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From: Peter Fogg (no email)
Date: Fri Jun 02 2006 - 03:39:09 EDT
Guy Schwartz wrote, on the 28 April:
"In the book 100 problems in Celestial Navigation. In the answer section
problem 1-2 it says "The LOPs have more spread than we would like, but we
rate the reliability as good"
In the perfect world all LOPs would cross at a given point, however the
system is not perfect, therefore when they say it has more spread than they
would like, how much spread is to much? Is there a certain distances that
relate to excellent, very good, good, fair and unuseable? Do these
distances relate to the reliability of the sights."
And I answered at the time. Now I would like to take advantage of being able
to include simple examples to illustrate this, and add to my response at
that time:
If these are two LOPs then the fix is at the intersection. Correctness is
quite dependent, among other things, on azimuth accuracy. So its a good idea
to shoot a third body if possible, ideally all three with a wide range of
azimuths. The third LOP might look like this:
The navigator would probably feel quite encouraged by this small enclosing
'cocked-hat' and might take it as an indication of a fairly accurate round
of sights, with the fix enclosed at the centre of the small triangle. And so
it might be. On the other hand, that third LOP could be a blunder. If the
mistake (poor sight, horizon, timing, calculation, take your pick) had not
happened, or was rectified, that more correct LOP might look like this
Now the navigator might feel disappointed, as the encompassing LOPs have
much more 'spread'. But if the actual position is located within them then
this is obviously a better, more accurate result despite the greater spread.
I guess the bottom line is that there is no hard and fast rule about LOP
spread, although as a generality a smaller encompassed area is more
encouraging than the converse.
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