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Subject: Old LOP's and the Celesticomp V
From: Richard B. Emerson (navsys@XXX.XXX)
Date: Sat Aug 05 2000 - 10:16:03 EDT
I posted a question about advancing or retiring LOP's over 12 hours
old relative to the current fix time. I also said there were some
other circumstances involved in the problem that developed from doing
so. Here we go...
Briefly restating the situation, on Tuesday, 20 June I did a noon sun line
and an afternoon sun line and a morning sun line on 21 June. Using a
Celesticomp V to advance Tuesday's LOP's to Wednesday morning, I
consistently come up with a 30-35 mile error in position (verified by
GPS and a bearing off Montauk Point, Long Island). An error of a
third to half of that would have been acceptable (if somewhat
disappointing).
In trying to recreate what went wrong, it appeared that the
Celesticomp was introducing the error. Reductions with a StarPilot
and plotting the LOP's manually both gave a more reasonable position,
within about 5 miles of our GPS position. I then wrote the author of
the Celesticomp about the matter and received an explanation of what
happened.
In the Celesticomp manual, on pages 19 and 34, there is a comment:
"...you can use any sight within 12 hours of fix time..." which I took
to be merely an admonition to avoid using older lines. Not so; in
fact the program reduces older (i.e., age +/- 12 hours from current
fix time) LOP's , actually the sight series, with the fix time's
Greenwich hour angle (GHA) and declination instead of the GHA and
declination for time of the original sights. This introduces,
depending on a number of factors, an error on the order of a few tens
of miles.
Now, before anyone writes "this calculator is trash", *that is not the
case!* The real problem lies in the manual which doesn't make this
circumstance clear. I have been using the Celesticomp in connection
with plotting sheets and was trained by people who do the same thing,
all with the expected accuracy *IF* each round of sights is plotted
immediately and all running fixes, etc. are done on the sheets.
Had I relied solely on the mathematical reduction of two or more
LOP's, I would have gotten my due reward. Blind faith in numbers
displayed on a screen or LCD panel is not justifiable. The result
must be tested for reasonableness and cross checked by all means
possible. When I advanced the LOP's down the DR track manually, I got
the result confirmed by direct observation (a bearing from Montauk Pt,
in this case, as well as a GPS fix).
My point in writing is to simply point out a lack of an important
warning in the Celesticomp's manual. I continue to believe the
Celesticomp V is a useful navigation tool.
Rick
S/V One With The Wind, Baba 35
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