Some units do "dead reckoning" calculations based on vehicle speed and distance travelled to compensate for places where the GPS signal is weakened or lost for whatever reason (urban canyons, tunnels, heavy foliage...). This information is gathered by the head unit from the cars speedometer and odometer reports...
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Driving on down the road in my 2001 CVT, going "Boogety Boogety" ...and until avatars are provided, my car looks just like the original silver Insight on the header, above... =)
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