Classically, atmospheric corrections for terrestrial gravimetry are computed from the local air pressure record and a conventional admittance factor of -3.0 nm/s^2/hPa is usually adopted to derive gravity effects. The reference level is based on a standard atmosphere in agreement with the Resolution N^∘ 1 of the International Association of Geodesy (IAG) of 2023, which defines the International Terrestrial Gravity Reference System (ITGRS). Roughly 90 nm/s^2 level and are typically altitude-dependent. In the present study, we aim to solve this problem by referring the time series to a common reference ensuring the compatibility with the conventions adopted for the ITGRS, and we show the advantage of using Atmacs for correcting absolute gravity observations.

The International Terrestrial Gravity Reference System (ITGRS) was adopted by the International Association of Geodesy (IAG) in 2023 for the use in geosciences and metrology. Its realization, the International Terrestrial Gravity Reference Frame (ITGRF), is established through a set of reference stations where a gravity reference function is available and allows to monitor temporal changes of the gravity field and control absolute gravimeters. The gravity laboratory of the Argentine-German Geodetic Observatory (AGGO) operates the absolute gravimeter FG5-227 and the superconducting gravimeter SG038. In this study, we revisit the first estimation of the instrumental drift of the SG038 by combination with an extended set of absolute gravity observations, now covering a period of more than four years. By this, a progressive discrepancy between both observations could be solved, and the new estimation allows for both time series to agree with each other within the standard uncertainty of the FG5. These results, together with the infrastructure at AGGO to conduct regional comparisons of absolute gravimeters and the link to international and other regional comparisons provided by the FG5-227, enable AGGO to be proposed as a core station of the future ITGRF.

Classically, the scale factor of superconducting gravimeters (SGs) can be determined by two approaches. A first estimate can be obtained by comparison of its observations with a theoretical Earth tide and precise ocean tide loading models, after correcting for the effects of polar motion and atmosphere on gravity. However, an independent estimation from existing tide models is preferred and can be achieved through the combination with parallel absolute gravity observations, usually performed with classical free-fall gravimeters. In this case, absolute gravity observations are carried out only over a few days to limit the mechanical wear of these instruments, and preferably during periods of spring tides to enhance the signal-to-noise ratio.In this study, we investigate the use of the quantum gravimeters AQG by Exail for the calibration of superconducting gravimeters at the Geodetic Observatory Wettzell, Germany. To do so, continuous absolute gravity measurements were carried out for different periods of time and up to three months and combined with the SG observations. Scale factors were estimated through a least-squares adjustment, considering different time windows. Although the AQGs have a much larger scatter of individual experiments than FG5 absolute gravimeters, their benefit is considerably longer and continuous observation epochs, that are proved to be important also by synthetic data. A good agreement of the calibration factors with previous estimations was found, demonstrating the potential of quantum gravimeters to calibrate SGs.

<p>Terrestrial gravimetry allows for integrative measurements of mass changes associated with water storage variations in all storage compartments above and below the Earth surface. Superconducting gravimeters (SGs) currently are the most precise instruments for continuous monitoring of gravity change. Their footprint typically covers a radius of about 1 km around the instrument, with most of the signal originating from within the first 100 meters. We installed a SG (iGrav033) in a mixed pine-beech-oak forest in the TERENO observatory in the lowlands of north-eastern Germany. It is housed in a small field enclosure with less than 1 m<sup>2</sup> base area, on top of a stable concrete pillar. Complementary hydro-meteorological monitoring data are available at the site, including a weather station, a groundwater monitoring well, clusters of soil moisture sensors along deep soil profiles, interception measurements and near-surface soil moisture from Cosmic Ray Neutron Sensing. For quantification and correction of the long-term instrumental SG drift, repeated measurements with an FG5 absolute gravimeter were carried out. The gravity residual time series (gravity measurements reduced to the local hydrological effect) covers a sequence of years with below average precipitation, from 2018 to 2022. We show the gravity-based water storage variations in the forested landscape throughout this period, indicating that storage depletion during summer in most years is not fully recovered by the subsequent wetter winter periods. The amplitudes of gravity-based water storage variations tend to exceed those observed by soil moisture sensors in the top meters of the soil and of groundwater. This indicates the value of terrestrial gravimetry in revealing dynamics of the deeper unsaturated zone water storage.</p>

Large ring lasers have improved significantly in recent years, such that we are now in the position to separate and mitigate error sources that are not directly related to the rotation sensing process from the Sagnac interferogram. As a result, we are now able to reduce the measurement error of the 16 m(2) G ring laser of the Geodetic Observatory Wettzell by a factor of two. Improvements in the measurement of the relevant parameters for the backscatter correction remove most of the sensor drift effects, so that the backscatter induced coupling is no longer a real concern. Now that we can separate the mechanisms of the error contributors much better, we can mitigate them in a more effective way. In this paper we report on the latest progress.

We describe the construction and operation of a large ring laser whose beam paths enclose an area of 6.25m2. The gyroscopic performance of this large laser interferometer was determined using laser operation at a wavelength of 632.8 nm. The laser cavity Q was inferred to be 1.1×1012 via a measured ring-down time of 375 µs, and the measured Sagnac frequency is 198.40 Hz due to Earth's rotation. The measured experimental sensitivity to rotation achieved is 7.9×10-12rad/s/Hz at an averaging interval of 512 s (being limited primarily by ambient building noise). The observation of microseismic activity in the 200 mHz region as well as local earthquakes is discussed.

SUMMARY 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 broad-band 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 multicomponent ring laser system are presented.

The analysis of the relative amplitudes of a passing seismic wave recorded on a single seismometer measuring six degrees of freedom of ground motion (translation and rotation) theoretically allows one to extract information on the wave that can conventionally only be obtained from receiver arrays. In the past, it has been shown on numerical data that the extension of conventional three-component (3C) polarization analysis techniques to six-components, allows one to unambiguously identify the wave type of a passing wave and characterise it in terms of its propagation direction (without the 180° ambiguity inherent in 3C data) and local wave speed. Additionally, due to the increase in the dimensionality of the data, two waves arriving at a station at the same time can be simultaneously characterised under ideal conditions (low noise). Attempts to apply such 6-C polarization analysis techniques to field data have so far been met with limited success. Varying noise levels on the individual components and complex wavefields (with more than two interfering waves arriving at the station at the same time) usually prevent the stable recovery of wave parameters using single-station 6-C polarization analysis. Here we discuss first attempts to overcome these issues. We (1) test the robustness of different wave parameter estimators (maximum likelihood, MUSIC) towards high levels of noise and (2) we try to reduce the number of interfering events in the analysis window by performing 6-C polarization analysis on time-frequency decomposed seismograms (i.e. spectrograms) using the S-transform. The new techniques are extensively tested on field data recorded on the high-performance ROMY ringlaser.

The ROMY ring laser was constructed with 4 non-orthogonal triangular-shaped cavities of 12 m side length in the Geophysical Observatory outside Munich, Germany, in 2016. The large dimensions of the individual rings have the benefit of allowing high sensitivity surpassing in principle the sensitivity of the G-ring at the Fundamentalstation Wettzell. However, the concrete construction of ROMY is geometrically less stable than the G-ring that is built on a rigid Xerodur plate. Each of the four rings has its own Sagnac frequency. The horizontal triangular ring laser at the top of the inverted tetrahedral ROMY structure allows direct comparison of teleseismic signals and noise with the G-ring at a distance of 200km. It also serves as redundant component. In principle, three orthogonal components of rotational ground motion can be obtained by linear combination from any combination of three rings, that - due to the variable Sagnac frequency - have different noise characteristics. We report on the behavior and observations of ROMY from a seismological point of view. It is fair to say that ROMY provides the most accurate direct 3-component rotational ground motion seismic observations to date. In combination with a collocated broadband seismometer as well as a surrounding small-scale seismic array, we analyse regional, teleseismic events, and ocean-generated noise and compare with array-derived rotation.

The ring laser gyroscope (RLG) technique has been investigated for over 20 years as a potential complement to space geodetic techniques in measuring Earth rotation. However, RLGs are also sensitive to changes in their terrestrial orientation. Therefore in this paper, we review how the high-frequency band (i.e. signals shorter than 0.5 cycle per day) of the known phenomena causing site deformation contribute to the RLG observable, the Sagnac frequency. We study the impact of solid Earth tides, ocean tidal loading and non-tidal loading phenomena (atmospheric pressure loading and continental hydrosphere loading). Also, we evaluate the differences between available models of the phenomena and the importance of the Love numbers used in modeling the impact of solid Earth tides. Finally, we compare modeled variations in the instrument orientation with the ones observed with a tiltmeter. Our results prove that at the present accuracy of the RLG technique, solid Earth tides and ocean tidal loading effects have significant effect on RLG measurements, and continental hydrosphere loading can be actually neglected. Regarding the atmospheric loading model, its application might introduce some undesired signals. We also show that discrepancies arising from the use of different models can be neglected, and there is almost no impact arising from the use of different Love numbers. Finally, we discuss differences between data reduced with tiltmeter observations and these reduced with modeled signal, and potential causes of this discrepancies.

Absolute rotation rate sensing with extreme sensitivity requires a combination of several large scale gyroscopes in order to obtain the full vector of rotation. We report on the construction and operation of a four-component, tetrahedral laser gyroscope array as large as a five story building and situated in a near surface, underground laboratory. It is demonstrated that reconstruction of the full Earth rotation vector can be achieved with sub-arcsecond resolution over more than six weeks.

The VLBI (Very Long Baseline Interferometry) technique can provide the full set of parameters needed for the transformation between celestial and terrestrial reference frames with high accuracy. Yet it has some limitations regarding temporal resolution and continuity, and the accuracy of the resulting Earth Orientation Parameters (EOP) varies depending on the network geometry. In this work we explore the benefit of combining VLBI observations with the measurements of the large ring laser gyroscope “G” in Wettzell for deriving highly resolved ERP (Earth Rotation Parameters, i.e. polar motion and universal time variations, δUT1). We examine the observations collected by two simultaneously operating VLBI networks during the 15 d of the CONT17 campaign. These two networks, of 14 globally distributed telescopes each, were designed for the estimation of Earth rotation variations, for which reason the resulting hourly ERP are appointed as benchmark in this investigation. To evaluate the advantage of a VLBI and ring laser combined solution, we create degraded versions of the original networks, containing only six stations. The ERP derived from those sparse networks and from the VLBI sparse plus ring laser solutions are then compared in terms of differences to the reference values. It should certainly be considered that these are relative numbers, since they are also determined by the number and selection of the stations remaining in the sparse networks. The root mean square of the difference to the benchmark is reduced by 24 % in case of δUT1 from one network. The polar motion yp component from the same network moves 14 % closer to the reference value due to the inclusion of the ring laser data. The impact on xp and on all ERP from the other network ranges between 2 % and 9 %. The research again confirms the feasibility and also the potential gain of a combined evaluation of VLBI and ring laser observations, but the full capacity of such a sensor fusion will emerge once the ring laser gyroscopes reach a level of accuracy similar to VLBI.

We review the current status of large ring laser gyroscopes having the potential to contributeto terrestrial measurements of general relativistic precessions. At this point in time, although thesedevices possess the raw sensitivity for such a measurement, they remain limited by long-term geometricinstability, detection noise and imperfections in the physical models required to isolate geophysicaleffects. Furthermore, minute non-reciprocal biases provide a null-shift error and therefore no currentlyconstructed laser system meets the requirement of absolute rotation rate sensing. Nevertheless, we are ofthe view that these are surmountable problems and the ability of ring laser gyroscopes to measure lowfrequency to DC signals has vastly increased in the last decade.