Heiner Igel, Ulrich K Schreiber, Andre Gebauer, Felix Bernauer, Sven Egdorf, Andrea Simonelli, Chin-Jen Lin, Joachim Wassermann, Stefanie Donner , Celine Hadziioannou, Shihao Yuan, Andreas Brotzer, Jan Kodet, Toshiro Tanimoto, Urs Hugentobler, Jon-Paul Wells

Single-component ring lasers have provided high-resolution observations of Earth's rotation rate as well as local earthquake- or otherwise-induced rotational ground motions. Here we present the design, construction, and operational aspects of ROMY, a four-component, tetrahedral-shaped ring laser installed at the Geophysical Observatory Fürstenfeldbruck near Munich, Germany. Four equilateral, triangular-shaped ring lasers with 12 m side length provide rotational motions that can be combined to construct the complete vector of Earth's rotation from a point measurement with very high resolution. Combined with a classic broadband seismometer we obtain the most accurate 6 degree-of-freedom ground motion measurement system to date, enabling local and teleseismic observations as well as the analysis of ocean-generated Love and Rayleigh waves. The specific design and construction details are discussed as are the resulting consequences for permanent observations. We present seismic observations of local, regional, and global earthquakes as well as seasonal variations of ocean-generated rotation noise. The current resolution of polar motion is discussed and strategies how to further improve long-term stability of the multi-component ring-laser system are presented.