We demonstrate a 16 m(2) helium-neon ring laser gyroscope with sufficient sensitivity and stability to directly detect the Chandler wobble of the rotating Earth. The successful detection of both the Chandler and the annual wobble is verified by comparing the time series of the ring laser measurements against the "C04 series" of Earth rotation data from the International Earth Rotation and Reference System Service.

Superconducting gravimeters (SG) measure temporal changes of the Earth's gravity field with high accuracy and long term stability. Variations in local water storage components (snow, soil moisture, groundwater, surface water and water stored by vegetation) can have a significant influence on SG measurements and-from a geodetic perspective-add noise to the SG records. At the same time, this hydrological gravity signal can provide substantial information about the quantification of water balances. A 4D forward model with a spatially nested discretization domain was developed to investigate the local hydrological gravity effect on the SG records of the Geodetic Observatory Wettzell, Germany. The possible maximum gravity effect was investigated using hypothetical water storage changes based on physical boundary conditions. Generally, on flat terrain, a water mass change of one meter in the model domain causes a gravity change of 42 µGal. Simulation results show that topography increases this value to 52 µGal. Errors in the Digital Elevation Model can influence the results significantly. The radius of influence of local water storage variations is limited to 1000 m. Detailed hydrological measurements should be carried out in a radius of 50 to 100 m around the SG station. Groundwater, soil moisture and snow storage changes dominate the hydrological gravity effect at the SG Wettzell. Using observed time series for these variables in the 4D model and comparing the results to the measured gravity residuals show similarities in both seasonal and shorter-term dynamics. However, differences exist, e.g. the range comparison of the mean modeled (10 µGal) gravity signal and the measured (19 µGal) gravity signal, making additional hydrological measurements necessary in order to describe the full spatio-temporal variability of local water masses.

a b s t r a c tThe Newtonian attraction of the atmosphere is a major source of noise in precise gravimetric measurements. A major part of the effect is eliminated using local air pressure records and constant admittance factors. However, vertical mass shifts under constant surface pressure or distant pressure anomalies are not covered by this technique although they affect the gravimeter. In order to improve the atmospheric correction and to evaluate the horizontal components of attraction as well, the Newtonian attraction is computed based on the spatial density distribution derived from three-dimensional weather models.Operational models from the German Weather Service (DWD) of various scales were used, supplemented by a global data set from the European Centre of Medium Weather Forecast (ECMWF) for comparison. The low temporal resolution and the improper point-mass assumption in the near field are tackled by a cylindrical local model by computing the attraction analytically based on local air pressure records with high temporal resolution.It is shown that a height of at least 50 km and global coverage is required to meet a threshold of 1 nm/s 2 . Neglecting the upper atmosphere leads to an overestimation of the seasonal gravity signal. At distances greater than 10 • the time consuming three-dimensional computation can be replaced by a two-dimensional surface pressure approach without significant error.The results show differences up to 20 nm/s 2 as compared to the linear regression method. The threedimensional atmospheric correction significantly reduces noise in the time series, giving more insight into other signals such as hydrological effects or deformation processes.

[1] We report the first direct measurements of the very small effect of forced diurnal polar motion, successfully observed on three of our large ring lasers, which now measure the instantaneous direction of Earth's rotation axis to a precision of 1 part in 10 8 when averaged over a time interval of several hours. Ring laser gyroscopes provide a new viable technique for directly and continuously measuring the position of the instantaneous rotation axis of the Earth and the amplitudes of the Oppolzer modes. In contrast, the space geodetic techniques (very long baseline interferometry, side looking radar, GPS, etc.) contain no information about the position of the instantaneous axis of rotation of the Earth but are sensitive to the complete transformation matrix between the Earth-fixed and inertial reference frame. Further improvements of gyroscopes will provide a powerful new tool for studying the Earth's interior.

Ring Laser gyroscopes exploit the Sagnac effect and measure rotations absolute. They do not require an external reference frame and therefore provide an independent method to monitor Earth rotation. Large-scale versions of these gyroscopes promise to eventually provide a similar high resolution for the measurement of the variations in the Earth rotation rate as the established methods based on VLBI and GNSS. This would open the door to a continuous monitoring of LOD (Length of Day) and polar motion, which is not yet available today. Another advantage is the access to the sub-daily frequency regime of Earth rotation. The ring laser ''G'' (Grossring), located at the Geodetic Observatory Wettzell (Germany) is the most advanced realization of such a large gyroscope. This paper outlines the current sensor design and properties.