); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] From: Dr Bruce Griffiths <[EMAIL PROTECTED]> Subject: Re: [time-nuts] Cs stability Date: Tue, 17 Jul 2007 21:30:03 +1200 Message-ID: <[EMAIL PROTECTED]>
> ); SAEximRunCond expanded to false > Errors-To: [EMAIL PROTECTED] > > 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. Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
