Abstract:
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.
