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How the Global Positioning System (GPS) works

By Pat, 28 November 2012 Activity

String tied around a tree trunk. The string is rolled out.

You should have a piece of string tied around each tree, with the other ends of each string meeting at your marked point.

Time to head outdoors! This simple activity explains the basics of how the Global Positioning System (GPS) works.

outdoor hazard iconSafety: This activity requires you to head outdoors. Ask an adult for permission, and make sure they know where you are. Protect yourself from the sun by wearing a hat, shirt, sunglasses and sunscreen. Wear insect repellent to avoid bites.

You will need

  • Ball of string
  • Pair of scissors
  • 3 trees in a triangle, or 3 tree-alternatives, such as telegraph poles, letterboxes etc.
  • Measuring tape
  • Marker to stick in the ground
  • Friend
  • ‘Treasure’ – for example, a small toy or similar

What to do

  1. Without your friend watching, select a spot somewhere between the three trees. Hide the treasure at this point. Make sure it can’t be easily seen. Mark this point.
    A tree trunk with string tied around it and bundled at the base.

    Bundle the string at the base of the trees so your friend can’t tell where they were laid out.

  2. Place the end of the string at the marker point, then unroll the string as you walk towards one of the trees.
  3. Tie the other end of the string around the tree and cut this piece from the rest of the ball of string.
  4. Repeat this for the other two trees. You should end up with a piece of string tied around each tree, with the other ends of each string meeting at your marked point.
  5. Leave the strings tied to the trees, but bundle the string at the base of the trees so your friend can’t tell where they were laid out.
  6. Remove the marker, but take note of where it was.
  7. Ask your friend to come out. Tell them that you’ve buried some treasure at a point between the three trees, and they need to use your string GPS to find it.
  8. Your friend should use the strings to find the point where you had the marker. There is only one possible point where all three strings will meet.
    The ends of the three pieces of string meeting on the ground.

    There is only one possible point where all the strings will meet.

What’s happening?

This method of finding a point is called ‘trilateration’, and is the same basic method that the Global Positioning System (GPS) uses to locate points on Earth.

Trilateration identifies one single point, based on measurements from three other fixed points.

If you are told a park is 1 km from your home, that won’t help you find it, because you don’t know which direction it is in. In fact, there are a whole lot of points that are 1 km from your home and they form a circle around your home with a radius of 1 km – that is, the distance from your house (the centre of the circle) to any point on the circle is 1 km.

If you were then told the skate park is 2 km from your school, that will narrow things down. You can draw another circle around your school, and the two circles will intersect (cross) at two points.

Add the distance from a third point, for example 1.5 km from your friend’s house, and bingo – you’ve finally got your answer. There’s only one point where three circles with different centres can all intersect.

In the case of GPS, the centres of the circles are actually satellites orbiting the Earth. A GPS receiver will contact satellites orbiting the Earth nearby. Each satellite will tell the receiver how far away they are, and the receiver will then calculate exactly where on Earth it is.

GPS receivers do this a lot faster than your friend with the strings. Fast enough, in fact, that they can accurately collect information about a player’s movements in a soccer game!

Applications

GPS was developed by the US Department of Defense and, through a law created by US Congress, will be provided free “for peaceful civil, commercial and scientific use on a continuous, worldwide basis”.

GPS can be used on land, at sea and in the air. Basically, GPS is usable everywhere except where it’s impossible to receive the signal, such as inside most buildings, in caves and other subterranean locations, and underwater.

Anyone who needs to keep track of where he or she is, to find his or her way to a specified location, or know what direction and how fast he or she is going can benefit from GPS.

GPS is popular in cars and in phones. It can tell people where they are located and suggest the way to get to their destination.

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1 comments

  1. Hi,

    Thanks for this great activity. I can use this with my Bushranger Cadets. I have yet to see it so easily explained.

      Reply

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