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.