How does Sat Nav continue to track your position underground?

Question:
We've got Sat Nav in our car and last night we were driving in a tunnel under a river and it was still able to tell us what turns to take. How does it do this?

had it up the rear?


Also, does anyone know how you get updates for Sat Nav systems? Our Sat Nav is the Audi installed one but the Audi dealer didn't have a clue when we asked about updates....

Answers:

Why isn't there an independent Air Conditioning system in a vehicle whereby fuel consumption is not affected?

factory fit sat/nav systems receive a square wave pulse
when the vehicle is moving and as it knows the direction you are heading when you enter the tunnel it calculates the speed at which you are moving,therefore it knows your location at any given time.

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Some posh sat nav systems can do this.

what does the black license plate w/ the blue line in the middle mean?


From the little i know.. the sat nav is also rigged up to some device in the wheels which tracks speed and movement of the car. So when the system loses its gps lock.. it takes an approximation from how the car is being driven / speed / turning.

How many miles can I expect a VW diesel engine to last?


I can't see it being accurate for too long underground - but useful to have for certain.. and would work fine underground for a while i bet.

Who knows the best first car? (Good gas mileage and no more than 3000$?

Mine (a tom tom) wont work in tunnels but i have to say if you get lost in a tunnel you really shouldnt be driving! As already described it takes into account the speed on entering the tunnel and assumes where you are based on that. I cant think of any tunnels in the UK with turns in them.
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