Recommendations for construction of a nonlinear international Terrestrial Reference Frame

The problems of ITRF2008,the latest International Terrestrial Reference Frame,are pointed out and analyzed as follows：（1） ITRF is not a mm-level Terrestrial Reference Frame;（2） the origin of ITRF is neither the Earth＇s center of mass （CM） nor the center of figure （CF）;（3） the scale of ITRF is not a uniform system in the sense of the gravitational theory of relativity.These problems result from the linear hypothesis used in the establishment and maintenance of ITRF,which includes the linear hypothesis of the coordinates definition of the ITRF reference stations,and the seven coordinate transformation parameters （three translation parameters,three rotation parameters,and one scale parameter） when the ITRF combine solution is constructed.The linear hypothesis of the ITRF construction leads to the current terrestrial reference frame only at the cm-level,which cannot satisfy the requirements of monitoring mm-level crust movements as well as the global environment.This article points out that the construction of a mm-level Terrestrial Reference Frame is actually a leap from linear to nonlinear.Therefore,according to the main characteristics of nonlinear changes of the crust＇s deformation,the geocenter motion and the overall height fluctuation of the Earth,the new ITRF station coordinates definition and the new observation equations of combined solutions are constructed for the realization of a mm-level nonlinear ITRF,which can solve the problems of the current ITRF.