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    Re: Celestial up in the air
    From: Gary LaPook
    Date: 2008 Jul 14, 20:01 -0700

    Gary LaPook wrote:
    You are absolutely right, the push is to compute fixes rapidly, not to 
    have short time intervals between fixes. FAR part 63 assumes cel fix 
    accuracy  (or uncertainty) of 10 NM. If the leg between a completely 
    accurate fix and a cel nav fix is 100 NM you  could could compute a 10 
    knot wind in any direction even if there was no wind at all, just due to 
    random variation in the fix itself.  Just draw a circle around the 
    second fix with a radius of 10 NM  and you should be somewhere inside 
    that circle ( though you are more likely to be near the center than near 
    the edge.)  Because of this uncertainty the wind could be from any 
    direction or any speed that results in a fix within the 10 NM circle. It 
    is even worse if the first fix was also a celnav fix with the same 10 NM 
    circle of uncertainty since they would combine to make a 20 NM 
    uncertainty in the derived wind vector. The longer the leg between fixes 
    the less the percentage error possible in the computed ground speed, 
    drift angle and winds speed. However, just like with the "cocked hat," 
    we still have to use the fix position to compute the winds keeping in 
    mind that there is a possible uncertainty in the computation.
    Ken Gebhart wrote:
    > Gary,
    > Your article on air navigation was excellent.  I hope people like you 
    > and Greg R keep up the good work to remind others that celestial is 
    > not forgotten.  I do have a couple of comments about air navigation 
    > and your article though.
    > Your admonishment to plot quickly due to high speed travel, may imply 
    > a misconception among navigators in general.  Navigation is not about 
    > knowing where you are, it is about knowing where you WERE, and where 
    > you are going to be.  Because it is between the fixes that one solves 
    > for the wind acting on his airplane, it is important that the fixes be 
    > a few hundred miles apart.  If taken too frequently, the errors in the 
    > fixes themselves can yield false wind values which when compared to 
    > weather prognosis charts, can give an erroneous analysis of the 
    > weather system affecting him.  This, in turn, causes a . wrong 
    > estimate of when and where he will show up on air traffic control 
    > radar at the end of the trip.
    > Ken Gebhart
    > On Jul 12, 2008, at 2:32 AM, Andres Ruiz wrote:
    >> Dear Gary,
    >> your article on Ocean Navigator is very interesting for me, I 
    >> practice astronavigation at sea and at shore, but never on an 
    >> aircraft. Your description of the process with the bubble sextant and 
    >> the HO249 is illustrative.
    >> Gary LaPook "Celestial up in the air":
    >> Thnaks to Richard for the link.
    >> -- 
    >> Andr�s Ruiz
    > >
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