Re: Relative Positioning

Hi Matt,

in principle you are right, the time difference would represent the change
in distance divided by the speed of light. So 1mm difference would
correspond to approximately 3ps time difference.

There are several sources of inaccuracies:
- the speed of light depends on the medium. In air, it's close to vacuum but
not quite, and depends somewhat on temperature, moisture, pressure etc.
- the circuits/equipment you use to measure such small time differences will
not be perfectly accurate, depending on how much you would want to spend on
this. In addition to initial inaccuracy (which you could calibrate out) the
errors might also depend on temperature, supply voltage, etc.
- the phase shift in the cables that connect the antennas to the equipment
often depends on e.g. bends in the cable and so on (unless you want to spend
many hundreds of dollars per yard of cable). So moving the antennas (and
therefore the cables) around might influence the measurement results
- at short distances (in the order of one wavelength or less) the coupling
between the antennas becomes somewhat complicated and involves other modes
than TEM waves - which will result in other phase shifts/time differences
than you would expect based on simple radio wave propagation
- however, the most important source of inaccuracies is probably the
reflections of radio waves off other objects and so multiple copies of the
radio signal will reach the receive antenna along different paths (this is
called multipath) and with different delays - very similar to what happens
with echo's and sound.

So, in summary, yes it will work, but it is not quite so easy and/or
accurate as you might imagine - sorry...

Peter

<mattrapoport@xxxxxxxxx> wrote in message
news:f213ea81-0546-4397-aa5f-53e4b9a28368@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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?

Thanks,

Matt



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