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MARINE VEHICLE CLASSIFICATION:
A COMPARISON WITH BIOLOGY
A COMPARISON WITH BIOLOGY
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Distinguishing between AUVs and other types of marine vehicles is not quite as straightforward. The field of taxonomy in biology faces a similar issue - what defining characteristics should be used to group different species? Biologists use a number of different methods - differences in size, rRNA, cell structure, life history, ecological (functional) role, etc. - and each biologist has his/her preferred classification method(s). Similarly, marine robotic vehicles can be classified by differences in size, propulsion, energy acquisition method and storage, depth capacity, decision-making abilities, etc.
Taxonomical definitions are also evolving; for example, many are familiar with the older "Kingdoms" system (which included the two, five, and six kingdoms) and (are possibly less familiar with) the "Three Domains" classification, which suggests that life can be classified by an organism's cell structure. An even earlier classification divided life into two types: prokaryotes, which are unicellular and do not have membranes to contain their organelles, and eukaryotes, which may be single or multicellular and have membranes. In 1977, an American biologist named Carl Woese suggested that prokaryotes be split into two groups: bacteria (containing all "typical bacteria") and archaebacteria (which, in 1977, only comprised of methanogens, or bacteria that could produce methane, but now includes more members and has been renamed to archaea), resulting in the "Three Domains" [1]. However, to this day, biologists continue to debate the various classification methods, with continued modifications occurring as we learn more about life on Earth. Likewise, marine robotic enthusiasts will each have their own methods of classifying marine vehicles. The following section focuses on distinguishing AUVs from other types of marine vehicles. For examples of different AUV types, see AUVS: TYPES. TEXTBOOK DEFINITION OF AN AUV
Autonomous Underwater Vehicles (AUVs) are self-propelled, intelligent robotic devices that carry instruments and sensors for sampling and surveying underwater areas. As their name suggests, they do not require manual operation, and thus are distinct from Remotely Operated Vehicles (ROVs) and other manned submersible vehicles (like submarines).
It is important to note that the acronym UAV refers to an Unmanned Aerial Vehicle (the aircraft equivalent of an AUV), and thus a UAV is NOT the same as an AUV (see MARINE VEHICLES: TERMINOLOGY). HOW CAN WE DISTINGUISH AUVS FROM OTHER VEHICLES?
As discussed in the previous section, the lines between the different types of oceanographic vehicles are still fairly blurred and there is no universal agreement. You might think that an AUV must be both autonomous and operate underwater, but this isn't always true. I've included some examples of different oceanographic vessels and their general classifications, based on a somewhat arbitrary balance between the ideas of "majority rules", "comparisons to biology", "inventor/researcher/institution terminology", and, of course, (personal) "logic".
AUTONOMY (AUV, UUV, ROV, HROV, BOTTOM CRAWLER, UGV)
Many people use the terms "AUV" and “UUV" (Unmanned Underwater Vehicle) interchangeably; however, they actually refer to different types of vehicles (see MARINE VEHICLES: TERMINOLOGY). "UUV" is a term coined by the US Navy in the 1990s and refers to both AUVs and ROVs - this can be confusing as ROVs are technically manned (they cannot work without a human operator). To add to the confusion, there are now additional types of vehicles that could be considered under the UUV umbrella, such as vehicles that act as both AUVs and ROVs - Hybrid Remotely Operated Vehicles (HROVs). An example of an HROV is the Nereus [2], which was developed by the world-renowned Woods Hole Oceanographic Institution (WHOI). Since HROVs have the capacity to act as AUVs, I have included these as an alternative variation on the traditional AUV. Vehicles that traverse the seabed are often referred to as Bottom Crawlers, and, if autonomous, Autonomous Bottom Crawlers - like the Benthic Rover [3] (currently one of the only autonomous Bottom Crawlers), developed by the illustrious Monterey Bay Aquarium Research Institute (MBARI). The naval/military community calls them 'UGVs' (Underwater Ground Vehicles), although this may also refer to their above-water counterparts, Unmanned Ground Vehicles (see MARINE VEHICLES: TERMINOLOGY for more details).
DEPTH CAPABILITY (ASV/USV, WAVE GLIDER, SEMI-SUBMERSIBLE AUV)
Autonomous Surface Vehicles (ASVs), which (to add to the confusion) are also called Unmanned Surface Vehicles (USVs), are vehicles that travel on the surface of the water and do not have a human operator. They often resemble boats (or, in the case of wave gliders, surfboards) and from far away, you might not be able to distinguish them between a remote-controlled toy boat or even a traditional motorboat. These are generally NOT considered AUVs, as they never go below the surface (unless something has gone terribly wrong). A specific example of an ASV is Liquid Robotics' Wave Glider [4] which sits on top of the water and uses wave energy as its primary mean of propulsion.
However, there are vehicles that can be considered AUVs although they are not fully underwater. AUVs that travel just underneath the ocean's surface are called semi-submersible AUVs, and were invented in 1981 by one of the world's foremost commercial AUV developers, International Submarine Engineering (ISE) [5]. The body of a semi-submersible AUV is fully underwater with only a mast protruding the surface. The mast sucks in air, which allows a diesel engine to be used, unlike all other AUVs, which operate on stored or "natural" energy (i.e. batteries, solar power, wave power). The mast also provides the ability to use GPS (which is extremely important as GPS doesn't work underwater and modern methods of underwater localization remain fairly inaccurate). Again, as these vehicles mostly function as shallow-water AUVs, I have included them as an alternative type of AUV. A LITTLE MORE ABOUT GLIDERS:
THE "PHYTOPLANKTON" OF THE MARINE ROBOTICS WORLD The term glider may refer to both wave glider (discussed above in the Autonomy section) and ocean glider. Unlike a wave glider, an ocean glider IS often considered an AUV as it is both autonomous and operates completely underwater. Ocean gliders use changes in buoyancy to move vertically, and (like wave gliders) use ocean currents to move horizontally. They are therefore similar to air gliders, except that ocean gliders have as little difficulty moving up as they do down. Like wave gliders, ocean gliders can travel for long distances as they do not require much power (only to make small adjustments in their buoyancy). A good explanation of an ocean glider and one of the most widely used types (the Slocum glider) can be found at a website [6] produced by the National Oceanic and Atmospheric Administration (NOAA).
Gliders are often considered as a separate oceanographic vehicle category, as they usually move passively; that is, they rely on buoyancy and water movement to travel, whereas other AUVs use active propulsion (i.e. using a diesel engine, batteries, solar energy). The idea of separating vehicles based on their ability to move independently is comparable to how we separate biology in the ocean: phytoplankton (from the Greek words for "plant" and "drifter") move in the same way as gliders, whereas their predators, zooplankton (from the Greek words for "animal" and "drifter") are capable of swimming and can thus propel themselves independently, similar to AUVs and ROVs. To clearly illustrate the difference between a propelled vehicle and an ocean glider, I have included ocean gliders as a subset of UUVs (NOT as examples of AUVs). It is also debatable whether ocean gliders are completely autonomous as they are reliant on water motion and are not as capable as propelled vehicles in reaching planned destinations - leading some experts (see [7]) to classify them separately from AUVs. Perhaps it is more appropriate to compare gliders to dinoflagellates, (mostly) marine plankton, who, due to their swimming and eating habits, are considered both phytoplankton and zooplankton [8]. SUMMARY
Figure 1 summarizes common instruments used to measure ocean properties. The grey boxes indicate the flow from general oceanographic measuring tools to different categories of AUVs.
Figure 1. Common oceanographic instruments
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REFERENCES
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Last revised: April 2, 2018
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