Absolute gravity observations in Estonia from 1995 to 2017

The establishment of a national gravity standard based on international metrological standards is a high priority for the Estonian geodetic, geophysical, and metrological community. With the presently available gravimetric instruments and models, geoscientific research at the level of 10 - 9 g is possible and requires a homogeneous performance of the definition of gravity standards and of measurements of gravity values at the regional to global scale. From 1995 to 2017, five absolute gravimetric measurement campaigns have been carried out to determine the absolute value of gravity acceleration at all of the seven Estonian gravity network points by deploying JILAg, FG5, and FG5X gravimeters. In this study, the absolute gravity (AG) data were collected and reprocessed to unify the corrections due to local vertical gravity gradient, the self-attraction, and diffraction of the absolute gravimeter. The full set of gravity observations was used to estimate the rates of secular gravity change on the periphery of the Fennoscandian postglacial rebound area, which is continuously deforming due to the glacial isostatic adjustment (GIA). The observed gravity rates, which have been estimated using a linear regression model, differ from the gravity rates that are derived from the vertical velocities of the continuous Global Navigation Satellite System (GNSS) stations and the land uplift model NKG2016LU of the Nordic Commission of Geodesy (NKG) for northern Europe. These differences could be the effect of an insufficient amount of data, seasonal, and inter-annual variation in the hydrology on the observed gravity rates, and the offsets of gravimeters. The discrepancies, nevertheless, are within the uncertainties of observed and derived gravity rates. Similarly, an estimated slope of a linear relation between observed gravity rates and vertical velocities is consistent with a GIA model prediction. The effect of possible offsets of gravimeters on Estonian AG data was corrected, based on the results of international comparisons of absolute gravimeters, as well as the regional analysis of Finnish AG data. The linear regression with corrected data did not improve the fit with the rates that were based on vertical velocities. Further, the linear relation between observed gravity and uplift rates deviated more from the GIA prediction. Therefore, our results did not confirm the positive effect of gravimeter offset correction. However, in order to potentially obtain conclusions t