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View Poll Results: Blue Water- is a Sextant Necessary?
Absolutely essential 24 18.90%
Desirable, but not essential 52 40.94%
Good fun, but little practical use these days 39 30.71%
Don't waste your money and time on this 11 8.66%
Sextants make excellent dingy anchors. 3 2.36%
Multiple Choice Poll. Voters: 127. You may not vote on this poll

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Old 30-10-2012, 11:26   #46
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Re: Poll-Blue water. Is a Sextant Necessary?

The answer to the question depends on how you feel about having backups. Different philosophies for different folks. I think it's more of a philosophical answer than a quantitative yes or no answer. Some people are okay with no backup or one backups for something, others want as many backups as is practical. There is no saying who is right and who is wrong. What level of risk tolerance is acceptable?...it depends on your beliefs. One could ask the same question about whether or not a life raft is necessary.
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Old 30-10-2012, 17:42   #47
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Re: Poll-Blue water. Is a Sextant Necessary?

Quote:
Originally Posted by Adelie View Post


but GPS time will do if you keep track of how much it has varied from UTC. UTC occasionally has leap seconds which GPS doesn't. Currently you subtract 16sec from GPS to get UTC (GPS, UTC, and TAI Clocks). within 200nm or so
Hi Adelie,

I really do appreciate your knowledge of nav, but are sure about this 16 sec difference on the time dsiplayed on a GPS receiver. There is a 16 sec difference in so called GPS Time and UTC but, so far as I am aware, the time difference between UTC and GPS time is included in the satellite message, and the GPS can perform a correction to display the correct time. Think there are only a few older models of GPS which cannot perform this correction. The only thing to be aware of is if the GPS has been turned off. When turned on, it may miss the message containing the correction and will display GPS time. After a few minutes, the correction message will be received and displayed time corrected.
See
How accurate is the TIME DISPLAY on my GPS?
Just to be sure, I checked the time on two Furuno GPS 150 units, and 2 Fugro Seatex GPS recievers, and all agree with UTC time signal

Cheers
Nigel
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Old 30-10-2012, 18:15   #48
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Re: Poll-Blue water. Is a Sextant Necessary?

The Polynesian navigators were the best, sailed all around the Pacific without mod-cons like sextants. Used their eyes. Sun & stars when skies were clear, wave patterns when it was not clear. Could sail to little Islands thousands of miles away. As they got close they read the reflected wave patterns to zero in on it.
Author David Lewis studied their methods and sailed with a traditional Polynesian navigator.
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Old 30-10-2012, 18:31   #49
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Re: Poll-Blue water. Is a Sextant Necessary?

For those interested in how much one can do without read "Beyond the Blue Horizon" by Brian Fagan. History of how pre-Columbian sailors discovered how to navigate.

They had NONE of these items.

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Old 30-10-2012, 18:59   #50
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Re: Poll-Blue water. Is a Sextant Necessary?

If you have a decent timepiece but can't receive time signals:

You can re-establish the time to support approximate LOPs simply by taking a number of observations for 15 to 30 minutes either side of Local Apparent Noon

Graph these observations, draw the smoothest symmetrical line of best fit, and call the midpoint/(=highest point of tangency to a horizontal line) Local Apparent Noon.

(Or if you still have a usable 'puter you could write an Excel spreadsheet to crunch that, it would take longer especially if you enforce symmetry, but arguably be more fun)
A cheating way if the seas are calm:
As the sun approaches the zenith, note times and altitudes. Then as it declines, prset the sextant to the same altitudes, in descending order, and take the time when it reaches each altitude. This makes the job considerably easier: just average the pair of times at each altitude, then average that list of mean times (which should all be very close to LAN) again. As always, throw out any outliers. If too many outliers, throw out the navigator. (Times very close to noon will not be accurate because the altitude is changing too slowly to be detectable. The definition of "very close" will vary with atmospheric and seastates)

Work out where you are from a running fix based on an assumed position at that noontime.

Now apply the time difference (= LHA) between the assumed position and that new "fix" to correct your Local Apparent Noon back to UTC.

Do as many iterations as you feel inclined to do - I'd call it done when the answer was only a second or two different from last time.

Now do the same over a few days, in each case recording (rather than applying) the successive new corrections.

When you have tired of the whole exercise, or land is in the offing, average those corrections, and apply them to the chronometer.

This is laborious but simple in essence (simpler than my explanation might suggest)

Lunar distances are one of the classic methods, and give a result straight away.

However it seems from my limited recollection that you would have to be able to see sun and moon simultaneously, (and I assume for accuracy they need to be relatively near to each other, so first quarter only? ) but I don't personally know what's involved and how realistic it would be to master the calculations in a post apocalyptic situation (even if only a one-boat apocalypse)

(Seeing UTC doesn't work as an acronym for either English OR French, I can never remember the reason for its adoption.
To save the fingerprints of any other enquiring but uninformed bunnies: I Googled and found the following explanation:
In 1970 the Coordinated Universal Time system was devised by an international advisory group of technical experts within the International Telecommunication Union (ITU). The ITU felt it was best to designate a single abbreviation for use in all languages in order to minimize confusion. Since unanimous agreement could not be achieved on using either the English word order, CUT, or the French word order, TUC, the acronym UTC was chosen as a compromise.)

Shades of '70's Frost Report, announcing the opening of the elevator connecting the French Channel Tunnel with the English Channel Bridge. (Or maybe it was the other way round.... )
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Old 30-10-2012, 20:15   #51
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Re: Poll-Blue water. Is a Sextant Necessary?

Quote:
Originally Posted by nigel1 View Post
Hi Adelie,

I really do appreciate your knowledge of nav, but are sure about this 16 sec difference on the time dsiplayed on a GPS receiver. There is a 16 sec difference in so called GPS Time and UTC but, so far as I am aware, the time difference between UTC and GPS time is included in the satellite message, and the GPS can perform a correction to display the correct time. Think there are only a few older models of GPS which cannot perform this correction. The only thing to be aware of is if the GPS has been turned off. When turned on, it may miss the message containing the correction and will display GPS time. After a few minutes, the correction message will be received and displayed time corrected.
See
How accurate is the TIME DISPLAY on my GPS?
Just to be sure, I checked the time on two Furuno GPS 150 units, and 2 Fugro Seatex GPS recievers, and all agree with UTC time signal

Cheers
Nigel
By GPS time I meant the time that the machine uses which I assumed it would also display. Apparently I was wrong about that in the case of the GPS's you use. Do you know if there is a universal agreement among manufacturer's to apply the correction? Whatever unit you are using it would be simple enough to check against UTC before setting off ore check the manual to see if the topic is discussed. Interesting issue, I will have to look into this more.
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Old 30-10-2012, 21:21   #52
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Re: Poll-Blue water. Is a Sextant Necessary?

Quote:
Originally Posted by Andrew Troup View Post
If you have a decent timepiece but can't receive time signals:

You can re-establish the time to support approximate LOPs simply by taking a number of observations for 15 to 30 minutes either side of Local Apparent Noon

Graph these observations, draw the smoothest symmetrical line of best fit, and call the midpoint/(=highest point of tangency to a horizontal line) Local Apparent Noon.

(Or if you still have a usable 'puter you could write an Excel spreadsheet to crunch that, it would take longer especially if you enforce symmetry, but arguably be more fun)
A cheating way if the seas are calm:
As the sun approaches the zenith, note times and altitudes. Then as it declines, prset the sextant to the same altitudes, in descending order, and take the time when it reaches each altitude. This makes the job considerably easier: just average the pair of times at each altitude, then average that list of mean times (which should all be very close to LAN) again. As always, throw out any outliers. If too many outliers, throw out the navigator. (Times very close to noon will not be accurate because the altitude is changing too slowly to be detectable. The definition of "very close" will vary with atmospheric and seastates)

Work out where you are from a running fix based on an assumed position at that noontime.

Now apply the time difference (= LHA) between the assumed position and that new "fix" to correct your Local Apparent Noon back to UTC.

Do as many iterations as you feel inclined to do - I'd call it done when the answer was only a second or two different from last time.

Now do the same over a few days, in each case recording (rather than applying) the successive new corrections.

When you have tired of the whole exercise, or land is in the offing, average those corrections, and apply them to the chronometer.

This is laborious but simple in essence (simpler than my explanation might suggest)




Lunar distances are one of the classic methods, and give a result straight away.

However it seems from my limited recollection that you would have to be able to see sun and moon simultaneously, (and I assume for accuracy they need to be relatively near to each other, so first quarter only? ) but I don't personally know what's involved and how realistic it would be to master the calculations in a post apocalyptic situation (even if only a one-boat apocalypse)
For Lunars you need to see the moon and several other bodies, the son may be one of them but it is not mandatory.
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Old 30-10-2012, 22:56   #53
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Re: Poll-Blue water. Is a Sextant Necessary?

I find these threads interminable and they usually involve me being called a Luddite, so I will just leave these points here:

1) GPS is a man-made system of daunting complexity. Past reliability (and there have been outages and problems in the past) is no guarantee of future reliability.

Anyone who doubts this can meet me tomorrow on a Manhattan subway.

2) GPS can be affected by solar storms, meteoric mishaps, failure to pay for the cost of replacing old satellites, wonky firmware upgrades, green-tinged battery contacts and cases of dropsy on deck.

3) I like GPS. I use GPS. But I am quite conscious that on a long passage I might not be aware of the "geopolitical situation", and that GPS is not just for me, but for the military of the United States, and it can be turned off, and not just SA.

4) Stars and sun and moon cannot always be seen, but they also cannot be turned off.

5) Sextant use requires practice, like parallel parking or braking on ice. I see it as a useful skill for navigation, but also for the mind. The dirty secret about passagemaking is that sometimes it's boring, if pleasant in the trades. Sextant use, verified by GPS, can be improved to the point where it is quite accurate.

6) Sextants can be used to estimate distance to the coast, height of objects from the ground and a number of other measurement through the magic of what used to be grade-school geometry. They can connect the prudent mariner to his/her local environment of sky and sea in a way not customary to those who stare at screens.

7) The sort of bone-headed nav errors that have happened since the widespread advent of GPS are nearly impossible with a sextant. If you find your numbers seem to indicate it's midnight at noon, or you are in the wrong hemisphere, it is immediately clear that you've made an error in your sight reduction. The GPS user (in the form of a chartplotter) will take the lat/lon. as gospel, even when the reef is munching his bow. The sextant user, having a more generous margin of error, will approach charted objects with more caution and prudence, and will pilot by eye rather than by "what the plotter says".

GPS is a wonderful thing. It is not the only thing, and has virtues beyond its function as a navigational tool. My .02.
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Old 31-10-2012, 02:39   #54
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Re: Poll-Blue water. Is a Sextant Necessary?

Quote:
Originally Posted by Adelie View Post
By GPS time I meant the time that the machine uses which I assumed it would also display. Apparently I was wrong about that in the case of the GPS's you use. Do you know if there is a universal agreement among manufacturer's to apply the correction? Whatever unit you are using it would be simple enough to check against UTC before setting off ore check the manual to see if the topic is discussed. Interesting issue, I will have to look into this more.

Not sure if there is some international agreement on the above. My own Raymarine unit does display the correct time as well.
I have read of some real old Garmin Units that cannot apply the correction, (although with a firmware update they might be able to)
Another website
NIST time
indicates that most GPS recievers can apply the difference.
I think anyone with a GPS and is in need of an accurate time, should compare the GPS time display with a time signal, that will confirm if their GPS is OK to use as an accurate time piece
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Old 31-10-2012, 04:31   #55
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Re: Poll-Blue water. Is a Sextant Necessary?

Over 50 posts but still no one has even tried to get any reliable data on what level of risk loosing GPS is for ocean cruising. "Heard of a couple of boats" doesn't cut it.

Such is the land of forums..
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Old 31-10-2012, 04:37   #56
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Re: Poll-Blue water. Is a Sextant Necessary?

people talk talk about the GPS system going down like if that were to happen there wasn't bigger issues, like the end of the world, going on

this IF and that IF

one of those living/sailing in fear things where the fear doesn't cross over into other safety issues

sailing has been going on much longer than GPS and sextants have been around
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Old 31-10-2012, 04:49   #57
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Re: Poll-Blue water. Is a Sextant Necessary?

REalistically, GPS won't go down. Or if it does, we probably all will have bigger problems that the fact the GPS doesn't work.

Having and using a sextant is probably more an idea of being self-sufficient or perhaps self-contained is the better word. Being able to cruise and exist without the need for too much civilization.

Or am I getting too philosophical here?
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Old 31-10-2012, 05:34   #58
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Re: Poll-Blue water. Is a Sextant Necessary?

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Originally Posted by conachair View Post
Over 50 posts but still no one has even tried to get any reliable data on what level of risk loosing GPS is for ocean cruising. "Heard of a couple of boats" doesn't cut it.

Such is the land of forums..
For the GPS system to provide a coninous fix virtually worldwide a minimum number of 24 satellites are needed.
The number of usable GPS satellites has been steadily rising from about 26 in 1998 to 30 at the moment.
Probably the biggest change to reliability has been the development of very cheap GPS. This means that boats can afford multiple GPS receivers.
With a well designed system and a couple of units sealed in a faraday cage I think boats can achieve a very high reliability in their GPS systems. No system is foolproof, but this is true of all boat systems.
If I hit a container and sink my by boat my life raft better work. This sort of single backup for critical system is not uncommon. I sometimes think people worrying about quite unlikely possibilities, in non critical systems, (such as the US government shutting down the GPS system) are not directed their concerns in the correct place.
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Old 31-10-2012, 05:53   #59
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Re: Poll-Blue water. Is a Sextant Necessary?

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Originally Posted by goboatingnow View Post
Firstly GPS ( and glonass, and hopefully Galileo , then the Chinese gnss etc) will never be turned off. " we" need it more then the baddies do.
Whilst I agree with that, my concern is more localised GPS blocking / jamming by da Gubberment (for Security of people (onboard and ashore) as much as for any vessels themselves) - would it be such a stretch to imagine that if a VIP was on vacation somewhere (unnnanouced?) that part of the security blanket would be to jam GPS to stop either a missile, a drone or a suicide attack from happening. And my guess is that outside Gubberment circles that a few people in this world do also have these things to worry about...........Mid ocean not such a biggie - but when something to actually bump into more of a problem if unexpected......not to say that a Sextant would be the answer (IMO being able to DR from the last known position would be the more useful skill - and for that the primary skill is probably being able to not have an attack of the vapours from suddenly becoming "lost").

BTW, I can't use a sextant - I would like to be able to, but it's on page 127 of my "to do list". But not being able to use one would not phase me from heading over the horizon (lots else would! - but not the Sextant thing).
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Old 31-10-2012, 06:42   #60
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Re: Poll-Blue water. Is a Sextant Necessary?

Localised, tempory, GPS jamming is more than just a possibility it is a reality although it is generally close to coast caused by car thieves.
As you say celestial navigation does not really provide an answer to this. Celestial navigation is onfen "jammed" by clouds and the jamming can last for days or more. Sailing in he days before GPS was very different to today. I would encourage everyone to try it. It's a great learning experience.
You will need a lot of willpower not to uncover the GPS when you have been hove too for a couple of days waiting for a break in the clouds.

One of the things to watch out for when carrying a sextant is that it can encourage people to accept a minimal amount of GPS redundancy. A passage without GPS reinforces what a miricale it is to have a continuous an accurate position update.

A sextant is an alternative to a GPS, but unfortunatly it is a very poor substitute.
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