The one-way ranging used in GPS is more complicated. It requires the use of two clocks. The broadcast signals from the satellites are collected by the receiver, not reflected. Nevertheless, in general terms, the full time elapsed between the instant a GPS signal leaves a satellite and arrives at a receiver, multiplied by the speed of light, is the distance between them. Unlike the wave generated by an EDM, a GPS signal cannot be analyzed at its point of origin. The measurement of the elapsed time between the signal’s transmission by the satellite and its arrival at the receiver requires two clocks, one in the satellite and one in the receiver. This complication is compounded because to correctly represent the distance between them, these two clocks would need to be perfectly synchronized with one another. Since such perfect synchronization is physically impossible, the problem is addressed mathematically.

In the image, the basis of the calculation of a range measured from a GPS receiver to the satellite, ρ, is the multiplication of the time elapsed between a signal’s transmission and reception, Δt, by the speed of light, c. A discrepancy of 1 microsecond, 1 millionth of a second, from perfect synchronization, between the clock aboard the GPS satellite and the clock in the receiver can create a range error of 300 meters, far beyond the acceptable limits for nearly all surveying work.

GPS ranging cannot take advantage of a two-way system. The signal comes from the GPS satellite and it goes down to the receiver. Nevertheless, the same measurement concept is used. The elapsed time and an electromagnetic signal of constant frequency are still the key components of determining the distance (the range). Of course, measuring the elapsed time perfectly would require that the clock, the oscillator, in the GPS satellite and the clock in the GPS receiver would have to be perfectly synchronized. That's awfully difficult to do.

So, you can see that this sort of perfect synchronization is not in the cards, so we have to solve for time with GPS.