Abstract:
The Global Positioning System is the most accurate positioning and
navigation system in use today. It uses the time of arrival of radio signals
transmitted from satellites placed in high altitude orbits around the globe.
The ideal GPS theory assumes free space radio propagation whereas in
reality, the signals have to propagate through the atmosphere. When
propagating through the atmosphere the finite refractive index of the various
layers of the atmosphere causes the electromagnetic waves to travel
distances that are longer than the corresponding free space distances. This
causes an error in the observed time of arrival which is carried on to the
positional computation.
The error due to refraction in the troposphere is of particular interest. Unlike
the ionosphere, the troposphere is non-dispersive. Its refractive properties
depend more on physical parameters such as pressure and temperature. Due
to this reason, the refractivity tends to depend on the location as well. It has
been shown that a good correlation exists between the refractivity at the
surface of the earth and the range error. Hence this error may be determined
using the refractivity at the surface of the earth.
In this dissertation, the effect of the troposphere on GPS observations made
within the geographical extent of Sri Lanka is studied. The range errors at
reference locations within Sri Lanka are determined for different parts of the
year. The results are compared with established results for the region and
reasons for discrepancies are briefly discussed.
The design of a conceptual GPS receiver processor that can self-correct
tropospheric range errors at the surface of the earth by sensing the pressure
and the temperature is proposed. A computer program is used to simulate the
operation of this receiver.
