Car: Jag XF 3.0
Join Date: 10th April 2006
Thanked 2,774 Times in 824 Posts
Skippy, I'm not sure whether it's clever enough (or indeed accurate enough) to do proper DR using latitude. The cal value is simply the 2.5 volt neutral position, so when the car is not turning then the thing reads zero.
I strongly suspect that the nav simply uses the yaw rate and wheel speed (which it gets from the wheel impulse generator factored by a tyre factor that it computes when the GPS is working) to DR for the small times there is no GPS signal (including just after system start). It's pretty accurate - it copes perfectly with the Mersey Tunnel, and it tells you to take the "next exit" off a roundabout immediately after the penultimate exit.
These two features alone make the system significantly nicer than a tomtom or similar. But it's not like having three sets of laser gyros...