Session: A.3 - GRACE-Follow On & Bridging the Gap
(Convener: )

Title: Application of the LIS-based OSSE platform for future gravity missions
Presenter: Rodell, Matthew
Co-Authors: S. Kumar; M. Rodell; S. Luthcke; D. Wiese; B. Loomis; C. Peters-Lidard

Abstract: Observing System Simulation Experiments (OSSEs) are often conducted to evaluate the worth of existing data and data yet to be collected from proposed new missions. As missions increasingly require a broader ``Earth systems'' focus, it is important that the OSSEs capture the potential benefits of the observations on targeted process variables and end-use applications. In this presentation, we will present results from OSSEs used for developing science requirements for future gravity missions, using the NASA Land Information System (LIS) system. The experiments quantify the impact of assimilating data from several future gravimetry mission scenarios on the accuracy of model-simulated terrestrial hydrological processes. These experiments also demonstrate the value of a comprehensive modeling environment such as LIS for conducting end-to-end OSSEs by linking satellite observations, physical models, data assimilation algorithms and evaluation tools in a single integrated framework.

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Modeling of Present-Day Atmosphere and Ocean Non-Tidal De-Aliasing Errors for Future Gravity Mission Simulations
Presenter: Dobslaw, Henryk
Co-Authors: I Bergmann-Wolf, T. Mayer-Gürr, E. Forootan, J. Kusche

Abstract: A realistically perturbed synthetic de-aliasing model consistent with the updated Earth System Model of the European Space Agency (Dobslaw et al., 2015) is now available for the years 1995 -- 2006. The data-set contains realizations of (i) errors at large spatial scales assessed individually for periods between 10 -- 30, 3 -- 10, and 1 -- 3 days, the S1 atmospheric tide, and sub-diurnal periods; (ii) errors at small spatial scales typically not covered by global models of atmosphere and ocean variability; and (iii) errors due to physical processes not represented in currently available de-aliasing products. The error magnitudes for each of the different frequency bands are derived from a small ensemble of four atmospheric and oceanic models.

In order to demonstrate the plausibility of the error magnitudes chosen, we perform a variance component estimation based on daily GRACE normal equations from the ITSG-Grace2014 global gravity field series recently published by the University of Graz. All 12 years of the error model are used to calculate empirical error variance-covariance matrices describing the systematic dependencies of the errors both in time and in space individually for five continental and four oceanic regions, and daily GRACE normal equations are subsequently employed to obtain pre-factors for each of those matrices. For the largest spatial scales up to d/o = 40 and periods longer than 24 h, errors prepared for the updated ESM are found to be largely consistent with noise of a similar stochastic character contained in present-day GRACE solutions. Differences and similarities identified for all of the nine regions considered will be discussed in detail during the presentation.

Dobslaw, H., I. Bergmann-Wolf, R. Dill, E. Forootan, V. Klemann, J. Kusche, and I. Sasgen (2015), The updated ESA Earth System Model for future gravity mission simulation studies, J. Geod., doi:10.1007/s00190-014-0787-8.

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Title: Comparing Different Approaches for GRACE Follow-On Data Analysis
Presenter: Bender, Peter L.
Co-Authors:

Abstract: A number of different approaches have been used in simulations of the data analysis process for GRACE Follow-On data. Assuming that the laser interferometry system works well, the resulting uncertainties in the geopotential height will be due mainly to some combination of temporal aliasing, acceleration noise, orbit errors, and limitations in the anti-aliasing models that are employed. Data arcs ranging from about 30 min to 1 day have been used, with empirical parameter sets for each arc to correct for satellite state vector errors and/or acceleration noise. In order to compare the different analysis approaches without the effects of temporal aliasing included, it appears useful to just evaluate the errors along track at satellite altitude. This has been done for the proposed ocean calibration analysis approach for the two 6-hour data arcs each day when 4 successive revolutions cross the equatorial Pacific [Bender and Betts, arXiv:1506.05169 (2015)]. The geopotential height uncertainty along track is 2 to 3 mm after corrections are made for the acceleration noise level expected for GRACE Follow-On. For a more standard analysis approach also using 4 rev arc lengths, the accuracy will depend on the effects of having to make empirical corrections for the acceleration noise and orbit errors, and on the uncertainties over the whole Earth in the geophysical models that are used for calibration in making those corrections. The reason for expecting that the results from this simplified comparison method will agree with those from more complete analyses will be given.

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Title: Orbit and Gravity Field Solutions from Swarm GPS Observations
Presenter: Dahle, Christoph
Co-Authors: A. Jaeggi, D. Arnold, U. Meyer

Abstract: Abstract: Although ESA’s Earth Explorer Mission Swarm is primarily dedicated to measure the Earth’s magnetic field, it may also serve as a gravity field mission. Equipped with GPS receivers, accelerometers, star-tracker assemblies and laser retro-reflectors, the three Swarm satellites are capable to be used as a high-low satellite-to-satellite tracking (hl-SST) observing system. As the GRACE mission will likely end before the planned launch of its follow-on mission GRACE-FO in August 2017, Swarm is therefore a good candidate to provide large-scale time-variable gravity field variations and to contribute to potential products bridging the gap between GRACE and GRACE-FO. The properties of the Swarm constellation with two lower satellites flying in a pendulum-like orbit and a slightly differently inclined third satellite at higher altitude represent a unique observing system raising expectations at least compared to CHAMP-derived time-variable gravity field solutions. Its success strongly depends on the quality of the Swarm Level 1b data as well as the quality of the derived Swarm orbits.

We assess the quality of the first 1.5 years of Swarm Level 1b data (excluding accelerometer data) for Swarm orbit determination and subsequent recovery of the Earth’s gravity field. Special emphasis is made to further investigate and eliminate systematic errors affecting the gravity field solutions along the Earth’s geomagnetic equator. We also discuss differences in the tracking behavior of the GPS receivers onboard Swarm and GRACE. Furthermore, a first preview of the capabilities of Swarm to recover time-variable signals of the Earth’s gravity field is given, showing that large seasonal signals such as in the Amazon basin can be recovered quite well, whereas the estimation of secular trends in Greenland seems to be a rather challenging task. The presented gravity field solutions are also compared to GRACE GPS hl-SST solutions based on the same amount of data and processing methods.

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Title: Development status of the electrostatic accelerometer flight models of the GRACE Follow-On Mission
Presenter: Christophe, Bruno
Co-Authors: Onera's GFO Team

Abstract: Last year, the accelerometer has successfully passed its Critical Design Review and the manufacturing of the flight units was immediately engaged. The presentation will provide the present status of the assembly, integration and tests of the flight models at a few months of their delivery for integration at the centre of the satellite. From the control measurements of the mechanical parts and from the verification of the electronics functions characteristics, the best current estimate of the instrument performance will be also presented.

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Title: What can be expected from the GRACE-FO Laser Ranging Interferometer for Earth Science applications?
Presenter: Flechtner, Frank
Co-Authors: K. Neumayer, C. Dahle, H. Dobslaw, A. Güntner, J. Raimondo, E. Fagiolini

Abstract: The primary objective of the Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) satellite mission is to continue the GRACE time series of global monthly gravity field models. For this, evolved versions of the GRACE microwave instrument (MWI), GPS-receiver, and accelerometer will be used. A secondary objective is to demonstrate the effectiveness of a laser ranging interferometer (LRI) in improving the low-low satellite-to-satellite tracking measurement performance.

In order to investigate the expected benefit of the LRI for Earth science applications, we performed a full-scale simulation in terms of spherical harmonics over the nominal mission lifetime of five years using a realistic orbit scenario and error assumptions for orbit, instrument and background model errors.

We will present results in the spectral and spatial domain showing moderate improvements when using LRI instead of MWI observations for global quality indicators. As these global indicators are not meaningful for Earth system applications which show a clear mass variation signal in regionally defined areas such as water mass changes in hydrological basins or melting of glaciers we have also additionally investigated how simulated seasonal, sub-seasonal, secular and instantaneous (Earthquake) signals are recovered when using GRACE-FO MWI or LRI data. Related results will be presented at the GSTM.

Analysis of the different individual error contributions to the overall monthly gravity model error has shown that dominant errors are still due to accelerometer noise and imperfect modeling of tidal and non-tidal mass variations. Consequently, these errors have to be further reduced when using LRI observations on Next Generation Gravity Missions.

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