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Go530 fault - positions car to one side of the road
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Apple
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PostPosted: Tue Jul 03, 2012 12:10 am    Post subject: Reply with quote

If Home throws out the error again when I try to update the 920, I'll copy and paste the message in to here...
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jaybea_0989375
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PostPosted: Tue Jul 03, 2012 3:18 pm    Post subject: Another 720 with problems Reply with quote

My 720 was fine on Friday but on Saturday was showing location out by .25 to .5 of a mile, rendering it all but useless (I had to rely on Google Navigation on my phone to navigate through lanes from the depths of Derbyshire).

I had not connected the 720 to the PC before setting off on Friday, so there were no GPSQuickFix updates, although it was connected to a mobile for Traffic so it might have downloaded it. The phone battery was flat on Saturday, so the 720 started in whatever state it was in on Friday when it was turned off.

As MrT asks, could this be a leap second problem? GPS positioning relies on accurate timekeeping at both ends, so a leap second issue could account for the difference. The location on my phone was spot-on, suggesting the problem is with the 720 rather than some rogue satellites. It might also explain why doing a clock sync works.

Anyway, I tested it again after a reset and an update (including GPSQuickFix) on Sunday, and it was the same.
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dhn
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PostPosted: Tue Jul 03, 2012 3:26 pm    Post subject: Reply with quote

Here is a long and detailed explanation of what caused the problem and what the solution is. From a TomTom staff person involved in analysing the issue:

Quote:
Hi everybody,

Our investigations yesterday have resulted in a definite root cause for this problem, and as a result, there is now also a good fix scenario that some of you have already uncovered at this point.

First, how do you fix your device when you've ended up in the bad state of 'getting a quick fix, but solidly end up hundreds of meters away from your actual position'?

- remove the cached QuickGPSfix data from your computer (for me, this is: C:\Documents and Settings\behm\My Documents\TomTom\HOME\Downloads\complete\ephemeris ) - remove the entire 'ephemeris' folder
- remove the 'ephem' folder from your device (or the content of the folder). Make sure that you remove both the prediction file itself (packedephemeris.ee) and the timestamp files
- connect your device to Home and download the latest QuickGPSfix file
- disconnect device from pc, maybe you need to do one more suspend/resume

Problem solved

What was going on?

All affected devices contain the same gps chip, the SirfStar III, which uses a proprietary QuickGPSfix format, called packedephemeris.ee. This file predicts the position of the satellites in the sky for the next seven days after it was created. It seems (we still need to get an official response from the supplier, hopefully this evening) that they made a mistake in their prediction in the way they take the leap second into account. For some reason, this results in bad positioning both before and after the leap second changeover at midnight (UTC) of 30th june (saturday).

The result is a fix in the wrong location that is nevertheless interpreted as a valid and accurate position. The consequence of that is that the system will never recover. Moreover, even after you remove the file, the problem remains. So apparantly, the chip caches this prediction data somewhere, and removing the file is not enough. Even pin reset and/or factory reset doesn't always solve the problem. I've had some success with repeated pin resets and factory resets, but I've yet to come up with a sequence that reliably removes this cache data.

So the most reliable solution is to overwrite the prediction data with new prediction data, i.e. download a new packedephemeris.ee file via Home. All files that were generated after the leap second event are ok, so this applies to all files we're currently serving via Home.

After next thursday, the last affected devices (that were impacted by connecting last saturday) will be elligeble for a new QuickGPSfix download according to our back end system that checks the expiry date in the metafiles in 'ephem' on the device. So after that, the recovery scenario simplifies to:

- Connect to Home and accept the QuickGPSfix download

All this means the following devices, and only these, are impacted by this problem:
'Anyone with a TomTom device with the SSIII gps, who connected to Home for any kind of update between 24 june and 30 june, and did NOT explicitly unselect the option to install the QuickGPSfix file, will have run into this problem. Also devices who can obtain QuickGPSfix over the air - which for Sirf-based devices will always be via pairing with bluetooth to your phone, I believe - may be affected.

This seems to fit well with almost all the issue reporting that I've seen regarding this problem: almost everybody states they 'recently updated their device', so they were connected to Home and would be exposed to this unfortunate download. This also explains why it seemed to be related to some kind of bad software update released by TomTom, when in fact it has been several months since we actually released anything for these types of devices.

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jaybea_0989375
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PostPosted: Tue Jul 03, 2012 3:46 pm    Post subject: Reply with quote

Thanks for the information David - it makes sense of why some people see the problems and others have not (those who don't use GPSQuickFix).

Anyway, it gave me an excuse to replace my old 720 (with much improved battery life since I replaced it) with a new LIVE model. It also means that I can finally get rid of the antique Motorola RAZR phone which I have had to keep hold of because it was the only one the 720 would connect to for Traffic data.
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Guivre46
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PostPosted: Tue Jul 03, 2012 3:47 pm    Post subject: Reply with quote

The previous Leap Second was in January 2009. I can't remember if we had this fuss or not then. But if there was no problem last time, why was there one this time? Or if there was a problem last time, why did not they learn from it and have it ready to roll this time?

Progress?? [it's becoming my favourite comment]

Edit: Perhaps early warning of this may help?
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Guivre46
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PostPosted: Wed Jul 04, 2012 12:23 am    Post subject: Reply with quote

A very honest TT employee on their forum now says that the problem is probably not connected to the leap second, and may be more persistent. If it happens again he'd like the full content of the ephemeris file emailed to him.
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M8TJT
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PostPosted: Wed Jul 04, 2012 6:15 am    Post subject: Reply with quote

So let's first blame someone else first (Sirf Starlll) then find out what the real problem is. Rolling Eyes
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AliOnHols
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PostPosted: Wed Jul 04, 2012 6:38 am    Post subject: Reply with quote

I must say that I remain sceptical about the leap second causing the troubles.

Given that our location is calculated by triangulation based on differences in the time stamps between the GPS receiver (SatNav) and the satellites. The speed of light is roughly 300,000km per second and the time stamps were 1 second out which would put us floating in space about 287,000km above Australia (three-quarters of the way to the moon!) and not just a couple of hundred metres away from the road we should be on.

Or am I applying my laymans logic to science yet again?

Edited by Ali.
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M8TJT
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PostPosted: Wed Jul 04, 2012 7:56 am    Post subject: Reply with quote

But all the time stamps would be one second out, and as you say, it's the difference in time stamps that gives the triangulation (simplistically), so your theory is in line with your final statement. However, the satellites will be in the wrong place for your time (if your time is UTC) and arrive at the predicted position one second earlier (it might be later, but I can't be assed to work it out at this time in the morning {plus/minus a second}). This positional error will be much less than 300,000km and will be related to the orbital velocity of the sat, and this itself will depend on the elevation angle of the sat, as the radial velocity increases with elevation angle.
The orbital velocity of a 12hour satellite is nearly 4km/Sec, so it would be about 4km 'out of orbital position' compared with the predicted data for that time. Quite how this much smaller orbital distance 'error' would affect the triangulated position, I don't have a clue. Shocked After all that, the satnav device gets the sat position data from transmissions from the sats from time to time, so should correct itself as soon as it gets this data. Further to this, the satnav device's clock must be synchronised to the satellite 'time' extremely accurately (hence the need for a 4 sat lock) so the clock should have sorted itself out as sat lock with 4 sats was aquired.

PS. I sometines come up with some fairly convincing rubbish. Anyone notice my deliberate mistake?
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AliOnHols
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PostPosted: Wed Jul 04, 2012 8:10 am    Post subject: Reply with quote

WOW! Shocked
Many thanks for that M8TJT, not sure that I understand it all yet. It shall probably take several reads.
As for finding your deliberate mistake? Not a Scoobies!
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M8TJT
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PostPosted: Wed Jul 04, 2012 8:31 am    Post subject: Reply with quote

Ali. Please notice the first sentence in my PS. There may be a bit of tuth in wot i rit, but there is deffo a deliberate mistake in the sem-convincing stuff Very Happy .
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Andy_P
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PostPosted: Wed Jul 04, 2012 4:28 pm    Post subject: Reply with quote

Ahh... I thought your deliberate mistake was in the PREVIOUS post.

..... The bit where you said TomTom would be able to work out what the problem was. Wink
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M8TJT
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PostPosted: Wed Jul 04, 2012 6:25 pm    Post subject: Reply with quote

No Andy, that was a stupid non-deliberate mistake. I suppose I should have written "So let's first blame someone else first (Sirf Starlll) Rolling Eyes "
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Guivre46
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PostPosted: Fri Jul 06, 2012 10:50 pm    Post subject: Reply with quote

Anita wrote:
I've never felt the need to download the QuickGPSfix. My 720 gets a satellite lock pretty quickly anyway.

This isn't the first time the file has screwed up devices, and I'd rather restrict the possible sources of problems caused by TomTom to essential files.


Anita, always thought the Sirfstar3 chip was the most sensitive of my three devices, so giving it a whirl. So far it seems to lock onto satellites quicker than when I had QgpsF installed!!
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Anita
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PostPosted: Fri Jul 06, 2012 11:05 pm    Post subject: Reply with quote

Thumbs Up I hope it lasts, Mike!
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