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Resolving azimuth ambiguity - sometimes relatively simple
From: Bill Noyce
Date: 2005 May 31, 11:33 -0400
From: Bill Noyce
Date: 2005 May 31, 11:33 -0400
George Huxtable writes:
> A simple and popular, but VERY faulty, method for calculating az. is-
>
> sin az. = sec Hc sin (dlong) cos dec.
>
> The serious problem with this is that sin 95 deg (say) is exactly the
same
> as sin 85 deg, and there is NO SIMPLE WAY to tell whether an azimuth,
near
> to East, should be North of East or South of East. This vital matter
is
> glossed-over in many texts.
This is indeed a problem in many methods. But if you're using Ageton's
method (is this HO211?) one of the intermediate results is the latitude
at which a perpendicular to the body crosses your meridian. If this
point is north of you, the azimuth has a northerly component; if it's
south of you, the azimuth has a southerly component. (This rule has to
be modified when the hour angle is not in the range -90..90, but I think
the azimuth for a visible body is always toward your local pole in that
case.)
The other problem with this formula is that the sin changes slowly near
due
east or west, so if your AP was way out, the error in the LOP's angle
could lead to a poor fix. At least with Ageton's method, it's possible
to
use your new fix as an AP and iterate, to see whether this is a serious
issue.
-- Bill
