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Author
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Topic: LAN estimates while underway (first and second estimate)
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Bill Brucato
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posted February 11, 2004 11:44 AM
David, This question is in regard to predicting LAN while enroute.
What formula should I be using to convert course, speed,and distance traveled to arrive at the correct dr pos.? While we're at it, what is the "second estimate of LAN" and how much more involved is it to calculate.
Bill
From: Celestial Navigation Home/Online
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David Burch
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posted February 17, 2004 11:37 PM
I am going to split this up into two topics. It is best to post one thought and one question per topic. it makes it much easier for everyone to follow the thread of a discussion when we are on one topic at a time. When the search enigine is used, it lists the results by topic, so it makes things simpler for everyone if the topics are to the point. thanks.
The DR part is now addressed in a topic called "Computed DR positions"
--david
From: Starpath, Seattle, WA
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David Burch
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posted February 17, 2004 11:50 PM
Although it is never needed underway in any practical situation -- in a 30 footer, or in a aircraft carrier -- the way to compute the predicted time of LAN or sunrise or sunset accurate to the minute from a moving vessel is to do it in two steps, often called the first and second estimate of LAN.
The process is covered in Bowditch and it is asked for on USCG cel nav tests.
First estimate is done assuming you are dead in the water at your present DR position, which would corresponds to some WT, just as we do in our course materials...
To make this more realistic, after you get that result, which will be a time, sometime in the future, you then DR from the present position to that position, and do the computation again, assuming dead in the water at the new position. The latter is the second estimate.
For testing purposes, i might add that when doing this for sunrise or sunset, you need not interpolate for lat on the first estimate, but you must interpolate for the right sunrise/set time for the second estimate. This does not enter into the case for LAN sights.
--david
From: Starpath, Seattle, WA
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