Page 433 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

426 CHAPTER 16 Acoustic Positioning
possessed the manned submersible capability to dive to the depth of the wreck site. But precision underwater navigation, through any other means but visual, was impossible with the available technology.
In the 1970s, as the search for hydrocarbons migrated into deeper water, repeatable high- accuracy bottom positioning became necessary to place the drill-string into the exact position refer- enced earlier through seismic instrumentation. Since radio frequency waves penetrate just a few wavelengths into water, some other form of precision navigation technology needed development. Thus was born acoustic positioning technology.
Today, water conditions in ports and harbors, as well as littoral areas around the world, are such that visual navigation below the surface is either difficult or impossible due to low visi- bility conditions. The need for underwater acoustic positioning remains high.
16.2 What is positioning?
According to Bowditch (2002), “positioning” is defined as “the process of determining, at a particular point in time, the precise physical location of the craft, vehicle, person or site.” The position determination can vary in quality (degree of certainty as to its accuracy), in relativity (positioning relative to any number of reference frames), and point reference (versus a line of position (LOP) that is a mathematical position referenced along a given line, circle, or sphere).
A position can be derived from any number of means, including deduced (also termed “dead reckoning”), resolved (resolving bearing referenced to known fixed or moving objects—also termed “geodetic” when resolved relative to known earth-fixed objects), esti- mated (also termed SWAG—normally claimed by helmsman immediately before striking sub- merged rocks/objects), and claimed (or announced
“I claim this island in the name of King George”).
16.3 Theory of positioning
All positioning is a simple matter of referencing a position relative to some other known position. From the earliest mariners, navigation was performed through “line of sight” with the coast. As explorers ventured farther from the sight of land, navigation with reference to the stars became common. Navigation with reference to the North Star for the determination of latitude was the ear- liest version of celestial navigation. Accurate determination of one’s latitude could be gained by measuring the angular height of the North Star above the visible horizon.
The determination of position upon a known line of latitude formed a rudimentary “line of posi- tion” (LOP) in that a known position is resolved. In order to gain a higher positional resolution, a second LOP and then a third (and so forth) will be required to intersect the lines of position and resolve for two- and three-dimensional accuracy. This theory works for celestial navigation, GPS positioning, and (of course) acoustic positioning.