The Utopian notion of position determination at sea is for an unlimited number of users to obtain an unambiguous, extremely accurate positioning facility, not only at the sea surface but also on the seabed or anywhere in between, 24 hours per day, continuously and in all weathers. This has traditionally implied such unattainable standards of technology that the notion has inspired little but cynical laughter. Now however, the notion has become reality for most of the practical situations encountered offshore. The question of how accurate a position should be can still evoke debate, of course, but only remote sensing and, perhaps, geodesy demand, on occasion, higher standards than are attainable by contemporary methods. This paper attempts to set in perspective the various methods employed to determine position offshore and to discuss in general terms the several approaches to the accuracy requirement.
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