Examensarbeten för masterexamen // Master Theses
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- PostModelling and Simulation of Tropospheric Water Vapour With Gaussian Random Fields-Time dependence beyond the frozen flow hypothesis(2018) Gingsjö, Henrik; Chalmers tekniska högskola / Institutionen för rymd-, geo- och miljövetenskap; Chalmers University of Technology / Department of Space, Earth and EnvironmentOne of the major sources of error in Very Long Baseline Interferometry (VLBI) is signal delay due to tropospheric water vapour. Turbulent convection makes it inherently unpredictable and it must therefore be measured directly or modelled stochastically. In particular, realizations of delay signals are necessary to simulate the performance of existing and future VLBI networks which, in turn, is needed to optimize them and reduce errors. In previous work, modelling of tropospheric delay has been performed only on the spatial structure of refractivity through phenomenological second order statistics derived from Kolmogorov theory. Time dependence has been introduced exclusively through the frozen-flow hyporthesis. In this thesis, refractivity fields are modelled as Gaussian random fields. Efficient software is implemented to generate realizations of such fields sampled on a 3D grid. To achieve realistic time evolution of such gridded fields, it turns out to be both necessary and natural to introduce intrinsic time dependence beyond the frozen-flow hypothesis. Such time dependence can easily be made compatible with the temporal structure of Kolmogorov turbulence. The novel contributions of this thesis are methods of obtaining two kinds of time dependence for refractivity fields beyond the frozen-flow hypothesis. Firstly: Intrinsic time dependence compatible with Kolmogorov theory. Secondly: Translation by horizontal wind with arbitrary height and time dependence. The latter may provide a more realistic description of the planetary boundary layer which has strong wind shear and contains about 15% of the total water vapour; corresponding to delays of several centimetres.
- PostMonitoring GNSS transmit power with small radio telescopes(2018) Malmquist, Marcus; Chalmers tekniska högskola / Institutionen för rymd-, geo- och miljövetenskap; Chalmers University of Technology / Department of Space, Earth and EnvironmentThe small radio telescopes SALSA (“Such A Lovely Small Antenna”) refers to the system consisting of two (2.3m) radio telescopes located at the Onsala Space Observatory and built with the aim of introducing students to the world of radio astronomy. In this project, SALSA has been extended with the possibility of tracking satellites that are part of the Global Navigation Satellite Systems (GNSS). This new feature of SALSA allows to carry out observations in an automatic fashion and store all the necessary information for subsequent data analysis. The latter stage aims at deriving important satellite-based parameters, namely the transmit power of GNSS satellites as well as their antenna gain patterns. Good knowledge of transmit power of GNSS satellites is required for an enhanced GNSS orbit modelling and GNSS orbit determination. In addtion, satellite antenna gain patterns are vital for the callibration of data from GNSS reflectometry (GNSS-R). The following report describes the technical specifications, system calibration procedure and measurement performance of SALSA with emphasis on the hardware modifications and software tools required to upgrade an uncalibrated system designed for astronomical observations at 1.4 GHz to a system that can be utilized for satellite observations at frequencies between 1 and 2 GHz. This project can form a foundation of a new routine service, regularly monitoring GNSS signals and providing users with the aforementioned satellite-based products.