The Global Positioning System
Use your Global Positioning System (GPS) receiver to access a space-based navigation system that uses 24 satellites in geosynchronus orbits (the satellites stay above one place on the Earth) to obtain information on your position, speed and your vessel's heading.
All the system's satellites simultaneously transmit signals, received by GPS units on earth. Even though the signals are transmitted simultaneously, the receiver receives the signals at slightly different times. The farther away a satellite is, the longer the interval between the time of the signal's transmission and the time of its reception. The speed of the signal---the speed of light---is known, the time from transmissin to recpetion is known and this allows the receiver to calculate the distance to each satellite.
As long as the GPS receiver is acquiring data from at least 3 satellites, the receiver will triangulate your position using the data from those 3 satellites.
Activate your GPS unit, and you will be able to generally determine your location. GPS, originally a military system, was made available for civilian use in 1993, and the signal was degraded somewhat, reducing its accuracy to within several hundred yards of a location. This was called "Selective Availability," but the policy of degrading the GPS signal was officially revoked in May, 2000; however, because of the terrorist attacks of September 11, 2001, there is a possibility of unannounced intermittent selective signal degredations for reasons of national security.
Other factors affecting the accuracy of GPS in boat navigation include the boat's speed, the strength of the signals received by the GPS unit and environmental factors, such as the geomagnetic or gravitational anomalies that appear at several locations around the globe.
Signal strength will vary depending on the distance of the satellite from the receiver and how close the satellite is to being "over the electronic horizon." On start up, the receiver will evaluate all the satellite signals that it receives and use from three to five of the strongest satellite signals for navigation.
Specialized Receivers and Special Uses
Use a chartplotter, a specialized GPS display, to display your boat's progress. Chartplotters use an Electronic Chart Display and Information System (ECDIS) to display a chart, or map, of the area you are in and show your position relative to land or other permanent features in your vicinity. A chartplotter might make your voyage seem like a very slow-moving video game.
Link your GPS unit to the radar unit on your boat and it will show a "track line" on your radar display representing the most direct route to your destination or one waypoint along a route involving multiple legs.
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