History

• The U.S. military originally used GPS satellites for operational assistance. Availability of the technology to the public then followed. The Air Force still controls and monitors the satellite system. The placement of the first GPS satellite in space occurred in 1978. By 1994, the system was complete, with 24 satellites in space.
  
  

The Satellites

• The GPS satellite system is composed of 24 satellites orbiting the planet. The satellites complete two orbits in less than one day. They are located approximately 12,000 miles above the Earth and have a motion speed of about 7,000 miles per hour. GPS satellites run on solar energy. The life span of a satellite is 10 years. New satellites replace the old ones when they cease to function. Each satellite weighs about 2,000 pounds and is 17 feet long.
  
  

Segments

• There are three segments of the GPS satellite system: The space, control and user segments. The space segment consists of the system itself and its functionality in space. The control segment is made up of stations on Earth that monitor and maintain the satellites. The duties of the control segment include ensuring that the satellites continue in a correct orbit. The user segment refers to the various devices that receive the transmissions provided by the satellites. These devices include all items used by the civilian population that utilize GPS.
  
  

The Process

• The satellites send a signal to Earth. The GPS receivers triangulate the location of the user based upon this signal. Time plays an integral role in the process. The receiver records the time a signal reaches the user, and compares it to the time of transmission by the satellite. Based on the time difference, the receiver can determine a person's exact location. Navigation information and instructions are then readily available.