Update to the conventional model for rotational deformation

Rotational deformation (also called the "pole tide") is the deformation resulting from the centrifugal effect of polar motion on the solid earth and ocean, which manifests itself as variations in ocean heights, in the gravity field and in surface displacements. The model for rotational deformation assumes a primarily elastic response of the Earth to the centrifugal potential at the annual and Chandler periods and applies body tide Love numbers to the polar motion after removing the mean pole. The original model was conceived when the mean pole was moving (more or less) linearly, largely in response to glacial isostatic adjustment. In light of the significant variations in the mean pole due to present-day ice mass losses, an `appropriately' filtered mean pole was adopted for the conventional model, so that the longer period variations in the mean pole were not included in the rotational deformation model. However, the elastic Love numbers should be applicable to longer period variations as well, and only the secular (i.e. linear) mean pole should be removed. A model for the linear mean pole is recommended based on a linear fit to the IERS C01 time series spanning 1900 to 2015: in milliarcsec, Xp = 55.0+1.677*dt and Yp = 320.5+3.460*dt where dt=(t-t0), t0=2000.0 and assuming a year=365.25 days. The consequences of an updated model for rotational deformation for site motion and the gravity field are illustrated.