GPS and Loran are similar in that they both provide calculated numeric positions fixes. When compared to a nautical chart the fix will give the user his present position.

Loran calculates position fixes from a system of land based towers. Loran is a US system only. Depending on weather conditions accuracy varies from 600 feet to several miles. Although once you have marked a Loran waypoint, you can return to it with incredible accuracy, regardless of the fix accuracy as compared to a nautical chart. This is called "repeatable accuracy" and is extremely useful for fishing and scuba diving. Loran is suppose to be turned off sometime after the year 2006.

GPS calculates position information from a group of satellites orbiting the earth and provides global coverage. GPS provides accuracy from 50 feet to 200 feet. The error is randomly changed by the service providers (many thanks to the U.S. Military) for this reason GPS has a very high "Absolute Accuracy" (as compared to a chart), but lower "repeatable accuracy" (the ability to return accurately to a previously marked spot).

Enter Differential GPS! This system solves GPS repeatable accuracy problems. A series of land based towers transmit the GPS error corrections to a second antenna on board your vessel. Your GPS unit (if it is capable) then applies the correction factors to your GPS fix and now you have accuracy from 3 to 75 feet. To our knowledge, Differential GPS is available throughout North America and some areas of Europe.