Activity: How Global Positioning System (GPS) works

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

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

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

There are lots of places that are 1km from your house.

With two reference points, the possibilities are narrowed down to two - but which is the right spot?

With three reference points, there's only one option. We've found the skateboard park!

You will need

• A ball of string
• Three trees in a triangle, or three tree-alternatives, such as telegraph poles, letterboxes, stakes, or fenceposts
• A measuring tape
• A marker to stick in the ground, a stick will work fine.
• A friend (or three)

What to do – preparation

1. 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, but take note of where it was.

What to do – treasure hunt

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 of the strings will meet.
9. Don’t forget to hide some treasure or, in GPS terminology, a cache.

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 1km 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 1km from your home and they form a circle around your home with a radius of 1km – 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 2km 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 1.5km 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 the global positioning system.

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

A company called uLocate Communications in the United States connects mobile phones, the Internet and GPS technology to allow parents to pinpoint their child’s location at all times. The mobile phone that is given to the child sends the map coordinates to a computer that allows the location to be viewed on a map by somebody at home. Big Brother has just become big mother!

For those who like the thrill of adventure, you could try Geocaching. Using your own GPS receiver, Geocaching involves hiding a cache filled with small prizes or treasures. The geographical coordinates of the cache are first recorded via a hand-held GPS receiver. These coordinates are then posted on the web. Often clues are provided as to the location of the cache. Then the treasure hunt is on to locate the concealed cache.

The basic idea is to have individuals and organisations set up caches all over the world and share the locations on the Internet. The caches could contain maps, books, software, hardware, CDs, videos, pictures, money, jewelry, tickets, antiques, tools or games. Preferably, such items would be wrapped in a way as to be protected from the elements.

More information

Geocaching - a game of hide and seek via satellite can be found on several sites, such as: http://geocaching.com.au/ or just type Geocaching into your search engine.