Global Positioning System

• The Global Positioning System, usually called GPS, is the only
  fully-functional satellite navigation system. A constellation of more
  than two dozen GPS satellites broadcasts precise timing signals by
  radio to GPS receivers, allowing them to accurately determine their
  location (longitude, latitude, and altitude) in any weather, day or
  night, anywhere on Earth.
• GPS has become a vital global utility, indispensable for modern
  navigation on land, sea, and air around the world, as well as an
  important tool for map-making, and land surveying. GPS also provides an
  extremely precise time reference, required for telecommunications and
  some scientific research, including the study of earthquakes.
• United States Department of Defense developed the system, officially
  named NAVSTAR GPS (Navigation Signal Timing and Ranging GPS), and the
  satellite constellation is managed by the 50th Space Wing at Schriever
  Air Force Base. Although the cost of maintaining the system is
  approximately US$400 million per year, including the replacement of
  aging satellites, GPS is available for free use in civilian
  applications as a public good.
• In late 2005, the first in a series of next-generation GPS satellites
  was added to the constellation, offering several new capabilities,
  including a second civilian GPS signal called L2C for enhanced accuracy
  and reliability. In the coming years, additional next-generation
  satellites will increase coverage of L2C and add a third and fourth
  civilian signal to the system, as well as advanced military
  capabilities.