MEMS: why INS + GPS?
Each sensor has specific characteristics:


Accelerometer: Provides detailed information about linear acceleration. Can also measure direction of gravity (partial orientation) under some circumstances. High sample rate, but subject to scale and offset errors. Integration to get velocity and position leads to drift.
Gyroscope: Provides detailed information about rotation rates.  High sample rate, but subject to scale and offset errors. Integration to get orientation leads to drift.
Magnetometer: Provides drift-free information about orientation, but absolute accuracy is low, depending on knowledge of local magnetic field and the distortion effects of nearby iron. Moderate sample rate.
GPS: Provides drift-free information about position and velocity. Interferometry can be used to get information about orientation as well. Low sample rate.


The INS provides high sample rate data, but is subject to drift. Therefore, high-accuracy systems combine that data with magnetometer and GPS data to eliminate the drifts and get the best possible answer at a high sample rate.