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The GPS included a feature called Selective Availability (SA) that introduces intentional errors between 0 meters and up to a hundred meters (300 ft) into the publicly available navigation signals, making it difficult to use for guiding long range missiles to precise targets. Additional accuracy was available in the signal, but in an encrypted form that was only available to the United States military, its allies and a few others, mostly government users.
Some Guides where written before the GPS SA system was turned off in 2000. If you look at he publication dates and when the information for the guide were wrtitten. SA typically added signal errors of up to about 10 meters (30 ft) horizontally and 30 meters (100 ft) vertically. The inaccuracy of the civilian signal was deliberately encoded so as not to change very quickly, for instance the entire eastern U.S. area might read 30 m off, but 30 m off everywhere and in the same direction. In order to improve the usefulness of GPS for civilian navigation, many civilian GPS receivers to greatly improve accuracy used Differenal GPS.
None of the Guides that we buy use Mapping grade GPS. Those receivers use the carrier wave data from only the L1 frequency, but have a precise crystal oscillator, which reduces errors, related to receiver clock jitter. This allows positioning errors on the order of one meter or less in real-time, with a differential GPS signal received using a separate radio receiver. By storing the carrier phase measurements and differentially post processing the data, positioning errors on the order of 10 centimeters are possible with these receivers. Post processing is way Kool!
Also do an Internet search to check for updates and error corrections for the Guides. Mexico Boating Guide by Captains Pat and John Rains has corrections on line. One Yahoo group is setup to provide a link with updated material on the Explorer Chart book series for the Near Bahamas, Exumas and Far Bahamas.
On the chart that you are using check the datum WGS84 NAD27 for North America OSGB36 for Great Britain and ED50 for Europe. Using WGS84 error is generally less than 200 meters, but in some parts of the world errors of up to three miles has been reported.
Accuracy can also be affected by the geometry of the current satellites visible above the receiver's mask angle with respect to user receiver's antenna. Antenna type and location is important. Also be sure that you have the latest update to the GPS software.
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