Atmospheric refraction and turbulence in VLBI data analysis

The progress in further improving the quality of results derived by space-geodetic techniques observing in the radio frequency domain, such as Very Long Baseline Interferometry (VLBI) or Global Navigation Satellite Systems (GNSS), is limited by rapid changes in the neutral part of the atmosphere. In particular, insufficient knowledge of the temporal and spatial refractivity variations restrict the attainable accuracy of the derived VLBI and GNSS target parameters. In the current model describing the additional propagation delay due to the neutral part of the atmosphere, only annual to hourly long periodic variations are taken into account. In contrast, small-scale fluctuations mainly originating from turbulent motions are generally neglected, although they form a serious error source for electromagnetic wave propagation. Dynamic processes in the neutral atmosphere additionally induce physical correlations in space and time, which are also largely ignored so far. Particularly with regard to future requirements, as, for instance, defined within the framework of the Global Geodetic Observing System established by the International Association of Geodesy, the current tropospheric model is not sufficient and needs to be improved. High rate GNSS data of 1 Hz sampling and below, and the VLBI Global Observing System with faster telescopes result in a better sampling of the atmosphere. However, new challenges emerge with respect to improved and proper analysis strategies, in particular to model the stochastic properties of atmospheric refraction, which represents a crucial issue in research and the main objective of this thesis. Quantifying and assessing the small-scale behavior of atmospheric refraction is extremely challenging, since small-scale characteristics of atmospheric refraction cannot be analyzed without sufficient knowledge of the stability of the VLBI observing system. An optimal experimental setup for both, investigations in atmospheric refraction and system stability issues, emerges from the commissioning phase of the twin radio telescope at the Wettzell Geodetic Observatory in Germany. Specially designed so-called WHISP sessions are scheduled, observed and analyzed within this thesis allowing to quantify the individual components of the observing system, in part for the first time. On this basis, refractivity fluctuations are quantified which are found to be in the range of 1-3 millimeters. A number of noteworthy conclusions has been drawn which would not have been possible without the novel observing approach. Special emphasis is also given to the development of an atmospheric turbulence model, which stochastically describes small-scale refractivity fluctuations due to turbulent motions in the neutral atmosphere. The results have produced an important contribution to the modeling of refraction effects in the neutral atmosphere now considering temporal and spatial correlations between the observations in a physical and meteorological way. By analyzing 2700 VLBI sessions including traditional and local observing networks, it is demonstrated that the incorporation of the newly devised model into the VLBI data analysis leads to an improvement of the solutions compared to the standard strategies of the International VLBI Service for Geodesy and Astrometry, or other strategies refining the stochastic model of VLBI observations. Compared to other approaches addressing the issue of atmospheric turbulence, the model developed within this thesis has the advantage to be operationally efficient for routine mass analysis of VLBI observing sessions. Since the current atmospheric model reveals severe deficiencies with respect to the estimation of atmospheric parameters, new modeling and adjustment strategies are introduced to better describe the behavior of the neutral atmosphere. It is demonstrated that, in particular, the least squares collocation method ensures an improved modeling of the stochastic properties of the neutral atmosphere, which allows a zenith wet delay estimation in more meaningful and appropriate sense. The main achievements of this thesis are the development of an atmospheric turbulence model to improve the stochastic model of VLBI observations and the quantification of local atmospheric refraction variations in space and time. Both allows for new interpretations and model improvements in a stochastic and deterministic sense

Atmospheric refraction and turbulence in VLBI data analysis

The progress in further improving the quality of results derived by space-geodetic techniques observing in the radio frequency domain, such as Very Long Baseline Interferometry (VLBI) or Global Navigation Satellite Systems (GNSS), is limited by rapid changes in the neutral part of the atmosphere. In particular, insufficient knowledge of the temporal and spatial refractivity variations restrict the attainable accuracy of the derived VLBI and GNSS target parameters. In the current model describing the additional propagation delay due to the neutral part of the atmosphere, only annual to hourly long periodic variations are taken into account. In contrast, small-scale fluctuations mainly originating from turbulent motions are generally neglected, although they form a serious error source for electromagnetic wave propagation. Dynamic processes in the neutral atmosphere additionally induce physical correlations in space and time, which are also largely ignored so far. Particularly with regard to future requirements, as, for instance, defined within the framework of the Global Geodetic Observing System established by the International Association of Geodesy, the current tropospheric model is not sufficient and needs to be improved. High rate GNSS data of 1 Hz sampling and below, and the VLBI Global Observing System with faster telescopes result in a better sampling of the atmosphere. However, new challenges emerge with respect to improved and proper analysis strategies, in particular to model the stochastic properties of atmospheric refraction, which represents a crucial issue in research and the main objective of this thesis. Quantifying and assessing the small-scale behavior of atmospheric refraction is extremely challenging, since small-scale characteristics of atmospheric refraction cannot be analyzed without sufficient knowledge of the stability of the VLBI observing system. An optimal experimental setup for both, investigations in atmospheric refraction and system stability issues, emerges from the commissioning phase of the twin radio telescope at the Wettzell Geodetic Observatory in Germany. Specially designed so-called WHISP sessions are scheduled, observed and analyzed within this thesis allowing to quantify the individual components of the observing system, in part for the first time. On this basis, refractivity fluctuations are quantified which are found to be in the range of 1-3 millimeters. A number of noteworthy conclusions has been drawn which would not have been possible without the novel observing approach. Special emphasis is also given to the development of an atmospheric turbulence model, which stochastically describes small-scale refractivity fluctuations due to turbulent motions in the neutral atmosphere. The results have produced an important contribution to the modeling of refraction effects in the neutral atmosphere now considering temporal and spatial correlations between the observations in a physical and meteorological way. By analyzing 2700 VLBI sessions including traditional and local observing networks, it is demonstrated that the incorporation of the newly devised model into the VLBI data analysis leads to an improvemen of the solutions compared to the standard strategies of the International VLBI Service for Geodesy and Astrometry, or other strategies refining the stochastic model of VLBI observations. Compared to other approaches addressing the issue of atmospheric turbulence, the model developed within this thesis has the advantage to be operationally efficient for routine mass analysis of VLBI observing sessions. Since the current atmospheric model reveals severe deficiencies with respect to the estimation of atmospheric parameters, new modeling and adjustment strategies are introduced to better describe the behavior of the neutral atmosphere. It is demonstrated that, in particular, the least squares collocation method ensures an improved modeling of the stochastic properties of the neutral atmosphere, which allows a zenith wet delay estimation in more meaningful and appropriate sense. The main achievements of this thesis are the development of an atmospheric turbulence model to improve the stochastic model of VLBI observations and the quantification of local atmospheric refraction variations in space and time. Both allows for new interpretations and model improvements in a stochastic and deterministic sense.Atmosphärische Refraktion und Turbulenzin der VLBI-Auswertung Die stetige Weiterentwicklung und Qualitätsverbesserung von Ergebnissen aus weltraum-geodätischen Verfahren im Radiofrequenzbereich, wie beispielsweise VLBI (Very Long Baseline Interferometry) oder GNSS (Global Navigation Satellite Systems), ist durch schnelle Veränderungen in der neutralen Atmosphäre limitiert. Die zu erreichende Genauigkeit von Stationskoordinaten, Erdrotationsparametern oder anderen Zielparametern wird durch die unzureichende Kenntnis räumlicher oder zeitlicher Variationen in der Refraktivität maßgeblich begrenzt. Das aktuelle Atmosphärenmodell in der Auswertung weltraum-geodätischer Verfahren sieht ausschließlich die Berücksichtigung langperiodischer Signale vor. Kleinskalige, überwiegend durch turbulentes Verhalten in der Atmosphäre hervorgerufene Fluktuationen werden hingegen weitestgehend vernachlässigt, obwohl sie einen nicht unerheblichen Einfluss auf die Ausbreitung elektromagnetischer Wellen haben. Des Weiteren induzieren dynamische Prozesse in der neutralen Atmosphäre sowohl räumliche als auch zeitliche Korrelation zwischen den Beobachtungen, die ebenfalls weitestgehend ignoriert werden. Insbesondere im Hinblick auf die von der IAG (International Association of Geodesy) formulierten GGOS (Global Geodetic Observing System) Ziele genügt das aktuelle Atmosphärenmodell nicht den zukünftigen Anforderungen. Zwar führen hoch aufgelöste GNSS-Daten mit Abtastfrequenzen von bis zu 1 Hz und eine neue Generation von schnelleren und präziseren sogenannten VGOS (VLBI Global Observing System) Radioteleskopen zu einer besseren Abtastung der Atmosphäre, jedoch entstehen auch neue Herausforderungen hinsichtlich einer verbesserten und geeigneteren Modellierung der stochastischen Eigenschaften atmosphärischer Refraktion, welche allgemein eine zentrale Fragestellung darstellt und folglich die wesentliche Aufgabe dieser Arbeit repräsentiert. Die Quantifizierung und Bewertung des Verhaltens der atmosphärischen Refraktion stellt eine große Herausforderung dar. Da insbesondere das kleinskalige Verhalten der atmosphärischen Refraktion eng mit den Stabilitätseigenschaften des VLBI-Beobachtungssystems zusammenhängt, müssen diese ausreichend gut bekannt sein. Durch die Inbetriebnahme des weltweit ersten Twin-Teleskops am Geodätischen Observatorium Wettzell in Deutschland entstanden optimale Voraussetzungen für die Detektion der Stabilitätseigenschaften des Beobachtungssystems sowie der atmosphärischen Refraktion. In dieser Arbeit wurden spezielle WHISPExperimente entworfen, die es erlauben, einzelne Komponenten des Beobachtungssystems zum Teil erstmalig zu quantifizieren. Auf dieser Grundlage wird auch der Einfluss von Variationen in der Refraktivität bestimmt, dem eine Größenordnung von 1-3 Millimetern zugerechnet wird. Ein besonderer Fokus liegt außerdem auf der Entwicklung eines Turbulenzmodells, welches zum einen zeitliche und räumliche Korrelationen zwischen den Beobachtungen berücksichtigt und zum anderen kleinskalige Fluktuationen in der Refraktivität stochastisch sowie physikalisch und meteorologisch sinnvoll beschreibt. Auf Basis der Auswertung von 2700 VLBI-Beobachtungssessionen unterschiedlicher Netzwerkgröße wird gezeigt, dass die Einführung des neuen Turbulenzmodells in die VLBI-Auswertung für die operationelle Auswertung geeignet ist und zu Verbesserungen gegenüber der Standardlösung des IVS (International VLBI Service for Geodesy and Astrometry) sowie alternativer Ansätze zur Verfeinerung des stochastischen Modells führt. Da das routinemäßig verwendete Atmosphärenmodell einige Defizite hinsichtlich der Schätzung atmosphärischer Parameter aufweist, werden in dieser Arbeit einige Modellierungs- und Ausgleichungsstrategien eingeführt, um die neutrale Atmosphäre besser zu charakterisieren. Es wird gezeigt, dass insbesondere die Kleinste-Quadrate-Kollokation eine verbesserte Modellierung der stochastischen Eigenschaften der neutralen Atmosphäre erlaubt und somit zu einer aussagekräftigeren und geeigneteren Schätzung der Atmosphärenparameter führt. Die Haupterrungenschaften dieser Arbeit sind die Entwicklung eines Turbulenzmodells zur Verbesserung des stochastischen Modells sowie die verbesserte Quantifizierung lokaler Refraktionseigenschaften in Raum und Zeit. Beides resultiert in neuen Interpretationsmöglichkeiten und Modellverbesserungen in deterministischer und stochastischer Hinsicht

Initial Results Obtained with the First TWIN VLBI Radio Telescope at the Geodetic Observatory Wettzell

Geodetic Very Long Baseline Interferometry (VLBI) uses radio telescopes as sensor networks to determine Earth orientation parameters and baseline vectors between the telescopes. The TWIN Telescope Wettzell 1 (TTW1), the first of the new 13.2 m diameter telescope pair at the Geodetic Observatory Wettzell, Germany, is currently in its commissioning phase. The technology behind this radio telescope including the receiving system and the tri-band feed horn is depicted. Since VLBI telescopes must operate at least in pairs, the existing 20 m diameter Radio Telescope Wettzell (RTW) is used together with TTW1 for practical tests. In addition, selected long baseline setups are investigated. Correlation results portraying the data quality achieved during first initial experiments are discussed. Finally, the local 123 m baseline between the old RTW telescope and the new TTW1 is analyzed and compared with an existing high-precision local survey. Our initial results are very satisfactory for X-band group delays featuring a 3D distance agreement between VLBI data analysis and local ties of 1 to 2 mm in the majority of the experiments. However, S-band data, which suffer much from local radio interference due to WiFi and mobile communications, are about 10 times less precise than X-band data and require further analysis, but evidence is provided that S-band data are well-usable over long baselines where local radio interference patterns decorrelate

The IVS data input to ITRF2014

2015ivs..data....1N - GFZ Data Services, Helmoltz Centre, Potsdam, GermanyVery Long Baseline Interferometry (VLBI) is a primary space-geodetic technique for determining precise coordinates on the Earth, for monitoring the variable Earth rotation and orientation with highest precision, and for deriving many other parameters of the Earth system. The International VLBI Service for Geodesy and Astrometry (IVS, http://ivscc.gsfc.nasa.gov/) is a service of the International Association of Geodesy (IAG) and the International Astronomical Union (IAU). The datasets published here are the results of individual Very Long Baseline Interferometry (VLBI) sessions in the form of normal equations in SINEX 2.0 format (http://www.iers.org/IERS/EN/Organization/AnalysisCoordinator/SinexFormat/sinex.html, the SINEX 2.0 description is attached as pdf) provided by IVS as the input for the next release of the International Terrestrial Reference System (ITRF): ITRF2014. This is a new version of the ITRF2008 release (Bockmann et al., 2009). For each session/ file, the normal equation systems contain elements for the coordinate components of all stations having participated in the respective session as well as for the Earth orientation parameters (x-pole, y-pole, UT1 and its time derivatives plus offset to the IAU2006 precession-nutation components dX, dY (https://www.iau.org/static/resolutions/IAU2006_Resol1.pdf). The terrestrial part is free of datum. The data sets are the result of a weighted combination of the input of several IVS Analysis Centers. The IVS contribution for ITRF2014 is described in Bachmann et al (2015), Schuh and Behrend (2012) provide a general overview on the VLBI method, details on the internal data handling can be found at Behrend (2013)