An interesting development, although not hugely applicable to aircraft GPS systems one would imagine (or hope).
"Swiss company u-blox just devised a chip that keeps you on course when the satellites drop out, using the most old-fashioned of navigation techniques: dead reckoning. The u-blox 3D Dead Reckoning chip uses a bevy of accelerometers, gyroscopes, and speed sensors to calculate a car's exact location, no GPS signal needed."
https://www.youtube.com/watch?v=ykR0TWL-_oA