NavList:

   

Reply

Lu & Gary,

Yes, there are cases where the LOP accuracy is less than the accuracy
of the estimated track.  But I said at the outset that I'm considering
the opposite case, where the LOP accuracy is significantly greater
than the track accuracy.  This occurs quite frequently with running
fixes, such has in a one, or more, day's run with no new LOPs.

In the first attached figure (labeled Uncertainties) regions of
uncertainty are show in blue, the LOPs are drawn as lines, indicating
that the uncertainty perpendicular to them is negligible.  When LOP1
was acquired, the uncertainty surrounding EP1 was effectively
collapsed to a line by LOP1.  After a run of some length, the dark
blue line at EP1 expands equally in all directions as shown.  A MAJOR
OBSERVATION is that the distance from DR1 to EP2 is always less than
the distance from DR1 to RFIX.  (These distances are equal if the two
LOPs are perpendicular.)

That is, unless more is known about the shape of the uncertainty
region surrounding DR1, EP2 is always a better estimate than is RFIX.
Dropping a perpendicular from DR1 to LOP2 is a better estimate than
the traditional running fix.

The second figure (labeled Weird Uncertainties) shows an example where
by some weird circumstance the DR uncertainty just happens to have
grown, known to the navigator, more along the advanced LOP1 than
perpendicular to it.  It's difficult to image a practical situation
where this would happen.

All this is simply said by noting that the Estimated Position concept
fully honors the new LOP2 while retaining the information in the DR
that is not contradicted by LOP2.

And Lu, the two tracks in my previous figure need not be the same
length.  They are just an example of two tracks that have the same
perpendicular distance between LOP1 and its advance (speeds and even
times can be unrelated).

JK



--
NavList message boards: www.fer3.com/arc
Or post by email to: NavList@fer3.com
To , email NavList+@fer3.com

File:

File:

   

Reply