NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Old LOP's and the Celesticomp V
From: Richard B. Emerson
Date: 2000 Aug 05, 8:16 AM
From: Richard B. Emerson
Date: 2000 Aug 05, 8:16 AM
I posted a question about advancing or retiring LOP's over 12 hours old relative to the current fix time. I also said there were some other circumstances involved in the problem that developed from doing so. Here we go... Briefly restating the situation, on Tuesday, 20 June I did a noon sun line and an afternoon sun line and a morning sun line on 21 June. Using a Celesticomp V to advance Tuesday's LOP's to Wednesday morning, I consistently come up with a 30-35 mile error in position (verified by GPS and a bearing off Montauk Point, Long Island). An error of a third to half of that would have been acceptable (if somewhat disappointing). In trying to recreate what went wrong, it appeared that the Celesticomp was introducing the error. Reductions with a StarPilot and plotting the LOP's manually both gave a more reasonable position, within about 5 miles of our GPS position. I then wrote the author of the Celesticomp about the matter and received an explanation of what happened. In the Celesticomp manual, on pages 19 and 34, there is a comment: "...you can use any sight within 12 hours of fix time..." which I took to be merely an admonition to avoid using older lines. Not so; in fact the program reduces older (i.e., age +/- 12 hours from current fix time) LOP's , actually the sight series, with the fix time's Greenwich hour angle (GHA) and declination instead of the GHA and declination for time of the original sights. This introduces, depending on a number of factors, an error on the order of a few tens of miles. Now, before anyone writes "this calculator is trash", *that is not the case!* The real problem lies in the manual which doesn't make this circumstance clear. I have been using the Celesticomp in connection with plotting sheets and was trained by people who do the same thing, all with the expected accuracy *IF* each round of sights is plotted immediately and all running fixes, etc. are done on the sheets. Had I relied solely on the mathematical reduction of two or more LOP's, I would have gotten my due reward. Blind faith in numbers displayed on a screen or LCD panel is not justifiable. The result must be tested for reasonableness and cross checked by all means possible. When I advanced the LOP's down the DR track manually, I got the result confirmed by direct observation (a bearing from Montauk Pt, in this case, as well as a GPS fix). My point in writing is to simply point out a lack of an important warning in the Celesticomp's manual. I continue to believe the Celesticomp V is a useful navigation tool. Rick S/V One With The Wind, Baba 35