Jack Smith wrote:
> Clever idea. Most AM radio stations should have decent short-term phase
> stability. From my experience in the industry, it's quite common to see
> the frequency to stay within 0.5 Hz of the assigned frequency for days or
> even weeks at a time and when the frequency moved, it was a very slow
> drift, like 0.1 Hz/day.

But even small short term phase noise can be a problem.  That's why the base
station records the same data as the mobile units.  Any frequency or slow
phase drift is factored out.

> Some of the technical problems are interesting. You have to strip off the
> sidebands to measure the carrier, so perhaps a limiter as found in an FM
> receiver might be used. Or, PLL to the carrier with a long time constant
> and measure the PLL.

You really don't need anything very complicated.  The effect of the side
bands is just slow (compared to the carrier frequency) change in the carrier
amplitude.  The amplitude isn't important anyway, as long as the signal is
strong enough in the first place.  All you are looking for are the carrier
zero crossings.

> If you were going to use this technique for long distance measurements, I
> can see quite a few other difficulties, but for distances of a few hundred
> yards, those wouldn't be a problem.

I think it would work reasonably well for a few miles from the base station
before location got a bit distorted.  Of course if you can pass a known
location once or twice with the mobile unit, then the correctable range can
be greatly improved.  For example, mapping a 10 mile loop into a wilderness
area may have some error at the far distances, but a 10 mile trail that ends
at a known location should be quite accurate because you've got known anchor
points at each end.

> (To take one, you are measuring from
> the phase center of the antenna. In a directional array, that changes with
> azimuth. But if you are a few miles from the station and only move a short
> distance, you can take the phase center as a constant location.)

Most AM antennas are just big sticks in the sky.  A list of known
uni-directional stations would help.  There aren't that many of them that a
complete list in some PROM would be a problem.  Even so, as you said, the
phase center of a directional antenna isn't going to change much compared to
the typical distance you are from the antenna.


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