ALBERT

Description: This study presents an enhancement to the Global Navigation Satellite Systems (GNSS) by integrating low Earth orbiters (LEOs) to a joint precise orbit determination (POD) processing. The Global Position System (GPS) operated by the United States is studied as a representative of all GNSS. The LEOs equipped with GNSS receivers supplement the receivers of the ground stations, especially for regions with a limited number of employed stations, which can be caused by various reasons. Due to the altitude and high velocity of LEOs, they not only contribute with additional observations, but also with a rapidly-changing observation geometry. Moreover, space-based observations have additional advantages over ground-based observations, e.g., signals are received without the impact of the troposphere. LEOs not only act as kinematic stations for GNSS satellites, but also bring additional orbit dynamics to the integrated system. The constraints caused by these orbit dynamics have an important impact on the determination of the orbits of the GNSS satellites and other parameters beyond that. In this thesis, the following topics are presented: 1) Background information and the basic principles related to the POD of GNSS satellites and LEOs, 2) the separated POD of GNSS satellites and LEOs, 3) the integrated POD, 4) the determination of the antenna phase center offsets (PCOs) of the GPS satellites and other geodetic parameters in the integrated POD. The orbit modeling and processing configuration used in this study for GNSS satellites and LEOs are verified to be compatible with state-of-the-art studies by the separated POD. The orbits of the GNSS satellites and LEOs reach an accuracy of a few centimeters and are comparable with the state-of-art studies. A more efficient outlier detection method has been developed to improve the position determined by using pseudo-range observations. In the study about the enhancement of the GPS orbits by integrating LEOs, a 26-station ground network in a global and sparse distribution is supplemented by different subsets of seven LEOs including GRACE-A/B, OSTM/Jason-2, Jason-3 and, Swarm-A/B/C. A 34% improvement of the GPS orbit in 1D-mean RMS (from 37.5 mm to 23.9 mm) is achieved by including the seven LEOs. Both the number of space-based observations and the LEOs' orbital geometry affect the GPS orbits where the orbital geometry is shown to be more important. The estimated GPS PCOs are also improved by including LEOs. For the x- and y-components of the GPS PCOs, the formal error is reduced significantly due to the additional observations and expanded nadir angle coverage brought by the LEOs during the periods of large solar-elevation angle. The z-component of the GPS PCOs (z-PCO) are strongly correlated with the scale of the terrestrial reference frame. By introducing the orbit dynamics of the seven LEOs to the processing without applying a no-net-scale constraint, the correlation coefficients between the GPS z-PCOs and the scale are reduced from 0.85 to 0.30. Consequently, the GPS z-PCOs can be estimated independently from the a-priori scale and a purely GNSS-based scale can be determined as well. A system-specific -25.5 cm offset of the GPS z-PCOs relative to the values offered by the International GNSS Service (IGS) is computed based on the seven-LEO-integrated solution. Another approach based on Galileo also solves this problem. The GPS satellites, multi-GNSS stations, and Galileo satellites with ground calibrated PCOs are processed jointly to calibrate the GPS z-PCOs and simultaneously determine a Galileo-based scale simultaneously. Based on the comparison and cross-check, a good agreement is shown between the LEO-based and Galileo-based methods. There is a slight improvement in the geocenter when including three Swarm satellites to the processing with about 80 ground stations over a half year. Based on the analysis in theory and the results derived from real data, an obvious enhancement to various aspects of GNSS by the integrated processing with LEOs is shown. More LEOs equipped with GNSS receivers and carefully calibrated PCOs are expected for further missions or even the next generation of GNSS.

Description: In dieser Arbeit wird eine Verbesserung der globalen Satellitennavigationssysteme (GNSS) durch die Einbindung von Satelliten in niedrigen Erdumlaufbahnen (LEOs) in eine gemeinsame präzise Bahnbestimmung (POD) vorgestellt. Das von den Vereinigten Staaten betriebene Global Positioning System (GPS) wird stellvertretend für alle GNSS untersucht. Die mit GNSS-Empfängern ausgestatteten LEOs ergänzen die Empfänger der Bodenstationen, vor allem in Regionen, in denen aus verschiedenen Gründen nur wenige Stationen verfügbar sind. Aufgrund der Orbithöhe und schnellen Bewegung der LEOs tragen diese nicht nur mit zusätzlichen Beobachtungen bei, sondern auch mit einer sich schnell verändernden Beobachtungsgeometrie. Darüber hinaus haben weltraumgestützte Beobachtungen zusätzliche Vorteile gegenüber bodengestützten Beobachtungen, z. B. werden Signale ohne den Einfluss der Troposphäre empfangen. LEOs stellen nicht nur kinematische Stationen für die GNSS-Satelliten dar, sondern bringen auch eine zusätzliche Bahndynamik in das integrierte System ein. Die durch diese Bahndynamik gegebenen Beschränkungen sind sowohl für die Bahnbestimmung der GNSS Satelliten als auch für weitere Parameter äußerst relevant. In dieser Arbeit werden die folgenden Themen behandelt: 1) Hintergrundinformationen und Grundprinzipien der POD von GNSS-Satelliten und LEOs, 2) eine separate POD von GNSS-Satelliten und LEOs, 3) eine integrierte POD, 4) die Schätzung der Antennen-Phasenzentrumsversätze (PCOs) der GPS-Satelliten und anderer geodätischer Parameter in der integrierten POD. Die separaten PODs bestätigen, dass die in dieser Studie verwendete Bahnmodellierungs- und Prozessierungskonfiguration der GNSS-Satelliten und LEOs mit dem aktuellen Stand der Forschung kompatibel ist. Die Bahnen der GNSS-Satelliten und LEOs erreichen eine Genauigkeit von wenigen Zentimetern. Es wurde eine effizientere Methode zur Erkennung von Ausreißern entwickelt, um die mit Hilfe von Pseudo-Range-Beobachtungen ermittelte Position zu verbessern. Ein Bodennetz mit 26 global dünn verteilten Stationen wird verwendet, um die Verbesserung der GPS-Bahnen durch die Integration von verschiedenen Teilgruppen der sieben LEOs GRACE-A/B, OSTM/Jason-2, Jason-3 und Swarm-A/B/C zu untersuchen. Bei der Einbeziehung aller sieben LEOs ergibt sich eine Verbesserung des 1D RMS Mittelwertes der GPS-Orbits von 34 % (von 37,5 mm auf 23,9 mm). Sowohl die Anzahl der weltraumgestützten Beobachtungen als auch die Geometrie der Bahnen der LEOs beeinflussen die GPS-Bahnen, wobei die Orbitgeometrie sich als der wichtigere Faktor erweist. Die geschätzten GPS PCOs werden durch die Einbeziehung von LEOs ebenfalls verbessert. Der formale Fehler der x- und y-Komponenten der GPS PCOs wird durch die zusätzlichen Beobachtungen und die größere Abdeckung des Nadirwinkels, den die LEOs während Perioden eines großen Sonnenstandswinkels mit sich bringen, erheblich reduziert. Die z-Komponente der GPS PCOs (z-PCO) ist mit dem Maßstabsfaktor des terrestrischen Referenzrahmens stark korreliert. Durch die Berücksichtigung der Bahndynamik der sieben LEOs in der Prozessierung werden ohne Fixierung des Maßstabes (d.h. ohne eine No-Net-Scale Bedingung) die Korrelationskoeffizienten zwischen den GPS z-PCOs und dem Maßstabsfaktor von 0,85 auf 0,30 reduziert. Folglich können zum einen die GPS z-PCOs unabhängig von einem externen Maßstab geschätzt werden und zum anderen kann ein rein GNSS-basierter Maßstabsfaktor bestimmt werden. Mit der integrierten Lösung mit sieben LEOs ergibt sich ein systemspezifischer Versatz der GPS z-PCOs von -25,5 cm relativ zu den vom International GNSS Service (IGS) veröffentlichten Werten. Ein anderer Ansatz basierend auf Galileo löst dieses Problem ebenfalls. Die GPS Satelliten, Multi-GNSS Bodenstationen und Galileo Satelliten mit bodenkalibrierten PCOs werden gemeinsam prozessiert, um die GPS z-PCOs zu kalibrieren und gleichzeitig einen Galileo-basierten Maßstabsfaktor zu bestimmen. Ein Vergleich zur Überprüfung zeigt eine hohe Übereinstimmung der LEO- und Galileo-basierten Methoden. Die Einbeziehung von drei Swarm Satelliten in eine Prozessierung mit etwa 80 Bodenstationen über ein halbes Jahr hinweg zeigt eine leichte Verbesserung des Geozentrums. Auf der Grundlage der theoretischen Analyse und der aus realen Daten abgeleiteten Ergebnisse zeigt sich eine deutliche Verbesserung verschiedener Aspekte der GNSS durch die Integration von LEOs. Es ist zu erwarten, dass mehr LEOs, ausgestattet mit GNSS-Empfängern und sorgfältig kalibrierten PCOs, für künftige Missionen oder sogar die nächste GNSS Generation eingesetzt werden.

Description: Improving and homogenizing time and space reference systems on Earth and, more specifically, realizing the Terrestrial Reference Frame (TRF) with an accuracy of 1 mm and a long-term stability of 0.1 mm/year are relevant for many scientific and societal endeavors. The knowledge of the TRF is fundamental for Earth and navigation sciences. For instance, quantifying sea level change strongly depends on an accurate determination of the geocenter motion but also of the positions of continental and island reference stations, such as those located at tide gauges, as well as the ground stations of tracking networks. Also, numerous applications in geophysics require absolute millimeter precision from the reference frame, as for example monitoring tectonic motion or crustal deformation, contributing to a better understanding of natural hazards. The TRF accuracy to be achieved represents the consensus of various authorities, including the International Association of Geodesy (IAG), which has enunciated geodesy requirements for Earth sciences. Moreover, the United Nations Resolution 69/266 states that the full societal benefits in developing satellite missions for positioning and Remote Sensing of the Earth are realized only if they are referenced to a common global geodetic reference frame at the national, regional and global levels. Today we are still far from these ambitious accuracy and stability goals for the realization of the TRF. However, a combination and co-location of all four space geodetic techniques on one satellite platform can significantly contribute to achieving these goals. This is the purpose of the GENESIS mission, a component of the FutureNAV program of the European Space Agency. The GENESIS platform will be a dynamic space geodetic observatory carrying all the geodetic instruments referenced to one another through carefully calibrated space ties. The co-location of the techniques in space will solve the inconsistencies and biases between the different geodetic techniques in order to reach the TRF accuracy and stability goals endorsed by the various international authorities and the scientific community. The purpose of this paper is to review the state-of-the-art and explain the benefits of the GENESIS mission in Earth sciences, navigation sciences and metrology. This paper has been written and supported by a large community of scientists from many countries and working in several different fields of science, ranging from geophysics and geodesy to time and frequency metrology, navigation and positioning. As it is explained throughout this paper, there is a very high scientific consensus that the GENESIS mission would deliver exemplary science and societal benefits across a multidisciplinary range of Navigation and Earth sciences applications, constituting a global infrastructure that is internationally agreed to be strongly desirable.

Description: SKS and SKKS shear waves recorded on the INDEPTH III seismic array deployed in central Tibet during 1998 - 1999 have been analysed for the direction and extent of seismic polarization anisotropy. The 400-km-long NNW trending array extended south to north, from the central Lhasa terrane, across the Karakoram-Jiali fault system and Banggong-Nujiang suture to the central Qiangtang terrane. Substantial splitting with delay times from 1 to 2 s, and fast directions varying from E-W to NE-SW, was observed for stations in the Qiangtang trerrane and northernmost Lhasa terrane. No detectable splitting was observed for stations located farther south in the central Lahsa terrane. The change in shear wave splitting characteristics occurs at 32°N, approximately coincident with the transcurrent Karakoram-Jiali fault system but ~40 km south of the surface trace of the Banggong-Nujiang suture. This location is also near the southernmost edge of a region of high Sn attenuation and low upper mantle velocities found in previous studies. The transition between no measured splitting and strong anisotropy (2.2 s delay time) is exceptionally sharp (=15 km), suggesting a large crustal contribution to the measured splitting. The E-W to NE-SW fast directions are broadly similar to the fast directions observed farther east along the Yadong-Golmud highway, suggesting that no large-scale change in anisotropic properties occurs in the east-west direction. However, in detail, fast directions and delay times vary over lateral distances of ~100 km in both the N-S and E-W direction by as much as 40° and 0.5-1 s, respectively. The onset of measurable splitting at 32° N most likely marks the northern limit of the underthrusting Indian lithosphere, which is characterized by negligible polarization anisotropy. Taken in conjunction with decades of geophysical and geological observations in Tibet, the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern Tibet is being squeezed and sheared between the advancing Indian lithosphere to the south and the Tsaidam and Tarim Lithospheres to the north and west, resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability. In northern Tibet, crustal deformation clearly follows this large-scale deformation pattern.

Description: The GPS satellite transmitter antenna phase center offsets (PCOs) can be estimated in a global adjustment by constraining the ground station coordinates to the current International Terrestrial Reference Frame (ITRF). Therefore, the derived PCO values rest on the terrestrial scale parameter of the frame. Consequently, the PCO values transfer this scale to any subsequent GNSS solution. A method to derive scale-independent PCOs without introducing the terrestrial scale of the frame is the prerequisite to derive an independent GNSS scale factor that can contribute to the datum definition of the next ITRF realization. By fixing the Galileo satellite transmitter antenna PCOs to the ground calibrated values from the released metadata, the GPS satellite PCOs in the z-direction (z-PCO) and a GNSS-based terrestrial scale parameter can be determined in GPS + Galileo processing. An alternative method is based on the gravitational constraint on low earth orbiters (LEOs) in the integrated processing of GPS and LEOs. We determine the GPS z-PCO and the GNSS-based scale using both methods by including the current constellation of Galileo and the three LEOs of the Swarm mission. For the first time, direct comparison and cross-check of the two methods are performed. They provide mean GPS z-PCO corrections of −186±25 mm and −221±37 mm with respect to the IGS values and +1.55±0.22 ppb (parts per billion) and +1.72±0.31 in the terrestrial scale with respect to the IGS14 reference frame. The results of both methods agree with each other with only small differences. Due to the larger number of Galileo observations, the Galileo-PCO-fixed method leads to more precise and stable results. In the joint processing of GPS + Galileo + Swarm in which both methods are applied, the constraint on Galileo dominates the results. We discuss and analyze how fixing either the Galileo transmitter antenna z-PCO or the Swarm receiver antenna z-PCO in the combined GPS + Galileo + Swarm processing propagates to the respective freely estimated z-PCO of Swarm and Galileo.

Description: While many studies suggested that thick-skinned deformation may occur in the interior of a mountain belt and a thin-skinned contraction wedge is predominant near the foothills, we report the coexistence of both thin- and thick-skinned deformation beneath the foreland basin in southwestern Taiwan based on the high-resolution microearthquake distribution observed by a dense seismic array. We revealed three major tectonic elements in our study area, which are 1. A basal decollement of the thin-skinned wedge at a depth of about 5 km beneath the foothills and coastal plain. 2. A westward-dipping backthrust extending from the foothills to a depth of about 15 km beneath the Coastal Plain. 3. Foreland-vergence blind thrusts rooting on top of the backthrust at depths below ca. 8-15 km. The foreland-dipping back thrust may result from reactivating an ancient normal fault for being associated with an inverted half-graben. Our numerical modeling further demonstrated that this pre-existing normal fault becomes a passive roof thrust during later orogenic contraction. The rotation and westward movement of this backthrust would compress the rocks in front of it and create blind thrusts beneath the Coastal Plain. This thick-skinned deformation may also have affected the overlying tectonic wedge by building a ramp on the shallow decollement. As a result, the proto-Dajienshan fault might form at the ramp where stress concentrates.

Description: The condensation of organic vapors along with water vapor (co-condensation, CC) in adiabatic cloud parcel model (CPM) simulations enhances the number of aerosol particles that activate to form cloud droplets. The boreal forest is rich in organic vapors, but their role in CC has not been systematically investigated. In this work, we characterize conditions under which strong CC-driven cloud droplet number concentration enhancements (ΔCDNC) would be expected over the boreal biome. For this purpose, we performed a suite of CPM simulations, initialized with a set of concurrent state-of-the-art aerosol observations from the boreal forest of Finland (spring 2014). The degree to which CC impacts droplet formation is shown to be dependent on the initialization of the updraft velocity, aerosol size distribution, organic vapor concentration and volatility distribution. The predicted ΔCDNC is ~20%, which corresponds to activating particles 12–16 nm smaller in dry diameter, that would otherwise remain as interstitial aerosol. The highest ΔCDNC are predicted in the presence of a nascent ultrafine aerosol mode. Such aerosol size distributions are observed frequently in the studied boreal forest environment in spring and fall. UK Earth System Model simulations are further used to evaluate the frequencies to which such distributions are experienced by an Earth System Model over the whole boreal biome. They are lower than those observed at the measurement site and are modeled only during spring with large spatial variability. For accurate predictions of CC effects on cloud droplet number, the representation of the aerosol size distribution is of essence.

Description: High-resolution Thermospheric Neutral Density (TND) measurements from Low Earth Orbit (LEO), are valuable to accurately estimate the short-term atmosphere abrupt disturbances, triggered by magnetospheric forcing. For our study, the GRACE-FO satellites (Gravity Recovery and Climate Experiment Follow-On) 10s accelerometer-derived TND measurements were processed. We also derived the TND by determining the atmospheric drag force through centimeter-level reduced-dynamic Precise Orbit Determination (POD) using onboard Global Navigation Satellite System (GNSS) observations. We compare the resulting densities with those of the NRLMSIS-2.0 empirical atmosphere model to examine the status of solar cycle 25 between August 29 and December 31, 2020. Our investigations indicate that the POD-derived density shows a good agreement with accelerometer-derived density, which exhibited a more sensitive response to magnetic storms compared with NRLMSIS-2.0. The intensity of neutral densities enhancement in the Southern hemisphere surpasses that in the Northern one. The observed 4-month enhancing disturbances suggest a shift from relatively quiescent to a much more active phase of solar activity, revealing unexpected dependencies on the temporal and spatial characteristics. Furthermore, the Wygant coupling function was applied to model the response of the geomagnetic field changes in the solar wind, and quantify the impact of geomagnetic activities on the density of the upper atmosphere. All these findings hold the potential to improve our understanding of LEO orbital drag.