Follow these instructions to make a basic GPS and create a treasure hunt for your friends.

You will need

• ball of string
• three trees in a triangle, or three tree-alternatives, such as telegraph poles, letterboxes, stakes, or fence posts
• measuring tape
• marker to act as 'treasure'
• a friend.

What to do

1. A Global Positioning System (GPS) uses trilateration to locate points on Earth, it identifies one single point, based on measurements from three other fixed points (satellites)

Without your friend watching, select a spot somewhere between the three trees. Mark this point.
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, and hide the treasure at this point.

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 skateboard park is 1 km from your home, that won't help you find it, because you don't know which direction it is. 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 1km.

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!