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
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