ALBERT

Description: A possible new Geodetic Reference System, successor to the GRS80, has mostly been discussed in terms of updating the fundamental constants in the Somigliana-Pizzetti theory. The update would replace the geocentric gravitational constant GM and the second-degree zonal coefficient J2 of the GRS80 with the current best estimates, and adopt the W0 of the International Height Reference System as U0, the potential at the ellipsoid, the scale constant instead of the semi-major axis of the ellipsoid. Such an update is thought to advance the unification of different fields of geodesy.Nevertheless, any unifying effects could be negligible as long as a quantity of the same size as the update receives a different treatment in those different fields. This quantity is the permanent tide. The International Height Reference System IHRS is mean-tide, the International Terrestrial Reference Frame ITRFxx is (conventional) tide-free, and terrestrial gravity as exemplified by the International Terrestrial Gravity System is zero-tide. For the new GRS to serve a unifying purpose, its reference potential should include the mean tide, in addition to the Newtonian attraction and centrifugal potential. The surface of the ellipsoid would then be the equipotential surface of the sum of these three potentials, an enlarged Somigliana-Pizzetti theory (Vermeer and Poutanen 1997). Mean-tide coordinates and mean-tide gravity would be adopted. Whether such unity would be worth the major disruption caused, and whether it would be favourably received by the user community might well be questioned.

Description: In the year 2021 a milestone was reached in the absolute gravity research at the Finnish Geospatial Research Institute, when the expansion of the absolute gravity network in Finland was completed. The stations in the network are co-located with the FinnRef permanent GNSS network. This enables studying long-term gravity change, including the Fennoscandian Post-glacial rebound phenomenon, in parallel with deformation monitoring and monitoring of the 3D reference frame. The first absolute gravity measurements in Finland were made in the 1970s and 1980s in Metsähovi, Vaasa and Sodankylä. When the FinnRef network was built in the 1990s, facilities for absolute gravity measurements were established at 5 co-located sites. In 2012-2018 the FinnRef network was modernized and gradually expanded. At the same time, more absolute gravity facilities were built, raising the number of absolute gravity stations from 8 to 19. The latest absolute gravity station added was the Finnström station in the Åland Archipelago, closing a major gap in the Nordic absolute gravity site coverage.The FinnRef permanent GNSS network now consists of 47 stations. The subset of 19 stations contains facilities for absolute gravity measurements. These stations, homogeneously distributed over the country, are repeatedly measured in a 3-year cycle with the FG5X-221 absolute gravimeter, which is the national standard of free-fall acceleration in Finland. The stations also form the zero-order absolute gravity network, to which the renewed First Order Gravity Net (FOGN) is tied. We will describe the absolute gravity stations and the measurements made at them.

Description: The Finnish Geospatial Research Institute (FGI) maintains the First Order Gravity Net of Finland (FOGN). The purpose of the net is to provide easily accessible reference stations for gravity surveys in geodesy and applied geophysics. During 2009-2011 the FOGN was renovated in cooperation with the Institute of Geodesy and Cartography (IGiK). With the A10-020 of the IGiK altogether 51 FOGN stations were occupied. The FOGN stations are outdoors, mostly attached to monumental buildings. We describe the specifics of the field campaign and the accuracy achieved. The highest-order gravity network in Finland is the absolute-gravity (AG) network of the FGI. In this “zero-order” network, time series of AG measurements are maintained at 19 locations in Finland with the FG5X-221 absolute gravimeter, which is the national standard of free-fall acceleration in Finland. The stations are co-located with stations of the permanent GNSS network FinnRef. The FOGN was attached to the AG network and through it to the International Terrestrial Gravity Reference Frame ITGRF using multiple A10-020 measurements at AG stations during the FOGN campaign. We discuss the properties of the method and the propagation of uncertainty with respect to the Comparison Reference Values (CRVs) of international comparisons of absolute gravimeters. We compare the uncertainty with the alternative of bypassing the AG network, i.e. tying the FOGN to CRVs using only the results of the A10-020 at comparisons.

Description: Over the past several years, a definition for a modern International Terrestrial Gravity Reference System (ITGRS) has been developed by the IAG Joint Working Group 2.1.1. Now a resolution will be proposed to IAG to adopt the ITGRS. The ITGRS is based on the instantaneous acceleration of free-fall, expressed in the International System of Units (SI). Consequently, the International Terrestrial Gravity Reference Frame (ITGRF) will be its realization and will be based on measurements with absolute gravimeters at an accuracy level of a few microGal〈sup〉 〈/sup〉(1 µGal = 10〈sup〉-8〈/sup〉 m/s〈sup〉2〈/sup〉). Conventional models for the correction of time dependent effects in these measurements are proposed which are compatible with previous standards. An adequate infrastructure then allows for replacement of IGSN71 which no longer meet the today’s requirements. The details of the concept are outlined and summarized, specifically the proposal to include constant components of temporal gravity corrections as part of the system definition and the importance of instrument comparisons at different levels to ensure a common reference level and the traceability to the SI. Reference stations to monitor the stability of absolute gravimeters will play a key role in the ITGRF. Colocation with space geodetic techniques will provide a link to the International Terrestrial Reference Frame and to GGOS. Establishing compatible first order gravity networks and sharing information on absolute gravity observations will provide access to users but will require support and collaboration with national authorities worldwide.