scholarly journals Determining and Evaluating the Hydrological Signal in Polar Motion Excitation from Gravity Field Models Obtained from Kinematic Orbits of LEO Satellites

2019 ◽  
Vol 11 (15) ◽  
pp. 1784 ◽  
Author(s):  
Justyna Śliwińska ◽  
Jolanta Nastula
Keyword(s):  
Gravity Field ◽  
Polar Motion ◽  
Low Earth Orbit ◽  
Data Sets ◽  
Leo Satellites ◽  
Hydrological Angular Momentum ◽  
Academy Of Sciences ◽  
Gravity Field Models ◽  
Polar Motion Excitation ◽  
Background Models

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.

scholarly journals Prograde and Retrograde Terms of Gravimetric Polar Motion Excitation Estimates from the GRACE Monthly Gravity Field Models

2020 ◽  
Vol 12 (1) ◽  
pp. 138 ◽  
Author(s):  
Jolanta Nastula ◽  
Justyna Śliwińska
Keyword(s):  
Angular Momentum ◽  
Gravity Field ◽  
Polar Motion ◽  
Temporal Variations ◽  
Short Term ◽  
Grace Data ◽  
Hydrological Angular Momentum ◽  
Gravity Field Models ◽  
Polar Motion Excitation

From 2002 to 2017, the Gravity Recovery and Climate Experiment (GRACE) mission’s twin satellites measured variations in the mass redistribution of Earth’s superficial fluids, which disturb polar motion (PM). In this study, the PM excitation estimates were computed from two recent releases of GRACE monthly gravity field models, RL05 and RL06, and converted into prograde and retrograde circular terms by applying the complex Fourier transform. This is the first such analysis of circular parts in GRACE-based excitations. The obtained series were validated by comparison with the residuals of observed polar motion excitation (geodetic angular momentum (GAM)–atmospheric angular momentum (AAM)–oceanic angular momentum (OAM) (GAO)) determined from precise geodetic measurements of the pole coordinates. We examined temporal variations of hydrological excitation function series (or hydrological angular momentum, HAM) in four spectral bands: seasonal, non-seasonal, non-seasonal short-term, and non-seasonal long-term. The general conclusions arising from the conducted analyses of prograde and retrograde terms were consistent with the findings from the equatorial components of PM excitation studies drawn in previous research. In particular, we showed that the new GRACE RL06 data increased the consistency between different solutions and improved the agreement between GRACE-based excitation series and reference data. The level of agreement between HAM and GAO was dependent on the oscillation considered and was higher for long-term than short-term variations. For most of the oscillations considered, the highest agreement with GAO was obtained for CSR RL06 and ITSG-Grace2018 solutions. This study revealed that both prograde and retrograde circular terms of PM excitation can be determined by GRACE with similar levels of accuracy. The findings from this study may help in choosing the most appropriate GRACE solution for PM investigations and can be useful in future improvements to GRACE data processing.

Comparing and evaluating GRACE and GRACE-FO mascon data in the study of polar motion excitation

2020 ◽  
Author(s):  
Justyna Śliwińska ◽  
Małgorzata Wińska ◽  
Jolanta Nastula
Keyword(s):  
Polar Motion ◽  
Temporal Variations ◽  
Great Success ◽  
Spectral Bands ◽  
The Earth ◽  
Hydrological Angular Momentum ◽  
High Consistency ◽  
Gravity Recovery ◽  
Grace Mission ◽  
Polar Motion Excitation

<p>The Gravity Recovery and Climate Experiment (GRACE) mission has provided global observations of temporal variations in mass redistribution at the surface and within the Earth for the period 2002–2017. Such measurements are commonly exploited to interpret polar motion changes due to variations in the Earth’s surficial fluids, especially in the continental hydrosphere. Such impacts are usually examined by computing the so-called hydrological polar motion excitation (Hydrological Angular Momentum, HAM). The great success of the GRACE mission and the scientific robustness of its data contributed to the launch of its successor, GRACE Follow-On (GRACE-FO), which begun in May 2018 and continues to the present.</p> <p>This study compares the estimates of HAM computed from GRACE and GRACE-FO mascon data provided by three data centers: Jet Propulsion Laboratory (JPL), Center for Space Research (CSR), and Goddard Space Flight Center (GSFC). The analysis of HAM is performed for different spectral bands. A validation of different HAM estimates is conducted here using precise geodetic measurements of the pole coordinates and geophysical models (so-called geodetic residuals or GAO).</p> <p>Comparison of HAM computed from different mascon data sources indicates high consistency between the solutions provided by JPL and CSR, and low consistency between the GSFC solution and other data. The reason for this may be that the strategy used for GSFC mascons computation is different than methodology exploited by CSR and JPL teams. This study also indicates that HAM computed using CSR and JPL solutions are characterized by the highest consistency with GAO in all considered spectral bands.</p>

New Global Gravity Field Models from Selected CHAMP Data Sets

2003 ◽  
pp. 120-127 ◽  
Author(s):  
Christoph Reigber ◽  
Georges Balmino ◽  
Peter Schwintzer ◽  
Richard Biancale ◽  
Albert Bode ◽  
...  
Keyword(s):  
Gravity Field ◽  
Data Sets ◽  
Global Gravity ◽  
Gravity Field Models ◽  
Champ Data

scholarly journals GRAVL: a new satellite mission concept aiming to detect earthquakes with a magnitude of 6.5 Mw and higher

2020 ◽  
Author(s):  
Jerome Woodwark ◽  
Marcel Stefko ◽  
Keyword(s):  
Gravity Field ◽  
Spatial Resolution ◽  
Summer School ◽  
Engineering Students ◽  
Low Earth Orbit ◽  
Laser Ranging ◽  
Earth Orbit ◽  
Satellite Mission ◽  
Seismic Mass ◽  
Leo Satellites

<p>Data from the US and German Gravity Recovery And Climate Experiment (GRACE) showed indications of pre-, co-, and post-seismic mass redistributions associated with earthquakes down to a magnitude of 8.3 Mw. These demonstrated state-of-the-art capabilities in obtaining high spatial resolution space-based gravimetry, and helped to improve understanding of mantle rheology, potentially even providing a route to developing early warning capabilities for future seismic events. We describe a new mission concept, GRAvity observations by Vertical Laser ranging (GRAVL), which aims to extend the earthquake detection limit down to magnitude 6.5 Mw, significantly increasing the number of observable events.</p><p>GRAVL directly measures the radial component of the acceleration vector via “high-low” inter-satellite laser ranging, increasing gravity field sensitivity. A constellation of Low-Earth Orbit (LEO) satellites act as test masses, equipped with reflectors and high precision accelerometers to account for non-gravitational forces. Two or more larger satellites are placed above these, in Geostationary or Medium Earth Orbit (GEO / MEO), and measure the distance to the LEO satellites via time-of-flight measurement of a laser pulse. To do this, the GEO/MEO spacecraft are each equipped with a laser, telescope and detector, and additionally require highly  accurate timing systems to enable ranging accuracy down to sub-micron precision. To detect co-seismic mass redistribution events of the desired magnitude, we determine a gravity field measurement requirement of order 0.1 µGal at a spatial resolution of approximately 100 km over a 3-day revisit interval. These are challenging requirements, and we will discuss possible approaches to achieving them.</p><p>The GRAVL mission concept was developed during the FFG/ESA Alpbach Summer School 2019 by a team of science and engineering students, and further refined using the Concurrent Engineering approach during the Post-Alpbach Summer School Event at ESA Academy's Training and Learning Facility at ESEC-Galaxia in Belgium.</p>

scholarly journals Evaluating Gravimetric Polar Motion Excitation Estimates from the RL06 GRACE Monthly-Mean Gravity Field Models

2020 ◽  
Vol 12 (6) ◽  
pp. 930 ◽  
Author(s):  
Justyna Śliwińska ◽  
Jolanta Nastula ◽  
Henryk Dobslaw ◽  
Robert Dill
Keyword(s):  
Polar Motion ◽  
Temporal Variations ◽  
Spectral Bands ◽  
Mass Redistribution ◽  
Temporal Models ◽  
Gravity Recovery ◽  
Grace Mission ◽  
Gravity Field Models ◽  
Polar Motion Excitation ◽  
Level Of Agreement

Over the last 15 years, the Gravity Recovery and Climate Experiment (GRACE) mission has provided measurements of temporal changes in mass redistribution at and within the Earth that affect polar motion. The newest generation of GRACE temporal models, are evaluated by conversion into the equatorial components of hydrological polar motion excitation and compared with the residuals of observed polar motion excitation derived from geodetic measurements of the pole coordinates. We analyze temporal variations of hydrological excitation series and decompose them into linear trends and seasonal and non-seasonal changes, with a particular focus on the spectral bands with periods of 1000–3000, 450–1000, 100–450, and 60–100 days. Hydrological and reduced geodetic excitation series are also analyzed in four separated time periods which are characterized by different accuracy of GRACE measurements. The level of agreement between hydrological and reduced geodetic excitation depends on the frequency band considered and is highest for interannual changes with periods of 1000–3000 days. We find that the CSR RL06, ITSG 2018 and CNES RL04 GRACE solutions provide the best agreement with reduced geodetic excitation for most of the oscillations investigated.

The influence of Antarctic and Greenland ice loss on polar motion: an assessment based on GRACE and multi-mission satellite altimetry

2021 ◽  
Author(s):  
Franziska Göttl ◽  
Andreas Groh ◽  
Maria Kappelsberger ◽  
Undine Strößenreuther ◽  
Ludwig Schröder ◽  
...  
Keyword(s):  
Gravity Field ◽  
Polar Motion ◽  
Satellite Altimetry ◽  
Global Climate ◽  
Pole Position ◽  
Position Vector ◽  
Random Errors ◽  
Laser Altimeter ◽  
Excitation Functions ◽  
Polar Motion Excitation

<p>Increasing ice loss of the Antarctic and Greenland Ice Sheets (AIS, GrIS) due to global climate change affects the orientation of the Earth’s spin axis with respect to an Earth-fixed reference system (polar motion). Ice mass changes in Antarctica and Greenland are observed by the Gravity Recovery and Climate Experiment (GRACE) in terms of time variable gravity field changes and derived from surface elevation changes measured by satellite radar and laser altimeter missions such as ENVISAT, CryoSat-2 and ICESat. Beside the limited spatial resolution, the accuracy of GRACE ice mass change estimates is limited by signal noise (meridional error stripes), leakage effects and uncertainties of the glacial isostatic adjustment (GIA) models, whereas the accuracy of satellite altimetry derived ice mass changes is limited by waveform retracking, slope related relocation errors, firn compaction and the density assumption used in the volume-to-mass conversion.</p><p> </p><p>In this study we use different GRACE gravity field models (CSR RL06M, JPL RL06M, ITSG-Grace2018) and satellite altimetry data (from TU Dresden, University of Leeds, Alfred Wegener Institute) to assess the accuracy of the gravimetry and altimetry derived polar motion excitation functions. We show that due to the combination of individual solutions, systematic and random errors of the data processing can be reduced and the robustness of the geodetic derived AIS and GrIS polar motion excitation functions can be increased. Based on these investigations we found that AIS mass changes induce the pole position vector to drift along the 60° East meridian by 2 mas/yr during the study period 2003-2015, whereas GrIS mass changes cause the pole vector to drift along the 45° West meridian by 3 mas/yr.</p>

scholarly journals GNSS RTK Positioning Augmented with Large LEO Constellation

2019 ◽  
Vol 11 (3) ◽  
pp. 228 ◽  
Author(s):  
Xingxing Li ◽  
Hongbo Lv ◽  
Fujian Ma ◽  
Xin Li ◽  
Jinghui Liu ◽  
...  
Keyword(s):  
Low Earth Orbit ◽  
Global Navigation Satellite Systems ◽  
Convergence Time ◽  
Current Status ◽  
Initial Assessment ◽  
Satellite Systems ◽  
Long Baseline ◽  
Leo Satellites ◽  
Global Navigation Satellite ◽  
Long Baselines

It is widely known that in real-time kinematic (RTK) solution, the convergence and ambiguity-fixed speeds are critical requirements to achieve centimeter-level positioning, especially in medium-to-long baselines. Recently, the current status of the global navigation satellite systems (GNSS) can be improved by employing low earth orbit (LEO) satellites. In this study, an initial assessment is applied for LEO constellations augmented GNSS RTK positioning, where four designed LEO constellations with different satellite numbers, as well as the nominal GPS constellation, are simulated and adopted for analysis. In terms of aforementioned constellations solutions, the statistical results of a 68.7-km baseline show that when introducing 60, 96, 192, and 288 polar-orbiting LEO constellations, the RTK convergence time can be shortened from 4.94 to 2.73, 1.47, 0.92, and 0.73 min, respectively. In addition, the average time to first fix (TTFF) can be decreased from 7.28 to 3.33, 2.38, 1.22, and 0.87 min, respectively. Meanwhile, further improvements could be satisfied in several elements such as corresponding fixing ratio, number of visible satellites, position dilution of precision (PDOP) and baseline solution precision. Furthermore, the performance of the combined GPS/LEO RTK is evaluated over various-length baselines, based on convergence time and TTFF. The research findings show that the medium-to-long baseline schemes confirm that LEO satellites do helpfully obtain faster convergence and fixing, especially in the case of long baselines, using large LEO constellations, subsequently, the average TTFF for long baselines has a substantial shortened about 90%, in other words from 12 to 2 min approximately by combining with the larger LEO constellation of 192 or 288 satellites. It is interesting to denote that similar improvements can be observed from the convergence time.

scholarly journals Evaluation of hydrological and cryospheric angular momentum estimates based on GRACE, GRACE-FO and SLR data for their contributions to polar motion excitation

2021 ◽  
Vol 73 (1) ◽  
Author(s):  
Justyna Śliwińska ◽  
Jolanta Nastula ◽  
Małgorzata Wińska
Keyword(s):  
Angular Momentum ◽  
Spherical Harmonic ◽  
Polar Motion ◽  
Spectral Band ◽  
Water Storage ◽  
Terrestrial Water Storage ◽  
Terrestrial Water ◽  
High Consistency ◽  
Polar Motion Excitation

AbstractIn geodesy, a key application of data from the Gravity Recovery and Climate Experiment (GRACE), GRACE Follow-On (GRACE-FO), and Satellite Laser Ranging (SLR) is an interpretation of changes in polar motion excitation due to variations in the Earth’s surficial fluids, especially in the continental water, snow, and ice. Such impacts are usually examined by computing hydrological and cryospheric polar motion excitation (hydrological and cryospheric angular momentum, HAM/CAM). Three types of GRACE and GRACE-FO data can be used to determine HAM/CAM, namely degree-2 order-1 spherical harmonic coefficients of geopotential, gridded terrestrial water storage anomalies computed from spherical harmonic coefficients, and terrestrial water storage anomalies obtained from mascon solutions. This study compares HAM/CAM computed from these three kinds of gravimetric data. A comparison of GRACE-based excitation series with HAM/CAM obtained from SLR is also provided. A validation of different HAM/CAM estimates is conducted here using the so-called geodetic residual time series (GAO), which describes the hydrological and cryospheric signal in the observed polar motion excitation. Our analysis of GRACE mission data indicates that the use of mascon solutions provides higher consistency between HAM/CAM and GAO than the use of other datasets, especially in the seasonal spectral band. These conclusions are confirmed by the results obtained for data from first 2 years of GRACE-FO. Overall, after 2 years from the start of GRACE-FO, the high consistency between HAM/CAM and GAO that was achieved during the best GRACE period has not yet been repeated. However, it should be remembered that with the systematic appearance of subsequent GRACE-FO observations, this quality can be expected to increase. SLR data can be used for determination of HAM/CAM to fill the one-year-long data gap between the end of GRACE and the start of the GRACE-FO mission. In addition, SLR series could be particularly useful in determination of HAM/CAM in the non-seasonal spectral band. Despite its low seasonal amplitudes, SLR-based HAM/CAM provides high phase consistency with GAO for annual and semiannual oscillation.

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