Re: Relative Positioning

Gordon McComb wrote:
mattrapoport@xxxxxxxxx wrote:
If I take an RF transmitter (A) and an RF receiver (B) and I precisely
record the time it takes for the the signal to get from A to B, I
wonder how accurate the distance measurement would be. If I moved A
1mm away from B, would the recorded time accurately represent the
change in distance?

You'd probably want to use a modulated RF signal, and compare the phase
of the returned signal (from another sensor) with the original. This is
how some surveyors transits work. The circuitry is simpler, as you are
not measuring speed of light. You could probably achieve 1mm accuracy
depending on the frequencies used. How to make all this so that each
sensor is stand-alone and independent is another matter. If you use more
than two sensors you need a way of timing the pulses from each. There
will need to be some coordination between the sensors. There are other
problems to overcome that are left to the OP.

-- Gordon

I built a LIDAR that way once, around 1990. Surprisingly similar to
an FM radio, IF cans and all. But nobody does it that
way any more. (Well, the Swiss Ranger does, which allows them to use
LEDs as the light source). Today, you usually just count, really fast.
Here's a Bosch laser range finder for $129:
"http://www.amazon.com/Bosch-DLR165K-Laser-Rangefinder-Kit/dp/B000T7LISM";.
Accuracy is 1-2 mm.

The main problem with working in this area is that while the
parts are cheap (they're derived from cell phone technology),
the test instruments required are expensive.

If you want to work in this area, you'll need shelves full
of RF engineering books, some really good RF test gear, and the
software for designing boards with gigahertz transmission lines.
There are advanced radio hams who do such things.

Look up "A UWB based Localization System for Indoor Robot Navigation"
for some hints on current (2007) thinking on the subject.

This is a basically good idea which takes significant engineering
effort to make work, but probably won't be expensive to replicate.

John Nagle
.