A Global Navigation Satellite System plus Inertial Measurement Units provides a stable navigation solution. IMU measurements in general are very clean measurements, but are prone to drift over time, temperature, and vibration. Global Positioning System (GPS) measurements in general are very stable, but can be blocked or jammed. Modern navigation systems combine Inertial sensors and GNSS (including GPS) and use each system to calibrate the errors out of each other. The result is a very clean, stable position and attitude measurement.