Prof. Dr. Thomas Hobiger

@article{ATLAS2025,

title = {Motivation, structure and goals of the Collaborative Research Centre 1667: Advancing Technologies of very Low-Altitude Satellites—ATLAS},

author = {Stefanos Fasoulas and Adam S. Pagan and Constantin Traub and Björn Annighöfer and Stefanie Barz and Andrea Beck and Torbjørn Cunis and Thomas Dekorsy and Stephanie Essig and Walter Fichter and Bernd Flemisch and Georg Herdrich and Thomas Hobiger and Ingmar Kallfass and Johannes Kästner and Sabine Klinkner and Grazia Lamanna and Stefan Loehle and Marcel Pfeiffer and Rico Poser and Johannes Roth and Michael Saliba and Martin Schneider and Nico Sneeuw and Gerd Wagner},

url = {https://doi.org/10.1007/s12567-025-00687-8},

doi = {10.1007/s12567-025-00687-8},

year  = {2025},

date = {2025-01-01},

journal = {CEAS Space Journal},

abstract = {The Collaborative Research Centre (CRC) 1667 ``Advancing Technologies of Very Low Altitude Satellites—ATLAS''was established in April 2024 with the scientific goal of addressing the fundamental challenges of making satellite operations in Very Low Earth Orbits (VLEO) sustainable. These orbits are beneficial for satellite services that have become indispensable to our modern society. Moreover, access to VLEO offers the opportunity to operate satellites without exposure or contribution to the increasing contamination of traditional orbits with space debris. Seventeen highly interlinked research projects have been selected to investigate and advance accurate numerical and experimental methods for gas–surface interactions, novel concepts utilising the residual atmosphere and minimising the satellite sizes, and mission-related challenges of a selected scenario. In addition, support projects cover topics related to public outreach and academic exchange and assist in achieving the strategic goal of positioning the University of Stuttgart as a key contributor to this internationally very important research area. In summary, the CRC ATLAS aims to constitute a research-oriented profile-building measure at the University of Stuttgart with a strong international reputation.},

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tppubtype = {article}

}

@article{TOPP2025,

title = {INSTINCT: a flow-based open-source PNT framework for satellite navigation and sensor fusion},

author = {Thomas Topp and Marcel Maier and Thomas Hobiger and Doris Becker},

url = {https://doi.org/10.1007/s10291-025-01927-4},

doi = {10.1007/s10291-025-01927-4},

year  = {2025},

date = {2025-01-01},

journal = {GPS Solutions},

volume = {29},

number = {4},

pages = {171},

abstract = {INS toolkit for integrated navigation concepts and training (INSTINCT) is an open-source positioning, navigation and timing (PNT) framework for global navigation satellite system (GNSS) navigation and sensor fusion written in C++. It uses flow-based programming to encapsulate functionality, enforce clean interfaces and promote reusability. Not only multi-constellation, multi-frequency single point positioning (SPP) and real-time kinematic positioning (RTK) algorithms are available, but also inertial navigation system (INS)/GNSS sensor fusion. Moreover, innovative concepts like multi inertial measurement unit (IMU) arrays and factor graph optimization are featured. Furthermore, most file formats common in the PNT field can be read with the software and converted between them. Also, simulation of trajectories and IMU data with different error models is possible. A graphical user interface allows the user to directly set parameters and analyze results in plots, which enables rapid prototyping and testing. A developer can easily extend the functionality with own algorithms and sensor interfaces building upon the existing modules. In order to evaluate the performance of the algorithms two experiments were performed. Analysis of a static dataset shows that the position accuracy of the RTK algorithm of INSTINCT is comparable to RTKLIB. Additionally, a dynamic dataset was generated using a Spirent GNSS simulator and INSTINCT's IMU simulation capabilities. In-depth assessment confirms the high accuracy of the results and demonstrates that the INS/GNSS loosely coupled Kalman filter can compensate for GNSS outages.},

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tppubtype = {article}

}

@article{GHRIBI202549,

title = {Receiver Autonomous Integrity Monitoring for LEO Satellites: Outlier Detection and Exclusion},

author = {Adam Ghribi and Marvin B. Stucke and Thomas Hobiger and Stefan Winkler},

url = {https://www.sciencedirect.com/science/article/pii/S2405896326000674},

doi = {https://doi.org/10.1016/j.ifacol.2026.01.060},

issn = {2405-8963},

year  = {2025},

date = {2025-01-01},

journal = {IFAC-PapersOnLine},

volume = {59},

number = {31},

pages = {49-54},

abstract = {Robust Precise Orbit Determination (POD) is essential for Low Earth Orbit (LEO) satellite missions. Based on observational data from Global Navigation Satellite System (GNSS), the orbit parameters are estimated by means of an adjustment process. To achieve accurate results, highly reliable navigation data are required. In this work, we investigate the application of Receiver Autonomous Integrity Monitoring (RAIM)-based methods — originally developed for aviation — to detect and exclude faulty GNSS measurements in a post-processed POD framework. Our approach utilizes redundant GNSS code measurements and evaluates the residuals from a weighted least squares (WLSQ) orbit solution. Based on a predefined false alarm rate, we set a threshold and compare it to a residual-based test statistic. The method enables detection and exclusion of outliers in post-processed solutions. Simulation and real GRACE-FO data results demonstrate the effectiveness of the applied RAIM-based method. It enhances the integrity and reliability of GNSS-based POD solutions for LEO satellites.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{He24GPSSol,

title = {Improving PPP positioning and troposphere estimates using an azimuth-dependent weighting scheme},

author = {Shengping He and Thomas Hobiger and Doris Becker},

url = {https://link.springer.com/article/10.1007/s10291-024-01754-z},

doi = {10.1007/s10291-024-01754-z},

year  = {2024},

date = {2024-01-01},

journal = {GPS Solutions},

volume = {24},

number = {212},

pages = {1-14},

abstract = {Asymmetric troposphere modeling is crucial in Precise Point Positioning (PPP). The functional model of the asymmetric troposphere has been thoroughly studied, while the stochastic model lacks discussion. Currently, there is no suitable stochastic model for asymmetric tropospheric conditions, potentially degrading the positioning accuracy and the reliability of Zenith Total/Wet Delay (ZTD/ZWD) estimates. This paper introduces an Azimuth-Dependent Weighting (ADW) scheme that utilizes information from asymmetric mapping functions to adaptively weight Global Navigation Satellite System (GNSS) observations affected by azimuth-dependent errors. The concept of ADW has been validated using Numerical Weather Prediction data and International GNSS Service data. The results indicate that ADW effectively improves the coordinate repeatability of the PPP solution by approximately 10% in the horizontal and 20% in the vertical direction. Additionally, ADW appears to be capable to improve the ZWD estimates during the PPP convergence period and yields smoother ZWD estimates. Consequently, it is recommended to adopt this new weighting scheme in PPP applications when an asymmetric mapping functions is employed.},

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pubstate = {published},

tppubtype = {article}

}

@article{He24JoG,

title = {The B-spline mapping function (BMF): representing anisotropic troposphere delays by a single self-consistent functional model},

author = {Shengping He and Thomas Hobiger and Doris Becker},

url = {https://doi.org/10.1007/s00190-024-01864-z},

doi = {10.1007/s00190-024-01864-z},

year  = {2024},

date = {2024-01-01},

journal = {Journal of Geodesy},

volume = {98},

number = {6},

pages = {1432-1394},

abstract = {Troposphere’s asymmetry can introduce errors ranging from centimeters to decimeters at low elevation angles, which cannot be ignored in high-precision positioning technology and meteorological research. The traditional two-axis gradient model, which strongly relies on an open-sky environment of the receiver, exhibits misfits at low elevation angles due to their simplistic nature. In response, we propose a directional mapping function based on cyclic B-splines named B-spline mapping function (BMF). This model replaces the conventional approach, which is based on estimating Zenith Wet Delay and gradient parameters, by estimating only four parameters which enable a continuous characterization of the troposphere delay across any directions. A simulation test, based on a numerical weather model, was conducted to validate the superiority of cyclic B-spline functions in representing tropospheric asymmetry. Based on an extensive analysis, the performance of BMF was assessed within precise point positioning using data from 45 International GNSS Service stations across Europe and Africa. It is revealed that BMF improves the coordinate repeatability by approximately 10% horizontally and about 5% vertically. Such improvements are particularly pronounced under heavy rainfall conditions, where the improvement of 3-dimensional root mean square error reaches up to 13%},

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pubstate = {published},

tppubtype = {article}

}

@article{Wang24,

title = {Improving GNSS Meteorology by Fusing Measurements of Several Colocated Receivers on the Observation Level},

author = {Rui Wang and Grzegorz Marut and Tomasz Hadas and Thomas Hobiger},

url = {https://ieeexplore.ieee.org/abstract/document/10480123},

doi = {10.1109/JSTARS.2024.3381792},

year  = {2024},

date = {2024-01-01},

journal = {IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing},

volume = {17},

pages = {7841-7851},

abstract = {Zenith wet delay (ZWD) estimation is a key component for the global navigation satellite system (GNSS) meteorology. At present, the zenith hydrostatic delay can be computed with sufficient accuracy by means of empirical models, while the ZWD, which is induced by water vapor with the nature of highly spatio-temporal variability, is typically estimated as an unknown parameter in precise point positioning (PPP). Due to GNSS receiver noise and the system biases of GNSS receivers, the accuracy as well as the precision of ZWD estimates is limited. In this study, we propose a novel fusion model based on undifferenced GNSS pseudorange and carrier-phase observations for sites, which have several receivers connected to a single antenna or which are separated horizontally by only a few meters. By fusing GNSS measurements collected by multiple receivers on the observation level, our model can provide common ZWD estimates with a high temporal resolution, which can then be used for more accurate and reliable meteorologic applications on a local scale. According to results with simulated and real data, it is revealed that such combined ZWD estimates are superior to single receiver estimates in terms of precision and accuracy. On the other hand, it is confirmed that the estimation of a common ZWD parameter leads to an improvement in positioning accuracy and precision, especially in the vertical component.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wang23,

title = {Stochastic modeling with robust Kalman filter for real-time kinematic GPS single-frequency positioning},

author = {Rui Wang and Doris Becker and Thomas Hobiger},

url = {https://link.springer.com/10.1007/s10291-023-01479-5},

doi = {10.1007/s10291-023-01479-5},

year  = {2023},

date = {2023-01-01},

journal = {GPS Solutions},

volume = {13},

number = {3},

pages = {153},

abstract = {The centimeter-level positioning accuracy of real-time kinematic (RTK) depends on correctly resolving integer carrier-phase ambiguities. To improve the success rate of ambiguity resolution and obtain reliable positioning results, an enhanced Kalman filtering procedure has been developed. Based on a posteriori residuals of measurements and state predictions, the measurement noise variance–covariance matrix for double-differenced measurements is adaptively estimated, rather than approximated by an empirical function which uses satellite elevation angle as input. Since, in real-world situations, unexpected outliers and carrier-phase outages can degrade the filter performance, a stochastic model based on robust Kalman filtering is proposed, for which the double-differenced measurement noise variance–covariance matrix is computed empirically with a modified version of the IGG (Institute of Geodesy and Geophysics) III method in order to detect and identify outliers. The performance of the proposed method is assessed by two tests, one with simulated data and one with real data. In addition, the performance of F-ratio and W-ratio tests as proxies for the success of ambiguity fixing is investigated. Experimental results reveal that the proposed method can improve the reliability and robustness of relative kinematic positioning for simulation scenarios as well as in a real urban test.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Purnell20,

title = {Quantifying the Uncertainty in Ground-Based GNSS-Reflectometry Sea Level Measurements},

author = {David Purnell and Natalya Gomez and Ngai Ham Chan and Joakim Strandberg and David M. Holland and Thomas Hobiger},

url = {https://ieeexplore.ieee.org/abstract/document/9144372},

doi = {10.1109/JSTARS.2020.3010413},

year  = {2020},

date = {2020-01-01},

journal = {IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing},

volume = {13},

pages = {4419-4428},

abstract = {Global Navigation Satellite System reflectometry (GNSS-R) tide gauges are a promising alternative to traditional tide gauges. However, the precision of GNSS-R sea-level measurements when compared to measurements from a colocated tide gauge is highly variable, with no clear indication of what causes the variability. Here, we present a modeling technique to estimate the precision of GNSS-R sea-level measurements that relies on creating and analyzing synthetic signal-to-noise-ratio (SNR) data. The modeled value obtained from the synthetic SNR data is compared to observed root mean square error between GNSS-R measurements and a colocated tide gauge at five sites and using two retrieval methods: spectral analysis and inverse modeling. We find that the inverse method is more precise than the spectral analysis method by up to 60% for individual measurements but the two methods perform similarly for daily and monthly means. We quantify the contribution of dominant effects to the variations in precision and find that noise is the dominant source of uncertainty for spectral analysis whereas the effect of the dynamic sea surface is the dominant source of uncertainty for the inverse method. Additionally, we test the sensitivity of sea-level measurements to the choice of elevation angle interval and find that the spectral analysis method is more sensitive to the choice of elevation angle interval than the inverse method due to the effect of noise, which is greater at larger elevation angle intervals. Conversely, the effect of tropospheric delay increases for lower elevation angle intervals but is generally a minor contribution.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JSTARS-Hadas20,

title = {Benefits of using Galileo for Real-Time GNSS Meteorology},

author = {Tomasz Hadas and Thomas Hobiger},

url = {https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9141353},

doi = {10.1109/LGRS.2020.3007138.},

year  = {2020},

date = {2020-01-01},

journal = {IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing},

abstract = {Remote sensing of water vapor using Global Navigation Satellite Systems (GNSS) is a well-established tool for weather and climate monitoring. Current challenges of GNSS meteorology are real-time performance and the inclusion of emerging GNSS, such as Galileo. We demonstrate that real-time GPS-only, Galileo-only and GPS+Galileo solutions are consistent among each other. However, our results show that Galileo-only solutions tend to underestimate Zenith Total Delay (ZTD) with respect to GPS. The Galileo-only real-time ZTD is less accurate as the one from GPS. The combination of both GNSS leads to a superior product. The daily solution availability increases by up to 50%, and the overall gain is 0.7% over entire year. The accuracy improves by 3.7% to 8.5% and uncertainty is reduced by a factor of 1.5 to 2. A combined GPS and Galileo solution suppresses artifacts in a real-time ZTD product which otherwise would be attributed to high frequencies orbital effects.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{GPSSol-Hadas20,

title = {Considering different recent advancements in GNSS on real-time zenith troposphere},

author = {Tomasz Hadas and Thomas Hobiger and Pawel Hordyniec},

url = {https://doi.org/10.1007/s10291-020-01014-w},

doi = {10.1007/s10291-020-01014-w},

year  = {2020},

date = {2020-01-01},

journal = {GPS Solutions},

volume = {24},

number = {4},

abstract = {Global navigation satellite system (GNSS) remote sensing of the troposphere, called GNSS meteorology, is already a well-established tool in post-processing applications. Real-time GNSS meteorology has been possible since 2013, when the International GNSS Service (IGS) established its real-time service. The reported accuracy of the real-time zenith total delay (ZTD) has not improved significantly over time and usually remains at the level of 5–18 mm, depending on the station and test period studied. Millimeter-level improvements are noticed due to GPS ambiguity resolution, gradient estimation, or multi-GNSS processing. However, neither are these achievements combined in a single processing strategy, nor is the impact of other processing parameters on ZTD accuracy analyzed. Therefore, we discuss these shortcomings in detail and present a comprehensive analysis of the sensitivity of real-time ZTD on processing parameters. First, we identify a so-called common strategy, which combines processing parameters that are identified to be the most popular among published papers on the topic. We question the popular elevation-dependent weighting function and introduce an alternative one. We investigate the impact of selected processing parameters, i.e., PPP functional model, GNSS selection and combination, inter-system weighting, elevation-dependent weighting function, and gradient estimation. We define an advanced strategy dedicated to real-time GNSS meteorology, which is superior to the common one. The a posteriori error of estimated ZTD is reduced by 41%. The accuracy of ZTD estimates with the proposed strategy is improved by 17% with respect to the IGS final products and varies over stations from 5.4 to 10.1 mm. Finally, we confirm the latitude dependency of ZTD accuracy, but also detect its seasonality.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JoG-KLopotek20,

title = {Geodetic VLBI for precise orbit determination of Earth satellites: a simulation study},

author = {Grzegorz Klopotek and Thomas Hobiger and Ruediger Haas and Toshimichi Otsubo},

url = {https://doi.org/10.1007/s00190-020-01381-9},

doi = {10.1007/s00190-020-01381-9},

year  = {2020},

date = {2020-01-01},

journal = {Journal of Geodesy},

volume = {91},

number = {52},

abstract = {Recent efforts of tracking low Earth orbit and medium Earth orbit (MEO) satellites using geodetic very long baseline interferometry (VLBI) raise questions on the potential of this novel observation concept for space geodesy. Therefore, we carry out extensive Monte Carlo simulations in order to investigate the feasibility of geodetic VLBI for precise orbit determination (POD) of MEO satellites and assess the impact of quality and quantity of satellite observations on the derived geodetic parameters. The MEO satellites are represented in our study by LAGEOS-1/-2 and a set of Galileo satellites. The concept is studied on the basis of 3-day solutions in which satellite observations are included into real schedules of the continuous geodetic VLBI campaign 2017 (CONT17) as well as simulated schedules concerning the next-generation VLBI system, known as the VLBI Global Observing System (VGOS). Our results indicate that geodetic VLBI can perform on a comparable level as other space-geodetic techniques concerning POD of MEO satellites. For an assumed satellite observation precision better than 14.1 mm (47 ps), an average 3D orbit precision of 2.0 cm and 6.3 cm is found for schedules including LAGEOS-1/-2 and Galileo satellites, respectively. Moreover, geocenter offsets, which were so far out of scope for the geodetic VLBI analysis, are close to the detection limit for the simulations concerning VGOS observations of Galileo satellites, with the potential to further enhance the results. Concerning the estimated satellite orbits, VGOS leads to an average precision improvement of 80% with respect to legacy VLBI. In absolute terms and for satellite observation precision of 14.1 mm (47 ps), this corresponds to an average value of 17 mm and 7 mm concerning the 3D orbit scatter and precision of geocenter components, respectively. As shown in this study, a poor satellite geometry can degrade the derived Earth rotation parameters and VLBI station positions, compared to the quasar-only reference schedules. Therefore, careful scheduling of both quasar and satellite observations should be performed in order to fully benefit from this novel observation concept.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JoG-Nievinski19,

title = {SNR-based GNSS reflectometry for coastal sea-level altimetry – Results from the first IAG inter-comparison campaign},

author = {Felipe Nievinski and Thomas Hobiger and Rüdiger Haas and Wei Liu and Joakim Strandberg and Sajad Tabibi and Sibylle Vey and Simon Williams and Jens Wickert},

url = {https://doi.org/10.1007/s00190-020-01387-3},

year  = {2020},

date = {2020-01-01},

journal = {Journal of Geodesy},

volume = {94},

number = {70},

abstract = {Ground-based Global Navigation Satellite System Reflectometry (GNSS-R) is quickly maturing as a viable alternative for operational coastal sea-level (SL) altimetry in a geocentric reference frame. SL has immense societal implications related to climate change, both in its mean and extreme values. Of particular interest is the exploitation of existing GNSS networks for reflectometry by means of signal-to-noise ratio (SNR) observables. We report results from the first inter-comparison campaign on SNR-based GNSS-R. The goal was to cross-validate retrieval solutions from independent research groups under comparable conditions. This action was an initiative of the International Association of Geodesy working group 4.3.9 (2015-2019 term). Data collected at the Onsala Space Observatory for a one-year period (2015-2016) were compared to a co-located tide gauge (TG). SNR data for the GPS L1-C/A signal were processed by four groups, in Sweden, Luxembourg/Brazil, Germany, and the UK. Semi-diurnal tidal constituents showed good agreement between TG and all GNSS-R groups. Diurnal and lower-frequency components were also well captured by all series. Most solutions exhibited spurious tones at integer fractions of the GPS satellite revisit period (one sidereal day). Band-pass filtering between 3- and 30-hours confirmed that the dominant tidal components were well captured by most GNSS-R solutions. The higher-frequency band (< 3 h) is poorly represented by GNSS-R. The most discrepant solution neglects a correction associated with the rate of change in sea level and satellite elevation. Overall there was excellent performance, with correlation coefficients exceeding 0.9 and RMSE at a few centimeters.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{GPSSol-Strandberg19,

title = {Real-time sea-level monitoring using Kalman filtering of GNSS-R data},

author = {Joakim Strandberg and Thomas Hobiger and Rüdiger Haas},

url = {https://doi.org/10.1007/s10291-019-0851-1},

doi = {10.1007/s10291-019-0851-1},

year  = {2019},

date = {2019-01-01},

journal = {GPS Solutions},

volume = {23},

number = {61},

pages = {1},

abstract = {Current GNSS-R (GNSS reflectometry) techniques for sea surface measurements require data collection over longer periods, limiting their usability for real-time applications. In this work, we present a new, alternative GNSS-R approach based on the unscented Kalman filter and the so-called inverse modeling approach. The new method makes use of a mathematical description that relates SNR (signal-to-noise ratio) variations to multipath effects and uses a B-spline formalism to obtain time series of reflector height. The presented algorithm can provide results in real time with a precision that is significantly better than spectral inversion methods and almost comparable to results from inverse modeling in post-processing mode. To verify the performance, the method has been tested at station GTGU at the Onsala Space Observatory, Sweden, and at the station SPBY in Spring Bay, Australia. The RMS (root mean square) error with respect to nearby tide gauge data was found to be 2.0 cm at GTGU and 4.8 cm at SPBY when evaluating the output corresponding to real-time analysis. The method can also be applied in post-processing, resulting in RMS errors of 1.5 cm and 3.3 cm for GTGU and SPBY, respectively. Finally, based on SNR data from GTGU, it is also shown that the Kalman filter approach is able to detect the presence of sea ice with a higher temporal resolution than the previous methods and traditional remote sensing techniques which monitor ice in coastal regions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{EPS-Klopotek19,

title = {Position determination of the Chang’e 3 lander with geodetic VLBI},

author = {Grzegorz Klopotek and Thomas Hobiger and Rüdiger Haas and Zhongkai Zhang and Songtao Han and Axel Nothnagel and Laura La Porta and Frederic Jaron and Alexander Neidhardt and Christian Plötz},

url = {https://doi.org/10.1186/s40623-019-1001-2},

doi = {10.1186/s40623-019-1001-2},

year  = {2019},

date = {2019-01-01},

journal = {Earth, Planets and Space},

volume = {71},

number = {1},

pages = {23},

abstract = {We present results from the analysis of observations of the Chang'e 3 lander using geodetic Very Long Baseline Interferometry. The applied processing strategy as well as the limiting factors to our approach is discussed. We highlight the current precision of such observations and the accuracy of the estimated lunar-based parameters, i.e., the lunar lander's Moon-fixed coordinates. Our result for the position of the lander is $$44.12193^backslashcirc backslashhbox N$$44.12193∘N, $$-backslash,19.51159^backslashcirc backslashhbox E$$-19.51159∘Eand $$-backslash,2637.3$$-2637.3 m, with horizontal position uncertainties on the lunar surface of 8.9 m and 4.5 m in latitude and longitude, respectively. This result is in good agreement with the position derived from images taken by the Narrow Angle Camera of the Lunar Reconnaissance Orbiter. Finally, we discuss potential improvements to our approach, which could be used to apply the presented concept to high-precision lunar positioning and studies of the Moon."},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JOG-Klopotek17,

title = {Geodetic VLBI with an artificial radio source on the Moon - A simulation study},

author = {Grzegorz Klopotek and Thomas Hobiger and Rüdiger Haas},

url = {https://doi.org/10.1007/s00190-017-1072-4},

doi = {10.1007/s00190-017-1072-4},

year  = {2018},

date = {2018-01-01},

journal = {Journal of Geodesy},

volume = {92},

number = {5},

pages = {457–-469},

abstract = {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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{GRSL-Strandberg17,

title = {Coastal sea ice detection using ground-based GNSS-R},

author = {Joakim Strandberg and Thomas Hobiger and Rüdiger Haas},

url = {http://ieeexplore.ieee.org/document/7993065/},

doi = {10.1109/LGRS.2017.2722041},

year  = {2017},

date = {2017-01-01},

journal = {IEEE Geoscience and Remote Sensing Letters},

volume = {14},

number = {9},

pages = {1552 – 1556},

abstract = {Determination of sea ice extent is important both for climate modeling and transportation planning. Detection and monitoring of ice is often done by SAR imagery, but mostly without any ground truth. For the latter purpose, robust and continuously operating sensors are required. We demonstrate that signals recorded by ground-based GNSS receivers can detect coastal ice coverage on nearby water surfaces. Beside a description of the retrieval approach, we discuss why GNSS reflectometry is sensitive to the presence of sea ice. It is shown that during winter seasons with freezing periods, GNSS-R analysis of data recorded with a coastal GNSS installation clearly shows the occurrence of ice in the bay where this installation is located. Thus, coastal GNSS installations could be promising sources of ground truth for sea ice extent measurements.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{TGRS-Liu17,

title = {Coastal Sea Level Measurements Based on GNSS-R Phase Altimetry: A case study at the Onsala Space Observatory, Sweden},

author = {Wei Liu and Jamila Beckheinrich and Maximilian Semmling and Markus Ramatschi and Sibylle Vey and Jens Wickert and Thomas Hobiger and Rüdiger Haas},

url = {https://doi.org/10.1109/TGRS.2017.2711012},

doi = {10.1109/TGRS.2017.2711012},

year  = {2017},

date = {2017-01-01},

journal = {IEEE Transactions on Geoscience and Remote Sensing},

volume = {55},

number = {10},

pages = {5625–5636},

abstract = {The characterization of global mean sea level is important to predict floods and to quantify water resources for human use and irrigation, especially in coastal regions. Recently, the application of Global Navigation Satellite System Reflectometry (GNSS-R) for water level monitoring has been successfully demonstrated. This paper focuses on the retrieval of sea surface height within a field experiment, that was conducted at the Onsala Space Observatory (OSO) using the phase-based altimetry method. A continuous phase tracking algorithm, which relies on the GNSS amplitude and phase observations is proposed and works even under rough sea conditions at OSO’s coast. Factors, impacting the phase-based altimetry model, i.e. atmospheric propagation effects of the GNSS signals and influence of the GNSS-R observation instrument, are discussed. The relationship between the yield of coherent GNSS-R compared to the overall recorded events and the wind speed is investigated in detail. Ground-based sea level measurements from June 10 to July 3, 2015 demonstrate, that altimetric information about the reflecting water surface can be obtained with a Root Mean Square Error (RMSE) of 4.37 cm with respect to a reference tide gauge dataset. The sea surface changes, derived from our field experiment and the reference tide gauge, are highly correlated with a correlation coefficient of 0.93. The altimetric information can be retrieved even when the sea surface is very rough, corresponding to wind speeds up to 13 m/s. Moreover, the use of inexpensive conventional GNSS antennas shows that the system is useful for future large-scale sea level monitoring applications including numerous low-cost coastal ground stations.},

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tppubtype = {article}

}

@article{EPS-Kareinen17,

title = {Identifying optimal tag-along station locations for improving VLBI Intensive sessions},

author = {Niko Kareinen and Grzegorz Klopotek and Thomas Hobiger and Rüdiger Haas},

url = {http://earth-planets-space.springeropen.com/articles/10.1186/s40623-017-0601-y},

doi = {10.1186/s40623-017-0601-y},

year  = {2017},

date = {2017-01-01},

journal = {Earth, Planets and Space},

volume = {69},

number = {16},

pages = {1–9},

abstract = {Very Long Baseline Interferometry (VLBI) is a unique space-geodetic technique capable of direct observation of the Earth's phase of rotation, namely Universal Time (UT1). The International VLBI Service for Geodesy and Astrometry(IVS) conducts daily 1-h Intensive VLBI sessions to determine rapid variations in the difference between UT1 and Coordinated Universal Time (UTC). The mainobjective of the Intensive sessions is to provide timely UT1-UTC estimates.These estimates are especially crucial for Global Navigation Satellite Systems (GNSS). The monitoring of rapid variations in Earth rotation also provide insight into various geophysical phenomena. There is an ongoing effort to improve the quality of the UT1-UTC estimates from single-baseline Intensive sessions to realise the expected accuracy and to bring them to a better agreement with the 24-hour VLBI sessions. In this paper, we investigate the possibility to improve the Intensives by including a third station in tag-along mode to these regularly observed sessions. The impact of the additional station is studied via extensive simulations using the c5++ analysis software. The location of the station is varied within a pre-determined grid. Based on actual Intensive session schedules a set of simulated observations are generated for the two original stations and each grid point. These simulated data are used to estimate UT1-UTC for every Intensive session scheduled during the year 2014 on the Kokee–Wettzell and Tsukuba-Wettzell baselines, with the addition of a third station. We find that in tag-along mode when a third station is added to the schedule we can identify areas where the UT1-UTC estimates are improved up to 67 % w.r.t. the original single-baseline network. There are multiple operational VLBI stations in these areas, which could with little effort be included in a tag-along mode to the currently scheduled Intensive sessions, thus providing the possibility to improve the UT1-UTC estimates by extending the observation network.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JOG-Haas16,

title = {Ultra-rapid earth rotation determination with VLBI during CONT11 and CONT14},

author = {Rüdiger Haas and Thomas Hobiger and Shinobu Kurihara and Tetsuya Hara},

url = {http://link.springer.com/article/10.1007/s00190-016-0974-x},

doi = {10.1007/s00190-016-0974-x},

year  = {2017},

date = {2017-01-01},

journal = {Journal of Geodesy},

volume = {91},

number = {7},

pages = {831–837},

abstract = {We present earth rotation results from the ultra-rapid operations during the continuous VLBI campaigns CONT11 and CONT14. The baseline Onsala–Tsukuba, i.e., using two out of the 13 and 17 stations contributing to CONT11 and CONT14, respectively, was used to derive UT1-UTC in ultra-rapid mode during the ongoing campaigns. The latency between a new observation and a new UT1-UTC result was less than 10 min for more than 95% of the observations. The accuracy of the derived ultra-rapid UT1-UTC results is approximately a factor of three worse than results from optimized one-baseline sessions and/or complete analysis of large VLBI networks. This is, however, due to that the one-baseline picked from the CONT campaigns is not optimized for earth rotation determination. Our results prove that the 24/7 operation mode planned for VGOS, the next-generation VLBI system, is possible already today. However, further improvements in data connectivity of stations and correlators as well in the automated analysis are necessary to realize the ambitious VGOS plans.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{EPS-Otsubo16,

title = {Effective expansion of satellite laser ranging network to improve global geodetic parameters},

author = {Toshimichi Otsubo and Koji Matsuo and Yuichi Aoyama and Keiko Yamamoto and Thomas Hobiger and Toshihiro Kubo-oka and Mamoru Sekido},

url = {http://dx.doi.org/10.1186/s40623-016-0447-8},

doi = {10.1186/s40623-016-0447-8},

year  = {2016},

date = {2016-01-01},

journal = {Earth, Planets and Space},

volume = {68},

number = {1},

pages = {1–7},

abstract = {The aim of this study is to find an effective way to expand the ground tracking network of satellite laser ranging on the assumption that a new station is added to the existing network. Realistic numbers of observations for a new station are numerically simulated, based on the actual data acquisition statistics of the existing stations. The estimated errors are compared between the cases with and without a new station after the covariance matrices are created from a simulation run that contains six-satellite-combined orbit determination. While a station placed in the southern hemisphere is found to be useful in general, it is revealed that the most effective place differs according to the geodetic parameter. The X and Y components of the geocenter and the sectoral terms of the Earth's gravity field are largely improved by a station in the polar regions. A middle latitude station best contributes to the tesseral gravity terms, and, to a lesser extent, a low latitude station best performs for the Z component of the geocenter and the zonal gravity terms.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JSTARS-Hobiger16,

title = {Software defined radio direct correlation GNSS reflectometry by means of GLONASS},

author = {Thomas Hobiger and Rüdiger Haas and Johan Löfgren},

url = {http://ieeexplore.ieee.org/document/7425137/},

doi = {10.1109/JSTARS.2016.2529683},

year  = {2016},

date = {2016-01-01},

journal = {IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing},

volume = {9},

number = {10},

pages = {4834–4842},

abstract = {Ground-based GNSS reflectometry systems can be realized by different means. The concept of correlation between direct and reflected GNSS signals is basically possible with all GNSS systems. However, using signals from the Russian GLONASS system simplifies the signal processing so that software defined radio components can be used at replace expensive hardware solutions. This paper discusses how such a solution, called GLONASS-R, can be realized by using entirely off-the-shelf components. Field tests with such a system demonstrate the capability to monitor sea surface heights with a precision of 3~cm or better even with a sampling rate of 1.5~Hz. The flexibility of a software defined radio and the simple concept of GLONASS-R allows to build such a system with low costs and adapt it to the needs of any ground-based GNSS-R problem.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JGD-Kareinen16,

title = {Automated ambiguity estimation for VLBI Intensive sessions using L1-norm},

author = {Niko Kareinen and Thomas Hobiger and Rüdiger Haas},

url = {http://www.sciencedirect.com/science/article/pii/S0264370716300679},

doi = {10.1016/j.jog.2016.07.003},

year  = {2016},

date = {2016-01-01},

journal = {Journal of Geodynamics},

volume = {102},

pages = {39–46},

abstract = {Very Long Baseline Interferometry (VLBI) is a space-geodetic technique that is uniquely capable of direct observation of the angle of the Earth's rotation about the Celestial Intermediate Pole (CIP) axis, namely UT1. The daily estimates of the difference between UT1 and Coordinated Universal Time (UTC) provided by the 1-hour long VLBI Intensive 

sessions are essential in providing timely UT1 estimates for satellite navigation systems and orbit determination. In 

order to produce timely UT1 estimates, efforts have been made to completely automate the analysis of VLBI Intensive 

sessions. This involves the automatic processing of X- and S-band group delays. These data contain an unknown 

number of integer ambiguities in the observed group delays. They are introduced as a side-effect of the bandwidth 

synthesis technique, which is used to combine correlator results from the narrow channels that span the individual 

bands. In an automated analysis with the c5++ software the standard approach in resolving the ambiguities is to 

perform a simplified parameter estimation using a least-squares adjustment (L2-norm minimisation). We implement 

L1-norm as an alternative estimation method in c5++. The implemented method is used to automatically estimate the 

ambiguities in VLBI Intensive sessions on the Kokee-Wettzell baseline. The results are compared to an analysis setup 

where the ambiguity estimation is computed using the L2-norm. For both methods three different weighting 

strategies for the ambiguity estimation are assessed. The results show that the L1-norm is better at automatically 

resolving the ambiguities than the L2-norm. The use of the L1-norm leads to a significantly higher number of good 

quality UT1-UTC estimates with each of the three weighting strategies. The increase in the number of sessions is 

approximately 5 % for each weighting strategy. This is accompanied by smaller post-fit residuals in the final UT1-UTC 

estimation step.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RS-Strandberg16,

title = {Improving GNSS-R sea level determination through inverse modeling of SNR data},

author = {Joakim Strandberg and Thomas Hobiger and Rüdiger Haas},

url = {http://onlinelibrary.wiley.com/doi/10.1002/2016RS006057/abstract},

doi = {10.1002/2016RS006057},

year  = {2016},

date = {2016-01-01},

journal = {Radio Science},

volume = {51},

number = {8},

pages = {1286–-1296},

abstract = {This paper presents a new method for retrieving sea-surface heights from GNSS-R data by inverse modeling of SNR observations from a single geodetic receiver. The method relies on a B-spline representation of the temporal sea-level variations in order to account for its continuity. The corresponding B-spline coefficients are determined through a non-linear least-squares fit to the SNR data and a consistent choice of model parameters enables the combination of multiple GNSS in a single inversion process. This leads to a clear increase in precision of the sea-level retrievals which can be attributed to a better spatial and temporal sampling of the reflecting surface. Tests with data from two different coastal GNSS sites and comparison with co-located tide gauges show a significant increase in precision when compared to previously used methods, reaching standard deviations of 1.4 cm at Onsala, Sweden, and 3.1 cm at Spring Bay, Tasmania.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{EPS-Kareinen15,

title = {Automated analysis of Kokee–Wettzell Intensive VLBI sessions - algorithms, results, and recommendations},

author = {Niko Kareinen and Thomas Hobiger and Rüdiger Haas},

url = {http://www.earth-planets-space.com/content/67/1/181},

doi = {10.1186/s40623-015-0340-x},

year  = {2015},

date = {2015-01-01},

journal = {Earth, Planets and Space},

volume = {67},

number = {1},

pages = {181},

abstract = {The time-dependent variations in the rotation and orientation of the Earth are represented by a set of Earth Orientation Parameters (EOP). Currently, Very Long Baseline Interferometry (VLBI) is the only technique able to measure all EOP simultaneously and to provide direct observation of universal time, usually expressed as UT1-UTC. To produce estimates for UT1-UTC on a daily basis, 1-h VLBI experiments involving two or three stations are organised by the International VLBI Service for Geodesy and Astrometry (IVS), the IVS Intensive (INT) series. There is an ongoing effort to minimise the turn-around time for the INT sessions in order to achieve near real-time and high quality UT1-UTC estimates. As a step further towards true fully automated real-time analysis of UT1-UTC, we carry out an extensive investigation with INT sessions on the Kokee-Wettzell baseline. Our analysis starts with the first versions of the observational files in S- and X-band and includes an automatic group delay ambiguity resolution and ionospheric calibration. Several different analysis strategies are investigated. In particular, we focus on the impact of external information, such as meteorological and cable delay data provided in the station log-files, and a priori EOP information. The latter is studied by extensive Monte Carlo simulations.Our main findings are that it is easily possible to analyse the INT sessions in a fully automated mode to provide UT1-UTC with very low latency. The information found in the station log-files is important for the accuracy of the UT1-UTC results, provided that the data in the station log-files are reliable. Furthermore, to guarantee UT1-UTC with an accuracy of less than 20 mus, it is necessary to use predicted a priori polar motion data in the analysis that are not older than 12 h.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{MET-HOBIGER15,

title = {Combining GPS and VLBI for inter-continental frequency transfer},

author = {Thomas Hobiger and Carsten Rieck and Rüdiger Haas and Yasuhiro Koyama},

doi = {10.1088/0026-1394/52/2/251},

year  = {2015},

date = {2015-01-01},

journal = {Metrologia},

volume = {52},

number = {2},

pages = {251–261},

abstract = {For decades the Global Positioning System (GPS) has been the only space geodetic technique routinely used for inter-continental frequency transfer applications. In the past Very Long Baseline Interferometry (VLBI) has also been considered for this purpose and the method's capabilities were studied several times. However, compared to GPS current VLBI technology only provides few observations per hour, thus limiting its potential to improve frequency comparisons. We therefore investigate the effect of combining GPS and VLBI on the observation level in order to draw the maximum benefit from the strength of each individual technique. As a test-bed for our study we use the CONT11 campaign observed in 2011. First we review the frequency transfer performance that can be achieved with independent technique-specific analyses, both with individual software packages and with the multi-technique software c5++. With this analysis approach both techniques, GPS and VLBI, show similar frequency link instabilities at the level of 1e-14 to 1e-15 (MDEV) on inter-continental baselines for averaging times of one day. Then we use the c5++ software for a combined analysis of GPS and VLBI data on the observation level. We demonstrate that our combination approach leads to small but consistent improvements for frequency transfer of up to 10~%, in particular for averaging periods longer than 3000 s.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JOG-HOBIGER14,

title = {Combination of GPS and VLBI on the observation level during CONT11 - common parameters, ties and inter-technique biases},

author = {Thomas Hobiger and Toshimichi Otsubo},

doi = {10.1007/s00190-014-0740-x},

year  = {2014},

date = {2014-01-01},

journal = {Journal of Geodesy},

volume = {88},

number = {11},

pages = {1017–1028},

abstract = {Multi-technique space geodetic analysis software has been developed which allows to combine data on the observation level. In addition to local tie information, site-wise common parameters, i.e. troposphere and clocks, can be estimated with this software. Thus, it will be discussed how common parameters have to be estimated and where biases/offsets need to be taken into account. In order to test such a novel concept, Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI) data from the CONT11 campaign are being utilized. Since the VLBI baselines of this campaign extend over several thousands of kilometers, GPS data is processed in precise point positioning (PPP) mode and satellite orbits and clocks are kept fixed to the IGS final products. From the obtained results it can be shown that the combination of space geodetic data on the observation level leads to a consistent improvement of station position repeatability as well as nuisance parameters like troposphere estimates. Furthermore, estimation of common parameters (troposphere or clocks) at co-located sites helps to improve the solution further and derive an utmost physically consistent model of the concerned parameters.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RS-HOBIGER14,

title = {GLONASS-R: GNSS reflectometry with a Frequency Division Multiple Access-based satellite navigation system},

author = {Thomas Hobiger and Rüdiger Haas and Johan Löfgren},

doi = {10.1002/2013RS005359},

year  = {2014},

date = {2014-01-01},

journal = {Radio Science},

volume = {49},

number = {4},

pages = {271-282},

abstract = {The information from reflected Global Navigation Satellite System (GNSS) signals can become a valuable data source, from which geophysical properties can be deduced. This approach, called GNSS Reflectometry (GNSS-R), can be used to develop instruments that act like an altimeter when arrival times of direct and reflected signals are compared. Current GNSS-R systems usually entirely rely on signals from the Global Positioning Service (GPS), and field experiments could demonstrate that information from such systems can measure sea level with an accuracy of a few centimeters. However, the usage of the Russian GLONASS system has the potential to simplify the processing scheme and to allow handling of direct and reflected signals like a bistatic radar. Thus, such a system has been developed and deployed for test purposes at the Onsala Space Observatory, Sweden, that has an operational GPS-based GNSS-R system. Over a period of 2 weeks in October 2013, GPS-based GNSS-R sea level monitoring and 

measurements with the newly developed GLONASS-R system were carried out in parallel. In addition, data from colocated tide gauge measurements were available for comparison. It can be shown that precision and accuracy of the GLONASS-based GNSS-R system is comparable to, or even better than, conventional GPS-based GNSS-R solutions. Moreover, the simplicity of the newly developed GLONASS-R system allows to make it a cheap and valuable tool for various remote sensing applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JASR-HOBIGER14,

title = {Observation Level Combination of SLR and VLBI with c5++: a case study for TIGO},

author = {Thomas Hobiger and Toshimichi Otsubo and Mamoru Sekido},

doi = {10.1016/j.asr.2013.10.004},

year  = {2014},

date = {2014-01-01},

journal = {Advances in Space Research},

volume = {53},

number = {1},

pages = {119-129},

abstract = {A multi-technique space geodetic analysis software named c5++ has been developed and allows one to combine data on the observation level. With SLR and VLBI modules being ready and tested, this software has been used to compute coordinate time series of the geodetic fundamental station TIGO, located near Concepción, Chile. It can be shown that the combination of space geodetic data on the observation level leads to a significant improvement of station position repeatability, which is an important measure for the stability of a station in the terrestrial reference frame. Moreover, it could be demonstrated that the geophysical signal of the post-seismic tectonic plate movement is usually more complete than detected by any of the two single-technique solutions. In addition, it has been confirmed that so-called nuisance parameters, which are relying on data from a single technique, are not biased when combing observations from different space geodetic techniques.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{IEICE-GOTOH14,

title = {Development of a Software Based GPS Time Transfer Receiver (in Japanese)},

author = {Tadahiro Gotoh and Thomas Hobiger and Jun Amagai and Huan-Bang Li},

url = {http://search.ieice.org/bin/pdf_link.php?category=B&lang=J&year=2014&fname=j97-b_4_363&abst=},

year  = {2014},

date = {2014-01-01},

journal = {電子情報通信学会論文誌 B},

volume = {J97-B},

number = {4},

pages = {363-370},

abstract = {GPSやGlonassなどに代表される衛星測位システムは，国際原子時決定の手段としても重要な役割を担っている．しかし，市販の受信機の多くは測量目的のため，時刻比較に使用できる受信機の種類は限られており，価格も測量のみを目的としたものに比べると高価である．筆者らは，汎用のA/D変換器とソフトウェア無線技術を使用することで，時刻比較目的に使用できるGPS受信機の開発を行った．開発した受信機は，新しい測距信号であるL2C信号を受信することで電離層の影響を相殺した高精度な時刻比較を可能とする．ソフトウェア受信機と市販受信機の組合せによる，国内基線での時刻比較結果では10-13乗台の比較が行えることを実証した．また，衛星双方向方式との比較により2 ns以内の一致を得た．},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JOG-TEKE13,

title = {Troposphere delays from space geodetic techniques, water vapor radiometers, and numerical weather models over a series of continuous VLBI campaigns},

author = {Kamil Teke and Tobias Nilsson and Johannes Boehm and Thomas Hobiger and Peter Steigenberger and Susana Garcia-Espada and Rüdiger Haas and Pascal Willis},

doi = {10.1007/s00190-013-0662-z},

year  = {2013},

date = {2013-01-01},

journal = {Journal of Geodesy},

volume = {87},

number = {10-12},

pages = {981-1001},

abstract = {Continuous VLBI (Very Long Baseline Interferometry) campaigns over two weeks have been carried out repeatedly, i.e. CONT02 in October 2002, CONT05 in September 2005, CONT08 in August 2008, and CONT11 in September 2011, to demonstrate the highest accuracy the current VLBI is capable at that time. In this study, we have compared zenith total delays (ZTD) and troposphere gradients as consistently estimated from the observations of VLBI, Global Navigation Satellite Systems (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) at VLBI sites participating in the CONT campaigns. We analyzed the CONT campaigns using the state of the art software following common processing strategies as closely as possible. In parallel, ZTD and gradients were derived from numerical weather models, i.e. from the global European Centre for Medium-Range Weather Forecasts (ECMWF) analysis fields, the High Resolution Limited Area Model (HIRLAM, European sites), the Japan Meteorological Agency 

(JMA) - Operational Meso-Analysis Field (MANAL, over Japan), and the Cloud Resolving Storm Simulator (CReSS, Tsukuba, Japan). Finally, zenith wet delays were estimated from the observations of water vapor radiometers (WVR) at sites where the WVR observables are available during the CONT sessions. The best ZTD agreement, interpreted as the smallest standard deviation, was found between GNSS and VLBI techniques being about 5 to 6 millimeters at most of the co-located sites and CONT campaigns. We did not detect any significant improvement on the ZTD agreement between various techniques over time, except for DORIS and MANAL. On the other hand, the agreement and thus the accuracy of the troposphere parameters mainly depend on the amount of humidity in the atmosphere.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{IEEE-TIM13,

title = {Dissemination of UTC(NICT) by means of QZSS},

author = {Thomas Hobiger and Yasuhiro Takahashi and Maho Nakamura and Tadahiro Gotoh and Shinichi Hama and Takashi Maruyama and Tsutomu Nagatsuma and Hiroyuki Noda and Motohisa Kishimoto and Motohisa Nakayama and Yasuhiro Ohki},

doi = {10.1109/TIM.2012.2225920},

year  = {2013},

date = {2013-01-01},

journal = {IEEE Transactions on Instrumentation and Measurement},

volume = {62},

number = {2},

pages = {1537-1544},

abstract = {The Japanese Quasi-Zenith Satellite System (QZSS) offers the possibility to transmit information with an unprecedented bit rate of 2000 bps via the L-band experimental (LEX) signal. This feature can be used to disseminate Japan Standard Time, i.e. UTC(NICT) to any user capable of receiving the new QZSS signal. Various timing transmission modes as well as a dedicated ionosphere correction model allow users to instantaneously realize UTC(NICT) across Japan with an uncertainty of a few ns. However, a sophisticated real-time ionosphere correction model also needs to be transmitted to the user to compensate for dispersive ionosphere delays which are the largest contributor to the total error budget of the system.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{RS-HOBIGER13,

title = {A correction model of dispersive troposphere delays for the ACES microwave link},

author = {Thomas Hobiger and Dirk Piester and Phillipe Baron},

doi = {10.1002/rds.20016},

year  = {2013},

date = {2013-01-01},

journal = {Radio Science},

volume = {48},

number = {2},

pages = {131-142},

abstract = {The Atomic Clock Ensemble in Space (ACES) will be a future ESA experiment which utilizes ultra-stable clocks on-board the International Space Station (ISS). This mission is expected to perform tests of fundamental physics (relativity, possible drift of fundamental constants with time) and at the same time allows to compare the ACES time reference with respect to ground stations by using a novel microwave link concept. However, uncorrected dispersive troposphere delays pose the risk of degrading the performance of this microwave link over longer integration periods. Thus, a semi-empirical correction model has been developed which is only based on input from meteorologic sensors at the ground stations. The proposed model has been tested with simulated ISS overflights at different potential ACES ground station sites and it could be demonstrated that this model is capable to remove biases and elevation dependent features caused by the dispersive troposphere delay difference between the up-link and 

down-link. The model performs well at all sites by reducing the impact on all reasonable averaging time scales by at least one order of magnitude. Similar studies like this might be of importance for other time and frequency transfer instruments or future space geodetic instruments.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JOG-Kinoshita13,

title = {Are numerical weather model outputs helpful to reduce tropospheric delay signals in InSAR data?},

author = {Youhei Kinoshita and Masato Furuya and Thomas Hobiger and Ryuichi Ichikawa},

year  = {2013},

date = {2013-01-01},

journal = {Journal of Geodesy},

volume = {87},

number = {3},

pages = {267-277},

abstract = {Interferometric Synthetic Aperture Radar (InSAR) phase data include not only signals due to crustal movements but also those associated with microwave propagation delay through the atmosphere. In particular, the effect of water vapor can generate apparent signals on the order of a few centimeters or more, and prevent us from detecting such geophysical signals as those due to secular crustal deformation. In order to examine if and to what extent numerical weather model (NWM) outputs are helpful to reduce the tropospheric delay signals at spatial scales of 5 to 50 km wavelengths, we compared three approaches of tropospheric signal reduction, using 54 interferograms in central Hokkaido, Japan. The first approach is the conventional topography-correlated delay correction that is based on the regional digital elevation model (DEM). The second approach is based on the Japan Meteorological Agency's operational meso-scale analysis model (MSM) data, where we compute tropospheric delays and subtract them 

from the interferogram. However, the MSM data are available at predefined epochs, and their spatial resolution is about 10 km, and therefore we need to interpolate both temporally and spatially to match with interferograms. Expecting to obtain a more physically plausible reduction of the tropospheric effects, we ran a 1-km mesh high-resolution numerical weather model WRF (Weather Research and Forecasting model) by ourselves, using the MSM data as the initial and boundary conditions. The third approach is similar to the second approach except that we make use of the WRF-based tropospheric data.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JGS2012,

title = {宇宙測地技術における大気電波伝播遅延の高精度補正},

author = {Thomas Hobiger},

url = {https://www.jstage.jst.go.jp/browse/sokuchi/-char/ja/},

year  = {2012},

date = {2012-01-01},

journal = {測地学会誌},

volume = {58},

number = {1},

pages = {1-8},

abstract = {The atmospheric excess path delay is a major contributor to the error budget of space geodetic applications and should therefore be reduced to the maximum possible extent. Numerical weather models are undergoing improvements with regard to their spatial resolution, which enables compensation of troposphere propagation errors by applying corrections obtained from ray-tracing 

through three-dimensional meteorologic fields. Such correction can be applied utilized for GNSS positioning, removal of troposphere artifacts in InSAR images or even help to improve Earth orientation parameters determined from VLBI measurements.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{ASR-Hobiger2012,

title = {A real-time GNSS-R system based on software-defined radio and graphics 

	processing units},

author = {Thomas Hobiger and Jun Amagai and Masanori Aida and Hideki Narita},

doi = {10.1016/j.asr.2012.01.009},

year  = {2012},

date = {2012-01-01},

journal = {Advances in Space Research},

volume = {49},

number = {7},

pages = {1180-1190},

abstract = {Reflected signals of the Global Navigation Satellite System (GNSS) 

	from the sea or land surface can be utilized to deduce and monitor 

	physical and geophysical parameters of the reflecting area. Unlike 

	most other remote sensing techniques, GNSS-Reflectometry (GNSS-R) 

	operates as a passive radar that takes advantage from the increasing 

	number of navigation satellites that broadcast their L-band signals. 

	Thereby, most of the GNSS-R receiver architectures are based on dedicated 

	hardware solutions. Software-defined radio (SDR) technology has advanced 

	in the recent years and enabled signal processing in real-time, which 

	makes it an ideal candidate for the realization of a flexible GNSS-R 

	system. Additionally, modern commodity graphic cards, which offer 

	massive parallel computing performances, allow to handle the whole 

	signal processing chain without interfering with the PC’s CPU. Thus, 

	this paper describes a GNSS-R system which has been developed on 

	the principles of software-defined radio supported by General Purpose 

	Graphics Processing Units (GPGPUs), and presents results from initial 

	field tests which confirm the anticipated capability of the system.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Tseng2012,

title = {First International TWSTFT Experiment by Employing Dual Pseudo-random 

	Noise Codes},

author = {Wen-Hung Tseng and Yi-Jiun Huang and Tadahiro Gotoh and Jun Amagai and Thomas Hobiger and Miho Fujieda and Shinn-Yan Lin and Huang-Tien Lin and Kai-Ming Feng},

doi = {10.1109/TUFFC.2012.2224},

year  = {2012},

date = {2012-01-01},

journal = {IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control},

volume = {59},

number = {3},

pages = {531 -538},

abstract = {Two-way satellite time and frequency transfer (TWSTFT) is one of the 

	major techniques to compare atomic time scales over long distances. 

	In order to both improve the precision of TWSTFT and decrease the 

	satellite link fee, a new software-defined modem with dual pseudo-random 

	noise (DPN) codes has been developed. In this paper, we demonstrate 

	the first international DPN-based TWSTFT experiment over a period 

	of 6 months. The results of DPN exhibit an excellent performance, 

	which is competitive with the GPS precise point positioning (PPP) 

	technique in the short term and consistent with the conventional 

	TWSTFT in the long term. The time deviations of below 75 ps are achieved 

	for averaging times from 1 s to 1 day. Moreover, the DPN data has 

	less diurnal variations than that of the conventional TWSTFT. Since 

	the DPN-based system has advantages of higher precision and lower 

	bandwidth cost, it is one of the most promising methods to improve 

	the international time transfer links.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}