GPS receivers are specified for conditions with a clear view of the sky, no obstructions and all satellites in view. In automotive applications these conditions are present only part of the time. In mountainous terrain, under dense foliage and around high rise buildings, GPS reception is compromised. In dense urban canyons, GPS radio signals bounce off buildings creating "multipath" reception, sometimes fooling the GPS receiver into supplying a position with a large error.
Adding inertial sensors and processing using a Kalman Filter provides a dead reckoning (DR) position solution. Combining the DR with GPS gives more consistent navigation.
GPS signals originate about 12,000 miles above the Earth. They are very weak when they reach ground level so are easily shielded. Buildings, passing vehicles, RF absorbent material and dense foliage can interfere with signals from some satellites. At the same time a bounced GPS signal due to multipath gives a false distance reading to the satellite, throwing off the position calculation (at the speed of light, a 3.3 ns delay is approximately equal to a position error of 1 meter).
Adding inertial sensors and sophisticated dead reckoning calculations provide a method of keeping the navigation and positioning system on track. The combination of DR with GPS increases availability of positioning solutions in mines, tunnels, under dense foliage, in urban canyons, in mountains and under bridges or overpasses.