Re: How to choose among position lines?
Subject: Re: How to choose among position lines?
From: Royer, Doug (doug.royer@XXX.XXX)
Date: Wed Jun 11 2003 - 13:54:34 EDT
Federico,in theory one divides the number of observations one is planning to
make into 360* to find the proper Zn spread.So,if 8 bodies are to be
observed for a round the Zn spread used to minimize error would be 45*
between each one.The more sights in a round the less error for the round in
theory.In practice it is entirely differant.I've spent the last 20 years in
the U.S.M.M.,the last 5 as 2nd officer,and can tell you that with the other
duties required there is not time to plan or take that many sights in a
round.The maximum I've ever done was a 4 sight round with the Zn's spaced as
close to 90* apart as conditions allowed.We've trained and used the 3 sight
round because for all practical purposes that spread nulls out most errors
and cuts observation times down so other duties can be completed.If the Zn's
of the round are not spaced evenly it is advised that one bisects the LOP's
to form an MPP.Most of the time a single sight is taken,reduced,advanced and
combined with later sights to form a MPP.From: Federico Rossi
[mailto:fedeastro.rossi@XXX.XXX]
Sent: Wednesday, June 11, 2003 10:08
To: NAVIGATION-L@XXX.XXX
Subject: How to choose among position lines?
Hallo everybody,
I want to submit you a doubt I often encounter when I draw position lines to
find the position.
Usually, in the Italian merchant marine they used to plot 4 position lines,
aproximately 90° with respect to each other, and this configuration doesn't
leave space to doubt when it's time to draw the bisecants in order to reduce
the systematic errors.
The Americans used to be even more practical and adopted 3 lines spaced 120°
(obviously this in the ideal condition when you can choose which bodies to
shoot).
In "100 problems in Celestial Navigation", which I consider a very good
exercise book and I advise to everybody, instead the author often uses 5, 6,
7 or sometimes even 8 bodies, differently spaced in azimuth. In this cases,
are there general criteria to choose the lines to use (since the choice can
bring to significantly different positions)?
Thanks,
Federico Rossi
