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Subject: Re: Compact Data for Navigation and Astronomy
From: J.C.Sutherland (jcs@XXX.XXX)
Date: Thu Sep 09 1999 - 15:59:17 EDT
Hello Andres,
No, I am not using the program in The 'Compact Data for Navigation and
Astronomy'.
However, I would recommend this book as a very good alternative to the
'Nautical Almanac'. It is cheaper and the data covers five years. Also
the program and data on the CD that comes with it when loaded into a PC
will provide a complete Astro sight reduction package. It will evaluate
a position fix, obtained from combinations of sights, using any of the
navigational bodies and give a statistically calculated Ellipse of
Probability for the complete fix. Unfortunately the program on the disk
cannot be used by a calculator and for this purpose it is necessary to
use the data printed in the book instead. The book gives all the
formulae required and the data is easy to use.
A yacht navigator would not wish to rely on a single sextant
observation of the Sun or other body at sea as this would give no
indication of its reliability.
By taking several sights through the sextant one after the other within
a few minutes and by plotting the trend on graph paper, the linearity of
the group can be assessed. Any blunders can be rejected and the most
suitable sight selected for subsequent reduction to a P/L. This method
has been in use for many years but without some calculator help the
sights cannot be analysed statistically.
Using the Ti-82, this plotting can be done on the calculator screen, the
raw data compared with a plotted Linear(or a Quadratic)'Least Squares
Fit' equation evaluated by the Ti-82 and a statistical 'Figure of Merit'
calculated. See Note below.
Once this procedure has been run on the Ti-82, using a sea horizon and
data corrected for Index Error and Watch Error, the selected least
squares fit equation can then be used in my sight reduction programs.
these will first calculate the 'Most Probable Altitude' for the 'Mean
Time of Sight' derived from this input data. This combination of TIME
and ALTITUDE is used to calculate an INTERCEPT and AZIMUTH using the
'Compact Data' to evaluate GHA and DEC etc. Finally the 'figure of
merit' of the sight group used directly to indicate the Probable Error
of the plotted P/L.
I have written some sight reduction programs specifically for the
Ti-82,and based on the 'Compact data' above. If there is sufficient
interest I will send a sample of them to the group for an opinion as
soon as I have dotted the T's and crossed the I's.
FOOTNOTE,
For a training exercise, since it is not necessary for the sights to be
'Reduced' to a real P/L in order to find the 'Figure of Merit', IT IS
NOT NECESSARY FOR A REAL HORIZON TO BE USED to gain experience in using
the marine sextant. Timed sights can be taken ashore using a distant
roof or hilltop, for example, as a dummy horizon. Provided that this
dummy horizon is horizontal and distant and is vertically below the Sun
or Moon at the time of the sight.
Nor is it necessary to apply any corrections (except perhaps Index Error
if a plastic sextant is used). The Figure of Merit calculated by the
Ti-82 would be the measure of the observers skill. The aim of the
exercise would be to gain practice by attempting to reduce its value
while at the same time reducing the span of time needed to acquire a set
of observations.
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