[time-nuts] Important parameters for a GPS/GNSS antenna

Attila Kinali attila at kinali.ch
Thu May 7 10:23:52 EDT 2015


On Sun, 03 May 2015 07:29:30 +0000
"Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:

> >When you post-process raw GPS data you get to include antenna phase
> >center / gain / az/el corrections for free.
> 
> Speaking of which...
> 
> I wonder if anybody ever made a rotating GPS antenna to average out the
> X-Y phase-center offset ?

There is a severe mechanical problem with that. Moving contacts
are very hard to keep electrically stable. It works for simple
power and digital signal wires, as there the only considerations
are resistance over the joint and sparks. If you need to transmit
analog signals you generally convert them into a form that does
not depend on the amplitude of the signal (either going digital
or doing a voltage to frequency conversion).

Of course, this does not really work with a gps antenna, unless you
put the whole receiver onto the rotary table. But then you shift
the problem onto the PPS output (note: amplitude noise translates
into phase noise).

The best system i know about, for such rotating contacts with analog
(possibly high frequency) signals is a small pot with mercury. But
i guess you can see the problems that causes.

The second best, but which only works with high frequency signals,
is to use a hollow waveguide. There you need "only" to ensure that
the waveguide walls are properly connected and you have a large
area to use for that. If you can ensure that there is only one mode,
you can make it such, that there is no current flowing over the gap.


			Attila Kinali
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson


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