NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Accuracy of sextant observations at sea
From: Jeremy C
Date: 2010 Sep 23, 14:56 EDT
From: Jeremy C
Date: 2010 Sep 23, 14:56 EDT
Hi Gary,
I'll quote myself here [NavList 9887]:
On Sep 23, 2009 3:40 pm, Anabasi...---com wrote:
> I shot 70 star fixes on my trip from Japan to the USA and here are a few
> interesting statistics.
>
> 1) average fix error from GPS: 0.566 nm (low of 0.0 and high of 2.7 nm
> error)
>
> 2) 88.6% of the fixes were under 1.0 nm fix error.
>
> 3) 60.0% of the fixes were under 0.5 nm fix error
>
> 4) two fixes had errors under 0.05 nm (under 100 meters).
>
> My next project is to determine the average intercept based on the GPS fix
> as the DR but that is over 700 data points so might be awhile.
>
> Jeremy
> I shot 70 star fixes on my trip from Japan to the USA and here are a few
> interesting statistics.
>
> 1) average fix error from GPS: 0.566 nm (low of 0.0 and high of 2.7 nm
> error)
>
> 2) 88.6% of the fixes were under 1.0 nm fix error.
>
> 3) 60.0% of the fixes were under 0.5 nm fix error
>
> 4) two fixes had errors under 0.05 nm (under 100 meters).
>
> My next project is to determine the average intercept based on the GPS fix
> as the DR but that is over 700 data points so might be awhile.
>
> Jeremy
As you can see, these are position errors, not LOP errors. I also ran
standard Dev for my fixes and it was 0.5813 nm. I will add that my worst
fix was 2.7 nm off under awful conditions.
The trouble I have with LOP statistics is that there are so many variables
at play as render them only interesting but not very useful. With a good
sky and horizon I can pinwheel every time (intercepts well under 1 nm), if the
horizon and/or sky is terrible, I can have errors of several miles. Also,
can someone expect the aforementioned statistics in a bouncing sailboat as
opposed to a large ship? Probably not.
I just ran the statistics for the individual LOPs, 551 in all.
These were shot in a wide variety of conditions including the haze of the East
China Sea, near Cape Hope, and in the tropics. There is a wide variation
of error from day to day and twilight to twilight depending on the
environment.
I only ran them on the computer reduced sights as these use GPS fixes for
the AP as opposed to my tabular reductions. (planets, the moon, and stars)
I didn't use the computer for most of my sunlines, but there are a few included
in there.
Standard deviation for all 551 sights was .800225.
I did shoot 1 moonlight fix. The position error was about 2.5 nm
which would be good enough in a pinch, but well below my standard.
Jeremy