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>How to measure distance with lasers?
>I've been thinking a lot about the problem, and my conclution is:

>  It's impossible  (at least with submillimeter accuracy)

>Still it's done. And it's done by cheap devices...

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Light travels about one foot per nanosecond, so a laser rangefinder 
with a 1' resolution would need a 500-MHz timebase if it works by 
measuring the time it takes for the light to return from the target.

Laser rangefinders which measures in micro-millimeters over ranges of 
100m or less use optical interferometry techniques.

For longer distances, as in surveying, one way to do it precisely 
without direct counting is the same principle used in time-domain 
reflectometers for inspecting transmission lines.  It's a sampling 
technique, a form of correlation; simple concept but hard to 
describe-- I saw a good explanation in the instruction book for 
a Tektronix sampling oscilloscope.

Another way to do it, used in some surveying instruments, is to 
transmit a continuous light beam from a solid-state laser (infrared), 
which is modulated with a sinewave RF carrier having precisely 
controlled frequency, say, 10 MHz, which has a wavelength of 30 
meters.  The receiver has a phase detector which compares outgoing and 
incoming phase of the carrier. The same solution occurs each 
wavelength, so the instrument uses other frequencies to solve the 
ambiguity.  It's all taken care of by the on-board computer, which 
measures the distance several times and indicates an error if the 
variance is too great (which could be caused by target movement, air 
turbulence, etc.).

Before lasers and solid-state stuff, there were vacuum-tube microwave 
devices for measuring long distances in surveying. They were manually-
operated, some with internal oscilloscopes.  A transponder was placed 
at the distant point. Operation was tedious, followed by laborious 
mathematical data reduction (but much faster and easier than measuring 
several miles with a tape).

--

Frank Reid
reid@gold.bacs.indiana.edu
PZZ