The Global Positioning System (GPS) is a collection of satellites that orbit Earth and send signal information to allow people with ground receivers (GPS units) to identify their exact geographic location. The GPS is free, works in any weather, and is accessible 24 hours a day. Owned and operated by the United States Government, GPS was originally intended for military use, but in the 1980s it was made available for global civilian use.
The GPS is made up of three important components:
1) Space component is made up of the GPS satellites orbiting the Earth. Each satellite contains a computer, a radio, and an atomic clock. As a satellite moves it broadcasts its changing position and time. Satellites are well spaced so that from any location on the Earth’s surface, four satellites will be above the horizon. Each satellite circles the same path around the Earth two times each day. These satellites are solar-powered, with back-up batteries in case of a solar eclipse that blocks the sun’s rays. The space component includes old and new satellites that are built to last approximately 10 years. The oldest satellites were launched in 1978 and the newest GPS III satellites Fig. 2. (image to the left) are scheduled to launch in 2018.
2) Control component is the global monitoring and control stations that maintain and update the GPS satellites. This includes one master control station, an alternate master control station, 11 command and control antennas, and 16 monitoring sites.
3) User component is the GPS unit, which receives the radio signal from the GPS satellites. A GPS unit has a computer that determines its geographic position (longitude and latitude) from at least three satellites. A fourth satellite allows the GPS unit to determine altitude for the location. GPS units can also calculate other information including speed, bearing, trip distance, track, distance to destination, sunrise and sunset time.
A GPS unit can calculate its position by measuring its distance from multiple GPS satellites. The satellites send radio signals that contain information on the satellite’s current time (measured by its highly accurate atomic clock) and position. The GPS unit receives these signals and calculates how long it took the signal to travel from the satellite to the GPS unit. Distance can then be calculated as follows:
Distance = Speed of light * Time difference (time taken for the signal to travel)
By calculating this distance to three or more satellites, a GPS unit can continuously identify its location on the Earth, including latitude, longitude, and altitude. A GPS receiver can thus be used for navigation, recording data points/tracks in the field, surveying, and a number of other capabilities.
Fig. 4. Diagram of how the GPS determines geographic location. A GPS unit compares the time a signal was sent by a GPS satellite (monitored by ground control stations) with the time it was received by the unit. The time difference tells how far away the satellite is. Additional distance measurements from other satellites allow the unit to determine its exact location (Image modified from PDH Center)
Accuracies for common handheld GPS units range from about 5 to 15 meters (altitude estimations are less accurate) and are not affected by weather conditions or time of day. The GPS satellite signal can travel through clouds, glass, and plastic, but cannot travel through most solid objects such as buildings, mountains, or thick forest canopies. These obstructions can decrease accuracy by limiting communication between the GPS satellites and the GPS unit. If possible, the GPS unit should be in an open location with a clear view of the sky for highest accuracy.
GPS points and tracks from the field can be imported into a GIS for data processing and visualization. DNRGPS and Garmin Basecamp are both free software available for importing GPS data and saving it to your computer. These files can be viewed using Google Earth (kml/kmz format) or imported as an ArcGIS shapefile.