[time-nuts] To use or not to use transmission line splitters for GPS receivers

paul swed paulswedb at gmail.com
Tue Oct 9 14:43:14 EDT 2012


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 <dennis.c.ferguson at gmail.com
> wrote:

>
> On 9 Oct, 2012, at 12:48 , "Bob Camp" <lists at rtty.us> 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
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