AIMS: An alternative to the traditional method for modeling kinematics of the
Earth's rotation is proposed. The purpose of developing the new approach
is to provide a self-consistent and simple description of the Earth's
rotation in a way that can be directly estimated from observations
without using intermediate quantities.
METHODS: Instead of estimating time series of pole coordinates,
differences UT1--TAI, their rates, and daily offset of nutation, it is
proposed to estimate coefficients of the expansion of parameters of
a small perturbational rotation into basis functions. The resulting
transformation from the terrestrial coordinate system to the celestial
coordinate system is formulated as a product of an a priori matrix of
a finite rotation and an empirical vector of a residual perturbational
rotation. In the framework of this approach the specific choice of the
a priori matrix is irrelevant, provided the angles of the residual
rotation are small enough to neglect their squares. The coefficients of
the expansion into the B-spline and Fourier bases together with estimates
of other nuisance parameters are evaluated directly from observations of
time delay or time range in a single least square solution.
RESULTS: This approach was successfully implemented in a computer program
for processing VLBI observations. The dataset from 1984 through 2006 was
analyzed. It is shown that the new procedure adequately represents the
Earth's rotation, including slowly variating changes in UT1-TAI and polar
motion, the forced nutations, the free core nutation, and the high
frequency variations of polar motion and UT1.