There are three basic positioning techniques that are practical for use on ROVs: dead reckoning, Global Positioning System (GPS), and Ultra-Short Baseline (USBL). We will look at each of them in turn.
Dead reckoning is the navigational method used by mariners all over the globe before the invention of compasses, chronometers and sextants. In dead reckoning, the navigator starts with a known (or estimated) position, and from there attempts to gauge the vehicle’s position by applying the vehicle’s heading and speed. Depending on the tools available, dead reckoning can be a surprisingly effective form of navigation. In the modern era, where the starting position of an ROV may be known with great precision and the heading and speed of the vehicle are also known quantities, dead reckoning can be sufficient for many applications. However, dead reckoning has one fatal defect: it cannot account for currents in the water. If the current is significant, the position of the ROV at the end of a run can be vastly different than the reckoned position.
The widespread availability and accuracy of global positioning systems (GPS) has made terrestrial navigation a breeze for motorists and surface vessels. Anyone with a line of sight to four or more GPS satellites – which effectively means anyone on the surface of the Earth – can use an inexpensive computerized navigation device to locate themselves, with an accuracy of as close as ten feet. GPS is cheap, effective, and nearly foolproof – so it’s the perfect solution for ROV navigation, right? Unfortunately, no – GPS is almost useless for most ROV applications. The problem is that the satellite communication signal is severely blocked by fresh water, and totally blocked by sea water. Underwater vehicles cannot use GPS without coming to the surface. There are some specialized use cases for GPS on an ROV, but they are few.
Ultra-Short Baseline (USBL) is a technique for determining a vehicle’s position underwater. It is a fairly complex system which involves a base station/transceiver, usually mounted on a ship, a transponder/transceiver mounted on the ROV, and a sophisticated computer system (usually located with the base station). The base station sends out a powerful acoustic pulse, similar to a sonar “ping”. This pulse is picked up by the transponder on the ROV, and a response pulse is sent from the ROV back to the base station. When the base station receives the response pulse, it calculates the duration between the initial signal and the response, and determines the range between the two transceivers based on that time. A set of transducers in the transceivers permits the calculation of a bearing between the two transceivers; the combination of the range and the bearing gives a precise position for the ROV relative to the base station.
USBL is extremely effective and accurate, but it is also expensive - a USBL installation for an ROV can cost upwards of $20,000. It also has a technical weakness in that under “busy” sonar conditions – a crowded harbor, for example – can cause problems for the system. However, for now, USBL represents the state of the art in ROV navigation systems.