Stopwatch for Astro Nav

ChiPete

ChiPete

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I've just read Alastair Scott's excellent article on astro in this months PBO. I has actually answered a couple of issues re PL's that I've been struggling with and has fleshed out other areas of uncertainty I had.

Now, the question I have is regarding the use of a stopwatch. Is it the case that you start the stop watch at (for example) at the stroke of mid-day from your known good time source, stop it when you have your sight and use the delay to calculate the actual time? Apologies if this is elementary to those of you who know, but I have the same blackboard rubber affliction Alastair suffered from during maths at school!

Any assistance would be hugely appreciated.

Cheers,
Pete
 
Yes you're right.Or you can get a cheap reliable electronic watch and just use that.Stopwatches were used in the days of expensive chronometers that had to stay tucked away in the cabin.
 
I've never astro-navigated in a small boat ... so treat this with a degree of caution.

Seeing your post I had a glance at the article. I see Alastair Scott managed an error of 11.7 miles, which I think was pretty impressive under the conditions.

However, watches are so easy to synchronise nowadays it does occur to me that this might be an option? If the watch - or even a spare watch(!) - were synchronised with the good time source it would avoid the need for a correction. That way there's one less step in the calculation.

As above, a cheap electronic watch is also a good bet ... but they are worth checking against the time source.

By way of comparision, when we took sun and star sights we would walk from the bridge wing to the (clockwork!) chronometer counting "thousand one ... thousand two ... ". We claimed (there was no way of checking!) that we were accurate to about a mile in good conditions. Given that two of us had to take and work the sights separately we often agreed to less than a mile. Of course our platform was pretty stable!
 
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Thanks for the replies, much appreciated.

I have a cheap'n'cheerful £10 waterproof Casio that lives on the binnacle with the hour beep set to remind us to update the log on passage. If I were to synchronise that to the Greenwich time signal (Radio 2 or 4) when in the UK, then log the time changes observed per day over a period of days after setting, perhaps this could be the 'chronometer'?

I have to say, I like the idea of removing any steps in the calculation - the numbers just swim around at the mo!

I have read that there are time signals available on VHF so that you can sychronise watches anywhere in the world but can't seem to find any way of receiving them - does anyone have any ideas here?

Cheers,
Pete
 
ChiPete said:
I have read that there are time signals available on VHF so that you can sychronise watches anywhere in the world but can't seem to find any way of receiving them - does anyone have any ideas here?
Click to expand...

You can get the time from a GPSR :D

Boo2
 
I had read that you can't use the GPS time and now I understand why - thanks Porthandbuoy. Fascinating site BTW.

I suppose the question may be - you can use GPS time, but factor in the 15s difference?
 
ChiPete said:
Thanks for the replies, much appreciated.

I have a cheap'n'cheerful £10 waterproof Casio ... If I were to synchronise that to the Greenwich time signal (Radio 2 or 4) when in the UK, then log the time changes observed per day over a period of days after setting, perhaps this could be the 'chronometer'?
Click to expand...


Well back in the old days ... that's pretty much what we used to do!

I've often looked at the watches in garages and thought they were more accurate than our ships' chronometers. At £10 you could have a back up.

Our gimballed clockwork chronometer (wound exactly so may turns at the same time EVERY day - the only reason it's glass topped box was usually opened) had its own error book. If the radio officer was lucky you might get a time-signal deep-sea to up date the error book.

Then you had to apply the chronometer error to the time - so you're back to another step in your calcs.

But as we mid-ocean would sometimes go 5-6 days without a sight (at 17 knots) who cared about a few seconds time error? We didn't!

Until we closed the coast we were happy with a pretty general idea. Closing the coast the pressure was on - even watching the cloud for a break for the moon to shine through! Not every coast even had radio stations and they had their own problems!

I'm beginning to feel old - but it was only the '70s
 
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alan_d said:
Yes, that is what the satellites use, but I thought a correction was transmitted so that your GPS receiver will show UTC.
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Y'know . . . I don't know. Further research required methinks. A simple check against a radio time signal will confirm. Though don't use Digital Radio; they're a law unto themselves with all sorts of delays.
 
Thanks again Ancient. Good to understand how it used to work, though with all this talk of bridge wings, gimballed chonometers and 17 knots, I get the impression you maybe didn't have to lash yourself to the mast to get a sight....surely that's cheating old boy??!! ;-)

Agreed Porthand, some more research is required here methinks too. You'll find me on the boat tomorrow at 0800 for the R2 time signal and a spot of Casio synchronisation!

The Digital switchover debate also clearly has implications as I understand that the 'lag' in the transmission is entirely up to the quality of the available signal and the receiver. ''If'' (hopefully) rather than ''when'' the digital switchover happens, will they cease the pips?
 
ChiPete,
Pardon my ignorance, what does your signature mean -

"Happiness is V1 at LOS"

I've been sitting here for half an hour and I still can't figure it out.

:confused:
 
ChiPete said:
... all this talk of bridge wings, gimballed chonometers and 17 knots, I get the impression you maybe didn't have to lash yourself to the mast to get a sight....surely that's cheating old boy??!! ;-)
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Certainly didn't have to lash ourselves to the mast. That would be undignified and might soil our whites.

The serious point to my correspondence is that:
- if you know that you are a long way from anywhere,
- does it really matter (in navigation rather than safety terms) if you don't know exactly where you are?

Therefore my view is that you can usually be quite relaxed about your accuracy. Be as accurate as you can, but proportionate to the risks.

This includes generating your EP. We had much greater scope to generate EP errors at 17 knots than most sailing vessels will have. We always used to get where we were going.

Sextant work is very rewarding.

I have to admit when sailing in good visibility I only started turning the GPS on when I got a DSC VHF!
 
ChiPete said:
I had read that you can't use the GPS time and now I understand why - thanks Porthandbuoy. Fascinating site BTW.

I suppose the question may be - you can use GPS time, but factor in the 15s difference?
Click to expand...

I have on my wrist a Garmin GPS/watch. There is no way of setting the time, it uses GPS. It is absolutely spot-on against time signals. So (of course) GPS can know the 'true' time and I assume a simple constant offset will correct for it. And for some receivers that is already built-in.


But a cheapo Casio digital is easily accurate enough if it is set vs a time signal every now and then. I used one for years that was unbelievably accurate until it died.
 
For my astro nav practice, I bought a radio-controlled wristwatch (Precision). This is updated every hour from a time signal: there are two transmitters that cover Europe, one (MSF-60) located in Anthorn, Cumbria and one (DCF-77) located in Mainflingen, Germany.

Both transmitters cover a radius of approx 1500km from the base station, this can be affected to some extent by terrain and prevailing atmospheric conditions. A further transmitter (WWVB) in Fort Collins, Colorado, serves all of the continental USA and Japans J-Ga-AS service provides further coverage.

My watch will pick up all four different frequencies, but this is probably not much use mid-Atlantic, though in the absence of a time signal, it is still an accurate quartz watch, with better specs than a 'chronometer'.

I usually count the secs between taking the sight and looking at the watch. What we really need is a radio-controlled watch with world coverage, that you can 'pause' the time as soon as you take the sight. Although my GPS seems only 1 sec out? No, it's my iPhone that is 1 sec out!
 
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Ancient, thanks for that perspective, that really does set expectations; I was stressing about getting to within a mile with every sight!

You've also confirmed what a number of sources have said; sextant work is very rewarding. I just look forward to the day (sometime soon I hope) that I'll be able to put it all together and start to output my position to the nearest ocean!

Neil and Whipper, cheers for the posts, good options for the mechanics of timing the sight. A spot of trial and error is on the cards.
 
I don't see the point of using a stop watch although I haven't seen the PBO article. Perhaps I'll go and make my way to the nearest high street newsagent with a common surname and do some shelf reading ;)

Like others have said, if you are so inclinded, using a sextant is hugely rewarding and extremely good fun.

I keep two, cheap, digital wrist watches for the purpose. About a month prior to a trip, calibrate against BBC analaogue radio pips. These watches are generally remarkably accurate but every few days record any difference between pips and watch.

To smooth out any errors, I take, say 5 sights over a few minutes and plot them on paper. Choose any on the goodness of fit line, perhaps most convenient for whatever sight reduction process you follow.
 
Porthandbuoy said:
Y'know . . . I don't know. Further research required methinks. A simple check against a radio time signal will confirm. Though don't use Digital Radio; they're a law unto themselves with all sorts of delays.
Click to expand...

GPS depends on an extremely accurate time being broadcast from each satellite. So, GPS time is fundamentally a very accurate time-source indeed, and is used as such in applications like the time source of network systems and so on.

The real problem is that a NAVIGATIONAL GPS prioritizes the navigation processing, and the displayed time may be a few seconds out. The difference between UTC and GPS time is readily compensated for, and is not a problem.

GPS units that are prioritized for time output are available, are used in Network time services (amongst others) and are extremely accurate. But they aren't cheap, because they are specialized, low volume pieces of kit.

All done to death at THIS
 
GPS time

GPS time is different from UTC, as the speed (for want of a better word) of UTC varies slightly. However the time correction is transmitted down to the receivers, and they output UTC.
This is generally accurate to about a second - there is a small delay between starting calculating the fix and updating the display, as most GPS receivers show the time of the fix, and this is also what is output in most NMEA sentences (though there are some for time per se, these are seldom output).
 
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