These operate on the principle that transmitter at the master station sends modulated continuous carrier wave to a receiver at the remote station from which it is returned. The instruments measure slope distance D between transmitter and receiver. It is done by modulating the continues carrier wave of different frequencies and the measuring the phases difference at the master station between the outgoing and incoming signals.
Let the distance between P and Q be D which is to be measured. A wave is transmitted from the transmitter at station ‘p’ with certain phase angle. There is reflector at the other end Q which consists of the prism. The wave strikes on reflection at Q and then gets reflected from Q.
It is received back at the transmitting end at ‘P’ with a different phase. For finding the distance, the phase difference between transmitted wave and the reflected wave is measured and converted into distance.
Working of E.D.M