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

John Ackermann N8UR jra at febo.com
Tue Oct 9 19:27:05 UTC 2012


Here's a link to a USNO paper that measured the tempco of three GPS 
amplifiers: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA490830

They found that amplifier filtering was the prime cause of tempco, and 
the narrowest bandpass amplifier they looked at had a group delay range 
of 4 nanoseconds over the range of -15 to +45 degrees C.

And for what it's worth, I used a VNA to measure the electrical length 
of an ~80 foot piece of LMR-400 laying on the dark roof of my Georgia 
house, and saw no measurable difference between cool pre-sunrise and hot 
mid-afternoon on a sunny summer day.

There's also a brief paper about coax tempco from Haystack:
http://www.haystack.mit.edu/tech/vlbi/mark5/mark5_memos/069.pdf

John
----
On 10/9/2012 2:38 PM, Dennis Ferguson 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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