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    Re: Trans Atlantic
    From: Antoine Couëtte
    Date: 2012 Nov 8, 07:08 -0800

    REF : [NavList] TRANS ATLANTIC
    From: Kevin Coulombe
    Date: 25 Oct 2012 14:00
    STAR FIX

    *******
    From Kermit : own results hereunder, assuming that I have correctly understood information given by you Kevin (see my "decoded" values between ### ... ### 's). I am also assuming that TT-UT = 68.0 seconds of time. As for your 22 Oct 2012 former example, Intercepts and Azimuths are given from each DR position individually derived/computed from "Reference DR Elements".
    *******
    Date : 25.10.2012 Assumed Position : L 42-00 N Lo 034-10 W Course 097 T , SOA 20.8 kt ### Note : I am assuming that this position the one for 17:56:20 LT (or 19:56:20 UT), which is the hour of the ALPHERATZ Observation. All data here-above constitute "Reference DR Elements".###
    HEIGHT OF EYE 30 METERS , BAROMETER 1010, AIR TEMP 18.5C ZD: +2 , WATCH ERROR: 5 s fast , INDEX ERROR 0.8 OFF THE ARC

    BODY: ALPHERATZ
    WATCH TIME: 175625 LT ### UT = 19:56:20.0 ###
    SEXTANT ALTITUDE 38-37.3 ### H = 38°38'1 (residual error not taken in account)
    ### i ALPHERATZ = -5.8 (A) NM , Z ALPHERATZ = 083°0

    BODY: ALTAIR
    WATCH TIME: 175838 LT ### UT = 19:58:33.0s ###
    SEXTANT ALTITUDE 57-58.4 ### H = 57°59'2 (residual error not taken in account)
    ### i ALTAIR = + 57.9 (T) NM , Z ALTAIR = 184°0

    BODY: KOCHAB
    WATCH TIME: 180159 ### UT = 20:01:54.0s ###
    SEXTANT ALTITUDE 43-22.9 ### H = 43°23'7 (residual error not taken in account) ### i KOCHAB = +0.2 (T) NM, Z KOCHAB = 338°4

    BODY: MOON
    WATCH TIME: 175430 ### UT = 19:54:25.0s ###
    SEXTANT ALTITUDE 29-21.5 UPPER LIMB ### H = UL 29°22'3 (residual error not taken in account) ### i MOON = +1.5 (T) NM, Z MOON = 121.6°

    First computation run Fix observed in Azimuth 205° and at a distance of 35.0 NM from DR Position with LOP's Dispersion of 18.6 NM. (not good)

    Hence a first computation run "updated" observed position at N 41°28'4 W 034°30'0 for LT = 17:56:20

    From this "updated" position, and time get :

    ALPHERATZ i = +13.0 (T) NM , Z = 082°4
    ALTAIR i = +25.3 (T) NM , Z = 183°5
    KOCHAB i = +24.1 (T) NM , Z = 338°6
    MOON UL i = - 2.3 (A) NM , Z = 121°1

    Second shot Fix observed in Azimuth 070° and at a distance of 0.4 NM from first computation run "updated" observed position, but still with a LOP's Dispersion of 18.7 NM, which is not good.

    After a closer look at your data, Kevin, the first observed intercept for ALTAIR is "strange" (57.9 NM) since it is so close from 60 NM. Accordingly it should reasonably be attributable to a classical/typical Typo blunder. Let us then entirely rework your example with ALTAIR sextant height set to 56°59'2 (vs 57°59'2) and see what happens.

    From you initial DR position get :
    ALPHERATZ i = -5.8 (A) NM , Z = 083°0
    ALTAIR i = -2.1 (A) NM , Z = 184°0
    KOCHAB i = +0.2 (T) NM , Z = 338°4
    MOON UL i = +1.5 (T) NM , Z = 121°6

    Observed position is in Azimuth 263° and at distance of 2.5 NM from DR Position, with a LOP's dispersion of 2.7 NM, which is fair.

    Hence at 17:56:20 LT, DR position is N 42°00'0 W034°10'0 , and
    Observed Position is N 41°59'7 W034°13'4
    Final notes :
    * (Most) Probably actual ALTAIR Sextant height was 56°59'2 (Kevin, please be so kind as to confirm that it makes sense to you). And
    * Any GPS position on file corresponding to your DR ? If so, it will be interesting to rate the quality of your observations. And
    *I have most often observed that it is difficult to get good fixes so high from the sea level (from Aircraft Carriers in my case), since - as you must have noticed - the horizon will often be "hazy" and not sharply defined from so high above the water, while it will often be "reasonably good" seen from a lower height.

    Best Regards to all

    Kermit
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