The Global Positioning System


The Global Positioning System (GPS) is a satellite-based navigation system owned by the US.  It is a space-based utility that provides users with positioning, navigation, and timing (PNT) services. The Global Positioning System is a made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense.  The system was originally intended for military applications, but in the 1980s, the US government made the system available for civilian use.  The GPS consists of three segments: the space segment, the control segment, and the user segment. The U.S. Air Force develops, maintains, and operates the space and control segments.  GPS works in any weather conditions, anywhere in the world, 24 hours a day.  There are no subscription fees or setup charges to use GPS.

How GPS works

Each of the GPS satellite circles the earth twice a day in a very precise orbit and transmits navigational information to earth-based receivers. These receivers use the information to calculate the user's exact location by triangulation. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference allows the GPS receiver to calculate the distance to the satellite. With distance measurements from four or more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be able to receive and process signals from at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position which includes the latitude, longitude and altitude. Once the user's position is determined, the GPS receiver can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time etc.

Accuracy of GPS Positioning

GPS receivers available in the market today are extremely accurate, thanks to their multi-channel design which allows them to receive and process signals from a dozen or more satellites simultaneously. Garmin's 12 channel receivers are quick to lock onto satellites when first turned on and they maintain strong locks, even in dense foliage or urban settings with tall buildings. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. Most GPS receivers are accurate to within 15 meters on average.

GPS receivers equipped with Wide Area Augmentation System (WAAS) capability can improve accuracy to less than three meters on average. WAAS is an air navigation aid, which uses a network of ground-based reference stations, to measure small variations in the GPS satellites' signals. No additional equipment or fees are required to take advantage of WAAS, however it is only available in North America and Hawaii. Users can also get better accuracy with Differential GPS or DGPS, which is a form of using a number of GPS receivers in a manner which allows much greater positional accuracy compared to a single GPS receiver. 

The GPS satellites

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above the surface. Each of these satellites travels at speeds of approximately 7000 miles an hour and makes two complete orbits in less than 24 hours.  The satellites are powered by solar energy and have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small booster rockets on each satellite allow the ground control to keep them in the desired orbit.

NAVSTAR is the official name given to the GPS by the US Department of Defense.  The following are some interesting facts about GPS satellites:

  • The first GPS satellite was launched in 1978.
  • A full constellation of 24 satellites was achieved in 1994.
  • Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
  • A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
  • Transmitter power is only 50 watts or less.


Even as the burgeoning number of users is demanding more advanced capabilities, most of the GPS satellites that are now 1n orbit have surpassed their operational design life. To sustain and modernize the constellation, the U.S. Air Force and Lockheed Martin Corporation are developing the next generation satellites to augment the system, known as GPS III.

GPS III will improve position, navigation and timing services and provide advanced anti-jam capabilities yielding superior system security, accuracy and reliability.  In general, the GPS III will:

  • Deliver signals three times more accurate than current GPS satellites.
  • Provide military users up to eight times improved anti-jamming capabilities.
  • Due to its advanced design, GPS IIIís operational life is also expected to go upto 15 years or more (25 percent longer than the newest Block IIF satellites).
  • Civil users will find greatly enhanced global connectivity in the future, when GPS III becomes the first GPS satellite to host a civil L1C signal, which will be shared by other emerging international Global Navigation Satellite Systems (GNSS).

Notes & Handouts

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