What's the time?

[Carduelis]

I'm doing my first Transat shortly and I'm wondering what is the latest thing in timepieces suitable for astro nav. The options that occur to me are:

1. Quartz watch - I already have one, and its reasonably accurate. I guess this can be corrected by radio time signals (when we can hear them) in the traditional way.

2. Radio callibrated (so called "atomic") clocks. I'm sure that these are very accurate when they can receive a signal, but how much do they drift when out of range?

3. Use the GPS clock. The textbooks warn against this because while the system is internally very precise, apparently it is drifting away from 'real' time. . .

Any ideas/recommendations anyone?

 

[Malabar]

Best use a proven quartz clock, checked whenever possible by radio time signal. If you use GPS as a time check you need to convert GPS time to UTC by subtracting, currently, 15 seconds. Background information is available at http://leapsecond.com/java/gpsclock.htm

 

[bedouin]

I should think that the Radio controlled clocks have a range of at most a few hundred miles so aren't much use on a transat.

For really precise times then use GPS, but make sure you make the correct adjustments. However relying on GPS to give the time for Astro is a little perverse, if the GPS is working then you don't need Astro.

More realistic is probably a set of Quartz clocks / watches. They are so cheap that you might as well have three. Ideally you should have owned these for long enough to have calibrated them.

You can of course pick up time signals on radio to calibrate your clocks at sea

 

[geoid96]

..... If you use GPS as a time check you need to convert GPS time to UTC by subtracting, currently, 15 seconds.

Beware you don't apply the UTC / GPS offset twice. The GPS signal includes the current offset value and most receivers apply this automatically and display UTC time, not GPS time.

 

[glynnffc]

Time

If you have internet access you could run this:

http://www.yugop.com/ver3/stuff/03/fla.html

 

[Carduelis]

So the GPS time is likely to be accurate then?

Beware you don't apply the UTC / GPS offset twice. The GPS signal includes the current offset value and most receivers apply this automatically and display UTC time, not GPS time.

So you are saying that the displayed time on a modern GPS is likely to be accurate? That's good news. Next time I get on board I'll verify it against the Pips.

Agree it is a little perverse using GPS for this purpose, but actually I'm doing the astro just "for fun" anyway - actually its for a YMO qualifying passage. Doubtless the GPS readings will be used for the real thing.

 

[geoid96]

If you have internet access you could run this:

http://www.yugop.com/ver3/stuff/03/fla.html

This application just displays the time on your PC clock. It is not linked to a time server and if your PC is out, so will be the display.

I keep my PC syncronised with UTC using the following free application:
http://www.symmetricom.com/resources/downloads/symmtime/

 

[sarabande]

if you are using a digital radio to check the pips, there is a delay of several hundred milliseconds.

And as we know, seconds mean miles.

 

[maxi77]

I would suggest that the standard quartz watch which you have calibrated the error rate on is the only answer, any other depends on outside contact.

This may be usefull to.

http://www.amazon.com/Emergency-Navigation-Pathfinding-Techniques-Inquisitive/dp/0071565582

 

[jetwake]

Forgive my ignorance, I have never used astronav for real , but is the likely accuracy REALLY going to be so badly affected by a few seconds of time?

I struggle to believe that an astronavigator in the mid atlantic would require accuracy of his chronometer to the second to be able to plot a position. Surely the star shots will be less accurate than any clock by a factor of "lots".

David

 

[blackdogsailing]

I have always used my Casio Quartz watch which has Dual time. 1 permanently set to GMT ( or UT {whatever} ) and 1 to local. Over the 3-4 week period without a time signal, very accurate.

Chris

 

[sarabande]

Hi Jetwake

being unsure of your position by a couple of miles in the middle of the Atlantic is not likely to have much impact on your state of well being. Which is more than can be said for a similar position uncertainty when in coastal waters.

Very very roughly, one second is one mile.

 

[jason -and the arguenauts]

Best solution is to forget the primitive technology of sextant and astro, and equip yourself with 3 good GPS sets and plenty of batteries.

GPS sets dont mess up the maths. They still work in 100% cloud. They dont get knocked out of calibration when dropped. They dont require you to try to operate balance and outside on deck. They give fixes to metres not miles. They are quick. And they are in my experience 100% reliable.

If you want to do astro for fun and as a sentiomental link to the past, then a normal quart watch is accurate enough for mid ocean.

Good luck with the trip by the way

 

[Deleted member 36384]

Cheepo Digital Watches

I bought 4 cheepo digital watches, set the time on them to GMT by the speaking clock and set sail. All 4 were kept in the chart table and only taken out for sights. Not an issue and Barbados popped up on the nose as expected. I cant remember if there was any drift on the watches but there probably was. I only used the one watch. the rest were back ups.

If you are serious about Navigating with astro only then buy a proper time piece for the job.

Later when I had a Sat Nav I found that my astro with the watches was always south of my Sat Nav plots by 5 to 10 miles.

Astro is still a valid method of navigating and isn't consigned to fun and sentimentality, then again Morse code is also a valid method of communication!

 

[peterb]

More realistic is probably a set of Quartz clocks / watches. They are so cheap that you might as well have three. Ideally you should have owned these for long enough to have calibrated them.

I believe that quartz watches are more stable when worn as a wristwatch than when kept in, for instance, a chart table. The reason is that the crystal cut used in most watches gives a noticeable shift of frequency with temperature, and contact with the wrist gives a good stabilised temperature.

 

[bilbobaggins]

And they are in my experience 100% reliable.

If you want to do astro for fun and as a sentiomental link to the past....

And your experience of transocean navigation is both professional and extensive? As for sentimental links to the past, I take it you are similarly scathing about the nonsense of using sails, spars and cordage to manoeuvre a vessel about the seas....

Bolleaux!

 

[blackdogsailing]

Bolleaux!

Well said

 

[jetwake]

And your experience of transocean navigation is both professional and extensive? As for sentimental links to the past, I take it you are similarly scathing about the nonsense of using sails, spars and cordage to manoeuvre a vessel about the seas....

Bolleaux!

Well that WOULD be a valid argument too. Commercial vessels dont use sail for good reason. The airliners I fly don't use piston engines for good reason. It doesnt invalidate the other options though: using sail for the reason "because i like it" is as valid as "i use diesels because they are convenient".

The use of astronav these days IS arcane and quaint. It doesnt mean it doesnt wstill work as good as ever. I fly my airliners on GPS not astro for very good reasons, the same reasons most sailors use GPS I am sure. But it doesnt invalidate astro. So, lets lay off the Bolleaux bit.

David

 

[smartcom]

With all the inaccuracies of taking a sight on a small boat bobbing around in the ocean, a standard quartz wristwatch is perfectly accurate.

As already said, a GPS receive will show the time in UTC, using the leap second correction at the time the firmware was put together - which may be a few years ago, so you could look up the difference between then and now if you wanted to.

If you want the ultimate, a friend of mine has a company "Quartzlock" that makes super accurate clocks. You advance and retard the clock a nano second at a time, and they are accurate to about 1 second in the life of the universe!

Coming back down to earth, you could take a look at our WinAstro software to do all the sight reduction calcs, using its built-in perpetual almanac.

 

[lw395]

What I would do is
Set wristwatch to FM radio pips, a month before departure.
wear continuously
each week record the error, to establish the drift rate.
Set to radio pips on departure, when out of LW range, extrapolate the drift rate.
Keep a second watch/clock in the chart table and record its error daily.
There is much to be said for keeping a system independent of gps, but no harm in checking the time against gps.