Brenden Bobby gets it right when he says “GPS stands for Global Positioning System,” (“Mad About Science,” September 1, 2016), but after that his article is riddled with serious errors: The satellites are not geosynchronous; they do not “stay put;” they don’t have to be within line-of-sight of each other; they do not rely on Wi-Fi, cellular, or any other public network; your phone does not send a request to the satellites; and poor internet service cannot explain poor satellite service.
The GPS satellites fly at about 12,500 miles altitude and circle the earth twice a day. They needn’t see each other to provide a fix; instead, they communicate periodically with ground-based control stations. Nor does a user need a network connection, whether cellular or Wi-Fi, to use the satellites. This is easily demonstrated by my 20-year old Garmin and by the nav system in my new Subaru, neither of which has either. The mapping program in your phone works just fine outside cellular or Wi-Fi range, too, as long as you have the map itself preloaded. A GPS receiver is just a receiver; no talking is required. This magic is accomplished by the receiver listening to a satellite’s signal, which contains encoded time and orbit data. The receiver can determine where the satellite was at a point in time. With a clock in the receiver and similar information from three other satellites the receiver can compute the four unknowns: latitude, longitude, altitude, and time of day. Thank some extremely clever scientists and engineers for making this possible.
Easy to understand descriptions of the system are available from Garmin (the receiver manufacturer) at http://www8.garmin.com/aboutGPS/ or from NASA (the folks that put them there) at http://spaceplace.nasa.gov/gps/en/.
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