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    Re: Air vs. water: MOB and MOO table corrections in HO 249
    From: David Pike
    Date: 2016 Jan 8, 12:55 -0800

    Mark.  MOO and MOB are mainly tied up with air navigation when pre-computation is frequently used.  They allow you to ‘rescue’ a shot taken at some time other than the calculated time and also to shoot a number of bodies using a single assumed position and LHA.

    The mnemonic “Late, Back, West” comes to mind.  If the shot is late, you move the position line back along your track by an amount equal to the product of the time difference and your groundspeed (the MOO correction) and west at a rate which varies from 15° per hour at the equator to zero at the Poles (the MOB correction).  Using the LBW convention, you write the original pre-calculation time against the corrected position line.  Looking at the MOO and MOB tables, you’ll see you can produce exactly the same result by adjusting the observed or pre-calculated altitude.  You can use the same technique for 3PL fixes although it’s probably best to use the time of the final shot as the pre-calc time, otherwise you end up with position and time of the fix on the chart a long way behind real time.
    I’m not an expert, but as you say, unless you’re navigating a speedboat, MOO can probably be neglected in a marine setting. 
    MOB’s probably little mentioned, because in the marine setting the technique is probably to take a shot when you get a chance, and then work out the calculations afterwards using the time of the actual observation.
    We never used MOO and MOB on the V Force.  We used two star sandwich fixing about a centre time, and with four people to remind you to get up there, you invaribly got shooting on time, and there wasn't much cloud at FL430 DaveP

    From: Mark Coady
    Date: 2016 Jan 8, 09:10 -0800

    Speaking of air vs water navigation and pre planned increments.....HO 249 contains correction tables for movement of observer and movement of body.

    If you are doing let's say a group of divergent star sights over a period of say five or ten minutes, which would give you crossing LOP's and thus a fix.  Does it make sense if you have HO 249 handy, to use the tables to adjust the three LOP's to the center sight time based on the "movement of body" table?  "Movement of observer" is probobly less a concern, due to relatively slow body speeds (at least my body is assummed to be on a slow boat).

    I've seen lots of triangulations done, but never saw noted the use of these tables in my reading on marine navigation.  I tend to be methodical and slow in shooting, hence three quick sights in succession might be longer for me than the more experienced or more adept navigators, and have a greater impact on crossing LOP's.


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