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Satellite altimetry observations have provided a significant contribution to the understanding of global sea surface processes, particularly allowing for advances in the accuracy of ocean tide estimations. Currently, almost three decades of satellite altimetry are available which can be used to improve the understanding of ocean tides by allowing for the estimation of an increased number of minor tidal constituents. As ocean tide models continue to improve, especially in the coastal region, these minor tides become increasingly important. Generally, admittance theory is used by most global ocean tide models to infer several minor tides from the major tides when creating the tidal correction for satellite altimetry. In this paper, regional studies are conducted to compare the use of admittance theory to direct estimations of minor tides from the EOT20 model to identify which minor tides should be directly estimated and which should be inferred. The results of these two approaches are compared to two global tide models (TiME and FES2014) and in situ tide gauge observations. The analysis showed that of the eight tidal constituents studied, half should be inferred (2N2,
Michael G. Hart-Davis; Denise Dettmering; Roman Sulzbach; Maik Thomas; Christian Schwatke; Florian Seitz. Regional Evaluation of Minor Tidal Constituents for Improved Estimation of Ocean Tides. Remote Sensing 2021, 13, 3310 .
AMA StyleMichael G. Hart-Davis, Denise Dettmering, Roman Sulzbach, Maik Thomas, Christian Schwatke, Florian Seitz. Regional Evaluation of Minor Tidal Constituents for Improved Estimation of Ocean Tides. Remote Sensing. 2021; 13 (16):3310.
Chicago/Turabian StyleMichael G. Hart-Davis; Denise Dettmering; Roman Sulzbach; Maik Thomas; Christian Schwatke; Florian Seitz. 2021. "Regional Evaluation of Minor Tidal Constituents for Improved Estimation of Ocean Tides." Remote Sensing 13, no. 16: 3310.
Information on sea level and its temporal and spatial variability is of great importance for various scientific, societal, and economic issues. This article reports about a new sea level dataset for the North Sea (named North SEAL) of monthly sea level anomalies (SLAs), absolute sea level trends, and amplitudes of the mean annual sea level cycle over the period 1995–2019. Uncertainties and quality flags are provided together with the data. The dataset has been created from multi-mission cross-calibrated altimetry data preprocessed with coastal dedicated approaches and gridded with an innovative least-squares procedure including an advanced outlier detection to a 6–8 km wide triangular mesh. The comparison of SLAs and tide gauge time series shows good consistency, with average correlations of 0.85 and maximum correlations of 0.93. The improvement with respect to existing global gridded altimetry solutions amounts to 8 %–10 %, and it is most pronounced in complicated coastal environments such as river mouths or regions sheltered by islands. The differences in trends at tide gauge locations depend on the vertical land motion model used to correct relative sea level trends. The best consistency with a median difference of 0.04±1.15 mm yr−1 is reached by applying a recent glacial isostatic adjustment (GIA) model. With the presented sea level dataset, for the first time, a regionally optimized product for the entire North Sea is made available. It will enable further investigations of ocean processes, sea level projections, and studies on coastal adaptation measures. The North SEAL data are available at https://doi.org/10.17882/79673 (Müller et al., 2021).
Denise Dettmering; Felix L. Müller; Julius Oelsmann; Marcello Passaro; Christian Schwatke; Marco Restano; Jérôme Benveniste; Florian Seitz. North SEAL: a new dataset of sea level changes in the North Sea from satellite altimetry. Earth System Science Data 2021, 13, 3733 -3753.
AMA StyleDenise Dettmering, Felix L. Müller, Julius Oelsmann, Marcello Passaro, Christian Schwatke, Marco Restano, Jérôme Benveniste, Florian Seitz. North SEAL: a new dataset of sea level changes in the North Sea from satellite altimetry. Earth System Science Data. 2021; 13 (8):3733-3753.
Chicago/Turabian StyleDenise Dettmering; Felix L. Müller; Julius Oelsmann; Marcello Passaro; Christian Schwatke; Marco Restano; Jérôme Benveniste; Florian Seitz. 2021. "North SEAL: a new dataset of sea level changes in the North Sea from satellite altimetry." Earth System Science Data 13, no. 8: 3733-3753.
Coastal studies of wave climate and evaluations of wave energy resources are mainly regional and based on the use of computationally very expensive models or a network of in-situ data. Considering the significant wave height, satellite radar altimetry provides an established global and relatively long-term source, whose coastal data are nevertheless typically flagged as unreliable within 30 km of the coast. This study exploits the reprocessing of the radar altimetry signals with a dedicated fitting algorithm to retrieve several years of significant wave height records in the coastal zone. We show significant variations in annual cycle amplitudes and mean state in the last 30 km from the coastline compared to offshore, in areas that were up to now not observable with standard radar altimetry. Consequently, a decrease in the average wave energy flux is observed. Globally, we found that the mean significant wave height at 3 km off the coast is on average 22% smaller than offshore, the amplitude of the annual cycle is reduced on average by 14% and the mean energy flux loses 38% of its offshore value.
Marcello Passaro; Mark A. Hemer; Graham D. Quartly; Christian Schwatke; Denise Dettmering; Florian Seitz. Global coastal attenuation of wind-waves observed with radar altimetry. Nature Communications 2021, 12, 1 -13.
AMA StyleMarcello Passaro, Mark A. Hemer, Graham D. Quartly, Christian Schwatke, Denise Dettmering, Florian Seitz. Global coastal attenuation of wind-waves observed with radar altimetry. Nature Communications. 2021; 12 (1):1-13.
Chicago/Turabian StyleMarcello Passaro; Mark A. Hemer; Graham D. Quartly; Christian Schwatke; Denise Dettmering; Florian Seitz. 2021. "Global coastal attenuation of wind-waves observed with radar altimetry." Nature Communications 12, no. 1: 1-13.
In a previous paper, Glomsda et al. (2020) revisited the impact of distinct parts of non-tidal loading in the analysis of geodetic Very Long Baseline Interferometry (VLBI). The loading is represented by displacements of the reference positions of the observing VLBI antennas, which are variables of a corresponding Gauss-Markov model for estimating various geodetic target parameters. These displacements were applied at two different levels of the model, the observation and the normal equation level, and quite similar results were obtained for both cases. In this paper, the authors provide a more detailed theoretical discussion of the application of site displacements at the distinct levels, which also comprises the a posteriori application at the solution level. For each case, the respective formulas and implications for the Gauss-Markov model are derived, and equations for assessing the differences between the estimated parameters are established. In this way, the authors aim to create a deeper understanding of the results of the previous paper, which show the capability of the normal equation level to approximate the application of site displacements at the observation level with less effort and prerequisites, and to provide evidence for the claims made in that paper (for VLBI only): (i) the chosen reference frame of the site displacements is basically irrelevant except for the solution level; (ii) the Jacobi matrix does not change substantially; (iii) the loss of temporal resolution of the site displacements is more important than the linear approximation of the functional model at the normal equation level; and (iv) the station coordinate estimates for all three levels strongly depend on the regularizing (datum-) conditions of the model. The theoretical results are substantiated with numerical examples, which consider site displacements generated from non-tidal atmospheric loading by the Earth-System-Modelling group of the Deutsches GeoForschungsZentrum (GFZ). However, the results are valid for site displacements of any source.
Matthias Glomsda; Mathis Bloßfeld; Manuela Seitz; Florian Seitz. Correcting for site displacements at different levels of the Gauss-Markov model – A case study for geodetic VLBI. Advances in Space Research 2021, 68, 1645 -1662.
AMA StyleMatthias Glomsda, Mathis Bloßfeld, Manuela Seitz, Florian Seitz. Correcting for site displacements at different levels of the Gauss-Markov model – A case study for geodetic VLBI. Advances in Space Research. 2021; 68 (4):1645-1662.
Chicago/Turabian StyleMatthias Glomsda; Mathis Bloßfeld; Manuela Seitz; Florian Seitz. 2021. "Correcting for site displacements at different levels of the Gauss-Markov model – A case study for geodetic VLBI." Advances in Space Research 68, no. 4: 1645-1662.
Information on sea level and its temporal and spatial variability is of great importance for various scientific, societal and economic issues. This article reports about a new sea level dataset for the North Sea (named NorthSEAL) of monthly sea level anomalies (SLA), absolute sea level trends and sea level mean annual amplitudes over the period 1995–2019. Uncertainties and quality flags are provided together with the data. The dataset has been created from multi-mission cross-calibrated altimetry data, preprocessed 5 with coastal dedicated approaches and gridded with innovative methods to a 6–8 km wide triangular mesh. The comparison of SLA and tide gauge time series shows a good consistency with average correlations of 0.85 and maximum correlations of 0.93. The improvement with respect to existing global gridded altimetry solutions amounts to 8–10 %, and it is most pronounced in complicated coastal environments such as river mouths or regions sheltered by islands. The differences in trends at tide gauge locations depend on the vertical land motion model used to correct relative sea level trends. The best 10 consistency with a median difference of 0.04 ± 1.15 mm/year is reached by applying a recent glacial isostatic adjustment (GIA) model. With the presented sea level dataset, for the first time, a regionally optimized product for the entire North Sea is made available. It will enable further investigations of ocean processes, sea level projections and studies on coastal adaptation measures. The NorthSEAL data is available at https://doi.org/10.17882/79673 (Müller et al., 2021).
Denise Dettmering; Felix L. Müller; Julius Oelsmann; Marcello Passaro; Christian Schwatke; Marco Restano; Jérôme Benveniste; Florian Seitz. NorthSEAL: A new Dataset of Sea Level Changes in the North Sea from Satellite Altimetry. 2021, 2021, 1 -28.
AMA StyleDenise Dettmering, Felix L. Müller, Julius Oelsmann, Marcello Passaro, Christian Schwatke, Marco Restano, Jérôme Benveniste, Florian Seitz. NorthSEAL: A new Dataset of Sea Level Changes in the North Sea from Satellite Altimetry. . 2021; 2021 ():1-28.
Chicago/Turabian StyleDenise Dettmering; Felix L. Müller; Julius Oelsmann; Marcello Passaro; Christian Schwatke; Marco Restano; Jérôme Benveniste; Florian Seitz. 2021. "NorthSEAL: A new Dataset of Sea Level Changes in the North Sea from Satellite Altimetry." 2021, no. : 1-28.
EOT20 is the latest in a series of empirical ocean tide (EOT) models derived using residual tidal analysis of multi-mission satellite altimetry at DGFI-TUM. The amplitudes and phases of seventeen tidal constituents are provided on a global 0.125-degree grid based on empirical analysis of seven satellite altimetry missions and four extended missions. The EOT20 model shows significant improvements compared to the previous iteration of the global model (EOT11a) throughout the ocean, particularly in the coastal and shelf regions, due to the inclusion of more recent satellite altimetry data as well as more missions, the use of the updated FES2014 tidal model as a reference to estimated residual signals, the inclusion of the ALES retracker and improved coastal representation. In the validation of EOT20 using tide gauges and ocean bottom pressure data, these improvements in the model compared to EOT11a are highlighted with the root-square sum (RSS) of the eight major tidal constituents improving by ~3 cm for the entire global ocean with the major improvement in RSS (~3.5 cm) occurring in the coastal region. Concerning the other global ocean tidal models, EOT20 shows an improvement of ~0.2 cm in RSS compared to the closest model (FES2014) in the global ocean. Variance reduction analysis was conducted comparing the results of EOT20 with FES2014 and EOT11a using the Jason-2, Jason-3 and SARAL satellite altimetry missions. From this analysis, EOT20 showed a variance reduction for all three satellite altimetry missions with the biggest improvement in variance occurring in the coastal region. These significant improvements, particularly in the coastal region, provides encouragement for the use of the EOT20 model as a tidal correction for satellite altimetry in sea-level research. All ocean and load tide data from the model can be freely accessed at https://doi.org/10.17882/79489 (Hart-Davis et al., 2021).
Michael Geoffrey Hart-Davis; Gaia Piccioni; Denise Dettmering; Christian Schwatke; Marcello Passaro; Florian Seitz. EOT20: A global ocean tide model from multi-mission satellite altimetry. 2021, 2021, 1 -23.
AMA StyleMichael Geoffrey Hart-Davis, Gaia Piccioni, Denise Dettmering, Christian Schwatke, Marcello Passaro, Florian Seitz. EOT20: A global ocean tide model from multi-mission satellite altimetry. . 2021; 2021 ():1-23.
Chicago/Turabian StyleMichael Geoffrey Hart-Davis; Gaia Piccioni; Denise Dettmering; Christian Schwatke; Marcello Passaro; Florian Seitz. 2021. "EOT20: A global ocean tide model from multi-mission satellite altimetry." 2021, no. : 1-23.
Our earlier work on assessment of altimeter significant wave height (SWH) algorithms
Florian Schlembach; Marcello Passaro; Graham Quartly; Andrey Kurekin; Francesco Nencioli; Guillaume Dodet; Jean-François Piollé; Fabrice Ardhuin; Jean Bidlot; Christian Schwatke; Florian Seitz; Paolo Cipollini; Craig Donlon. Correction: Schlembach, F., et al. Round Robin Assessment of Radar Altimeter Low Resolution Mode and Delay-Doppler Retracking Algorithms for Significant Wave Height. Remote Sens. 2020, 12, 1254. Remote Sensing 2021, 13, 1182 .
AMA StyleFlorian Schlembach, Marcello Passaro, Graham Quartly, Andrey Kurekin, Francesco Nencioli, Guillaume Dodet, Jean-François Piollé, Fabrice Ardhuin, Jean Bidlot, Christian Schwatke, Florian Seitz, Paolo Cipollini, Craig Donlon. Correction: Schlembach, F., et al. Round Robin Assessment of Radar Altimeter Low Resolution Mode and Delay-Doppler Retracking Algorithms for Significant Wave Height. Remote Sens. 2020, 12, 1254. Remote Sensing. 2021; 13 (6):1182.
Chicago/Turabian StyleFlorian Schlembach; Marcello Passaro; Graham Quartly; Andrey Kurekin; Francesco Nencioli; Guillaume Dodet; Jean-François Piollé; Fabrice Ardhuin; Jean Bidlot; Christian Schwatke; Florian Seitz; Paolo Cipollini; Craig Donlon. 2021. "Correction: Schlembach, F., et al. Round Robin Assessment of Radar Altimeter Low Resolution Mode and Delay-Doppler Retracking Algorithms for Significant Wave Height. Remote Sens. 2020, 12, 1254." Remote Sensing 13, no. 6: 1182.
Launched in 1992, the TOPEX/Poseidon (T/P) mission is one of the first major altimetry missions. It is the predecessor of the Jason satellites which orbit the Earth on a very similar orbit. The geodetic space technique SLR (Satellite Laser Ranging) provides observations of this mission by targeting the Laser Retroreflector Array (LRA) mounted on the spacecraft. The T/P LRA is extremely large and not optimally designed. It thus causes big variations in the LRA phase center. These variations are a significant limiting factor of the orbit accuracy which makes it essential to apply a measurement correction for precise orbit determination. Up to now, only tabulated LRA corrections are available which require an interpolation.
In this contribution, we present a new approach to determine station-dependent LRA corrections to improve the phase center variations. The approach is based on a continuous analytical correction function which only uses the observation azimuth and zenith angle in combination with four parameters. These parameters are computed within an estimation process for each observing SLR station. Therefore, uncorrected SLR residuals based on raw SLR normal point observations are used. The correction value is added to the SLR measurement and counteracts the LRA phase center variations.
The advantages of this method are the continuous functional, which is easy to implement in existing software packages, as well as the avoidance of an interpolation between tabulated values. Furthermore, the differences between orbits determined with and without the LRA correction will be presented. Station coordinate time series and orbit comparisons with external T/P orbits are investigated in order to prove the high quality of the obtained LRA corrections.
Julian Zeitlhöfler; Mathis Bloßfeld; Sergei Rudenko; Florian Seitz. Estimation of station-dependent LRA correction parameters for the TOPEX/Poseidon mission. 2021, 1 .
AMA StyleJulian Zeitlhöfler, Mathis Bloßfeld, Sergei Rudenko, Florian Seitz. Estimation of station-dependent LRA correction parameters for the TOPEX/Poseidon mission. . 2021; ():1.
Chicago/Turabian StyleJulian Zeitlhöfler; Mathis Bloßfeld; Sergei Rudenko; Florian Seitz. 2021. "Estimation of station-dependent LRA correction parameters for the TOPEX/Poseidon mission." , no. : 1.
The project OPTIMAP is at the current stage a joint initiative of BGIC, GSSAC and DGFI-TUM. The development of an operational tool for ionospheric mapping and prediction is the main goal of the project.
The ionosphere is a dispersive medium. Therefore, GNSS signals are refracted while they pass through the ionosphere. The magnitude of the refraction rate depends on the frequencies of the transmitted GNSS signals. The ionospheric disturbance on the GNSS signals paves the way of extracting Vertical Total Electron Content (VTEC) information of the ionosphere.
In OPTIMAP, the representation of the global and regional VTEC signal is based on localizing B-spline basis functions. For global VTEC modeling, polynomial B-splines are employed to represent the latitudinal variations, whereas trigonometric B-splines are used for the longitudinal variations. The regional modeling in OPTIMAP relies on a polynomial B-spline representation for both latitude and longitude.
The VTEC modeling in this study relies on both a global and a regional sequential estimator (Kalman filter) running in a parallel mode. The global VTEC estimator produces VTEC maps based on data from GNSS receiver stations which are mainly part of the global real-time IGS network. The global estimator relies on additional VTEC information obtained from a forecast procedure using ultra-rapid VTEC products. The regional estimator makes use of the VTEC product of the real-time global estimator as background information and generates high-resolution VTEC maps using real-time data from the EUREF Permanent GNSS Network. EUREF provides a network of very dense GNSS receivers distributed alongside Europe.
Carrier phase observations acquired from GPS and GLONASS, which are transmitted in accordance with RTCM standard, are used for real-time regional VTEC modeling. After the acquisition of GNSS data, cycle slips for each satellite-receiver pair are detected, and ionosphere observations are constructed via the linear combination of carrier-phase observations in the data pre-processing step. The unknown B-spline coefficients, as well as the biases for each phase-continuous arc belonging to each receiver-satellite pair, are simultaneously estimated in the Kalman filter.
Within this study, we compare the performance of regional and global VTEC products generated in real-time using the well-known dSTEC analysis.
Eren Erdogan; Andreas Goss; Michael Schmidt; Denise Dettmering; Florian Seitz; Jennifer Müller; Ernst Lexen; Barbara Görres; Wilhelm F. Kersten. Real-time regional VTEC modeling based on B-splines using real-time GPS and GLONASS observations. 2021, 1 .
AMA StyleEren Erdogan, Andreas Goss, Michael Schmidt, Denise Dettmering, Florian Seitz, Jennifer Müller, Ernst Lexen, Barbara Görres, Wilhelm F. Kersten. Real-time regional VTEC modeling based on B-splines using real-time GPS and GLONASS observations. . 2021; ():1.
Chicago/Turabian StyleEren Erdogan; Andreas Goss; Michael Schmidt; Denise Dettmering; Florian Seitz; Jennifer Müller; Ernst Lexen; Barbara Görres; Wilhelm F. Kersten. 2021. "Real-time regional VTEC modeling based on B-splines using real-time GPS and GLONASS observations." , no. : 1.
Increasing ice loss of the Antarctic and Greenland Ice Sheets (AIS, GrIS) due to global climate change affects the orientation of the Earth’s spin axis with respect to an Earth-fixed reference system (polar motion). Ice mass changes in Antarctica and Greenland are observed by the Gravity Recovery and Climate Experiment (GRACE) in terms of time variable gravity field changes and derived from surface elevation changes measured by satellite radar and laser altimeter missions such as ENVISAT, CryoSat-2 and ICESat. Beside the limited spatial resolution, the accuracy of GRACE ice mass change estimates is limited by signal noise (meridional error stripes), leakage effects and uncertainties of the glacial isostatic adjustment (GIA) models, whereas the accuracy of satellite altimetry derived ice mass changes is limited by waveform retracking, slope related relocation errors, firn compaction and the density assumption used in the volume-to-mass conversion.
In this study we use different GRACE gravity field models (CSR RL06M, JPL RL06M, ITSG-Grace2018) and satellite altimetry data (from TU Dresden, University of Leeds, Alfred Wegener Institute) to assess the accuracy of the gravimetry and altimetry derived polar motion excitation functions. We show that due to the combination of individual solutions, systematic and random errors of the data processing can be reduced and the robustness of the geodetic derived AIS and GrIS polar motion excitation functions can be increased. Based on these investigations we found that AIS mass changes induce the pole position vector to drift along the 60° East meridian by 2 mas/yr during the study period 2003-2015, whereas GrIS mass changes cause the pole vector to drift along the 45° West meridian by 3 mas/yr.
Franziska Göttl; Andreas Groh; Maria Kappelsberger; Undine Strößenreuther; Ludwig Schröder; Veit Helm; Michael Schmidt; Florian Seitz. The influence of Antarctic and Greenland ice loss on polar motion: an assessment based on GRACE and multi-mission satellite altimetry. 2021, 1 .
AMA StyleFranziska Göttl, Andreas Groh, Maria Kappelsberger, Undine Strößenreuther, Ludwig Schröder, Veit Helm, Michael Schmidt, Florian Seitz. The influence of Antarctic and Greenland ice loss on polar motion: an assessment based on GRACE and multi-mission satellite altimetry. . 2021; ():1.
Chicago/Turabian StyleFranziska Göttl; Andreas Groh; Maria Kappelsberger; Undine Strößenreuther; Ludwig Schröder; Veit Helm; Michael Schmidt; Florian Seitz. 2021. "The influence of Antarctic and Greenland ice loss on polar motion: an assessment based on GRACE and multi-mission satellite altimetry." , no. : 1.
EOT20 is the latest in a series of empirical ocean tide (EOT) models derived using residual tidal analysis of multi-mission satellite altimetry at DGFI-TUM. The amplitudes and phases of seventeen tidal constituents are provided on a global 0.125-degree grid based on empirical analysis of eleven satellite altimetry missions. The EOT20 model shows significant improvements compared to the previous iteration of the global model (EOT11a) throughout the ocean, particularly in the coastal and shelf regions, due to the inclusion of more recent satellite altimetry data as well as more missions, the use of the updated FES2014 tidal model as a reference to estimated residual signals, the inclusion of the ALES retracker and improved coastal representation. In the validation of EOT20 using tide gauges and ocean bottom pressure data, these improvements in the model compared to EOT11a are highlighted with the root-square sum (RSS) of the eight major tidal constituents improving by ~3 cm for the entire global ocean with the major improvement in RSS (~3.5 cm) occurring in coastal regions (<1 km to the coast). Compared to the other global ocean tidal models, EOT20 shows a clear improvement of ~0.4 cm in RSS compared to the closest model (FES2014) in the global ocean. Compared to the FES2014 model, the RSS improvement in EOT20 is mainly seen in the coastal region (~0.45 cm) while in the shelf and open ocean regions these two models only vary in terms of RSS by ~0.005 cm. The significant improvement of EOT20, particularly in the coastal region, provides encouragement for the use of the EOT20 model as a tidal correction of satellite altimetry in coastal sea level research.
Michael Hart-Davis; Denise Dettmering; Gaia Piccioni; Christian Schwatke; Marcello Passaro; Florian Seitz. EOT20: A new global empirical ocean tide model derived from multi-mission satellite altimetry. 2021, 1 .
AMA StyleMichael Hart-Davis, Denise Dettmering, Gaia Piccioni, Christian Schwatke, Marcello Passaro, Florian Seitz. EOT20: A new global empirical ocean tide model derived from multi-mission satellite altimetry. . 2021; ():1.
Chicago/Turabian StyleMichael Hart-Davis; Denise Dettmering; Gaia Piccioni; Christian Schwatke; Marcello Passaro; Florian Seitz. 2021. "EOT20: A new global empirical ocean tide model derived from multi-mission satellite altimetry." , no. : 1.
Vertical land motion (VLM) at the coast is a substantial contributor to relative sea level change. In this work, we present a refined method for its determination, which is based on the combination of absolute satellite altimetry (SAT) sea level measurements and relative sea level changes recorded by tide gauges (TGs). These measurements complement VLM estimates from the GNSS (Global Navigation Satellite System) by increasing their spatial coverage. Trend estimates from the SAT and TG combination are particularly sensitive to the quality and resolution of applied altimetry data as well as to the coupling procedure of altimetry and TGs. Hence, a multi-mission, dedicated coastal along-track altimetry dataset is coupled with high-frequency TG measurements at 58 stations. To improve the coupling procedure, a so-called “zone of influence” (ZOI) is defined, which confines coherent zones of sea level variability on the basis of relative levels of comparability between TG and altimetry observations. Selecting 20 % of the most representative absolute sea level observations in a 300 km radius around the TGs results in the best VLM estimates in terms of accuracy and uncertainty. At this threshold, VLMSAT-TG estimates have median formal uncertainties of 0.58 mm yr−1. Validation against GNSS VLM estimates yields a root mean square (rmsΔVLM) of VLMSAT-TG and VLMGNSS differences of 1.28 mm yr−1, demonstrating the level of accuracy of our approach. Compared to a reference 250 km radius selection, the 300 km zone of influence improves trend accuracies by 15 % and uncertainties by 35 %. With increasing record lengths, the spatial scales of the coherency in coastal sea level trends increase. Therefore, the relevance of the ZOI for improving VLMSAT-TG accuracy decreases. Further individual zone of influence adaptations offer the prospect of bringing the accuracy of the estimates below 1 mm yr−1.
Julius Oelsmann; Marcello Passaro; Denise Dettmering; Christian Schwatke; Laura Sánchez; Florian Seitz. The zone of influence: matching sea level variability from coastal altimetry and tide gauges for vertical land motion estimation. Ocean Science 2021, 17, 35 -57.
AMA StyleJulius Oelsmann, Marcello Passaro, Denise Dettmering, Christian Schwatke, Laura Sánchez, Florian Seitz. The zone of influence: matching sea level variability from coastal altimetry and tide gauges for vertical land motion estimation. Ocean Science. 2021; 17 (1):35-57.
Chicago/Turabian StyleJulius Oelsmann; Marcello Passaro; Denise Dettmering; Christian Schwatke; Laura Sánchez; Florian Seitz. 2021. "The zone of influence: matching sea level variability from coastal altimetry and tide gauges for vertical land motion estimation." Ocean Science 17, no. 1: 35-57.
The ionosphere is one of the main error sources in positioning and navigation; thus, information about the ionosphere is mandatory for precise modern Global Navigation Satellite System (GNSS) applications. The International GNSS Service (IGS) and its Ionosphere Associated Analysis Centers (IAAC) routinely provide ionospheric information in terms of global ionosphere maps (final GIM). Typically, these products are modeled using series expansion in terms of spherical harmonics (SHs) with a maximum degree of n=15 and are based on post processed observations from Global Navigation Satellite Systems (GNSS), as well as final satellite orbits. However, precise applications such as autonomous driving or precision agriculture require real-time (RT) information about the ionospheric electron content with high spectral and spatial resolution. Ionospheric RT-GIMs are disseminated via Ntrip protocol using the SSR VTEC message of the RTCM. This message can be streamed in RT, but it is limited for the dissemination of coefficients of SHs of lower degrees only. It allows the dissemination of SH coefficients up to a degree of n=16. This suits to most the SH models of the IAACs, but higher spectral degrees or models in terms of B-spline basis functions, voxels, splines and many more cannot be considered. In addition to the SHs, several alternative approaches, e.g., B-splines or Voxels, have proven to be appropriate basis functions for modeling the ionosphere with an enhanced resolution. Providing them using the SSR VTEC message requires a transfer to SHs. In this context, the following questions are discussed based on data of a B-spline model with high spectral resolution; (1) How can the B-spline model be transformed to SHs in order to fit to the RTCM requirements and (2) what is the loss of detail when the B-spline model is converted to SHs of degree of n=16? Furthermore, we discuss (3) what is the maximum necessary SH degree n to convert the given B-spline model and (4) how can the transformation be performed to make it applicable for real-time applications? For a final assessment, we perform both, the dSTEC analysis and a single-frequency positioning in kinematic mode, using the transformed GIMs for correcting the ionospheric delay. The assessment shows that the converted GIMs with degrees n≥30 coincide with the original B-spline model and improve the positioning accuracy significantly.
Andreas Goss; Manuel Hernández-Pajares; Michael Schmidt; David Roma-Dollase; Eren Erdogan; Florian Seitz. High-Resolution Ionosphere Corrections for Single-Frequency Positioning. Remote Sensing 2020, 13, 12 .
AMA StyleAndreas Goss, Manuel Hernández-Pajares, Michael Schmidt, David Roma-Dollase, Eren Erdogan, Florian Seitz. High-Resolution Ionosphere Corrections for Single-Frequency Positioning. Remote Sensing. 2020; 13 (1):12.
Chicago/Turabian StyleAndreas Goss; Manuel Hernández-Pajares; Michael Schmidt; David Roma-Dollase; Eren Erdogan; Florian Seitz. 2020. "High-Resolution Ionosphere Corrections for Single-Frequency Positioning." Remote Sensing 13, no. 1: 12.
Despite increasing interest in monitoring the global water cycle, the availability of in situ gauging and discharge time series is decreasing. However, this lack of ground data can partly be compensated for by using remote sensing techniques to observe river stages and discharge. In this paper, a new approach for estimating discharge by combining water levels from multi-mission satellite altimetry and surface area extents from optical imagery with physical flow equations at a single cross-section is presented and tested at the Lower Mississippi River. The datasets are combined by fitting a hypsometric curve, which is then used to derive the water level for each acquisition epoch of the long-term multi-spectral remote sensing missions. In this way, the chance of detecting water level extremes is increased and a bathymetry can be estimated from water surface extent observations. Below the minimum hypsometric water level, the river bed elevation is estimated using an empirical width-to-depth relationship in order to determine the final cross-sectional geometry. The required flow gradient is derived from the differences between virtual station elevations, which are computed in a least square adjustment from the height differences of all multi-mission satellite altimetry data that are close in time. Using the virtual station elevations, satellite altimetry data from multiple virtual stations and missions are combined to one long-term water level time series. All required parameters are estimated purely based on remote sensing data, without using any ground data or calibration. The validation at three gauging stations of the Lower Mississippi River shows large deviations primarily caused by the below average width of the predefined cross-sections. At 13 additional cross-sections situated in wide, uniform, and straight river sections nearby the gauges the Normalized Root Mean Square Error (NRMSE) varies between 10.95% and 28.43%. The Nash-Sutcliffe Efficiency (NSE) for these targets is in a range from 0.658 to 0.946.
Daniel Scherer; Christian Schwatke; Denise Dettmering; Florian Seitz. Long-Term Discharge Estimation for the Lower Mississippi River Using Satellite Altimetry and Remote Sensing Images. Remote Sensing 2020, 12, 2693 .
AMA StyleDaniel Scherer, Christian Schwatke, Denise Dettmering, Florian Seitz. Long-Term Discharge Estimation for the Lower Mississippi River Using Satellite Altimetry and Remote Sensing Images. Remote Sensing. 2020; 12 (17):2693.
Chicago/Turabian StyleDaniel Scherer; Christian Schwatke; Denise Dettmering; Florian Seitz. 2020. "Long-Term Discharge Estimation for the Lower Mississippi River Using Satellite Altimetry and Remote Sensing Images." Remote Sensing 12, no. 17: 2693.
The Kalman filter (KF) is widely applied in (ultra) rapid and (near) real-time ionosphere modeling to meet the demand on ionosphere products required in many applications extending from navigation and positioning to monitoring space weather events and naturals disasters. The requirement of a prior definition of the stochastic models attached to the measurements and the dynamic models of the KF is a drawback associated with its standard implementation since model uncertainties can exhibit temporal variations or the time span of a given test data set would not be large enough. Adaptive methods can mitigate these problems by tuning the stochastic model parameters during the filter run-time. Accordingly, one of the primary objectives of our study is to apply an adaptive KF based on variance component estimation to compute the global Vertical Total Electron Content (VTEC) of the ionosphere by assimilating different ionospheric GNSS measurements. Secondly, the derived VTEC representation is based on a series expansion in terms of compactly supported B-spline functions. We highlight the morphological similarity of the spatial distributions and the magnitudes between VTEC values and the corresponding estimated B-spline coefficients. This similarity allows for deducing physical interpretations from the coefficients. In this context, an empirical adaptive model to account for the dynamic model uncertainties, representing the temporal variations of VTEC errors, is developed in this work according to the structure of B-spline coefficients. For the validation, the differential slant total electron content (dSTEC) analysis and a comparison with Jason-2/3 altimetry data are performed. Assessments show that the quality of the VTEC products derived by the presented algorithm is in good agreement, or even more accurate, with the products provided by IGS ionosphere analysis centers within the selected periods in 2015 and 2017. Furthermore, we show that the presented approach can be applied to different ionosphere conditions ranging from very high to low solar activity without concerning time-variable model uncertainties, including measurement error and process noise of the KF because the associated covariance matrices are computed in a self-adaptive manner during run-time.
Eren Erdogan; Michael Schmidt; Andreas Goss; Barbara Görres; Florian Seitz. Adaptive Modeling of the Global Ionosphere Vertical Total Electron Content. Remote Sensing 2020, 12, 1822 .
AMA StyleEren Erdogan, Michael Schmidt, Andreas Goss, Barbara Görres, Florian Seitz. Adaptive Modeling of the Global Ionosphere Vertical Total Electron Content. Remote Sensing. 2020; 12 (11):1822.
Chicago/Turabian StyleEren Erdogan; Michael Schmidt; Andreas Goss; Barbara Görres; Florian Seitz. 2020. "Adaptive Modeling of the Global Ionosphere Vertical Total Electron Content." Remote Sensing 12, no. 11: 1822.
Vertical land motion (VLM) at the coast is a substantial contributor to relative sea level change. In this work, we present a refined method for its determination, which is based on the combination of absolute satellite alimetry (SAT) sea level measurements and relative sea level changes recorded by tide gauges (TG). These measurements complement VLM estimates based on GNSS (Global Navigation Satellite System) by increasing their spatial coverage. Trend estimates from SAT and TG combination are particularly sensitive to the quality and resolution of applied altimetry data as well as to the coupling procedure of altimetry and tide gauges. Hence, a multi-mission, dedicated coastal along-track altimetry dataset is coupled with highfrequent tide gauge measurements at 58 stations. To improve the coupling-procedure, a so-called `Zone of Influence’ is defined to identify coherent zones of sea level variability on the basis of relative levels of comparability between tide gauge and altimetry observations. Selecting 20 % of the most representative absolute sea level observations in a 300 km radius around the tide gauges results in the best VLM-estimates in terms of accuracies and uncertainties. At this threshold, VLM_SAT-TG estimates have median formal uncertainties of 0.59 mm/year. Validation against GNSS VLM estimates yields a root-mean-square (RMS_VLM) of VLM_SAT-TG and VLM_GNSS differences of 1.28 mm/year, demonstrating the level of accuracy of our approach. Compared to a reference 250 km radius selection of sea level anomalies, the 300 km Zone of Influence improves trend accuracies by 12 % and uncertainties by 28 %. With progressing record lengths, the spatial scales of coastal sea level trend coherency increase. Therefore the relevance of the ZOI for improving VLM_SAT-TG accuracies decreases. Further individual Zone of Influence adaptations offer the prospect of bringing the accuracy of the estimates below 1 mm/year.
Julius Oelsmann; Marcello Passaro; Denise Dettmering; Christian Schwatke; Laura Sanchez; Florian Seitz. The Zone of Influence: Matching sea level variability from coastal altimetry and tide gauges for vertical land motion estimation. 2020, 2020, 1 -32.
AMA StyleJulius Oelsmann, Marcello Passaro, Denise Dettmering, Christian Schwatke, Laura Sanchez, Florian Seitz. The Zone of Influence: Matching sea level variability from coastal altimetry and tide gauges for vertical land motion estimation. . 2020; 2020 ():1-32.
Chicago/Turabian StyleJulius Oelsmann; Marcello Passaro; Denise Dettmering; Christian Schwatke; Laura Sanchez; Florian Seitz. 2020. "The Zone of Influence: Matching sea level variability from coastal altimetry and tide gauges for vertical land motion estimation." 2020, no. : 1-32.
In this study, a new approach for estimating volume variations of lakes and reservoirs using water levels from satellite altimetry and surface areas from optical imagery is presented. Both input data sets, namely water level time series and surface area time series, are provided by the Database of Hydrological Time Series of Inland Waters (DAHITI), developed and maintained by the Deutsches Geodätisches Forschungsinsitut der Technischen Universität München (DGFI-TUM). The approach is divided into three parts. In the first part, a hypsometry model based on the new modified Strahler approach is computed by combining water levels and surface areas. The hypsometry model describes the dependency between water levels and surface areas of lakes and reservoirs. In the second part, a bathymetry between minimum and maximum surface area is computed. For this purpose, DAHITI land-water masks are stacked using water levels derived from the hypsometry model. Finally, water levels and surface areas are intersected with the bathymetry to estimate a time series of volume variations in relation to the minimum observed surface area. The results are validated with volume time series derived from in-situ water levels in combination with bathymetric surveys. In this study, 28 lakes and reservoirs located in Texas are investigated. The absolute volumes of the investigated lakes and reservoirs vary between 0.062 km 3 and 6.041 km 3 . The correlation coefficients of the resulting volume variation time series with validation data vary between 0.80 and 0.99. Overall, the relative errors with respect to volume variations vary between 2.8% and 14.9% with an average of 8.3% for all 28 investigated lakes and reservoirs. When comparing the resulting RMSE with absolute volumes, the absolute errors vary between 1.5% and 6.4% with an average of 3.1%. This study shows that volume variations can be calculated with a high accuracy which depends essentially on the quality of the used water levels and surface areas. In addition, this study provides a hypsometry model, high-resolution bathymetry and water level time series derived from surface areas based on the hypsometry model. All data sets are publicly available on the Database of Hydrological Time Series of Inland Waters.
Christian Schwatke; Denise Dettmering; Florian Seitz. Volume Variations of Small Inland Water Bodies from a Combination of Satellite Altimetry and Optical Imagery. Remote Sensing 2020, 12, 1606 .
AMA StyleChristian Schwatke, Denise Dettmering, Florian Seitz. Volume Variations of Small Inland Water Bodies from a Combination of Satellite Altimetry and Optical Imagery. Remote Sensing. 2020; 12 (10):1606.
Chicago/Turabian StyleChristian Schwatke; Denise Dettmering; Florian Seitz. 2020. "Volume Variations of Small Inland Water Bodies from a Combination of Satellite Altimetry and Optical Imagery." Remote Sensing 12, no. 10: 1606.
Radar altimeters have been measuring ocean significant wave height for more than three decades, with their data used to record the severity of storms, the mixing of surface waters and the potential threats to offshore structures and low-lying land, and to improve operational wave forecasting. Understanding climate change and long-term planning for enhanced storm and flooding hazards are imposing more stringent requirements on the robustness, precision, and accuracy of the estimates than have hitherto been needed. Taking advantage of novel retracking algorithms, particularly developed for the coastal zone, the present work aims at establishing an objective baseline processing chain for wave height retrieval that can be adapted to all satellite missions. In order to determine the best performing retracking algorithm for both Low Resolution Mode and Delay-Doppler altimetry, an objective assessment is conducted in the framework of the European Space Agency Sea State Climate Change Initiative project. All algorithms process the same Level-1 input dataset covering a time-period of up to two years. As a reference for validation, an ERA5-based hindcast wave model as well as an in-situ buoy dataset from the Copernicus Marine Environment Monitoring Service In Situ Thematic Centre database are used. Five different metrics are evaluated: percentage and types of outliers, level of measurement noise, wave spectral variability, comparison against wave models, and comparison against in-situ data. The metrics are evaluated as a function of the distance to the nearest coast and the sea state. The results of the assessment show that all novel retracking algorithms perform better in the majority of the metrics than the baseline algorithms currently used for operational generation of the products. Nevertheless, the performance of the retrackers strongly differ depending on the coastal proximity and the sea state. Some retrackers show high correlations with the wave models and in-situ data but significantly under- or overestimate large-scale spectral variability. We propose a weighting scheme to select the most suitable retrackers for the Sea State Climate Change Initiative programme.
Florian Schlembach; Marcello Passaro; Graham D. Quartly; Andrey Kurekin; Francesco Nencioli; Guillaume Dodet; Jean-François Piollé; Fabrice Ardhuin; Jean Bidlot; Christian Schwatke; Florian Seitz; Paolo Cipollini; Craig Donlon. Round Robin Assessment of Radar Altimeter Low Resolution Mode and Delay-Doppler Retracking Algorithms for Significant Wave Height. Remote Sensing 2020, 12, 1254 .
AMA StyleFlorian Schlembach, Marcello Passaro, Graham D. Quartly, Andrey Kurekin, Francesco Nencioli, Guillaume Dodet, Jean-François Piollé, Fabrice Ardhuin, Jean Bidlot, Christian Schwatke, Florian Seitz, Paolo Cipollini, Craig Donlon. Round Robin Assessment of Radar Altimeter Low Resolution Mode and Delay-Doppler Retracking Algorithms for Significant Wave Height. Remote Sensing. 2020; 12 (8):1254.
Chicago/Turabian StyleFlorian Schlembach; Marcello Passaro; Graham D. Quartly; Andrey Kurekin; Francesco Nencioli; Guillaume Dodet; Jean-François Piollé; Fabrice Ardhuin; Jean Bidlot; Christian Schwatke; Florian Seitz; Paolo Cipollini; Craig Donlon. 2020. "Round Robin Assessment of Radar Altimeter Low Resolution Mode and Delay-Doppler Retracking Algorithms for Significant Wave Height." Remote Sensing 12, no. 8: 1254.
The ionosphere is one of the largest error sources in GNSS (Global Navigation Satellite Systems) applications and can cause up to several meters of error in positioning. Especially for single-frequency users, who cannot correct the ionospheric delay, the information about the state of the ionosphere is mandatory. Dual- and multi-frequency GNSS users, on the other hand, can correct the ionospheric effect on their observations by linear combination. However, real-time applications such as autonomous driving or precision farming, require external high accuracy corrections for fast convergence. Mostly, this external information is given in terms of grids or coefficients of the vertical total electron content (VTEC). Globally distributed GNSS stations of different networks, such as the network of the International GNSS Services (IGS), provide a large number of multi-frequency observations which can be used to determine the state of the ionosphere. These data are used to generate Global Ionosphere Maps (GIM). Due to the inhomogeneous global distribution of GNSS real-time stations and especially due to the large data gaps over oceanic areas, the global VTEC models are usually limited in their spatial and spectral resolution. Most of the GIMs are mathematically based on globally defined radial basis functions, i.e., spherical harmonics (SH), with a maximum degree of 15 and provided with a spatial resolution of 2.5 ° × 5 ° in latitude and longitude, respectively. Regional GNSS networks, however, offer dense clusters of observations, which can be used to generate regional VTEC solutions with a higher spectral resolution. In this study, we introduce a two-step model (TSM) comprising a global model as the first step and a regional model as the second step. We apply polynomial and trigonometric B-spline functions to represent the global VTEC. Polynomial B-splines are used for modelling the finer structures of VTEC within selected regions, i.e., the densification areas. The TSM provides both, a global and a regional VTEC map at the same time. In order to study the performance, we apply the developed approach to hourly data of the global IGS network as well as the EUREF network of the European region for St. Patrick storm in March 2015. For the assessment of the generated maps, we use the dSTEC analysis and compare both maps with different global and regional products from the IGS Ionosphere Associated Analysis Centers, e.g., the global product from CODE (Berne, Switzerland) and from UPC (Barcelona, Spain), as well as the regional maps from ROB (Brussels, Belgium). The assessment shows a significant improvement of the regional VTEC representation in the form of the generated TSM maps. Among all other products used for comparison, the developed regional one is of the highest accuracy within the selected time span. Since the numerical tests are performed using hourly data with a latency of one to two hours, the presented procedure is seen as an intermediate step for the generation of high precision regional real-time corrections for modern applications.
Andreas Goss; Michael Schmidt; Eren Erdogan; Florian Seitz. Global and Regional High-Resolution VTEC Modelling Using a Two-Step B-Spline Approach. Remote Sensing 2020, 12, 1198 .
AMA StyleAndreas Goss, Michael Schmidt, Eren Erdogan, Florian Seitz. Global and Regional High-Resolution VTEC Modelling Using a Two-Step B-Spline Approach. Remote Sensing. 2020; 12 (7):1198.
Chicago/Turabian StyleAndreas Goss; Michael Schmidt; Eren Erdogan; Florian Seitz. 2020. "Global and Regional High-Resolution VTEC Modelling Using a Two-Step B-Spline Approach." Remote Sensing 12, no. 7: 1198.
Polar motion is caused by mass redistribution and motion within the Earth system. The GRACE satellite mission observed variations of the Earth’s gravity field which are caused by mass redistribution. Therefore GRACE time variable gravity field models are a valuable source to estimate individual geophysical mass-related excitations of polar motion. Since GRACE observations contain erroneous meridional stripes, filtering is essential in order to retrieve meaningful information about mass redistribution within the Earth system. However filtering reduces not only the noise but also smooths the signal and induces leakage of neighboring subsystems into each other.
We present a novel approach to reduce these filter effects in GRACE-derived equivalent water heights and polar motion excitation functions which is based on once and twice filtered gravity field solutions. The advantages of this method are that it is independent from geophysical model information, works on global grid point scale and can therefore be used for mass variation estimations of several subsystems of the Earth (e.g. continental hydrosphere, oceans, Antarctica and Greenland). In order to validate this new method, we perform a closed-loop simulation based on a realistic orbit scenario and error assumptions for instruments and background models, apply it to real GRACE data (GFZ RL06) and show comparisons with ocean model results from ECCO and MPIOM.
Franziska Göttl; Michael Murböck; Michael Schmidt; Florian Seitz. Reducing filter effects in GRACE-derived polar motion excitations. 2020, 1 .
AMA StyleFranziska Göttl, Michael Murböck, Michael Schmidt, Florian Seitz. Reducing filter effects in GRACE-derived polar motion excitations. . 2020; ():1.
Chicago/Turabian StyleFranziska Göttl; Michael Murböck; Michael Schmidt; Florian Seitz. 2020. "Reducing filter effects in GRACE-derived polar motion excitations." , no. : 1.