Re: Distance Measurement Device

The tellurometer, invented in the 1950s I think, was an electronic
distance measuring instrument for surveyors. It used microwave beams
transmitted between instruments at each end of the line. Max range was
50+ km and in favorable conditions the devices were accurate to a few
parts per million.

Laser instruments displaced tellurometers many years ago, but you can
still find them from time to time on eBay. I got an early 1980s set
that way.

All tellurometers use phase comparison rather than measuring time of
flight directly like a radar. That is, a continuous microwave beam is
modulated by a crystal-controlled oscillator, usually at 7.5 or 10
MHz. The remote unit at the far end of the line re-transmits the
signal back to the master, where the modulation is recovered and its
phase compared to the oscillator.

Of course, one phase measurement only tells you the fractional
wavelength. You don't know how many complete cycles are between you
and the remote unit. The same reading occurs every 20 meters (for 7.5
MHz modulation). The solution is to use several different modulating
frequencies to resolve the ambiguity.

The microwave carrier is around 3, 10, or 18 GHz, depending on the
model. The 10 GHz tellurometers have been used by hams since its
carrier falls in one the amateur bands. A precise carrier frequency is
not necessary; it's the modulation frequency that determines the
instrument's accuracy.

All tellurometers, even the early models, have an interesting feature:
voice communication over the microwave beam. Back in the day, that
must have come in handy when measuring lines many miles long between
mountain peaks.

http://www.gmat.unsw.edu.au/currentstudents/ug/projects/f_pall/html/edm.html
http://www.photolib.noaa.gov/historic/c&gs/theb1683.htm
http://www.photolib.noaa.gov/historic/c&gs/theb1685.htm

--
Paul Hirose <jvcmz89uwf@xxxxxxxxxxxxxxxxxxxx>
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