NavList:

   

Reply

Geoffrey,

How do you determine the index error of a bubble
sextant?
I understand that some bubble sextants have two
optical paths, one of them for the horizon observations.
But if I understand correctly what sextant you are using,
it does not have
this option. So how do you determine its index error?

In particular, I don't understand the sentence in your message
"can be easily determined at the start of a round of sights".
How?

Alex.

On Fri, 6 Apr 2007, Geoffrey Kolbe wrote:

>
>
> 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
>
>
>
>
>
>
> >
>


--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply