Airborne geoid mapping techniques may provide the opportunity to improve the geoid over vast areas of the Earth, such as polar areas, tropical jungles and mountainous areas, and provide an accurate "seam-less" geoid model across most coastal regions. Determination of the geoid by airborne methods relies on the development of airborne gravimetry, which in turn is dependent on developments in kinematic GPS. Routine accuracy of airborne gravimetry are now at the 2 mGal level, which may translate into 5–10 cm geoid accuracy on regional scales. The error behaviour of airborne gravimetry is well-suited for geoid determination, with high-frequency survey and downward continuation noise being offset by the low-pass gravity to geoid filtering operation. In the paper the basic principles of airborne geoid determination are outlined, and examples of results of recent airborne gravity and geoid surveys in the North Sea and Greenland are given.
Scandinavia is a key study region for the research of glacial isostasy, and, in addition, it offers a unique opportunity for validating and testing the results of GRACE. Over a period of five years, the expected life time of GRACE, a temporal geoid variation of 3.0 mm is expected in the centre of the Fennoscandian land uplift area, corresponding to a gravity change of about 100 nm/s2. This is expected to be within the detection capabilities of GRACE. With terrestrial absolute gravimetry, the gravity change due to the land uplift can be observed with an accuracy of ±10 to 20 nm/s2 for a 5-year period. Thus, the terrestrial in-situ observations (groundtruth) may be used to validate and test the GRACE results. Since 2003, absolute gravity measurements have been performed in Scandinavia at about 30 stations covering Norway, Sweden, Finland and Denmark. Four groups with FG5 absolute gravimeters (BKG, FGI, IfE, NLH) are engaged to survey the uplift network annually by a mutually controlled procedure. Nearly all absolute stations are co-located with permanent GPS stations. From the 2003 and 2004 comparisons between the instruments, an overall accuracy of ±30 nm/s is indicated for a single absolute gravimeter and a single station determination. This is in full agreement with the project goal. 1 Determination of the Fennoscandian land uplift In Fennoscandia, the Earth’s crust is rising continuously since the last glacial maximum due to the deloading of the ice. The region is dominated by the Precambrian basement rocks of the Baltic Shield and comprises mainland Norway, Sweden, Finland, the Kola Peninsula, and Russian Karelia. The maximum spatial extension is about 2000 km in northeast-southwest direction; see Fig. 1 for the approximate shape and location (after Ekman 1996). Geophysical approaches to study the postglacial rebound are associated with the evidence for past sea lev-
Abstract. The Zugspitze Geodynamic Observatory Germany has been set up with a worldwide unique installation of a superconducting gravimeter at the summit of Mount Zugspitze. With regard to hydrology, this karstic high-alpine site is largely dominated by high precipitation amounts and a long seasonal snow cover period with significant importance for water supply to its forelands, while it shows a high sensitivity to climate change. However, regarding the majority of alpine regions worldwide there is only weak knowledge on temporal water storage variations due to only sparsely distributed hydrological and meteorological point sensors and the large variability and complexity of alpine signals. This underlines the importance of well-equipped areas such as Mount Zugspitze serving as natural test laboratories for an improved monitoring, understanding and prediction of alpine hydrological processes. The observatory superconducting gravimeter OSG 052 supplements the existing sensor network as a novel hydrological sensor system for the direct observation of the integral gravity effect of total water storage variations in the alpine research catchment Zugspitze. Besides the experimental setup and the available datasets, the required gravimetric prerequisites are presented such as calibration, tidal analysis and signal separation of the superconducting gravimeter observations from the first 2 years. The snowpack is identified as primary contributor to seasonal water storage variations and thus to the gravity residuals with a signal range of up to 750 nm/s2 corresponding to 1957 mm snow water equivalent measured at a representative station at the end of May 2019. First hydro-gravimetric sensitivity analysis are based on simplified assumptions of the snowpack distribution within the area around Mount Zugspitze. These reveal a snow-gravimetric footprint of up to 4 km distance around the gravimeter with a dominant gravity contribution from the snowpack in the Partnach spring catchment. This study already shows that the hydro-gravimetric approach can deliver important and representative integral insights into this high-alpine site. This work is regarded as a concept study showing preliminary gravimetric results and sensitivity analysis for upcoming long-term hydro-gravimetric research projects.
Summary): Within a cooperation between the Institut fur Astronomische und Physikalische Geodasie (IAPG), Munchen, and the Institut fur Erdmessung (IfE), Hannover, four new absolute gravity stations were established in the German Alps during the autumn season of 2004, a period with minimum snow coverage. Two stations are located close to the summit of the mountain Zugspitze, while the other two stations are situated at the foot and at the top of the mountain Wank. The four stations cover an elevation range of 2200 m and a gravity range of 0.00522 m/s2 (≡ 0.522 Gal). The accuracy of each station determination is assumed to be within ±20 to 40 nm/s2. The main purpose of the new gravity base net is to serve as a high-precision long-range gravity calibration line for the determination of linear and quadratic calibration terms of modern relative gravimeters. The precision of the calibration line is in the order of ±5×10 which also considers uncertainties from unmodelled temporary gravity changes due to snow accumulation over a year.
During November 2011 a EURAMET key comparison of absolute gravimeters was organized in the Underground Laboratory for Geodynamics in Walferdange, Luxemburg. The comparison assembled 22 participants coming from 16 countries and four different continents. The comparison was divided into two parts: a key comparison that included six National Metrology Institutes or Designated Institutes, and a pilot study including all participants. The global result given by the pilot study confirms that all instruments are absolutely coherent with each other. The results obtained in the key comparison confirm a good agreement between the NMI instruments. Finally, a link to ICAG-2009 shows also that the NMI gravimeters are stable in time. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).
<p>Geodetic observations on Earth accurate to better than a part per billion require a common reference for the same precision as described in the goals of the Global Geodetic Observing System. The International Gravity Reference System (IGRS) is proposed as a new reference for terrestrial gravity observations (Wziontek et al. 2021).</p><p>The International Gravity Reference Frame (IGRF) as the realization of IGRS is represented by absolute gravity measurements traceable to the SI. Due to the lack of a natural reference, absolute gravimeters need to be compared and the gravity reference is realized based on a set of measurements by a group of absolute gravimeters and the functional model for their processing.</p><p>We present the international comparison of absolute gravimeters WET-CAG2021 hosted at the Geodetic Observatory Wettzell in autumn 2021. This comparison is classified as an additional comparison following the strategy paper of the Consultative Committee for Mass and related quantities (CCM) and IAG. Seven FG5/X absolute gravimeters and two AQG quantum gravimeters have observed up to four individual piers over a period of twelve weeks. The individual observation epochs are connected by recordings of the continuously operating superconducting gravimeter GWR OSG 030 in the same laboratory.</p><p>We show the procedure of data analysis following P&#225;link&#225;&#353; et al. (2021) and discuss the results also with respect to the latest regional metrological EURAMET comparison 2018 at the same location.</p><p>&#160;</p><p>Marti, U., Richard, P., Germak, A., Vitushkin, L., P&#225;link&#225;&#353;, V., Wilmes, H.: CCM-IAG Strategy for Metrology in Absolute Gravimetry, 11 March 2014</p><p>P&#225;link&#225;&#353;, V., Wziontek, H., Va&#318;ko, M. et al.: Evaluation of comparisons of absolute gravimeters using correlated quantities: reprocessing and analyses of recent comparisons. <em>J Geod</em> 95,<strong> </strong>21 (2021). https://doi.org/10.1007/s00190-020-01435-y</p><p>Wziontek, H., Bonvalot, S., Falk, R. et al.: Status of the International Gravity Reference System and Frame. <em>J Geod</em> 95, 7 (2021). https://doi.org/10.1007/s00190-020-01438-9</p>
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