General principle

IMU captures attitude of the aircraft in real time, about 20x (200x?) a second.

• there is some variable error in this measurement, particularly if the IMU isn't shaken up regularly

GPS receiver records signals from the visible GPS satellites, allowing a position fix approximately every second (?)

• there is inherent error in this measurement, due to atmospheric conditions, etc delaying the signal from the satellites

In postprocessing, the position information is normally combined with a fixed ground station to remove/reduce local error. Position fixes are still once every second or so.

Following this, the IMU and GPS information are combined to produce an SBET (Smoothed Best EsTimate) file, which uses the IMU information to interpolate a predicted best path through the GPS position samples. This path will not necessarily be the true path. The SBET contains the interpolated navigation at high resolution and has inextricably mixed the GPS and IMU information.