GPS: What is GPS
In 500 words or less...
The Global Positioning System (GPS) is a system of 30+ satellites maintained by the US Department of Defense for navigational and security purposes. These satellites orbit the earth at approximately 12,000 miles and broadcast continuous radio signals communicating their location. GPS receivers are able to determine location by tracking and comparing signals from multiple satellites. Receivers measure how long each signal took to travel from the satellite to the receiver (measured as a delay in the signal’s arrival time), and translate this time interval to a distance estimate. By comparing the distances to multiple satellites the receiver is able to approximate location (both horizontal and vertical) using mathematical trilateration.
This system requires extremely precise timing and communication between the satellites and the receivers. It also requires a clear line-of-sight between the receiver and each satellite. Tree canopy, hills, buildings, etc. can limit the ability of a receiver to accurately 'see' satellite(s) and thus lessens the ability to determine position. In addition, the configuration of the satellites themselves is sometimes better or worse for a given location. For example, if all of the satellites are directly overhead, it is very difficult to get accurate trilateration measurements. Conversely, if the satellites are extremely low on the horizon then their signal passes thru so much of the earth’s atmosphere that the signal delay (and thus the distance estimate) is harder to predict. What is desired is a large number of satellites, widely scattered across the viewable horizon but not too low (higher than 15°).
Current receivers are (typically) able to determine horizontal location to within 5 meters. This accuracy can be improved by a technique known as Differential Correction whereby readings from a field receiver are compared to those of a base station GPS receiver at a known location. WAAS (the Wide Area Augmentation System) is a continental correction service that uses multiple base stations to improve GPS receiver estimates in real-time. Survey grade receivers, using high-precision differential correction are able to produce sub-meter accuracy.
Most receivers can record points, lines or areas. For points, a longer duration of GPS recording will yield a more accurate location due to the power of averaging (the receiver records many positions and averages them to estimate location).
GPS is just one of a number of Global Navigation Satellite Systems (GNSS). Other systems include the Russian GLONASS, the EU's Galileo and China's BeiDou. These systems similarly use their own constellations of satellites. Newer receivers are able to receive data from two or more systems for improved location estimation. GPS is the oldest and most widely used GNSS.