GAGAN (GPS Aided Geo Augmented Navigation)
Modeling of tropospheric delay correction for GPS navigation
The Global Positioning System (GPS) finds important application in satellite-based navigation. This application requires a priori modeling of tropospheric refraction of GPS signal, which is in the microwave domain. The tropospheric delay for microwaves, estimated by ray tracing through the Earth’s atmosphere, is modeled in terms of measurable surface meteorological parameters such as atmospheric pressure, temperature, water vapour partial pressure as well as columnar water vapour for different locations over the Indian subcontinent using the upper air data. Different forms of the empirical relation were examined to develop these models. Site-specific models are first generated for different locations of selected climetological conditions over the Indian subcontinent. Taking all these models into account a unified model has been developed, which is applicable for the entire Indian region. Though the unified model is slightly inferior to site-specific models, the deviations are within the allowable limits for this specific application. The merit of unified model, however, lies in the fact that a single model can be used for any location over the subcontinent. The model predictions are compared with the tropospheric delay derived from ground-based GPS measurements to establish the model accuracy.
In a remarkable development, using the measurements of total electron content (TEC) of the ionosphere at 18 different stations across the length and breadth of India (covering the crest and trough of the Equatorial Ionization Anomaly) under GAGAN (GPS Aided Geo Augmented Navigation), a model has been developed which would successfully predict the TEC between 8-30deg. latitude and 60-100deg. longitude zone. In this model, observations at 77deg. longitude zone are considered as reference and the solar zenith/neutral wind control are applied to estimate changes in TEC at different longitude sectors from that at 77deg. longitude emphasizing a first principle based the Parametric Ionospheric Model (PIM). The model has now been modified to include the variabilities in Kp and F10.7 cm flux to represent the impact of geomagnetic storms at low latitude. A case study for the severe geomagnetic storm of August 24, 2005 (Dst -158 nT; Kp ~9) revealed that ISRO-TEC model successfully predicted temporal variations in TEC even at longitudes far away from the reference longitude zone at 77deg. The model is now being used to generate super-truth data for GAGAN certification process. A detailed evaluation of the performance of the model with that developed by Raytheon has shown that SPL developed TEC model to be superior. A comparison of ISRO-TEC model generated TEC with the observed measurements and IRI-2007 Storm model for the storm event of August 24, 2005 at two mid-latitude Indian stations viz., Guwahati (26.2deg. N, 91.7deg. E) and Kolkata (22.6deg. N, 88.4deg. E ) is shown in the figure. Needless to say, the ISRO-TEC model represents the real Indian ionosphere better.