems have additional communications and environmental condition challenges, especially when operating in the open ocean.
Some of the factors affecting an AUV are visibility under water, gravity and buoyancy, stability, hydrodynamic damping, hydrostatic pressure and environmental forces. The visibility underwater is affected by different factors like the absorption of light travelling through and the amount of particles in the water.The relation between the buoyancy and weight, the stability of the AUV, the dragging force, the Coriolis and the pressure are the key in navigating at a certain depth level.
The underwater system must have packaged electronics that are water tight at depth (versus splash-proof), use materials that are resistant to aggressive corrosion, and be able to localize underwater without the aid of conventional communications (such as GPS or WiFi). This environmental challenge drives a need to adopt a prevention and maintenance strategy for life cycle management that involves resilient or special materials selection and/or design methods to enable easy maintenance or replacement.
AUVs need to cruise in different sea depths according to different situations. For example, AUVs are required to float near the surface to receive GPS signals regularly so as to ensure proper position and heading. At this point, the force and motion characteristics of AUVs affected by the sea are different from those of the deep dive state. The mathematical model of AUVs’ motion is highly nonlinear because of currents, waves’ interaction changing with time and location. Furthermore, the motion control system of AUVs is a multi-input and multi-output system, which has a contradiction between system complexity and control accuracy.
Operational depth presents another significant and unique challenge to AUVs. As a system travels deeper under water, the pressure increases. Unlike aerial vehicles that can operate effectively at 10 feet or 1000 feet above the surface, the hulls and pressure vessels for the AUV must be designed for the appropriate depth. This makes the vehicles more mission and depth-specific than their aerial counterparts.
An AUV must have the ability in both power and agility to overcome complex hydrodynamic forces. While analogous to aerodynamic forces in many ways, hydrodynamics in unsteady, obstacle-laden environments is often more complex and not well understood. These forces change over time and can act upon the AUV over large durations of time (continuous currents) or in short bursts (gusts and turbulence). Therefore, for normal operation (not including storms and extreme weather conditions) the AUV has high demands on its power supply for normal operation and even simple station keeping.
There are many challenges of achieving autonomy with unmanned systems, for instance, these systems need obstacle detection and avoidance (ODOA), situational awareness (“What is happening around me?”), decision-making capacity (“Should I respond aggressively or passively?”), and health monitoring (“Is everything OK with me?”). An area of focus is making these systems act more like biological systems.
In addition, although any autonomous system requires collision detection and avoidance capability, the AUV must contend with more challenging limitations. Unmanned Aerial Vehicles (UAVs) have similar collision avoidance sensory needs, but the technologies for in-air sensing are much further advanced and can effectively sense objects hundreds and thousands of feet away. Usable AUV sensory capability is lagging, leaving effective sensing to less than 50 feet and many times less than 10 feet depending on the level of debris in the water.
Communicating Under Water is also challenge as Water significantly attenuates sound and many spetra of light. Therefore, the accomplishments of the technology community in wireless communications technology (WiFi, GPS, etc.) does not work in the water. This presents the AUV with a significant challenge in localization and navigation as well as transfer of information to and from other systems.
A 2014 Rand Corp. research paper on Designing Unmanned Systems with Greater Autonomy looked at how AI is being implemented — and researched for future application — across the board for unmanned systems. “Achieving and maintaining communication with underwater vehicles and even with surface vehicles is technically challenging, especially at longer ranges. Water attenuates radio waves and other wireless signals that can easily be used at long range in air-to-ground or air-to-air communications,” the report stated. “This means that high-bandwidth communications underwater are largely impractical using traditional communication technologies.”
“Although there has been some experimentation with laser communications for underwater applications, laser communications systems are expensive and consume considerable amounts of power. Because of these communications limitations, UUVs that do not require continuous communications links are essential. For example, autonomous path planning is needed to avoid underwater obstacles and unanticipated terrain features.”
AUV navigation and localization is a challenging problem due primarily to the rapid attenuation of higher frequency signals and the unstructured nature of the undersea environment. Above water, most autonomous systems rely on radio or spread spectrum communications and global positioning. However, underwater such signals propagate only short distances and acoustic based sensors and communications perform better.
However, until these technologies advance for underwater applications, the most effective means for overcoming the challenge includes having the ability to return quickly and often covertly to the surface where the system can leverage in-air wireless and geo-positioning technology.
The growth in AUV use is in part driven by continuing improvements in AUV technology and capability. In the last two years, new AUV models have been launched and these can gather more data, over longer periods and more accurately. Many of these AUV systems are more compact than their predecessors despite their increased capacity.
Better endurance, improved communications, more accurate navigation, enhanced imaging, artificial intelligence and big data are all contributors. Recent advances in energy density, spearheaded by the mobile phone industry, have helped improve AUV endurance. In parallel, communications, navigation and payload instruments are becoming more effective.
Artificial Intelligence enhancing Autonomy
Autonomy will bring significant changes to undersea warfare, from longer range and endurance attack and surveillance operations amid minefields, enemy submarines and UUVs, to being able to strike undersea, surface, or land targets with improved targeting at long ranges.
Such missions will require new and advanced capabilities, from seamless, real-time communications among UUVs and motherships, to an advanced undersea “GPS-like” capability, currently being developed by the U.S. Defense Advanced Research Projects Agency (DARPA) and BAE Systems.
The Embracing Artificial Intelligence in Undersea Warfare session at the Navy Submarine League’s Submarine Technology Symposium in May summarized the U.S. Navy’s increasing focus on AI — especially undersea: “As the current submarine force trusts mechanical and electrical technology to execute the mission, the future force will need to trust AI to extract and exploit actionable patterns among an ocean of data,” the session says. “The advent of big data and deep learning technology has rendered signal detection and classification an increasingly automated process. Furthermore, advances in autonomous navigation have enabled unmanned platforms to operate alone or in swarms.
Multi-agent systems (MAS) are being designed to address significant challenges in intelligent control in collecting simultaneous data points from a large ocean volume as part of a coordinated mine sweep. Those include automated planning and replanning, context-sensitive reasoning, unanticipated event handling, inter-agent communication, autonomous organization, and reorganization of multi-autonomous undersea vehicle (AUV) systems and task allocation.
“Advances in artificial intelligence and global technology proliferation are driving the rapid evolution and global adoption of autonomy, which is creating economic, social and military disruption,” wrote Mike Griffin, U.S. undersecretary of defense for research and engineering.
Electro-energetic System/Energy management
In the propulsion matter, it’s common for AUVs to use thrusters because they provide more accuracy and a faster response. Relying on the autonomy characteristic, the AUV must provide its own power for long term operation. Since all the systems onboard the AUV are electric, batteries are the choice for the power supply. Electric propulsion comes with the advantages of silent operation, ease of speed control and simplicity.
AUVs use secondary batteries, meaning rechargeable ones, lithium-ion being the most common choice nowadays. There is also a trend to combine solar power with lithium-ion batteries for very long operation. Another new idea is to achieve neutral buoyancy by adding extra batteries instead of dead weight therefor increasing the autonomy in order to operate more time than a given application requires.
For accurate navigation purpose, it’s recommended for AUVs to work in conjunction with surface vessels. In these cases the location of the underwater vehicle is calculated by measuring the acoustic range relative to the known GPS position of the surface support ship. If the mission scenario does not include a surface support ship, the AUV will take its own GPS position when at surface and when submerged it will use its inertial navigation system onboard to
measure acceleration and velocity, therefore the rate of travel, in order to determine a final navigation solution.
Considering that in literature exist three navigation methods known as (1) dead-reckoning and inertial navigation, (2) acoustic navigation, and (3) geophysical navigation techniques the configuration of navigation sensors must take into consideration the mission needs.
In parallel, navigation performance is improving thanks to new inertial navigation systems that can dead-reckon with as much as twice the certainty of what was possible even two years ago. This is possible by combining latest generation gyros and acoustic aiding from Doppler velocity logs as part of a single instrument.
Sensor System and Signal Processing
The AUV platform is equipped with a sensor and sensing systems to perform static and dynamic experiments, to navigate autonomously and map feature of the ocean. The main sensors to estimate its position are a depth sensor, a compass and a speed sensor but a Doppler velocity log is recommended to increase the accuracy of the estimates. Instead of simple speed sensors, it would be recommended to install an inertial navigation system with laser or fiber optic gyroscopes. Sonar and underwater cameras are used for obstacle detection and classification.
SW relies on separating tiny submarine signals from background ocean noise, primarily by using active and passive acoustic sensing (sonar) and magnetic anomaly detection (MAD), and it looks likely that these will remain the most important signals in the near future. However, the range of signals may grow as sensor resolution, processing power and machine autonomy reach the necessary thresholds to reliably separate other, ‘quieter’ kinds of signal. As Bryan Clark notes, ‘While the physics behind most [non-acoustic detection] techniques has been known for decades, they have not been exploitable until very recently because computer processors were too slow to run the detailed models needed to see small changes in the environment caused by a quiet submarine.’ However, he adds there’s now ‘the capability to run sophisticated oceanographic models in real time’.
No breakthroughs have been publicly disclosed, though an independent investigation by British Pugwash in 2016 identified light detection and ranging, or LIDAR, using blue–green lasers; anti-neutrino detection; and satellite wake detection as signal types that may merit further examination. Higher processing power can also enable digital sensor fusion, whereby different kinds of signal are synthesized and analyzed together, and better simulations of the baseline ocean environment, which would show up anomalies in greater contrast.
It’s a known fact that the underwater environment imposes electro-magnetic constraints both on communications as it does to navigation. The most common application that involves AUV communication is done at surface by radio for mission upload, status and monitoring or data download. When submerged, an AUV mission involves minimal or no communication because of the limitation of the underwater environment. In these cases acoustic means are employed even though they have limited range or bandwidth and very limited data rate and depend on factors such as depth, bottom type, temperature, salinity, and sea state.
The latest advances in signal design are being used to make acoustic communications travel further and carry more data, using less power. Other techniques like free space optical modems are also enabling large amounts of data to be transferred through-water to AUV systems, using the visible light spectrum at distances of up to 150m. More than ten thousand times more data can be transferred this way than is possible with acoustics.
When thinking about a mission, operators can choose from lasers, sonars and even stereoscopic high-definition video. Electronically scanned sonar systems are now manufactured in all sizes – even as small as a GoPro camera – for every application. Some produce stunning imagery at 5m range using high frequencies. Others, working at lower frequencies, can spot obstacles at ranges of over 1000m.
For inspection missions, video and laser are combining to provide stunning pictures of the subsea environment as it has never been seen before; at centimetric resolution and in full colour. When it comes to survey, operators are not just limited to side-scan sonar imagery and multibeam bathymetry. There is now a new generation of multi aperture sonar systems capable of extending range and producing three-dimensional bathymetry.
Synthetic aperture sonar has also proved popular for large AUV systems. This is an industry generating more data than ever before. Fortunately, it is at a time when storage solutions have become more prevalent, and machine learning and big data techniques are becoming wide-spread.
UUV & AUV Swarms
Recently, the cooperative control of the multi-UUVs has been becoming a hot topic in various applications, including ocean exploration, submarine rescue, minesweeping, and other fields. Another development is the proliferation of low cost, small AUV units, which collaborate and work together for one common goal. This is typically referred to as a swarm of AUV systems. There have been some well publicised trials by several companies, and other work has been published but not discussed in the open media.
Compared to the individual UUV which has the limited processing power and operational capability to complete a single task, the multi-UUVs as a whole can perform the complex task, thus having more superiority in the harsh ocean environment, such as enhancing reliability and reducing cost. Designing the cooperative control law for the multi-UUVs is a current challenge, which utilizes the UUV location information relative to its neighbours, such that the multi-UUVs agree on the specific quantities of the interest, namely, reaching the consensus control.
Networks of this type could be greater than the sum of their parts, with nodes able to carry heterogeneous sensors, cross-reference positive signals from multiple directions and domains, and move and respond to get a better look at signals using real-time swarming. It’s easy to imagine fleets of MDUSVs being used in the same way, potentially much further apart. Some technical challenges remain, including scaling up to blue water and improving underwater communication, autonomous decision-making, self-location and battery life, but none appear insurmountable and some of the physical limitations felt by a single vehicle can be mitigated by swarming.
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