Time Signals

[BobE]

Hi,
I took my Garmin 12 XL hand held into the garden this morning, switched on and noticed that when it acquired the first satellite the time displayed changed (presumably updated to correct time on acquisition of signal)… I then went indoors and turned on the TV.
I checked the text pages… Both BBC and ITV and they were 4 second out from the GPS, which was now running in "simulation" mode. Then I, just for the hell of it, checked Sky text… needless to say it was also out… But would you credit it? It was over a minute adrift.
Sky satellite signals are always a few seconds behind the "terrestrial" signals but this error was about 1 min 20 secs.
So I just wonder about listening to the BBC time signal when doing astro navigation..
Maybe I shouldn't get the sextant that I've always hankered after..

Happy New Year
Bob E… (r.egerton@virgin.net)

 

[tcm]

GPS time is not accurate

GPS time is not accurate - all the corrections to GMT due to there not being exactly 60 X 60 x 24 x 365.25 seconds per year have not been caried out. BBC time clock on the other hand will be accurate, use that as correct time though not sure you need quite this degree of accuracy when using sextant!

 

[kingfisher]

If time=wrong, then pos=wrong?

I always assumed that it's the most accurate clock on board. GPS uses the time signal of different satellites to calculate your position. So if the GPS clock is wrong, your position is wrong. Unless off course all the satellites have the same error?

Group of people on the pontoon: skipper is the one with the toolbox.
http://sirocco31.tripod.com

 

[tcm]

Re: no, all same time

i too had assumed the same! But they all have the same error, and not using corrected time which we need for making sure that high noon is indeed when sun is top dead centtre, so to speak, which GPS doesn't careabout solong as all sats have same error. Bet there's a good article or fifty about this nerdy issue on the web somewhere.

 

[Nicolas]

Correct me if i'm wrong, but I understood GPS time is accurate when it's got several satelites

It receives the time when the satelites sent the signal, and calculates the distance to that satelite (assuming speed of signal) with difference to GPS time. Repeats this operation for all satelites, which gives its position as being the intersection of all these different spheres.

If this intersection does not happen to be a point, it will calibrate its internal clock to get this as close as to a point as possible.

That's how I understood it, so the GPS clock can only be accurate if you have several satelites, did you have only one (not sure about the "simulation" mode)


happy new year!
Nicolas

 

[bedouin]

Re: no, all same time

A common misconception - the sun is not (usually) directly overhead the prime meridian at noon GMT. That is why it is called MEAN time. Clocks do not need to be adjusted constantly to keep them in step - they just throw in the odd "leap second" every few years.

 

[alex_rogers]

This article discusses exactly what you observed.

http://joe.mehaffey.com/gpstime.htm

Alex Rogers
<A target="_blank" HREF=http://www.YachtsAtSea.com> www.YachtsAtSea.com </A>

 

[J_Priedkalns]

For accurate time signals tune in on your SW radio dial at 5MHz, 10MHz, 15MHz, 20MHz and 25MHz and listen to the WWV signals and compare it to your GPS

 

[VMALLOWS]

How are you working out the number of times the short wave signal has bounced up to the ionosphere and back? We're obviously into precise accuracy here.

 

[LadyInBed]

Railway Time

When the railways first started they had a similar problem, the more E or W the trains went from Greenwich the more the LOCAL time differs from Greenwich – Obvious, everyone knows that.
So unless your house is on the Greenwich meridian then the LOCAL time given to you by your GPS will lag or lead Greenwich proportionate to the distance E or W of Greenwich.

 

[TheBoatman]

Re: Railway Time

Nige
I thought that had been sorted by us (the UK and Ireland) adopting GMT as standard across the whole of the UK/Eire, not withstanding BST. Therefore if its 13.00 hrs in Greenwich then its 13.00 in Bristol or Cork or anywhere else for that matter?
Which raises another interesting question, if your sailing across the pond when does the GPS correct itself for local, does it suddenly jump 1 hour?

 

[Twister_Ken]

GPS Time Standard

Can't speak for all GPSs but my Garmins work on UTC, unless I enter an offset for local time.

 

[jollyjacktar]

In my workshop I have several working radios, and sometimes am desperate to get a good program to listen to, so tune in them all to different stations. [Bit like being in a pub with everyone talking at once, the ear can then tune into the station/conversation that it wants.] Come the hour and the beeps for the time signals, it is rare that they all beep at the same time, seems that various stations just put on the recording when they wish and try to pretend that it is 'on the hour' and that they are on time.

Don't know who to believe, usually consider that BBC is the correct time. But what the heck, use the Mark I eyeballs to figure out where you are. I was under the misconception that GPS device gave you an accurate fix within a yard or two. So who cares if the time feature is a second or two out, the altitude feature if you have one on your GPS is notoriously inaccurate. At sea level, I have rarely seen them register 0, or whatever, making allowance for instrument height, Yet it is obvious to me that I am at sea level, so why bother with the feature, my boat doesn't climb hills and mountains.

Use the proper approved marine radio time signals if you have to [see the other threads]. Although goodness knows why you need precise accurate time yourself.

I know exactly where I am at all times [I'm here], often just can't quite figure out where the rest of the world is in relation to this place. Fair weather, happy days and plane sailing.

 

[LadyInBed]

Re: Railway Time

There's a thought, just goes to show that you can't take anything for granted. I assumed the GPS would reflect the time precisely coincident with the Longitude, not allowing that some devious softy would program the time to be consistent through a time zone.
If that is the case, it would do a jump on the TZ boundary, but come to think of it, my GPS doesn't. I set the TZ in the set-up, and it stays on that time.