# Acceleration of the nacelle in the global frame (inertial frame)

Hello @Jason.Jonkman,

I have a couple of questions regarding the output list parameters.

In fact, i am looking for the acceleration of tower top in fore-aft direction in the global frame. When using the Excel file named OutList Parameters.xlsx, i found in the sheet named ElastoDyn a section called Internal degrees of freedom. Among the outputs, there is Acceleration of 1st tower fore-aft bending mode DOF named QD2TFA1. I just wanna make sure that this output is computed in the global frame (i.e. inertial frame) ?

My second question is : tower top acceleration is equal to nacelle acceleration when the yaw dof of the nacelle is disabled. Do you confirm ?

My third question : is there an output for tower top acceleration relative to the undeflected position and expression in xt-direction ?

Best Regards,

I donâ€™t see that ElastoDyn has an output that expresses the absolute tower-top acceleration expressed in the global-inertial frame. The closest to that is ElastoDyn outputs `YawBrTAxp`, etc., which are the absolute tower-top-acceleration expressed in the tower-top / base-plate coordinate system, which you could reorient to global coordinates based on the rotation/orientation of the tower top. You could also output the position of the tower-top in global coordinates (ElastoDyn outputs `TwHt1TPxi`, etc., assuming you place a tower-analysis node near the tower-top) and compute its second time derivative.

I agree that the tower-top and nacelle accelerations are equivalent for a point in the nacelle located at the tower top.

Iâ€™m not sure what you mean by â€śtower top acceleration relative to the undeflected position and expression in xt-directionâ€ť, but ElastoDyn output `QD2FA1` is the second time-derivative of the tower-top degree of freedom (DOF), and ElastoDyn uses relative coordinates for its DOFs (i.e., the tower DOFs are expressed in a frame that moves with the floating platform), so, perhaps that is what you want?

Best regards,

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Dear @Jason.Jonkman,

In fact, I am looking for what you have meant in your last point. In fact, if i understand well, the internal degrees of freedom in ElastoDyn are computed using the relative coordinate for its DoFs. Thatâ€™s mean the ElastoDyn output QD2FA1 is computed in the xt frame which is located at tower base and moves with the platform. Am i right ?
For platform velocities and accelerations, they are computed with respect to the global frame (fixed at Still Water Level) which does not move with the platform. Right ?

Second, when i want to know about each frame in OpenFAST, i used to looking in FAST v7 documentation. Is that the right place ? If not, could you please tell me where to look ?

Best Regards,