3 ms·

I think your conceptualization is a bit off. Time of flight as you describe could work at the transmitter, but could not work at the Lighthouse receiving phot

by beambot 11y ago

I think your conceptualization is a bit off. Time of flight as you describe could work at the transmitter, but could not work at the Lighthouse receiving photodiode. To measure time of flight, you need to know (very precisely) when the light was initially transmitted. The synchronization flash is insufficient for this, as an error of just 1ns results in a 1ft error -- modulation or not. Even with modulation, the receiving photodiode doesn't have another signal with which to compare for ToF measurements. Normal laser rangefinders are transmitting the light (modulated), and looking for the reflected return (modulated) at the same location as the transmitter. They use a PLL to determine the phase difference between the TX and RX (modulated) signals -- since the transmitter has both versions readily at hand. The phase difference corresponds to a time difference => distance. Note that the "reflector" (where the photodiode sits in Lighthouse) is not part of the equation. In other words, laser rangefinder ToF measurements require coherent demodulation at the location of transmission.

nomel 11y ago
This doesn't use time of flight, it's simple "lighthouse was at this angle when diode triggered".
- beambot 11y ago
  I know how Lighthouse works -- I wrote the article. :) I was responding to the GP: "You would be able to get some distance information via the modulation on the signal." I was telling GP, (1) how Lighthouse cannot use time of flight, and (2) how time of flight works for laser rangefinders.
gallamine 11y ago
I wasn't referring to ToF measurement, I was referring to two diodes measuring the difference in phase of a subcarrier intensity modulated signal. Essentially the subcarrier is a intensity modulation of the light. At XX MHz the optical output power will go from 100% to 0% and then back. If two diodes receive the light at the same distance then difference in phase will be zero (i.e. correlating the two signals will have a 0 lag). If the two diodes are separated by a distance, then one diode will receive a greater optical signal and one will receive a smaller optical signal. Correlating the two received signals (or measuring the phase difference) will directly correspond to the difference in distance between the two diodes - this is because the light travels at a fixed speed and the intensity of the signal directly corresponds to the physical distance. Obviously there isn't a unique solution since the diodes could be spaced at multiples of the wavelength, but for a fixed area this isn't a problem. This only requires knowledge of the subcarrier modulation frequency, as any diode can be picked as the reference and the others matched to it. This isn't an ideal way of doing distance measurement because noise will greatly effect the estimate, but it's certainly do-able.

by beambot 11y ago

nomel 11y ago

This doesn't use time of flight, it's simple "lighthouse was at this angle when diode triggered".

beambot 11y ago
I know how Lighthouse works -- I wrote the article. :) I was responding to the GP: "You would be able to get some distance information via the modulation on the signal." I was telling GP, (1) how Lighthouse cannot use time of flight, and (2) how time of flight works for laser rangefinders.

gallamine 11y ago

I wasn't referring to ToF measurement, I was referring to two diodes measuring the difference in phase of a subcarrier intensity modulated signal. Essentially the subcarrier is a intensity modulation of the light. At XX MHz the optical output power will go from 100% to 0% and then back. If two diodes receive the light at the same distance then difference in phase will be zero (i.e. correlating the two signals will have a 0 lag). If the two diodes are separated by a distance, then one diode will receive a greater optical signal and one will receive a smaller optical signal. Correlating the two received signals (or measuring the phase difference) will directly correspond to the difference in distance between the two diodes - this is because the light travels at a fixed speed and the intensity of the signal directly corresponds to the physical distance. Obviously there isn't a unique solution since the diodes could be spaced at multiples of the wavelength, but for a fixed area this isn't a problem. This only requires knowledge of the subcarrier modulation frequency, as any diode can be picked as the reference and the others matched to it. This isn't an ideal way of doing distance measurement because noise will greatly effect the estimate, but it's certainly do-able.