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    Re: 12 Feb 2011 Unorthodox Jupiter Lunar from a moving platform
    From: Antoine Couëtte
    Date: 2011 Feb 20, 14:06 -0800

    RE:
    http://www.fer3.com/arc/m2.aspx?i=115706
    [NavList] Re: 12 Feb 2011 Unorthodox Jupiter Lunar from a moving platform
    From: lars.h.bergman---com
    Date: 20 Feb 2011 06:44

    Hey Lars : GREAT JOB AGAIN !

    You published - ahead of everybody this time - your position as follows :

    " 46d42'N 2d30'W, with the watch some 16h24m16s slow on GMT "

    which I can "decode" as being the position at the Averaged Lunar Distance UT which happened at UT = 01h50m21.4s + 16h24m16s = 18h14m37.4s

    For UT = 18h14m37.4s the Ship Navigation System indicated : N46°36'.6 W002°25'.6

    (My computations give the following results : Watch correction : 16h23m41.0s UT of average Sextant Distance = 18h14m02.4s Position at UT of average Sextant Lunar Distance = N46°36'6 W002°30'.4 From the Ship Navigation System, Position at UT = 18h14m02.4s was N46°36'8 W002°25'8)

    *******

    From the Ship Logbook (unlike Aircraft Logbooks, all headings true here) it is interesting to replay our route on that evening, which interested / curious readers can easily follow on Google Earth.

    We departed our Pornic Harbor anchorage following a 255° bottom track at a moderate speed (12 kts for the Engines to fully warm up and due to the sea state (everybody's comfort + I did not want to take any chance to get drenched).

    At UT=16h35m we were at position N47°03.'4 W002°24'.7, i.e. north of Phare du Pilier (north of Ile de Noirmoutier).

    We were to transit West of Ile d'Yeu in order to keep the horizon clear to the West. In addition this would avoid any conflicting route with the regular Mainland / Ile d'Yeu shuttles. Therefore from that 16:35 UT Position we turned left and set course to follow a bottom track 184° . Speed increased to 16.2 kts both because Engines had warmed up and because the Sea state also permitted it (2 ft waves and 4 ft swell from W/NW while she gently and nicely rolled). After we had left Ile d'Yeu on our port side, at UT=18h10m we turned again left to a bottom track of 135° . We had following seas then and it was quite comfortable to increase speed to 22 kts ( she can reach over 35 kts …). No more roll then and she was perfectly steady for the Lunar Distances.

    *******

    Now, Lars, back to your results and let us see how things look in terms of LOP's. I think it is worth doing such check since you indicated " I haven't been able to fit the Jupiter altitude into my solution very well, maybe I have done some arithmetical error somewhere."

    If you do not feel fully confident in your results derived so far, why not considering them with a different view-point ?

    May I suggest that you use your results here-above simply as DR and perform a traditional 3 LOP fix. It will then be possible for you to check your LOP's "fit" and especially the one from Jupiter. If the 3 LOP's are reasonably "tight", you can them adopt this updated position as your final solution. And, Cherry on the Cake, if you want to "iterate" (just) once in an easy manner, and since your "Moving Observer" computations are certainly much more complex now, how about "iterating" with the help of Frank's on-line computer this time ?

    Let's go for it then !

    STEP 1 : LET US COMPUTE A TRADITIONAL FIX FROM YOUR RESULTS.

    Starting from your position and Watch time correction, we "reconstruct" the entire trip as follows :

    First leg course speed 255°/12.0 kts
    UT11 = 16h33m10.6s SunL = 07°46.'7
    UT = 16h36m16.0 s change course/speed from 255°/12.0 kts to 184°/16.2kts
    UT21 = 17h51m13.9s MoonL = 61°02.'9
    UT31 = 18h06m17.8s Jupiter = 28°28.'8
    UT = 18h11m16.0s change course speed from 184°/16.2kts to 135°/22.0 kts
    UT41 = 18h14m37.4s Moon Distance = 66°12.'9 Observed Lunar Position1 46d42'N 2d30'W

    Let us first determine your position at Time = UT41 from the hereabove data.

    We get the following results :
    UT11 = 16h33m10.6s (DR) Position : N47°08'7 W002°28.'0 SunL 07°46'.7 intercept = +8.0 NM (i.e. Toward) azimut = 240.°2
    UT21 = 17h51m13.9s (DR) Position : N46°48'.3 W002°30'.8 MoonL 61°02.'9 intercept = 0.0 NM azimut = 137°1
    UT31 = 18h06m17.8s (DR) Position : N46°44'.2 W002°31'.2 Jupiter 28°28.'8 intercept = +8.3 NM azimut = 236°.4

    Get an Updated fix in Azimuth 227° from DR Position and at a distance of 8.3 NM with LOP's SDEV = 0.2 NM (so the Jupiter LOP should fit pretty well with the SUN and MOON LOP's).

    Since the JUPITER is fully consistent with the SUN and MOON LOP's, then let us consider that the position derived from these 3 LOP's is an improvement to your earlier results.

    THEN YOUR IMPROVED POSITION WITH A TRADITIONAL 3 LOP FIX becomes :

    UT41 = 18h14m37.4s N46°36'.4 W002°38'.9 This is now MUCH BETTER, especially for your Latitude.

    IF YOU CHOOSE TO FURTHER REFINE YOUR RESULTS WITH JUST ONE "SIMPLE" ITERATION …

    What does Frank Computer say for UT41, position and Jupiter Moon Distance = 66°12'9 ?

    Error in Lunar: -0.4'
    Approximate Error in Longitude: 0° 11.6' , which more accurately translates into Error in Lunar = -.348'

    We then need to compute (or extract from a table) the hourly change in the Jupiter Moon geocentric distance since this quite helpful data is not available on Frank's Computer (Frank, are you here ?). We find that the Geocentric Jupiter-Moon distance changes at a rate of + 31'.41 / hour. Therefore the "-.348' error in Lunar" translates into a -39.9 s time correction to all your previous results. And in particular your updated Watch Correction becomes +16h23m36.1s

    Since this time interval correction is small and for simplicity, you can simply decrease all times hereabove by 39.9 s and shift your position to the East by 39.9s/4 in arc minutes, i.e. 10'.0 to the East.

    Therefore your "updated" time for this Lunar becomes UT = 18h14m37.4s - 30.9s = 18h13m57.5s and your Updated position for such time becomes :
    N46°36'4 W002°28'9

    Well ! Now this looks really wonderful ! No UT chronometer, just a Watch in "elapsed time" mode, and you can position yourself within better than 5 miles by reference to your true position on a fast moving platform with successive changes and Speed and/or course ...

    Isn't Life Beautiful ???


    Thanks to all, and to you Lars, AGAIN CONGRATULATIONS FOR YOUR EXCELLENT RESULTS, and

    Best Regards to you all

    Kermit


    Antoine M. "Kermit" Couëtte

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