No description available.

Thesis (M. Phil.)--University of Hong Kong, 2009. / Includes bibliographical references (leaves 120-122) Also available in print.

Thesis (M. Sc.)--University of Canterbury, 2006. / Typescript (photocopy). Includes bibliographical references (leaves 60-62). Also available via the World Wide Web.

The growth of satellite communication industry and the need to offer new and better services, to compete with optical fiber systems, have driven the satellite industry to search for higher bandwidth and improved cost of services. Higher frequency bands, such as Ka and V-band, came into play. These frequency bands have the advantage of efficient use of spectrum, the ability to transfer data at much higher rates, and the use of smaller antennas resulting in lower cost for the user. A major drawback though, is the severity of propagation impairments. Scintillation becomes important for low-fade margin systems operating at high frequencies and with low elevation angles and therefore must be taken into account in the design of satellite communication systems. The first part of the thesis deals with the analysis of beacon data and presentation of various statistics, namely short and long-term distributions of scintillation amplitudes and intensity. Their correlations with meteorological parameters are also presented. A dynamic model of tropospheric scintillation is developed, which permits the generation of scintillation time series based on known scintillation characteristics, such as spectral shape and probability density function. The model employs readily obtainable input parameters (antenna diameter, path elevation angle, frequency, ground temperature and humidity) and yields statistics that compare extremely well with existing prediction models as well as measured scintillation data (up to the fourth moment). Important applications of the new model are for simulating the performance of scintillation-degraded communication links and predicting required scintillation fade margins. The second part of the thesis investigates the effects of scintillation on digital satellite communication systems using simulations implemented in COSSAP. Simulation results show that the higher the order of the modulation that is being used, the larger is the effect of scintillation on the bit error rate performance of the link. In the third part of the thesis, fade mitigation techniques to overcome the effects of scintillation fading are discussed. Analysis of an adaptive modulation system using three modulation techniques is presented and its advantages demonstrated. The adaptive system can be used to mitigate scintillation fading and at the same time increase the capacity of the system, decreasing system outage time through an efficient utilisation of available resources. In the last part, data transfer scenarios in VSAT systems incorporating adaptive modulation, ARQ and adaptive up-link power control are presented. The implementation of the adaptive system in a COSSAP simulation platform is discussed and individual adaptive components are simulated and presented in detail.

No description available.

No description available.

published_or_final_version / Physics / Master / Master of Philosophy

<p>The PAMELA space experiment is scheduled for launch towards the end of 2004 on-board a Russian Resurs DK1 satellite, orbiting Earth at an altitude of 300– 600 km. The main scientific goal is a study of the antimatter component of the cosmic radiation. The semipolar orbit (70.4◦) allows PAMELA to investigate a wide range of energies for antiprotons (80 MeV–190 GeV) and positrons (50 MeV– 270 GeV). Three years of data taking will provide unprecedented statistics in this energy range and will set the upper limit for the ratio He/He below 10−7. PAMELA is built around a permanent magnet silicon spectrometer, surrounded by a plastic scintillator anticoincidence shield built at KTH. The anticounter scintillators are used to aid in the rejection of background from particles not cleanly entering the acceptance of the tracker. Information from the anticounter system will be included as a veto in a second level trigger, to exclude the acquisition of events generated by false triggers.</p><p>An LED-based monitoring system has been developed for the anticounter system. The LEDs mimic the light signal produced in the scintillator by an ionising particle. This allows the functionality of the AC system to be verified in-orbit. The development and testing of the monitoring system are presented and comparisons have been made with independent radioactive source-based calibration methods. The anticounter system has also been extensively tested with cosmic rays and particle beams. Most of these tests have been performed with the anticounters integrated with the other PAMELA subdetectors in a flight-like configuration.</p>

Neste trabalho, construímos um novo e versátil forno de crescimento de cristais pela técnica de Bridgman, o qual pode operar em temperaturas de até 2000 &#176;C em alto-vácuo ou atmosferas inertes. Visto que o problema mais importante após o crescimento de um cristal por esta técnica é a aderência entre o mesmo e as paredes do caidinho, construímos também um sistema para tratamento do material de partida que eliminou este fenômeno. Este procedimento permitiu a obtenção de monocristais cilíndricos de NaI(Tl) transparentes, com alta qualidade e com cerca de 60 mm de comprimento e diâmetro de 20 mm. Utilizando a técnica de Czochralski, crescemos também vários monocristais cilíndricos transparentes de NAI(Tl) com comprimentos de 50 mm e diâmetros de 30 mm. Descrevemos também um método para eliminar o excesso de iodo dissolvido na fase líquida durante o processo de crescimento do cristal. Medidas de absorção ótica realizadas com os cristais de NAI(Tl) à temperatura ambiente e do nitrogênio líquido, mostram bandas de absorção características do Tálio próximas de 304, 291, 257, 249 e 236 nm. Através da relação entre a densidade ótica da banda de absorção em 304 nm e a concentração de tálio, estimamos o perfil da concentração deste elemento ao longo de um cristal crescido por Cz e por Bridgman. Através de espectroscopia de fluorescência encontramos, à temperatura ambiente, uma banda de emissão centrada em 425 nm. Protótipos de detetores foram construídos para avaliar as resoluções de energia dos monocristais crescidos pelos métodos de Bridgman e Czochralski. A performance dos detetores também foi analisada / In this work we constructed a new versatile Bridgman crystal growth furnace that can operate to 2.000 &#176;C in high vacuum or in inert atmospheres. Since the most important post-growth problem in this technique is related with the adherence between crystal-crucible, we also developed an initial materials-treatment system that could prevent this phenomenon, by previously saturating with HI gas. These procedures allowed us to get clear and high quality NaI(Tl) cylindric single crystals up to 60mm length and 20mm diameter. Using the Czochralski technique, we also grew several clear NaI(Tl) single crystals with dimensions up to 50 mm length and 30 mm diameter. A method to prevent the excess of Iodine in the liquid phase during the crystal growth process is also described. Optical absorption measurements in NaI(Tl) single crystals, at room and liquid nitrogen temperatures, show characteristic Thallium bands near 304, 291, 257, 249 and 236 nm. Through the relationship between optical density of the absorption band at 304 nm and Thallium concentration, we could estimate the concentration profile through the entire crystal, grown by each method: Bridgman and Czochralski. By fluorescence spectroscopy we also find, at room temperature, one emission band near to 425 nm. Radiation detector prototypes were developed to evaluate the energy resolution of the single crystals grown by Bridgman and Czochralski methods. The performance of the radiation detectors is also analyzed

Neste trabalho, construímos um novo e versátil forno de crescimento de cristais pela técnica de Bridgman, o qual pode operar em temperaturas de até 2000 &#176;C em alto-vácuo ou atmosferas inertes. Visto que o problema mais importante após o crescimento de um cristal por esta técnica é a aderência entre o mesmo e as paredes do caidinho, construímos também um sistema para tratamento do material de partida que eliminou este fenômeno. Este procedimento permitiu a obtenção de monocristais cilíndricos de NaI(Tl) transparentes, com alta qualidade e com cerca de 60 mm de comprimento e diâmetro de 20 mm. Utilizando a técnica de Czochralski, crescemos também vários monocristais cilíndricos transparentes de NAI(Tl) com comprimentos de 50 mm e diâmetros de 30 mm. Descrevemos também um método para eliminar o excesso de iodo dissolvido na fase líquida durante o processo de crescimento do cristal. Medidas de absorção ótica realizadas com os cristais de NAI(Tl) à temperatura ambiente e do nitrogênio líquido, mostram bandas de absorção características do Tálio próximas de 304, 291, 257, 249 e 236 nm. Através da relação entre a densidade ótica da banda de absorção em 304 nm e a concentração de tálio, estimamos o perfil da concentração deste elemento ao longo de um cristal crescido por Cz e por Bridgman. Através de espectroscopia de fluorescência encontramos, à temperatura ambiente, uma banda de emissão centrada em 425 nm. Protótipos de detetores foram construídos para avaliar as resoluções de energia dos monocristais crescidos pelos métodos de Bridgman e Czochralski. A performance dos detetores também foi analisada / In this work we constructed a new versatile Bridgman crystal growth furnace that can operate to 2.000 &#176;C in high vacuum or in inert atmospheres. Since the most important post-growth problem in this technique is related with the adherence between crystal-crucible, we also developed an initial materials-treatment system that could prevent this phenomenon, by previously saturating with HI gas. These procedures allowed us to get clear and high quality NaI(Tl) cylindric single crystals up to 60mm length and 20mm diameter. Using the Czochralski technique, we also grew several clear NaI(Tl) single crystals with dimensions up to 50 mm length and 30 mm diameter. A method to prevent the excess of Iodine in the liquid phase during the crystal growth process is also described. Optical absorption measurements in NaI(Tl) single crystals, at room and liquid nitrogen temperatures, show characteristic Thallium bands near 304, 291, 257, 249 and 236 nm. Through the relationship between optical density of the absorption band at 304 nm and Thallium concentration, we could estimate the concentration profile through the entire crystal, grown by each method: Bridgman and Czochralski. By fluorescence spectroscopy we also find, at room temperature, one emission band near to 425 nm. Radiation detector prototypes were developed to evaluate the energy resolution of the single crystals grown by Bridgman and Czochralski methods. The performance of the radiation detectors is also analyzed