GravIS: mass anomaly products from satellite gravimetry
Christoph DahleEva BoergensIngo SasgenThorben DöhneSven ReißlandHenryk DobslawVolker KlemannMichael MurböckRolf KoenigRobert DillMike SipsUlrike SyllaAndreas GrohMartin HorwathFrank Flechtner
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Abstract. Accurately quantifying global mass changes at the Earth's surface is essential for understanding climate system dynamics and their evolution. Satellite gravimetry, as realized with the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) missions, is the only currently operative remote sensing technique that can track large-scale mass variations, making it a unique monitoring opportunity for various geoscientific disciplines. To facilitate easy accessibility of GRACE and GRACE-FO (GRACE/-FO in the following) results (also beyond the geodetic community), the Helmholtz Centre for Geosciences (GFZ) developed the Gravity Information Service (GravIS) portal (https://gravis.gfz.de, last access: 21 January 2025). This work aims to introduce the user-friendly mass anomaly products provided at GravIS that are specifically processed for hydrology, glaciology, and oceanography applications. These mass change data, available in both a gridded representation and as time series for predefined regions, are routinely updated when new monthly GRACE/-FO gravity field models become available. The associated GravIS web portal visualizes and describes the products, demonstrating their usefulness for various studies and applications in the geosciences. Together with GFZ's complementary information portal https://www.globalwaterstorage.info/ (last access: 21 January 2025), GravIS supports widening the dissemination of knowledge about satellite gravimetry in science and society and highlights the significance and contributions of the GRACE/-FO missions for understanding changes in the climate system. The GravIS products, divided into several data sets corresponding to their specific application, are available at https://doi.org/10.5880/GFZ.GRAVIS_06_L2B (Dahle and Murböck, 2019), https://doi.org/10.5880/COST-G.GRAVIS_01_L2B (Dahle and Murböck, 2020), https://doi.org/10.5880/GFZ.GRAVIS_06_L3_ICE (Sasgen et al., 2019), https://doi.org/10.5880/COST-G.GRAVIS.5880/GFZ.GRAVIS_01_L3_ICE (Sasgen et al., 2020), https://doi.org/10.5880/GFZ.GRAVIS_06_L3_TWS (Boergens et al., 2019), https://doi.org/10.5880/COST-G.GRAVIS_01_L3_TWS (Boergens et al., 2020a), https://doi.org/10.5880/GFZ.GRAVIS_06_L3_OBP (Dobslaw et al., 2019), and https://doi.org/10.5880/COST-G.GRAVIS_01_L3_OBP (Dobslaw et al., 2020a).Keywords:
Gravimetry
Anomaly (physics)
Gravimetry
Satellite geodesy
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Recent studies of the Australian Geodetic Datum indicated that it is possible to establish a world geodetic system from the gravity data available at the present time. The necessary formulae for doing so are developed in such a manner that it is possible for each major datum to be fixed independently in relation to the system provided the global gravity field was represented by an adequate combination of satellite data and surface gravimetry. The major uncertainty is one of zero degree, being equivalent to that which exists in the knowledge of the dimensions of that ellipsoid which has the same volume as the geoid. The permanence of such a system is also discussed.
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The basic theory of Airborne Scalar Gravimetry is that two series accelerations are different to yield the acceleration of gravimetry which then can be downloaded to the surface of earth.One acceleration is sensed by INS (inertial navigation system)or gravimeter,the other is deduced from GPS observations.High accuracy velocity is required for the calculation of the Etvs corrections (namely Coriolis acceleration caused by the rotation of earth and the movement of airforce) and the determination of the vertical acceleration.And if we want to get the accuracy at mGal level in airborne gravimetry,the kinematic velocity accuracy at cm/s level is very necessary. Several ways of evaluating velocity of moving_base are available by using GPS.Usually the velocity is determined by solving for the position of the vehicle relative to a base station and subsequently taking one time derivative of the three components,which has been successfully used in airborne gravimetry of Switzerland and others.The alternative method is that it can be determined by using GPS Doppler observations,which has advantage of avoiding resolution ambiguity over the first one but isn't as straight forward as it.Due to the largely unpredictable receiver_clock errors and the imposition of the selective availability degradation,double difference Doppler observations are used to obtain the relative vehicle velocity. In this paper the latter theory is discussed,its accuracy is estimated,and the accuracy requirement for the position of airforce or satellite and velocity of satellite is given.Then two tests are tried to validate the reliability and stability of this method and to evaluate the accuracy of this way. First is static test,in which two Trimbel 4000SST GPS receivers are used and respectively mounted on two stations of which coordinate is known precisely,one is looked as base_station,the other as “moving” station whose true velocity is zero.Surveying continued two hours as kinematic survey mode,then observation files are transferred from binary to RINEX format,and the RINEX files are processed by using VAES software(velocity and acceleration estimation system),therefore the result is achieved.We compare the result with the true value and the statistical result indicates that the accuracy are at mm/s level.Second is kinematic test,in which three Trimbel 4000SST GPS receivers are used.One is mounted on the airforce,the others respectively are set on two different base_stations whose coordinate is known precisely.Surveying continued two hours as kinematic survey mode,then two base_stations observation files are respectively processed together with the airforce observation file by using VAES software,and the two results of airforce velocity are achieved.The two series velocities are compared with each other,and the statistic indicates that the accuracy is the same as the first test. Both the theory and two test results indicate that the accuracy at mm/s level of moving_base velocity can be achieved by using this means discussed in the paper.
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Gravity field modeling using airborne vertical component gravimetry has made significant strides over the last decade. We demonstrate the feasibility of extending this to three‐dimensions using data from inertial navigation systems (INS) and the Global Positioning System (GPS). A significant advantage of measuring the horizontal gravity components is that the geoid can be determined in profiles by direct along‐track integration, thus not only adding strength to conventional methods, but reducing the required area of survey support, especially along model boundaries. As such, the ultimate limitation of the method is in the quality of the INS and GPS sensors. In our test case, all three components of the gravity vector were determined over a profile in the Canadian Rocky Mountains. Differences between available truth data and the computed gravity components have standard deviations of 7–8 mGal (horizontal) and 3 mGal (vertical). These standard deviations include uncertainties in the truth data (<5 mGal, for horizontal; 1.3 mGal, for vertical). The resolution in the computed values is about 10 km. These analyses have demonstrated for the first time that the total gravity vector can be determined from airborne INS and GPS to reasonable accuracy and resolution, without any external orientation information, nor prior statistical hypothesis on the gravity signature, using medium‐accuracy INS and geodetic quality GPS receivers.
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