The tellurometer was the first successful microwave electronic distance measurement equipment. The name derives from the Latin tellus, meaning Earth.
The original tellurometer, known as the Micro-Distancer MRA 1, was introduced in 1957. It was invented by Dr. Trevor Lloyd Wadley of the Telecommunications Research Laboratory of the South African Council for Scientific and Industrial Research (CSIR), also responsible for the Wadley Loop receiver, which allowed precision tuning over wide bands, a task that had previously required switching out multiple crystals.
The tellurometer emits a microwave-frequency radio wave: the remote station reradiates the incoming wave in a similar wave of more complex modulation, and the resulting phase shift is a measure of the distance travelled. The results appear on a cathode ray tube with circular sweep. The instrument penetrates haze and mist in daylight or darkness and has a normal range of 30–50 km but can extend up to 70 km.
The tellurometer design yields high accuracy distance measurements over geodetic distances, but it is also useful for second order survey work, especially in areas where the terrain was rough and/or the temperatures extreme.
Examples of remote locations mapped using Tellurometer surveys are Adams Bluff, Churchill Mountains, Cook Mountains, Jacobsen Glacier, Mount Albright, Mount Predoehl, Mount Summerson, Sherwin Peak and Vogt Peak.
They were used by the Army of the Republic of Vietnam in the late 1960s.
Plessey, the British electronics company, formed a new subsidiary known as Tellurometer (Pty) Limited in the 1960s to manufacture the product and to develop and sell derivatives. The Company subsequently introduced numerical displays, solid state transmitters, integrated circuits and eventually microprocessors for the product.