DATELINE :- 4th July 2000
GPS IMPROVED
GPS now as accurate as Decca
Navigational accuracy of the standard satellite global positioning system (GPS) over 95 per cent of the time has improved from 100m to just 15m.
That's the view of major GPS sets manufacturer Simrad, following the decision by the US Government to cease its intentional degradation of GPS at the beginning of May. Simrad, which produces marine and terrestrial GPS navigation systems for military, commercial and leisure use, came to its conclusions following a series of test fixes.
It also assessed the accuracy of Differential GPS (DGPS), available to civilians willing to pay a premium. This system sifts out wayward signals distorted while travelling through the earth's ionosphere and any erroneous data from satellites, and Simrad says it now provides exceptional accuracy to within a range of 2-5m, 95 per cent of the time. But while DGPS represents phenomenal fixing for divers requiring pinpoint location of sites, they no longer need go to the expense of buying it to be reasonably certain of hitting their mark.
"If the level of accuracy of the new standard GPS service is indeed proved, over a period of time, to be in the region of 15m, it will be very much on a par with the accuracy we used to enjoy with the [radio beacon-based] Decca system before its disbanding," Alan Cherry, Chairman of the Royal Institute of Navigation's Small Craft Group, told Diver.
"To my knowledge, divers generally found Decca sufficiently accurate to allow final location of a wreck by echo-sounder. If GPS performs similarly, it will allow the same."
But Cherry had a warning on general navigation and seamanship. "The danger of more accurate position fixing is that the temptation to cut things fine - to approach narrow, rock-lined waters in poor visibility, say - increases, where previously prudence would have been the order of the day.
"Even if a system is accurate most of the time, that still leaves periods when it's not."
In the past the US degraded GPS signals so that potential aggressors would not be able to use it as an accurate weapons guidance system. Now, the US will limit the accuracy of GPS for perceived aggressors by applying its accuracy-reducing Selective Availability function on a regional basis.