GPS tropospheric modelling: new developments and insights
Date
2017
Authors
Masoumi, Salim
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Abstract
GPS is widely used to monitor temporal and spatial variations of
Earth’s crust, oceans
and atmosphere. Of particular interest to this research is the
use of GPS for studying
variations in the Earth’s lower atmosphere. While there have
been significant advances
in the techniques and models used in GPS analyses over the past
two decades,
there is still room for improvement. In particular, observations
at very low elevation
angles still suffer greatly from modelling errors. These
low-elevation observations
provide useful information about the moisture content of the
atmosphere and its
variability around a GPS station, and are thus valuable data for
meteorological studies
if properly modelled.
The main focus of this thesis is on optimization of the
techniques and models
used in GPS analysis for more accurate estimates of the
tropospheric delays. Particular
attention is paid to modelling low-elevation observations and
challenging
weather conditions. Throughout the thesis, we investigate several
different aspects
of modelling techniques and how each of them affect the
tropospheric estimates.
By applying a previously developed empirical model [Moore, 2015],
the site-specific
errors are shown to have large impacts on the tropospheric delay
estimates:
empirical mitigation of site-specific errors leads to improved
repeatabilities of heights
and tropospheric zenith delays for the majority of the stations
in our analysis. The
empirical site-specific model also significantly reduces the
sensitivity of tropospheric
zenith delay estimates to the choice of elevation cut-off.
Another important potential source of error, the GPS estimates of
tropospheric
horizontal gradients are shown to be more accurate than the model
values currently
available. However, the conventional two-axis planar model of
gradients does not
accurately represent the actual gradients of the refractivity
under weather conditions
with asymmetric horizontal changes of refractivity. Such abnormal
conditions
may occur due to topography-driven gravity waves in the
troposphere, and the mismodelled
tropospheric horizontal gradients induce errors in the parameter
estimates,
sometimes leading to skewed position time series and inaccurate
tropospheric zenith
delays. A new parametrization of tropospheric gradients whereby
an arbitrary number
of gradients are estimated as discrete directional wedges is
shown via both simulations
and real case studies to largely improve the accuracy of
recovered tropospheric
zenith delays in asymmetric gradient scenarios. The new
directional model
significantly improves the repeatabilities of the station height
time series in asymmetric
gradient situations while causing slightly degraded
repeatabilities for the stations
in normal symmetric gradient conditions.
The constraints on the temporal variations of the tropospheric
delays are also
investigated. It is shown via simulations and real experiments
that it is generally
preferable to avoid constraints on both tropospheric zenith
delays and horizontal gradients. However, since the conventional
model of horizontal gradients oversimplifies
the horizontal variations of the refractivity in asymmetric
gradient conditions,
it is important to use a more complete model of gradients like
the directional gradient
model introduced in this thesis in conjunction with the relaxed
constraints to
avoid errors caused by the simplifying assumption of symmetric
gradients by the
conventional model.
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Keywords
Global Positioning System, Tropospheric delay, Site-specific errors, Tropospheric horizontal gradients, Directional gradient model, Temporal constraints
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