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

Dennis Ferguson dennis.c.ferguson at gmail.com
Tue Oct 9 14:38:36 EDT 2012

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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