Quality Assessment Of Satellite Based Global Gravity Field Models

This book provides a sound theoretical basis for the the different gravity field recovery methods and the numerics of satellite-to-satellite tracking data. It represents lectures given at the ‘Wilhelm and Else Heraeus Autumn School’ in Bad Honnef, Germany, October 4-9, 2015. The emphasis of the school was on providing a sound theoretical basis for the different gravity field recovery methods and the numerics of data analysis. The approaches covered here are the variational equations (classical approach), the acceleration approach and the energy balance approach, all of which are used for global gravity field recovery on the basis of satellite observations. The theory of parameter estimation in satellite gravimetry and concepts for orbit determination are also included. The book guides readers through a broad range of topics in satellite gravimetry, supplemented by the necessary theoretical background and numerical examples. While it provides a comprehensive overview for those readers who are already familiar with satellite gravity data processing, it also offers an essential reference guide for graduate and undergraduate students interested in this field.

In July 1995 the XXI General Assembly of the International Union of Geodesy and Geophysics was held in Boulder, Colorado. At this meeting the International Association of Geodesy (lAG) organized a number of symposia to discuss scientific developments and future directions in a number of areas. One of these symposia was G3, Global Gravity Field and Its Temporal Variations. This symposium consisted of four invited and 36 contributed papers. The contributed papers were given as oral or poster presentations. This proceedings volume represents the written contributions of the four invited papers (appearing as the first four papers in the volume) and 19 additional papers. The authors were asked to limit the length of their paper to approximately ten pages, which, in some cases, did limit what an author wanted to say. The papers in this volume have been placed in the same order as they were presented at the ruGG meeting. A key theme of the symposium is given in the paper by Nerem, Klosko, and Pavlis where they discuss applications of gravity field information in geodesy and oceanography. The significant achievements in determining the gravity field in the ocean areas from satellite altimeter data is discussed by Sandwell, Yale, McAdoo, and Smith. A review of time changes of the Earth's gravity field from terrestrial measurements is given by Lambert et aI. , and from satellite perturbation techniques by Eanes and Bettadpur. A description of new geopotential models is given in the paper by Tapley et al.

Modelling the gravity field of the Earth is important for many scientific disciplines. Global gravity models allow for the investigation of long-wavelength properties of the gravity field. Global models derived from satellite observations provide an additional benefit: they are uncorrelated with any error contaminating regional terrestrial gravity information; this makes them ideal for combination with terrestrial gravity data in order to formulate high-precision regional geoid models. This dissertation investigates several possible areas of improvement to both the formulation and evaluation of satellite-only global gravity models. The first major barrier is due to what is known to the geodetic community as the “polar-gap problem”: the lack of data collected by the satellites over the poles due to the inclination angle of their orbit. The second is the rigorous application of these models inside of the topographical masses (and most pertinent, on the surface of the geoid). These problems are addressed in three articles. The first presents a mathematical tool that can be used in order to address the polar-gap problem by performing the global integration making use of the additivity property of Riemann integrals. The second article presents a computational scheme that allows for the evaluation of various quantities derived from global gravity models inside the topographical masses. Finally, the third article describes the production and validation of a 2D global topographical density model that is required for the rigorous evaluation of the gravity field as prescribed in the second article.

Over the last two decades, satellite gravimetry has become a new remote sensing technique that provides a detailed global picture of the physical structure of the Earth. With the CHAMP, GRACE, GOCE and GRACE Follow-On missions, mass distribution and mass transport in the Earth system can be systematically observed and monitored from space. A wide range of Earth science disciplines benefit from these data, enabling improvements in applied models, providing new insights into Earth system processes (e.g., monitoring the global water cycle, ice sheet and glacier melting or sea-level rise) or establishing new operational services. Long time series of mass transport data are needed to disentangle anthropogenic and natural sources of climate change impacts on the Earth system. In order to secure sustained observations on a long-term basis, space agencies and the Earth science community are currently planning future satellite gravimetry mission concepts to enable higher accuracy and better spatial and temporal resolution. This Special Issue provides examples of recent improvements in gravity observation techniques and data processing and analysis, applications in the fields of hydrology, glaciology and solid Earth based on satellite gravimetry data, as well as concepts of future satellite constellations for monitoring mass transport in the Earth system.

In the recent years, space-based observation methods have led to a subst- tially improved understanding of Earth system. Geodesy and geophysics are contributing to this development by measuring the temporal and spatial va- ations of the Earth’s shape, gravity ?eld, and magnetic ?eld, as well as at- sphere density. In the frame of the GermanR&D programmeGEOTECHNO- LOGIEN,researchprojectshavebeen launchedin2002relatedto the satellite missions CHAMP, GRACE and ESA’s planned mission GOCE, to comp- mentary terrestrial and airborne sensor systems and to consistent and stable high-precision global reference systems for satellite and other techniques. In the initial 3-year phase of the research programme (2002-2004), new gravity ?eld models have been computed from CHAMP and GRACE data which outperform previous models in accuracy by up to two orders of m- nitude for the long and medium wavelengths. A special highlight is the - termination of seasonal gravity variations caused by changes in continental water masses. For GOCE, to be launched in 2006, new gravity ?eld analysis methods are under development and integrated into the ESA processing s- tem. 200,000 GPS radio occultation pro?les, observed by CHAMP, have been processed on an operational basis. They represent new and excellent inf- mation on atmospheric refractivity, temperature and water vapor. These new developments require geodetic space techniques (such as VLBI, SLR, LLR, GPS) to be combined and synchronized as if being one global instrument.

Contents Abstract xi Samenvatting xv Curriculum vitae xix Acknowledgements xxii 1. Introduction 1 2. Gravity field modeling from SST data: an overview 9 3. Gravity field modeling from CHAMP data 53 4. Gravity field modeling from GRACE hl-SST data 81 5. Gravity field modeling from GRACE ll-SST data 91 6. Analysis of results obtained from the 3RC approach 133 7. Summary, conclusions and recommendations 203 Bibliography 209 A. Autocorrelation 223 B. Gaussian Filtering 225

For the past three decades, it has been possible to measure the earth's static gravity from satellites. Such measurements have been used to address many important scientific problems, including the earth's internal structure, and geologically slow processes like mantle convection. In principle, it is possible to resolve the time-varying component of the gravity field by improving the accuracy of satellite gravity measurements. These temporal variations are caused by dynamic processes that change the mass distribution in the earth, oceans, and atmosphere. Acquisition of improved time-varying gravity data would open a new class of important scientific problems to analysis, including crustal motions associated with earthquakes and changes in groundwater levels, ice dynamics, sea-level changes, and atmospheric and oceanic circulation patterns. This book evaluates the potential for using satellite technologies to measure the time-varying component of the gravity field and assess the utility of these data for addressing problems of interest to the earth sciences, natural hazards, and resource communities.

IAG Symposium, Cairns, Australia, 22-26 August, 2005