[time-nuts] Dithering vs. locking all the clocks to the OCXO?

[Magnus Danielson]

From: "Bill Hawkins" <bill at iaxs.net>
Subject: Re: [time-nuts] Dithering vs. locking all the clocks to the OCXO?
Date: Tue, 4 Jul 2006 12:18:34 -0500
Message-ID: <006901c69f8d$e1616760$0500a8c0 at darius.domain.actdsltmp>

> > However, a quick guess would be the delay caused by atmospheric effects (I
> > don't think thermal noise would play a big role since the antenna is
> looking
> > straight up).
> 
> Seems to me that thermal noise depends only on the resistive
> impedance of the antenna and input circuit. See Johnson
> noise. Orientation of the antenna would affect reception of
> external noise radiation. Don't think cosmic background
> radiation is a problem for the usual GPS ground antenna.

There will indeed be a difference due to orientation, as will the weather.
If you have unobstructive clear sky condition by night, you will have the
background temperature of the universe, about 2,7 K. If you have clouds, they
are way hotter and thus will contribute more noise as a result. This is the
difference between direct parabola antennas and offset parabolas, the edges
contribute less noise into the antenna in the offset antenna since it sees
clear sky - if you mounted it properly.

> What causes the "atmospheric effects"? I'd expect radiation to
> slow down a bit as it passes through water.

Not much. One of the interested parties was the US marine, and their vessels
see a fair amount of water. The selection of the L-band was done with this in
mind.

> Are there heat
> effects as well? Does the density of the atmosphere change
> enough to make a 10E-10 second delay possible? If so, it
> seems there would be non-negligible changes due to turbulence
> over a kilometer or so.

The really big error is the ionosphere, where the amount of ionised atoms will
cause a shift in phase.

> I have two each HP 58532A antenna, 58535A distribution amplifier,
> and Z3801A receiver connected to a laptop running a common (not
> HP) program that provides receiver status reports. The antennae
> are mounted 4 feet apart on a sturdy pole (6 inch plastic pipe).
> There are nearby trees above the pole. One has to live with one's
> mate when it comes to unnatural things that are visible from the
> street.
> 
> I ran it for a while and got diverted to other matters, but I
> remember seeing relative variations in altitude and position.
> A Racal-Dana 1992 counter set for phase angle showed a constantly
> increasing phase difference between the receivers, with readings
> taken hours apart. I have not automated data collection yet.
> 
> Is this setup able to quantify atmospheric effects or are there
> too many system errors?

For me it sounds like you are measuring difference in local reflections and
obstructions.

Cheers,
Magnus