Designing Earth Gravity Field Missions for the Future: A Case Study
Gravity field changes due to mass changes in the Earth system have been observed successfully by the GRACE mission. Having a single tandem like GRACE limits the achievable resolution of observing such mass changes both in time and space. A simulation stud
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Designing Earth Gravity Field Missions for the Future: A Case Study P.N.A.M. Visser
Abstract Gravity field changes due to mass changes in the Earth system have been observed successfully by the GRACE mission. Having a single tandem like GRACE limits the achievable resolution of observing such mass changes both in time and space. A simulation study was carried out to make a first assessment of the impact of different gravity satellite formations on the retrieval of temporal gravity, in this case caused by hydrology. These formations include polar formations of one, two and four GRACE-type tandems and a formation that includes one polar and one non-polar tandem. A comprehensive force modeling was used including gravity field changes due to ocean tides and hydrological, atmospheric, oceanographic, solid-earth and ice mass change processes. The impact of errors in these models in conjunction with observation errors by the space-borne gravity instruments was assessed. First results indicate that having more than one tandem helps to reduce the impact of errors in background models such as ocean tides, provided that instrument observation errors are sufficiently low.
18.1 Introduction The current GRACE mission has successfully observed mass changes in the Earth system due to continental hydrology and Greenland ice mass losses,
P.N.A.M. Visser () Faculty of Aerospace Engineering, Delft Institute of Earth Observation and Space Systems (DEOS), Delft University of Technology, Delft, The Netherlands e-mail: [email protected]
although great care has to be taken with interpreting the results since large uncertainties in observed mass change rates still exist (Schrama et al. 2007; Tapley et al., 2004; Velicogna and Wahr, 2006). In order to extract these mass changes from the GRACE observations, a comprehensive modeling is required that takes into account all sources of the Earth’s gravity field that are within the sensitivity of the observing system. A distinction is made between sources that are to be observed and monitored, and sources that need to be included in the prior models (referred to as background models). Modeling errors are defined as errors of – or omissions in – the background models. For GRACE, examples are errors of ocean tide models, uncertainties in atmospheric mass change models, and gravity field omission errors such as unmodeled (known or unknown) mass changes and high spatial resolution parts of the gravity field. The point has been reached where the accuracy of mass changes observed by GRACE are not limited by instrument observation errors, but more by its spatio-temporal sampling and errors in the background models. Gravity field recovery errors typically show up as stripes in geographical representations of the solved for models (Schrama et al., 2007; Wahr et al., 2006). There is strong support in the Earth sciences community for follow-on gravity satellite missions that will as a minimum continue the time series of gravity changes as currently observed by GRACE, but preferably enhances no
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