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SEC: UNCLASSIFIED - Re: North (and pointing antennas at it)



In 
AMSAT-BB-digest      Tuesday, January 30 2001      Volume 2001 : Number 080
Bill VE7WFG at DO00jk asked about North.


Speaking as a Navy man, a trained 'navigator', I can assert, "North is
North". 
However, "Magnetic North is only rarely North".

An explanation of determining the various components of North is at
http://www.terrax.org/geography/compass/compass.html
<http://www.terrax.org/geography/compass/compass.html>  

This is an explanation for 'yachties', and for the Amateur purposes, our
'course' is generally 'True' North/South. 

Magnetic Deviation is compass error... a particular compass' inability to
point North because of its own in-built magnetic forces and these vary
depending on what direction it is pointing. This is determined by 'swinging'
the compass (eg in a boat/ship) and recording the errors. There is also an
effect from movable, nearby material and it is THIS effect that is of most
concern for us as Amateurs because we can often do little about needing to
use the compass near a steel-sided garage, a clothes line, or above an iron
sewer pipe that we don't know is there.

Magnetic Variation is 'Earth' magnetism error (comprising both a horizontal
and a vertical factor, but you can typically ignore the vertical error
unless you are near a Magnetic Pole) ... because the magnetic poles are not
even near the Earth's point of rotation, and... the Magnetic Poles MOVE!
Therefore, it is only along very particular lines that Magnetic North is
aligned to North. The difference everywhere else is called Deviation. 

TRY THIS SITE>>>
VERY USEFUL APPLETS for Variation, position, and 
GREAT CIRCLE, are reached via http://www.pangolin.co.nz/almanac/index.asp
<http://www.pangolin.co.nz/magvar.html>  .

Tables of Variation are at http://geomag.usgs.gov/chartsdo.html
<http://geomag.usgs.gov/chartsdo.html>  .

Or you can look at maps. Good ones show the Variation, with a date and rate
of change.
You should take into account the date, and calculate using its RATE to
determine the PRESENT Variation. 

*	Note the 'sign' of the Variation [and Deviation] (whether East or
West of North) and account for it with common sense. 
*	Remember: TVMDC, remember "TV Makes Dull Company." T=True Course,
V=Variation, M=Magnetic Course, D=Deviation, C=Compass Course to work out
what compass direction you must observe to equal North.
*	However, this must be worked backwards (CDMVT) from a compass
direction to a true direction.

Hint: When you work out where to find North/South, look along that direction
and find an object as far away as possible that you can aim at after that.
It becomes your 'natural compass' FOR THAT SITE. (Sorry, you can't go to a
mate's place in the next suburb and use it.) Yet, the angular error between
antennas even within a ~20-30m of each other at your QTH will be swamped by
the distance to the 'natural compass' if it is more than a few hundred
metres away. 

Peter VK1KEP

 


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