Frank Flechtner

System Earth via Geodetic-Geophysical Space Techniques

CHAMP and GRACE.- More Accurate and Faster Available CHAMP and GRACE Gravity Fields for the User Community.- The CHAMP/GRACE User Portal ISDC.- Improvements for the CHAMP and GRACE Observation Model.- The Release 04 CHAMP and GRACE EIGEN Gravity Field Models.- Orbit Predictions for CHAMP and GRACE.- Rapid Science Orbits for CHAMP and GRACE Radio Occultation Data Analysis.- Parallelization and High Performance Computation for Accelerated CHAMP and GRACE Data Analysis.- GRACE.- Improved GRACE Level-1 and Level-2 Products and Their Validation by Ocean Bottom Pressure.- The GRACE Gravity Sensor System.- Numerical Simulations of Short-Term Non-tidal Ocean Mass Anomalies.- Improved Non-tidal Atmospheric and Oceanic De-aliasing for GRACE and SLR Satellites.- Global Gravity Fields from Simulated Level-1 GRACE Data.- ITG-GRACE: Global Static and Temporal Gravity Field Models from GRACE Data.- Validation of GRACE Gravity Fields by In-Situ Data of Ocean Bottom Pressure.- Antarctic Circumpolar Current Transport Variability in GRACE Gravity Solutions and Numerical Ocean Model Simulations.- GOCE.- Gravity and Steady-State Ocean Circulation Explorer GOCE.- GOCE Data Analysis: From Calibrated Measurements to the Global Earth Gravity Field.- GOCE and Its Use for a High-Resolution Global Gravity Combination Model.- Spectral Approaches to Solving the Polar Gap Problem.- Regionally Refined Gravity Field Models from In-Situ Satellite Data.- Quality Evaluation of GOCE Gradients.- Validation of Satellite Gravity Field Models by Regional Terrestrial Data Sets.- Comparison of GRACE and Model-Based Estimates of Bottom Pressure Variations Against In Situ Bottom Pressure Measurements.- SEAVAR.- Sea Level Variations - Prospects from the Past to the Present (SEAVAR).- Radar Altimetry Derived Sea Level Anomalies - The Benefit of New Orbits and Harmonization.- Combining GEOSAT and TOPEX/Poseidon Data by Means of Data Assimilation.- Reanalysis of GPS Data at Tide Gauges and the Combination for the IGS TIGA Pilot Project.- Sea Level Rise in North Atlantic Derived from Gap Filled Tide Gauge Stations of the PSMSL Data Set.- Using ARGO, GRACE and Altimetry Data to Assess the Quasi Stationary North Atlantic Circulation.- A 15-Year Reconstruction of Sea Level Anomalies Using Radar Altimetry and GPS-Corrected Tide Gauge Data.- TIVAGAM.- Continental Water Storage Variations from GRACE Time-Variable Gravity Data.- Surface Mass VariabilitySurface mass variability from GRACE and Hydrological Models Hydrological model : Characteristic PeriodsPeriods characteristic and the Reconstruction of Significant SignalsReconstruction of significant signals .- Time-Space Multiscale AnalysisTime-Space Multiscale Analysis Multiscale analysis and Its Application to GRACE and Hydrology Data.- Mass Variation Signals in GRACE Products and in Crustal Deformations crustal deformation from GPS: A Comparison.- Monthly and Daily Variations of Continental Water Storage and Flows.- Calibration of a Global Hydrological Modelglobal hydrological model with GRACE Data.- NRT-RO.- Near-Real-Time Provision and Usage of Global Atmospheric Data from CHAMP and GRACE (NRT-RO): Motivation and Introduction.- Global Atmospheric Data from CHAMP and GRACE-A: Overview and Results.- Near-Real Time Satellite Orbit Determination for GPS Radio Occultation with CHAMP and GRACE.- The Operational Processing System for GPS Radio Occultation Data from CHAMP and GRACE.- Assimilation of CHAMP and GRACE-A Radio Occultation Data in the GME Global Meteorological Model of the German Weather Service.- MAGFIELD.- The Earths Magnetic Field at the CHAMP Satellite Epoch.- GGOS-D.- Integration of Space Geodetic Techniques as the Basis for a Global Geodetic-Geophysical Observing System (GGOS-D): An Overview.- GGOS-D Data Management - From Data to Knowledge.- GGOS-D Consistent, High-Accuracy Technique-Specific Solutions.- GGOS-D Global Terrestrial Reference Frame.- GGOS-D Consistent and Combined Time Series of Geodetic/Geophyical Parameters.- GGOS-D Integration with Low Earth Orbiters.

Seasonal variation of ocean bottom pressure derived from Gravity Recovery and Climate Experiment (GRACE): Local validation and global patterns

The Gravity Recovery and Climate Experiment (GRACE) processing centers at the GeoForschungsZentrum Potsdam (GFZ) and the University of Texas Center for Space Research (UTCSR) provide time series of monthly gravity field solutions covering the period since mission launch in March 2002. Although the achieved accuracy still remains an order of magnitude below the missions baseline goal, these time series have successfully been used to study terrestrial phenomena such as water storage variations. Over the oceans, the monthly gravity field solutions can be converted into estimates of the fluctuating ocean bottom pressure (OBP), which is the sum of atmospheric and oceanic mass variations. The GRACE products may be validated against in situ OBP observations which are available from a ground truth site in the tropical northwest Atlantic Ocean. Large differences are observed between the in situ and GRACE-derived OBP which are investigated by comparing the tidal and nontidal ocean models used at GFZ and UTCSR for dealiasing short-term (<2 months) mass variations from satellite measurements. Results show that the barotropic nontidal and tide models need improvement at periods shorter than 1 day and longer than 2 weeks. On a global scale the monthly OBP fields from GRACE generally overestimate the variability compared to ocean general circulation models, especially in tropical regions. This may be attributed to continuing deficiencies in GRACE data processing. Nevertheless, there is some initial evidence that GRACE possesses the potential to observe large-scale averages of bottom pressure fluctuations.

Changes in total ocean mass derived from GRACE, GPS, and ocean modeling with weekly resolution

[1] We derive changes in ocean bottom pressure (OBP) and ocean mass by combining modeled ocean bottom pressure, weekly GRACE-derived models of gravity change, and large-scale deformation patterns sensed by a global network of GPS stations in a joint least squares inversion. The weekly combination allows a consistent estimation of geocenter motion, loading mass harmonics up to degree 30, and a spatially uniform mass correction term, which serves as a correction for forcing of the ocean model. We provide maps and time series of ocean mass and bottom pressure variations. Furthermore, we discuss the estimated geocenter motion and the estimated model correction. Our results indicate that the total ocean mass change is predominantly annual, with a maximum amplitude corresponding to 7.4 mm in October, which is in line with earlier work. The mean ocean bottom pressure (i.e., ocean plus atmospheric mass) shows an annual amplitude of 8.7 mm and is shifted forward by about 1.5 months. In addition, the solution exhibits typical autocorrelation times of about 2 weeks. A comparison with in situ bottom pressure time series in the southern Indian Ocean shows a good agreement, with correlations of 0.7-0.8. Based on these comparisons, we see that our results monitor realistic submonthly variations, which are strongest at high latitudes. The addition of GRACE data in the inversion is found to improve these high-latitude variations and enables better separability of the geocenter motion from other unknowns. Increasing the OBP model error from 3 cm to 4.8 cm affects mainly the higher-degree coefficients.

Comparing seven candidate mission configurations for temporal gravity field retrieval through full-scale numerical simulation

The goal of this contribution is to focus on improving the quality of gravity field models in the form of spherical harmonic representation via alternative configuration scenarios applied in future gravimetric satellite missions. We performed full-scale simulations of various mission scenarios within the frame work of the German joint research project “Concepts for future gravity field satellite missions” as part of the Geotechnologies Program, funded by the German Federal Ministry of Education and Research and the German Research Foundation. In contrast to most previous simulation studies including our own previous work, we extended the simulated time span from one to three consecutive months to improve the robustness of the assessed performance. New is that we performed simulations for seven dedicated satellite configurations in addition to the GRACE scenario, serving as a reference baseline. These scenarios include a “GRACE Follow-on” mission (with some modifications to the currently implemented GRACE-FO mission), and an in-line “Bender” mission, in addition to five mission scenarios that include additional cross-track and radial information. Our results clearly confirm the benefit of radial and cross-track measurement information compared to the GRACE along-track observable: the gravity fields recovered from the related alternative mission scenarios are superior in terms of error level and error isotropy. In fact, one of our main findings is that although the noise levels achievable with the particular configurations do vary between the simulated months, their order of performance remains the same. Our findings show also that the advanced pendulums provide the best performance of the investigated single formations, however an accuracy reduced by about 2–4 times in the important long-wavelength part of the spectrum (for spherical harmonic degrees

De-aliasing of Short-term Atmospheric and Oceanic Mass Variations for GRACE

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Derivation of the Topographic Potential from Global DEM Models

<50), compared to the Bender mission, can be observed. Concerning state-of-the-art mission constraints, in particular the severe restriction of heterodyne lasers on maximum range-rates, only the moderate Pendulum and the Bender-mission are beneficial options, of course in addition to GRACE and GRACE-FO. Furthermore, a Bender-type constellation would result in the most accurate gravity field solution by a factor of about 12 at long wavelengths (up to degree/order 40) and by a factor of about 200 at short wavelengths (up to degree/order 120) compared to the present GRACE solution. Finally, we suggest the Pendulum and the Bender missions as candidate mission configurations depending on the available budget and technological progress.

Static and Time-Variable Gravity from GRACE Mission Data

This study examines present-day changes of the Antarctic ice sheet (AIS) by means of different data sets. We make use of monthly gravity field solutions acquired by the Gravity Recovery and Climate Experiment (GRACE) to study mass changes of the AIS for a 10-year period. In addition to ‘standard’ solutions of release 05, solutions based on radial base functions were used. Both solutions reveal an increased mass loss in recent years. For a 6-year period surface-height changes were inferred from laser altimetry data provided by the Ice, Cloud, and land Elevation Satellite (ICESat). The basin-scale volume trends were converted into mass changes and were compared with the GRACE estimates for the same period. Focussing on the Thwaites Glacier, Landsat optical imagery was utilised to determine ice-flow velocities for a period of more than two decades. This data set was extended by means of high-resolution synthetic aperture radar (SAR) data from the TerraSAR-X mission, revealing an accelerated ice flow of all parts of the glacier. ICESat data over the Thwaites Glacier were complemented by digital elevation models inferred from TanDEM-X data. This extended data set exhibits an increased surface lowering in recent times. Passive microwave remote sensing data prove the long-term stability of the accumulation rates in a low accumulation zone in East Antarctica over several decades. Finally, we discuss the main error sources of present-day mass-balance estimates: the glacial isostatic adjustment effect for GRACE as well as the biases between laser operational periods and the volume–mass conversion for ICESat.

GNSS navigation and positioning for the GEOHALO experiment in Italy

GFZ is responsible for routine calculation of atmospheric and oceanic mass variations which have to be considered during GRACE precise orbit determination and calculation of gravity field partial derivatives. This Level-1B Atmosphere and Ocean De-aliasing product (AOD1B) is made available to the GRACE Science Data System and user community in terms of spherical harmonic coefficients with a maximum time delay of about 3–4 days dependent on the availability of required ECMWF meteorological fields. The spatial and time-variable vertical structure of the atmosphere is taken into account by vertical integration of the atmospheric masses. Oceanic mass variations are derived from a barotropic ocean model (PPHA) which was provided by JPL. The individual atmospheric and oceanic contributions as well as the processing strategy to derive the combined AOD1B product are described in the first part of this paper.