Standby attitude estimation

Abstract

In most military aircraft an inertial navigation system (INS) is the main source for attitude and heading data. Aircraft attitude and heading are important quantities, and in the case of a failure in the INS, backup values are needed. We here present how these can be calculated using other sensors in the aircraft. In order to determine attitude and heading with respect to a reference coordinate system, the same quantities in both the aircraft body coordinates as well as in the reference coordinate system are needed. These can then be used to determine the rotation from reference to body coordinates and thus the attitude and heading of the aircraft. We show how data from various sensors in the Saab Gripen aircraft, can be used to generate quantities exhibiting such relations. These relations are analyzed using recorded flight data. Using vector relations the QUEST algorithm yields a least squares fit of the quaternion of rotation from reference to body coordinates. We show here an alternative and simpler approach to such an algorithm in quaternion space. The REQUEST algorithm is an extension to the QUEST algorithm that processes measurements recursively. In the REQUEST algorithm angular velocities can also be used between measurements to update the quaternion of rotation. We extend our alternate approach in a similar way, but we also solve the case were additional states besides the quaternion rotation are introduced. The angular velocities used in a system of this type are often of poor quality, and states representing e.g. bias can thus be introduced.