This book deals with the refraction of GPS signal in the tropical atmosphere conditions prevailing over the Indian subcontinent, with the aim of quantifying the positioning error in GPS based navigation. In this process the tropospheric zenith delay is modeled in terms of atmosphere parameters based on geographical location and day-of-year. This work is highly suitable for the India?s GPS based aircraft navigation (GAGAN) and Indian Regional Navigation Satellite System (IRNSS). The GPS data is also used for estimating Precipitable Water (PW) over different locations in India to study its spatial and temporal features. The high resolution time series of zenith delay derived from GPS data is also used to probe the potential of GPS measurements for atmosphere dynamics studies.