Travel Reference
In-Depth Information
systematic error. While our method cannot compensate for such systematic errors, the data
we collect in the process can help us identify and remedy them.
Assuming we are practicing celestial navigation for our own enjoyment and still have ac-
cess to GPS data, we can use our GPS position at the time of the fix as our Assumed Posi-
tion (AP) for the sight reduction calculation. If our sights were perfect, our intercepts
would come out at zero, resulting in an LOP that runs through our GPS position. We can
then tell immediately from our intercept values whether our sight series appear to be shif-
ted consistently above or below zero rather than being scattered around this central value.
If we spot such a bias, we can track down the root cause and start to correct for it, success-
ively perfecting our technique.
Early into a four-week passage between the Galapagos Islands and the Marquesas, we
found our sights to be high by one to two arc minutes on average. After carefully checking
the sextant and eliminating misalignment as a possible error source, the problem was
eventually tracked down to gripping the sextant too tightly, resulting in a skewed
swinging motion. This in turn led to systematically overstated altitude readings. Loosen-
ing the grip (after adding a wrist-loop to allay our fear of losing the precious piece over-
board) showed an immediate and permanent improvement in sight accuracy. Sometimes
simple solutions are the best. The ability to easily spot these biases is also the main reason
why we use a sorted series of intercepts rather than simply picking the median among
fully corrected sextant altitudes (which would save us eight subtractions in the process).
Another valuable exercise is to mark the best perceived sight in each series and then check
the actual accuracy based on the GPS position. It took us a while to figure out what makes
a “good sight,” but in the process we learned to judge the quality of our sights as we take
them. This becomes particularly valuable when we have to make do with a single sight,
snuck in between the clouds, without the luxury of a full five-sight series.
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