Title: How GPS Works

Title image:  Under the title, there is a small diagram of the GPS constellation, including icons of satellites in six orbits around the Earth.

Intro text:  GPS is a constellation of 24 or more satellites flying 20,350 km above the surface of the Earth.  Each one circles the planet twice a day in one of six orbits to provide continuous, worldwide coverage.

Main image:  GPS satellites are shown in four locations across a starry sky.  Each emits a straight beam pointing to the center of the poster.  The four beams converge at a single point on the roof of a green tractor.  A farmer can be seen driving the tractor through very tight rows of crops.

Main captions:

1.  GPS satellites broadcast radio signals providing their locations, status, and precise time (t1) from on-board atomic clocks.

2.  The GPS radio signals travel through space at the speed of light (c), more than 299,792 km/second.

3.  A GPS device receives the radio signals, noting their exact time of arrival (t2), and uses these to calculate its distance from each satellite in view.

To calculate its distance from a satellite, a GPS device applies this formula to the satellite's signal: distance = rate x time, where rate is (c) and time is how long the signal traveled through space.

The signal's travel time is the difference between the time broadcast by the satellite (t1) and the time the signal is received (t2).

4.  Once a GPS device knows its distance from at least four satellites, it can use geometry to determine its location on Earth in three dimensions.

Side images:  On the left, two satellite dishes are shown on the horizon.  An inset photo shows two Air Force officers operating a computer console, along with the Air Force Space Command logo.  On the right, a rocket launches into space.

Side captions:

The GPS Master Control Station tracks the satellites via a global monitoring network and manages their health on a daily basis.

Ground antennas around the world send data updates and operational commands to the satellites. 

The Air Force launches new satellites to replace aging ones when needed.  The new satellites offer upgraded accuracy and reliability.

Bottom caption:  How does GPS help farmers? Learn more about the Global Positioning System and its many applications at www.gps.gov.

Credits:  This poster is a product of the National Coordination Office for Space-Based Positioning, Navigation, and Timing, an official body of the United States Government.  Rocket image courtesy of ULA.