[time-nuts] Cs stability

Magnus Danielson magnus at rubidium.dyndns.org
Tue Jul 17 09:24:19 EDT 2007

From: Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz>
Subject: Re: [time-nuts] Cs stability
Date: Tue, 17 Jul 2007 21:30:03 +1200
Message-ID: <469C8C1B.50807 at xtra.co.nz>

> ); SAEximRunCond expanded to false
> Errors-To: time-nuts-bounces+magnus=rubidium.dyndns.org+magnus=rubidium.dyndns.org at febo.com
> Tom Van Baak wrote:
> >> I guess that is why some GPS antenna cables is temperature-stabilized as well
> >> as the cement-pidestal for the GPS antenna as it stands on solid rock. The same
> >> place have controlled environment for the cesiums and hydrogens, together with
> >> UPS and disel-engine that kicks in for longer runs.
> >>
> >> Or I could be wrong... :)
> >>     
> >
> > Magnus,
> >
> > This is true (temperature stabilization) for sites that do mm
> > level survey and ps level time transfer, using all the tricks
> > in the GPS book.
> >
> > On the other hand, I think for most of us that play at the
> > meter and ns levels with cheap OEM receivers and plastic
> > L1 antennas the coax cable temperature issue is quite
> > overblown. Or if I'm wrong, show me the data.
> >
> > /tvb
> >
> >
> >   
> Tom
> Calculations are easy:
> Cable delay tempco is at worst 100ppm/K.
> with 100ns of cable delay tempco will be 10ps?K or less.
> With a 20K temperature change change in delay will be 200ps or less.
> The group delay tempco of the antenna components (bandpass filters 
> amplifiers etc ) is likely to be much greater.

Tom's point is that while we are toying around with L1 C/A code-tracking
receivers the effective noise is so large that more finegrained compensations
in the GPS solution is note taken out so other finegrained offsets will also be
lost. I agree with him. When you toy with dual frequency carrier-tracking
receivers you gain a much better precision in ionspheric correction. Operating
in mm or sub-mm RMS noise on pseudo-ranges noise sources as change of
temperature in cables and mountning stands certainly comes in, as well as the
phase stability of the antenna, quality sites compensates for antenna phase
deviation as they change over the satelite coarse. But that's not very
meaningfull for a L1 C/A code-tracking only receiver, infact such efforts are
lost in the noise and uncompensated biases you have. Especially considering a
cheezy TCXO.

So while it is easy to calculate and compensate, the meaningfullness depends on
the situation. Compensating for cable delay may however be worth it.


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