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Zero altitude sights (Hs = 0) can work out well but the problem is that 
you don't know if this sight is an accurate one or an inaccurate one due 
to the vagaries of refraction that the observer won't know about. 
Because of this you have to allow a larger uncertainty band around any 
LOP taken this way. I have taken such observations in flight and they 
work out well considering the lower accuracy inherent in flight 
navigation and I have carried a table in my wallet for 30 years for this 
purpose, it is posted here:

http://www.fer3.com/arc/img/104652.ho%20table.jpg

On my recent Atlantic crossing I took three zero altitude sights. The 
semi-diameter of the sun was 16.1', parallax in altitude was .1'  and 
standard refraction correction was 34' . The temperature for all the 
sights was about 70 F but I do not know the actual atmosphere pressure. 
So using the average pressure from the pilot chart of 1017.5 mb the 
refraction correction for non-standard conditions is + 2.3'. So for the 
upper limb observations this totals to a -47.7' for all of the 
observations not including the dip correction .

The first was sunrise on October 23, 2009 at 06:53:25 Z at GPS position 
37 11.2 N, 9 32.1 W. Height of eye was 24 feet so dip was -4.7' so the 
observed altitude (Ho) was -52.4'. Using the GPS position and then using 
the navy website at:

http://aa.usno.navy.mil/data/docs/celnavtable.php

the computed altitude (Hc) was -52.3' resulting in an intercept of .1 NM 
away. Not too bad.

The second was at sunset the same day, October 23, 2009 at 17:49:58 Z at 
GPS position 35 15.3 N, 8 43.4 W. The height of eye was the same so the 
Ho was also -52.4'. Hc was -50.6' resulting in an intercept of 1.8 NM away.

The third was sunset on October 24, 2009 at 17:46:27 Z at GPS position 
33 42.9 N, 7 34.6 W. Height of eye was 26 feet giving a dip of -4.9' 
resulting in an Ho of -52.6'. Hc was -54.9' resulting in an intercept of 
2.3 NM toward.

These all provide useful LOPs  keeping in mind the larger band of 
uncertainty.

gl










Greg Rudzinski wrote:
>
> Jeremy,
>
> I worked your upper and lower limb sunset examples using a palm 
> program to derive a standard LOP azimuth and intercept with the given 
> GPS position of 16°33'N 129°7'E as A.P.,Height of eye 100ft., Pressure 
> /Temperature correction from A4 of +3'. No index error applied.
>
> Sun L.L. 18:58:16 ZT
> Hs 0°0'
> Azimuth 294.3°
> Intercept 3.9' away
>
> Sun U.L. 19:00:48 ZT
> Hs 0°0'
> Azimuth 294.5°
> Intercept 2.2' away
>
> This shows that your observations with the naked eye were very good. 
> The sunset Longitude solution should work out if a precise table or 
> program is used.
>
> GRudzinski
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