In order to measure the distance between stations A and B (Figure 10.3) , let an EDM be placed at A and a reflector be at B. The modulated beam transmitted from A travels to B and returns back. Let the received signal is out of phase from the transmitted signal by 
f, as measured by a phase meter. The equivalent linear distance, d is
Thus, the distance (D) between the stations is
where n is the integral number of wavelength, l in the double path.
The ambiguity of n is resolved commonly in EDM instruments by employing multiple-frequency technique based on decade modulation. It is usually built into the EDM device, permitting automatic determination of n and thus a direct readout of the distance.