Each GPS satellite has atomic clocks on board. Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver.

The GPS receiver is located at sphere surfaces centered at each satellite.

So the location is at the intersection for those three sphere surface.

The intersection for two sphere surface is a circle.

And the intersection for the above circle and the other sphere surface can be at two intersect points. However, only one of the point is on the earth surface (the other one will be on the outer space, not on the earth surface). You unique location can be determined.

This following applet shows how the GPS works. It needs at least three satellites.

You can drag those three blue dots to change the location for satellite.

Check "satellite 1/2" two display intersection for two satellite (yellow circle).

Then un-check "satellite 1/2" to find the intersection between circle and the third sphere surface (the red dot).

I hope this applet can help you understand how satellite works.

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**Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop.**This work is licensed under a Creative Commons Attribution 2.5 Taiwan License

- Please feel free to post your ideas about how to use the simulation for better teaching and learning.
- Post questions to be asked to help students to think, to explore.
- Upload worksheets as attached files to share with more users.

http://www.youtube.com/watch?v=_8P3DrKFjYc