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Abstract:
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Global Positioning System (GPS), with its high integrity, continuous availability and
reliability, revolutionized the navigation system based on radio ranging. With four or more GPS satellites in view, a GPS receiver can find its location anywhere over the globe with
accuracy of few meters. High accuracy - within
centimeters, or even millimeters is achievable by correcting the GPS signal with external
augmentation system. The use of satellite for critical application like navigation has become
a reality through the development of these augmentation systems (like W AAS, SDCM, and
EGNOS, etc.) with a primary objective of providing essential integrity information needed
for navigation service in their respective regions. Apart from these, many countries have
initiated developing space-based regional augmentation systems like GAGAN and IRNSS of
India, MSAS and QZSS of Japan, COMPASS of China, etc. In future, these regional
systems will operate simultaneously and emerge as a Global Navigation Satellite System or
GNSS to support a broad range of activities in the global navigation sector.Among different types of error sources in the GPS precise positioning, the
propagation delay due to the atmospheric refraction is a limiting factor on the achievable
accuracy using this system. The WADGPS, aimed for accurate positioning over a large area
though broadcasts different errors involved in GPS ranging including ionosphere and
troposphere errors, due to the large temporal and spatial variations in different atmospheric
parameters especially in lower atmosphere (troposphere), the use of these broadcasted
tropospheric corrections are not sufficiently accurate. This necessitated the estimation of
tropospheric error based on realistic values of tropospheric refractivity. Presently available
methodologies for the estimation of tropospheric delay are mostly based on the atmospheric
data and GPS measurements from the mid-latitude regions, where the atmospheric
conditions are significantly different from that over the tropics. No such attempts were made
over the tropics. In a practical approach when the measured atmospheric parameters are not
available analytical models evolved using data from mid-latitudes for this purpose alone can
be used. The major drawback of these existing models is that it neglects the seasonal
variation of the atmospheric parameters at stations near the equator. At tropics the model
underestimates the delay in quite a few occasions. In this context, the present study is afirst
and major step towards the development of models for tropospheric delay over the Indian
region which is a prime requisite for future space based navigation program (GAGAN and
IRNSS). Apart from the models based on the measured surface parameters, a region specific
model which does not require any measured atmospheric parameter as input, but depends on latitude and day of the year was developed for the tropical region with emphasis on Indian
sector.Large variability of atmospheric water vapor content in short spatial and/or temporal
scales makes its measurement rather involved and expensive. A local network of GPS
receivers is an effective tool for water vapor remote sensing over the land. This recently
developed technique proves to be an effective tool for measuring PW. The potential of using
GPS to estimate water vapor in the atmosphere at all-weather condition and with high
temporal resolution is attempted. This will be useful for retrieving columnar water vapor
from ground based GPS data. A good network of GPS could be a major source of water
vapor information for Numerical Weather Prediction models and could act as surrogate to
the data gap in microwave remote sensing for water vapor over land. |