[time-nuts] Choke Rings and Points North

Tom Van Baak tvb at LeapSecond.com
Mon Dec 15 23:10:26 EST 2014

> With all the discussion about surveys & position accuracy, I have a question 
> about my choke ring antenna.  There is an arrow marked "N" on the underside 
> of the rings.  How accurately does the alignment need to be to "N"orth? 
> True north or magnetic north (my thinking says True North)?
> Does the directional accuracy affect the precision survey?  I'm assuming 
> that it would have no effect on the accuracy of the 10 MHz frequency output. 
> Or am I completely off base?
> Dave M 


I agree with the others. If you're using dual-frequency GPS with post-processing it might make a few mm or ps difference. This only matters for absolute measurements, or multiple measurements where you plan to compute differentials.

It also depends on the antenna. You can find the phase vs. angle profiles at http://facility.unavco.org

For one example of an antenna calibration see:

The GPS pros take their mm very seriously. If all you're using is a fixed location and commodity GPSDO then it makes no difference at all, not even close.

But to prove us wrong, put the antenna on a 17 hour turn-table, collect data for 6 months, and then see if you see any 17h peaks in the FFT!


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