Retrograde diurnal motion of the instantaneous rotation axis observed by a large ring laser gyroscope

Ring laser gyroscope technique directly senses the Earth’s instantaneous rotation pole (IRP), whose polar motion contains strong retrograde diurnal components induced by external torques due to the gravitational attraction of the Moon and Sun. The first direct measurement of this retrograde diurnal motion with three large ring lasers was reported by Schreiber et al. (J Geophys Res 109(B18):B06405, 2004). Since then many technical improvements led to a significant increase in precision and stability of ring laser gyroscopes; however, precise determination of amplitude and phase at main partial waves has not been given in the literature. In this paper, I will report on determination of the retrograde diurnal motion of the IRP at main partial waves (\(Oo_1, J_1, K_1, M_1, O_1, Q_1\)) by the ring laser “G”, located in Wettzell, Germany, which is the most stable one amongst the currently running large ring laser gyroscopes.