A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Observing Celestial bodies
From: Gary LaPook
Date: 2013 Mar 19, 23:29 -0700
|RAIM (Receiver Autonomous Integrity Monitoring) is probably not known to surface navigators but is vitally important to pilots who end up flying GPS instrument approaches to the runway and descending to within 100 feet of the ground without being able to see anything outside the plane because the plane is inside the clouds. The GPS must provide accurate information to ensure that the plane is descending along a safe path that lines the plane up with the unseen runway. There must also be a way to ensure that the system is working properly so that the pilot can be sure that the GPS is providing the required level of accuracy and this is where RAIM comes in. |
An airborne GPS requires signals from four satellites to determine a position while surface GPS only requires three. RAIM then uses the signal from a fifth satellite to check the accuracy of the GPS
position and if it is not accurate enough for safe flight warns the pilot.
I have suggested that a celestial navigator take a two body fix and then use the third LOP just to check the accuracy of that two body fix and to catch any errors, just like RAIM. See:
Go to page 76 of the PDF of the AIIM. http://www.faa.gov/air_traffic/publications/ATpubs/AIM/aim.pdf
--- On Tue, 3/19/13, Byron Franklin <byronink---.com> wrote:
From: Byron Franklin <byronink---.com>
Subject: [NavList] Observing Celestial bodies
Date: Tuesday, March 19, 2013, 5:34 PM
Byron: A person who understands the nature of errors avoids many pitfalls. Thus the magnitude of the error of individual lines of position is not a reliable indication of the size of the error of the fix
obtain from them. The size of the triangle formed by three lines of position has often been used as a guide to the accuracy of the fix, although a large triangle might be the result of a large constant error if the objects observed are equally spaced in azimuth. On the other hand, two lines of position with small errors might produce a fix having mach larger errors if the line crosses at a small angle.”Bowditch.
Byron “Let us use a large angle nearest to 90 degrees for accuracy for the best
What I have seen and read in the course books on Navigation is bad for the beginner.
The real needed information is not complete, sure you can get the job done using what is in the books,
but if you want to get the best fix you must understand how to think, the books won’t help.
In my Franklin Piloting Technique the navigator takes 3 Navaids two Navaids are selected for the fix. In this case the two closer Navaid become the ships position. The farthest Navaid become an indicator of accuracy and can give a direction + or – and the amount of error in degrees the Navigator can solve compass error and get the good fix. It has been taught at navy schools including the Naval Academy as well the Navigation Bowditch and Chief Quartermaster Manuel.
Taking a celestial round of stars has some similarity in that a round of sights with near to ninety degrees cannot be improved upon as far as the angle of cut, but you must have an
indicator as to accuracy, the same as the piloting technique. You must add another star or more. They will not improve the fix position, but can give a clue to the accuracy. If they do not support the near 90 degree best fix, chose the next best near 90 intersection. Often you will see an example in the book of three stars one with East –West Line of position No. two with a 045-225 No 3 with 000-180 and the fix in the center of the three. The 90 degree position should be the best choice. The present books will pick the center of the three lines as the idea choice! This is alright, I will not argue about this, but for myself I will pick the best intersection near 90 degree as my fix. I will be correct more times than the book. We are nit picking, but my experience on board a Radar Picket Liberty Hull for 5 years using sextant and loran A for fixes while tracking aircraft coming to the US shore. I had to get ships positions by sun-lines and shooting
stars for l fixes for reference tracking point at sea I had 27 years of at sea Navigation and Navigation teaching at Naval schools
My third ship a Liberty Hull with it old bad equipment and lack of on board knowledge taught me well.
The encluded is from my old Navy QM book on Navigation.
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