Loosely Coupled GPS-Aided Inertial Navigation System for Range Safety

The Autonomous Flight Safety System (AFSS) aims to replace the human element of range safety operations, as well as reduce reliance on expensive, downrange assets for launches of expendable launch vehicles (ELVs). The system consists of multiple navigation sensors and flight computers that provide a highly reliable platform. It is designed to ensure that single-event failures in a flight computer or sensor will not bring down the whole system. The flight computer uses a rules-based structure derived from range safety requirements to make decisions whether or not to destroy the rocket.

By combining the inertial navigation system (INS) with Global Positioning System (GPS), the GPS signal can be used to check error growth of the INS and, due to the small, short-term errors of the INS, the system is more accurate than the sensor alone. The fused system helps to solve the common cause failures, and also provides the benefit of graceful degradation of system performance should a failure occur.

This innovation has algorithms developed specifically with range safety applications in mind. The INS and Kalman filter algorithms, including the linearized error model, for integrating the two systems were developed and simulated to determine their performance. The system calculates the errors in the IMU and provides information on the quality of the data it outputs to aid the AFSS system in determining what level of trust to give the data.

The filter is designed in such a way that there is always position and velocity output. Loss of GPS will not cause the INS to go unstable, or to cease information output. Also, covariance estimates and the error states are available to the user for further use in determining data quality.

This work was done by Scott Heatwole and Raymond J. Lanzi for Goddard Space Flight Center. For further information, contact the Goddard Innovative Partnerships Office at (301) 286-5810. GSC-15549-1