Astro - navigation

[lw395]

You shouldn't use a navigational GPS as a time source; it may be off by a second or two. Although the embedded time signal is very precise, the navigational processing (connected, I think, with the cycle of the GPS transmissions) means that the displayed time is not so precise. There is also the issue of "leap seconds" - thesemay not be corrected, leading to errors of 12+ seconds. GPS time sources (used by web servers and such like) are different beasts from the positional GPS we all use.

However, a modern quartz watch - even a cheap one - will probably be amply accurate for navigational use.

I've never noticed my GPSs drift relative to my watch.
I think you'd be unlucky to be 12s out.
And few of my fixes would be made much worse by anything less than 12 seconds.

My mobile phone OTOH, varies a lot.

 

[AntarcticPilot]

That rather depends on the receiver manufacturer. Although typical receivers output position every second, they are well capable of outputting accurate time, converted from GPS system time to UTC including leap second corrections.

My point is that unless you know it IS accurate, you have to assume it may not be. And it is more than the receiver update cycle; it is also to do with the update cycle of the GPS data downlinked from the satellites. As you say, GPS receivers certainly can output accurate time; as I noted, specialized GPS receivers are used as highly accurate time sources (+/- 40 nS) in critical applications like web services and banking. But consumer GPS systems don't usually bother.

I am not so certain of this, but isn't it the case that GPS time error will always be slow? I mean, it will always display a time that has already passed.

All rather academic as the positional error from a 1 or 2 second time error is small compared with the other likely sources of error! Very roughly, I reckon the maximum error is about 1/4 of a nautical mile per second error.

 

[RAI]

I am not so certain of this, but isn't it the case that GPS time error will always be slow? I mean, it will always display a time that has already passed.

Again depends on the receiver manufacturer. The processor itself will be cycling at about 1023 times a second IIRC and takes its time from the satellite atomic clocks. The satellite messages include such stuff as correction to UTC, so its not short on info to work out what time to display each second. Those scientists interested in frequency and time have never had it so good. There are more calibrated atomic clocks in orbit than there are on the ground these days and they are all bleating away.

Sorry about the thread drift, longitude is still all about time I guess. An EMP that would take out GPS satellites would probably also give the average quartz watch a bit of shock. So a well calibrated self winding mechanical watch would be nice to compliment a sextant. My Seiko is in for repairs.

 

[BurnitBlue]

About half a dozen SSB frequencies for various bands give continuous time pips. Can't recall the freqs but a radio manual will give them. The US coastguard also give out storm warnings before the pips. Again don't know freqs anymore. Not very useful post I guess.

 

[Uricanejack]

I have no idea why? on several occasions I have seen a time error on GPS. for some strange reason it was a temporary error of exactly 1 minute. it later updated and corrected itself. The only reason I noticed the error was I thought the clock was wrong. It did seam strange both the clock and my watch were wrong. Once upon a time long ago I would check my time from the BBC. Now there is a delay on some if retransmitted through a satellite. I used Denver for years. Now I tend to use GPS to check my clock. and watch perhaps the old time signal was a better idea.

 

[capnsensible]

One of my favourite parts of an ocean passage on a small yacht is when you get close enough to start getting FM radio stations.

Music, news and a time signal each hour to check my 8 quid Casio F91W which is normally only a couple of seconds out after three weeks!

Then ones mind turns to consideration of, err, rum!

 

[alant]

One of my favourite parts of an ocean passage on a small yacht is when you get close enough to start getting FM radio stations.

Music, news and a time signal each hour to check my 8 quid Casio F91W which is normally only a couple of seconds out after three weeks!

Then ones mind turns to consideration of, err, rum!

Strange, isn't rum spelled starting with C?

 

[oldvarnish]

Once upon a time long ago I would check my time from the BBC. Now there is a delay on some if retransmitted through a satellite. I used Denver for years. Now I tend to use GPS to check my clock. and watch perhaps the old time signal was a better idea.

Don't make the mistake I made and use the time signal from BBC World Service of SSB in mid Atlantic. Although the transmission is an analogue set up, the distribution from studio to transmitter - which might be half a world away - is digital. This introduces considerable delays of several seconds. Enough to screw up an otherwise good fix.

 

[Uricanejack]

I have. A nice digital Sony shortwave radio. It's in a box in the basement. Finding it might take a while.
I used to get the time signal and world service on. WWV from 2500 KHz 5000 10000 15000 20000. depending on time of day. WWV is in Fort Collins in Boulder Colorado. It's probably deep under a mountain and safe rom a zombie apocalypse. There is also one in Hawaii.

A good short wave Radio is not something you see much nowadays. I suppose you can still get them.

 

[snowleopard]

If anyone is interested I can post how to check you compass by amplitude. Its quite simple.

I recall having to do that as part of the 'day's work' I submitted for my YM Ocean.