[terry]

Have you ever noticed that you still have navigation even when you enter a tunnel or in underground parking. How does the system do that ? Obviously it doesn't have any satellites coverage. Any idea

[mjh93sa]

Dead reckoning based on input from the car... wheel speed and so on.

[terry]

makes sense. Thanks. What about "heading". Is there a sensor for wheels direction ?

[SteveMD]

Dead reckoning. RPMs maybe, vehicle speed?

I doubt that it has direction sensors. It likely assumes that you are still on the road you were on when you lost contact with the satellites. i.e. if you go through a tunnel, where else could you go?

[X5FX]

Garmin has been doing this for years with their Street Pilots:

Dead Reckoning

Unlike the 7200, the 7500 also includes dead reckoning sensors to track your vehicle's course when your GPS signal is blocked for example in urban areas or tunnels. If you lose GPS coverage in areas with tall buildings or tunnels, the StreetPilot 7500 keeps on navigating. Minimal installation is required for the dead-reckoning feature.

The older units had sensors and were hard wired to spedo

[mjh93sa]

If you start with a good GPS lock then all you would need is a few accelerometers to determine any change of detection made. From this your new position can then be computed.

Given the amount of data being gathered and monitored for things like DSC all of this is available within car (from speed, to steering angle and accelerations) and could be fed to the nav system.

[BK]

Originally Posted by terry

makes sense. Thanks. What about "heading". Is there a sensor for wheels direction ?

Most dead reckoning units use an electronic compass (like the one built in your mirror with the premium package) to tell which way the car is facing, combined with wheel speed sensors. I assume BMW does the same.

FWIW, I love dead reckoning units. It's cool to see the map turning as you turn the car in a parking garage -- this does not happen with a portable GPS. When portable units lose sat lock, most of the good ones "guess" that you're still going the same direction and speed and will continue to update the map under that assumption until it hooks up with the sats again, or until a certain amount of time passes, at which point it gives up until it finds the sats again.