Synthetic attitude and heading reference for Saab Gripen

In future versions of Saab Gripen, the mechanical artificial horizon will be replaced by a computer calculated attitude and heading, independent of the inertial navigation system (INS). The system uses data from sensors already existing in the aircraft, which are easily available in a highly integrated, 4th generation combat aircraft such as the Gripen. The sensor information used is a three-axis magnetic detector, true airspeed, angle of attack, barometric altitude, flight control rate gyros and load factor. The sensor data is fused together in an extended Kalman filter (EKF). Each sensor by itself is of relatively poor quality. For instance, the accuracy of the rate gyros is in the order of degrees per second, rather than degrees per hour as is the case in gyros dedicated for navigation use. However, when all data are combined, they provide an attitude and heading estimate with sufficient quality for its purpose; to cross-monitor the INS, and to serve as a backup in case the INS fails or data can not be displayed. The system is called synthetic attitude and heading reference system (SAHRS), and is a Saab patent. A similar system is developed and operational in the Saab Viggen.