Satellites
GPS devices vary widely in their design, features and intended uses. However, they all use the same basic operation to determine your location. What varies is how they apply your position to other functions. GPS (Global Positioning System) satellites orbit the Earth twice each day on a fixed orbital path. There are 24 individual GPS satellites in operation at any given time. Each satellite sends transmissions to Earth, which include satellite identification, location and time and date information. A GPS receiver captures the satellite transmissions and gauges the distance of each satellite by comparing the difference in the time the information was sent from the satellite and the time it was received. Since the speed of transmission is a constant (speed of light), the receiver is able to calculate exactly how far away the satellite is.
Trilateration
Knowing the distance to one satellite would not provide the user with very useful information. Essentially, you would know how far that you are from the satellite, but have no idea what direction you were in in terms of this distance. In other words, it's like realizing that you're 400 miles from New York City, but having no idea which direction it's in. You'd have a reference, but a useless one in determining exactly where you are. This is why a GPS receiver uses transmissions from several satellites to trilaterate the location of the receiver. Three satellites are needed to determine 2D positioning (longitude and latitude), and four or more are needed to calculate 3D positioning (includes altitude). An inherent factor in the trilateration calculation is that times must be completely in-sync because the calculation relies upon time to get distance. Satellites operate on an atomic clock that is extremely accurate. However, it is not feasible to install an atomic clock into a handheld receiver. Therefore, the receiver also takes the potential inaccuracy of its clock into its calculations when using trilateration. By automatically adjusting for this inaccuracy and adjusting your location accordingly, the GPS receiver is able to provide an accurate location, generally within at least 10 or 20 meters. Different GPS receivers take different amounts of time to register your position and offer different accuracy levels. Trialaterating position is the first thing that a user must do in order to use GPS effectively.
Waypoints
The simplest GPS units are those that merely allow the user to enter a series of waypoints. Waypoints are basically locations that you've been to or are heading to that allow you to trace a route. Waypoint coordinates can be determined by your GPS and saved when you are at the location or entered in manually before travelling. You can then use the waypoint to determine the direction and distance that you need to travel in order to get to a destination.
Maps
More advanced GPS units use maps. Mapping GPS units essentially use the location determined by satellites to show exactly where you are on a map. Your location is overlaid onto the map. It is much like a "You are Here" sticker on a park map or mall directory. You can then see exactly how you need to travel on the map. GPS with turn by turn directions includes detailed street information that allows the GPS to offer specific directions from a map.
Tags: atomic clock, clock into, exactly where, from satellite, need travel, receiver able, your location