You might want to look at 'radio altimeter' methods.
A swept RF frequency produces a beat frequency proportional to
distance of the reflecting source. With an additional heterodyne
down to a low IF frequency you could probably use the PIC
timers directly in accumulate mode.

The old Hewlett Packard survey units used an RF modulated
LED to determine range by measuring the phase shift in the
detected retroreflected signal.

Both of these methods avoid the use of 'measuring nanosecods.'

Robert

David Minkler wrote:
>
> Robert,
>
> More like 6 or 7 ns for a radar meter.  c is ballpark of 30cm/ns and it
> has to go both ways for radio ranging applications.
>
> Dave
>
> Robert B. wrote:
>
> >The application (so far) is just a thought experiment regarding
> >medium-distance ranging accurate to within a meter (about 1ns for
> >speed-of-light radio reflections), but it's something I'm pretty fascinated
> >with and if I can come up with a viable system I'd love to build it as an
> >educational project.

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