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

Bill Hawkins bill at iaxs.net
Tue Jul 4 13:18:34 EDT 2006


> 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.

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

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?

Regards,
Bill Hawkins




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