Q matrix tuning - Kalman filtering

Hi there,
I am using a Kalman filter to determine trajectory and attitude of a
terrestrial vehicle, with the measurements of an INS/GPS system.

I have some difficulties in choosing the right values of the Q matrix.

Are there any methods to choose them? I heard about Bartlett's method but
I don't know how to make it work properly...

Thanks a lot for your help and sorry for my English!

Res