GRACE Science Team Meeting 2008

Session: A.1 - GRACE Geodesy

Part - 1, Friday, 9:40 - 10:25

(An improved 10-day time series of the geoid from GRACE and LAGEOS data)
Richard Biancale

(GFZ EIGEN-GRACE05S Weekly Gravity Field Time Series)
Christoph Dahle

(DEOS Mass Transport Model (DMT-1) Based on GRACE Satellite Data)
Pavel Ditmar

(Global Mascon Recovery from GRACE)
David Rowlands

Part - 2, Friday, 10:45 - 12:00

(Assessing Signal Content in GRACE)
Sean Swenson

(High-resolution analysis of GRACE sensor data)
Jakob Flury

(The use of regularization for global GRACE solutions)
Himanshu Save

(Analysis of the Stripe-like Noise in GRACE is Static and Monthly Gravity Fields)
Jianliang Huang

(Decorrelated GRACE Time-Variable Gravity Solutions by GFZ, and their Validation using a Hydrological Model)
Juergen Kusche

(Low-Degree Geopotential Harmonics from SLR and GRACE)
John Ries

(GRACE Models in Support of SLR Analysis for LARES and the ITRF)
Erricos C. Pavlis

Title: An improved 10-day time series of the geoid from GRACE and LAGEOS data
Session: A.1 - GRACE Geodesy
First Author: Richard Biancale
Presenter: Richard Biancale
Co-Authors: J.-M. Lemoine, S. Bruinsma, S. Gratton, S. Bourgogne

Abstract: Recently the CNES/GRGS geodesy team has reprocessed all GRACE data available in an improved modeling context. Based on these data and LAGEOS SLR data as well, new time variable geoid models have been finalized in 10-day steps from June 2002 till September 2008. A particular care have been brought to the stabilization process in order to get rid as much as possible of the meridian artifacts in the individual 10-day geoid solution expanded up to the spherical harmonic degree 50.

Based on these models for the long wavelength part, a mean field has been derived as well. It was computed up to the degree 160 but includes also annual periodic and semi-periodic terms as well as drifts up to degree 50 and adjusted over the 2002-2008 period.

All these models are now available through the BGI web site (

The presentation will particularly emphasize the improvements from the processing to the models.

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Title: GFZ EIGEN-GRACE05S Weekly Gravity Field Time Series
Session: A.1 - GRACE Geodesy
First Author: Christoph Dahle
Presenter: Christoph Dahle
Co-Authors: F. Flechtner; J. Kusche; R. Rietbroek

Abstract: GFZ Potsdam, as part of the GRACE Science Data System, has processed almost the complete GRACE mission data. The resulting time series of 69 monthly gravity field solutions, covering the period August 2002 till September 2008, is called EIGEN-GRACE05S (or RL04 in the SDS nomenclature) and exhibits various time-varying mass signals in the system Earth such as the continental hydrological cycle, ice mass change in Antarctica and Greenland or the Sumatra Earthquake in December 2004.

In addition to the monthly gravity field solutions, a time series of 7-daily (aligned to GPS weeks) solutions up to spherical harmonic degree and order 30 has been processed at GFZ. These weekly GRACE products will be published via GFZ is ISDC by the end of 2008. The presentation will focus on the processing strategy and the spatial resolution of the solutions; comparisons with standard monthly GFZ RL04 solutions as well as with other independent data, such as modeled and in-situ ocean bottom pressure will be shown.

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Title: DEOS Mass Transport Model (DMT-1) Based on GRACE Satellite Data
Session: A.1 - GRACE Geodesy
First Author: Pavel Ditmar
Presenter: Pavel Ditmar
Co-Authors: X. Liu, C. Siemes, C. Slobbe, E. Revtova, R. Klees, Q. Zhao, R. Van Beek. and M. Bierkens

Abstract: A model of temporal mass variations in the Earth's system has been computed at the Delft Institute of Earth Observation and Space Systems (DEOS) of the Delft University of Technology. The model is based on GRACE satellite data and consists of 46 monthly solutions covering the time interval from Feb. 2003 to Dec. 2006 (excluding Jun. 2003). Each solution consists of a set of spherical harmonic coefficients complete to degree 120. The solutions are produced with an original methodology based on average inter-satellite accelerations, which are derived from GRACE KBR data. Each solution is post-processed by applying a statistically optimal Wiener filter based on full signal and noise covariance matrices. Remarkably, the solutions do not show a contamination with along-track stripes, which are typical for most of other GRACE-based solutions. At the same time, the models have an enhanced spatial resolution, which allows one to detect small-scale mass variations like the decreasing water level of the Lake Victoria in Africa (68,800 kmē) and to separate geographically close signals, e.g. due to ice loss in the Kangerdlugssuaq and Helheim glaciers at the South-East coast of Greenland.

For the purpose of an independent validation, DMT-1 is compared with the hydrological model PCR-GLOBWB of Utrecht University, which describes the total in-land water storage variations (including snow cover, interception, soil moisture, groundwater, and open water bodies). For a number of rivers, the mean water storage per basin is computed as a function of time. It turns out that the best agreement between the GRACE-based and hydrological model is observed in North America, Europe, and Siberia. For example, the Rms differences for the Mississippi, Rhine, and Ob River basins do not exceed 1.8 cm, whereas the correlation coefficients stay above 89%. In tropical regions, however, the differences are larger, which can be explained both by inaccuracies in the data from the ECMWF Operational Archive (which are being used as a forcing to the hydrological model) and by a reduced accuracy of GRACE-based models in equatorial areas.

Finally, we have used the DMT-1 to estimate the rate of ice mass loss in Greenland, both per drainage system and totally. Our estimation of the secular trend in 2003 - 2006 for the entire Greenland is -150 to -170 Gton per year, which agrees reasonably well with estimations published earlier.

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Title: Global Mascon Recovery from GRACE
Session: B.5 - Progress in Inter-Disciplinary Applications
First Author: David Rowlands
Presenter: Frank Lemoine
Co-Authors: S.B. Luthcke, D.S. Chinn, J.J. McCarthy, J.P. Boy

Abstract: We have examined a different approach for improvement of the global spherical harmonic solutions. For example, MASCONs can be estimated as lumped sets of spherical harmonic coefficients. When a global set of MASCON parameters is estimated in this way, a global set of spherical harmonic coefficients is natural by-product. Global spherical harmonics estimated as MASCONs can be obtained with submonthly temporal resolution and require no post-solution smoothing. We describe the methodology of solution, and compare global spherical harmonic solutions estimated as global MASCONs to more standard solutions.

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Title: Assessing Signal Content in GRACE
Session: A.1 - GRACE Geodesy
First Author: Sean Swenson
Presenter: Sean Swenson

Abstract: We compare GRACE solutions from different processing centers with the aim of understanding the spatial resolution of the current generation of products.

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Title: High-resolution analysis of GRACE sensor data
Session: A.1 - GRACE Geodesy
First Author: Jakob Flury
Presenter: Jakob Flury
Co-Authors: B.D. Tapley, S. Bettadpur

Abstract: Analysis and processing of GRACE observations has undergone a continuous improvement process. However, the accuracy and resolution of K-band ranging (KBR) and accelerometer (ACC) sensors are still not fully exploited as of today, leaving room for improvement for gravity field determination, for determination of non-gravitational forces, and towards a full understanding of in-orbit performance of the GRACE sensor-platform system. The sensor data time series contain effects due to several types of disturbances which are only partially understood and modeled. Further on, the parameterization currently use for the modeling of systematic measurement errors (bias, drift, scale factors) is probably not optimal. In this paper, the current status for identification, step-wise separation and modeling of satellite-induced disturbances based on high-resolution ACC and KBR data is presented. In addition, approaches for mutual validation of sensors onboard GRACE and for monitoring of measurement errors with high resolution are discussed. This includes conclusions on the design of sensor-platform configurations for follow-on missions.

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Title: The use of regularization for global GRACE solutions
Session: A.1 - GRACE Geodesy
First Author: Himanshu Save
Presenter: Himanshu Save
Co-Authors: Srinivas Bettadpur; Byron D. Tapley

Abstract: We use regularization techniques to reduce the noise in the higher degrees including stripes while preserving the signal in the gravity field products. This paper discusses the characteristics and statistics of a 5-year time-series of regularized gravity field solutions. The solutions show markedly reduced stripes, are of uniformly good quality over time, and leave little or no systematic observation residuals, which is a frequent consequence of signal suppression from regularization. Up to degree 14, the signal in regularized solution shows correlation greater than 0.8 with the un-regularized CSR Release-04 solutions. We also explore the use of regularization in giving insights into GRACE data characteristics. For example, regularization tailored for the global KBR-only solutions, which are excessively noisy otherwise; provide insight into relative contributions of GPS and KBR to the global gravity products.

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Title: Analysis of the Stripe-like Noise in GRACE is Static and Monthly Gravity Fields
Session: A.1 - GRACE Geodesy
First Author: Jianliang Huang
Presenter: Jianliang Huang

Abstract: The stripe-like noise (in short, the stripe noise) stretching from Arctic to Antarctic in GRACE is global gravity field models poses a major difficulty for its applications. Its power can reach many times as strong as that of the Earth's gravity change signal. Numerous studies have been contributing to interpretation and elimination of the stripe noise. However, validity of any elimination method is arguable one way or another because of the fact that the stripe noise intrinsically originates from insufficiently spatiotemporal coverage of observations and inaccuracy of geophysical background models. Objectives of this work are to characterize geospatial, spectral and temporal properties of the stripe noise, therefore to seek a procedure that can extract the gravity change signal from GRACE is models in an efficient way. In particular, it tries to discuss and answer whereas possible the following questions: How does the geospatial resolution of the GRACE gravity fields vary geographically? Which Spherical Harmonic (SH) coefficients cause the stripe noise? How does the stripe noise contaminate the SH coefficient time series? Is it possible to improve the GRACE static (or mean) gravity model by reducing the stripe noise in the monthly gravity fields?

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Title: Decorrelated GRACE Time-Variable Gravity Solutions by GFZ, and their Validation using a Hydrological Model
Session: A.1 - GRACE Geodesy
First Author: Juergen Kusche
Presenter: Juergen Kusche
Co-Authors: R. Schmidt, S. Petrovic, R. Rietbroek

Abstract: We have analyzed recent GRACE RL04 monthly gravity solutions, using a new decorrelating post-processing approach. We find very good agreement with mass anomalies derived from a global hydrological model (WGHM). The post-processed GRACE solutions exhibit only little amplitude damping and an almost negligible phase shift and period distortion for relevant hydrological basins. Furthermore, these post-processed GRACE solutions have been inspected in terms of data fit with respect to the original inter-satellite ranging and to SLR and GPS observations. This kind of comparison is new. We find variations of the data fit due to solution post-processing only within very narrow limits. This confirms our suspicion that GRACE data does not firmly 'pinpoint' the standard unconstrained solutions. Regarding the original Kusche (2007) decorrelation and smoothing method, a simplified (order-convolution) approach has been developed. This simplified approach allows to realize a higher resolution, as required, e.g., for generating computed GRACE observations, but in particular for the applications of the GRACE-based surface mass anomalies. In addition, the computational effort of the new approach is significantly reduced as far less filter coefficients need to be stored.

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Title: Low-Degree Geopotential Harmonics from SLR and GRACE
Session: A.1 - GRACE Geodesy
First Author: John Ries
Presenter: John Ries
Co-Authors: M. Cheng, S. Bettadpur, D. Chambers

Abstract: GRACE provides estimates of the temporal variations in the Earth's gravity field with extraordinary precision, but as with any non-synoptic measurement system, the problem of aliasing of high-frequency signals errors is an important concern. In the case of GRACE, errors in the models for the diurnal and semi-diurnal (solid earth and ocean) tides will alias into long-period variations in the low-degree geopotential harmonics. This issue is particularly apparent for the degree-2 zonal harmonic, C20. In addition, after removing the tidal aliases, the GRACE C20 series is still noisier than SLR by about a factor of 3. As a consequence, GRACE data product users typically replace the C20 estimates from GRACE with estimates derived independently (but using consistent modeling) from SLR tracking of several geodetic satellites. In this paper, we will discuss the characteristics of the GRACE and SLR time series for the low-degree harmonics. In particular, we discuss the SLR and GRACE estimates for the harmonic terms describing the orientation of the Earth's principal figure axis, C21 and S21, and compare them to the conventional model for the long-term evolution of these terms.

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Title: GRACE Models in Support of SLR Analysis for LARES and the ITRF
Session: A.1 - GRACE Geodesy
First Author: Erricos C. Pavlis
Presenter: Erricos C. Pavlis
Co-Authors: I. Ciufolini; G. Sindoni; P. M. Hinkey

Abstract: GRACE monthly fields are now available for just over six years and provide a reasonably stable basis to derive accurate and robust estimates of the annual, semi-annual and seasonal signals in temporal gravity variations for the low degree harmonics of the field. These can in turn be used to reduce with higher accuracy the precise Satellite Laser Ranging (SLR) data from geodetic targets like the LAGEOS satellites for the definition of the International Terrestrial Reference Frame (ITRF) and for testing relativistic predictions such as the Lense-Thirring (L-T) frame-dragging) effect predicted by General Relativity. Accuracy requirements for the ITRF are becoming increasingly more stringent, especially with regards to its origin definition and its scale stability. SLR contributes unique information on the origin, and along with VLBI, for its absolute scale. The precise monitoring of the geocenter and its variations over increasingly shorter intervals is a priority goal in order to meet the requirements of the Global Geodetic Observing System (GGOS) of 1 mm for the definition of the TRF origin at epoch and the 0.1 mm/y limit in its temporal evolution. Such stringent requirements are dictated by the geophysical signals that we are trying to observe with confidence, such as sea level variations. Advances in our understanding of the coupling between the sub-components of the Earth system will improve the definition of both of these attributes. With the recent release of numerous products from global circulation models and, satellite and terrestrial observations, we are able now to examine the effect of such improved modeling in the analysis of several years of SLR data. As GRACE models mature, we can also determine more robust accuracy estimates for these models, thence place bounds on the errors associated with our results in measuring the L-T signal with SLR data now and in the near future. ASI's LARES mission is to be launched in less than a year from now; we thus examine what the current error contributions are and what that means in terms of % error of the L-T parameter estimate. We will also present a brief status report on the LARES mission which in addition to enabling an even more accurate L-T measurement, it is expected to contribute significantly in the definition of the ITRF through the expansion of the LAGEOS-like satellite constellation.

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