Advanced accelerometer/gradiometer concepts based on atom interferometry

M. Schmidt, A. Senger, M. Hauth, S. Grede, C. Freier, A. Peters
Humboldt Universität zu Berlin, AG Optische Metrologie, Hausvogteiplatz 5-7, 10117 Berlin, Germany

Since 1992, matter wave interferometry has been used in many laboratories for a variety of fundamental physics experiments, e.g. measurement of the fine-structure and gravity constants. However, due to the complexity of these experiments, they were confined to laboratory environments. In recent years, however, efforts have been undertaken to develop mobile atom interferometers. These new sensors open up the possibility to perform on-site high-precision measurements of rotations, gravity gradients as well as absolute accelerations.

We present the basic principles underlying atom interferometers. Furthermore, we report on the status of different projects that are currently investigating and testing possible applications of mobile atom interferometers. These possibilities include earthbound mobile gravimeters as well as future employment of this technology in earth observation satellite missions. Finally, we discuss a variety of possible sensor configurations for earth orbit gravity measurements in GRACE / GOCE follow-up missions.

Transportable optical clocks: towards gravimetry based on the graviational redshift

A. Görlitz(1), P. Lemonde(2), C. Salomon(3), S. Schiller(1), U. Sterr(4), G. Tino(5) and the SOC team
(1) Univ. Düsseldorf,
(2) SYRTE Paris,
(3) ENS Paris,
(4) PTB Braunschweig,
(5) Univ. Firenze & LENS

The frequency difference between atomic clocks located at different locations in a gravitational field is fundamentally determined by the difference in gravitational potential. With the new generation of atomic clocks now under development, optical atomic clocks, it should be possible to achieve an equivalent height resolution at the 1 cm level. This type of measurement is a local measurement, complementary to the well-established satellite gravimetry approach. A useful implementation would be a reference optical clock on a satellite, to which a network of (transportable) optical clocks on the ground are compared. Transportable optical clocks demonstrators based on ensembles of neutral atoms in an optical lattice are under development within an ESA/DLR-sponsored project, as a first step towards space clocks. The status and perspectives of the project will be presented.

Long Lifetime Orbits for the German Lunar Exploration Orbiter

Peter F. Gath
ASTRIUM GmbH Satellites, Friedrichshafen

In the context of the German Lunar Exploration Orbiter mission LEO, one mission objective is the improvement of the lunar gravity field models. Current models are based on tracking data from the very first lunar probes, the Apollo program, and more recently the Lunar Prospector mission. A comparison of the historic lunar gravity models with the more recent models that include the Lunar Prospector data reveals large differences in the zonal harmonic coefficients. The largest errors that are contained in the current gravity field models occur on the backside of the Moon due to missing ranging data.

In order to achieve a significant improvement, a pair of satellites forming a GRACE-like constellation is considered as one element of the LEO mission. Such an approach can significantly improve the orbit modeling on the far side of the Moon. In order to obtain a global coverage for the gravity field, the satellites need to fly in a low altitude polar orbit around the Moon. Due to the asymmetry in the Moon's gravity field, such orbits are known to be highly unstable due to a large disturbance that changes the eccentricity vector and thus leads to a deformation of the orbit which ultimately yields a crash of the satellite into the Moon's surface.

In the context of the LEO Phase 0 and A studies, a range of almost polar orbits with an average altitude of 50 km were identified which exhibit a good stability and long lifetime. Such orbits are known as frozen repeat cycle orbits since their average eccentricity vector is not changing and their ground track is repeating. Due to the very slow rotation of the Moon, the repeat cycle is at least one lunar month (approx. 28 days). As illustrated in Figure 1, the eccentricity vector is changing during this period, but the pattern is closed after one lunar month. The same applies for the ground track. A 3D representation of the orbit is shown in Figure 2 with the altitude scaled by a factor of 40. The orbit is designed for a nominal lifetime of 4 years. It was analyzed in more detail with a Monte Carlo simulation in which the different gravity field parameters were varied according to the reported accuracies in the lunar gravity field model. It turns out that a lifetime of at least 100 days can be guaranteed in the presence of the assumed uncertainties and that a the chances of surviving a full year are around 80% if no orbit maneuvers are applied (see Figure 3).

Figure 1: Eccentricity vector diagram for the baseline reference orbit shown for one lunar month

Figure 2: 3D representation of the reference orbit (altitude scaled by a factor of 40)

Figure 3: Survival rate of the LEO reference orbit without orbit maintenance applied

The mission lifetime can be extended by applying eccentricity vector correction maneuvers at least once per lunar month and an inclination correction maneuver approximately every 10 lunar months. The inclination correction maneuvers become necessary due to changes in the Moon's rotational axis orientation which effectively yields a small change in the ground track and thereby in the inclination w.r.t. a coordinate frame that is fixed to the Moon's current rotational axis. An extensive analysis was carried out for LEO's main satellite for which a lifetime of four years must be guaranteed. This results in a fuel consumption of approx. 1.1 m/s per lunar month. Of course, the lifetime of the sub-satellites performing the gravity field measurements could also be extended by using the same approach, depending on the fuel that is finally available for orbit maintenance maneuvers. However, the number of maneuvers for the gravity mission should be minimized in order not to disturb the measurements too often. In conclusion, a reference baseline orbit has been found that is suitable for gravity field measurements that allow for a global coverage of the Moon's gravity field and require no orbit maintenance for the first 100 days.

Local vs Global Methodologies for Gravity Recovery: A GRACE Perspective

F. G. Lemoine, D. D. Rowlands, S. B. Luthcke, J. P. Boy, T. J. Sabaka, R. D. Ray, S.C. Han

The recovery of time-variable gravity from a GRACE-type measurement system poses challenges. The desired signal (usually taken to be hydrology, the cryosphere, ocean mass, or the solid earth) is embedded in other high-frequency signals such as the atmosphere, high-frequency variations in the oceans, and ocean tidal variations. The approach to recover these mass variations has been to forward model as best as possible these ‘extraneous’ mass variations. Herein we concentrate on the methodology of recovering the time-variable signal. We compare a local recovery (mascons) using embedded spatial constraints with a global recovery using unconstrained spherical harmonics and show the advantages of this approach. We illustrate how appropriate forward modeling can reduce the aliased signal and leakage appearing in the gravity solutions, regardless of technique. We provide a brief update on tidal modeling in the era of GRACE and show how even a GRACE spherical harmonic time series can still provide a useful time series of the low degree coefficients, such as C_20. Finally we offer a perspective on the resolution of geophysical signals, with a view of what a future GRACE-Follow-On mission might attempt to recover.

Methodological aspects related to gravity analysis from future missions

Torsten Mayer-Gürr, Annette Eicker, Enrico Kurtenbach, Jürgen Kusche
Institute for Geodesy and Geoinformation, Bonn University

Experience with CHAMP and GRACE has shown that ‘alternative’ methodologies to process orbit and inter-satellite ranging data to gravity field models are competitive to the conventional dynamic method. We expect this will be the case for data analysis from future (i.e. GRACE-FO) missions as well. After a brief review on the ‘Bonn method’ (that essentially solves a series of temporal boundary value problems for an orbit or for inter-satellite observables, instead of initial-value problems), its implementation and current results, we will discuss aspects related to future missions where the data will be of even higher accuracy.

Can geodetic data be used as a complement to satellite gravity data in the future?

van Dam, T.
University of Luxembourg, Luxembourg

Terrestrial and space geodetic data are sometimes used to address spatial and temporal sampling deficiencies in dedicated gravity missions. The most obvious example is the contribution of space geodesy to determining degree-1. Other examples include degree-2 from SLR (which is peculiar to GRACE data usage) and the more general use of extracting a load from observed surface displacement.

As we move towards longer time-series and towards more precise gravity missions, the requirements on these types of space geodetic data products will change. This presentation will attempt to address a few questions regarding the future role of space geodetic data as a complement to dedicated gravity missions including: What are the realistic prospects for improving the supplementary space geodetic data? What areas need work to make this happen? What are the significant challenges?

Background models used in geodetic data processing

P. Gegout (1), F. Lyard (2), J.-M. Lemoine (3)
(1) DTP/GS (2) LEGOS (3) CNES/GS, Observatoire Midi-Pyrenees, 14, avenue Edouard Belin, 31400 Toulouse

The atmosphere, non-tidal oceans and hydrological variations fall into a class of models that can only be represented in empirical time-series; and are generally not used in a predictive sense in geodetic data processing. There are several notable common attributes of these series that affect their use in geodetic data processing. Their quality depends on location. They are outcomes of non-geodetic activities, so the attributes important for their use in geodesy are not always safeguarded (e.g. continuity, uniformity of standards, or long-term trends). This presentation surveys what we know of these issues as relevant to geodesy, and examines the contrast between the availability and the requirements for future gravity missions.

LOCAL ANALYSIS APPROACH FOR SHORT WAVELENGTH VARIATIONS IN THE GEOPOTENTIAL

Bender, P. L. (1), and Wiese, D. N. (2)
(1) JILA, Univ. of Colorado and Nat. Inst. of Standards & Technology, Boulder, CO 80309, USA
(2) Dept. of Aerospace Engineering Science and Colorado Center for Astrodynamics Research, Univ. of Colorado, Boulder, CO 80309, USA

The value of global spherical harmonic analyses for determining 15 day to 30 day changes in the Earth's gravity field has been demonstrated extensively using data from the GRACE mission and previous missions. However, additional useful information appears to be obtainable from local analyses of the data. A number of such analyses have been carried out by various groups. In the energy approximation, the changes in the height of the satellite altitude geopotential can be determined from the post-fit changes in the satellite separation during individual one-revolution arcs of data from a GRACE-type pair of satellites in a given orbit. For a particular region, it is assumed that the arcs crossing that region during a time T of interest would be used to determine corrections to the spherical harmonic results. The shortest wavelengths that can be determined are the most important for investigating the spatial distribution of mass changes with high resolution.

While the longer wavelength variations are affected by mass distribution changes over much of the globe, the shorter wavelength ones hopefully will be determined mainly by more local changes in the mass distribution. The method of putting together results from different arcs crossing the region of interest with the energy approach has been used frequently before. It can be referred to as along-track analysis. The advantages of this approach can be seen most easily for an orbit with moderate inclination, such that the crossing angle between south-to-north (S-N) and N-S passes is fairly large over most regions well away from the poles. In that case, after filtering to pass the shorter wavelengths, the results for a given time interval can be combined to give the short wavelength W-E variations in the geopotential efficiently.

Questions have been raised about the effectiveness of local analyses, since changes in the geopotential at satellite altitude over a given region come from mass distribution changes over the whole globe. But, the main issue in considering higher measurement accuracy in future missions is how much improvement in spatial resolution can be achieved. For this, it is the shortest wavelengths in the geopotential spatial variations that are most important. To check on the effects of local versus global mass distribution changes, some preliminary calculations have been carried out for a roughly 2,000 by 2,000 km region in western and central North America. The results from these calculations indicate that most of the variations for wavelengths less than 1400 km will come from mass changes in the region considered.

Future missions are expected to have much higher accuracy for measuring changes in the satellite separation than GRACE. However, how large an improvement in the derived results in hydrology will be achieved is still very much a matter of study, because of the uncertainty in time variations in the atmospheric and oceanic mass distribution. To be specific, we will assume that improving the spatial resolution is the top objective, the satellite altitude is in the range of roughly 300 to 360 km made possible for long missions by drag-free operation, and ask first what information can be obtained for a single pass across a given Gaussian disk of water.

This single disk measure of performance is clearly much too optimistic because it ignores the complication of not knowing the geometry of the water distribution in the rest of the area of interest. However, it appears to give some insight into the spatial resolution issue, pending further studies. The major limitation is that the wavelengths of the variations in the satellite separation depend mainly on the altitude, until the half-thickness radius R of the disk becomes comparable with the altitude. Thus the measurement accuracy necessary in order to determine R is considerably higher than to just determine the total mass of the disk.

Some preliminary and approximate results will be given for both K-band and laser interferometer distance measurement systems, with and without some atmospheric mass variation uncertainty included. Although these results have been obtained only for a single pass across the region of interest, similar results for crossing S-N and N-S paths can be combined to give the short wavelength W-E variations when the crossing angles are fairly large. But more complete studies including information on expected patterns in the spatial distribution of the hydrological mass changes certainly are needed. A preliminary model probably would be uniform water depth changes in individual river basins. However, it is hoped that the resolution would be high enough to detect something like different changes in different distance ranges from the midlines of the basins, at least for the larger basins.

Can we achieve GRACE data continuity? France’s contribution.

S. Hosford (1), R. Biancale(1), G. Balmino(1), S. Bruinsma(1), A. Laurens(1), M. Diament (2), I. Panet(2)
(1) CNES, 18 avenue Edouard Belin, 31401 Toulouse, France (e-mail: steven.hosford@cnes.fr,
(2) Institut de Physique du Globe de Paris, Géophysique spatiale et planétaire - Bâtiment Lamarck, Case 7011, 5 rue Thomas Mann, 75205 Paris Cedex 13, FRANCE

As a contributor to geophysics missions over the past 15 years, the French space agency (CNES) has a long established procedure for deciding missions in response to the needs of the national scientific community defined during regular "Future Missions Workshops." Following a short description and history of this process, the priority missions resulting from the latest such workshop which took place in Biarritz in March 2009 will be briefly described. In the context of the French national space budget where bilateral missions are now the norm, we will discuss the potential for international collaboration on a future gravity mission and the steps being taken in France and by CNES to facilitate this.

A Satellite Laser Interferometry Gravity Mission Study

Yun-Kau Lau
Institute of Appl. Maths., Chinese Academy of Sciences, Beijing, China.
On behalf of the Space Advanced Gravity Measurements (SAGM) working group. Current member institutes:
Academy of Mathematics and System Sciences, CAS,
CAS Nanocenter, Beijing,
Dong Fang Hong Satellite Company,
Huazhong University of Science and Technology,
Institute of Atmospheric Physics, CAS,
Institute of Mechanics, CAS,
Institute of Physics, CAS,
Shanghai Engineering Center for MicroSatellites, CAS,
Wuhan Institute of Geodesy and Geophysics, CAS,
Wuhan Institute of Physics and mathematics, CAS.

This presentation outlines an ongoing, informal feasibility study of a low-low satellite-to-satellite laser interferometry tracking scientific satellite mission to map the Earth’s temporal gravity field within the Chinese microgravity community. This prospective mission is planned to be launched after the NASA/DLR Gravity Recovery and Climate Experiment (GRACE), or concurrent with or after the scheduled NASA GRACE Follow on mission. Unlike the current GRACE mission, we intend to use two co-orbiting drag-free satellites with laser ranging in place of microwave ranging to enhance the precision of satellite-to-satellite tracking measurement, to infer and to measure variations of the Earth’s gravity field. The scientific justification of the satellite mission includes but not limited to, quantification of the Asian continent and global hydrology, co-seismic and post-seismic deformations resulting from earthquakes and continental geodynamics, and observables driving or resulting from global climate change. At present, the preliminary baseline design parameters under consideration include: (1) the distance between the two co-orbiting spacecrafts: 50–100 km, (2) the precision of the laser interferometric measurement: 1–10 nm, (3) the altitude with drag-free control: 300–350 km, (4) the residual acceleration: 10-10 to 10-12 ms-2(Hz)-1 at around 0.1 Hz, and (5) nominal mission time: four to five years. In-depth instrument error analysis being conducted includes laser metrology, pointing stability and development of reference sensors. Sensitivity analysis is ongoing and one of the critical issues being addressed is the temporal aliasing of signals from errors from background forward models (for example, barotropic atmosphere and ocean responses and tides) as well as the desired signals to be measured. With potential international collaborative partners, collectively one could address of feasibility of formation flights or optimal constellations involving two or even three pairs of GRACE-type satellites to significantly minimize observability, temporal or spatial aliasing problems.

A brief introduction about promoting geo-gravity survey and satellite geo-gravity in China

Ye S.H. (1), Ping J.S. (1), Hu X.G. (1), Shen X.H. (2), Li H. (3)，Wang J.S. (4), Wu Y. (3), and Others (5)
(1) Shanghai Astronomical Observatory, CAS
(2) China Earthquake Administration
(3) Hubei Earthquake Administration, CEA
(4) Satellite Weather Research Center, CMA
(5) Prospective Additional Team Members from Other AgenciesShanghai Astronomical Observatory, CAS

Satellite geodetic and remote sensing techniques have been routinely used and play important roles in weather forecasting, surveying resources and monitoring/mitigating natural hazards in China. These satellites include meteorological satellites, the oceanographic satellite (HY-2) and another geodetic satellite, which are scheduled to launch in 2010. The Chinese Academy of Sciences (CAS), in potential future collaborations with other agencies, has identified a wide range of scientific rationales and applications within China and globally which requires a long range planning of satellite geodetic and remote-sensing missions, including satellite gravity missions. One such program is China’s Geo-Hazards Monitoring Program. Under the framework of this Program, the planned satellite missions include the electromagnetic satellite mission to monitor space weather, to study co-/post-seismic earthquake signals, and to explore the feasibility of the potential detectability of earthquake pre-cursor signals; as well as a near-infrared remote sensing satellite for geo-hazard imaging. A GRACE-type satellite gravity mission with enhanced accuracy and spatial resolution directly addresses some of the objectives of the Geo-Hazards Monitoring Program.

The overall identified scientific rationale for a long-term plan for gravity mapping satellite missions include, but not limited to, the efficiency, accuracy and potential timeliness from such global gravity field mapping satellites, the monitoring and potential mitigations of geo- and environmental hazards, the accurate observables enabling monitoring and quantification of anthropogenic climate change, water balance budget and water resource availability, especially within China. This presentation include the articulation of scientific interest within the Chinese scientific and other community to take part or to undertake GRACE-type satellite gravity field mission, and a first-cut plan for a possible roadmap and mechanism to support the realization of a GRACE Stop-Gap mission in the appropriate time frame to minimize data loss between GRACE and GRACE follow-on missions. The plan includes the following up of the Workshop Roadmap to discuss with international partners including the US, Germany, France and others to potentially realize a GRACE Stop-Gap mission or a GRACE Stop-Gap constellation mission, on: (1) a possible offer of a couple of 10–12 m radio tracking ground support system for the GRACE Stop-Gap mission(s), (2) the possibility of launch support of the GRACE Stop-Gap satellites, and (3) the feasibility of more than one pair of enhanced GRACE-type satellites with distinct inclinations and at lower altitudes. The ongoing discussion of formal collaborative agreement between Chinese Academy of Sciences and NASA on specific cooperation on space geodetic infrastructures and satellite missions should help the possibility of a potential collaboration of GRACE Stop-Gap satellite mission. The planned GRACE laser interferometric tracking instrument development (talk by Professor Lau in this Session, Project Leader: Professor Hu, Institute of Mechanics, CAS) will be part of the long-range plan to develop satellite gravity missions.

NASA's Plans

Michael Watkins
Jet Propulsion Laboratory, Pasadena, CA, USA

In this talk we will discuss NASA's current and future plans for the measurement of the Earth's mean and time varying gravity field. This strategy consists largely of 4 elements: 1) the maintenance of GRACE in extended mission for as long as possible 2) a robust Research and Analysis program for the analysis of both GRACE data and any and all related studies, including satellite altimetry, GPS, GOCE, and many others 3) Analysis and technology development related to both short and long term future gravity missions, including both science requirement and engineering development such as sub-nanometer laser interferometry, and 4) specific implementation of follow-on missions to GRACE, per the decadal survey and as part of multi-national efforts. This is focused currently on a fairly short term continuation of the critical climate-related data being provided by GRACE, per the recommendation of several NASA study teams, but also includes analysis of complex concepts for later implementation.

ESA’s Perspective

Roger Haagmans
European Space Agency, Roger.Haagmans@esa.int

The aim of the presentation is to shortly introduce the activities run by the Agency related to potential future gravity field mission concepts. Furthermore, the programmatic aspects for proposing missions in the competitive ESA Earth observation context and the guiding science strategy for future missions will be reviewed.

Thursday, October 1, 2009

Mission scenarios and system architecture concepts for a Next Generation Gravity Mission

Alberto Anselmi(1), Stefano Cesare(1), Miguel Aguirre(2)
(1) Thales Alenia Space Italia, Torino, Italy
(2) ESA-ESTEC, The Netherlands

Since 2003, the European Space Agency has promoted assessment studies and technology developments in preparation of a future space gravity mission based on the Satellite-to-Satellite Tracking technique with laser metrology. Thales Alenia Space Italia has led three of these studies in which, in particular, different mission scenarios and mission architectures have been investigated, a first measurement error budget has been established and a preliminary top-down requirements allocation to the satellite elements has been performed.

The simplest mission scenario capable to potentially achieve the required amplitude resolution in the gravity field temporal variation monitoring consists of two satellites flying along the same sun-synchronous, circular orbit at 10 km relative distance and 325 km mean altitude for at least 6 years. In this scenario, a payload (laser metrology and accelerometers) capable to measure the inter-satellite distance with an error spectral density , and the non gravitational accelerations on the satellites with an error spectral density , will in theory enable the determination of the geoid height variation with an error = 0.1 mm/year at .

Since this basic mission scenario still does not meet the objectives of improving the temporal resolution in the gravity field temporal variation monitoring and the separability of the geophysical signals, more complex scenarios have been assessed. They consists essentially in flying more satellite pairs on orbits with different inclinations and/or adopting different formation geometries (e.g. cartwheel, pendulum-type). Any of these deviations from the basic scenario has important implications on the satellite configuration, the payload instruments, the formation, drag-free and attitude control system, the mission operations, and thus on the complexity and cost of the mission. Therefore a careful trade-off between the scientific return and the “affordability&lrquo; of each scenario is mandatory for identifying the best way of implementing such Next Generation Gravity Mission.

Laser interferometry for future satellite gravity missions

Benjamin Sheard, Marina Dehne, Christoph Mahrdt, Gerhard Heinzel and Karsten Danzmann
Albert-Einstein-Institut Hannover, Callinstraße 38, 30167 Hannover, Germany

The application of laser interferometry for use in satellite gravimetry using satellite-to-satellite tracking will be discussed. At present we are focusing on a `GRACE-like' configuration with two satellites where the inter-spacecraft distance is measured using an on-axis heterodyne interferometer with an active transponder. The feasibility of an off-axis interferometer design and operation without drag-free attitude control is also being investigated. The status of a laboratory demonstrator of the interferometer concepts will be given.

Performance of various satellite gravimetry concepts for monitoring mass transport in the Earth's system

R. Klees, J. de Teixeira da Encarnacao, P. Ditmar, and B.Gunter
Delft University of Technology, Faculty of Aerospace Engineering, Delft Institute of Earth Observation and Space Systems (DEOS)

The Gravity Recovery and Climate Experiment (GRACE) satellite mission, which has been acquiring data since 2002, demonstrated that satellite gravimetry is a powerful tool to monitor mass transport in the Earth's system. Models based on GRACE data are extensively used in various Earth sciences, including hydrology, climatology, oceanography, physics of the solid Earth, and others. However, the GRACE mission is characterized by some intrinsic limitations. First of all, errors in the models based on GRACE data show a strongly anisotropic pattern. This can be explained by the fact that the main observation technique of the GRACE mission - K-band ranging - delivers data, which can be interpreted as measurements of gravitation difference between the locations of the two satellites forming the GRACE mission. Since the satellites follow each other in a nearly polar orbit, they are located most of the time at nearly the same meridian. Consequently, the collected observations describe North-South variations of the gravitational field (and mass transport) much better than East-West variations. Secondly, the GRACE satellites re-visit each geographical location, roughly speaking, only once per month. This does not allow rapid mass transport processes to be monitored and, even worse, results in temporal aliasing, i.e., an unpredictable and destructive propagation of high-periodic signals into the monthly gravity field models.

A number of alternative satellite gravimetry concepts can be designed to overcome the limitations of the GRACE mission. Some of them have been introduced already a number of years ago. For instance, the so-called "pendulum" configuration allows information about East-West spatial variations to be collected. Alternatively, the "cartwheel" configuration implies that both along-track and vertical gravitation differences are measured.

We propose two novel satellite gravimetry concepts. Firstly, we consider a pair of satellites separated by a vertical offset and connected by a tether. Secondly, we discuss a constellation of multiple cheap satellites equipped with high-quality GPS receivers, so that a spatially isotropic "snapshot" of the Earth's gravitation and mass distribution can be generated as frequently as once in a few days or even hours.

The performance of various satellite gravimetry concepts and their synergetic effects are analyzed by mean of numerical simulations. In this way, we are able to draw conclusions regarding strong and weak points of each of these concepts.

Enhancing GRACE/GRACE Follow-on Products for Hydrological Applications

Matt Rodell
NASA Goddard Space Flight Center

The potential of GRACE and GRACE Follow-on missions to benefit hydrological research and water resources applications is enormous, because no other measurement method has ever provided global maps of the integrated quantity of water stored on and below the land surface. However, three factors have limited the acceptance of GRACE as a tool for such applications: (1) its spatial and temporal resolutions are low relative to other hydrological observing systems; (2) the current product latency greatly diminishes its value for operational applications; (3) total terrestrial water storage is a measurement unfamiliar to hydrologists used to dealing with discrete measurements of specific elements of the water cycle. These issues might be alleviated by hardware and processing improvements in a GRACE Follow-on mission, but they can also addressed using innovative methods such as data assimilation into a model which synthesizes observations of multiple variables from a variety of sources.

GRACE impact in geodesy and geophysics

R Biancale (1), M. Diament (2)
(1) GRGS-CNES Toulouse
(2) IPG Paris

After more than 7 years in orbit the GRACE satellites continue to successfully fulfil their mission in providing exceptional quality data of the Earth gravity field variations. Several groups among JPL, UTEX, GFZ, GRGS, GSFC… are currently delivering maps of these changes which provide us estimates of mass transfers at large scales (down to 400 km at best) over 10-day or monthly time spans.

The low Earth Satellite-to-Satellite Technique (SST) has then proved its ability to accurately monitor the geoid temporal variations. These are controlled by all mass transfers in the Earth system, the dominating ones being of hydrological origin. Beside the hydrology and glaciology sciences, geodesy and geophysics have benefited as well from the GRACE advances.

The improved static gravity field models of the Earth (by a factor 100 up to spherical harmonic degree 150 compared to former models) and slow temporal variations of low degree (< 50) coefficients allow to reduce now the modelling of gravitational Earth perturbations to millimetric level for low Earth orbiting satellites, leaving the non-gravitational perturbation errors preponderant. The recently launched GOCE mission will complete homogeneously the good resolution of models up to spherical harmonic degree 250, without to compete with GRACE for low degrees anyway. &In conjunction with other types of geodetic data GRACE has allowed as well to tackle two particular challenges in geophysics: discriminating between GIA signal and ice mass changes; studying the co-seismic mass displacement caused by large earthquakes such the Sumatra ones and associated post-seismic variations. This brought very new information on the mantle viscosity and the seismic cycle. &We propose to discuss these aspects and to outline as well desirable improvements and data product requirements for a follow-on mission.

Products based on regional modelling approaches for future satellite gravity missions

Isabelle Panet(1), Annette Eicker(2), Shin-Chan Han(3), Guillaume Ramillien(4), Michael Schmidt(5), C.K. Shum(6)
(1) Institut Géographique National, Marne-la-Vallée, France
(2) Bonn University, Institute of Theoretical Geodesy, Bonn, Germany
(3) NASA GSFC, Planetary Geodynamics Laboratory, Greenbelt, Maryland, USA
(4) Géodésie Spatiale, CNRS DTP UMR 5562, OMP, Toulouse, France
(5) Deutsches Geodätisches Forschungsinstitut, München, Germany
(6) Ohio State University, Division of Geodetic Science, Columbus, Ohio,USA

For many scientific applications of satellite gravity, one needs to derive the gravity field over an area of interest with the best possible precision and resolution, and to separate a local geophysical signal under study from the other sources of variability. For that aim, regional models of the gravity field and its sources expressed in terms of equivalent water thickness have been developed over the last years by different research teams following various approaches, and applied to the GRACE data. In this presentation, we first summarize and compare the different techniques to derive regional or global/regional solutions, using harmonic localized functions or fully discrete parameterizations: splines, hybrid spherical harmonics/splines models, multiresolution representations using wavelets, mascons, grids... Then, we discuss the interest and the use of the products derived from the abovementioned approaches by direct regularized modelling of the satellite gravity measurements or by post-processing of the global solutions, for the geoscientific user’s communities.

Filtering techniques and their potential application for generating new products for GRACE / GRACE-FO

Jürgen Kusche
Institute for Geodesy and Geoinformation, Bonn University

Experience with GRACE has shown that scientific applications of the standard Level-2 products (monthly geopotential models) require a spatial averaging. This is most likely due to background modelling errors in combination with the measurement geometry, and with an amplification of errors which is inherent to the problem of converting potential change at altitude into surface mass change.

Spatial averaging is usually implemented by isotropically or anisotropically filtering the coefficients, i.e. forming linear combinations of the original coefficients. A number of strategies have been proposed for finding weighting factors. The same effect can be achieved through regularizing the solutions on the level of normal equations.

We will review these techniques, and the new products that were generated by their application, for GRACE. We will consider their potential application to products that might be derived from GRACE-FO missions.

GRAVITY FIELD MISSIONS: STATUS & STUDIES FOR FUTURE MISSIONS

Visser, P.N.A.M.
Delft Institute of Earth Observation and Space Systems (DEOS) Delft University of Technology Kluyverweg 1, 2629 HS, Delft, The Netherlands

The ability of observing gravity from space with ever increasing spatial and temporal resolution has been and is being demonstrated by missions such as CHAMP (launch July 2000, proof of concept for GPS high-low satellite-to-satellite tracking - SST - in combination with a precise accelerometer), GRACE (launch March 2002, microwave low-low SST plus GPS and accelerometers), and GOCE (launch March 2009, ultra-precise accelerometers composing a gradiometer plus GPS and drag-free control).

The GRACE mission is already going strong for more than seven years (status August 2009) and is producing longer and longer time series of temporal gravity at typical spatial scales of 500-1000 km and temporal resolution of 10-30 days. Striking results are obtained in observing mass changes e.g. due to continental hydrology and due to ice mass changes in polar areas (Greenland in particular). In addition, the accumulation of a long time series of GRACE observations allows the modeling of a mission mean gravity field with increasing spatial resolution. GOCE is foreseen to enter the first operational observation phase soon (again status August 2009) and is ready to provide observations that will significantly further enhance the spatial resolution down to 100 km or better. The observation of gravity in terms of spatial and temporal resolution by missions like GRACE and GOCE is not only limited by the very nature of their sensor complement and orbital characteristics, but also by imperfections in the post-processing of the observations such as uncertainties in so-called de-aliasing products. Whereas the impact of the latter can be (and is) mitigated by ongoing efforts to improve models and processing schemes, the impact of the first can only be overcome by improving sensor systems and/or mission characteristics (e.g. tuning orbital parameters, flying more satellites simultaneously, etc.).

A large scientific and user community has expressed the need for further improving the temporal resolution of observing mass changes in the Earth system at enhanced spatial scales, significantly beyond the capabilities of the (combination of) GRACE and GOCE. Many activities are going on to study new sensor technologies (e.g. low-low SST by laser, improved accelerometers and drag-free control systems, etc.) and mission concepts (e.g. optimization of orbits, formations such as multiple GRACE-type tandems interlaced in time or space, cartwheel and pendulum). Typically, teams of researchers from universities and institutes, highly qualified engineers from industry and colleagues from space agencies work together to define and design new gravity field missions. Defining and designing future gravity field missions is a complicated process in which many trade-offs have to be made (e.g. minimum requirements for scientific and societal outcome, cost efficiency, technological feasibility, etc.).

Friday, October 2, 2009

Rooms: All plenary session will take place in room BE01. Breakout session will be in rooms AE01, A111, and A306.