Unmanned underwater/undersea vehicles, or UUVs, are vehicles that are capable of operating while submerged underwater without the need for a human to be physically occupying the vehicle. They can be divided into two main categories – Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs).
ROVs were developed to overcome the limitations of human divers and human-occupied diving vehicles. They are operated remotely by a crew on board a nearby vessel, and connected to their base of operations via an umbilical link that provides both a tether and a power, communications and data link. ROVs that operate in deeper or rougher water will also be fitted with a Tether Management System (TMS), which regulates the length of the tether to minimize the effects of underwater drag on the cable.
ROVs can be divided into different classes according to their size, weight or function. There are several different classification schemes.
The smallest ROVs are often known as observation or inspection-class vehicles, the simplest of which may carry only a single camera. More sophisticated vehicles of this class may also be equipped with sonar and other sensors. Typical operational depths may range from several hundred to several thousand feet depending on vehicle size.
At the other end of the spectrum are the work-class ROVs. These are fitted with a hydraulic pump to provide the extra power needed for additional equipment and for propulsion at greater depth.
Work-class ROVs are able to operate more powerful equipment such as manipulators and cable-layers. Vehicles of this class have been designed to operate at depths of up to and even exceeding 30,000 feet.
Unlike ROVs, Autonomous Underwater Vehicles are capable of functioning without real-time control from a human operator. They can be programmed to collect data along a predetermined route, and some are able to make decisions and potentially change their mission parameters according to the received data.
Due to the attenuation of radio waves in water, AUVs cannot rely on direct GPS navigation once submerged. Dead reckoning can be used to establish an estimate of position, although errors compound quickly. This positional estimate can be further improved by additional data from underwater acoustic transponders, GPS positions of nearby surface references, or temporary surfacing to establish a GPS fix. Acceleration and velocity estimates can be calculated using an Inertial Measurement Unit, and improved with a Doppler Velocity Log, a device similar to sonar which measures the rate of travel over the sea floor.
The most common propulsion methods for AUVs are propeller-based thrusters and Kort nozzles, usually powered by electric motors. AUVs may also “glide” through the water – by changing buoyancy, they are able to alter their depth, and use airfoil wings to convert this movement into forward motion.
AUVs are used extensively for surveying by the oil and gas industry and in scientific research. They are also used for naval defense applications such as mine detection, payload delivery and surveillance. Military AUVs have been networked with UAVs (unmanned aerial vehicles) to provide a cross-domain ISR (Intelligence, Surveillance and Reconnaissance) solution for the battlefield.