Abstract: Electromagnetic distance measurement, preferably utilizing optical signals. Signals from a main transmitter are transmitted from a first position to a second position and returned to the first position and simultaneously from the main transmitter along a first fixed length path. Two separate receiving channels are provided to receive the two signals. The phase difference between the two received signals is determined. A further auxiliary transmitter is provided and signals are subsequently transmitted from the auxiliary transmitter along second and third fixed length paths to the first and second receiving channels respectively. The phase difference between the received second and third path signals is determined, and the difference between the two phase differences obtained. This difference is then solely dependent on the distance between the two positions and the lengths of the first, second and third paths.

Abstract: This invention relates to a method and apparatus for determining the distance between two positions utilizing electromagnetic waves (EMW's) and the Tellurometer principle. However, instead of utilizing a separate auxiliary EMW at the remote position for each measuring EMW transmitted from the master position a single auxiliary EMW is generated at the remote position and this single auxiliary EMW is utilized with several measuring EMW's. The measuring EMW's have frequencies that differ from that of the auxiliary EMW by similar amounts. Further, the measuring EMW's have frequencies of suitable values so that pattern frequencies and their phase shifts can be derived directly from some of the measuring EMW's and from the difference between appropriate measuring EMW's. The invention is extended, where a large number of measuring EMW's are needed to provide sufficient pattern frequencies for the degree of accuracy required, by providing a plurality of auxiliary EMW's at suitably different frequencies.