NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Jeremy C
Date: 2010 Dec 13, 23:53 EST
Recent
discussions on the Navlist have centered on fix precision as well as various
methods to reduce random errors in various sights. As I interpret these
discussions, the given LOP’s are assumed to be equally likely to be an accurate
LOP. This is not true in most
practical navigation.
What
seems to be missing is a discussion is the quality of LOP's based more on
intrinsic or environmental factors that may affect the value of that LOP to
the navigator, rather than random errors. These issues are not usually
discussed except as a footnote or aside when they really need to be given more
thought by an active navigator.
A quality LOP is one that the navigator
is very confident as to being a true line of position, i.e., that the position
of the navigator is actually on the line as opposed to being off of the line by
a significant margin.
I
want to start out with the easy ones which can be found in piloting. The
selection of navaids should center on availability and the type of LOP that can
be acquired from the particular aid. The easiest example of this is when
using buoys for LOP's. Even in the most basic navigation courses this is a
discouraged practice. Visual bearing lines and radar ranges can be
acquired from floating navaids, but they offer a much lower quality than a fixed
navaid due to the fact that they have a watch circle or can be entirely off
station. Both of these intrinsic problems lead to a low quality
LOP.
Another
example of low quality LOP is trying to take radar ranges on low-lying and
swampy coastlines such as are found on the US East Coast. The radar range,
even on the 3 CM radar can have error due to fact that swamps don't reflect
radar waves as well as rockier coast. A prudent navigator gives these
LOP's less weight than a strong return from a single spire of rock (the latter
being an excellent candidate for the Franklin Radar plotting
technique).
The
best quality visual navaids are fixed, dedicated navaids found in nautical
publications. These are precisely surveyed and of known characteristics to
the navigator. Slightly lower in quality are "other" fixed conspicuous
objects such as church spires and water towers. They are easy to observe
and fixed, but not necessarily as accurately surveyed on a chart as a dedicated
navaid. This is especially true in less developed
nations.
The
rules for piloting/terrestrial navaids are easy to remember and often taught in
the course of a class in navigation. The rules are less succinct in
celestial navigation. There are four factors that can affect the quality
of a celestial LOP: equipment, personal, reduction methodology, and
environmental.
The
first factor is fairly straight forward. If have a high quality, properly
aligned instrument (usually a sextant) you can get a fairly high quality LOP out
of it. Modern metal sextants offer excellent performance. Some
features of sextants can also affect sight quality under certain
conditions. Better shade selection,
especially with the use of polarizing shades can better define both horizon and
limb with certain bodies. Higher
power scopes can also offer superior results over low power scopes or sight
tubes from stable platforms. When
using a metal sextant that is properly aligned, you can be fairly certain that
you will have an excellent foundation for a high quality
LOP.
The
personal factor comes mostly from a bit of talent, and lot of practice. A
beginner will not usually have the ability to properly handle a sextant to give
a high quality LOP. Various errors are introduced in the use of the
sextant and reading of the sextant that adversely affect the quality of the
LOP. Fortunately with some instruction and a bit of practice, consistent,
high quality LOP's can be obtained.
Reduction
methodology these days is a matter of choice more often than
not. You can use slide rules, various tables, hand held
calculators, or computers to convert observations and NA data to useful numbers
to create LOP's. On the high end of quality are computers and
calculators. Using accurate programming to reduce spherical triangles, you
can easily exceed the precision of the instrument used in the observation.
Lower in quality are the more complicated tables like HO 229. These tables
are precise to 0.1', or a little better than metal sextants. Less accurate tables such as HO 249 are
in use and offer 1.0' precision. Slide rules offer varying degrees of
precision depending on their construction.
Lastly
are environmental issues. These are the most variable and least
predictable. Foremost of these is the quality of the visible
horizon. This can vary twilight by twilight and also from one area to
another. Under excellent conditions highly accurate LOP's can be obtained,
under very hazy conditions, you are looking around wondering if it is even worth
trying. The various conditions of the horizon should be mentally noted by
the navigator and LOP's judged by the existing conditions. One of the most
trying is when you have poor horizon contrast due to dark clouds at the horizon
at certain azimuths so that you can have high confidence in the quality of a LOP
from one star, and then low confidence in another star due to a poor
horizon. This particular environmental issue, unlike most other
errors and mistakes, is nearly impossible to see on a post sight analysis
unless notes were taken during the shooting.
The
platform on which you are observing can also affect your LOP quality. Steady platforms such as land or large
ships in calm seas offer high quality sights. When on a small boat tossing in heavy
seas, the quality of observation can drop dramatically.
Another
environmental issue is slight cloud cover. If there are high altitude
clouds, they might scatter starlight so that an indistinct and/or dimmer point
of light is reflected and may be difficult to place on the horizon.
The
most predictable environmental issue is the brightness of the body you are
shooting. You can be reasonably sure that the quality of a lower order
star will not be as good as a brighter star. This is due not only to the
lower contrast of the light itself, but also because when the star is visible,
the quality of the horizon is generally lower. Both of these factors
should factor into a decision about the quality of a particular
LOP.
Position
of the body in the night sky is also a factor. Bodies that are very low to
the horizon usually suffer from non-standard refraction. Very high
altitude stars make it difficult to determine the exact azimuth and can lead to
false observations.
In
the end, the navigator must decide if a LOP is of a high enough quality to use
in navigation. The navigator must decide how sure he is of the LOP,
weighing the type of navigational aid and the conditions under which it was
observed. This process can be used to weed out misleading information
in order to navigate safely.
At
this point, if I have low confidence in a particular LOP, I throw it out from my
reduction. The question for you
math centric folks is how we can “bias” our statistical analysis to give
different weights to high quality LOP’s while minimizing the statistical impact
of low quality LOP’s without completely dismissing them.
Your thoughts?
Jeremy