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.
David
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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