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    Re: The Nonsensical Running Fix
    From: Gary LaPook
    Date: 2010 Oct 26, 02:45 -0700
    I think you have to agree with me, using the example I gave in my prior post that with a zero length DR leg between the two LOPs (a traditional fix), that you must accept the fix as the most accurate location even though it is far from EP1 due to a larger than anticipated error in DR1. You would then start the second DR leg from this fix.

    I believe you will agree again that you would use the fix as you actual location if the second DR leg is only one minute long (still a traditional fix) and again you would ignore EP1.

    But, at some point, either in time or length of the second DR leg, you then switch to your method and use EP1 to start the DR leg and then determine EP2 using your method. Does this change happen at some discontinuity or do you gradually start to move away from the running fix towards EP2, for example, perhaps at some point using a point on LOP2 half way between the traditional running fix and EP2? And then after some further DR run you then completely ignore the running fix and go only with EP2?

    Can you give us a formula to use (perhaps something like the Air Force formula in my original post) so that we can determine when we should make this switch over?

    I think the problem that I have with your method is that you start the DR leg from an EP and not from a fix. The EP is the best estimate of the actual location but it is only that, an estimate, and may be in error due to unexpected errors in the DR as you must accept in the case of an actual fix located some distance from the EP.

    The diagram that you attached to your prior post:



    show a situation where EP2 is not even in the uncertainty ellipse and the most probable position (MPP)  is located further along LOP2 which appears to confirm a problem with your method.

    Don't get me wrong, I am not denigrating your method, I am just trying to understand when it is appropriate and when it is not and how to make that determination while underway.


    On 10/25/2010 12:58 PM, John Karl wrote:

    Yes Gary, you do have a absurd result – because your zero-time run violates the conditions for using the EP running fix. Those are that EP1 was the best estimate after LOP1 was acquired, and that the run, from LOP1 to LOP2, has degraded DR2 to have much more uncertainty than the uncertainty in LOP2.

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