[time-nuts] Choke Rings and Points North

David McGaw n1hac at dartmouth.edu
Mon Dec 15 22:57:33 EST 2014

To answer which "North", it is True North, not Magnetic.  Orbits, 
including GPS, are specified relative to the geographic pole. Magnetic 
North moves noticeably over time and place.  True North moves somewhat 
over time but only very slightly.


On 12/15/14 9:05 PM, Jim Lux wrote:
> On 12/15/14, 5:46 PM, Dave M wrote:
>> 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?
> If you're using a standard antenna, they've characterized them for the 
> change in phase center with respect to the direction the signals are 
> coming from. It's assumed you'll install it level, so elevation is 
> taken care of.  The remaining uncertainty is the azimuth, hence the 
> north arrow.
> Now we can find out how much of a nut you really are.  On choke ring 
> antennas, I think the maximum shift in phase center with look 
> direction is on the order of single digit millimeters, or a few ps.
> And how accurately do you know what direction is "north".  That could 
> be a whole project in itself, ranging from moss on trees, to shadows 
> of sticks and rocks, to observations of Polaris through a theodolite, 
> and so forth.
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