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    From: Antoine Couëtte
    Date: 2021 Nov 5, 08:19 -0700

    Good day to you Peter,

    In reply to a recent remark by Bill Ritchie you indicated that:

    The Navigation data given by ICE will be the positions of the body centre, and the refraction given will be for the altitude (Hc) of the body centre. But for Sun and Moon work we will want a refraction figure for the upper or lower limb we observe; and this refraction will have to be different than the ICE value given.

    This statement does not seem correct for the reasons given to Bill as a comment to his remark

    Instead of giving corrections for SUN or MOON Centers, ICE seems to rather give height corrections for their Lower Limbs.

    I am ready to accept this very last statement as an incorrect one. Hence Users with much ICE experience, please have a go.

    On the other hand your remark opens up an interesting discussion. For the SUN and MOON the Software Programmer was left with 3 choices:

    - Corrections for UL, or:

    - Corrections for LL, or:

    - Corrections for Body Centers. Thinking it over, this one does not really seem an option since with a classical NAVY sextant you do not observe Body Centers.

    We are then left with either UL corrections or LL corrections. Since - most probably - the vast majority of Observations, for the SUN at least, are the LL ones, it should make sense if ICE processes LL sights only.

    Outside Low altitudes, the differential refraction between LL and UL is certainly negligible. This is not quite so for either SUN or MOON at the horizon (Remark 2).

    For the SUN which has a small HP, this should not be a problem provided the Users correct - in the right direction - the ICE height corrections with twice the SUN SD.

    The Users should then be able to retain full accuracy even with UL's.

    For the MOON the same should hold more or less true in spite of the differential parallax between UL and LL (see note below) which cannot be taken in account by ICE.

    As a recap, we should consider that ICE performs corrections on LL's with excellent and reliable results which are to remain a quite solid reference for many years if not decades to come provided it is used with the right Delta-T values. UL sights corrections should be adequately performed through subtracting twice the augmented SD values from the ICE overall "Sum" result. For Low alt MOON, this procedure might bring some extra uncertainties which nonetheless are most probably buried inside the Low Alt refraction values noise. 

    Best Navigational Regards,

    Antoine M. "Kermit" Couëtte

    Note: I have not investigated the effects such differential parallax.

    ICE seems to always compute the Parallax for a body center subsequently assumed to be above the recorded Limb.

    Anyway, we should not forget that this specific discussion is probably irrelevant because of the well known Refraction fluctuations at Low Altitudes.

    Hence this note is certainly pure speculative and frivolous computation overkill.

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