What is GPS?
The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS works in any weather conditions, anywhere in the world, 24 hours a day. GPS is free of charge.
How it works
GPS satellites circle the earth twice a day in a very precise orbit and transmit signal to earth. GPS receivers take this signal and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.
A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.
How accurate is GPS?
Modern 12 parallel channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Most GPS receivers are accurate to within 15 meters on average.
Newer GPS receivers with WAAS (Wide Area Augmentation System) or EGNOS capability can improve accuracy to less than three meters on average.
Users can also get better accuracy with Differential GPS (DGPS), which corrects GPS signals to within an average of three to five meters. DGPS correction system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.
Clear vision to the sky is preferred
GPS does not work well inside buidings or vehicles. The signal is lost or quality is lowered by obstacles. Antennas placed on car roofs will give good view of all satellites, if not shadowed by trees or buildings.