<p>Since the 1960s, the Institute of Earth Physics and Space Science (EPSS) in Sopron has been using different types of tiltmeters with nanoradian sensitivity to observe geodynamic phenomena (e.g. tides). In principle, the high sensitivity and the long term mechanical stability of the recently developed sensors (e.g. Lippmann-type tiltmeters) make it possible to detect rock tilts related to small seismo-tectonic deformations. On this research field the extensive use of networks of these devices is expected in the future. For the correct comparision of tilt values measured by different sensors, the instruments must be calibrated by suitable devices realizing simple and standardized metrological principles. Due to the high sensitivity of Lippmann tiltmeters, traditional comparators (e.g. level balances) can only be used to determine the sensors' characteristics on average over the entire measurement range. In the range below microradian theoretically the Newtonian (gravitational) method can be used to test the capabilities of the tilt sensors. The poster on the one hand shows the Lippmann tiltmeters and level balance joint measurement results. On the other hand the modell computation are discussed, which shows that the off-axis variation of the gravitational vector generated by the vertical movement of the cylindrical ring mass of the Mátyáshegy moving mass calibration device can provide for calibration a sufficiently accurate reference signal having (15±0.02) nrad peak-to-peak amplitude. It is just in the range of tilt induced by earth tide effect, which is a “standard” signal component in the time series recorded in observatory environment. </p>

The idea of moving mass calibration (MMC) of relative gravity meters dates back to the seventies of the last century. Probably the MMC apparatus built in the underground Mátyáshegy Gravity and Geodynamics Laboratory Budapest has been used most extensively and several spring type instruments (LaCoste and Romberg and Scintrex) have been investigated and calibrated by it. Its test mass is a cylindrical ring having a weight of 3 tons. Its main advantage is simplicity in terms of metrology. The same principle and technology can be used to test compact tilt sensors having nanoradian resolution capability. Up to now rigorous testing methods below microradian range were not available in practice. The analysis of the so-called off-axis variation of the gravitational vector generated by the vertical movement of the cylindrical ring mass of the Mátyáshegy MMC device, however, showed that a sufficiently accurate reference signal having (15 ± 0.02) nrad peak-to-peak amplitude can be provided for calibration. It is just in the range of tilt induced by earth tide effect, which is a “standard” signal component in the time series recorded in observatory environment. In the first part of the paper, a discussion of the proposed methodology of tilt meter calibration is given. Then the analysis of the effect of volumetric discretization of the cylindrical ring on the accuracy of calibration is provided. Finally, possible material inhomogeneities of the ring mass and their gravitational effects are investigated by forward simulations and inversion. For this purpose the results of 300 gravimeter calibration experiments, analysed and published earlier, were utilized.