Very Long Baseline Interferometry (VLBI) is exclusively sensitive to the complete rotation matrix from the terrestrial to the quasi-inertial frame, while ring lasers being sensitive to the instantaneous Earth rotation vector in an Earth- fixed frame, i.e., to polar motion and length of day variations. Additionally, assuming a perfect knowledge of mass attraction variations, superconducting gravimeters (SG) permit to detect Earth rotation variations from the magnitude of the local gravity. The formula of the Sagnac frequency and gravity variation caused by the motion of the Earth's body w.r.t. the instantaneous Earth rotation vector will be presented and compared to VLBI parameterization. The potential of a combination of the three independent types of observables will be examined for sub-diurnal polar motion and universal time variations.
