I may be going crazy here but I believe that several months ago I found a document somewhere on (or linked from) the paparazzi site that did an excellent job of describing the use of gyro, accelerometers, and quaternions for the purpose of Kalman filter for attitude estimation. It seemed to be what the 7-state Kalman filter was based on. Does anyone know if this document still exists, what its called, or where I can find it?