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

Jim Lux jimlux at earthlink.net
Sat May 9 11:20:50 UTC 2015


On 5/8/15 11:37 PM, Poul-Henning Kamp wrote:
> --------
> In message <FC02A5E8-5396-4474-A307-546E10909846 at n1k.org>, Bob Camp writes:
>
>> The “put the antenna up and rotate it to see what happens” experiment
>> has indeed been done. The objective was not correcting the antenna’s
>> issues, but validating that their model of the antenna’s phase
>> center was correct. They were trying to see if anechoic chamber
>> data really gave correct answers in free space.
>
> So this could be a realistic way for us to calibrate the phase-center
> of an antenna ?
>
>

Yes.. actually, the best way in the long run would be to collect many 
hours of GPS satellite data (carrier phase)  with the antenna in one 
position.  Then rotate the antenna to a new position, collect a bunch 
more data, repeat, etc.


Then, you post process using the known position of the satellite, which 
gives you a direction of arrival relative to the antenna.

You probably don't need so have a real precise position for the antenna: 
the apparent motion of the phase center as a function of az/el is 
probably fairly slow.

Isn't that how they collected phase center data for all those antennas 
on the UNAVCO site:
http://facility.unavco.org/kb/questions/458/UNAVCO+Resources%3A+GNSS+Antennas

one of the reports has this interesting statement:
Antenna rotation tests work well to identify inconsistencies in mean 
phase center offsets. By occupying a short baseline (less than 10 
meters) and rotating the antenna orientation 180 degrees it is possible 
to see changes in the baseline length caused by the antenna phase 
center. For antennas of the same type, the rotation tests will highlight 
variations of an individual antenna relative to the pool of antennas.




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