Search

The GOCE mission provides a unique gravity gradient dataset, which is used by most state-of-the-art global gravity field models. The quality of the gravity gradients of previous data releases degraded during the mission lifetime, which was often attributed to the increasing drag forces towards the end of the mission due to the approaching solar maximum and also, starting in August 2012, a sequence of five orbit lowerings. An imperfect calibration of the gravity gradiometer data was often suspected as root cause for the degradation. We present the novel method for the calibration of the GOCE gravity gradiometer data that was used for the official reprocessing of the GOCE gravity gradients in 2018. It is based on a comprehensive calibration model that includes quadratic factors and angular acceleration couplings as new calibration parameters. The calibration parameters are estimated from star tracker angular rates, gravity gradients calculated from a GRACE gravity field model and the condition that the three gradiometer arms measure the same non-gravitational acceleration. In addition to the GOCE data generated during science mode operations, we also use the data collected during satellite shaking mode operations. We demonstrate that applying the new method removes systematic errors to a large extent and, consequently, yields gravity gradients of superior and constant quality compared to previous data releases. [ABSTRACT FROM AUTHOR]

Abstract We analyse the inter-boresight angles (IBA) measured by the star trackers on board the GOCE satellite and find that they exhibit small offsets of 7–9″ with respect to the ones calculated from the rotation of the star tracker reference frames to the satellite reference frame. Further, we find small variations in the offsets with a peak-to-peak amplitude of up to 8″, which correlate with variations of the star trackers' temperatures. Motivated by these findings, we present a method for combining the attitude quaternions measured by two or more star trackers that includes an estimation of relative attitude offsets between star trackers as a linear function of temperature. The method was used to correct and combine the star tracker attitude quaternions within the reprocessing of GOCE data performed in 2018. We demonstrate that the IBA calculated from the corrected star tracker attitude quaternions show no significant offsets with respect to the reference frame information. Finally, we show that neglecting the star tracker attitude offsets in the processing would result in perturbations in the gravity gradients that are visible at frequencies below 2 mHz and have a magnitude of up to 90 mE. The presented method avoids such perturbations to a large extent. [ABSTRACT FROM AUTHOR]

The GOCE gravity gradiometer measured highly accurate gravity gradients along the orbit during GOCE's mission lifetime from March 17, 2009, to November 11, 2013. These measurements contain unique information on the gravity field at a spatial resolution of 80 km half wavelength, which is not provided to the same accuracy level by any other satellite mission now and in the foreseeable future. Unfortunately, the gravity gradient in cross-track direction is heavily perturbed in the regions around the geomagnetic poles. We show in this paper that the perturbing effect can be modeled accurately as a quadratic function of the non-gravitational acceleration of the satellite in cross-track direction. Most importantly, we can remove the perturbation from the cross-track gravity gradient to a great extent, which significantly improves the accuracy of the latter and offers opportunities for better scientific exploitation of the GOCE gravity gradient data set. [ABSTRACT FROM AUTHOR]

The exploitation of data from the Global Ocean Circulation Explorer (GOCE) mission still offers unique insights in Earth system processes despite the fact that the mission has been completed more than eight years ago. The use of GOCE data requires tedious and cumbersome processing in order to extract them from their original format, process them in the various reference frames and filter them to derive final products at the orbital altitude. GeoGravGOCE is an open-source software, developed in MATLAB, which aims to provide a friendly Graphical User Interface (GUI) to process the original GOCE gradiometric observations, i.e., second-order derivatives of the gravitational potential as provided by the European Space Agency. It is composed of four main tabs, which first allows the GOCE gradient pre-processing and the transformation of gradients from a geopotential models to the gradiometric reference frame. Then, the spectral filtering and the mutual transformations of gravity gradients between the four reference systems used in satellite gravimetry can be carried out.. Three main filtering methods are implemented within the software, referring to: Finite Impulse Response (FIR), Infinite Impulse Response (IIR), and Wavelet Multi-Resolution Analysis (WL-MRA) filters. The user is free to select the desired degree of filtering and evaluate the derived results in terms of both the power spectral density of the filtered signals and their spatial distribution. The present work gives an analytical outline of the program, along with its various functionalities, while results derived from the processing of original GOCE data are presented as well. GeoGravGOCE is a free software under the GNU license and can be freely accessed via the project webpage http://olimpia.topo.auth.gr/GeoGravGOCE/. • We designed a software for GOCE SGG data processing, reference frame transformations, orbit interpolation and filtering. • The software allows the user to experiment and test FIR, IIR and Wavelet filtering of GOCE gradients. • The software includes of a graphical user interface. • The software delivers residual gravity gradients for [immediate] application[s] in geodesy, oceanography, and geophysics. [ABSTRACT FROM AUTHOR]

The most common approach for the processing of data of gravity field satellite missions is the so-called time-wise approach. In this approach satellite data are considered as a time series and processed by a standard least-squares approach. This approach has a very strong flexibility but it is computationally very demanding. To improve the computational efficiency and numerical stability, the so-called torus and Rosborough approaches have been developed. So far, these approaches have been applied only for global gravity field determinations, based on spherical harmonics as basis functions. For regional applications basis functions with a local support are superior to spherical harmonics, because they provide the same approximation quality with much less parameters. So far, torus and Rosborough approach have been developed for spherical harmonics only. Therefore, the paper aims at the development and testing of the torus and Rosborough approach for regional gravity field improvements, based on radial basis functions as basis functions. The developed regional Rosborough approach is tested against a changing gravity field produced by simulated ice-mass changes over Greenland. With only 350 parameters a recovery of the simulated mass changes with a relative accuracy of 5% is possible. [ABSTRACT FROM AUTHOR]

The issue of combining high-resolution gravity models, based on observations taken on the Earth surface, with those derived from satellite-only observations is of increasing importance, due to the new data provided by gravity satellite missions, CHAMP, GRACE and GOCE. The paper addresses this issue with a twofold purpose. On the one hand, it is an attempt to discuss and assess general concepts, well known in literature, such as achievable resolution, regularization in the least-squares sense or in an infinite dimensional setup, combination criteria, symmetry and block diagonal structures. In particular, as for the symmetry question, a well-defined result, generalizing known facts, is derived. On the other hand, the outcomes of the general discussion are specifically applied to the combination of a high-resolution model (e.g. EGM08) with a GOCE gravity model estimated by the so-called space-wise approach. Small numerical examples are developed to clarify the property of the proposed solution. [ABSTRACT FROM AUTHOR]

The electrostatic space accelerometers mounted on board the three satellites of the two last geodesy missions, CHAMP [C. Reiberg, CHAMP a challenging micro-satellite payload for geophysical research and application, GFZ Final Report, Potsdam, Germany, 1995] and GRACE [E. Davis, et al., The GRACE Mission: Meeting the Technical Challenges, 50th International Astronautical Congress, IAF-99-B.2.05 Conference, AAS 90-034, 1999], respectively launched in 2000 and 2002, take continuous and accurate measurements of the satellite drag acceleration. At the altitude between 400 and 500 km, this drag is induced by the residual atmosphere and the Sun and Earth radiation pressures. With a resolution up to several pico-g, these data provide also a good survey of the satellite vibration behaviour and of the operation of the attitude and orbit control. Beside the scientific interest of both missions with the fine and global recovery of the Earth''s gravity field, the analysis of the collected acceleration data is very interesting for the future space missions like GOCE [ESA, Gravity Field and Steady-State Ocean Circulation Mission, ESA SP-1233(1), Report for mission selection of the four candidate earth explorer missions, European Space Agency, 1999, p. 217], MICROSCOPE [P. Touboul, et al., C. R. Acad. Sci. Ser. IV 2 (9) (2001) 1271–1286] or LISA [LISA Study Team, LISA Laser Interferometer Space Antenna: a Cornerstone Mission for the Observation of Gravitational Waves, ESA-SCI(2000)11, European Space Agency, 2000] which envisage much more accurate acceleration measurements. The definitions of these missions and others in the domain of fundamental physics and solid Earth are all based on drag-free satellites and similar electrostatic inertial sensors. Benefits of the previous results are presented in parallel to the status of these developments. [Copyright &y& Elsevier]

One of the aims of the Earth Explorer Gravity Field and Steady-State Ocean Circulation (GOCE) mission is to provide global and regional models of the Earth's gravity field and of the geoid with high spatial resolution and accuracy. Using the GOCE error model, simulation studies were performed in order to estimate the accuracy of datum transfer in different areas of the Earth. The results showed that with the GOCE error model, the standard deviation of the height anomaly differences is about one order of magnitude better than the corresponding value with the EGM96 error model. As an example, the accuracy of the vertical datum transfer from the tide gauge of Amsterdam to New York was estimated equal to 57 cm when the EGM96 error model was used, while in the case of GOCE error model this accuracy was increased to 6 cm. The geoid undulation difference between the two places is about 76.5 m. Scaling the GOCE errors to the local gravity variance, the estimated accuracy varied between 3 and 7 cm, depending on the scaling model. [ABSTRACT FROM AUTHOR]