How the global positioning system works

What the Activity is for

This is a class activity, making a physical model of the GPS that uses three measurements to plots positions.

What to Prepare

• 3 lengths of rope, red, green and blue, knotted at 0.5 metre intervals
• a space approximately 3 metre square in the middle of the classroom
• drawing compasses, pencil and paper

What Happens During this Activity

Explain that you can find your place in two dimensions by knowing how long three signals take to reach you. Place three pupils around the perimeter of the open area, with firm instructions not to move. They will be the red, green and blue satellites. Their job is to pay out the red, or blue, or green rope, keeping it taut. Appoint one pupil as the explorer, he or she enters the open square holding one end of each of the knotted ropes. The other end is held by the appropriate satellite.

Instruct the explorer to take a fix every so often (3 to 7 fixes might be appropriate, depending on the patience and ability of the class), as they move around the space. To do this they need only count the number of knots of rope between them and the satellite. This represents the trip time for a signal from the satellite. A recorder writes down these values.

Then the class can reconstruct the journey using the fixed locations of the three satellites and a pair of compasses. You might like to prepare a sheet containing the fixed locations, with a scale for the knots on the bottom of the sheet, to allow different attempts to be overlaid for easy comparison.

The brave will note that this whole experiment can be scaled up to work outside.

Further questions to explore with the class:

• Does it matter where the satellites are, so long as we know where they are? [no]
• What happens if the satellites move? [model it with one – all readings from that satellite drift]
• What happens if the clocks in the satellites do not send out their signals at the same time? [model it with one – all readings from that satellite gain or lose an extra metre of rope]
• What do we need to know in order to fix the explorer's position more accurately? [more knots in the rope, to be able to work out the times more accurately]