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Computation of Calibration Gradients and Methods for In-Orbit Validation of Gradiometric GOCE Data

Prof. Dr. Jürgen Müller(1), Dr Heiner Denker(1) , Mr Focke Jarecki(1) , and Mrs Karen Insa Wolf(1)

(1) University of Hannover, Schneiderberg 50, 30167 Hannover, Germany


The accuracy planned for GOCE gravity gradiometry requires special independent concepts for calibration and validation of the measured gravitational gradients. Although the gradiometer will be calibrated internally on ground (pre- flight) and prior to the measurement phase itself in orbit (in- flight), further external calibration is obligatory to establish the relationship of the measurements to the earth's gravitational field at the required accuracy level. To keep the accuracy of the satellite data, a region with correspondingly well known gravity field parameters has to be selected for calibration. Central Europe has been identified as such a region where the Institut für Erdmessung has collected a database of 5' by 5' gridded gravity anomalies. For calibration and validation purposes, reference gravitational gradients at GOCE altitude are computed from these terrestrial gravity anomalies. The upward continuation is carried out by least squares collocation and integral formulas. The calibrated gravitational gradients have to pass a further quality assessment. Even without the availability of independent data, some conclusions on quality and consistency of the collected data can be drawn by comparing measurements in the same satellite position, i.e. satellite track cross-overs or repeat orbits. Unfortunately, due to the orbit characteristics of the mission, identical repeat positions are reached seldom throughout the mission lifetime, so one has to take into account satellite ground track cross- overs as well. For those, a reduction concept has to be applied to consider the differences caused by different satellite altitudes and orientations. It is shown here, that present global gravity field models meet the accuracy and resolution requirements of the reduction concept. This relative validation procedure obviously just allows the identification of parts of the possible errors. So a further validation step has to be integrated to obtain absolute differences between the calibrated measurements and the earth's real gravitational field. If this comparison is done with terrestrial data in the same well observed region mentioned above, it has to be performed in close coordination with the calibration step to avoid misinterpretation. But in contrast to the gradients' calibration, the absolute validation step can also be carried out after further processing of the measurements, e.g. based on new regional or global gravity field models or the use of oceanographic data.


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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