How does Sat/Nav work in tunnels???
01-07-2008, 10:34 PM
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My friend has a TomTom satNav and it does not update its position when it goes into the Sydney Harbour Tunnel (about 2KM long), while mine continues to update (apparently accurately) in the tunnel. So how does the BMW Sat/Nav work when it loses it's signal from the satellite eg when it is in a tunnel?
TIA
TIA
01-07-2008, 10:58 PM
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My nuvi loses satellite reception in tunnels, deep canyons etc., sometimes.... but generally I know where I am until it picks up reception again 
01-07-2008, 11:06 PM
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unlike tomtoms, our navis have a direct connection to the speed sensor, so therefore the our navis can predict where we are given when we were at the beginning of the tunnel it know at what speed you are going through the tunnel. If there are any changes in direction in the tunnel, our navis know this thanks to a gyro (like a compass), therefore all changes in direction are noted. I think the gyro is also missing in the tomtom..
01-08-2008, 12:04 AM
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Originally Posted by rsyed' post='514986' date='Jan 8 2008, 09:06 AM
unlike tomtoms, our navis have a direct connection to the speed sensor, so therefore the our navis can predict where we are given when we were at the beginning of the tunnel it know at what speed you are going through the tunnel. If there are any changes in direction in the tunnel, our navis know this thanks to a gyro (like a compass), therefore all changes in direction are noted. I think the gyro is also missing in the tomtom..
01-08-2008, 12:58 AM
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My N?vi uses dead reconing and this works for shorter tunnels up to a kilometer or so. If the tunnel is longer it tells me that it has lost reception of satelites.
01-08-2008, 01:32 AM
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Originally Posted by faarcanal' post='514980' date='Jan 8 2008, 08:34 AM
My friend has a TomTom satNav and it does not update its position when it goes into the Sydney Harbour Tunnel (about 2KM long), while mine continues to update (apparently accurately) in the tunnel. So how does the BMW Sat/Nav work when it loses it's signal from the satellite eg when it is in a tunnel?
TIA
TIA
Simple: it gets speed from differential or wheels .
01-08-2008, 08:36 AM
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Originally Posted by MacSid' post='514998' date='Jan 8 2008, 04:04 AM
+1 .. the built-in nav units dead-reckon when there is no reception, using speedometer, steering and a gyro as inputs.
SA is currently disabled and has been since some time in 2000 and is not likely to ever be enabled again...
01-08-2008, 09:02 AM
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My Ride: 2008 Porsche 911 Carrera S Convertible. Midnight Blue, 6 Speed.Retired - 2007 997 Carrera S, Midnight Blue, Grey leather, premium audioRetired - 2007 550i, Monaco Blue over Beige, Navigation, Logic 7, Cold Weather Pack, Comfort Access, Sport Package
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The more up to date TomToms do indeed dead reckon and are about as accurate as anything on the built in nav. They use accelerometers to monitor speed and directional changes when out of satellite coverage and it's uncannily accurate. This is something you'll find on most mid to high end aftermarket GPS systems. TomTom also now lets you send addresses from Google Maps to the device and also has weekly uploads with satellite locations so the unit can establish its position quicker on startup if it has limited satellite coverage (i.e. if it finds only one or two but knows where the rest should be at any given point in time thanks to the weekly upload it can still position itself with reasonable accuracy).
01-09-2008, 11:21 AM
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The new Tomtom 920 (I think 720 too) has the EPT (enhanced positioning technology) and it seems to work fine.
01-09-2008, 11:25 AM
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My Ride: 2008 Porsche 911 Carrera S Convertible. Midnight Blue, 6 Speed.Retired - 2007 997 Carrera S, Midnight Blue, Grey leather, premium audioRetired - 2007 550i, Monaco Blue over Beige, Navigation, Logic 7, Cold Weather Pack, Comfort Access, Sport Package
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Originally Posted by kchan' post='515590' date='Jan 9 2008, 12:21 PM
The new Tomtom 920 (I think 720 too) has the EPT (enhanced positioning technology) and it seems to work fine.