Dead reckoning is accomplished by monitoring your speed and direction and using this data to keep track of your position relative to a known starting location.
Different techniques can be used to measure speed and distance when GPS reception is lost:
* accelerometers to sense changes in your speed and direction
* sensors that measure the distance covered by the car's wheels
Sophisticated systems may combine data from sensors with map data to more accurately plot your course along curves and turns in a road.
The effectiveness of dead reckoning depends on how accurately a navigation system is able to monitor your speed and direction.