Originally Posted by richierich
The satnav does indeed work "underground" it tracked me all the way through the Dartford tunnel, the Heathrow spur tunnel and all those round the M25 up towards Hatfield. Does it use the radiosignal, or does it use the speed of the car. I know my TomTom used to keep you moving along the route if it lost a signal, but at a reduced rate.
No sat nav system will work in a tunnel. The time codes from the satallites simply cannot penetrate that far. What many sat nav system do is guess your position by assuming you are continuing at a steady speed and direction after signal loss, and waits for it to re-appear.
The system is working very well if it accurately judges your position til you exit the tunnel. The only time it does not help is when you do not carry on in a straight line or you stop. The system assumes you are still moving as you were when it lost signal, and can take you miles off course.
I was in London a while back, and I stopped at lights under a big bridge system. GPS lost signal, and presumed I was still travelling at 20mph. my 'position' was some distance away from where I really was, and the stupid thing kept telling me that i was off route, and kept recalculating!
Whether or not the system is linked to the car in terms of speed I do not know, but it certainly can't determine your direction after signal loss.