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
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