Superconducting gravimeter and seismometer shedding light on FG5’s offsets, trends and noise: what observations at Onsala Space Observatory can tell us

Ten-year worth of absolute gravity (AG) campaigns at Onsala Space Observatory (OSO), Sweden, are simultaneously reducedusing synchronous data from a superconducting gravimeter (SG). In this multi-campaign adjustment, the a priori modelscommonly applied for each setup in AG-alone experiments are sidestepped in favour of SG records and a model to estimateits drift. We obtain a residual (hourly samples) at the 5 nm/s2 RMS level, reducing the SG data with a range of ancillary datafor the site’s exposure to ocean and atmospheric loading, and hydrology effects. The target quantity in AG projects in theBaltic Shield area is the secular change of gravity dominated by glacial isostatic adjustment with land uplift as its major part.Investigating into the details of the associated processes using AG requires a long-term stable reference, which is the aim ofinternational comparison campaigns of FG5 instruments. Two of these have been campaigning at OSO since 2009 when theSG had been installed. In the simultaneous inversion of all sixteen campaigns, we identify weaknesses of AG observations,like varying systematic offsets over time, excess microseismic sensitivity, trends in the AG data and side effects on the SG’sscale factor when campaigns are evaluated one by one. The simultaneous adjustment afforded us an SG scale factor verynear the result from a campaign with a prototype quantum gravimeter.Whence, we propose that single-campaign results maybe biased and conjectures into their variation, let alone its causes misleading. The OSO site appears to present manageableproblems as far as environmental influences are concerned. Our findings advocate the use of AG instruments and proceduresthat are more long-term stable (reference realization), more short-term stable too (setup drifts), less service craving and moreresilient to microseismic noise

An Accuracy Assessment of Absolute Gravimetric Observations in Fennoscandia

We compare a suite of absolute gravimeters used to monitor the temporal changes of gravity at a number of sites in Fennoscandia. Direct comparisons are made from simultaneous observations at selected sites within and outside of the postglacial uplift region. We also compare results at sites visited by two instruments with some separation in time. We conclude from four years of data that gravity differences are obtained within an rms error of ± 3 Gal. The data reveal no systematic biases between the instruments, but occasional shifts from one year to another are noted. We consider that annual instrument comparisons are required to ensure data integrity in a regional observing program that extends over more than a decade

Geodetic SAR for Height System Unification and Sea Level Research—Observation Concept and Preliminary Results in the Baltic Sea

Traditionally, sea level is observed at tide gauge stations, which usually also serve as height reference stations for national leveling networks and therefore define a height system of a country. One of the main deficiencies to use tide gauge data for geodetic sea level research and height systems unification is that only a few stations are connected to the geometric network of a country by operating permanent GNSS receivers next to the tide gauge. As a new observation technique, absolute positioning by SAR using active transponders on ground can fill this gap by systematically observing time series of geometric heights at tide gauge stations. By additionally knowing the tide gauge geoid heights in a global height reference frame, one can finally obtain absolute sea level heights at each tide gauge. With this information the impact of climate change on the sea level can be quantified in an absolute manner and height systems can be connected across the oceans. First results from applying this technique at selected tide gauges at the Baltic coasts are promising but also exhibit some problems related to the new technique. The paper presents the concept of using the new observation type in an integrated sea level observing system and provides some early results for SAR positioning in the Baltic sea area

RG 2000 – the New Gravity Reference Frame of Sweden

The increased need for improved geoid models for Global Navigation Satellite Systems (GNSS) height determination calls for additional gravity observations and quality assurance of existing data. In this perspective, a modern gravity system and the renovation of an already existing high order gravity network is considered as a moderate strategic investment which provides a firm foundation for further activities. Here the new gravity reference frame RG 2000 for Sweden is presented. RG 2000 is realized by absolute gravity observations at 109 stations. The absolute points are connected via old and new relative gravity observations, including another 216 points. Points and observations have been chosen so that good overlap with the older Swedish reference frames, RG 62 and RG 82, is achieved, allowing to evaluate the older frames and transformations between them. RG 2000 is based on a zero permanent tide system with epoch 2000

Geodetic SAR for Height System Unification and Sea Level Research—Results in the Baltic Sea Test Network

Coastal sea level is observed at tide gauge stations, which usually also serve as height reference stations for national networks. One of the main issues with using tide gauge data for sea level research is that only a few stations are connected to permanent GNSS stations needed to correct for vertical land motion. As a new observation technique, absolute positioning by SAR using off the shelf active radar transponders can be installed instead. SAR data for the year 2020 are collected at 12 stations in the Baltic Sea area, which are co-located to tide gauges or permanent GNSS stations. From the SAR data, 3D coordinates are estimated and jointly analyzed with GNSS data, tide gauge records and regional geoid height estimates. The obtained results are promising but also exhibit some problems related to the electronic transponders and their performance. At co-located GNSS stations, the estimated ellipsoidal heights agree in a range between about 2 and 50 cm for both observation systems. From the results, it can be identified that, most likely, variable systematic electronic instrument delays are the main reason, and that each transponder instrument needs to be calibrated individually. Nevertheless, the project provides a valuable data set, which offers the possibility of enhancing methods and procedures in order to develop a geodetic SAR positioning technique towards operability