Earth Rotation Observed by Very Long Baseline Interferometry and Ring Laser

We present a systematic and unified treatment of Earth rotation from the geodetic, astronomical, and geophysical point of view. A precise terminology of precession-nutation and polar motion is of great importance for understanding the interacting physical phenomena leading to rotational irregularities of the Earth. In total, four poles of the equatorial plane are defined for the description of Earth rotation. The various components of the Earth Orientation Parameters, i.e., precession-nutation, polar motion, and universal time (or length of day) are summarized. This paper shows how very long baseline interferometry and ring laser differ in terms of Earth rotation and presents the current state-of-the-art measurement accuracy that can be achieved.