How Your GPS Works

By Yosef Solomon on December 1st, 2008

GPS is an acronym for Global Positioning System, but most don’t have a clue how global positioning really works. This is a satellite-based navigation system, consisting of more than 20 satellites and many supporting ground facilities, which provide accurate, three-dimensional position, velocity, and time, 24 hours a day, everywhere in the world, and in all weather conditions. It is compromised of three primary components.

The GPS ground control stations, the satellites, and the receivers are the three key players in how this complex system is brought to life. The ground control component includes the master control station at Falcon Air Force Base, Colorado Springs, Colorado and monitor stations at Falcon AFB, Hawaii, Ascension Island in the Atlantic, Diego Garcia in the Indian Ocean, and Kwajalein Island in the South Pacific. The measurements that are collected by the monitor stations predict the behavior of each satellite’s orbit and atomic clocks. The prediction data is linked up to the satellites for transmission to users. The control segment also ensures that GPS satellite orbits remain within limits and that the satellites do not drift too far from nominal orbits.

The space segment is compromised of satellites and the Delta rockets that launch the satellites from Cape Canaveral in Florida, United States. GPS satellites orbit in circular orbits at 17,440 km altitude, each orbit lasting 12 hours. The orbits are tilted to the equator by 55° to ensure coverage in Polar Regions. The satellites are powered by solar cells to continually orientate themselves to point the solar panels towards the Sun and the antennas towards the Earth. Each satellite contains four atomic clocks.

When you buy a GPS, you are actually buying only the GPS receiver and get free use of the other two main components. These are worth literally billions of dollars and are all yours, compliments of the Government of the United States. The ground stations send control signals to the GPS satellites, The GPS satellites transmit radio signals and the GPS receivers, receive these signals and use it to calculate its position.

The calculations that are used to determine your GPS receiver’s position are based on very mini time differences. These small differences are then used to calculate the distance from the receiver to the satellite. However, when receiving only one signal, you can only calculate how far away from the satellite you are. When receiving two signals, you can determine two likely positions where you are. There needs to be three satellite signals to determine the exact position on the earth’s surface.

The GPS satellite constellation is designed in such a manner as to guarantee that at least 4 satellites are visible from any place on earth at any moment in time. Most of the time (+95%) however, you should have at least 6 satellites visible. Many commercial GPS receivers can receive and process signals from 12 satellites for increased reliability and accuracy. GPS satellites carry atomic clocks that measure time to a high degree of accuracy.