[time-nuts] GPS antenna with direction orientation?

[Tom Van Baak]

> A couple of years ago I picked up a surplus Aeroantenna choke-ring GPS 
> antenna that I think was intended for surveying use.  I finally got it 
> installed today and noticed that it has an arrow on the bottom 
> indicating that the antenna should be oriented with the arrow facing north.
> 
> I'm trying to figure out why an omnidirectional antenna should care 
> about which way it is oriented.  The best I can figure is that perhaps 
> it is for repeatability in surveying, so that any minor offset in the 
> phase center would remain consistent when moving the antenna from site 
> to site.
> 
> Does anyone have a better answer?
> 
> John

That's my understanding too. Interpret the word "omnidirectional"
like the words "cable length"; to a first approximation it is obvious
and very constant, no worries; but if you look too many decimal
places to the right, well of course, you will see variations.

My Leica/Novatel/Areoantenna575 and Ashtech L1/L2 antennas
have the same arrow. I pointed mine north only by eyeball. My
hunch was that at this extreme level issues like cable tempco or
receiver temperature were more important so I used FSJ1 heliax
for the feed and kept the lab stable to 1C. I never revisited the
angle issue.

To be certain, and if your gear is sensitive enough, you can do
an experiment and plot the phase center variation as a function
of antenna angle. But given how the GPS constellation changes
over time I think it would take many weeks before you could get
 results. I'd guess the asymmetry is well below 1 ns. If you have
a spare ham antenna rotor then vary the angle over 360 degrees
every N hours and then after you've collected many days of data
look carefully for correlation between gps phase and antenna
angle right around tau N hours.

BTW, I gave up I gave up on this sort of problem when I realized
that plate tectonics (mm to cm per year here in the NW) and local
earthquakes had a greater effect on my long-term timing. See:
http://www.leapsecond.com/pages/quake/

If there are any cm-level surveyors on the list I'd like to hear their
experience in the matter. I could imagine John's question has an
effect only at the mm level, but not cm or meter level.

/tvb