Smithsonian - National Museum of American History, Behring Center

Physical Sciences Collection - Surveying and Geodesy

Browse Makers | Browse Instruments | Index


No image available

EDM (Tellurometer M/RA 1)

Catalogue number:



The first successful microwave EDM was invented by Col. Harry A. Baumann of the South African Trigonometrical Survey, developed by Trevor Lloyd Wadley of the Telecommunications Research Laboratory of the South African Council for Scientific and Industrial Research (CSIR), and manufactured by Tellurometer Pty. Ltd. in Cape Town. The Tellurometer was designed to yield geodetic accuracy over geodetic distances, but it was also useful for second order work, especially in areas where the terrain was rough and/or the temperatures extreme.

The original Tellurometer, known as the Micro-Distancer M/RA 1, used a continuous wave at 3,000 megahertz, modulated by 10 megahertz and three other nearby frequencies. The remote station reradiated the incoming wave in a similar wave of more complex modulation, and the resulting phase shift was a measure of the distance traveled. The results appeared on a cathode ray tube with circular sweep. This instrument could penetrate haze and mist in daylight or darkness, and had a normal range of 30-50 km. It was covered by patents in at least eleven countries granted to Wadley and assigned to the CSIR; the U.S. patent (#2,907,999) was entitled "Determining Relative Position by Means of Transit Time of Waves."

Further Information: