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In an otherwise correctly adjusted marine or 
bubble sextant, the main source of systematic 
error is Index Error, which can be easily 
determined at the start of a round of sights. 
Index Error in bubble sextants cannot be so 
easily determined. So it is necessary to adopt 
strategies to ensure that Index Error does not 
does not affect the accuracy of the final position.

I spent some weeks in the Western Desert in Egypt 
in March 2006 and had a lot of trouble getting 
good fixes to start with. The problem was that 
the desert is lot hotter than the Borders of 
Scotland, where my A12 bubble sextant usually 
resides, and so the Index Correction was no 
longer the -7' that it had been for the past 20 
years! Worse still, it seemed to change from day 
to day. I finally came round to adopting a 
strategy of taking sights on stars and the sun 
when in the Cardinal points. The problems went 
away and I started getting reliable fixes.

On my return, further research revealed that 
Royal Air force navigators used exactly the same 
strategy to ensure accurate fixes when using a 
bubble sextant. Too, surveyors of old who had to 
rely on a transit theodolite to determine their 
position by stellar observations, also used the same strategy.

Last month I returned to the Western Desert, 
again with an A12 bubble sextant - but not the 
same instrument I had used last year. Back home 
in Scotland, I had determined that an IC of +5' 
had to be applied to this sextant to get the 
correct altitudes. From previous experience, I 
did not expect this to be a good IC in the Sahara 
desert - and I was not disappointed!

Here is a round of sights 
http://www.pisces-press.com/desert/14thMarch.jpg 
I took when we reached Jebel Uweinat, the 6000 ft 
mountain in the South West corner of the Western 
desert. My estimated position was 21N55,25E10. I 
did rounds of sights on the sun (morning and 
evening), Polaris, Canopus and Regulus. I applied 
the usual IC of +5' to the resulting altitudes. 
Azimuths are in blue, position lines are green, 
the red dot is where the GPS said I was when I 
consulted it after I had obtained a fix by traditional means.

As can be seen, the box formed by the resulting 
position lines is rather large, about 10' on a 
side. From this, I determined that my IC should 
have been more like +10' as the sextant was 
reading about 5' too low. The resulting fix is 
not bad however, just a couple of minutes away from the GPS position.

A few days later and a bit further North, when 
camping next to Wadi Sora, (the "Cave of 
Swimmers" made famous in the movie, "The English 
Patient"), I did another round of sights, this time applying +10' of IC.

As can be seen 
http://www.pisces-press.com/desert/19thMarch.jpg 
this is altogether more satisfactory. All the 
sightings (averaged and reduced) produced 
position line intercepts less than 1' from the 
estimated position. The GPS position subsequently 
revealed that my EP was only about one minute 
away from my actual position. With this 
instrument, this result is really the best that 
could be hoped for. But it should be noted that 
the accuracy of my final fix is not actually much 
better than it was when my IC was an unknown quantity.

In conclusion, I was able to get reliable fixes 
far more quickly by moving away from the 
traditional fix using three objects about 120� 
apart with its resultant cocked hat, and adopting 
a strategy of taking sightings on four objects, 
each near one of the Cardinal points. I know that 
if the box formed by the position lines is 
substantially square, the errors are far more 
likely to be dominated by systematic rather than 
random errors. With a tradition 
three-position-fix you never know if random or 
systematic errors are dominating, so you can 
never be sure just how accurate your fix is.

Geoffrey Kolbe






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