GPS adrift again?

[Andy_P]

UPDATE: It was wrong on the way in to work again this morning, and had me 200 metres too far north when I was in the carpark.
So I ran inside, hooked up to a PC and deleted the contents of the "ephem" (ephemeris) folder.
Went for a walk back in the carpark and so far... no change.

Still a mystery then!

[Snudge]

[StanStill]

I don't think it is a mystery. What you have proved is that it is not quickgps fix that is the problem, it is the signal from the satellite that is giving you the wrong position probably because the ionosphere (which is affected by solar activity) slows down the time signal from the satellite. Ground stations try to compensate for these errors but it seems that they are not 100% successful. Or so it says in wikipedia.

[Andy_P]

It's not that I don't believe you, but if it was solar activity, wouldn't it be much more widely seen and we would all know about it?

Actually from my drive home tonight, I've got another theory...
It might be Bluetooth related.
I only used to enable Bluetooth occasionally when I played with my wife's phone (mine has no BT capability).
but in the last couple of weeks I've got a new BT capable radio headunit, so have been playing music from the TT via it.
POSSIBLY, the problem occurs if BT is enabled on EITHER the TT or the radio.
I still can't be certain because if you turn BT off it doesn't instantly cure it.

[dhn]

Don't want to rain on your parade, Andy, but I've experienced 'drift' and I don't use Bluetooth.
_________________
David

[Andy_P]

Yeah, I wasn't convinced myself, I was just really keen to find a solution.

I've never had the "drift" for such a long period as this before. It's been pretty bad all week. I reckon if I was trying to use it in central London with lots of small streets close together, it would be unusable.

[Vortex]

Andy, could it be anything to do with your unit getting too hot? Most (all?) GPS hardware is temperature compensated to keep the internal clock steady. If your unit has been in the sun for a long time or near a heater vent, this could cause the drift.

[mikealder]

The internal clock used for time has no bearing on positional accuracy, try setting it to a stupid time and it won't do anything wrong, the high accuracy clocks are in the satellites orbiting the earth.
Getting the GPS too hot can shut down the battery charger though so its not a good idea though - Mike

[Andy_P]

OK, I was going to use a sledgehammer to crack a nut and try putting v8 on this machine to see if that would fix it.
Of course I had to go via v7.481 first, and when I had put that one on I had a quick look standing outside the house.
My location was now showing correctly, rather than one and a half streets away! Switched off and on again and saw it actually MOVE my assumed location away (as if I was driving) but now it only ended up about 50yards wrong.
Switched off and on again a second time and it looks right again.

I guess I will only see for certain when I take it out for a drive tomorrow.

(Would still love to know what goes wrong in its tiny brain that a re-boot doesn't sort out though).

[kichu1979]

I also had the same problem around similar date to yours, so I would bet on faulty gpsfix data. As soon as I saw this I did pinhole reset and everything was fine.

[Vortex]

Mike, when I mentioned the internal clock I was referring to the temperature controlled oscillator which has everything to do with the positional accuracy of the receiver. What the clock on the display shows is pretty much what you tell it to. The internal clock, however, has to be synchronised accurately with UTC which is what the satellite uses. Because temperature changes can affect the frequency of the oscillator in the receiver, temperature compensation circuits are used to keep the output stable. Big changes of temperature outside the receiver's normal operating range can affect the accuracy of the calculated position, hence my comment above.

[Andy_P]

After a couple of days use, it's still crap.
It's as if some internal calibration between the received co-ordinates and the map overlay has taken a hit. When it is only 50 yards or so out, the "snapping" to roads often disguises it, but it's still noticeable if I don't set a route and drive in places with nearby roads parallel to the one I'm on.

Does anyone know if such a callibration exists, even if we can't get at it ourselves?

Looks like it needs sending back, but I'm scared of a long, drawn out "no fault found" exchange.

[Darren]

It's very possible that the data is 'written' to the flash memory on the GPS receiver, the ephem folder simply being the download repository for the QuickGPS files.

So deleting the files woudn't flush the Ephemeris data from the GPS receiver itself.

I'm still betting this is corrupt Almanac/Ephemeris data, whether it's from QuickGPS or not is another matter but it's not caused by atmospheric anomalies.
_________________
Darren Griffin

[mikealder]

Try the clear flash tool and restore factory defalults then see what it does -Mike

[AlandEi]

I don't know if this has any relevance to this thread as I am a beginner, and have only owned my Go 520 for a few weeks, but on Monday evening I downloaded a GPS update via Home 2.3, and immediately after disconnecting from the PC, noticed my cursor position was now about 50 yds. away in the next street... I rebooted the sat-nav a number of times to no avail... so I re-installed my last Explorer backup which was 02-10-08, and immediately, all was well again.... I didn't have the opportunity to try the unit on the road at the time.

This is the first time I have seen this happen since owning the Go, and have downloaded and installed many of these GPS updates since purchasing it.

Alan.
_________________
Tom Tom Go 520 - Map UK ROI 720.1803 Navcore_8.204.9718.go40 - ( Loquendo TTS7 ) / RDS-TMC /Tyre... purchased the Go mainly for the itinerary feature

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