SAT NAV in tunnels

Ian,you have answered your own ?.However you nwill also find that you will lose reception when going through relatively not so dense forest.i.e tree tunnel on roadway as my gps does on occasions.Must have three sats to give a reading.
Cheers Bob.

if you have an in car system that has a feed of the vehicle speed to the GPS then it will still work, but if it is a stand alone system then when it looses the satellites it assumes you are in the same position even when you are traveling.
Robert Peppers book "how GPS works" will explain the use of a GPS.

Keith

Thanks Keith and Loiuse
Yes it is an in car system - bloody clever actually - I assumed it would lose me in the tunnel when it lost the satellites

IAN

Mirrors, Ian.

It's all done with mirrors!

hooroo,
Bob.

The Sat Nav is guestimating your location based on the direction the car is travelling and speed, then when its in range of the Satellite, its switchs back over to Sat.

Happens when you drive amongst tall buildings as wel.

My tomtom does the same when I go through the Melb tunnels. I assume it is estimating my position based on it's last speed reading from the satellite etc.

There are two systems which "work" in a tunnel -for a time.
The cheaper ones will assume you will follow the road in the tunnel for a certain amount of time at your existing speed. If you stop in the tunnel it will not know. Eventually they will come up with an error message when they don't find another satellite lock-on.

Some better and yes much more expensive ones have an "accelerometer" built in and can tell if you stop/start or even slow down and turn within reason, till they get another satellite signal.
(This is how inertial nav systems work on aeroplanes, and how the Apollo and LM orbiter were navigated to the moon.)

These will obviously work better in poor signal areas.
Cheers

the missus MIO just says signal lost when going through the polly farmer

It makes the usually safe assumption that you are in a car rather than a tunnel boring machine, so it assumes you will continue travelling along the tunnel.