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Tone modulated laser ranging

Nabla1

Feb 20, 2012
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Feb 20, 2012
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Hi, I was hoping that someone could explain to me the exact method of laser tone modulate ranging. (Otherwise known as beam modulation telemetry.

Essentially, I understand we need the path length of the wave to calculate the range (Half the path length), and we know the speed of light so, we can calculte the path length based on the time of flight.

However, this method modulates a laser, to give a continuous wave, and apparently calculates the range from the phase difference between the transmitted and received waves.

This however is ambiguous, since the path length is given by the phase difference PLUS the integer number of wavelengths of the modulating signal. So measuring phase difference alone tells us nothing about the number of wavelengths, and hence only gives us a "fine" measurement, which is useless without the number of wavelengths.

I'm certain that the integer number of wavelengths must also be measured somehow, but nothing I've read mentions this, and they just give a formula for range purely based on the modulating frequency and measured phase difference:

R = (c * dP) / (4 * pi * f_mod)

where:
R = range
c = speed of light
dP = measured phase difference
f_mod = frequency of modulating/measuring wave
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Following is a layman's guess.

For a single frequency, a particular phase variation points to a distance that is determined by the period of the modulating frequency.

If you use multiple frequencies you will have 2 sets of these potential distances. Only where they within some delta corresponding to your ability to resolve phase differences will you have a potential distance.

Choosing frequencies carefully you can eliminate a large number of the possibilities.

Knowing the signal attenuation, you can determine a maximum distance (since if this distance was exceeded you could not reliably detect the reflected signal.
 
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