Estimation of tropospheric time delay for Indian LAAS

In LAAS with airport pseudolites (APLs), error introduced in the signals due to tropospheric time delay proves to be the largest in pseudolite error budget. In this paper, tropospheric delay (TD) is estimated using Hopfield model and RTCA model for a typical LAAS scenario. Hopfield model is used to estimate delay in GPS signal and RTCA model is used to estimate delay in pseudolite signal. As the tropospheric delay is a function of meteorological parameters, these parameter values are collected from India Meteorological Department (IMD) for Hyderabad city, India, for a typical day. These data values are interpolated with respect to altitude for the estimation of TD at various heights. For an altitude of 1.94 m TD in APL signal is estimated as 7.2 m (approx.) and TD in GPS signal is estimated as 2.75 m. When the aircraft just touches the runway touchdown point, TD in APL signal reduces to insignificant value and TD in GPS signal increases to 3.25 m.