Abstract
This study evaluates the gravity field solutions based on high-low satellite-to-satellite tracking (hl-SST) of low-Earth-orbit (LEO) satellites: GRACE, Swarm, TerraSAR-X, TanDEM-X, MetOp-A, MetOp-B, and Jason 2, by converting them into hydrological polar motion excitation functions (or hydrological angular momentum (HAM)). The resulting HAM series are compared with the residuals of observed polar motion excitation (geodetic residuals, GAO) derived from precise geodetic measurements, and the HAM obtained from the GRACE ITSG 2018 solution. The findings indicate a large impact of orbital altitude and inclination on the accuracy of derived HAM. The HAM series obtained from Swarm data are found to be the most consistent with GAO. Visible differences are found in HAM obtained from GRACE and Swarm orbits and provided by different processing centres. The main reasons for such differences are likely to be different processing approaches and background models. The findings of this study provide important information on alternative data sets that may be used to provide continuous polar motion excitation observations, of which the Swarm solution provided by the Astronomical Institute, Czech Academy of Sciences, is the most accurate. However, further analysis is needed to determine which processing algorithms are most appropriate to obtain the best correspondence with GAO.
Highlights
The accurate determination of spatial and temporal changes of the Earth’s gravity field is essential in many applications, including solid Earth science, hydrology, oceanography, glaciology, and geodesy
To evaluate hydrological angular momentum (HAM) functions obtained from models based on kinematic orbits of LEO satellites, we compared them with two reference series: (i) the hydrological signal in geodetically observed polar motion (PM) excitation (GAO), and (ii) the HAM function obtained from the latest monthly Gravity Recovery and Climate Experiment (GRACE) gravity field model Institute of Theoretical Geodesy and Satellite Geodesy (ITSG) 2018
Based on numerous comparisons of different oscillations in HAM series computed from different GRACE solutions, we indicated that ITSG 2018 provides the highest consistency with GAO in the largest number of cases [51,52,53]
Summary
The accurate determination of spatial and temporal changes of the Earth’s gravity field is essential in many applications, including solid Earth science, hydrology, oceanography, glaciology, and geodesy. The gravity field varies in space and time due to disturbances in mass redistribution of Earth’s surficial fluids, which include the atmosphere, oceans, and the land hydrosphere These variations cause changes in Earth orientation parameters (EOPs), which describe the rotation of the planet. EOPs are essential for a number of applications, including precise positioning and navigation in space and on the Earth’s surface, pointing of astronomic instruments, and communication with deep space objects Such great importance of these parameters results from the fact that they are necessary variables for transforming coordinates between a terrestrial reference frame (in which coordinates of ground stations are commonly available and used in all surveying tasks) and a celestial reference frame (in which the coordinates of ground stations are determined from space geodesy techniques such as Global Navigation Satellite Systems (GNSS)) because they provide the rotation between these two frames as a function of time
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