The Global Positioning System (GPS) is a U.S. space-based global navigation satellite system. It provides reliable positioning, navigation, and timing services to worldwide users on a continuous basis in all weather, day and night, anywhere on or near the Earth.
GPS is made up of three segments: Space, Control and User. The Space Segment is composed of 24 to 32 satellites in Medium Earth Orbit and also includes the boosters required to launch them into orbit. The Control Segment is composed of a Master Control Station, an Alternate Master Control Station, and a host of dedicated and shared Ground Antennas and Monitor Stations. The User Segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. GPS satellites broadcast signals from space that GPS receivers use to provide three-dimensional location (latitude, longitude, and altitude) plus precise time.
After Korean Air Lines Flight 007 was shot down in 1983 after straying into the USSR's prohibited airspace, President Ronald Reagan issued a directive making GPS freely available for civilian use, once it was sufficiently developed, as a common good. The first satellite was launched in 1989, and the 24th and last satellite was launched in 1994.
Initially, the highest quality signal was reserved for military use, and the signal available for civilian use intentionally degraded ("Selective Availability", SA). Selective Availability was ended in 2000, improving the precision of civilian GPS from about 100m to about 20m.
Some civilian GPS devices sold by Garmin and others use the Wide Area Augmentation System (WAAS) to enhance GPS readings in North America to less than 3 meters of accuracy. WAAS consists of approximately 25 ground reference stations positioned across the United States that monitor GPS satellite data. Two master stations, located on either coast, collect data from the reference stations and create a GPS correction message. This correction accounts for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere. The corrected differential message is then broadcast through one of two geostationary satellites, or satellites with a fixed position over the equator. The information is compatible with the basic GPS signal structure, which means any WAAS-enabled GPS receiver can read the signal.
Users can either purchase a special GPS wristwatch (for around $120) or can use a cell phone with GPS capabilities. For example, MayMyRun has an app for the IPhone which uses the iPhone's GPS function to automatically map and log a user's training run routes. Users can then monitor the speed and amount of distance covered during a run and can download it into a computer for a log of training runs.
There are both advantages and disadvantages for a GPS system over competing technologies. For example the Nike+iPod counts strides to measure distance. To the extent that stride length has been mis-calibrated, the results will be inaccurate. On the other hand, GPS relies upon a clear line of sight to multiple satellites to measure a position and assumes a straight line path between satelllite reading locations. As a result, GPS is less accurate when running in among skyscrapers or under a lot of bridges.