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Sat-nav still works in tunnels ... how?
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01-16-2008, 10:41 AM | #1 |
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Sat-nav still works in tunnels ... how?
The BMW sat-nav continues to work even when there is no clear view of the sky, eg in tunnels and multi-storey car parks.
How? I have a handheld GPS which fails immediately under trees, even heavy cloud. But the BMW system works all the time. I have a theory that actually, satellite reception IS lost in tunnels, but the mapping software combined with the speed and direction of the travel is enough to keep the system running.
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01-16-2008, 10:58 AM | #2 |
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The BMW satnav also uses the ABS sensors on each wheel to determine the rotational speed, and therefore the distance travelled. That's why even in a tunnel the satnav keeps track of where you are...
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01-16-2008, 11:09 AM | #3 | |
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When I had TomTom the signal used to drop every time I was in London because I lost the line of sight to the satellite. |
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01-16-2008, 11:42 AM | #4 |
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Tomtoms also TRY to use dead reckoning as they have (or at least had) an accelerometer in them. It just doesn't work anything like a well as the integrated system in the BMW.
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01-16-2008, 11:53 AM | #5 |
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Now that is interesting. You learn something new everyday...
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01-16-2008, 12:22 PM | #6 |
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+1 a wealth of useful information
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01-16-2008, 12:30 PM | #7 | |
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Otherwise it can be quite sluggish.
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01-16-2008, 12:41 PM | #8 |
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I've noticed this with my Tomtom. It can't be that hard to predict where you are in a long tunnel. If you enter the Dartford tunnel at 60mph, the SatNav should assume you're still doing 60mph. Chances are you're not going to be turning left or right or stopping in the tunnel.
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01-16-2008, 12:56 PM | #9 | |
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01-16-2008, 01:50 PM | #10 |
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There are two types of dead-reckoning used in portable units (as I understand). If you spend enough money they'll put in some accelerometers, otherwise they'll just assume you keep moving (so an average or last known speed) along the route indicated.
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01-16-2008, 02:44 PM | #11 |
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That's interesting then, I wondered how come it still worked whilst driving through the Tyne tunnel a few weeks ago.
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01-16-2008, 02:53 PM | #12 |
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I take it that you are talking about the Nav system Helen and not any other type of electrical toy?
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01-16-2008, 02:58 PM | #13 |
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01-16-2008, 02:59 PM | #14 |
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Glad you keep your mind on the job in hand
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