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    Re: Hp by Doniol for Aircraft
    From: Gary LaPook
    Date: 2014 Dec 3, 00:54 -0800
    In flight navigation you apply the sextant corrections to the computed altitude to create Hp, "precomputed altitude." This then allows you to compare your raw sextant altitude to the Hp to obtain the intercept which is much quicker than the marine practice of making the corrections to the Hs after the shot to obtain Ho after the shot is taken. You apply the corrections to the Hc with the sign reversed from the marine practice.

    Let's say that the Hc is 40° 15' and the Hs is 40°  35' and the index error (IE)  is +6' (on the arc) and the height of eye is 16 feet.  [IC is IE with the sign reversed.]

    Marine method            flight method
    Hs  40°   35'                    Hc    40°   15'
    IC        -  6                     IE            + 6
    DIP       -4                      DIP          +4
    REF     -  1                     REF         +1
    --------------------                 ---------------------
    Ho    40°   24'                   Hp     40°   26'
    Hc    40°   15'                   Hs     40°   35'
    ----------------                     ---------------------------
    INT         9 T                   INT         9 T

    You get the same intercept either way. 

    So, when making a graph for precomputations you draw in the curve of Hp with all the sextant corrections already applied and compare the raw Hs with the graph.

    The only criticism is that the refraction correction is actually based on Ha not Hc so the correction taken from the table might fall on one side of the correct value or the other, but this is not a problem in real life. A difference of 0.1' from the Nautical Almanac refraction table of 1' from the Air Almanac table had no real consequence in the real world.  

    In fact, just memorize the refraction corrections to the nearest one minute, 5' above 10°  ; 4' above 12°  ; 3' above 16 ; 2' above 21°  ; 1' above 33°  and 0' above 63°. And for the dip correction just do the square root in your head, 2' for 4 feet; 3' for 9 feet; 4' for 16 feet; 5' for 25 feet, etc.




    From: Francis Upchurch <NoReply_Upchurch@fer3.com>
    To: garylapook---.net
    Sent: Tuesday, December 2, 2014 11:41 PM
    Subject: [NavList] Re: Hp by Doniol for Aircraft

    Thanks Gary,
    That’s very helpful and interesting. Could I ask you about sextant corrections? I was thinking of “reverse correcting” the pre-computed Hps and then plotting un-corrected Hs directly on the graph to save on mental arithmetic “in-flight” . Are you plotting Hs or, Ho or what on the charts? I suspect I am being rather dim here and missing a point or 2?
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Gary LaPook
    Sent: 02 December 2014 21:11
    To: francisupchurch---.com
    Subject: [NavList] Re: Hp by Doniol for Aircraft
    When you are planning your voyage estimate your time at each waypoint and for a couple of hours either side work up curves for the sun and moon for the day and for several stars at night. See:

    From: Francis Upchurch <NoReply_Upchurch@fer3.com>
    To: garylapook---.net
    Sent: Tuesday, December 2, 2014 11:36 AM
    Subject: [NavList] Re: Hp by Doniol for Aircraft
    I don't think any of us old romantics are seriously suggesting we do a Chichester as per 1931(he was crazy even then). I never go anywhere on my boat without multiple GPS back-ups, but just like to play with astro, celnav and the old Chichester stuff like Bygraves etc. In an emergency, the GPS rules.
    Anyways, this recent thread on precomputed Hps is fascinating and I intend to try it out on my next boat trip to Scillies. I've already worked out my "waypoints" with the Bygrave and will now also test the Brown-Nassau.
    Best wishes

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