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  • 1
    Publication Date: 2022-12-02
    Description: Temporal aliasing errors resulting from the undersampling of non-tidal atmospheric as well as oceanic mass variations constitute the largest limitation towards the retrieval of monthly gravity solutions based on GRACE and GRACE-FO satellite gravity missions. Their mitigation is thus a primary goal of current research. Unfortunately, the two-step co-parametrization approach proposed for application in Bender-type gravity retrieval scenario in Wiese et al. yields no added value for a single satellite pair. A detailed study of this parametrization strategy is carried out and it is shown that the reason for this is the flawed central assumption of the proposed method, that is that signals of different spatial wavelengths can be perfectly captured and separated with respect to their temporal extent. Based on this finding, we derive a multi-step self-de-aliasing approach (DMD) which aims to rectify the shortcoming of the Wiese et al. method specifically for the single-pair case while retaining its independence from background-model-based de-aliasing of non-tidal atmosphere and ocean (AO) signal components. The functionality and added value of this novel approach is validated within a set of numerical closed-loop simulations as well as in real GRACE and GRACE-FO data processing. The simulation results show that the DMD may improve the gravity retrieval performance in the high-degree spectrum by more than one order of magnitude if one aims to retrieve the full AOHIS (i.e. atmosphere, ocean, hydrology, ice, solid earth) signal, and by at least a factor 5 if a priori AO de-aliasing is applied. Simultaneously, the DMD is shown to degrade the retrieval of the low degrees, but it is also demonstrated that this issue can be mitigated by introducing a constraint into the processing scheme. The simulation results are widely confirmed by results obtained from applying the DMD to real GRACE/GRACE-FO data of the test years 2007, 2014 and 2019. The applicability of the DMD is further shown for Bender-type gravity retrieval. It is demonstrated that in case of a double-pair-based gravity retrieval this approach is at least equivalent to the Wiese et al. method.
    Type: info:eu-repo/semantics/article
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  • 2
    Publication Date: 2022-01-05
    Description: Ocean tide (OT) background models (BMs) used for a priori de-aliasing of GRACE/GRACE-FO observations feature distinct spatial uncertainties (primarily in coastal proximity and in latitudes above ±60°), and therefore pose one of the largest contributors to the overall retrieval error. The retrieval performance can be expected to increase if this underlying spatial error distribution is stochastically modelled and incorporated into the data processing chain. In this contribution, we derive realistic error variance-covariance matrices (VCM) based on a set of five state-of-the-art OT models. The additional value of using such VCMs is assessed through numerical closed-loop simulations, where they are rigorously propagated from model to observation level. Further, different approximations of the resulting VCM of observations are assumed, that is full, block-diagonal and diagonal, in order to evaluate the trade-off between computational efficiency and accuracy. It is asserted that correctly weighting the OT BM error can improve the gravity retrieval performance by up to three orders of magnitude, provided no further error contributors are considered. In comparison, the overall gain in retrieval performance is reduced to 75 per cent once instrument noise is taken into account. Here, it is shown that simultaneously modelling the OT BM and the instrument errors is critical, as each effect induces different types of correlations between observations, and exclusively considering covariance information based on the sensor noise may degrade the solution. We further demonstrate that the additional benefit of incorporating OT error VCMs is primarily limited by the de-aliasing performance for non-tidal mass variations of atmosphere (A) and oceans (O). This emphasizes the necessity of best-possible AO-de-aliasing (e.g. through optimized processing techniques and/or improved BMs) in order to optimally exploit the OT BM weighting.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 3
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    In:  XXVIII General Assembly of the International Union of Geodesy and Geophysics (IUGG)
    Publication Date: 2023-08-29
    Description: De-aliasing based on geophysical background models (BM) allows for a reduction of high-frequency, high-amplitude signal components in GRACE and GRACE-FO data processing. These are primarily related to the ocean tides (OT) and non-tidal variations within the atmosphere and the oceans (AO), and would otherwise superimpose the target signals stemming e.g. from the hydro- or the cryosphere. In the course of previous studies, it was shown both in closed-loop simulations (Abrykosov et al. 2021) as well as in real data processing (Panafidina et al. 2023, in preparation) that the incorporation of OT BM errors in terms of error variance-co-variance matrices (VCM) yields an enhanced gravity retrieval performance. In this contribution, we discuss the obvious next step, which is the consideration of stochastic properties of the underlying imperfections of the AO models. This task is not as trivial as in case of OT, since the spatial error features significant time variations. Thus, we first derive a static error VCM for the AO model and then refine it by adding temporal variations. The added value of both approaches is assessed in numerical closed-loop simulations by rigorously adding the error information into the data processing scheme. As the time-variable error VCM may quickly become too large to be handled, explicit investigations towards determining the optimal decorrelation period are carried out and discussed with regard to trade-off between computational efficiency and gravity retrieval accuracy.
    Language: English
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  • 4
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    In:  XXVIII General Assembly of the International Union of Geodesy and Geophysics (IUGG)
    Publication Date: 2023-08-29
    Description: The main objective of the joint ESA/NASA Mass-change And Geoscience International Constellation (MAGIC) is to extend the mass transport time series from previous gravity missions (GRACE, GRACE-FO) with significantly enhanced accuracy, spatial and temporal resolutions. The concept is based on a joint ESA/NASA Mission Requirements Document (MRD). The first (polar) pair (P1) of the constellation will be implemented via a Germany-USA fast-paced cooperation to ensure continuity of observations of GRACE-FO, with potential ESA in-kind contributions. The second (inclined) pair (P2) will be implemented via a Europe-USA cooperation with some potential NASA in-kind contributions. Within extensive full-fledged numerical simulations with realistic error assumptions regarding instrument performances and background model errors, the expected performance of the resulting gravity field products is evaluated. In this contribution, the main focus lies on the quantification of the added value of P2 and the relative contributions of P1 and P2 to the combined constellation solution. In particular, the achievable performance for 1- and 5-day solutions is assessed. Further, we analyze the stand-alone value of P2 the covered regions. Note that when using spherical harmonics as base functions, this requires also an adequate treatment of the polar gap areas. Finally, we match all results against the MRD requirements and evaluate the impact on various fields of science and service applications (continental hydrology, cryosphere, oceans, solid Earth, geodesy).
    Language: English
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  • 5
    Publication Date: 2023-03-10
    Description: The Gravity Recovery and Climate Experiment (GRACE) satellite mission has provided global long-term observations of mass transport in the Earth system with applications in numerous geophysical fields. In this paper, we targeted the in-orbit performance of the GRACE key instruments, the ACCelerometers (ACC) and the MicroWave ranging Instrument (MWI). For the ACC data, we followed a transplant approach analyzing the residual accelerations from transplanted accelerations of one of the two satellites to the other. For the MWI data, we analyzed the post-fit residuals of the monthly GFZ GRACE RL06 solutions with a focus on stationarity. Based on the analyses for the two test years 2007 and 2014, we derived stochastic models for the two instruments and a combined ACC+MWI stochastic model. While all three ACC axes showed worse performance than their preflight specifications, in 2007, a better ACC performance than in 2014 was observed by a factor of 3.6 due to switched-off satellite thermal control. The GRACE MWI noise showed white noise behavior for frequencies above 10 mHz around the level of 1.5×10−6 m/Hz−−−√. In the combined ACC+MWI noise model, the ACC part dominated the frequencies below 10 mHz, while the MWI part dominated above 10 mHz. We applied the combined ACC+MWI stochastic models for 2007 and 2014 to the monthly GFZ GRACE RL06 processing. This improved the formal errors and resulted in a comparable noise level of the estimated gravity field parameters. Furthermore, the need for co-estimating empirical parameters was reduced.
    Language: English
    Type: info:eu-repo/semantics/article
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  • 6
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    In:  XXVIII General Assembly of the International Union of Geodesy and Geophysics (IUGG)
    Publication Date: 2023-07-13
    Description: With the evolution of cold atom interferometry (CAI), an adaptation for spaceborne applications may become possible in the near future. One of the applications which may benefit from such CAI instruments are next-generation satellite gravity field missions (NGGMs), since they rely heavily on the accelerometer performance. Here, either future satellite-to-satellite tracking (SST) missions (such as GRACE/-FO) or satellite gravity gradiometry (SGG) missions (such as GOCE) are feasible. Until now, only electrostatic accelerometers have been used. However, all suffer from an increased long-term instability which affects the accuracy of the long wavelengths of the retrieved gravity field. In this contribution we investigate the impact of CAI sensors on various NGGM mission concepts (either SST or SGG variants) and quantify the instrument-only error separately from the full gravity field retrieval error (which is hampered by temporal aliasing). Knowing that temporal aliasing currently poses one of the main limiting factors, special attention is given to strategies which may help to minimize this error source. Therefore, in addition to investigating future instruments, also extended mission constellations containing several satellites/pairs and alternative satellite configurations are examined with respect to their time-variable gravity field retrieval performance. This work is supported by the ESA QSG4EMT study in collaboration with Politecnico di Milano, Delft University of Technology, HafenCity University Hamburg, University of Bonn and University of Trieste.
    Language: English
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  • 7
    Publication Date: 2023-08-31
    Description: The currently on-going NGGM preparatory activities encompass two parallel Phase A system studies, which are complemented by a science support study with the goal to identify of optimum set-up for a MAGIC constellation regarding science return, technical feasibility, and costs. As an extension of the Phase A science study and preparation for the upcoming Phase B, several aspects are treated in detail, such as to advance processing methods and algorithms towards an integrated L0-L3 processing, to demonstrate the scientific readiness of end-to-end simulations, to quantify the relative contributions of the two involved pairs to the total mission performance, to analyze specific aspects such the optimum set-up regarding short-term products for operational service applications, and to demonstrate the science impact of MAGIC in numerous application fields. Additionally, the study involves technical aspects such as the refinement of L0-L1b inter-satellite distance algorithm, or the in-flight calibration of the accelerometers. In this contribution will we will summarize the main finding of the first phase of the project and report on recent results of the extension phase. The underlying numerical E2E simulations are performed based on realistic error assumptions for the instruments and the involved background models. The simulation results are evaluated against the science requirements defined in the MRD and MRTD, with special focus on new application fields and short-term service applications.
    Language: English
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  • 8
    Publication Date: 2024-02-12
    Language: English
    Type: info:eu-repo/semantics/workingPaper
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