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?

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....

Magician - you cheeky so & so! Did it occur to you that I don't live in the UK? Why do some people assume that eveyone on here is in the UK? There is a long tunnel in Cologne which goes under the river our Sat Nav advised us to Keep Left and Turn Slighty Right whist we navigated that very tunnel last night - so ner!

duh - meant to say Dusseldorf not Cologne - am tired this week from excesses at the weekend!

Answer 1

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.

Answer 2

Some posh sat nav systems can do this.

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.

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.

Answer 3

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.