| View previous topic :: View next topic |
| Author |
Message |
C2k3
Occasional Visitor
Joined: Apr 21, 2004
Posts: 23
Location: UK
|
 Posted: Wed Jan 18, 2006 6:48 pm Post subject: Most accurate time - GPS/Time Server/??? |
 |
|
Bit anal this one, and of course it's splitting hairs.....
But having just bought a super accurate watch I started wondering.
What is the most accurate time reference I can delivered to me?
GPS tales time from the first sat it hooks onto. Does it take time from all sat fixes and do any computation for distance the signal travels?
Would it be any more accurate that my PC accessing a time server? I don't know if XP time sync allows and corrects for ping time.
I have SP TimeSync on my PDA which clearly shows the delay time. Seems very good.
I'm in the UK and have the BT speaking clock which is widely used as an authoritative time reference. Again I don't know, but doubt that it allows for transmission delay.
Then there are Radio Controlled clocks taking a signal from a transmitter (Derby in UK) - not played with one of these.
However, time servers, Sat and Speaking clock do not all agree with each other, and they are slightly inconsistent in their disagreement.
I know it's pretty academic anyway, but any thoughts anyone?
8O |
|
| Back to top |
|
 |
lbendlin
Pocket GPS Staff
Joined: 02/11/2002 22:41:59
Posts: 11878
Location: Massachusetts, USA
|
| Posted: Wed Jan 18, 2006 11:57 pm Post subject: |
|
|
What time do you want, GMT ot UTC? Your BT Talking clock will use GMT while the sats run on UTC. Those two times are quite a few seconds apart.
_________________
Lutz
Report Map Errors here:
TomTom/TeleAtlas NAVTEQ |
|
| Back to top |
|
|
kartracer
Frequent Visitor
Joined: 26/03/2003 20:15:33
Posts: 502
Location: United Kingdom
|
| Posted: Thu Jan 19, 2006 12:15 am Post subject: |
|
|
Your profile says you are in the UK, so I would suggest that you should go for the UK time signal funded by the Department of Trade and Industry, produced by the National Physical Laboratory, and transmitted from Rugby (not Derby) by British Telecom.
As Lutz says, GMT is currently about 2 seconds ahead of UTC. Unless you are calculating trajectories for space vehicles the difference is probably irrelevant, but theoretically we do still use GMT in the UK.
_________________
Kam |
|
| Back to top |
|
|
C2k3
Occasional Visitor
Joined: Apr 21, 2004
Posts: 23
Location: UK
|
| Posted: Thu Jan 19, 2006 12:16 am Post subject: |
|
|
Ah, the waters are even muddier than I thought!
I assumed that without Daylight savings they were the same.
I must google a tad more.
But either way - what do you suppose would be the most accurate time in either GMT or UTC as delivered to any locality?
Does Sat Nav software (lests say TT5 for instance) interpret any kind of transmission delay data?
C |
|
| Back to top |
|
|
C2k3
Occasional Visitor
Joined: Apr 21, 2004
Posts: 23
Location: UK
|
| Posted: Thu Jan 19, 2006 12:28 am Post subject: |
|
|
Hi Kam
Yes of course Rugby not Derby. And at something under 186,000 miles/second.
So I guess that the closest scource may be the best you could do. Transmission down copper must be slower. And routes via www are likely to be particularly long - but software will make corrections based on ping returns.
Still curious tho!  ) |
|
| Back to top |
|
|
kartracer
Frequent Visitor
Joined: 26/03/2003 20:15:33
Posts: 502
Location: United Kingdom
|
| Posted: Thu Jan 19, 2006 1:19 am Post subject: |
|
|
GPS receivers do know how long the signals have taken to reach them, and so should be able to calculate UTC to a high degree of precision. I imagine that software should be able to do the sums and set a clock most accurately from GPS satellite transmissions. However, if you are synchronising a watch by hand then a radio clock using the Rugby transmission is probably your best bet. I got a radio synchronised watch for Christmas, so I don't have to worry about this at all any more. :D
_________________
Kam |
|
| Back to top |
|
|
Skippy
Pocket GPS Verifier
Joined: 24/06/2003 00:22:12
Posts: 2946
Location: Escaped to the Antipodies! 36.83°S 174.75°E
|
| Posted: Thu Jan 19, 2006 2:15 pm Post subject: Re: Most accurate time - GPS/Time Server/??? |
|
|
| C2k3 wrote: | | GPS tales time from the first sat it hooks onto. Does it take time from all sat fixes and do any computation for distance the signal travels? |
Yes, this is the fundamental principle of how GPS works!
The GPS gets a rough time to within a few thousandths of a second from it's first sat. Once it has three or more sats, it can compute the time more accurately, typically within about 100 nano seconds (.0000001 seconds).
From this it can compute the distance to the satellites (which is knows the position of from the data they transmit) by measuring the propagation delay of the radio signals which travel at the speed of light. So it knows where the satellites are and how far away they are so it can triangulate it's position.
| C2k3 wrote: | | Would it be any more accurate that my PC accessing a time server? I don't know if XP time sync allows and corrects for ping time. |
The GPS and PC should agree. Mine do. The time sync using the NTP protocols on Windows and Unix systems does compensate for the ping delay.
Make sure you are using a RELIABLE time source. Try europe.pool.ntp.org (also available are north-america, oceania or asia .pool.ntp.org). See http://www.pool.ntp.org/use.html
| C2k3 wrote: | | However, time servers, Sat and Speaking clock do not all agree with each other, and they are slightly inconsistent in their disagreement. Any thoughts anyone? |
Segal's Law states that "A man with a watch knows what time it is. A man with two watches is never sure" 
As for the difference between GMT vs UTC vs GPS, there is some confusion over this.
Some background:
UTC is the official name for the time that we all use and is based on time measured by an extremely accurate atomic clock.
GMT is what we used to call time before Universal Time was invented about 100 years ago. Technically, GMT is based on the rotation of the earth and the presumption that each rotation takes 86400 seconds.
There is a problem though. The earth rotates once per day but the speed of rotation varies ever so slightly and overall it is slowing down. So in astronomical terms a day is just short of 86400 seconds and our extremely accurate atomic clock will eventually produce a time which is ahead of the actual rotation of the earth which GMT was based on. In practice, GMT can differ from UTC by up to .9 seconds.
So what we do is add a "leap second" to UTC to allow the earth to catch up. Indeed, 2005 was one second longer than normal and the time at 1 second to midnight was 23:59:60 on December 31 2005. If you set your watch on New Year's Eve then it would have been 1 second fast on New Years Day.
GPS time is different again. This was sync'd to UTC at midnight on 1st Jan 1980 and it is never adjusted with leap seconds. As of 1st Jan 2006, GPS time is 14 seconds ahead of UTC. This +14 second offset is transmitted as part of the GPS signal so your receiver knows how to compute the correct UTC time to display to the user. When new leap seconds are added, your GPS knows 24 hours in advance that a leap second will be added.
So, to summarise:
UTC is based on a highly accurate atomic clock and is the time we all use
GMT is based on the earth's rotation (UTC + up to .9 seconds ) but in reality, when the term GMT is used it refers to UTC not the celestial GMT.
GPS is based on an atomic clock without leap seconds (currently UTC +14 seconds). This offset is automatically used to correct the time shown on your GPS receiver.
For all intents and purposes baring extremely specialised applications in astronomy, any person, computer, time source, GPS receiver reports the UTC time NOT GMT or GPS time even if they call it GMT.
Yes, I am a geek for knowing all this.  However, accurate time it is critically important to many businesses including the one I work for!
_________________
Gone fishing! |
|
| Back to top |
|
|
C2k3
Occasional Visitor
Joined: Apr 21, 2004
Posts: 23
Location: UK
|
| Posted: Thu Jan 19, 2006 3:32 pm Post subject: |
|
|
Excellent - thanks.
Been to http://www.pool.ntp.org/use.html
downloaded ntp & installed it. Now I'm trying to figure out how I can find out what it is doing, how often & how well! Good fun in a geeky kind of way. |
|
| Back to top |
|
|
C2k3
Occasional Visitor
Joined: Apr 21, 2004
Posts: 23
Location: UK
|
|
| Back to top |
|
|
kartracer
Frequent Visitor
Joined: 26/03/2003 20:15:33
Posts: 502
Location: United Kingdom
|
| Posted: Thu Jan 19, 2006 9:03 pm Post subject: |
|
|
Skippy,
Wow! I found that very interesting, but it leaves me wondering now why my watch, synchronised with the transmission from Rugby, is reading a couple of seconds ahead of what my GPS receiver/software claims to be UTC time?
_________________
Kam |
|
| Back to top |
|
|
C2k3
Occasional Visitor
Joined: Apr 21, 2004
Posts: 23
Location: UK
|
| Posted: Thu Jan 19, 2006 9:58 pm Post subject: |
|
|
Hmm, see what you mean.
My GPS is one second ahead of Rugby....
Odd shaped balls maybe? Or processing time?
Think I can live with it tho......
I understand that US downgrade GPS data to be less accurate than it could be - maybe they do it by skewing the time - no idea.
Found this site pretty interesting:-
http://wwp.greenwichmeantime.com/info/index.htm |
|
| Back to top |
|
|
Skippy
Pocket GPS Verifier
Joined: 24/06/2003 00:22:12
Posts: 2946
Location: Escaped to the Antipodies! 36.83°S 174.75°E
|
| Posted: Fri Jan 20, 2006 4:07 pm Post subject: |
|
|
| C2k3 wrote: | My GPS is one second ahead of Rugby....
I understand that US downgrade GPS data to be less accurate than it could be - maybe they do it by skewing the time - no idea. |
Hmm, that's odd. How does it compare to the talking clock? How do you get the Rubgy time (I've never done it)?
The time accuracy is not due to any deliberate downgrading of the accuracy. A large number of applications rely on GPS to provide time to within a few thousandth's of a second so a 1 second drift would be noticed very quickly, I can assure you!
| kartracer wrote: | | it leaves me wondering now why my watch, synchronised with the transmission from Rugby, is reading a couple of seconds ahead of what my GPS receiver/software claims to be UTC time? |
If your GPS receiver has a fix then my bet would be that it has the correct time and your watch has some how failed to sync with the rugby clock. Maybe it has trouble receiving the rugby signal or only updates the time every now and then?
_________________
Gone fishing! |
|
| Back to top |
|
|
kartracer
Frequent Visitor
Joined: 26/03/2003 20:15:33
Posts: 502
Location: United Kingdom
|
| Posted: Fri Jan 20, 2006 5:38 pm Post subject: |
|
|
The time discrepancy I observed must be down to my TomTom receiver and/or software running on a PPC. The combination consistently reads just over a second slow. Using a Garmin handheld GPS to check I can see that it is indeed precisely in step with my Rugby synchronised timepieces (a clock and a watch).
_________________
Kam |
|
| Back to top |
|
|
jonajuna
Occasional Visitor
Joined: Mar 14, 2006
Posts: 56
|
| Posted: Sat Mar 25, 2006 2:15 pm Post subject: |
|
|
so.................
what times the football on?
|
|
| Back to top |
|
|
nej
Frequent Visitor
Joined: Jun 16, 2004
Posts: 454
Location: London, Ingerlund
|
| Posted: Mon Mar 27, 2006 10:38 am Post subject: |
|
|
| kartracer wrote: | | The time discrepancy I observed must be down to my TomTom receiver and/or software running on a PPC. The combination consistently reads just over a second slow. Using a Garmin handheld GPS to check I can see that it is indeed precisely in step with my Rugby synchronised timepieces (a clock and a watch). |
TomTom will always be a second or so behind. The TomTom receiver gets the signal and decodes it, but then has to transmit this to the PDA. NMEA messages are created once per second by the GPS Receiver, hence the 1 second delay. |
|
| Back to top |
|
|
|
![]() Posted: Today Post subject: Pocket GPS Advertising |
|
|
We see you’re using an ad-blocker. We’re fine with that and won’t stop you visiting the site.
Have you considered making a donation towards website running costs?. Or you could disable your ad-blocker for this site. We think you’ll find our adverts are not overbearing!
|
|
| Back to top |
|
|
|
Forum FAQ
Search
Usergroups
Profile
Log in for private messages
Log in
