Since the global positioning system (GPS) became fully operational in the mid-1990s, it’s widely used to navigate, map and survey land as well as study the small changes in movement of objects on Earth or of the Earth itself.
GPS is used to study earthquakes, for phone networks and is especially important for sensing messages that use 3G phones. GPS also extends our senses into space by making use of satellites.
GPS uses between 24 and 32 Earth Orbit satellites to send microwave signals to receivers on Earth that act as sensors. The signals are used to calculate the exact location, time and velocity of the satellites so the position of the receivers can be calculated too.
Where did GPS come from?
The idea for GPS came from the use of networked ground-based radio navigation systems and from US scientists tracking the launch of the Russian spacecraft Sputnik, the first artificial satellite orbiting the Earth, in 1957.
Scientists noticed that the frequency of the radio signal being transmitted by Sputnik got gradually higher as the satellite got closer and lower as the satellite moved away from them. This is because of the Doppler effect, which is a change in the frequency of a wave according to the relative motion of the source of the wave and the observer.
Since they knew their exact location on the Earth, they realised that they could map where the satellite was going by correcting for the distortion caused by the Doppler effect.
How does GPS work?
GPS receivers on Earth act as sensors and can calculate their own position by timing the signals sent by 4 satellites orbiting the Earth. You might think that 3 satellites would be enough, but data from 4 satellites is needed to allow for errors and give more accurate information about time.
Each satellite continually sends microwave signals giving information about the time the message was sent and the orbit it is taking. The receiver on Earth uses the arrival time of the messages from these multiple satellites to measure the distance to each one. The receivers use electronic systems to calculate the co-ordinates, which are then converted to latitude and longitude, and these are displayed digitally on a screen.
Nature of science
Often data collected in scientific investigations may need to be adjusted to take account of conditions and situations that could have given rise to distortion effects.
You may like to try this activity, Something creepy is happening, with your students. They will explore slow slips and also investigate a scientific application of GPS technology – what GPS is, how it is used and how important accurate measurement is.
Did you know that for over a decade, major earthquakes around the world have been interfering with our Earth-bound GPS sites? Find out how this could be introducing errors into our network of satellites and ground stations in the article, GPS skew-whiff.
This video explains how GPS works in more detail.