|
Author
|
Topic: Index Errors and running fixes
|
drlmk
|
posted February 23, 2007 04:43 PM
In a situation at sea with more or less constant temperature, how much change should I expect to see in the sextant index error from day to day? I understand this could depend on the make, materials and quality of the sextant, so let's say, for example, the Astra IIIB.
I'm asking because I recently used an Astra IIIB over a period of 2 weeks on a South Pacific cruise and the index error ranged from 1' (on) to 1.4' (off). It could have been my technique in determining the index error, but my position fixes agreed with the GPS position within 3NM.
From: Spokane
|
|
David Burch
|
posted February 23, 2007 09:21 PM
My experience is that the index error of a good metal sextant once settled in for some time, does not really change very much.
My guess is that variations in quality of the horizon (sharpness, sea-air color comparisons, etc) and the conditions of refraction could maybe cause a variation of some 0.5' but not what you observe — a variation of 2.4'.
So two more notes. first for the most accurate IC measurements, i would recommend the solar method we have notes for in our course and i think in the library somewhere. We even have a form for it. it uses the diameter of the sun. this gives a much better IC, but may not in fact be the best for an actual fix.
Second point, is, there maybe some issue with the actual measurements you are making since your fixes are only agreeing to within 3 nmi. This is a rather large error for well chosen sights done well. it depends of course on how fast you are moving at the time and other details of how you analyzed the sights, but this large uncertainly is a sign that there may be other factors to address beyond or including the IC error.
If you have more details, we can pursue it. In fact if you have all the data on the sights we can usually find out why they were off so much as 3 nmi.
From: Starpath, Seattle, WA
|
|
drlmk
|
posted February 26, 2007 08:30 AM
Thanks for your quick and helpful reply.
I'll look in my course book for the solar method for determining the IC.
As for the 3 mile discrepancy, I figure I just need more practice -- this was my first time doing sextant navigation out at sea. My fixes were all running fixes, using the sun only. I can probably narrow down the uncertainty with more practical experience.
Of course, the most likely explanation is that the GPS was off. I should have asked the captain to recalibrate his unit, so it would agree with me.
Thanks again for your help and your terrific courses.
From: Spokane
|
|
David Burch
|
posted February 26, 2007 10:00 AM
Running fixes are a challenge. We can look over the data if you like, but the key issue is good DR with accurate instruments in between the sights.
3 nmi uncertainly could indeed be perfectly reasonable for a running fix with less than excellent data.
here is the model to think on:
(1) record vessel course and speed and log reading
(2) take a quick series of 4 sights about one minute apart
(3) again record vessel course and speed and log reading.
(4) wait till the sun's bearing changes by at least 30 or 40° and then repeat (1) (2) and (3).
(5) be sure to have good DR records in between
(6) then, ideally, analyze the 8 sights using a calculator or software program like the StarPilot that will automatically advance all the lines.
in this scenario, your final fix will be limited mostly by the DR accuracy between sights.
-- to analyze by hand, use techniques in the course to choose the best sight of the 4 from each, and then advance the earlier to the later.
From: Starpath, Seattle, WA
|
|
drlmk
|
posted February 26, 2007 11:03 AM
I guess you're not buying my "recalibrate the GPS" excuse.
Thanks for the advice on running fixes. I calculated all my sight reductions by hand, but I did take multiple sights, etc. Still learning. I might have to look into the StarPilot, but I wanted to start out completely by hand, as the ultimate backup, for when the electronics fail. (I think you may have taught me that...)
Anyway, I certainly appreciate the sound advice from someone who has wrung more salt water out of his socks than I have sailed over in my entire career at sea.
From: Spokane
|
|
Dwayne Clark
|
posted February 27, 2007 07:14 AM
drlmk
Other than the IC issues do you like the Astra IIIb?
I know that is the sextant that David recommends and I have read good reviews elsewhere. I don't own one yet.
Sounds like you are doing great celestial to me!
Dwayne
From: Jacksonville, FL
|
|
drlmk
|
posted February 27, 2007 11:31 AM
I would say the Astra IIIB is an excellent value. As someone just getting started in celestial navigation, I didn't want to spend $1500 plus on a top-of-the-line model and -- on the other end of the price scale -- I was not impressed with plastic sextants.
You can certainly tell the scale on the Astra IIIB is not as finely marked as the high-end instruments, also, not as hefty, but it seems to work just fine. I figure any uncertainty in a fix is probably more likely a function of the technique and proficiency of the navigator than the instrument itself.
All in all, a good value. And why spend 1500 bucks on a sextant when you can get a darn good GPS unit for that price.
Avast, ahoy and all that,
-- lmk
From: Spokane
|
|
David Burch
|
posted February 27, 2007 12:15 PM
Yes, to confirm. We recommend the Astra IIIB deluxe model with a standard mirror. They sell for $510.
We would argue that no one regardless of experience will do better navigation with a $2,000 sextant, and that in fact the heavier expensive sextants are harder to use because of their weight. Most sellers of the heavy sextants argue that the weight is a virtue, but this is pure marketing. Even the top of the line sextant manufacturer at their peak—with the same heavy-is-good advertising from their resellers—had as the very top of their own line an alloy sextant praised for its low weight, but few resellers carried it because it was so expensive.
These heavier sextants weigh about the same as a half-gallon of milk. Hold this milk up in front of your eye for a few minutes, and maybe practice aligning the top edge of it with the top of a window across the street. A very simple experiment. Then do the same thing with a quart of milk.
I might add since the GPS was mentioned, that the Garmin Foretrex 201 is a remarkable device that sells for $140, so the comparison is even more stark.
From: Starpath, Seattle, WA
|
|
Dwayne Clark
|
posted February 27, 2007 12:44 PM
dr lmk and starpath...
Thanks for the feedback!!
drlmk...out of curiosity do you mind saying where you cruising in the South Pacific??
Thanks again,
Dwayne
From: Jacksonville, FL
|
|
drlmk
|
posted February 27, 2007 01:21 PM
Happy to tell my Tale of the South Pacific... Let's switch to the Cel nav Chat room channel.
From: Spokane
|
|
Dwayne Clark
|
posted February 28, 2007 07:00 AM
Ok..thanks.
Dwayne
From: Jacksonville, FL
|
|
drlmk
|
posted March 22, 2007 08:29 AM
I wanted to follow-up on Starpath's advice regarding my running fixes at sea. I purchased a calcculator and applied the sight analyzer function to my data. I then picked the best sight from each series and only plotted that one. My running fixes then fell dead on the GPS position points. No more 3-mile discrepancy.
Thanks for the sound advice and I'm having fun with calculated solutons. Now, if I could just get it to do my tax return...
From: Spokane
|
|
David Burch
|
posted March 22, 2007 11:16 AM
Yes, indeed. One the great values of a computer or calcualtor solutions is its ability to advance every single sight you take by the right amount automatically. With the StarPilot as long as you have the right course and speed entered into the device, every sight is advanced.
Even more valuable in the StarPilot is in the Sight Analyzer, we actually advance the theorectical values as well, so you are comparing the actual slope you should get when moving.
It is an interesting exercise to compare a set of sights with speed = 0 and then do it again with speed = say 20 kts (to enhance the effect) and look at the different results you would get when it comes to choosing the best sight from a series.
--david
From: Starpath, Seattle, WA
|
|
|
