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GPS Global Positioning Systems

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Global Positioning Systems (GPS) are used to find the exact location of things. They are a key part of digital mapping.

What is GPS?

GPS Stands for Global Positioning System. A GPS unit on the ground picks up signals sent from satellites.

The satellites transmit the exact time the signals are sent. By taking the time the signal was transmitted from the time it was received, the GPS can tell how far it is from each satellite. The GPS unit also knows the exact position of the satellites, at the moment they sent their signals.

How GPS works

Imagine you are somewhere in Christchurch and you are totally lost. You find a friendly local and ask, "Where am I?" He says, "You are 7km from Christchurch Airport”.

That is good evidence but it means you could be anywhere on a 7km circle around the airport.

 

You ask someone else where you are and she says, “you are 5km from Cathedral Square”.

 

Now you're getting somewhere. If you combine this information with the Christchurch Airport information, you have two circles that intersect. You now know that you must be at one of these two points (green dots).

A third person tells you that you are 3km from Redwood.

You can now take away one of the possibilities, because the third circle will only intersect with one of these points. You now know exactly where you are: Papanui!

Each of the three circles show the signals received from GPS satellites. The main difference is that rather than measuring to three known locations on the surface of the Earth, you are measuring to the known location of three GPS satellites.

A fourth signal is used to synchronize the low accuracy clock in the GPS unit with the high accuracy atomic clocks in the satellites.

Uses of GPS

GPS has many uses, for example:

  • Setting time - automatic updating cell phone times when daylight saving begins and ends.
  • Emergency services need GPS to find places.
  • Tracking a vehicle, aircraft, person or a pet which has been chipped.
  • Bus stops: to show how long the bus will take to arrive at a bus stop.
  • Navigation Personal Locator Beacons (PLBs): used to tell search and rescue experts where someone is.
  • Recreation: eg tramping, climbing, diving, sailing.
  • Surveying: Surveyors use GPS locations to make maps and work out property boundaries.
  • Tectonics: measuring the movement of faults during earthquakes can be measured.

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Watch: Take a look at this Youtube video to find out more about how GPS works.

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This is a picture of a GPS satellite, used to send location and time signals to help find a position. Image: NASA.

GPS devices are useful for activities such as tramping and climbing where it is important to know where you are. Image: LINZ.

Nic Donnelly from LINZ shows the GPS equipment used for finding exact locations during surveying work. Image: LEARNZ.

Car navigation systems use GPS to give directions. Image: LINZ.