# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: One Assumed position or Six?
From: Gary LaPook
Date: 2017 Feb 7, 18:33 -0800

The normal maritime celestial navigation process with inspection tables (AP3270, HO 249, etc.)  involves the use of separate assumed positions for each shot. However, the normal flight navigation procedure with HO 249 volume one utilizes only one assumed position and this can also be done by a surface navigator and by you. The secret is that you have to take your shots on a preplanned schedule, which is what flight navigators do. HO 249 volume one utilizes the LHA of Aries for determination of the assumed longitude so if you take your shots exactly four minutes apart you will end up with the same minutes of LHA for each shot, only the number of degrees will change. You can take the first shot at any time and then make sure you take your other shots at exactly four minute intervals. It is even more convenient if you shoot the first star at exactly on the hour and on the ten minute intervals tabulated in the Air Almanac.

An example will make this clear. On February 8, 2017 at 0120 Z you shoot your first star and the GHA of Aries is tabulated in the Air Almanac at that time as 158 - 20.7'. Say your D.R. longitude is 118 - 34.0' West so you assume a longitude of 118 - 20.7' since that is the closest to you. You then subtract the assumed longitude from the GHA Aries and derive the LHA Aries as 40 - 00.0'. You take your next shot at 0124 Z, exactly four minutes later. You look in the increment table for four minutes and take out the increment of 1 - 00'. Doing the normal computation, tabulated GHA Aries, 158 - 20.7', add the 1 - 00' increment and find the GHA Aries at 0124 Z of 159 - 20.7' Then subtract the same assumed longitude of 118 - 20.7 and determine the LHA Aries at 0124 Z is now 42 - 00.0'.

So that is how you can use just one AP.

You could also do it with Ho 249 volumes two and three but not so conveniently. You would have to plan your shot times to allow for the increments to the GHA of the bodies so as to make GHAs come out the the same number of minutes. Tricky but doable.

See my prior response to your previous question:

http://fer3.com/arc/m2.aspx/Hats-off-everyone-who-can-do-this-LaPook-feb-2017-g38144

gl

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HOWEVER... Either in a reply to one of my previous posts or in a PM I received someone said "use only one assumed position". Really confused as how to use the same assumed position for all the LOP's

Mike

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