Hi In the context of the original post, probably the right question is: do you have a hydrogen maser and an ensemble of cesiums to compare it to? That's the environment that gets front and center at these conferences.
Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of paul swed Sent: Tuesday, October 09, 2012 2:43 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] To use or not to use transmission line splitters for GPS receivers Boy all I can say is I measured the $7 satellite splitter and it matched the specs for fwd and rtn loss. Port to port loss using an HP network analyzer. So what can I say it worked and well. Actually surprisingly so. Regards Paul. On Tue, Oct 9, 2012 at 2:38 PM, Dennis Ferguson <[email protected] > wrote: > > On 9 Oct, 2012, at 12:48 , "Bob Camp" <[email protected]> wrote: > > If you are after sub ns level timing, things are a bit different than if > you > > are happy with tens of ns error. Few of us have an adequate survey of our > > location to *really* worry about sub ns numbers. If you are one of those > > lucky few that can worry about sub-ns, yes mismatch and voltage and a > whole > > long list of things matter. The temperature coefficient of your antenna > also > > gets onto that list at some point. > > I think you can get sub-nanosecond time (if you can arrange for a proper > equipment calibration) and sub-centimeter positioning on your own using > the IGS products and GPS Precise Point Positioning techniques. The gotchas > are that you need to have a high-priced dual-frequency, carrier phase > tracking receiver and the software you need seems to only be available to > the very rich (though there are free online services which will process > your data to determine the location for you). > > The antenna temperature thing is kind of indicative of just how much lore > and black art seems to be involved in arranging equipment for fine timing, > however. I have the ITU 2010 Handbook for "Satellite Time and Frequency > Transfer and Dissemination". In Chapter 12, when discussing GPS Common > View techniques, the document says this about antenna temperature > > 12.5.2 Temperature stabilized antennas > > It is now well documented, and generally admitted, that GPS > time-receiving > equipment, and more specifically its antenna, is sensitive to > environmental > conditions [Lewandowski and Tourde, 1990]. For conventional GPS > time-receiving > system this sensitivity could be expressed by a coefficient of about > 0,2 ns/°C and can approach 2 ns/°C. This was a major precluding > obstacle, > as it did, the goal of 1 ns accuracy announced earlier for GPS time > transfer. > > and goes on to recommend using an antenna with an oven keeping the > temperature > of the electronics constant. In Chapter 13, on the other hand, when > discussing > GPS PPP, it says this: > > There have been some poorly supported claims of strong variations of > geodetic clock estimates with temperature changes in some GPS antennas, > together with recommendations to use temperature-stabilized units. > While > this might apply to certain low-end, single-frequency units, direct > tests > of a standard AOA Dorne Margolin choke ring antenna have failed to > detect > any sensitivity of the clock estimates to antenna temperature > variations. > Ray and Senior [2001] placed an upper limit of 2 ps/°C on the > short-term > (diurnal) temperature sensitivity and later extended this to <10.1 > ps/°C > for any possible long-term component [Ray and Senior, 2003]. Even > smaller > sensitivities, 0.17 ps/°C or less, were determined by [Rieck et al., > 2003] > for an Ashtech choke ring model. > > So Chapter 13 says that what Chapter 12 said is bogus. It appears that > Chapter 12 > may have written been written by a European while Chapter 13 is an American > effort, so this may be some sort of cultural thing. Chapter 13 does later > go > on to point out how crappy the Canadian IGS stations are in the winter and > blames this on snow and ice in the near field below the antenna, so even > Chapter > 13 does find a use for heating at the antenna. Both chapters do agree > that keeping > the temperature of the receiver constant is good. > > I think the antenna splitter thing is probably the same kind of issue. > Someone, > somewhere, may have had a problem with an antenna splitter and published a > paper > on that, and this in turn reinforces the conservative assumption that you > should > leave anything out that doesn't absolutely need to be there, so it has > become > common wisdom that you should avoid splitters. Or something. > > Dennis Ferguson > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
