I had not heard of this before. Assuming that the nav unit is GPS based, I guess the motion sensor's purpose is to calculate the vehicle's position when the unit is unable to 'see' its satellites. When my Street Pilot III loses signal, such as when entering a tunnel, it announces loss of signal, freezes the display and updates itself when we pop out again at the other end. This yaw thingy is a neat device!Yaw Rate Sensor
The yaw rate sensor (located in the navigation unit) detects the direction change (angular speed) of the vehicle. The sensor is an oscillation gyro built into the navigation unit.
My Street Pilot is often, though not always, savvy enough to tell me exactly when to take the next off on a roundabout. I had sort of figured that it does this more on larger roundabouts and at slow speed on smaller ones. The map and numerical displays are very accurate in displaying time and distance to the next event. I don't know if it has yaw sensing, but if I have understood this right, I am intrigued at the possibility that it might! On the other hand I can also relate to Skippy's observation concerning lag, I have certainly noticed that and especially the delay inherent in recognising that you've gone "off route".- 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...
Crackers - portable navs don't have these DR (deduced reckoning) systems where they can tell where they are when the GPS system is unavailable. Portable systems are pretty good (I have two here), but a good built in system does have its advantages.
To do DR you need to measure speed (done from the sensor that drives the car speedo) and yaw (there are a few ways of doing this).