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09-09-2024, 13:09
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#16
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always in motion is the future
Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 19,682
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Re: Time Precision Needed for Celestial
Once I discovered the Eco-drive watches from Citizen, which dial face is actually a solar panel so you never need to replace a battery, I never looked back.
I have this one for 7 years now and it’s as good as day 1: https://www.amazon.com/gp/product/B00KCF7JL6
Yes, record daily, weekly and monthly accuracy in the logbook while using GPS time as the reference.
__________________
“It’s a trap!” - Admiral Ackbar.
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09-09-2024, 13:17
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#17
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Moderator
Join Date: Mar 2009
Location: Denmark (Winter), Cruising North Sea and Baltic (Summer)
Boat: Cutter-Rigged Moody 54
Posts: 34,989
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by s/v Jedi
The Garmin GPS19x that I got was by default configured for GPS only. It required being configured from a Garmin plotter to enable GLONASS and that is all it can do.
I believe my B&G Vulcan was also configured for GPS only.
Maybe most GNSS sensors in use by cruisers only use GPS because that is the default configuration. I also only know of a few that can do three constellations and only if those are GPS + GLONASS + a third.
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My Furuno SCX-20 can use four at once -- GPS, GLONASS, Galileo, and QZSS. Apparently does not use Beidu.
__________________
"You sea! I resign myself to you also . . . . I guess what you mean,
I behold from the beach your crooked inviting fingers,
I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
Walt Whitman
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09-09-2024, 13:33
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#18
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Moderator
Join Date: Mar 2009
Location: Denmark (Winter), Cruising North Sea and Baltic (Summer)
Boat: Cutter-Rigged Moody 54
Posts: 34,989
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by Jammer
The usual recommendation on navlist and elsewhere is to use inexpensive digital watches. The Casio F91W-1 is usually recommended for this purpose because it has been produced in enormous numbers, has a long battery life, and is available inexpensively anywhere:
https://www.amazon.com/gp/product/B000GAWSDG
If EMP or lightning is a concern people put them in a tightly lidded aluminum box.
Typically The F91W-1 is accurate within several seconds a week, which can be further improved by rating. Recommendations vary. One approach is to buy three watches, mark them so they can be identified, rate them by recording the time delta vs. an accurate source over a period of time, and then use the rating to determine an accurate time aboard. Using three watches makes it possible to identify a watch whose rate has changed for whatever reason and stop using it.
Other authorities (including Starpath.com) suggest that using a single watch is sufficient.
IMO the main problem with this line of reasoning is that the rate for an inexpensive watch is going to vary with temperature, often considerably. How much this matters depends on use case, I guess.
How accurate is accurate enough time for celestial? Prior practitioners of celestial before the GPS era usually peg the practical, achievable accuracy of celestial (assuming no time error) at 3nm-10nm depending on who you ask with the 3nm claims probably involving hubris or ideal conditions or both. This is for highly skilled practitioners with higher quality equipment. A four second error in time is 1 NM at the equator. It makes sense to think in terms of an error budget. 10 seconds, 2.5 NM, would contribute to inaccuracy but still be reasonable, for example. Then you can think about how long a voyage can be between time references, with no radio time setting ability maybe use 4 weeks, so 2.5 seconds a week would be tolerable. Maybe more.
Another way to look at it is to think about how far off your latitude can be and still be acceptable during an offshore passage with a GPS outage. Trying to find the Marquesas or Bermuda would be an example, you would typically run down the latitude but if out of sight of the islands would want to be able to determine whether you're east or west of them. In reasonable weather you would be able to see the terrain from at least 20 miles away so your latitude would have to be off by 40 miles = 40 minutes of latitude = 2 minutes 40 seconds time error for you to think you're on a different side than you actually are.
Of course, time can be obtained from other sources. GPS, even if functional only briefly. WWV. Some broadcast AM/FM stations state the time at the time of the hour. Or you can shoot a lunar and get within a minute or two.
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Great post; thanks.
My friend consistently does almost 1 mile -- I'm a witness to this. Using GPS time however, so cheating I guess.
To put all this in perspective I guess we don't really care at all about such accuracy, especially since ocean crossings being generally east-west or west-east, latitude is much more important than longitude.
I always thought that if I lost GPS on a crossing it would really not be a big deal. I would do dead reckoning and just sail on using the magnetic compass until land is raised. Then just sail up or down the coast until some landmark is visible, then just do normal pilotage by three point fixes.
So I guess this whole discussion is somewhat academic.
But I'm thinking more and more that a HAQ watch, synchronized with GPS once or twice a month and kept in a metal box is probably the optimum solution for time. These are temp compensated and accurate to 10 seconds a year, or sometimes 5 seconds a year. So if set to GPS time at least once a month should be good to the second, which is more than good enough for any conceivable purposes.
__________________
"You sea! I resign myself to you also . . . . I guess what you mean,
I behold from the beach your crooked inviting fingers,
I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
Walt Whitman
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09-09-2024, 14:43
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#19
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Moderator
Join Date: Jan 2010
Location: Minnesota
Boat: Tartan 3800
Posts: 5,339
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by Dockhead
I always thought that if I lost GPS on a crossing it would really not be a big deal. I would do dead reckoning and just sail on using the magnetic compass until land is raised. Then just sail up or down the coast until some landmark is visible, then just do normal pilotage by three point fixes.
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It is useful to consider specific voyages. Imagine you have departed Bermuda for Barbados. The trick with that voyage is that you really don't want to try to head east and run down the latitude because the eastern shore of Barbados is rocky dangerous and has no ports of entry. You don't want to try to approach from the west because you're heading straight into the trades. So you approach the island from the north where you need accurate longitude or you'll miss and end up in Venezuela or Guyana.
Quote:
But I'm thinking more and more that a HAQ watch, synchronized with GPS once or twice a month and kept in a metal box is probably the optimum solution for time. These are temp compensated and accurate to 10 seconds a year, or sometimes 5 seconds a year. So if set to GPS time at least once a month should be good to the second, which is more than good enough for any conceivable purposes.
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My experience and that of others is that HAQ watches are typically not as accurate as advertised (but are close). Mine was advertised as 10 seconds a year, and it's off by a little more than that. It runs fast. Right now it's 9 seconds fast, having last been reset for DST on March 10 (six months ago). Cold weather will throw it off if it is not worn on the wrist. I had it gain 23 seconds in one month during a cold Minnesota January when I had taken it off several times and left it outside while working on machinery in the cold; this has only happened once.
As for EMP, It's all hypothetical unless you're willing to pay for EMP testing but basic quartz wristwatches are considered to be relatively low risk for EMP damage. They're not connected to anything, don't have an antenna, and have a metal construction that shields the movement pretty well (metal back, metal dial).
__________________
The best part of an adventure is the people you meet.
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09-09-2024, 14:55
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#20
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Moderator
Join Date: Nov 2011
Location: aboard, in Tasmania, Australia
Boat: Sayer 46' Solent rig sloop
Posts: 29,627
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Re: Time Precision Needed for Celestial
When we first left the US, it was a drug-store purchased Casio we used for our celestial. And when we took noon nights, we took sights for about 15 min. before and after noon. I was in charge of recording the times, when he said, "mark". He did the sextant work. Jim had a calculator that did a parabolic fix from the combined shots. These taken from the somewhat bouncy platform of a 30 foot boat. You big boat guys will find it a piece of cake, unless bringing the sun down to the horizon makes you seasick, as it did me.
As we approached Oahu, Jim predicted we'd see the Molokai light at midnight. He sighted it at 0015.
We took star fixes, too. Had special paper for plotting and advancing positions. And learned to give the hard bits plenty of room.
Don't make it harder than it needs to be.
Ann
__________________
Who scorns the calm has forgotten the storm.
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09-09-2024, 15:08
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#21
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Registered User
Join Date: Aug 2013
Location: Back in the boat in Patagonia
Boat: Westerly Sealord
Posts: 8,340
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by Dockhead
That makes a lot of sense, but that's also a lot of workload. Did you try to keep the chronometer accurate to the second? Did you check it every day?
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Hardly a 'lot of workload'. These days you don't even have to tune to WWV/WWVH on your HF. Simply look at time on watch, look at time on GPS, note difference in deck log. Maybe 5 seconds a day.
It doesn't have to be 'accurate' - you just need to know the error and the rate. And you only need to know the rate when your GPS has gone TU.
First three lines in my sight books. Chron, Error +/-, Chron Corrected.
Note to self. I must rate my two cabin clocks. I know that when I rejoined after two years absence during the covid business they both seemed to be showing something close to the coreect time.
And yes - in the old fashioned days the chron error ( or errors, fancy ships had two ) was checked daily. You would open the little hatch connecting chartroom to radio room. Ask Sparkie for a time signal. Little speaker on chartroom bulkhead would start ticking away...... tick tick tick BEEP - job done.
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09-09-2024, 15:21
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#22
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Registered User
Join Date: Apr 2019
Posts: 1,431
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Re: Time Precision Needed for Celestial
we did a couple years of cruising before GPS. Used a nice plath and simple timex (ironman I believe). Worked just fine.
Was that watch optimal, idk, did not really matter. It was pretty bulletproof and had long battery life - so no fuss no muss ever. With rate adjustment it was well 'accurate enough'.
The bottom line is that I would suggest you overthink this. If you hanker after a high accuracy watch, fine just buy one, all good; but certainly not necessary.
I also suspect if the GPS sats are brought down, you will have much bigger problems to worry about.
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09-09-2024, 15:59
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#23
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Registered User
Join Date: Oct 2007
Location: Lakewood Ranch, FLORIDA
Boat: Alden 50, Sarasota, Florida
Posts: 3,604
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Re: Time Precision Needed for Celestial
What did Frank Worsley and Shackleton use? It was good enough for them.
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09-09-2024, 16:07
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#24
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Moderator
Join Date: Jan 2010
Location: Minnesota
Boat: Tartan 3800
Posts: 5,339
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by S/V Illusion
What did Frank Worsley and Shackleton use? It was good enough for them.
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An ordinary lever-escapement pocket watch:
https://www.spri.cam.ac.uk/archives/...s/N:_999a.html
__________________
The best part of an adventure is the people you meet.
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09-09-2024, 16:19
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#25
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Registered User
Join Date: Apr 2019
Posts: 1,431
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Re: Time Precision Needed for Celestial
btw, just as an aside - the apple watch is technically an HAQ.
I'm not recommending it or anything - I would not like that it needs such frequent charging.
But I could see the Ultra model might appeal to dockhead. He might get other actual uses for the fancy watch than just have it sit in an aluminum box for the next decade.
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09-09-2024, 17:40
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#26
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Moderator and Certifiable Refitter
Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: C.L.O.D.
Posts: 21,308
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Re: Time Precision Needed for Celestial
Almost all problems can be fixed with the application of money or labour.
If you prefer the money solution, buy an expensive chronometer.
If you wish for a cheaper solution, use labour and preferably use someone else's labour.
Keeping accurate time has been solved a long time ago (no pun intended).
You probably have everything you need already on board, just needs some additional labour from the crew (the watch-keepers; no pun intended).
You need a time reference - GPS - tick.
When you lose the time reference, you need secondary time source. I suggest you insist all crew have a personal wrist watch ( any make / model, their choice).
Have everyone set their personal watch against the GPS time once a week (month?). Have a dedicated log book in which everyone has to log the variation of their own watch against GPS once a day (say first watch of the day or at evening drinks or whenever suits).
You will soon observe the accuracy / precision of the each watch and which can be used once the GPS fails; perhaps several can be averaged.
It's an almost perfect solution, no money spent by you, not much labour needed by you except to ensure the crew are complying with the log entries.
Not an onerous imposition on the crew, after all they should have some personal interest in navigating safely after the GPS is fried. After all, they are watch keepers, make them earn their position.
If you want to be fancy, add a below deck quartz watch which you rate as an incentive for everyone to do their bit.
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
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09-09-2024, 17:51
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#27
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Moderator Emeritus
Join Date: Oct 2009
Location: Manchester, UK
Boat: Beneteau 473
Posts: 5,617
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by El Pinguino
First three lines in my sight books. Chron, Error +/-, Chron Corrected.
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We must be of the same era, my sight books have the same first three lines.
__________________
Nigel
Beneteau 473
Manchester, UK
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09-09-2024, 18:41
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#28
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Registered User
Join Date: Aug 2013
Location: Back in the boat in Patagonia
Boat: Westerly Sealord
Posts: 8,340
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by nigel1
We must be of the same era, my sight books have the same first three lines.
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Could be, went to sea in '63, retired 18 years ago.
Our chronometers were landed ashore to an instrument maker at the end of each voyage - voyage length could vary in that company between 6 weeks and 2 years. Came back after servicing 'zeroed', not touched in the interim.
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10-09-2024, 01:36
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#29
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Moderator
Join Date: Mar 2009
Location: Denmark (Winter), Cruising North Sea and Baltic (Summer)
Boat: Cutter-Rigged Moody 54
Posts: 34,989
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by Jammer
It is useful to consider specific voyages. Imagine you have departed Bermuda for Barbados. The trick with that voyage is that you really don't want to try to head east and run down the latitude because the eastern shore of Barbados is rocky dangerous and has no ports of entry. You don't want to try to approach from the west because you're heading straight into the trades. So you approach the island from the north where you need accurate longitude or you'll miss and end up in Venezuela or Guyana.
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Very good point. If your destination is an island, and you approach it from North or South, then I guess it's desirable to be able to make your longitude close enough to be able to sight the island -- so a few miles depending on the size of the island.
Quote:
Originally Posted by Jammer
. . . My experience and that of others is that HAQ watches are typically not as accurate as advertised (but are close). Mine was advertised as 10 seconds a year, and it's off by a little more than that. It runs fast. Right now it's 9 seconds fast, having last been reset for DST on March 10 (six months ago). Cold weather will throw it off if it is not worn on the wrist. I had it gain 23 seconds in one month during a cold Minnesota January when I had taken it off several times and left it outside while working on machinery in the cold; this has only happened once.
As for EMP, It's all hypothetical unless you're willing to pay for EMP testing but basic quartz wristwatches are considered to be relatively low risk for EMP damage. They're not connected to anything, don't have an antenna, and have a metal construction that shields the movement pretty well (metal back, metal dial).
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Thanks; really useful information.
__________________
"You sea! I resign myself to you also . . . . I guess what you mean,
I behold from the beach your crooked inviting fingers,
I believe you refuse to go back without feeling of me;
We must have a turn together . . . . I undress . . . . hurry me out of sight of the land,
Cushion me soft . . . . rock me in billowy drowse,
Dash me with amorous wet . . . . I can repay you."
Walt Whitman
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10-09-2024, 02:53
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#30
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always in motion is the future
Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 19,682
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by Dockhead
My Furuno SCX-20 can use four at once -- GPS, GLONASS, Galileo, and QZSS. Apparently does not use Beidu.
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Well that’s not just a GNSS sensor but rather a satellite compass with multiple receivers.
Also, it is only 3 real constellations… QZSS is only a handful of satellites to increase GPS accuracy around Japan. It’s not useful anywhere else in the world.
__________________
“It’s a trap!” - Admiral Ackbar.
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