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post #21 of 33 Old 02-17-2009
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I'd also point out that currently 20 or so of the 31 satellites used for GPS are beyond their expected working life, and 8 or 9 of those have no redundancy in critical systems. The GPS system requires 24 satellites to be working for the system to function properly... If only the 8 or 9 that have seriously compromised systems fail... GPS is down for the count...

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post #22 of 33 Old 02-17-2009
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Quote:
Originally Posted by Omatako View Post
Q) Should I be worried?

A) I don't think so.
So you would think, but this isn't about total failure, it's about gaps appearing in less-travelled parts of the world due to, as SD points out, the number of functioning satellites falling below the 24 needed for full coverage.

It's not that it might not be there, but it might not be there if you need it in mist at night near the sketchier sort of reef.

I'm not trying to be alarmist here, but simply realistic that GPS is a system reliant on a certain minimum number of functioning elements. Reduce or remove those elements, and coverage is also reduced along with accuracy. Three satellites is good (but not if they are also at 20 degrees above the horizon), but more locks are better, because of this very interconnectivity of the signal processing.

The way people talk about GPS, it's as permanent as the stars and as reliable as the sunrise. Actually, it's more like the electricity grid: 99% fine, but parts of it are pretty old and prone to failure, which is why you keep candles and flashlights in a drawer. Also, to extend the analogy, if the GPS constellation is missing a few functioning satellites, does this impair accuracy? Would the average sailor be aware if his "circle of confidence" went from five metres to five hundred because there were just 21 satellites and not 24 working? A subtle or intermittent degradation is in some ways worse than a straight outage.

Even if they did, how many voyagers today have such "candles and flashlights" aboard and can use them? Half the people here can't heave to.

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post #23 of 33 Old 02-17-2009
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My wife's first CN class is this evening...purely coincidental, I assure you.
Umm...sure it is...

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post #24 of 33 Old 02-17-2009 Thread Starter
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I would think that there is the real possibility that there will one day be so much debris in low earth orbit that high earth orbit becomes all but inaccessible. When a nav. satellites come to the end of its service life, there may not be a way to foist another one up to replace it.
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post #25 of 33 Old 02-17-2009
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Maybe it is time to break out the Star Wars stuff for a little target pratice.

A nuke test in the desert along with a ICBM long range test might remind some countries of some things they may have forgoten.

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post #26 of 33 Old 02-17-2009
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Quote:
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Maybe it is time to break out the Star Wars stuff for a little target pratice.

A nuke test in the desert along with a ICBM long range test might remind some countries of some things they may have forgoten.
Indeed it might; but be careful what you wish for.

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post #27 of 33 Old 02-18-2009
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If GPS was out in certain areas, how far would a person travel while they were taking a fix using a sextant and time ticks before they actually got their fix?

We know that a fix by sextant is not all that accurate, but it will get you in the ball park...but the ball park could be pretty big in rougher conditions.

Sailing in reefy areas using a sextant could land you on a reef by the time you figured out where you thought you should be, but the sextant is better than nothing.
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post #28 of 33 Old 02-18-2009
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You can always stop, or nearly so, and confirming latitude with a noon sight is a matter of seconds. Confirming latitude and longitude is less than 20 minutes with current methods, and a lot less than that if you are in practice.

So say you are in a light current of about two knots, and you are drifting with it in a 10 knot wind toward the suspected reef. With all sail down (or better yet, sailing to windward with reduced sail to get you practically treading water), you might move at worst 1 NM.

A good sight will give you accuracy of 1-2 NM, so yes, some planning is involved, but as some charts (including electronic ones) have an error of 1 NM or greater (especially the ones last done by Capt. Cook (!) although he was surprisingly good), I would hope you would be keeping a dead reckoning and start trying to determine your location a lot farther away than that.

Seeing discoloured water or breaking waves from five miles off with binoculars from halfway up a mast shouldn't be that hard in daylight, especially if you have a bearing in mind down which to look.

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post #29 of 33 Old 02-18-2009
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By the time GPS fails from old age and is inaccessible due to the LEO minefield, somebody will have produced a sufficiently realistic sailing video game, so that the electroboaters who relied solely on GPS can enjoy sailing without ever leaving their homes, while mariners who practiced CN will have our sextants to fall back on.

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post #30 of 33 Old 02-18-2009
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Also, I'd point out that CN techniques are primarily for use when in the open ocean, and that a mistake of even up to 5 NM is generally not a big problem given that you're hundreds of miles from land. If you're within sight of land, you should be using coastal pilotage skills, not celestial navigation.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
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—Cpt. Mal Reynolds, Serenity (edited)

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