When Selective Availability is activated, random errors (ie. clock dither) are inserted into the satillite code timing, thereby reducing the accuracy to approximately 100 meters.

If a receiver is located on the earth's surface, the center of the earth can serve as the fourth satellite.

In general, the LOP's rarely cross at a common point. The GPS receiver assumes that this situation is caused a timing error; and, therefore, resets the clock to the time which would result in a point fix.

Each satellite carries four atomic clocks and the clock time is transmitted to the receiver.

Differential GPS (DGPS) is an extension of GPS which improves accuracy. Positional accuracy of one meter is possible in theory.

The datum of the chart being used must be the same as the datum used in the GPS receiver because the receiver must convert every position measurement to latitude and longitude. The datum used for most US charts is WGS-84 which is the default datum for most GPS receivers.

A channel is a signal from a satellite. Four channels are received simultaneously and processed to obtain a three dimensional fix. the extra channels search for satellites which are in more favorable positions.

From a warm start, the aquisition time is usually less than one minute and updates are usually provided about once per second. A cold start means that the receiver does not have the satellite almanac and ephemeris data stored which require 10 to 30 minutes of loading time.