J
Justin - O'day Owner's Web
Some of you may already know this, but to date the principle limitingfactor in GPS accuracy has been an error intentionally introduced by thedepartment of defense as a safety precaution. This error is calledSelective Availability. President Clinton announced today that he hasordered that SA be turned off at midnight tonight. This will result inincreased GPS accuracy for both standard GPS units and those withdifferential units.GPS works by measuring the time it takes for a signal to travel from asatellite in an 11000 mile orbit to the GPS receiver. The elapsed timeestablishes the distance the receiver is from the satellite. Assumingwe know the position of the satellite, this narrows the possiblereceiver positions to a sphere. Add two more satellites to the equationand there are only two possible positions. One is your position onEarth. The other is either in the center of the Earth or out somewherein space. The receiver is programmed to assume you are on the surfaceof Earth and disregards the bogus position. (In reality, the systemuses a fourth satellite as an error checker and if possible up to twelvefor redundancy, but three is enough to start with)The possible accuracy of the GPS system is less than one centimeter,though the systems aboard our boats are not sophisticated enought to dobetter than a meter or so. More on boat accuracy later.So, the government was concerned that accuracy of the GPS system wouldallow miscreants to target weapons against the United States. Theydecided to make the accuracy of the GPS system varry periodically toprevent this. This varriance is the selective availability that isabout to be turned off (or may have been by the time you are readingthis).Differential GPS was designed to overcome the limites of SA. It workslike this: The DGPS sending station is attached to a GPS receiver whoseposition is known very precisely. The sending station then compares theposition given by SA to the known position and extrapolates the offset.This information is then sent to the DGPS receiver on the boats ofpeople with fewer student loans than I have, and the DGPS receiver fixesthe position it had been receiving from the satellites.DGPS is no longer needed to overcome SA, but it is still a good thing tohave aboard. In addition to SA, GPS accuracy is degraded by severalfactors including atmospheric transmission and local radiointerference. For this reason, it is typically not more accurate than15 yards even without SA. Under some conditions it is better, and someplaces experience those conditions more often than not (Casco Bay is oneof them) but you still cannot trust the unit. Unless you have DGPS.DGPS doesn't care if the inaccuracy is coming from SA or from aliens.It just filters the inaccuraccy out and passes on good data. With ityou can trust your GPS to a