Distance measurement with Rigol


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So ya wanna measure distance with 4 cheap radios & a cheap SRAM to measure propagation delay.  Before investing in a full system, let's try it with a laser reflection & a Rigol.

A laser diode is attached directly to a microcontroller pin via the shortest path possible.  It'll get 3.3V at 50% duty cycle, so it won't burn out while still delivering useful brightness.


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The laser rise time is given by the fall time on the cathode, which is extremely short.  There's no way to measure how long it takes to light up, in this apartment.


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Next, we have a photodiode receiving reflected laser light from various distances.

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A very carefully, painfully aligned mirror reflects the laser.  Should have used the heavier tripod for this.

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With the mirror 1ft away, the Rigol now becomes the Rigol of despair, as the rise time of the photodiode is nowhere close enough to the rise time of the laser to see a delay on the screen.

What if you take the 1/2 way points of the 2 rises?  That would get it to maybe 100ns, a distance of 30 meters.

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With the laser moved 15ft away, now the rise time of the photodiode is 4 times longer than before.  It has too much capacitance, causing the half way point to be more correlated with signal strength than propagation delay.


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Maybe a very low resistance pulling down the photodiode & a very high amplification would get rid of enough capacitance.

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The mighty capacitance fighting resistor did get the rise time down to a more useful range at 15ft, but still very erratic.  There still might be useful data with extreme averaging & the higher quality amplifier in a radio.

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At 1ft, there's no obvious change in propagation delay with the noise. 



A cheap radio would have automatic gain control & might have less capacitance, but the problems of longer rise time with a smaller signal & erratic half way point in the waveform would probably still be too great.

Another idea was to hack a laser tape measure to use a radio for part of the signal propagation. The same problems of capacitance & erratic rise time would apply.

The kind of components it would take to measure propagation delay of RF are going to be out of reach for a reasonable cost.  You'd think someone would invent a local area GPS system, which used a different frequency, but used the same components to have a cheap GPS system in a room.

Over the years, the ability to detect a 1/115200 second time difference with a simple chip radio has led to the idea of measuring distance with low cost components. A pair of radios operating on different frequencies & hard wired so 1 directly transmitted the voltage received by the other could theoretically allow the propagation time of a voltage change to & from the aircraft to be measured. It would use a bank of staggered comparators, all timed by a slower clock, to detect very small differences in time.

10 staggered comparators timed at 100Mhz resolution would have 30cm accuracy. Averaging hundreds of samples might get it down to 3cm. It really depends on how noisy the signal is. Rough knowledge of RF communication says the lower the bandwidth, the less precisely the arrival time of a signal should be known. If it has 256kbit of bandwidth, the arrival time should only be known to 1/256000 seconds. But GPS has only 1megabit of bandwidth, so that shouldn't be a problem.

The Rigol & a set of 4 radios could do a simple proof of concept, up to 3 meter accuracy.


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Anyways, the schedule is now to convert 1 new aircraft to autopilot every month, combined with several days of formalities required to run a business: meetings & traveling.  It's extremely ambitious & doesn't leave any time for experimenting.
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Comments

  • 100KM
    Jack,
    As usual, nice work. Have you tried a photo-resistor hooked up to a transistor in saturation mode ? Like what you see in fiber optics?
    Wayne
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