The Global Positioning Satellite (GPS) navigation system has quickly become the preferred choice for sailboat navigators. Position data is available anytime four of the twenty-four transmitting satellites are "within line of sight", and is normally accurate to within 150 feet. Cloud cover, distance from a transmitter, and weather are no longer issues, like in LORAN and celestial navigation systems, and basic models cost less than $100.

GPS navigation is based upon knowing exactly where each transmitting satellite is located, and the precise time. The GPS transmitters orbit 11,000 miles from the earth, where they send out coded messages at predetermined times. The timing of these transmissions, which is controlled by highly accurate atomic clocks aboard each satellite, is the key to finding your exact location. The GPS receiver listens to these signals, which contain detailed information about each satellites orbit, and calculates the distance to each clear satellite based on the time required for the signal to reach the receiver. Each distance calculation is converted to a Line of Position (LOP), much like in traditional celestial navigation. The intersection of these LOP’s is the receivers position.

In its most accurate mode, GPS can determine location within a fraction of a foot. This level of accuracy could pose a national security risk, according to military experts. To prevent misuse of GPS, the military implemented a feature in the GPS signal called "Selective Availability" (SA). When SA is "on", the accuracy of normal GPS receivers is degraded to about 150 feet by the inclusion of false position data. The civilian demand for more accurate GPS data resulted in the Federal Aviation Administration and Coast Guard implementing what is know as Differential GPS (DGPS). DGPS, which is accurate to within 2 meters, uses a receiver at a fixed location to broadcast corrections to nearby mobile receivers. Since these corrections eliminate the effect of SA, the military has agreed to eventually turn SA off. Until that happens, sailors with conventional GPS receivers will have to live with the reduced accuracy.

150-foot accuracy is adequate for basic navigation tasks such as setting in a harbor entrance waypoint and updating a dead reckoning plot, but it is not accurate enough for calculating velocity and heading information. The GPS receiver determines velocity and heading by calculating the change in position over time. Since SA is constantly adding false position information, velocity and heading data is always a little off.   Unfortunately for sailors, this effect is most pronounced at low speeds.