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    Re: The Noon Fix
    From: Gary LaPook
    Date: 2009 Apr 10, 20:38 +0200

    I think that it should be properly called a running fix because the
    accuracy of the resulting position degrades over time due to
    inaccuracies inherent in dead reckoning from the earlier sight like any
    running fix.
    But, on another point, nobody ever seems to discuss what level of
    accuracy is NECESSARY for a backup method. In aviation the level of
    accuracy required is specified for every segment of the route and for
    every navigation system based on proximity to hazards. Enroute accuracy
    is usually required to be within four nautical miles of the course line
    and the minimum altitudes specified look at hazards within this
    specified band. On instrument approaches the  required accuracy
    increases as you must fly closer to hazards so different navigation
    systems must be used that will provide the higher level of accuracy.
    Finally, on a precision approach to the runway, the accuracy required
    and provided is in mere feet.
    Celestial navigation is an enroute system so the accuracy needed while
    enroute, far from shore or other hazards, is not high so the "noon fix"
    method probably provides the requisite level of accuracy in most cases
    no matter what method you use to compute it. I have stated before that
    in the middle of the ocean one hundred mile accuracy may me sufficient.
    You may not plot the most direct course to your destination but you
    should be safe. When approaching the land or reefs then the level of
    accuracy needed increases until you reach the point that even normal
    celnav cannot provide the required level of accuracy and another method
    must then be used. You cannot use celnav to navigate up a river, to
    follow the Inter Coastal Waterway, or to enter a channel into a harbor.
    At some point you must switch to lights, bearings, ranges, buoys, or
    radar. Your celnav only needs to be accurate enough to get you to a
    point that you can switch to one of the other traditional methods. If
    you are using a backup method that does not provide the same level of
    accuracy as normal celnav then you must make your approach to land
    taking this into account. This might mean planning a different landfall
    than originally planned, waiting for daylight, calling for a pilot, etc.
    James N Wilson wrote:
    > George:
    > I have wrestled with this problem, and only reluctantly called the
    > position determination by the method a fix. Longitude /is /determined
    > at noon, but it is based on data taken before and after noon. It is
    > not a running fix, since no lines of position are advanced or
    > retarded. Or even determined. I do discuss a bit the error
    > sensitivity, but not as completely is in the reference, my 1985
    > /Navigation /paper.
    > I originally calculated Dhs at two different times bracketing LAN,
    > thus allowing for the different slope of the adjusted hs vs WT line. I
    > abandoned that in the interest of simplicity. For the example I used,
    > that made LAN eight seconds earlier. I considered that was not worth
    > the extra effort, considering other errors.
    > I didn't explore much at the extremes of Latitude and, as you have
    > pointed out, there are some where the Sun is never visible. So, the
    > method does have its limitations.
    > Thanks for your proper critique. As for the details of the arguments,
    > there's not much more in the two and a half page book. It was not
    > intended to be a treatise, but a description of a backup method. The
    > resultant surprise benefits were thoroughly examined to be sure that
    > they were true, but that's the extent of the effort.
    > Jim Wilson
    > >
    > ____________________________________________________________
    > Click here for great custom garage plans!
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