The Global Positioning System (GPS) has been used for more than a decade for the accurate determination of position on the earth's surface, as well as navigation. The system consists of approximately thirty satellites, managed by the US Department of Defense, orbiting at an altitude of 20 200 kilometres, as well as thousands of stationary ground-based and mobile receivers. It has become apparent from numerous studies that the delay of GPS signals in the atmosphere can also be used to study the amosphere, particularly to determine the precipitable water vapour (PWV) content of the troposphere and the total electron content (TEC) of the ionosphere. This dissertation gives an overview of the mechanisms that contribute to the delay of radio signals between satellites and receivers. The dissertation then focuses on software developed at the Hartebeesthoek Radio Astronomy Observatory's (HartRAO's) Space Geodesy Programme to estimate tropospheric delays (from which PWV is calculated) in near real-time. In addition an application of this technique, namely the improvement of tropospheric delay models used to process satellite laser ranging (SLR) data, is investigated. The dissertation concludes with a discussion of opportunities for future work. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2004.

The Global Positioning System (GPS) has been used for more than a decade for the accurate determination of position on the earth's surface, as well as navigation. The system consists of approximately thirty satellites, managed by the US Department of Defense, orbiting at an altitude of 20 200 kilometres, as well as thousands of stationary ground-based and mobile receivers. It has become apparent from numerous studies that the delay of GPS signals in the atmosphere can also be used to study the amosphere, particularly to determine the precipitable water vapour (PWV) content of the troposphere and the total electron content (TEC) of the ionosphere. This dissertation gives an overview of the mechanisms that contribute to the delay of radio signals between satellites and receivers. The dissertation then focuses on software developed at the Hartebeesthoek Radio Astronomy Observatory's (HartRAO's) Space Geodesy Programme to estimate tropospheric delays (from which PWV is calculated) in near real-time. In addition an application of this technique, namely the improvement of tropospheric delay models used to process satellite laser ranging (SLR) data, is investigated. The dissertation concludes with a discussion of opportunities for future work. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2004.

Radio signals from Global Navigation Satellite Systems (GNSS) satellites suffer delay as they propagate through the atmosphere (neutral and non-neutral) and this delay is partially driven by the water vapour content in the atmosphere. The delay component due to the non-neutral atmosphere (ionosphere) is removed through the use of dual frequency GNSS receivers. The main tropospheric parameter is the zenith tropospheric (or total) delay (ZTD), which is a widely accepted parameter with which to express the total delay in the signal from all satellites due to the neutral atmosphere. The ZTD is a measure of the integrated tropospheric condition over a GNSS receiver station. Accordingly, the integrated water vapour or precipitable water vapour (PWV) can be obtained from a portion of the ZTD, if the atmospheric pressure and temperature at the station are known through a concept often referred to as GNSS meteorology. A number of GNSS receivers have been deployed for mapping and geodetic services in Nigeria under the African reference frame initiative, but unfortunately most of these receivers do not have co-located meteorological sensors for pressure and temperature measurements. The prospect of incorporating GNSS meteorology into weather monitoring and climate analysis in Nigeria was investigated and is reported in this thesis. During the first task of this research, the technical basis for ground-based GNSS meteorology was reviewed and the potentials and challenges of the approach to meteorological activities in Africa (including Nigeria) were identified. Thereafter an in-depth analysis of the spatial and temporal variability of ZTD over Nigeria for the period of 2010-2014 was conducted; results revealed weak spatial dependence among the stations. Tidal oscillations (of the diurnal and semidiurnal components) were observed at the GNSS stations of which the diurnal ZTD cycles exhibited significant seasonal dependence, affirming the prospective relevance of ground-based GNSS data to atmospheric studies. Also in this research, the accuracy and suitability of using reanalysis datasets (ERA-Interim and NCEP/NCAR) and a GPT2 neutral model in retrieving PWV from GNSS observations over Nigeria were investigated; results showed that PWV can be retrieved to within a precision of about 1 mm, provided GNSS-derived ZTD is of high precision. A fundamental issue for GNSS meteorology in the West African region was yet again addressed in this research; this is the development of a weighted tropospheric mean temperature model for use in current and future GNSS meteorology activities in the region. A multitechnique comparison of PWV estimates showed good agreement between GNSS estimates and other techniques (i.e. the atmospheric infrared sounder, and ERAInterim reanalysis). This result is suggestive of the potential of assimilating GNSS atmospheric products into reanalysis and climate models. Diurnal and seasonal variability of GNSS PWV estimates exhibits strong correlation with weather events that influence the region (i.e. solar activity and rainfall events); this further demonstrated the immense contribution of the approach to efficient weather forecasting and climate monitoring for Nigeria. / Thesis (PhD)--University of Pretoria, 2017. / Geography, Geoinformatics and Meteorology / PhD / Unrestricted