Inertial Sensors Boost Smartphone GPS Performance

Because of the relatively slow bit rate for GPS communications, it tends to require a lot of power from battery powered handheld devices such as smartphones. One option is to sleep the GPS device, and use more energy efficient inertial sensors in the phone to approximate location in between GPS fixes - which can also be handy when you're in areas (like tunnels or specifically the Eisenhower tunnel in Colorado) that do not have a GPS fix available.

http://unbonmotgroundswell.blogspot.com/2013/11/inertial-sensors-boost-smartphone-gps.html