Investigation of the small-scale structure and dynamics of Uranus' atmosphere

This document constitutes the final technical report of the Uranus Analysis Program. Papers and/or abstracts resulting from this research are presented. The following topics are covered: (1) past and future of radio occultation studies of planetary atmospheres; (2) equatorial waves in the stratosphere of Uranus; (3) the atmosphere of Uranus- results of radio occultation measurements with Voyager 2; (4) Uranus' atmospheric dynamics and circulation; (5) small-scale structure and dynamics in the atmosphere of Uranus; (6) evidence for inertia-gravity waves in the stratosphere of Uranus derived from Voyager 2 radio occultation data; and (7) planetary waves in the equatorial stratosphere of Uranus

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

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

The deep space network, volume 6

Progress on Deep Space Network (DSN) supporting research and technology is presented, together with advanced development and engineering, implementation, and DSN operations of flight projects. The DSN is described. Interplanetary and planetary flight projects and radio science experiments are discussed. Tracking and navigational accuracy analysis, communications systems and elements research, and supporting research are considered. Development of the ground communications and deep space instrumentation facilities is also presented. Network allocation schedules and angle tracking and test development are included

GEOS-2 refraction program summary document

Data from an extensive array of collocated instrumentation at the Wallops Island test facility were intercompared in order to (1) determine the practical achievable accuracy limitations of various tropospheric and ionospheric correction techniques; (2) examine the theoretical bases and derivation of improved refraction correction techniques; and (3) estimate internal systematic and random error levels of the various tracking stations. The GEOS 2 satellite was used as the target vehicle. Data were obtained regarding the ionospheric and tropospheric propagation errors, the theoretical and data analysis of which was documented in some 30 separate reports over the last 6 years. An overview of project results is presented

Coastal altimetry for the computation of a Mean Dynamic Topography in the Mediterranean sea

Satellite Sea Level Anomaly (SLA) observations are crucial in an operational oceanographic system due to their high coverage on sea surface currents and elevation and their strong constraint on water column integrated steric contributions. The use of Sea Surface Height (SSH) measurements by altimeter satellites in the Mediterranean Forecasting System (MFS) requires an accurate Mean Dynamic Topography (MDT) field with a high horizontal resolution which must be added to SLA observations. Here a new MDT computed through a direct method is proposed to solve the main limitations to the current MDT, evaluated from a model-dependent first guess. The direct method consists in the difference between an altimetric Mean Sea Surface Height (MSSH) and a geoid model. Moreover, a novel altimetric dataset reprocessed near the coast is adopted in order to improve the representation of coastal dynamics. Altimetric data from a single satellite, Jason-2, are used to generate a SSH dataset. This is used along with the EGM2008 geoid model to compute along track MDT observations. Optimal Interpolation algorithms are used to regrid along track MDT on MFS model grid. Derived geostrophic velocities are then computed. The validation of the altimetric dataset against the operational dataset showed improved performances in terms of time series completeness and standard mean error. From the analysis of the MDT and the retrieved geostrophic velocities we can conclude that the direct method allowed us to reconstruct basin scale and large scale MDT features but not meso/small scale and coastal dynamics. Main limitations in our results are due to the low accuracy of geoid model and the Jason-2 tracks spacing

Über die GPS-basierte Bestimmung troposphärischer Laufzeitverzögerungen

One major problem of precise GPS data analysis is that of modeling wetdelays with high precision. All conventional models have to fail in this task due to the impossibility of modeling wet delays solely from surface measurements like temperature and relative humidity. Actually, the non-hydrostatic component of the tropospheric propagation delay is highly influenced by the distribution of water vapor in the lower troposphere which cannot be sufficiently predicted with sole help of surface measurements. A work-around is to include atmospheric parameters as additional unknowns in the analysis of GPS data from permanent monitor stations that turns out to improve the quality of position estimates. Moreover, knowledge of zenith wet delays allows to obtain a highly interesting value for climatology and meteorology: integrated or precipitable water vapor being important for the energy balance of the atmosphere and holds share of more than 60% of the natural greenhouse effect. GPS can thereby contribute to the improvement of climate models and weather forecasting. This work outlines the application of ground-based GPS to climate research and meteorology without omitting the fact that precise GPS positioning can also highly benefit from using numerical weather models for tropospheric delay determination for applications where GPS troposphere estimation is not possible, for example kinematic and rapid static surveys. In this sense, the technique of GPS-derived tropospheric delays is seen as mutually improving both disciplines, precise positioning as well as meteorology and climatology

Propagation of millimetric radio waves through the clear atmosphere

Open acces

The application of spaceborne GPS to atmospheric limb sounding and global change monitoring

This monograph is intended for readers with minimal background in radio science who seek a relatively comprehensive treatment of the mission and technical aspects of an Earth-orbiting radio occultation satellite. Part 1 (chapters 1-6) describes mission concepts and programmatic information; Part 2 (chapters 7-12) deals with the theoretical aspects of analyzing and interpreting radio occultation measurements. In this mission concept the navigation signals from a Global Positioning System (GPS) satellite that is being occulted by the Earth's limb are observed by a GPS flight receiver on board a low Earth orbiter (LEO) satellite. This technique can be used to recover profiles of the Earth's atmospheric refractivity, pressure, and temperature using small, dedicated, and relatively low-cost space systems. Chapter 2 summarizes the basic space system concepts of the limb-sounding technique and describes a low-cost strawman demonstration mission. Chapter 3 discusses some of the scientific benefits of using radio occultation on a suite of small satellites. Chapter 4 provides a more detailed discussion of several system elements in a radio occultation mission, including the launch system for small payloads, the LEO microsat, the GPS constellation, the GPS flight receiver payload, the mission operations ground control and data receiving system, the ground-based GPS global tracking network for precision orbit determination, and the central data processing and archive system. Chapter 5 addresses the various technology readiness questions that invariably arise. Chapter 6 discusses the overall costs of a demonstration mission such as GPS/MET (meteorological) proposed by the University Navstar Consortium (UNAVCO). Chapter 7 describes a geometrical optics approach to coplanar atmospheric occultation. Chapter 8 addresses major questions regarding accuracy of the occultation techniques. Chapter 9 describes some simulations that have been performed to evaluate the sensitivity of the recovered profiles of atmospheric parameters to different error sources, such as departure from spherical symmetry, water vapor, etc. Chapter 10 discusses horizontal and vertical resolution associated with limb sounders in general. Chapter 11 treats selected Fresnel diffraction techniques that can be used in radio occultation measurements to sharpen resolution. Chapter 12 provides brief discussions on selected special topics, such as strategies for handling interference and multipath processes that may arise for rays traveling in the lower troposphere