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Joined: Nov 24, 2003 Posts: 1441 Location: Swansea
Posted: Sun May 29, 2005 6:56 pm Post subject: GPS Error
I posted this query on a different forum, but had no reply. I wonder if anyone here has any ideas.
I was near the end of a longish drive of 240 miles, TomTom had behaved perfectly all the way. Then it started showing my position about 50 to 100 yards to the right of my true position. It was still showing my speed correctly, and if I zoomed out I could see the road I was actually on at the left of the screen. I stopped and did a soft reset, but it made no difference. I then switched to CoPilot, but that also showed the same error. As the direction of the road changed bearing, so did my recorded track, but always some distance to the right of where it should have been.
When I was out in open country there was little in the way of voice instructions, but when I entered my home town, there were often roads at my indicated position, and the software kept trying to make me turn round to get on the correct road (although I was already on it!). This continued all the way home.
The next day I tried TomTom on a short journey and it was absolutely normal.
I was getting a fix from 7 or 8 satellites most of the time this was happening.
I am mystified, I haven't had anything like this happen in nearly 2 years - if anyone has any ideas I would be grateful.
Robin
Joined: 02/11/2002 22:41:59 Posts: 11878 Location: Massachusetts, USA
Posted: Sun May 29, 2005 11:56 pm Post subject:
Your receiver may have worked with corrupt data for a while. This can be due to weather conditions, or satellite constellations, or some external noise source (like a car near you with a radio transmtter).
Your navigation programs were not at fault. but this is a good example that they can only provide assistance, and you should mostly rely on your own senses (which you did). _________________ Lutz
Joined: Nov 24, 2003 Posts: 1441 Location: Swansea
Posted: Mon May 30, 2005 12:02 pm Post subject:
I wondered if one of the satellites was sending out erroneous info. The thing which really puzzled me is that when my road turned through 90 degrees (eg from south to west) my indicated position was still some 50 to 100 yards to the right of where it should have been, ie the error changed from being, say, 50 yards too far west to 50 yards too far north!
I'm doing another 500 mile round trip later this week, so will see if it recurs after some time on the road.
Robin
Joined: Nov 24, 2003 Posts: 1441 Location: Swansea
Posted: Fri Jun 10, 2005 11:03 pm Post subject:
For the record, I repeated the journey a week later and TomTom behaved impeccably throughout. I believe there was a lot of sporadic E activity in the ionosphere - perhaps that caused a GPS error at that time
Robin
Joined: Aug 03, 2004 Posts: 137 Location: Essex, UK
Posted: Mon Jul 18, 2005 2:53 pm Post subject:
You were not on the A1 were you???....My Navman unit/SmartST would often get confused on the A1......when going to Alton Towers, or Keswick, or Durham......a couple of times it even wanted me to double-back on my self a couple of times, before I re-set it, and then got the "continue for 150mls"..
The A1 is a sod with the Navman....
I'm taking my new GlobalSat BT338 with TT5 up the A1 this Thursday....I wonder how it will cope?
....Then it started showing my position about 50 to 100 yards to the right of my true position.
...- if anyone has any ideas I would be grateful.
Robin
I have no difinitive answer but three things spring to mind:
1) Remember that the GPS is a military system - the US Military can introduce inaccuracies if needed - maybe they were sending a TOMTOMahawk to Osama...We wish....
2) The actual portion of that map in TT is ‘off track’
3) After watching the way the software reacts to certain circumstances and conditions, I have come to the conclusion that TomTom software has many algorithms that 'correct' the display even when the GPS data indicates that the receiver is not in the exact location that it is...
Example: You are travelling along a road - TT software puts the road on the position display marker on the screen. Because of the inherent /possible GPS errors the display could jitter back and forth giving an uncomfortable/unusable display. Therefore an algorithm could be used to (a) iron out the errors and/or (b) (using the logic that the car is on the road not driving in field/houses) The software ‘looks’ at the display and assumes you are on the road, therefore 'forcing' the road to be centred on the position cursor, even though GPS data may indicate the actual location being a few yards away.
Taking that summation as correct, it is also possible that the correction algorithm has got its knickers in a twist and 'forcing' a correction the wrong way. Hence putting you on a parallel track.
Have you ever noted that when driving off the known road, say into a large car park or into a field, that the position cursor tries its best to stay on the road for much longer than when tracking a change of direction along a normal road. It then Jumps to a position off the map.
Folks out there: this is only my own idea - take 'em with a pinch of salt and don't spread 'em around as the latest urban myth
_________________ Axim X50 & TT5, Arkon CM820 powered mount with built in sirf2 gps
Axim 5 sans GPS Rx
Ford Focus 2L Ghia, Wife, kids, dog, cats, mortgage, bank manager & taxman to support, debts, windows need painting - (I cut the grass! - its rained - it needs
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