Kids' Hangout

Budding Geographers

Q: How does a GPS work?

A: You've played Marco Polo, haven't you?  Close your eyes, your friend shouts "Marco!" and you try to figure out where the sound is coming from. Which direction, how far. And try to tag your friend with your eyes closed.

Now let's put a twist on that game. Pretend you had 4 friends, one in each corner of a room, each one singing a different song. You're blindfolded, but you've been told which friend is in which corner. Then you'd be able to walk around and, by listening to the sounds, figure out in which part of the room you are. If you hear Ben loudest, and he's right by the door, then you must be by the door. If you hear Jill, and you know she's standing by the closet, then you must be near the closet. If everyone's equally loud, you must be in the center of the room. Easy enough?

GPS works the same way. It's a Global Positioning System, so it works anywhere in the world. There are about 30 satellites in the sky, put there just for GPS. Each satellite sings a sort of tune, and the GPS listens. (Ok, they're radio waves, not tunes, but they work the same way). The GPS usually needs to hear 4 or 5 satellites and it can figure out where it is. It's quite accurate, give or take 10 metres with a $100 unit. So it knows whether it's in your yard or your neighbor's yard. But if you carried it in your right hand instead of your left hand, it couldn't tell the difference.

A GPS can get lost if it can't hear the tunes. Radio waves travel in straight lines, and they don't pass through metal or rock easily. So in a tunnel, or downtown surrounded by tall buildings, or even under heavy tree cover, a GPS can lose its position.

A GPS reports location using latitude and longitude. Latitude and longitude work like a graph. Longitude is like x, it runs from left (-180°) to right (+180°) on the map. Latitude is like y, it runs from the bottom (-90°, south pole) to the top (+90°, north pole). Every second or so, the GPS plots a point on that graph. By joining the dots, you can draw maps.

A GPS also reports time—very accurately. So it can calculate your speed. In fact the speed reading from a GPS is ±2 km/h, while the speedometer on your car can be off by ±10 km/h.

At present all the GPS satellites in the sky are owned by the United States and Russia. The Russian ones are called GLONASS. The Europeans are launching their own satellite system (Galileo), and other countries like Japan and India are planning some special purpose satellites too. If you're planning a space walk any time soon, you just might bump into one of them.

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