In modern GNSS, the position of the user is estimated as the intersection of three or more spheres whose center is the known position of a satellite and whose radius is given by the velocity of light multiplied by the travel time of the satellite signal, knowing when it started and measuring, by the local receiver, when it arrived.
Since the light velocity is a very large number, it is sufficient to allow an error of 1ns ($10^{−9}$ s) to obtain at least 30cm. of error in the estimated position. A time error of one nanosecond between a space-based to ground clock is difficult to achieve; an error of 100ns is more common, which would give a positioning error of 30m. This gives an understanding of why time metrology is vitally important in navigation.
