Journal: Journal of Geodesy

Abbreviation

J Geod

Publisher

Springer

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ISSN

0949-7714
1432-1394

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Publications 1 - 10 of 48
  • Downscaling GRACE-derived ocean bottom pressure anomalies using self-supervised data fusion
    Item type: Journal Article
    Gou, Junyang; Börger, Lara; Schindelegger, Michael; et al. (2025)
    Journal of Geodesy
    The gravimetry measurements from the Gravity Recovery and Climate Experiment (GRACE) and its follow-on (GRACEFO) mission provide an essential way to monitor changes in ocean bottom pressure (pb), which is a critical variable in understanding ocean circulation. However, the coarse spatial resolution of the GRACE(-FO) fields blurs important spatial details, such as pb gradients. In this study, we employ a self-supervised deep learning algorithm to downscale global monthly pb anomalies derived from GRACE(-FO) observations to an equal-angle 0.25◦ grid in the absence of high-resolution ground truth. The optimization process is realized by constraining the outputs to follow the large-scale mass conservation contained in the gravity field estimates while learning the spatial details from two ocean reanalysis products. The downscaled product agrees with GRACE(-FO) solutions over large ocean basins at the millimeter level in terms of equivalent water height and shows signs of outperforming them when evaluating short spatial scale variability. In particular, the downscaled pb product has more realistic signal content near the coast and exhibits better agreement with tide gauge measurements at around 80% of 465 globally distributed stations. Our method presents a novel way of combining the advantages of satellite measurements and ocean models at the product level, with potential downstream applications for studies of the large-scale ocean circulation, coastal sea level variability, and changes in global geodetic parameters.
  • GNSS signal ray-tracing algorithm for the simulation of satellite-to-satellite excess phase in the neutral atmosphere
    Item type: Journal Article
    Cegla, Adam; Rohm, Witold; Moeller, Gregor; et al. (2024)
    Journal of Geodesy
    Traditionally, GNSS space-based and ground-based estimates of tropospheric conditions are performed separately. It leads to limitations in the horizontal (e.g., a single space-based radio occultation profile covers a 300 km slice of the troposphere) and vertical resolution (e.g., ground-based estimates of troposphere conditions have spacing equal to stations' distribution) of the tropospheric products. The first stage to achieve an integrated model is to create an effective 3D ray-tracing algorithm for the satellite-to-satellite (radio occultation) path reconstruction. We verify the consistency of the simulated data with the RO observations from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC-1) Data Analysis and Archive Center (CDAAC) in terms of excess phase and bending angle. The results show that our solution provides an effective RO excess phase, with a relative error varying from 35% at the height of 25-30 km (1.0-1.5 m) to 0.5% at heights 5-10 km (0.1-1 m) and 14 to 2% at heights below 5 km (2-14 m). The bending angle retrieval on simulated data attained for high-resolution ray-tracing, bias lower than 2% with respect to the observed bending angle. The optimal solution takes about 1 s for one transmitter-receiver pair with a tangent point below 5 km altitude. The high-resolution processing solution takes 3 times longer.
  • Optimal VLBI baseline geometry for UT1-UTC Intensive observations
    Item type: Journal Article
    Schartner, Matthias; Kern, Lisa; Nothnagel, Axel; et al. (2021)
    Journal of Geodesy
    One of the main tasks of Very Long Baseline Interferometry (VLBI) is the rapid determination of the highly variable Earth’s rotation expressed through the difference between Universal Time UT1 and Coordinated Universal Time UTC (dUT1). For this reason, dedicated one hour, single baseline sessions, called “Intensives”, are observed on a daily basis. Thus far, the optimal geometry of Intensive sessions was understood to include a long east–west extension of the baseline to ensure a dUT1 estimation with highest accuracy. In this publication, we prove that long east–west baselines are the best choice only for certain lengths and orientations. In this respect, optimal orientations may even require significant inclination of the baseline with respect to the equatorial plane. The basis of these findings is a simulation study with subsequent investigations in the partial derivatives of the observed group delays τ with respect to dUT1 ∂τ/∂dUT1. Almost 3000 baselines between artificial stations located on a regular 10×10 degree grid are investigated to derive a global and generally valid picture about the best length and orientation of Intensive baselines. Our results reveal that especially equatorial baselines or baselines with a center close to the equatorial plane are not suited for Intensives although they provide a good east–west extension. This is explained by the narrow right ascension band of visible sources and the resulting lack of variety in the partial derivatives. Moreover, it is shown that north–south baselines are also capable of determining dUT1 with reasonable accuracy, given that the baseline orientation is significantly different from the Earth rotation axis. However, great care must be taken to provide accurate polar motion a priori information for these baselines. Finally, we provide a better metric to assess the suitability of Intensive baselines based on the effective spread of ∂τ/∂dUT1.
  • Deriving a global troposphere model for space geodetic simulations based on an ML ensemble featuring uncertainty quantification
    Item type: Journal Article
    Schartner, Matthias (2025)
    Journal of Geodesy
    This work presents a global, three-dimensional (latitude–longitude–time) model of the refractive index structure constant (Cn), enabling the spatiotemporally correlated simulation of tropospheric delays for space geodetic observations at radio frequencies. The model is based on an ensemble of 100 XGBoost models trained on 21 years of observations from 18,500 GNSS stations, using meteorological variables from ERA5 as features. It effectively captures high-frequency spatial and temporal variations, achieving a mean absolute error of 0.52m-1/3. To simplify the use of the model, monthly average Cn values are computed on a regular 2.5×2.5 degree grid, which are sufficiently accurate for most simulation studies. Besides, the model provides a Monte Carlo-based measure for the prediction uncertainty based on the XGBoost ensemble spread, which is revealed to be primarily driven by feature augmentation using ensemble spread information from ERA5. The model is validated both independently on 2500 GNSS stations over 3 years and externally through very long baseline interferometry simulations. The results demonstrate a significant improvement over current state-of-the-art simulation approaches.
  • The Geoid Slope Validation Survey 2014 and GRAV-D airborne gravity enhanced geoid comparison results in Iowa
    Item type: Journal Article
    Wang, Yanming; Becker, Colin; Mader, Gerald; et al. (2017)
    Journal of Geodesy
  • Optimizing schedules for the VLBI global observing system
    Item type: Journal Article
    Schartner, Matthias; Böhm, Johannes (2020)
    Journal of Geodesy
  • Geodetic VLBI with an artificial radio source on the Moon: a simulation study
    Item type: Journal Article
    Klopotek, Grzegorz; Hobiger, Thomas; Haas, Rüdiger (2018)
    Journal of Geodesy
    We perform extensive simulations in order to assess the accuracy with which the position of a radio transmitter on the surface of the Moon can be determined by geodetic VLBI. We study how the quality and quantity of geodetic VLBI observations influence these position estimates and investigate how observations of such near-field objects affect classical geodetic parameters like VLBI station coordinates and Earth rotation parameters. Our studies are based on today’s global geodetic VLBI schedules as well as on those designed for the next-generation geodetic VLBI system. We use Monte Carlo simulations including realistic stochastic models of troposphere, station clocks, and observational noise. Our results indicate that it is possible to position a radio transmitter on the Moon using today’s geodetic VLBI with a two-dimensional horizontal accuracy of better than one meter. Moreover, we show that the next-generation geodetic VLBI has the potential to improve the two-dimensional accuracy to better than 5 cm. Thus, our results lay the base for novel observing concepts to improve both lunar research and geodetic VLBI.
  • Confirming regional 1 cm differential geoid accuracy from airborne gravimetry
    Item type: Journal Article
    Smith, Dru A.; Holmes, Simon A.; Li, Xiaopeng; et al. (2013)
    Journal of Geodesy
  • 4D GPS water vapor tomography
    Item type: Journal Article
    Perler, Donat; Geiger, Alain; Hurter, Fabian (2011)
    Journal of Geodesy
  • CODE’s new solar radiation pressure model for GNSS orbit determination
    Item type: Journal Article
    Arnold, Daniel; Meindl, Michael; Beutler, Gerhard; et al. (2015)
    Journal of Geodesy
Publications 1 - 10 of 48