Use of an external cascade Kalman filter to improve the performance of a Global Positioning System (GPS) inertial navigator

The accuracy of in-air inertial navigation system (INS) alignment has been analyzed for several methods of GPS/INS integration. It is found that an external filter which operates on the outputs of the GPS receiver achieves a faster alignment than the GPS receiver alone. Steady-state accuracy is not significantly improved by the use of the external filter. The data rate of this external filter should be one update per second, which is the same as the GPS receiver´s update rate and is the fastest that the external filter could be updated. In-air alignment of a second INS can be performed as a transfer alignment. In this procedure, the INS, which is integrated with the GPS, transfers its alignment to the second INS using an external filter. The performance of this external filter is analyzed and it is found that for a medium-accuracy INS, and also for a low-accuracy attitude heading reference system (AHRS), the fastest update rate provides the best alignment response