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SEC: UNCLASSIFIED - RE: Compass corrections (Sun & surveyors)




Laura has this absolutely correct. I'll adapt this to the general...

*	The centre of [my] time zone is (X)XX degrees west/east. [My QTH] is
at (Y)YY degrees west/east. Each time zone is nominally 15 degrees wide, so
X-Y degrees is (Z.ZZ / 15) * 60 or (A) A minutes west/east [of the Zone
time], making local noon BBBB [taking account of summer time, zones with
half-hours, etc. REMEMBER: being west of the zone centre means you'll have a
later time for local noon.] The Handbook program uses a more elaborate
approach (modelling the Earth's elliptical orbit). 

Just like for a satellite pass, the Sun's rate of apparent change of
direction will change most at 'meridian passage' (local noon) but a FEW
minutes will make little difference in terms of antenna beamwidth, unless
you have a VERY sharp beam such as a multi-multi-beam or dish. 


METHOD 2
Also consider getting out a copy of your house plans, with the surveyor's
calculations. My own place is *exactly* north-south, but you can use any
other alignment to calculate North within a few degrees using a school
protractor. 

Say the side of the house is shown as aligned at 025' 30" (i.e. 25 ½
degrees), i.e. 25 degrees east of north. 
Put the protractor along the side of the house. The 90 degree mark points at
right angles. Rig a string-line out about 3-5m (10-15ft) so that it lays
across an angle 25 ½ degrees towards North. Now, reset the protractor to lie
along the string-line, so that you can see past the side of the house. Rig
another line along the 90 degree mark (i.e. North). Sight along this to find
a DISTANT point, say 1-5km away (1/2-3 miles) - the further the better,
within reason. It might be a big tree, a particular window on a house, a
notch between two trees, etc. Align the antenna to this 'natural' North that
you have established. The difference between this and the 'real' North will
be minimal.


Peter Ellis
VK1KEP
peter.ellis@cbr.defence.gov.au <mailto:peter.ellis@cbr.defence.gov.au> 

PS: We had a GREAT pass from UO-14 last night - right up the middle of
Australia - with the footprint covering the continent. The eastern seaboard
could talk to the western seaboard. I talked across the continent on my 5W
handheld with rubber duckie! Thanks, UO-14 team.


		-----Original Message-----
		From:	Laura Halliday [mailto:va3ldh@sympatico.ca]
		Sent:	Friday, 3 March 2000 6:33
		To:	amsat-bb@AMSAT.Org
		Subject:	Re: [amsat-bb] Compass corrections

		When I set up the rotator at school (VA3SFL) I tried a
compass,
		but found there was too much metal in the vicinity to get a
		good fix. So I went for the shadow-at-noon approach. I
computed
		local noon two different ways: one with a program that came
		with the ARRL Handbook, and a quick and dirty approximation
		from Toronto's longitude.

The centre of the time zone (North American Eastern) is 75 degrees
west. Toronto is at 79.33 degrees west. Each time zone is nominally
15 degrees wide, so 4.33 degrees is (4.33 / 15) * 60 or 17 minutes
west, making local noon 1317 (this was in the summer). The fancy
Handbook program used a more elaborate approach (modelling the
Earth's elliptical orbit) and said 1321. I sat up on the roof
with a can of bright pink spray paint at the appointed time and
noted that the difference between 1317 and 1321 was smaller than
the amount the antenna array flexed in the breeze...

		-- 
		Laura Halliday VA3LDH    "Laisse le vent tout emporter..."
		Grid: FN03gs                - Foly/Viennet
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