A Global Positioning System (GPS) receiver is a device that lets you figure out where you are by receiving timed radio signals from satellites. It works by measuring the travel time for the signals, which is related to the distance between you and the satellite. By finding the ranges to several different satellites in this way, it can pin down your location in three dimensions to within a few meters.

How accurate does the measurement of the time delay have to be in units of nanoseconds to determine your position to 20 m accuracy? 1 s = 10^9 ns