Electronic Surveying: Electronic Distance Measurements

Electronic instruments for the precise determination of short distances are well known, but the electronic basis for their operation has never been clearly outlined in a single publication. Details regarding the electronic circuits used in these instruments are reviewed and compared to indicate the similarities and the differences in the various methods of determining distance from a measurement of time. The relation between the travel time and the phase of a periodic signal is indicated. Methods for eliminating electronic zero errors and the use of signals with different frequencies to eliminate ambiguity in the measured distances are described. The effect of atmospheric refraction on the velocity of propagation and different methods for including corrections for atmospheric conditions are reviewed. Relative advantages of microwave and optical systems are compared and recommendations are suggested for an ideal instrument combining the best features of both types.