NavList:

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