Ships' Clock: Astro

[Skylark]

May I ask the panel for opinions, use and experience of a ships' clock?

Are there any examples of commonly available, reasonable cost, accurate, (digital?), timepieces?

Or is common use to have any old thing and log daily calibration versus a radio time signal. If so, what is a commonly used time signal.

Hopefully, you get my drift

Many thanks.

David

 

[jwilson]

Assuming you mean for astro - two cheap Casio waterproof digital watches: buy a few months before trip, make sure you can tell which is which, and rate both against BBC analog radio (not DAB or internet time) time pips, keep a record, then reset both to exact just before departure. Unless it's a very very long passage they will never be far enough out to even bother applying a correction.

 

[sarabande]

do those "Radio Clocks" work away from land ? (The ones that are advertised in colour supplement mags) They are supposed to be kept accurate to =<1 sec/day.

 

[webcraft]

GPS time

.
The GPS time signal is accurate to under one second - with the proviso in the quote below:

The GPS navigation message includes the difference between GPS time and UTC, which as of 2009 is 15 seconds due to the leap second added to UTC December 31, 2008. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message.

So you probably already have an accurate ships clock on board. This would allow you to calibrate a digital watch by checking it regularly against the GPS for gain or loss and tabulating the results. You would then have a reasonably well calibrated ships chronometer should the GPS set fail for any reason.

- W

 

[AntarcticPilot]

Assuming you mean for astro - two cheap Casio waterproof digital watches: buy a few months before trip, make sure you can tell which is which, and rate both against BBC analog radio (not DAB or internet time) time pips, keep a record, then reset both to exact just before departure. Unless it's a very very long passage they will never be far enough out to even bother applying a correction.

.
The GPS time signal is accurate to under one second - with the proviso in the quote below:

So you probably already have an accurate ships clock on board. This would allow you to calibrate a digital watch by checking it regularly against the GPS for gain or loss and tabulating the results. You would then have a reasonably well calibrated ships chronometer should the GPS set fail for any reason.

- W

Two things:

1) Internet time (if you are within reach of a connection!) is fine provided you use a proper Network Time Service with an appropriate client. MS Windows includes such a client for setting the system clock, but you'll need to change the default time server (which is run by Microsoft) to something sensible; ideally one in the same country as the place where your connection goes to. That should be accurate to better than a second against UTC; the NTS system has provision for assessing and removing delays in transmission. See this for more information and explanation. Note that the applet gives an estimate of the inaccuracy of the display, which (I guess) is arrived at by looking at variability on the transmission time between client and server. The protocol is, I understand, similar to "ping".

2) A NAVIGATIONAL GPS systems are NOT an accurate time source, at least, not to a second or so. This is not because of inaccuracies in the signal, which is an extremely precise time source, but because of the way the processing algorithms in the hardware prioritize processing the signal. The display on your GPS can be more than a second wrong quite easily. GPS time systems are rather specialized and expensive (because of low demand), but are used by Internet Service Providers.

I'd also suggest that a wall clock (these days!) is primarily for show, and that for precise independent time sources I'd agree with the suggestion of regularly calibrated Quartz watches. But no need to spend a fortune; Casio is as good as any!

Radio controlled clocks can be bought with world-wide capability, depending (I think) on WWV (or whatever it's called), but most commercial sets depend on relatively short-ranged transmitters giving national or slightly wider coverage.

 

[webcraft]

.
My apologies, AP is of course quite right - the time signal arriving at your GPS is spot-on but the display may vary by up to two seconds. I found this info:

A NOTE OF CAUTION ON INTERPRETING THE TIME DISPLAY ON HANDHELD GPS RECEIVERS

It has been found by many users in the USA, New Zealand and Australia that the LCD time display on navigation type GPS receivers (Garmin, Magellan, etc.) is often "pretty good", but can be early or late by as much as 1 or 2 seconds. This can be readily shown by comparing the LCD display on a handheld GPS receiver with another source of known accurate time. This discrepancy is probably caused by the relative priorities of the computing tasks performed by the microprocessor in the unit, which gives highest priority to computing position and velocity. The time discrepancy for any one receiver is not necessarily constant but may vary with the internal computing workload of the device, e.g. the number of satellites tracked at the time. In some models it has been shown to also depend on the power mode ('Battery Save', 'WAAS', or 'Normal').


It is important to only consider using the display on these receivers once they have a full 'lock' and advise that they are receiving signals from enough satellites for navigation. Note that before the GPS receiver achieves lock, the 'display' simply reads an internal crystal clock, using a time setting left over from the last time it was turned on. Only upon achieving lock is the internal clock synchronised with the GPS data, and even then there is a possibility of the above mentioned 1 or 2 second discrepancy.


Therefore, the LCD display on a standard handheld GPS receiver should not automatically be accepted as being 'accurate'. You have to get to know the characteristics of your particular receiver, and do so in a range of different circumstances. On the other hand its long term stability over extended periods is excellent, because even after many years it will still be within that same +/- approx. 2 sec range.

(Source)

Interesting stuff.

- W

 

[Pete7]

David, excellent question since I have just picked up a Mk15 sextant from esteemed fellow forumite Flobbergob

So if a watch with an error of say 15 seconds was used for a noon site, how far out of your true position might your plot be? 10 miles? in which case lets not get too excited. 100 miles? in which case fire up the Garmin before the memsahib realises you haven't a clue, or finally, your plot puts you in the middle of North Africa and you will need that Arabic phrase book.

Just trying to visualise the possible problem.

Pete

 

[AntarcticPilot]

David, excellent question since I have just picked up a Mk15 sextant from esteemed fellow forumite Flobbergob

So if a watch with an error of say 15 seconds was used for a noon site, how far out of your true position might your plot be? 10 miles? in which case lets not get too excited. 100 miles? in which case fire up the Garmin before the memsahib realises you haven't a clue, or finally, your plot puts you in the middle of North Africa and you will need that Arabic phrase book.

Just trying to visualise the possible problem.

Pete

Time is mainly important in fixing longitude. 24 hours = 360 degrees of longitude, so 1 hour = 15 degrees, 1 minute = 0.25 degrees (15 minutes), so 1 second error = 15 seconds of arc error. If the error is entirely on longitude (of course, it isn't) then a 1 second error in time introduces an error of about 1/4 of a nautical mile. 10 seconds will make you about 2.5 miles off, and so on.

All this at the equator - I'm too lazy to work it out for higher latitudes!

To put it in perspective, it is doubtful if a fix more accurate than 1 nautical mile can be obtained from a small boat with a sextant. Even land based surveyors with highly accurate theodolites can only get position to 1 or 2 hundred metres with astro techniques - it takes a fully equipped astronomical observatory to do better.

Another way of looking at it is that an error of 1 second in time is approximately equivalent to an index error of 15 seconds on your sextant.

The point is that it will be a systematic error in your position, and so it can't be averaged out by other methods. Taking multiple sights and averaging will simply give you a position that is still shifted by the amount caused by your time error.

 

[DJE]

David, excellent question since I have just picked up a Mk15 sextant from esteemed fellow forumite Flobbergob

So if a watch with an error of say 15 seconds was used for a noon site, how far out of your true position might your plot be? 10 miles? in which case lets not get too excited. 100 miles? in which case fire up the Garmin before the memsahib realises you haven't a clue, or finally, your plot puts you in the middle of North Africa and you will need that Arabic phrase book.

Just trying to visualise the possible problem.

Pete

Not really my field but I can't resist it. The earth rotates at 15 degrees per hour so 15 seconds of time equates to 3.75 minutes of longitude. I make that about 2.4 nautical miles at 50 degrees of lattitude. Is that right anyone?

 

[JayBee]

Another way of looking at it is that an error of 1 second in time is approximately equivalent to an index error of 15 seconds on your sextant.

Should perhaps point out that this is a maximum value and only applies on the equator, with observations of bodies bearing due east or west.

 

[AntarcticPilot]

Should perhaps point out that this is a maximum value and only applies on the equator, with observations of bodies bearing due east or west.

That is partially true for a simple noon-sight, but for other sights the time error will introduce other errors such as incorrect ephemeris data. However, in general, the error will decrease according to the cosine of the latitude, being maximum at the equator and zero at the Pole.

It is not true that it only applies to bodies due east or west; it is the rotational position of the EARTH that matters, not the position of the astronomical object. Consider for a moment - the simple noon shot (where the Sun is by definition due north or south) will be in error in longitude by precisely the error in the time used.

 

[JayBee]

That is partially true for a simple noon-sight, but for other sights the time error will introduce other errors such as incorrect ephemeris data. However, in general, the error will decrease according to the cosine of the latitude, being maximum at the equator and zero at the Pole.

It is not true that it only applies to bodies due east or west; it is the rotational position of the EARTH that matters, not the position of the astronomical object. Consider for a moment - the simple noon shot (where the Sun is by definition due north or south) will be in error in longitude by precisely the error in the time used.

I wouldn't argue with any of that. However, your statement that time errors are approximately equal to altitude measuring errors is incorrect in all circumstances other than the one I described.

 

[AntarcticPilot]

I wouldn't argue with any of that. However, your statement that time errors are approximately equal to altitude measuring errors is incorrect in all circumstances other than the one I described.

I did say I was being lazy in not giving the relationship with latitude ! And for the UK, the factor is between 65% and 50% of the figures I gave. So, even at our latitudes, an error of 10 seconds will give an offset comparable to the likely cumulative error in the fixing process.

I only compared the errors; I didn't state they were equivalent - at least, I hope I didn't. The point I was making was that 1 second time error was comparable (i.e. same order of magnitude) to the index error of a sextant.

 

[JayBee]

I had to learn this stuff when there was no other viable option for getting from one side of an ocean to the other.

Respect to all those who embark on a course of Astro Nav voluntarily, like the OP. It would be very easy to be deterred by threads such as this one. Don't be - it's simpler than it looks and very rewarding, IMO.

For anyone still interested, here's the relationship between change of altitude with time, dependent ONLY on latitude and azimuth:

Altitude change in one minute of time = 15' cos lat sin Az

 

[Cruiser2B]

.
The GPS time signal is accurate to under one second - with the proviso in the quote below:

So you probably already have an accurate ships clock on board. This would allow you to calibrate a digital watch by checking it regularly against the GPS for gain or loss and tabulating the results. You would then have a reasonably well calibrated ships chronometer should the GPS set fail for any reason.

- W

This is what I do and for most peep's astro-abilities is more than accurate enough. Put in perspective, the 1 or 2-second inaccuracy of your GPS's display is more accurate than what mariners used for centuries before the advent of atomic clocks and radio time signals. The beauty of the GPS is that you don't need to calibrate it daily and it's accuracy does not deteriorate. If you think it's important, then periodically check your GPS against the time signal to determine its error-range.

 

[nonitoo]

David, excellent question since I have just picked up a Mk15 sextant from esteemed fellow forumite Flobbergob

So if a watch with an error of say 15 seconds was used for a noon site, how far out of your true position might your plot be? 10 miles? in which case lets not get too excited. 100 miles? in which case fire up the Garmin before the memsahib realises you haven't a clue, or finally, your plot puts you in the middle of North Africa and you will need that Arabic phrase book.

Just trying to visualise the possible problem.

Pete

You don't use a chronometer for a noon sight. The noon sight is that where you measure that altitude of the sun at local noon (when the sun is due north or south of you) and from that calculate your latitude. The traditional method is to observe and increase the altitude until it stops and the sun is 'gone'. Simple to then calculate your latitude using the index error, declination and height of eye.

For an average sight calculated either using the "Marq St Hilare" or "longitude by chronometer" methods 15 seconds would probably put you about 4-6 miles away in error. Using the "ex-meridian" method would possibly reduce that error but not by much.

Don't forget that the noon position is essentially a running fix between two position lines, the first gained during your morning observations and the second gained at local noon.

Tom

 

[Lady Campanula]

My old navigation instructor used to say "If you can plot your sight on your passage-planning chart, you're doing most of it right. Use a sharper pencil, use a better watch, clean the salt-spray off the mirrors."

I suggest you keep a couple, or more, quartz watches in the same fixed position ( Subject to the bumps, accelerations and temperature changes. You might find this possible within the sextant box lid ) and 'rate' them by logging the error(s) regularly against a reliable radio time signal. 'Once a day' was the traditional marine routine. This permits you to see which had the most regular 'growth of error rate' - e.g. +3 secs per week,steady - which permits a correction to be applied to get 'as near as dammit' correct UTC time.

Reliable astro depends on 'whittling away' at all those small corrections, and I have little time for those who are keen to 'just not bother'. These are the same guys who, whenever the topic comes up, whine about it being a useless procedure 'cos they cannot get usable and accurate results.

Sure, they cannot. But YOU can....



And don't forget that 'Meridian Passage' ( MerPass ) shots for latitude can be taken, usually easily, on other Heavenly Bodies ( Lakey! ), and the resulting LOP resolved and plotted in less time than it takes to type.

Like Polaris.....

 

[nonitoo]

My old navigation instructor used to say "If you can plot your sight on your passage-planning chart, you're doing most of it right. Use a sharper pencil, use a better watch, clean the salt-spray off the mirrors."

Reliable astro depends on 'whittling away' at all those small corrections, and I have little time for those who are keen to 'just not bother'. These are the same guys who, whenever the topic comes up, whine about it being a useless procedure 'cos they cannot get usable and accurate results.

Sure, they cannot. But YOU can....

I could not agree more with you. Practice is the answer along with good preparation and taking great care in your observations. The more you do it the better you get (mind you, that applies to a lot of things, I suppose).

Tom

 

[lw395]

Assuming you mean for astro - two cheap Casio waterproof digital watches: buy a few months before trip, make sure you can tell which is which, and rate both against BBC analog radio (not DAB or internet time) time pips, keep a record, then reset both to exact just before departure. Unless it's a very very long passage they will never be far enough out to even bother applying a correction.

Since quartz frequency varies with temperature, the most accurate or consistent one will probably be the one on your wrist if you always wear it 24/7. Also the cheaper plastic front metal back types might be better than an all metal case (as the crystal will be closer to body temp than outdoors)...
Definitely agree with keeping a spare one, as I've trashed or lost a few on boats over the years...
Not sure I would reset the watch on day of departure either, you want to have several checks on it before you're out of VHF range.

 

[Pete7]

Folks thanks, although we have no real need for a Astro I thought it might be a fun thing to learn but didn't want to spend a fortune, hence the question. I like the idea of a cheap Casio and compare it regularly to the GPS.

Thanks Pete