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10-09-2024, 11:04
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#31
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Registered User
Join Date: Mar 2021
Posts: 348
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by Breaking Waves
….but certainly not necessary…..
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I would not trust navigation instruments for correct Local Time.
Changing Heading change time.
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10-09-2024, 13:29
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#32
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Registered User
Join Date: Jan 2009
Boat: Tartan 37 #6
Posts: 518
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Re: Time Precision Needed for Celestial
Casio digital worn on inside of wrist, so you just have to slightly move your eye,check occasionally with BBC time signal
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11-09-2024, 01:44
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#33
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Senior Cruiser
Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 51,146
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by steamgoat
Casio digital worn on inside of wrist, so you just have to slightly move your eye,check occasionally with BBC time signal
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“The history of the BBC pips” ~ by Dr Emily Akkermans, Curator of Time at the Royal Observatory
➥ https://www.rmg.co.uk/stories/topics...h-time-service
Do We Still Need the Pips? ➥ https://www.bbc.co.uk/programmes/m001w0yw
Canada’s National Research Council Time Signal was broadcast daily, on CNBC radio, at 1:00PM ET, until it was discontinued, due to accuracy concerns, on October 10, 2023.
"The end of the long dash: CBC stops broadcasting official 1 p.m. time signal" ~ by Dan Taekema
➥ https://www.cbc.ca/news/canada/ottaw...gnal-1.6988903
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
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11-09-2024, 05:58
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#34
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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 Baronkrak
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That's weird.
I certainly trust my Furuno SCX-20 (or any other GNSS receiver) to give atomic-clock level accurate UTC time. I'm not interested in local time for navigation.
__________________
"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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11-09-2024, 06:00
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#35
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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 GordMay
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This is highly relevant to the discussion -- can we get an accurate time signal over HF radio?
The answer seems to be YES:
https://www.dxinfocentre.com/time.htm
So if the HF radio survives the lightning strike or whatever EMP event took out the nav system -- I guess that's a big IF -- then this problem solves itself, as any old timepiece whatsoever will be good enough for a few hours between time signals.
__________________
"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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11-09-2024, 06:12
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#36
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Registered User
Join Date: Sep 2023
Location: Cruising
Posts: 453
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by Dockhead
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Never once managed to hear anything on those frequencies on east side of Atlantic. Hundreds of broadcast stations so easy enough to find something.
websdr.org
Imho though there are enough things to worry about offshore on a boat that might actually happen..
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13-09-2024, 09:55
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#37
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Marine Service Provider
Join Date: Feb 2012
Location: Kauai Hawaii
Boat: home built 31' Hartley Tasman
Posts: 307
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Re: Time Precision Needed for Celestial
That's the only type of navigating I ever did when I sailed about the world in 1985.
Celestion navigation is actually quite simple when you have a decent system worked out. Up to date almanac and a book of sight reduction tables, a decent sextant, I had an old Weems and Plath 1934 bronze sextant, and to keep very accurate time,....wait for it, a Casio lithium battery quartz wrist watch
That's all I ever used and all I ever needed... I even dived with it. What I used to check it's accuracy every month or so, was to use my old SSB Yaesu 7 radio receiver and one can tune in to the time channel and get a pin point accurate time signal...I used the radio to receive stations from around the world for entertainment and news as well (The voice of America, the BBC, Russia, Australia, New Zealand etc...)
I'm glad to see a return to this form of navigation. I sailed for many many thousands of miles, and never got lost
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13-09-2024, 11:34
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#38
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Registered User
Join Date: Mar 2012
Location: Michigan/Bocas del Toro Panama
Boat: Corbin 39
Posts: 260
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Re: Time Precision Needed for Celestial
I use an android app called "atomic clock & watch accuracy tool". This app makes it easy to keep track of the rate of change and develop a correction factor to use when you are without access to correct time. It allows you to track the accuracy of several time pieces.
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13-09-2024, 15:34
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#39
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Registered User
Join Date: Oct 2020
Location: New Zealand
Posts: 77
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Re: Time Precision Needed for Celestial
The answer to the question implied in the OP's title to this thread is that a four-second time error gives you at most a one mile position error.
You don't need accurate time for a noon latitude shot—just measure the sun's maximum altitude.
All I ever used for time was a cheap quartz wrist-watch, corrected occasionally from WWV on 10 or 15MHz.
More important than split-second time accuracy is an accurate knowledge of the current that has affected you since you plotted the position line derived from your previous sight. For example, after getting your latitude at noon, to get your actual position you must bring forward the position line from your morning sight to cross that latitude line. Bringing any position line forward requires you to know your course and distance made good, and that depends on the accuracy of your steering and distance through the water, together with the strength and direction of the current that has affected you. In the absence of GPS, the latter will seldom be better than an educated guess.
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13-09-2024, 16:26
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#40
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Registered User
Join Date: Jan 2010
Location: Portland, Oregon, USA
Boat: 31' Cape George Cutter
Posts: 3,325
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Re: Time Precision Needed for Celestial
I kept a cheap Casio watch on the handle of the sextant case. Of course keeping an eye on the rating is desirable. But consider that each second error is 1/4 mile error at the equator, and less as the latitude increases. You are kidding yourself if you think that you can get a sight from a small boat that is better than a few miles, so this source of error is not the sort of thing to lose sleep over.
Jedi's observation that it is not possible in many cases to get access to multiple GNSS results is true for most cases. The way to do that is to use a USB GNSS dongle, or an NMEA 0183 with RS-422 adapter, to connect to a computer. The GNSS receiver would need to be a uBlox 8 or 9, navigation grade receiver. And the computer would need to be Linux with GPSd software. Also, the maker of the dongle would have had to have provided some form of persistent memory, such as flash, to store the settings. Then you could use the uBlox tools that come with GPSd to choose which GNSS(s) you wish to use, and store the preferences. Hint: it's not worth it. I have activated (several years ago) GPS, GLONASS, and Galileo (or BeiDu); the uBlox receives them and then selects out poor performers until it has about a dozen satellites that it actively uses, while tracking many others. GLONASS gets dropped quickly - it is garbage. At that time GPS ended up being the exclusive source. Both Galileo and BeiDu should be of similar quality today but I haven't tested it again - it is a PITA to do and any accuracy gains ridiculously small. But if you wanted the fallback then buy one that is pre-programmed to receive 3 (or all 4) constellations and let the firmware choose.
As for time, it is possible to get lab-grade time out of GPS using GPSd again. Originally this was done using a separate PPS (pulse per second) signal that could be read by a serial input. These days the integral USB interface within the GPS module incorporates the timing, and GPSd can then provide it to the system. IIRC it can be sub-microsecond accurate, but the big problem is that modern microprocessors have too many interrupts happening to maintain anything like that accuracy. For that purpose a simple Raspberry Pi with minimal OS is hard to beat. GPS time is very useful for an internet (NTP) time server but I don't see the value on a boat. Cool stuff though.
Greg
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13-09-2024, 17:22
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#41
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Registered User
Join Date: Mar 2021
Posts: 348
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Re: Time Precision Needed for Celestial
Quote:
Originally Posted by Dockhead
That's weird.
I certainly trust my Furuno SCX-20 (or any other GNSS receiver) to give atomic-clock level accurate UTC time. I'm not interested in local time for navigation.
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Very British. At that location tide tables times are in local standard time.
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13-09-2024, 18:08
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#42
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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 CarinaPDX
.. But consider that each second error is 1/4 mile error at the equator, and less as the latitude increases. You are kidding yourself if you think that you can get a sight from a small boat that is better than a few miles, so this source of error is not the sort of thing to lose sleep over.
..
Greg
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At 45º the error would be 0.7 ( cosine of the Lat) x 0.25 = 0.175 miles.
So let us assume you haven't corrected your watch time for thirty days and it has a rate of 1 second a day - a reasonable assumption with a half decent watch. Your error in longitude will be a mighty 5.25 miles. As you say, neither here nor there in the grand scheme of things.
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13-09-2024, 18:45
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#43
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Registered User
Join Date: Aug 2021
Location: PNW
Boat: 35 Ft. cutter, custom
Posts: 2,728
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Re: Time Precision Needed for Celestial
Before the U.S. bacame involved in WWII, when Gen. George Patton was commissioning his new schooner, "When and If", he inquired about chronometers.
He was told that with one he could never be sure that it was correct.
So, he ordered two.
Then he was told that with two he would never know which one was right, so he ordered three.
A good friend of mine is a collector of watches, (all makes/models/price ranges,) my joke to him is that he never really knows what time it is.
__________________
Beginning to Prepare to Commence
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13-09-2024, 18:57
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#44
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Registered User
Join Date: Dec 2012
Location: Norfolk, VA USA
Posts: 720
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Re: Time Precision Needed for Celestial
Just remember: Joshua Slocum famously circumnavigated with celestial and "a cheap tin clock".
And you can always recover UT (GMT) by shooting a lunar.
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13-09-2024, 19:02
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#45
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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 Baronkrak
Very British. At that location tide tables times are in local standard time.
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I would suggest that local or zone time is used all the time except for when using time in celestial calculations, radio skeds, and a few other isolated situations.
I can't imagine calling up harbour control and giving an ETA in UTC.
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