GPS - Global Positioning System
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So, how does it work? The Nova* site gives a simplified explanation. For a more detailed explanation, check out Trimble's GPS tutorial*. In order for it to work, the satellites have to keep extremely accurate time, exact to a billionth of a second. How do they do this? By using atomic clocks. Why is it not necessary for the receiver to have an atomic clock, which would make the price of using the system prohibitive? A neat little trick of using a fourth satellite allows for the use of a quartz clock in the receiver. The positions of the satellite must also be known accurately. The Department of Defense monitors the satellites twice daily and measures their speed, altitude and position precisely.
After going through the two above sites, you may like to check out Peter H. Dana's GPS overview. This is a more in-depth discussion of GPS.
GPS is not just used as a navigational tool. Scientists use it for weather forecasting, surveying and in earthquake studies. The NASA web site explains how GPS can be use to measure earthquakes. Three activities at the end of the articles allow you to further explore the concepts.
A new online activity, called GeoCaching, is fun, educational and great exercise. Groups or individuals make caches (a collection of small items contained in a waterproof container) and hide them outside. Check out - Guide to Creating and Hiding a Cache - for tips on how to do this. Using a GPS receiver, record the cache's coordinates. Then post the coordinates of your cache on the geocaching web site for others to find. Have fun.
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