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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Altitude intercept (a-value)

   
Author Topic: Altitude intercept (a-value)
RTM


 - posted September 14, 2015 08:56 PM      Profile for RTM           Edit/Delete Post 
I recently started learning celestial navigation using David Burch's 2nd edition which I believe is a good book.
I'm trying to understand why if Hc is > than Ho then the LOP is away from the azimuth line by the difference in minutes from Hc to Ho. I have reviewed this relationship several times but I'm missing something. The sextant observed height measurement and the calculated height with respect to the why the LOP is away or towards the GP.

From: Langley, B.C.
Capt Steve Miller


 - posted September 15, 2015 09:05 AM      Profile for Capt Steve Miller           Edit/Delete Post 
I think that the attached drawing will clear up the question you have posted.


From: Starpath
David Burch


 - posted September 15, 2015 10:28 AM      Profile for David Burch           Edit/Delete Post 
You might also want to skip ahead to chapter 10 where the theory of these issues is explained. That way you see the fuller picture of what is involved. (The sketch above is a valuable schematic way to see what is going on in your question, but the full picture needs the concept of parallel light rays.)
From: Starpath, Seattle, WA
RTM


 - posted September 15, 2015 10:20 PM      Profile for RTM           Edit/Delete Post 
Steve, David, thank you for the feedback, it was helpful. The drawing is good I have been wondering which angle was the reference. I know now the reference angle at AP is the Hc. Therefore Ho > Hc = toward, Ho < Hc away. I have referred to Chapter 10 very helpful.
I would appreciate your comments on the following:

My understanding of AP is the assumed position of the observer after starting with his DR position and applying sight reduction techniques. After correct calculations and proper sighting procedure. The observer has an a-lat, a-lon, Zn GP bearing of the sun and an a value.
With this information he plots this information on a universal plotting sheet.
If the Hc is the correct calculated value why do I need to include the Ho sight? Why move the LOP with a Ho sight measurement that may be incorrect? When the LOP is increased or decreased with respect to Hc buy the difference of Hc and Ho why is this done?

From: Langley, B.C.
Capt Steve Miller


 - posted September 16, 2015 07:28 AM      Profile for Capt Steve Miller           Edit/Delete Post 
The Assumed Position is determined so that the Sight Reduction Tables can be used. The Hc is based on this position - you are very likely not at this position. Your actual sight is represented as Ho after corrections. The difference between the Ho (observed) and Hc (calculated) will give you your correct LOP on which you are actually located. The calculated Hc is a reference to which you relate the LOP that you are actually on.
From: Starpath
RTM


 - posted September 18, 2015 11:54 AM      Profile for RTM           Edit/Delete Post 
Thank you Steve. I appreciate your expertise. The concepts are coming together I need more practise. I fine celestial navigation fascinating.
From: Langley, B.C.


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