The Global Positioning System

What is GPS?

GPS stands for ‘Global Positioning System’ and is a system of US Government owned satellites that orbit the earth. It enables GPS receivers anywhere on, or near, earth to receive signals from at least 4 satellites which are then used to calculate an accurate location of the receiver.

The GPS satellite constellation consists of at least 24 satellites flying 12,500 miles above the earth. Each satellite circles the earth twice a day in one of six orbits to provide worldwide coverage.

The GPS project was launched by the US Department of Defense in 1973 with the intention of creating a global navigation system for the US Military. Civilian use of the system was authorised in the 1980s. In the 1990s the quality of the GPS system available for non-US Military users was degraded, however this was reversed by former President Bill Clinton in May 2000.

GPS Uses

The GPS system was originally designed to provide accurate location information to the US Army, Navy and Air Force. However, since its launch it has been used for many other applications and in many ways has revolutionised modern life.

  • Navigation – cars, boats, airplanes, cyclists and spacecraft all benefit from the GPS system.

  • Surveying and Mapping – mapping and surveying grade receivers allow very accurate positions to be calculated to the centimetre (but at a cost, with the receivers costing around $10,000).

  • Precision Agriculture – using GPS systems enables farmers to achieve higher crop yields.

How does GPS Work?

  1. GPS satellites broadcast radio signals. These radio signals contain information on their locations, status and the precise time that the signal was sent. The time is generated from the satellites’ on board atomic clock.

  2. The GPS radio signals travel through space at the speed of light.

  3. A GPS device receives the radio signal. By comparing the exact time that the signal arrived at the receiver with the time that the signal was sent it is possible to calculate the distance between the receiver and the satellite.

  4. Once the receiver as received signals from at least four satellites, it can calculate its position on earth, in three dimensions.

Other Systems

There are several other Global Navigation Satellite Systems (GNSS) which work along similar principals to GPS.

  • GLONASS is a Russian system that achieved global coverage in October 2011.

  • BeiDou is a Chinese system that plans to achieve global coverage in 2020.

  • Galileo is a EU system that’s planned to reach full operational capability in 2019.

There are other systems, such as the Indian Regional Navigation Satellite System, which are designed to provide regional positioning information.

Most GPS receivers are now capable of receiving signals from more than just the GPS system, which means that the system is more reliable.