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Author
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Topic: Questions on USCG Cel Nav Notes
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JimG
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posted January 28, 2006 01:14 AM
In the practice test FVXX1, July 18, 1989, question 4-(5), you are asked to do a running fix using 3 sun lines all of which were taken within 1 hr & 5min. Consequently the angle of intersection of all 3 LOPs is very shallow, & subject to error. Aren't we taught to wait till we have close to a 60 deg angle of intersection. Why would the Coast Guard ask this kind of question with 3 sun lines taken so close together??
From: Steamboat Sp, Co.
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David Burch
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posted February 02, 2006 06:59 PM
Yes these are indeed typical running fix problems from a USCG test — not just this one but all USCG cel nav tests—and you are right, we would not do 3 sunlines for a running fix this close together in time when actually underway.
First the intersection angles are too small (as you point out), which enhances plotting errors, and second we are better off taking more sights of just two suns bearing some 40 or so degrees apart. This could be sight sessions separated by over 2 hours apart, or even just a few minutes apart if the sun is near overhead. So it is not really the time interval that matters, but the intersection angles.
I must add, however, that if you do your sight reductions and plotting with a calculator or PC program that takes into account vessel motion (there are many free programs online), then the plotting uncertainty essentially goes away and these can be good fixes. This is in fact an advantage of computer cel nav. But the coast guard tests are manual plotting tests, and left with manual methods we would avoid this. And there is no virtue of combining 3 sights for a running fix. Any two would yield the same position accuracy... though this is a bit of a story to explain this.
Here is what is positive that can be said about these problems if we think about them just as test questions, ie designed to see if you know certain aspects of cel nav and can plot carefully.
First they isolate a couple steps in the process, since they give you Ho already (ie no sextant correcting tested) and they give you the GHA and Dec, so no almanac look up tested. They isolate out the step of choosing the assumed position and the use of the sight reduction tables and the plotting. If they gave you the full process, it would take too long. Also they are testing on your knowledge of a running fix.
So despite the risk they run of making new navigators think this is the way to do a running fix -- it is not -- they do test your ability. The ability to plot carefully is emphasized over and over again in USCG nav tests, and i must say we support that goal. it is fundamental to good navigation to be able to plot carefully and efficiently, which only comes from much practice.
Note that we do have several of these types of problems in our course materials as well (for the same reasons) and the ones we provide have the very many intermediate step answers provided. So if you do not get the correct answer, you can just start back the list of intermediate steps to see where you went wrong. If all the intermediate numerical values are correct, then you have a plotting error.
The most common error in these types of problems is not doing the DR to the time of the first sight, but instead using the DR given with the problem statement.
Next, if we put on the how-to-pass-the-test hat, note that the right answer will always be on the last LOP since that is the time they ask for the fix. Hence a short cut is to do the last LOP very carefully, then plot the answers. the one on the LOP is the right one, needless to say, we do not recommend solving the problems that way.
From: Starpath, Seattle, WA
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