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» Online Classroom   »   » Public Discussion of Cel Nav   » a = limits of reliability

Author Topic: a = limits of reliability

 - posted October 11, 2007 10:19 AM      Profile for DustyDan           Edit/Delete Post 
At what number should I begin to suspect my calculations when I get an 'a' value of, say greater than 40 miles?
From: North Dakota
David Burch

 - posted October 11, 2007 02:45 PM      Profile for David Burch           Edit/Delete Post 
the AP should be within 30 miles roughly of the DR, so a-values of 40 miles or so means your DR could be off some. In the normal course of events using assumed position you can get a values of 30' or so, so 40 is not off hand an alarm. The most likely cause underway of large a-values is poor DR, but in course work it could be just a mistake.

In several places in the course we recommend that for values of this size, say over 50 or so, then you should complete the fix, then use the fix value you get as a new DR, choose new aLat and aLon and do the reduction again. this will rule out math errors from the approximations we make in the procedure.

The best bet is always a set of star sights, since the intersection of 3 with the right time gives the right position, even if the a values are large, but the rule still applies. even if you do a 3 star fix and get a small triangle of intersections, it is best to call that fix the DR and redo the sight reduction.

so summary. 40' is the threshold into the concern region, 30' is quite possible with standard procedures, and over 50 means redo sight reduction.... if you redo the 3-star fix and the triangle is way big, then you have to do more sights, as something else is wrong.

From: Starpath, Seattle, WA

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