The objective of this thesis is to evaluate the sources of the differences between the NO2 satellite retrieval products provided by the Royal Dutch Meteorological Institute (KNMI) and the National Aeronautics and Space Administration (NASA). Ground studies have shown that although both products use the same satellite, these products yield different observations for NO2 tropospheric columns concentrations. This study does not validate either retrieval product, but rather indentifies the main sources for the discrepancy. There are several parameters which allow successful retrieval of NO2 vertical columns. For this study, only the difference between the radiative models and the a priori NO2 chemical transport models were considered relevant. All other parameters, such as cloud properties, slant columns, stratospheric serration and their assumptions, were held constant. Here, the models are referred to by their proprietor's acronym: "TOMRAD" refers to the radiative model used by NASA, "DAK" refers to the radiative model used by KNMI, "TM4" refers to the a priori chemical transport model used by KNMI, and "REAM" refers to the a priori chemical transport model maintained by the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. Mixing these parameters creates four retrievals for comparison. Many significant differences were identified after comparing these four retrievals. First, there are viewing geometry biases between the port side and the starboard side of the satellite retrieval for each swath. These viewing geometry biases lead to artificial periodicities in the retrievals of NO2 tropospheric vertical columns over a specific coordinate or site, such as a city. Furthermore, there were significant differences found after using different a priori NO2 chemical transport models. The low horizontal resolution of TM4 and the satellite retrieval/TM4 coupling effect compared to REAM leads to considerable questioning of the near real time application of the KNMI NO2 retrieval product. Though the TM4 model performs poorly, TM4 retrievals do perform nearly as well as REAM retrievals at capturing day-to-day variability and the spatial variability of the cities used as examples here. The retrievals using TOMRAD outperformed the retrievals using DAK when compared to the high resolution, hourly REAM a priori chemical transport model. In sum, these findings should lead to better optimizations of both the KNMI and NASA retrievals, and thus make their publicly available data products more reliable and accurate for general use.

This thesis brings statistical analyses techniques to bear on data derived from an extensive database of satellite launches and on-orbit anomalies and failures. The data collected is analyzed from two different perspectives and addresses, in two separate studies, two research objectives. The first study proposes to identify trends and cyclical patterns in the space industry, and to forecast the volume of launches for the next few years. Satellites have been rightfully described as the lifeblood of the entire space industry and the number of satellites ordered or launched per year is an important defining metric of the industry's level of activity. The structure of the space industry, its financial health and its workforce retention and development is dependent on the volume of satellites contracted. As such, trends and variability in this volume have significant strategic impact on the space industry. Over the past 40+ years, hundreds of satellites have been launched every year. Thus, an important data set is available for time series analysis and identification of trends and cycles in the various markets of the space industry. For the purpose of this first study, we collected data for over 6,000 satellites launched since 1960 on a yearly basis. We separated the satellites into three broad segments: 1) defense and intelligence satellites, 2) science satellites, and 3) commercial satellites. Several techniques are available for the analysis of time series data, both in the time domain and in the frequency domain. In this first study, we conducted spectral analysis of the time series for each of the three satellite populations and identified cycles contained in the data. In addition, once harmonic models were derived and fitted to the data, we built forecasting models of satellite launch volumes in the different market segments for the next few years. The potential implications of the results are discussed as a number of strategic matters for the space industry are contingent on the predictions or forecast of the volume of satellites contracted (the example of the U.S. auto industry is a solemn reminder of such possible strategic issues). The second study uses the previously collected launch data, confined to Earth-orbiting satellites launched between 1990 and 2008, and expanded with the failure information and retirement of each satellite to conduct a comparative analysis of satellite reliability in GEO, LEO, and MEO orbits. Reliability has long been recognized as an essential consideration in the design of space systems. However, there is limited statistical analysis of satellite reliability based on actual flight data. The objective of this second study is to conduct nonparametric satellite reliability analysis, with orbit type as a covariate, and to explore appropriate parametric fits (Weibull, lognormal, and mixture distributions). The results indicate for example that differences exist between the failure behaviors of satellites in different orbits, or that satellite infant mortality exists or dominates more clearly in a particular orbit type. The findings can be useful to satellite manufacturers as they would provide an empirical basis for reviewing and adjusting satellite testing and burn-in procedures.

Les tourbillons de mésoéchelle sont des structures relativement petites qui dominent la variabilité océanique et qui ont un impact sur la circulation de grande échelle, les flux de chaleurs et les processus biologiques dans l'océan. De nombreux tourbillons ont été observés dans la Méditerranée occidentale. Pourtant, il n'existe pas une caractérisation systématique de ces structures en raison des petites dimensions de ces structures dans la région où le rayon de déformation de Rossby qui caractérise la taille des tourbillons est petit (10-15 km). L'objectif de cette thèse est donc une caractérisation des tourbillons mésoéchelle dans la Méditerranée occidentale. Dans cette optique, des outils sont développés pour étudier les fines échelles dans le bassin. Une simulation « eddy-resolving » de la région est ainsi réalisée sur une période de 20 ans. Une évaluation à l'aide de données indépendantes et des connaissances actuelles montre un comportement réaliste de la simulation. L'étude des niveaux d'énergie de la simulation montre que les cartes altimétriques existantes sous-estiment le signal mésoéchelle. Une nouvelle méthode d'interpolation des traces altimétriques est alors développée pour mieux représenter la mésoéchelle. Cependant cette amélioration se fait au détriment de l'homogénéité de la résolution des cartes altimétriques. Dans un second temps, trois méthodes de détection et suivi des tourbillons sont appliquées aux cartes altimétriques, le modèle à haute résolution et un modèle à plus faible résolution pour extraire les caractéristiques des tourbillons. La taille des tourbillons varie grandement mais est en moyenne d'environ 25-30 km. Autour de 30 tourbillons sont détectés dans la région par jour avec une distribution spatiale très hétérogène. Au contraire d'autres régions du globe, ils sont principalement advectés par les courants moyens. En séparant les tourbillons selon leur durée de vie, on remarque que les ceux à longue durée de vie sont plus grands, plus intenses et ont un cycle saisonnier avec un pic en fin d'été, alors que ceux à plus courte durée de vie sont plus petits, moins intenses et plus présents en hiver. La profondeur des tourbillons a une grande variance mais la moyenne est de 300 m. Les tourbillons anticycloniques sont en moyenne plus profond et ont une forme plus conique que les tourbillons cycloniques. / Mesoscale eddies are relatively small structures that dominate the ocean variability and have large impact on large scale circulation, heat fluxes and biological processes. In the western Mediterranean Sea, a high number of eddies has been observed and studied in the past with in-situ observations. Yet, a systematic characterization of these eddies is still lacking due to the small scales involved in these processes in this region where the Rossby deformation radius that characterizes the horizontal scales of the eddies is small (10-15 km). The objective of this thesis is to perform a characterization of mesoscale eddies in the western Mediterranean. For this purpose, we propose to develop tools to study the fine scales of the basin. First, we develop an eddy resolving simulation of the region for the last 20 years. The performance of the simulation is evaluated with independent observations (drifters, satellites, hydrographic profiles) showing realistic behavior. This simulation shows that existing altimetry maps underestimate the mesoscale signal. Therefore, we attempt to improve existing satellite altimetry products to better resolve mesoscale eddies. We show that this improvement is possible but at the cost of the homogeneity of the fields; the resolution can only be improved at times and locations where altimetric observations are densely distributed. In a second part, we apply three different eddy detection and tracking methods to extract eddy characteristics from the outputs of the high-resolution simulation, a coarser simulation and altimetry maps. The results allow the determination of some characteristics of the detected eddies. The size of eddies can greatly vary but is around 25-30 km. About 30 eddies are detected per day in the region with a very heterogeneous spatial distribution. Unlike other areas of the open ocean, they are mainly advected by currents of the region. Eddies can be separated according to their lifespan. Long-lived eddies are larger in amplitude and scale and have a seasonal cycle with a peak in late summer, while short-lived eddies are smaller and more present in winter. The penetration depth of detected eddies has also a large variance but the mean depth is around 300 meters. Anticyclones extend deeper in the water column and have a more conic shape than cyclones.

In less than ten years, Aljazeera television has become the most popular satellite news service in the Arab world. Regimes around the region have regarded Aljazeera as a threat, while Aljazeera has consistently claimed that it is simply reporting the truth. Notwithstanding this, Aljazeera has successfully established its presence in the media world despite the controversies surrounding its professional approach and the hammering criticism that has been directed to it in both the Middle East and the West. This research explores the thesis that Aljazeera is a catalyst of democratisation and social and political change in the Arab world. As a recent media phenomenon, Aljazeera has been playing a critical role in changing the social and political values of societies in the Arab world and viewers’ perceptions of a range of social and cultural topics relating to human rights, equality, diversity, gender, employment and exploitation. It is said that through its persistent campaigns to raise the awareness of its increasingly broadening viewer base to these issues, Aljazeera has created a new public sphere in the Arab countries that are traditionally and historically non-democratic in the least and despotic and dictatorial in the extreme. It became “[the] arena within which debate occurs...” (Hartley, 2002, p.191) between viewers who share in the process of discourse to communicate and debate. In this context Aljazeera provided a public forum for Arab viewers to express their views and address a range of sensitive and controversial issues. Consequently, it is the perception of democracy that Aljazeera seems to be fostering in the Arab world, which is leading to a sense of empowerment at the individual level. The research sought to examine this phenomenon through a field study that garnered vital data from a representative sample of 600 viewers of Aljazeera, including 100 media professionals, in four Arab countries: Egypt, Jordan, United Arab Emirates, and Qatar, and amongst the Arab diaspora (with the Australian Arabs as a focus group). The data was analysed against a media model that was developed specifically for that purpose. The findings support the research hypothesis that Aljazeera is a catalyst of democratisation and socio-political change.

Thesis (M.S.)--University of Wisconsin--Madison, 1962. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaf 21).

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Η παρούσα εργασία περιλαμβάνει την μελέτη και τον χαρακτηρισμό του καναλιού ενός δορυφορικού καναλιού καθώς και την επίγεια διάδοση Αρχικά γίνεται μια εισαγωγή στα ασύρματα συστήματα τηλεπικοινωνιών . Ειδικά παρουσιάζεται το δορυφορικό σύστημα και αναλύονται τα μέρη αυτού. Γίνεται παρουσίαση των δορυφορικών ζεύξεων και μελετάται το περιβάλλον μέσα στο οποίο λαμβάνει χώρα μια σύνδεση. Αρχικά αναλύονται οι μέθοδοι διάδοσης της ηλεκτρομαγνητικής ακτινοβολίας και οι διάφορες εξασθενήσεις που υφίσταται το σήμα λόγω φαινομένων της ατμόσφαιρας καθώς και άλλων επιδράσεων. Γίνεται έπειτα μια εκτενής αναφορά στις διαλείψεις και στην ταξινόμησή τους. Στη συνέχεια γίνεται μια κατηγοριοποίηση των διαλείψεων αυτών με βάση τα χαρακτηριστικά τους. Ακολουθεί η στατιστική αναπαράσταση των καναλιών με διαλείψεις χρησιμοποιώντας τις βασικότερες κατανομές που συναντώνται στα τηλεπικοινωνιακά συστήματα. / This work includes the study and channel characterization for a satellite signal and also its terrestrial propagation. First we introduce the wireless telecommunications systems. We focus on the satellite system and its components are analyzed. A presentation of the satellite links follows and the propagation environment is studied. We analyze the propagation methods of electromagnetic radiation and the various losses that the signal endures due to atmospheric phenomena and other effects. There is then a reference to the fading channel and and its analysis. Then there is categorization of the fading according to its characteristics. The statistical representation of the fading channel follows, with the use of the key distributions found in telecommunication systems. Lastly, the satellite transmission is simulated for various orbit heights and conditions

Les tourbillons de mésoéchelle sont des structures relativement petites qui dominent la variabilité océanique et qui ont un impact sur la circulation de grande échelle, les flux de chaleurs et les processus biologiques dans l'océan. De nombreux tourbillons ont été observés dans la Méditerranée occidentale. Pourtant, il n'existe pas une caractérisation systématique de ces structures en raison des petites dimensions de ces structures dans la région où le rayon de déformation de Rossby qui caractérise la taille des tourbillons est petit (10-15 km). L'objectif de cette thèse est donc une caractérisation des tourbillons mésoéchelle dans la Méditerranée occidentale. Dans cette optique, des outils sont développés pour étudier les fines échelles dans le bassin. Une simulation « eddy-resolving » de la région est ainsi réalisée sur une période de 20 ans. Une évaluation à l'aide de données indépendantes et des connaissances actuelles montre un comportement réaliste de la simulation. L'étude des niveaux d'énergie de la simulation montre que les cartes altimétriques existantes sous-estiment le signal mésoéchelle. Une nouvelle méthode d'interpolation des traces altimétriques est alors développée pour mieux représenter la mésoéchelle. Cependant cette amélioration se fait au détriment de l'homogénéité de la résolution des cartes altimétriques. Dans un second temps, trois méthodes de détection et suivi des tourbillons sont appliquées aux cartes altimétriques, le modèle à haute résolution et un modèle à plus faible résolution pour extraire les caractéristiques des tourbillons. La taille des tourbillons varie grandement mais est en moyenne d'environ 25-30 km. Autour de 30 tourbillons sont détectés dans la région par jour avec une distribution spatiale très hétérogène. Au contraire d'autres régions du globe, ils sont principalement advectés par les courants moyens. En séparant les tourbillons selon leur durée de vie, on remarque que les ceux à longue durée de vie sont plus grands, plus intenses et ont un cycle saisonnier avec un pic en fin d'été, alors que ceux à plus courte durée de vie sont plus petits, moins intenses et plus présents en hiver. La profondeur des tourbillons a une grande variance mais la moyenne est de 300 m. Les tourbillons anticycloniques sont en moyenne plus profond et ont une forme plus conique que les tourbillons cycloniques. / Mesoscale eddies are relatively small structures that dominate the ocean variability and have large impact on large scale circulation, heat fluxes and biological processes. In the western Mediterranean Sea, a high number of eddies has been observed and studied in the past with in-situ observations. Yet, a systematic characterization of these eddies is still lacking due to the small scales involved in these processes in this region where the Rossby deformation radius that characterizes the horizontal scales of the eddies is small (10-15 km). The objective of this thesis is to perform a characterization of mesoscale eddies in the western Mediterranean. For this purpose, we propose to develop tools to study the fine scales of the basin. First, we develop an eddy resolving simulation of the region for the last 20 years. The performance of the simulation is evaluated with independent observations (drifters, satellites, hydrographic profiles) showing realistic behavior. This simulation shows that existing altimetry maps underestimate the mesoscale signal. Therefore, we attempt to improve existing satellite altimetry products to better resolve mesoscale eddies. We show that this improvement is possible but at the cost of the homogeneity of the fields; the resolution can only be improved at times and locations where altimetric observations are densely distributed. In a second part, we apply three different eddy detection and tracking methods to extract eddy characteristics from the outputs of the high-resolution simulation, a coarser simulation and altimetry maps. The results allow the determination of some characteristics of the detected eddies. The size of eddies can greatly vary but is around 25-30 km. About 30 eddies are detected per day in the region with a very heterogeneous spatial distribution. Unlike other areas of the open ocean, they are mainly advected by currents of the region. Eddies can be separated according to their lifespan. Long-lived eddies are larger in amplitude and scale and have a seasonal cycle with a peak in late summer, while short-lived eddies are smaller and more present in winter. The penetration depth of detected eddies has also a large variance but the mean depth is around 300 meters. Anticyclones extend deeper in the water column and have a more conic shape than cyclones.

Le monde des communications par satellite est dominé par les systèmes de diffusion de la télévision. Cependant, des satellites de communication offrent aussi des services de téléphonie et de données. Ils sont regroupés dans les familles des systèmes fixes et mobiles et ciblent des marchés de niche. Dans cette thèse, nous avons la volonté d’étendre les scénarios d’utilisation de ces systèmes. Notre vision nous dicte que leur développement est lié à l’utilisation de réseaux hybrides mobiles satellite-terrestre. En effet, une utilisation complémentaire des deux segments permet de s’affranchir d’une concurrence trop féroce des réseaux de télécommunications terrestres. Pour cela, nous optons pour deux applications qui nous paraissent prometteuses : un réseau mobile LTE (Long Term Evolution) avec des stations de base qui possèdent un backhaul satellite et un réseau MANET (Mobile Ad-hoc NETwork) qui s’interconnecte à des réseaux extérieurs grâce à des liaisons satellite. Nous soulevons l’un des problèmes les plus contraignants du réseau mobile LTE avec des backhauls satellite : la gestion de la mobilité. L’analyse du standard nous a conduits à conclure quant à la nécessité d’optimiser les procédures du handover. Ceux qui nécessitent des modifications surviennent entre des stations de base qui n’utilisent pas le même backhaul satellite et entre une station de base avec un backhaul satellite vers une avec un backhaul terrestre. Deux points nous ont semblé importants : la phase de préparation et le mécanisme qui permet d’éviter les pertes. Nous proposons donc une nouvelle phase de préparation qui prend en compte le retard induit par la liaison satellite ainsi qu’une phase de préparation à double décision combinée avec une préparation de multiples stations de base. Nous tentons ainsi de maximiser les chances de réaliser un handover avec succès. Puis, nous avons imaginé un mécanisme qui permet à la fois d’éviter les pertes lors de l’exécution du handover et de sauvegarder les précieuses ressources du satellite. Les réseaux MANET associés à des liaisons satellite offrent des caractéristiques très intéressantes pour les communications d’urgence, telles que l’indépendance vis-à-vis des infrastructures terrestres susceptibles d’être endommagées par des catastrophes ainsi qu’un déploiement rapide pour une intervention sur le théâtre des opérations. Nous avons souhaité améliorer l’un des points cruciaux dans le cadre d’une hybridation : la sélection de la passerelle satellite. Nous avons donc développé un mécanisme qui prend en compte la charge sur les passerelles satellite ainsi que le phénomène d’oscillation de passerelle souvent négligé dans la littérature. Ces optimisations ont pour but de favoriser le développement de réseaux hybrides satellite terrestres en améliorant les performances de ces réseaux. L’avenir nous semble prometteur quant à l’utilisation de la technologie LTE avec un backhaul satellite pour lequel nous avons proposé une nouvelle gestion de la mobilité qui est primordiale pour son développement. / Satellite communications are leaded by television broadcasting. Yet, fixed and mobile satellite systems provide voice services as well as IP-based applications. In this thesis, we try to develop user scenarios in order to extend their targeted market. Our vision to reach this objective consists to use hybrid satellite and terrestrial mobile networks. This network design avoids a competition between both segments in which a satellite success is difficult to imagine. Furthermore, hybrid networks may draw benefits from both segments. Two promising scenarios have been selected. The first one consists in a mobile LTE network (Long Term Evolution) with base stations backhauled by satellite links whereas the second scenario is composed of a Mobile Ad-hoc Network (MANET) connected to external networks thanks to satellite systems. One of the main problems in the hybrid LTE scenario is caused by mobility procedures. As a consequence of the standard analysis, we have decided to optimize the mobility management in two cases: a handover between two base stations for which the backhaul is provided by two different satellite terminals and a handover from a base station with a satellite backhaul to one with a terrestrial backhaul. Two procedures have drawn our attention: the preparation phase and the loss avoidance mechanism during the execution phase. First of all, we design a new procedure for the preparation which takes into account the delay induced by the satellite link. This new phase is based on a twofold decision preparation associated with multiple preparations. This solution leads to an increase of handover success. The second optimization aims to avoid losses during the execution phase and, at the same time, save satellite resources. MANET and satellite hybridization leads to very interesting characteristics for public safety communications. Indeed, these networks are independent of terrestrial infrastructures that can be impaired or destroyed. Furthermore, they can be rapidly deployed in the theater of operation. Gateway selection is a crucial problem linked to hybrid MANET. Therefore, we have focused our work on this mechanism taking into account the measured load on the satellite links as well as an oftenneglected phenomenon, the gateway flapping. These optimizations tend to promote hybrid satellite and terrestrial networks improving their performance. A promising future is foreseen for the hybrid LTE technology and we have proposed a solution to a problem that may be very detrimental to its deployment.

Les systèmes de communication par satellite d’aujourd’hui reposent principalement sur le multiplexage temporel pour optimiser leurs performances. Chaque utilisateur utilise le canal pendant une fraction de temps connu. Pendant cette période, la modulation et le taux de codage sont choisis de manière à transmettre le plus d’information possible. En pratique, ce schéma est facile à mettre en œuvre ce qui justifie sa popularité. Cependant, il est désormais bien connu que la répartition temporelle n’est pas optimale en termes d’efficacité spectrale offerte aux récepteurs. En effet, la stratégie qui consiste à superposer des données offre de meilleures performances que le multiplexage temporel. C’est dans ce contexte que s’inscrit la problématique de cette thèse. Le travail réalisé propose des applications du codage par superposition dans le domaine des communications par satellite. Tout d’abord, nous étudions la modulation hiérarchique qui est une implémentation du codage par superposition au niveau de la modulation. Les performances de ce type de modulation sont évaluées d’un point de vue théorique et pratique. Dans un deuxième temps, nous quantifions l’amélioration en termes d’efficacité spectrale que peut apporter la modulation hiérarchique pour les systèmes de communication par satellite. Les standards de diffusion par satellite DVB-SH et DVB-S2 fournissent un cadre pratique. Nous montrons que des gains non négligeables sont envisageables selon la configuration du système. Le dernier point abordé concerne un système où des utilisateurs communiquent entre eux à l’aide d’un satellite qui sert de relais. Nous proposons un schéma de communication où plusieurs utilisateurs émettent en même temps en coordonnant leur puissance de transmission. Ainsi, les signaux vont naturellement se superposer. Les récepteurs utilisent deux mécanismes pour le décodage des signaux : le codage réseau couche physique et la démodulation de constellations superposées. Finalement, les gains de performance obtenus dans les différents domaines par le codage par superposition ouvrent des perspectives pour des travaux futurs. / Modern satellite communication systems mainly rely on time sharing to optimize the throughput. Each receiver uses the channel during a given fraction of time. During this period, the transmission parameters (i.e., the modulation and the coding rate) are chosen in order totransmit as much information as possible. The scheme is easy to implement which explains its popularity. However, it is today well established that time sharing is not optimal in terms of spectrum efficiency offered to the receivers. Indeed, the scheme that consists in sending superposed data offers better performance than the time sharing. This thesis investigates the application of superposition coding in satellite communication systems. First of all, we study the performance of hierarchical modulation which is an implementation of superposition coding at the modulation level. We propose a performance evaluation method for such modulations. We also compare the performance of hierarchical and non hierarchical modulations in terms of spectrum efficiency and link unavailability. These two criteria are very important for broadcast system and we show that hierarchical modulations often offer better performance than non hierarchical modulations.Then, we study the performance improvement in terms of spectrum efficiency when using hierarchical modulation in satellite communication systems. Two issues are addressed. The first one is how to group the receivers in pairs in order to transmit data with a hierarchical modulation. The second issue is the computation of the spectrum efficiency. We show that significant gains are possible depending on the system configuration. The last part considers a system where multiple users communicate through a satellite. The satellite acts as a relay in our scenario. We propose a communication scheme where several users emit at the same time with appropriate transmitting power. Thus the signals naturally superpose and generate interference. The receivers use two mechanisms for decoding the signals: physical layer network coding and demodulation of superposed constellations. Finally, we explain how the performance improvements obtained by superposition coding in several scenarios open perspectives for future work.