bit of a delay on this thread but...
I've implemented an indirect Kalman Filter for attitude estimation, where the KF estimates the error in orientation and error in gyro bias, rather than the orientation and bias itself (these are kept external to the filter). Works pretty well - no problems with singular covariance matrix from using quaternions in the state etc.
However, i only get to model the gyro bias (using qerrdot = -gyro x qerr - 0.5 * bias). I'd like to model the gyro scaling parameters too, perhaps even including cross axis (i.e. a full 3x3 scaling matrix). This makes it all non-linear though.
Don't suppose anyone has seen this worked though into a set of EKF equations, or can point me to any papers of people doing this...?