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Why Not To Teach Running Fixes
From: John Karl
Date: 2009 Dec 15, 22:51 -0600
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From: John Karl
Date: 2009 Dec 15, 22:51 -0600
No Frank, It's given (i.e., proposed) that the two vessels start out at P1. But the two tracks, P2 and P4, and RFIX all have errors (uncertainties). Perhaps introducing two vessels makes the discussion too confusing.
I'm a bit surprised that no one can agree (or can see) that the standard running fix uses only the perpendicular component of the advance of LOP1, while completely ignoring the component of the estimated track parallel to LOP1. This is easily seen in the figure below, it is central to the construction of the standard running fix, and it depends on absolutely nothing else.
The only time that this would make sense is if the navigator had absolutely no idea where she was along LOP1, or that her uncertainty along LOP1 is very large. Usually in practice P1 is an estimated position (call it DR, EP, whatever) whose uncertainty has been reduced to a line by acquiring LOP1, but still retains the uncertainly along LOP1 that it had before acquiring LOP1. This normally is of the order of DR uncertainties. The acquisition of LOP2 reduces this along-LOP1 uncertainty by an amount that depends on the orientation of LOP1 and LOP2. I discuss this in post #11174 and the two figures there.
JK
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