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RoystonB Occasional Visitor
Joined: Aug 16, 2006 Posts: 19
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Posted: Mon Apr 21, 2008 11:44 am Post subject: Long long, Lock-on time when in Portugal |
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Hi All,
I just read a thread relating to the time it takes for the TT to lock on to satellite signals.
I have a 710 with all the latest updates that I can find from TT.
My problem is that although it only takes a minute or so to lock on when I’m in England, when I travel to Portugal it can take up to an hour. I usually switch it on as soon as I get the Car hire premises but it can take a long, long time to get a fix.
I’ve stopped downloading the QuickGPSfix in case that was the problem and I’ve purchased an extension aerial which I pop out the side window, in case of heated windscreen blockage.
I’m not sure what QuickGPSfix actually downloads, I’m in the London area. When QuickGPSfix is downloaded will my tt only search for those satellites visible from there, or is it a world wide sat. position fix?
Any thoughts on this problem would be most appreciated. |
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DennisN Tired Old Man
Joined: Feb 27, 2006 Posts: 14893 Location: Keynsham
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Posted: Mon Apr 21, 2008 5:21 pm Post subject: |
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Presumably you fly there with the 710 switched off. My 710 took a few minutes to lock on when I went to Corfu last summer - the important thing is to leave it with a clear sky and not moving. I sat mine on a table on the verandah until it locked - don't know how long because I knew it wouldn't be instant and went to check if a beer in the fridge needed drinking before it went sour - by the time I'd done the bottle opener thing and tested the taste, I had a lock. _________________ Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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Darren Frequent Visitor
Joined: 11/07/2002 14:36:40 Posts: 23848 Location: Hampshire, UK
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Posted: Mon Apr 21, 2008 6:16 pm Post subject: |
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The delay when you initially switch on in Portugal is because you have moved thousands of miles since its last fix.
The satellites it expects to see are not there and so the device will download a full set of Ephemeris and Almanac data. Give it time to do this and it'll all run fine there after.
This is a good primer worthy of a quick read:
http://www.pocketgpsworld.com/howgpsworks.php _________________ Darren Griffin |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Tue Apr 22, 2008 12:46 pm Post subject: |
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Darren, just for interest's sake, I looked up your link to see what the difference was between the 'ephemeris' and the 'almanac' data.
Unfortunately...
Quote: | The GPS signal contains three different bits of information — a pseudorandom code, ephemeris data and almanac data.
{snip}
2. Ephemeris data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system.
3. The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.
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... looks like the same sentence repeated? ...
Any more knowledge or info? |
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Darren Frequent Visitor
Joined: 11/07/2002 14:36:40 Posts: 23848 Location: Hampshire, UK
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Posted: Tue Apr 22, 2008 12:49 pm Post subject: |
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Andy_P2002 wrote: | Darren, just for interest's sake, I looked up your link to see what the difference was between the 'ephemeris' and the 'almanac' data.
Unfortunately...
Quote: | The GPS signal contains three different bits of information — a pseudorandom code, ephemeris data and almanac data.
{snip}
2. Ephemeris data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system.
3. The almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.
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... looks like the same sentence repeated? ...
Any more knowledge or info? |
Bu**er it, that's because I chose to re-visit the page having posted the link yesterday and tidy it up a bit, it had been untouched (and correct!0 since 2002 but not now I've mauled it!
I'll sort it out this afternoon _________________ Darren Griffin |
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DennisN Tired Old Man
Joined: Feb 27, 2006 Posts: 14893 Location: Keynsham
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Posted: Tue Apr 22, 2008 12:57 pm Post subject: |
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Well I looked and believed it all so much I nearly went out to buy three. _________________ Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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Darren Frequent Visitor
Joined: 11/07/2002 14:36:40 Posts: 23848 Location: Hampshire, UK
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Posted: Tue Apr 22, 2008 1:29 pm Post subject: |
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I've re-written the entire section to clearly explain the differences between almanac and ephemeris data and tried to explain cold/warm starts and the issues that occur when you have moved hundreds of miles between position fixes. _________________ Darren Griffin |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Tue Apr 22, 2008 2:28 pm Post subject: |
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Brill... all clear now!
But... (there's always a "but" )
From your description, it sounds like TomTom's "GPSQuickFix" downloads must be 'Almanac' data, whereas they always refer to it as 'Ephemeris' and the downloads even go in a folder called that.
Wikipaedia seems to agree with you... Are TT wrong again? |
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Darren Frequent Visitor
Joined: 11/07/2002 14:36:40 Posts: 23848 Location: Hampshire, UK
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Posted: Tue Apr 22, 2008 2:31 pm Post subject: |
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IIRC QuickFix uses a calculation algorithm to calculate extended validity ephemeris data, I'll see if I can find a reference to it. _________________ Darren Griffin |
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Darren Frequent Visitor
Joined: 11/07/2002 14:36:40 Posts: 23848 Location: Hampshire, UK
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Posted: Tue Apr 22, 2008 6:46 pm Post subject: |
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From our resident tech guru:
Quote: | The NOAA have an algorithm that predicts trajectories based on solar wind etc. The resulting data is passed to companies, it has a validity of two to ten days. Companies then package it and release via QuickGPS. |
So in essence it is an extended validity Ephemeris dataset calculated using predicted orbit data. The idea being that it reduces the time a device spends calculating. HTH? _________________ Darren Griffin
Last edited by Darren on Tue Apr 22, 2008 8:34 pm; edited 2 times in total |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Tue Apr 22, 2008 7:27 pm Post subject: |
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Curiosity satisfied , thanks!
(potters off to learn more about solar wind.....) |
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Calomax Lifetime Member
Joined: Sep 30, 2005 Posts: 988 Location: St Martin's, Guernsey
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Posted: Wed Apr 23, 2008 8:54 am Post subject: |
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RoystonB wrote Quote: | I’m not sure what QuickGPSfix actually downloads, I’m in the London area. When QuickGPSfix is downloaded will my tt only search for those satellites visible from there, or is it a world wide sat. position fix? |
Anybody know the answer to this please? Does it only help if the device has been turned off for a long time, and is turned on in the same location as the computer that downloads the info, or does it help if you travel to another country? _________________ TT Go Essential |
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Darren Frequent Visitor
Joined: 11/07/2002 14:36:40 Posts: 23848 Location: Hampshire, UK
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Posted: Wed Apr 23, 2008 9:02 am Post subject: |
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Calomax wrote: | Anybody know the answer to this please? Does it only help if the device has been turned off for a long time, and is turned on in the same location as the computer that downloads the info, or does it help if you travel to another country? |
QuickFix will be a world-wide data solution.
As explained above, it is an extended validity ephemeris orbit data-set calculated using a prediction program. It is not as exact as the ephemeris broadcast by the satellite but will in most cases speed up acquisition regardless of location.
Of course, if your device still has valid ephemeris data (<4hrs since last fix) it will make no difference at all. _________________ Darren Griffin
Last edited by Darren on Wed Apr 23, 2008 9:04 am; edited 1 time in total |
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Calomax Lifetime Member
Joined: Sep 30, 2005 Posts: 988 Location: St Martin's, Guernsey
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Posted: Wed Apr 23, 2008 9:03 am Post subject: |
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Thanks Darren _________________ TT Go Essential |
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