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GOCE gravity gradients for use in Earth sciences

Johannes Bouman(1), Sietse Rispens(1) and Radboud Koop(1)

(1) SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, Netherlands

Abstract

One of the most interesting Level 1b products derived from the GOCE observations are the gravity gradients (GG). These GG are provided in the so-called Gradiometer Reference Frame (GRF) and are internally calibrated (‘in-flight calibration’, by comparison with artificial but known signals). In order to use these GG for application in Earth sciences some additional pre-processing needs to be done, including external calibration (by comparison with existing external gravity field information), corrections for temporal gravity field signals (in order to isolate the static gravity field part), and additional screening for outliers.

These three steps, outlier detection and/or data gap interpolation, corrections for temporal gravity field variations and external calibration are part of the GOCE GG pre-processing, one of the tasks of the GOCE High-level Processing Facility (HPF). In our presentation this pre-processing is discussed with emphasis on those characteristics of the GG that are most important for end users who want to use the GG in scientific applications. The GOCE gradiometer has been designed such that the GG error characteristics are optimal in the so-called MBW (measurement bandwidth) between 0.005 and 0.1 Hz, exactly corresponding to the medium to high wavelength gravity field features that are to be resolved from the novel gradiometer instrument on GOCE. Below the MBW the error PSD is expected to show a dominant 1/f behaviour. For use of the final GOCE gravity field models the very high quality SST data will complement this degraded long-wavelength error behaviour of the GG, but a direct comparison of the GG with existing external gravity data, which is used in the outlier detection and external calibration, will become more cumbersome. The pre-processing algorithms that overcome this problem are presented.

The pre-processed GG, or Level 2 gradients, can be used in gravity field analysis, that is, a global gravity field model may be derived from these gradients also in combination with satellite-to-satellite tracking data. In addition, the Level 2 gradients themselves may be directly used in Earth sciences, typically for local and regional applications focusing on the smaller wavelengths. Since the GG are given in the GRF, which is not directly linked to the Earth, a derived product provided from the pre-processing facility is, therefore, GG in an Earth-fixed reference frame (EFRF). If all Level 2 gradients would have a comparable accuracy, then the transformation from the GRF to the EFRF would be straightforward. However, four out of the six gradients have a high accuracy in the MBW, whereas the other two are not very accurate in the MBW. A direct rotation from the GRF to EFRF would project the larger errors of the two inaccurate gradients to all gradients, which is undesirable. Therefore least-squares collocation (LSC) will be used to do the GRF – EFRF transformation and the accuracy of the gravity gradients in the EFRF will not be severely degraded as compared to the accuracy in the GRF. We will present our processing strategy which should result in accurate gravity gradients in the EFRF for further use in geophysical and oceanographic applications.

 

Workshop presentation

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry