What is it?
Originally developed for identifying exat locations on Earth for the militarty, this a high cost, high technology solution.
However, once the satellite infrastructure is in place and the receiving units have gone into bulk production, the cost to the individual is really fairly trivial.
Many satellites are in low Earth polar orbits and they each transmit a time signal on a specific radio frequency. The
receivers pick up these signals and track them. By measuring the differences in the time that each time signal is received it is possible to calculate the position of
the receiver in terms of latitude, longitude and altitude.
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