G31C-0798 INVITED 0800h
Viscoelastic response of an eccentrically-nested liquid-core model to surface-toroidal traction
We present a semi-analytical solution to the 2-D forward modelling of viscoelastic relaxation in a heterogeneous sphere consisting of a concentrically-nested elastic lithosphere and a viscoelastic mantle, and an eccentrically-nested liquid core. Several numerical methods for 2-D and 3-D viscoelastic relaxation modelling have been applied recently, including finite-element and spectral-finite-difference schemes. The present semi-analytical approach provides a model response that can be used to validate more general numerical algorithms. The solution for the eccentrically-nested liquid core has been tested by comparing it to the analytical solution for viscoelastic relaxation in concentrically-nested spheres and excellent agreement has been obtained.
G31C-0799 0800h
Estimation of time-varying geopotential coefficients using Satellite Laser Ranging data
Due to the long record of accurate and continuous laser ranging observations, Satellite Laser Ranging (SLR) is known as the only current space technique capable to monitor the long time variability of the Earth's gravity field. Geodetic satellites have been providing the low frequency part of the geopotential models used for precise orbit determination purposes (e.g. JGM3, EGM96,...). Nevertheless they can be used to estimate the temporal variation of selected coefficients, helping to clarify the complex interrelations in the earth-ocean-atmosphere system. In this work we present our analysis of SLR data (spanning two decades) from several geodetic satellites (including Lageos I-II, Stella, Starlette, Ajisai) to recover fortnightly estimates of low degree geopotential coefficients; the results are obtained analysing the satellites in proper combination.
G31C-0800 0800h
Absolute Gravity Measurements of Vertical Crustal Movements in the UK
Beginning in 1995/6, we started to make absolute gravity(AG) measurements at 3 sites in the UK using the Micro-g Solutions, Inc., absolute gravimeter FG5-103. These sites are at Newlyn in the south-west of England, Aberdeen in north-east Scotland and Lerwick in the Shetland Islands. The sites were chosen to be on bedrock, in order to reduce the effects of any possible local hydrological variations. The objective is to obtain long time series of absolute gravity values, which can be used for determining the vertical crustal movements at these sites, accurate to a few tenths of a millimeter per year. The measurements will be used to correct the mean sea level trends determined from the nearby tide gauges for crustal movements, in order to find the climate related changes in mean sea levels. Newlyn and Lerwick are core tide gauges in the Global Sea Level Observing System (GLOSS). The AG measurements show that both Newlyn and Lerwick are subsiding and this is consistent with GIA models, which show that the maximum subsidence rates in the British Isles following de-glaciation are in the extreme south-west of England and in the Shetland Islands. Compared to previous models of the ice distribution at the last glacial maximum, the recently published ICE-5G model has major modifications in ice thickness in Scotland and particularly in the North Sea around the Shetland Islands. The AG measurements will be important for testing the validity of the various models.
G31C-0801 0800h
Secular variations in the low degree zonal harmonics from 28 years of SLR data
The secular changes in the Earth's gravity field are the consequence of long-term mass redistribution within the Earth system. The major components of these phenomena are due to the readjustment of the Earth to glaciation, including post-glacial rebound and the mass balance of the polar ice sheets and subpolar glaciers. Satellite Laser Ranging (SLR) data have recorded the global nature of these gravity variations, through their effect on the orbits of geodetic satellites, for almost three decades. A solution for the secular variations in the zonal harmonics up to degree 7 were determined from the SLR data from 8 geodetic satellites over the time period spanning 28 years from 1976 to 2003. This solution is consistent with the result within the range of uncertainty determined using data from 1976 to 1995 by Cheng, Shum and Tapley [1997]. Analysis of the monthly estimates of the second degree zonal (J2) indicates that, in addition to the secular drift, the 18.6 year tide and the seasonal variations, J2 has significant interannual variations with time scales of 4-6 years and ~21 years. These longer period variations significantly affect the estimate of the secular drift, with the most recent SLR data affecting the estimate much more than the earlier data. Such effects are less significant for the higher degree (> 2) zonal rates. In addition, comparison of the monthly solution of lower degree and order geopotential coefficients shows a good agreement except for J2 derived from recent SLR and GRACE data. Detailed comparisons and analysis will be presented.
G31C-0802 0800h
Estimating mass balances of the global water reservoirs by GRACE satellite gravimetry
According to global hydrology models, the total water storage on the continents continuously decreases with time. In order to verify this scenario of a global and progressive transfer of water mass between the atmosphere, the oceans and the continents, we estimated and analysed the time-variations of the water mass in these water mass reservoirs for a recent period of time by space gravimetry. For this purpose, we used the monthly GRACE geoids recently released by CSR and GFZ (04/2002-05/2004). The spatial resolution of the GRACE solutions was unfortunately limited to degree 10-15 (around 2000 km) by the presence of noise for the higher harmonic degrees. The water mass changes were also analysed using Empirical Othogonal Functions (EOFs) decompositions for characterizing the main modes of mass variability for each water reservoirs at seasonal and inter-annual time scales.
G31C-0803 0800h
Climate-Driven Deformation of the Solid Earth from GRACE and GPS
Analysis of the global gravity field measured with the GRACE tandem satellites from the period 2002.3--2004.3 indicates large seasonal variations of gravity which are assumed to be related to climate-driven fluxes of surface water. A seasonal redistribution of surface mass should deform the Earth, and our calculations based on the GRACE data suggest radial (i.e., vertical) deformations with an amplitude of $\sim$13~mm in the region of greatest flux, the Amazon River Basin. To test the GRACE gravity-hydrology connection, we analyzed data for the same time period acquired by continuously operating GPS receivers located in this region. After adjusting for the degree-one (i.e., center of mass) difference between the geometric (GPS) and geodynamic (GRACE) reference frames, we find that annual amplitudes of radial deformation measured with GPS correlate significantly with predictions calculated from GRACE gravity variations. These results experimentally confirm the variations in surface water sensed by GRACE, which have been shown to be significantly larger than those predicted by hydrology models. These results also demonstrate that GRACE can be an important tool for monitoring deformation of the Earth, and suggests that comparison of GRACE and GPS data may be a useful approach for estimation of geocenter variations.
G31C-0804 0800h
Effects of GRACE orbit decay on the gravity field recovery
Effects of satellite ground track changes of GRACE on monthly gravity field recoveries are investigated. Although GRACE provides monthly gravity field solutions through out the mission lifetime, it may not maintain to keep the same orbit due to the thruster fuel limitation. It causes temporal variation of ground tracks. Globally uniform ground tracks provide a preferable condition for the solutions, however large gaps in the ground tracks may be appeared at some specific orbit altitude. In case of a gravity field recovery using relatively short period of a month or so, the variation of ground tracks affects the precision of the gravity field solutions. It is a serious problem when the solutions are employed for detecting temporally gravity changes which are at almost detection limits. In this study, the recoveries of four-weekly gravity fields are simulated and the relation between the recovery precision and the ground track is investigated. The result shows that the GRACE ground track of year 2003 was in good condition for four-weekly gravity field recovery, but it will sometimes appear worse cases as the decay of orbit altitude. In those cases, the global standard deviations of geoid height errors will be about 4 times worse than the best case. We discuss a preferable temporal resolution in order to suppress such difference of the recovery precision per time period and to obtain the solutions of temporally homogeneous quality. A method to correct the large error solutions using ones in other periods is also discussed.
G31C-0805 0800h
GRACE KBR and Accelerometer Data Reduction and Calibration
The Gravity Recovery And Climate Experiment (GRACE), launched on March 17, 2002, represents the state-of-the-art in geodetic observations of the static and time varying components of the Earth's geopotential field. The fundamental measurement used to observe gravity is the inter-satellite range and range rate between two coplanar, low altitude satellites obtained from a K-band ranging (KBR) system. In addition to the K-band ranging system, each satellite possess a SuperSTAR Accelerometer, a GPS receiver/antenna package, Star Cameras and a Laser Retro Reflector (LRR) to complete the compliment of science instruments. The GRACE project has now released two years of Level 1B data derived from the science instruments and sensors. An integral component of our time variable gravity research is the reduction, calibration and analyses of these Level 1B data. In particular we have analyzed several months of K-band ranging (KBR1B), accelerometry (ACC1B) and GPS navigation (GNAV1B) data. Accelerometer calibration and KBR data reduction methodology and results will be presented. We discuss the impact of these analyses on the recovery of time variable gravity.
G31C-0806 0800h
Sensitivity of GIA Models With a Low-Viscosity Earth Layer to the Ice-Load History in Relation to the Resolving Power of GOCE
The GOCE satellite mission, which is planned by ESA for launch in 2006, is designed to map the static global gravity field with centimeter accuracy in geoid height at a resolution of about 100 kilometer. Features in such a high resolution gravity field can be associated with geophysical processes that act on a regional scale, as the response of shallow low-viscosity layers to loading and unloading of the crust in glacial isostatic adjustment (GIA). In GIA studies it is generally assumed that continental lithosphere has very high viscosity and is therefore effectively elastic. It can be expected however that parts of the lower crust have low viscosity. In a recent study we have shown that such a crustal low-viscosity zone (CLVZ) introduces variations in geoid height up to 1 meter with spatial scales down to hundred kilometers underneath and just outside formerly glaciated areas. We have shown that the response is sensitive to both changes in the properties of the CLVZ and the ice-load history. In this study we will further investigate this sensitivity by using two existing ice-load histories. To investigate the depth-dependence of the sensitivity, we will also compute the response for an asthenospheric low-viscosity zone (ALVZ) just below the lithosphere. We will show, using degree amplitudes, that for a CLVZ the differences are above the expected GOCE performance up to degree 120 and for an ALVZ up to degree 60. The latter is also true for the realized GRACE performance (GGM01C). This means that GRACE, but especially GOCE, might be able to provide information on the ice-load history in the presence of an LVZ. To extract information about the LVZ from the degree amplitudes, we show that it is possible to partly remove the influence of the ice-load history and thus compute spectral signatures for different properties of the LVZ. We will focus on the continental shelf-areas of western Europe and eastern North America to show the sensitivity to the inclusion of an LVZ of GIA corrections to estimates of present-day sea level rise.
G31C-0807 0800h
ESA's Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) Mission
The Earth's gravity field is the fundamental physical force for every dynamic process on its surface. With the Gravity Field and steady-state Ocean Circulation Explorer (GOCE) Mission as its first Earth Explorer core mission, the European Space Agency (ESA) is playing an important role in this `geopotential decade' by preparing for acquisition of a high quality, high spatial resolution gravity field and geoid for future scientific applications. GOCE combines an innovative new three-axis gravity gradiometer (EGG) instrument (comprising three x, y, z pairs of accelerometers with a baseline separation of 0.5 m) with a drag-compensating ion-propulsion system to measure for the first time the full gravity gradient tensor along its orbit at 250 km altitude. GOCE will carry a GPS satellite-to-satellite tracking navigation system for 3-dimensional positioning, star trackers for precise pointing knowledge, and a laser retroreflector for ground laser tracking. GOCE is specifically designed to make accurate and precise measurements of the stationary gravity field and gravity anomalies (to 1 mGal) at high spatial resolution (100 km). The data will facilitate the computation of a high spatial resolution (100 km) global geoid model to 1-2 cm accuracy. Applications of these products will be illustrated using examples in oceanography, solid-earth physics and geodesy. After a successful completion of the design consolidation phase, the construction phase for the GOCE satellite is presently underway, with an anticipated a launch in late 2006.
http://www.esa.int/livingplanet/goce
G31C-0808 0800h
Mass redistribution from inversion of global GPS timeseries and global time-varying gravity: inverse methodology, estimability and truncation
Monitoring global mass redistributions through their integrated gravitational attraction is the primary aim of the GRACE mission. There are, however, indications that is possible to achieve this independently through geometrically measuring the elastic response of the Earth to the loading effect of these (hydrological and oceanic) masses, eg with the IGS network. In any case, the analyst is forced to constrain any solved-for mass configuration either by low-degree truncation, by spatial averaging, and/or regularization operations. Combining satellite gravity and geometrical displacements in a joint inversion might help in improving the reliability of estimates, but in particularly it offers prospects for cross-validation and for a more realistic quality assessment. We lay out a concept for such a joint inversion. For the time being, we have analyzed global GPS time series publicly made available as IGS combination solutions. One can easily show that parametrization issues (truncation, spatial aliasing due to inhomogeneous station distribution) affect the estimation of low-degree mass redistributions like geocenter motion and seasonal $J_2$ changes from these data. We show how a biased estimation technique that applies only physically motivated constraints over oceanic areas renders the truncation problem in a less serious one. Linear combinations of spherical harmonic coefficients beyond degree 4 contain significant information. We quantify what the effect of adding GRACE monthly solutions in a joint inversion would be, dependend on their quality.
G31C-0809 0800h
Time-variable gravity anomaly arising from dynamical mass transport in the outer core and core-mantle interaction
The geomagnetic field has been generated and maintained by convective flow in the Earth's outer core through much of the Earth's history. The core convection varies on time scales of a few decades and longer. This process contributes to time-variable gravity mainly in two ways: (1) redistributing small density anomalies within the entire outer core, and (2) producing varying non-hydrostatic pressure loading on the core-mantle boundary (CMB). The combined gravity effect shall be displayed at the Earth's surface. Our previous studies indicate that the density anomalies in the core alone could produce a time-variable gravity signal observable at the Earth's surface. However, the effect of mantle deformation depends not only on the core pressure field on the CMB, but also the properties of the mantle. It is therefore necessary to understand the net effect via our MoSST core dynamics model. To achieve this, we add two "plug-in" modules to the system: one for gravity variation due to density anomalies in the core, and one on mantle deformation due to pressure field on the CMB. This study shall help identify the geophysical contributors to the time-variable gravity observed at the Earth's surface, thus facilitating multidisciplinary studies of core dynamics and interactions of the core with other components of the Earth.
http://mosst.gsfc.nasa.gov
G31C-0810 0800h
GRACE L2 Science Data Processing Flow at UTCSR
The release of GRACE science products (Level-2) and associated data products (Level-1B) took place on August 9, 2004. To support this activity this poster illustrates the process flow employed at the University of Texas, Center for Space Research (UTCSR) for the steps encompassing: ingesting, analyzing, calibrating, processing, verifying and the exporting associated with these products. The interested reader will be able to see a graphical representation of how the GRACE mission sensor and measurement data flow though the interconnected analysis processes which constitute the UTCSR Science Data System (SDS). This will provide a clearer understanding of the procedures used in generation of the GRACE science data products. Side notes, providing representative data characterizations and processing statistics, are included for illustration.
G31C-0811 0800h
GRACE Data Processing and Distribution at JPL PO.DAAC
The JPL Physical Oceanography Distributed Active Archiving Center (PO.DAAC) is supporting the GRACE mission as the component of the GRACE Science Data System that performs the Level 0 to Level 1B data processing. After retrieving the satellite telemetry from the Raw Data Center in Neustrelitz and processing it, PO.DAAC sends the resulting Level 1B data to both UTCSR and GFZ/ISDC, where each produce gravity fields. These gravity fields, as well as lower level data, are then archived and distributed by JPL PO.DAAC and the GFZ/ISDC. The estimates of time variations in the gravity field obtained from GRACE, in conjunction with other satellite and in-situ data, and geophysical models, will provide improved measurements of deep ocean currents, ocean bottom pressure, sea level rise, sea ice mass variation and distribution, soil moisture, groundwater transport, and land density. PO.DAAC provides global oceanographic data from spaceborne instruments and produces higher-level data products. In addition to ocean surface topography, sea surface temperature, and ocean vector winds, other holdings include data on ocean wave height, ionospheric electron content, atmospheric moisture, land/sea ice, and heat flux, as well as in-situ data related to the main satellite holdings. Instruments that provide these data include the following: Topex/Poseidon and Jason altimeters and radiometers, SeaWinds on QuikSCAT and ADEOS-II scatterometers, NASA Scatterometer (NSCAT), NOAA Advanced Very High Resolution Radiometer (AVHRR), Seasat scatterometer and altimeter, and the MODIS radiometer on the NASA Terra and Aqua satellites.
http://podaac.jpl.nasa.gov/grace
G31C-0812 0800h
Issues in the Comparison of Ground Gravity with GRACE Data
Agreement in general has previously been established between superconducting gravimeter (SG) data of the Global Geodynamics Project, GRACE satellite data, and hydrology models of soil moisture for a region in central Europe. Nonetheless, there are several issues still to be resolved before one can be convinced of the robustness of the correlation. In this paper we extend the previous work (years 2000-2002) with the most recent SG and GRACE data available. From the point of view of ground gravity, our previous atmospheric corrections for the European stations were insufficient. Here we include a 3-D atmospheric model that incorporates data on vertical structure and that is known to contribute a seasonal component of about 1 microgal into the gravity residuals. We also discuss the use of GPS data to represent a vertical displacement of the surface that affects grounds measurements but is not seen by the satellite. Perhaps the most difficult and controversial aspect is how to incorporate local hydrology into the SG data reduction. The spatial averaging of many ground-based stations, although in principle equivalent to a satellite-derived smoothing of the hydrology, is clearly problematic when using only a handful of stations. We here discuss the role of local topographic and hydrological models, especially where there is mass above the station as in an underground installation, in the analysis of the SG data. Using Strasbourg (J9) as an example installation, we present the results of initial modeling that show the potential benefit of using a sub-array of gravity measurements around an SG station to solve the problem.
G31C-0813 0800h
Filters for GARCE Data Analysis
Conversion from differential GRACE geoid to near surface mass variation is an unbounded high-pass filter generated by a singular spherical cap with a divergent harmonic spectrum. Low-pass smoothing windows have to be used for control of error amplification due to the spectral divergence of this so-called inverse geoid filter (IGF) We present the criteria for under-smoothing and over-smoothing. A properly smoothed IGF is characterized by a nearly symmetric dome-shaped harmonic spectrum. Ascending from the lower end of the spectrum preserves the high-pass nature of IGF, while descending from the peak of the spectrum controls the error. Smoothed IGF with dome-shaped spectrum represents a class of wiggled spherical caps that are no longer positive definite over the surface. We demonstrate that a properly smoothed IGF that is also robust in resolving the geometry of sub-surface mass sheets should have zero-mean over the surface. The zero-mean nature makes the smoothed IGF a mother wavelet of proper scale. We propose a reversed process for the design of smoothing windows, starting from an effective spherical wavelet with an arbitrary multiplier as the end-result IGF. The low-pass smoothing window is recovered from the end-result IGF by modulating it with the exact IGF, and the multiplier can be determined by scaling the amplitude of the smoothing window against mass conservation. We developed a new smoothing window from an effective analytical spherical wavelet based upon the new procedure. Comparative studies are conducted with the new IGF on GRACE data and predicted ocean bottom pressure series over several oceanic and Ocean-land mixed regions.
G31C-0814 0800h
Assessment of GRACE Time-Variable Gravity Observables: A New Filtering Technique to Enhance Signal Spatial Resolutions
A contemporary filter based on the Gaussian smoother assumes a degree dependent error in the satellite geopotential coefficient estimates (Wahr et al., 1998). It effectively damps the power of ill-determined higher spatial frequency estimates. Wahr et al. (2004) shows that the current release of GRACE L2 monthly geopotential data product has the sensitivity up to degree and order 15 in spherical harmonics, and that GRACE error dominates beyond degree 15. Here, we study the correlations between the GRACE estimates and combination of hydrology and ocean models, with an objective to investigate the possibility of preserving higher frequency GRACE signals from the L2 data product. In particular, we have developed a filter to optimize the degree and order dependence of the GRACE geopotential error. The filter is dependent on the correlation between the GRACE L2 and predictions based on geophysical models (i.e., hydrological and ocean models). In order to retain the GRACE coefficients showing reasonable correlation with the models, we applied Gaussian smoothers with different averaging radii to the different orders. It provides higher resolution in latitude and the same resolution in longitude as the standard Gaussian smoother. We present the results based on both filters and discuss the advantage of the new filter by processing two years of GRACE L2 geopotential coefficients.
G31C-0815 0800h
CURTIN SYNTHETIC EARTH GRAVITY FIELD MODEL (CurtinSEGM) - A GLOGAL FORWARD GRAVITY FIELD MODEL
Curtin Synthetic Gravity Field Model (CurtinSEGM) describes the Earth's gravity field by forward gravity modelling using mass-density information of topography, bathymetry (JGP95E), crust (Crust 2.0) and mantle (S12WM13). CurtinSEGM is parameterised by a set of fully normalised spherical harmonic coefficients for the disturbing potential and the corresponding geoid height. Although it is a synthetic (simulated) gravity field model the aim of CurtinSEGM is to represent the Earth's gravity field as realistic as possible. Therefore it also can be used for geophysical interpretation of the models used to describe the global mass-density distribution. On a global scale CurtinSEGM represents the general structure of the observed gravity field implied by the EGM96 geopotential model. However in some regions (e.g. Bay of Bengal) the model shows greater differences to EGM96, which perhaps can be attributed mainly to mass-density anomalies in the upper to middle mantle. This paper describes the construction of CurtinSEGM and presents numerical results for the geoid height and (free-air) gravity anomalies as induced by the model. The agreement/disagreement with the observed gravity field (EGM96) gives an additional insight in possible supporting mechanisms for these structures, as well as the capability of obtaining the (external) gravity field by forward gravity field modelling only.
G31C-0816 0800h
AUSTRALIAN SYNTHETIC EARTH GRAVITY FIELD MODEL (AUSSEGM) -V A RGIONAL EARTH GRAVITY FIELD MODEL
Australian Synthetic Earth Gravity Model (AusSEGM) is a regional source-effects model SEGM over the area of Australia. It uses a global geopotential model (GGM, here EGM96) as effects model to supply the long wavelength component, and forward modelling results of a regional simulated mass distribution of the topography with a 3-arc-sec by 3-arc-sec resolution as source model to supply the finer resolution. Both the GGM as well as the simulated local/regional mass distribution are assumed to be perfectly known and error-free. AusSEGM provides exact and self-consistent high-resolution simulated gravity-field functional (such as geoid heights and gravity observations). Thus AusSEGM is ideal for validating theories, techniques and computer software for regional geoid determination. This paper describes the construction of AusSEGM as well as first results. The coarse structure of the field was taken from EGM96 (Nmax = 360), whilst the finer structure was computed from a 3-arc-sec by 3-arc-sec simulated digital elevation model (DEM) over Australia. This first version of AusSEGM uses a constant density distribution of ? = 2670 kg/m3 for the topographic masses. The global 30-arc-sec by 30-arc-sec DEM GLOBE is the basis for the high resolution DEM over Australia, where the higher resolution has been obtained by adding a fractal surface. From these data, the effect on gravity field (potential and gravitational attraction) has been determined using Newton's integration. Furthermore, these effects have been separated into a long- and short wavelength part using a spherical harmonic expansion (up to degree Nmax). The high-pass filtered short-wavelength constituent has then been added to the long-wavelength information of EGM96. AusSEGM provides gravity values at the Earth's surface and geoid heights at regular geographic grid nodes (1-arc-min by 1-arc-min) as well as arbitrary points with a similar distribution as measured gravity points. The precision of the synthetic gravity and geoid data (after a first iteration) is estimated to be better than 30 YGal and 3 mm, respectively.
G31C-0817 0800h
GRAVITY FIELD CHANGES DUE TO LONG-TERM SEA LEVEL CHANGES
Long-term sea level changes caused by climatic changes (e.g. global warming) will alter the system Earth. This includes the redistribution of ocean water masses due to the migration of cold fresh water from formerly ice-covered regions to the open oceans mainly caused by the deglaciation of polar ice caps. Consequently also a change in global ocean circulation patterns will occur. Over a longer timescale, such mass redistributions will be followed by isostatic rebound/depression due to the changed surface un/loading, resulting in variable sea level change around the world. These, in turn, will affect the gravity field, location of the geocentre, and the Earth's rotation vector. This presentation focuses mainly on gravity field changes induced by long-term (hundredths to many thousand years) sea level changes using an Earth System Climate Model (ESCM) of intermediate complexity. In this study, the coupled University of Victoria (Victoria, Canada) Earth System Climate Model (Uvic ESCM) was used, which embraces the primary thermodynamic and hydrological components of the climate system including sea and land-ice information. The model was implemented to estimate changes in global precipitation, ocean mass redistribution, seawater temperature and salinity on timescales from hundreds to thousands years under different greenhouse warming scenarios. The sea level change output of the model has been converted into real mass changes by removing the steric effect, computed from seawater temperature and salinity information at different layers also provided by Uvic ESCM. Finally the obtained mass changes have been converted into changes of the gravitational potential and subsequently of the geoid height using a spherical harmonic representation of the different data. Preliminary numerical results are provided for sea level change as well as change in geoid height.
G31C-0818 0800h
Airborne LaCoste&Romberge Gravimetry; an Alternative Computation Approach
The problem of separation of the accelerations sensed by an airplane and the gravity signal at flight height is the main problem of airborne gravimetry. The generally used method is the low pass filtering of the measured parameters. This is based on the assumption that the airplane induced accelerations have a high frequency nature while the gravity acceleration is a low frequency signal. The cut-off frequency is usually not higher than 0.01 Hz. This corresponds to 6 km at a mean speed of 60 m/s, meaning that the gravity acceleration with lower wavelength is overfiltered. This is basically because of the numerical differentiation of the time series. We have developed a method based on integral equations and least squares estimation. By taking into account the covariance matrices of the observations and the a priori unknowns, as well as the the mathematical formulation of the gravimeter as a spring-damper system, we obtain results with an improved contents on the high frequency band. The method is applied to the data acquired over the French Alpes mountains.
G31C-0819 0800h
Comparison of GRACE mean gravity fields with airborne gravity measurements
Validation of the mean GRACE gravity field is difficult because of the lack of sufficiently long-wavelength surface gravity measurements with which to compare. One measurement technique that can provide such a comparison is airborne gravity conducted from long-range aircraft. The Naval Research Lab (NRL) has conducted extensive aerogravity surveys over the Arctic Ocean and the Gulf of Mexico that provide opportunities for comparison with the mean GRACE field. Such surveys can provide 1-D comparisons with the longest airborne tracks (1200 - 1600 km long) and 2-D comparisons over gridded regions. We present the results of comparisons with both the GGM02S fields at degree and order 160 and truncated to degree/order 120 and with the GGM02C field.
G31C-0820 0800h
Combining GRACE Data With Sea Surface and Terrestrial Gravity Data for Global Geopotential Modeling
The optimal combination of the very accurate gravity information from GRACE with terrestrial and marine gravity information is a challenging task. One aspect of the challenge is to preserve the very high accuracy of the low-degree field estimates from GRACE while blending this information with terrestrial gravity data. A second aspect is to use a technique that requires no assumptions about density for reducing data to a single surface, or puts any restrictions on data type, geographic distribution, or weighting. Using a new out-of-core algorithm, a rigorous estimate of a full 360x360 field (with full covariance) has been obtained using a single IBM P690 node with only 32 GB of memory. Unlike traditional techniques, this method does not use block diagonal or quadrature-like methods and is not limited in the maximum degree of the full covariance. The combination field is shown to successfully blend the accurate, low-degree information from GRACE with high degree information from other gravity data. Its evaluation with respect to its application to oceanography, leveling and orbit determination shows it to be superior to previously available mean Earth gravity fields.
http://www.csr.utexas.edu/grace