Yes BOAC, this could be the answer.
Maybe in "Attitude" there is no correction for the earth's rotation as the system doesn't know the lateral position. Off course change of variation also plays, but normally it is less significant.
If I have enogh time next in an airplane on ground (long slot etc.), I'll put the stby. system into "attitude" and I will check the deviation for given time. Then I can calculate the the degrees/hours deviation, and I can compare it to the theoretical value.
Just I need a table or formula for this.
Anyway I think that I got the answer for my original question.
Thanks for all of you,
Lotetu