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What nuclear particles and general relativity has to do with your GPS? [Science]
posted February 12 2013 21:55.44 by Giorgos Lazaridis

Nuclear particles? What has to do with my GPS? One may ask. Well, first lets see how a GPS works. Any GPS needs at least 4 GPS satellites to operate. 3 of these satellites transmit a regular signal of their position in space, and the fourth satellite transmits a time signal. Your GPS will then trilaterate the position using these signals. But the time signal has to be very accurate, accurate down to the nano-second! And there is indeed one time-base generator that can achieve such an accuracy, the atomic clock. Using a canon that fires radioactive Cesium, the atomic clock can achieve extremely high accuracies. The rotation of the Earth itself actually is less precise than modern atomic clocks used as time standards.

Image: Global Positioning System (GPS) Time Dissemination for Real-Time Applications by PETER H. DANA [link]

And what about general relativity? Einstein proved that time is not a constant through the universe. A clock that is placed in a higher altitude will run faster than a clock on the surface of earth. This effect is called "Gravitational time dilation", according to which, the lower the gravitational potential (the closer the clock is to the source of gravitation), the more slowly time passes, and the higher the gravitational potential (the furthest the clock is to the source of gravitation) the fastest time passes. Therefore, due to general relativity and gravitational time dilation, the clock on the GPS satellites has to be constantly compensated. Leaving the clock without this correction will result in positioning errors within 2 minutes, and every day this error will increase by 10km! Each nanosecond of time-error results into 0.3meter of distance-error. Each year, the clock of a GPS satellite is corrected by about 12 seconds! This would have resulted in a distance-error of roughly the size of Europe, from East to West!

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