Teaching Physics with the Physics Suite

Edward F. Redish

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Speed of light and the GPS system

Although light appears to travel at a speed that is for all practical purposes infinite, for some modern purposes the time delay due to light travel time is of great importance. The Global Positioning System (GPS) allows you to determine your position from comparison of the time delays between radio signals from 4 satellites at a height of 20,000 km above the surface of the earth. (There are actually 24 of these satellites. Your GPS picks out the closest 4 to your current position.) In order to get some idea of how important the speed of light is in establishing your position with one of these gadgets, let's make some simple assumptions. Assume a satellite is almost directly overhead. Then let's figure out how far the satellite will move in the time it takes light (the radio signal) to get from the satellite to your GPS receiver. This estimates how far off the reading of your position would be if your device didn't include the speed of light in its calculations. To do this:

  1. Figure out what speed the satellite must be going in to be in a circular orbit.

  2. Estimate the time it would take for a radio signal to get from the satellite to your receiver.

  3. Estimate how far the satellite would move in that time. If you ignored light travel time, this tells about how wrong you would get the satellites postion (and therefore how wrong you would get your position).

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Page last modified October 23, 2002: OP02