Closed-Loop Operation of GPS Aided INS
Gregory B. Johnson, Zdzislaw H. Lewantowicz · 1990
The Global Positioning System (GPS) aided inertial nav- igation systems (INS) that are implemented in fighter aircraft must be able to operate in a high dynamic and jammed signal environment. This paper investigates op- eration of a GPS aided INS during horizontal and verti- cal maneuvers of up to 9 g’s During these maneuvers, the GPS receiver carrier tracking loop is expected to loose lock and the code tracking loop is expected to re- ceive Doppler aiding from the INS velocity signal. These maneuvers are simulated using a nominal and decreased signal-to-noise ratio (SNR). The decreased SNR repre-sents jamming of the GPS receiver signal. The present implementation in the F-16 (and other aircraft) of the GPS aided INS (LN-39) is a suboptimal filter-driving- filter configuration. This use of two Kalman filters and a decreased code loop bandwidth for increased jamming resistance tends to destabilize the closed-loop operation of the GPS aided INS. The performance of three Kalman filter configurations is examined using time simulation analysis. The three configurations are an optimal full- ordered filter, single reduced-ordered filter, and the F- 16 filter-driving-filter configuration. All three configura- tions are evaluated in closed-loop and open-loop modes. Each configuration is simulated using nominal and de- creased SNRs. Results show that the GPS aided INS is stable for nominal operations. However, the closed-loop system becomes unstable in filter-driving-filter configu- ration with a low SNR.