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Old 31-01-2011, 06:44   #1
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Acceptable Accuracy in Celestial Navigation

I've heard that intercepts off 12nm or so from true position aren't bad, so the fact that I'm getting consistent sun sight intercepts of 4.5nm (and one of .02nm!) off is heartening.

What do others consider an "accurate" sight?
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Old 31-01-2011, 06:58   #2
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I was taught by an old unlimited skipper who used to get oil tankers around the world with a sextant.

On a sailboat he said he could get 2 mile accuracy or better but that I should be happy with 4-5 mile accuracy.
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Old 31-01-2011, 07:03   #3
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When I sailed cargo ships deep sea, could usually get between .5 to 2 miles accuracy, all depends on the ships motion, atmospherics etc.
I used a sextant on my B473 while teaching one of the crew astro nav, it was is own sextant, one of those plastics ones, I got an intercept of 5 miles, he managed an intercept of 15, but this was his first time taking a sight. I noticed that on the plastic sextant, the index error and side error seemed to change everytime the instrument was picked up.
If your getting intercepts of 4.5nm, then I think you have got the art of the sextant cracked, congratulations.
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Old 31-01-2011, 07:05   #4
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I get about 10-14 miles at sea on a small boat.

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Old 31-01-2011, 07:14   #5
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[QUOTE=sneuman;What do others consider an "accurate" sight?[/QUOTE]

My own personal definitions:
"Accurate" would be within a mile radius of actual position, hard to regularly achieve with a sextant on a sailboat.
"Acceptable" would be within about 5 miles of actual position, not hard to achieve with practice.

Most navigators compare their sun sight LOPs with a running DR, and I would be suspect of any sight that puts me more than 15 or more miles from my DR position.
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Old 31-01-2011, 07:28   #6
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15 miles out and I would check my sextant to make sure I didn't read a 4 for a 6, and/or I would check my math on the "add/subtract" forms I tend to use. There's a couple of steps in CN where a small error (or even a slip of the finger in the almanac) would give you an answer that was wrong, but not so far wrong that you question it.

Other errors, of course, put you in the wrong hemisphere, which even a newbie would be expected to notice.
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Old 31-01-2011, 07:51   #7
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Quote:
Originally Posted by sneuman View Post
I've heard that intercepts off 12nm or so from true position aren't bad, so the fact that I'm getting consistent sun sight intercepts of 4.5nm (and one of .02nm!) off is heartening.

What do others consider an "accurate" sight?
Depends on conditions - in a gale just getting it in the right hemisphere is a good starter!

Seriously, if those are sights from a yacht in normal conditions then that is fine. The point about astro for the normal mariner is to get close enough to land to be able to recognise land features.

The other point is to recognise your level of inaccuracy, and make sure you stay at least twice that from any dangerous feature when just using astro.

Dont forget that the echo sounder is still a useable tool when using astro.
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Old 31-01-2011, 07:56   #8
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An expert navigator, with a steady hand & accurate sextant and timepiece, can determine their position to within half a mile. These accuracies are usually achieved on shore.

If you're bobbing around on a small boat, an expert would be very happy to get to within a mile or two of the correct position. In fact, most of us would be satisfied with the previously mentioned 4 - 5 nm accuracy.

Since sextant angles are measured in degrees and minutes of arc (1/60th of a degree), measuring this angle to an accuracy of 1 minute of arc (1') will result in a positional accuracy of 1 nautical mile.

Accurate sextants can measure this angle to an accuracy of 0.2'. This suggests that, theoretically, one could determine their position to 1/5 of a mile.

Additionally, a good clock is required to accurately compute the GP of the celestial body. An error of 1 second in the clock will create a positional error of up to 1/4 of a mile.
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Old 31-01-2011, 08:06   #9
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4 to 5 Nm accuracy is acceptable. Does the landfall show up on the horizon where and when expected?
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Old 31-01-2011, 08:09   #10
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I should clarify that I'm usiing an artificial horizon on dry land.
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Old 31-01-2011, 08:10   #11
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You should be able to trim that down a bit.
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Old 31-01-2011, 08:23   #12
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also, I have found that using a small pool of water on a black parking lot is easier that a proper artificial horizon. I can't think of a reason not to do it that way.
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Old 31-01-2011, 08:27   #13
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Quote:
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I've heard that intercepts off 12nm or so from true position aren't bad, so the fact that I'm getting consistent sun sight intercepts of 4.5nm (and one of .02nm!) off is heartening.

What do others consider an "accurate" sight?
In considering an answer to this question, I think it's necessary to disambiguate your definition of 'intercept' and the method you are using to determine your LOPs. Using the long-hand Marcq St-Hilaire method, the assumed position that is necessary to use could easily give you an intercept of 25nm, while giving a fix accuracy of 2 miles or less. The intercept is the distance between the calculated altitude and the observed altitude. If you are using a computer program and are able to use your GPS position as the assumed position, then you should hope the intercepts are very small, as depending on how the program calculates this, it could very well represent your absolute sight accuracy.
Assuming this last case, then I would consider 5 NM as reasonably acceptable, and pretty darn good in a small boat. In a large ship, 2NM would be what I would consider the expected level of accuracy (ie. without another method to verify my LOP, I would trust it within 2 NM) although the real accuracy would generally be less than 1 NM.
I would suggest that if you are cultivating celestial skill in the event that GPS goes down, then you should practice using it with a less-accurate Assumed Position. Run a DR independent from GPS fixing, or offset from the GPS position to the nearest round lat/lon. and plot your intercepts and LOPs. When I've taken starfixes, I've usually tried to shoot 5 or more stars; 1 or 2 are usually omitted for poor accuracy or mis-identification; this leaves 3 or 4 LOPs that give a decent cross-fix (sometimes) or (more often) a small 'cocked-hat'. From this 'fix' I would assume a 1 NM accuracy, and GPS verification usually put us somewhere between 0.5-1 miles away, although I would consider any fix that had us within 2NM to be a good one. This is on board a destroyer - likely on a small boat, this accuracy would be harder to achieve.
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Old 31-01-2011, 08:35   #14
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i would say that accuracy is achieved when you can see land and know exactly where you are. if it is too cloudy to be accurate with a sextant, then its too cloudy for celestial, then you better hope that you can see where you are going.
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Old 31-01-2011, 08:46   #15
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In considering an answer to this question, I think it's necessary to disambiguate your definition of 'intercept' and the method you are using to determine your LOPs. Using the long-hand Marcq St-Hilaire method, the assumed position that is necessary to use could easily give you an intercept of 25nm, while giving a fix accuracy of 2 miles or less. The intercept is the distance between the calculated altitude and the observed altitude. If you are using a computer program and are able to use your GPS position as the assumed position, then you should hope the intercepts are very small, as depending on how the program calculates this, it could very well represent your absolute sight accuracy.
Assuming this last case, then I would consider 5 NM as reasonably acceptable, and pretty darn good in a small boat. In a large ship, 2NM would be what I would consider the expected level of accuracy (ie. without another method to verify my LOP, I would trust it within 2 NM) although the real accuracy would generally be less than 1 NM.
I would suggest that if you are cultivating celestial skill in the event that GPS goes down, then you should practice using it with a less-accurate Assumed Position. Run a DR independent from GPS fixing, or offset from the GPS position to the nearest round lat/lon. and plot your intercepts and LOPs. When I've taken starfixes, I've usually tried to shoot 5 or more stars; 1 or 2 are usually omitted for poor accuracy or mis-identification; this leaves 3 or 4 LOPs that give a decent cross-fix (sometimes) or (more often) a small 'cocked-hat'. From this 'fix' I would assume a 1 NM accuracy, and GPS verification usually put us somewhere between 0.5-1 miles away, although I would consider any fix that had us within 2NM to be a good one. This is on board a destroyer - likely on a small boat, this accuracy would be harder to achieve.
I am in fact using a GPS pos as my AP. This is just for practice to see whether my sights are "in the ballpark", so to speak.
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