The satellite based navigation system is one of the main components of the satellites based CNS/ ATM system adopted by ICAO for worldwide implementations. Satellite navigation uses two-core constellation of medium orbiting satellites i.e. GPS and GLONASS. The positioning services offered by these two constellations for civilian use including civil aviation falls short of accuracy, integrity availability and continuity requirements of air navigation services. Indian air space in between Europe on the West and Japan on the East occupies a very critical position and, hence there is a need to have a system to bridge the gap between the coverage of EGNOS and MSAS and to facilitate seamless navigation of the aircraft from East to West and vice versa. The expertise available with Indian Space Research organization (ISRO) will be gainfully utilized in implementation of GPS / GLONASS Aided Geo- Stationary Satellite Augmented navigation (GAGAN) to provide the coverage over Indian airspace to the users. Indian augmentation with Indian payloads on GSAT-4 satellites, which are controlled by India, will offer some amount of control and flexibility on the position accuracies available to strategic users.

At present there is no SBAS service provider over Indian airspace. Even if in the future MSAS or EGNOS is in a position to provide SBAS services at a charge over India airspace, tt will be far cheaper and strategically important to have indigenous SBAS in view of the indigenous expertise in satellites technology available with ISRO. India can then become a SBAS service provider in neighboring countries in Asia-Pacific. Since GAGAN is a technology driven project, in which the technology is still evolving, it has been envisaged that DOS / ISRO will be in the right position to drive the project with AAI providing the requisite financial, manpower and the rest of supplementary support for the project. The national plan envisages implementation of a full operational capability SBAS in three phases.

This phase requires implementation of a minimum configuration system
which would demonstrates the capability of the system to support up to Precision Approach Cat-I over a limited region of the Indian airspace and will serve as proof of concept. The performance objective of this system is to meet the ICAO SARPs requirements. The TDS will broadly consist of adequate number of RIMS, an Indian MCC, NLES, space segment, IONO-TROPO modeling, required communication links necessary software for navigation and communication.

In this phase, TDS will be expanded to cover the entire Indian airspace
and requisite redundancies will be added to the system. SBAS elements
not covered in the TDS would be incorporated in the system and their redundancies built up. The system will be available for trial operation and the data collected by AAI during such trail operation would be analyzed by ISRO to further improve the system wherever considered necessary to achieve compliance of ICAO SARPs. The IEP will be completed in a period of one year after TDS.

During this phase, GAGAN program would have matured. Extensive tests would be carried out to establish the system stability of various element of the system as a whole. The responsibility and roles of ISRO and AAI would continue to be as the case of initial experimental phase. The system will be extensively used for its evaluation with respect to ICAO SARPs before declaring system operational. The FOP will be completed in a period of one year after IEP.

Finally we wish to add that an SBAS provider only guarantees adequate service in his service volume. In addition to this SBAS broadcast signals will be available anywhere in their Geo-stationary Earth Orbiting Satellites (GEOS) foot print coverage areas. In the case of GAGAN, its footprints overlap those of European Geo-Stationary Navigation Overlay System (EGNOS) and Multi Functional Transport Satellites (MTSAT) Satellite - Based Augmentation System (MSAS). Indian airspace falls on the edge of the footprint of EGNOS & MSAS, GEOS. It means that the level of service over Indian airspace is not defined by EGNOS & MSAS. Consequently degraded signal will be available. GAGAN has been designed taking this into account. The service area of worldwide SBASs is presented in Fig. 1. It is observed from the figure that the EGNOS (A) and WAAS (E) coverage areas are large due to two or more number of GEOS at different locations whereas GAGAN (B) and MSAS (D) have less as they use only one GEOS.

This part of the project, is a challenging task for Indian engineers and scientist to develop the model suitable for GAGAN service area. From the study point of view, the world can be divided into three regions i.e. mid latitudes, the auroral and polar caps, and the equatorial and equatorial anomaly region. The equatorial region extends upto ± 30 deg. Geo-magnetic latitude which is 50% of the earth’s surface. Entire Indian sub continent and many other countries fall in this region.

In the mid latitude regions, SBAS is expected to provide a PA service upto Approach Vertical Guidance (APV) -I & II using a standardized grid i.e. (5°x5°) to communicate ionospheric delay information to single frequency users. This strategy works well in mid latitude regions where the spatial and temporal changes in the structure of the ionosphere are fairly well documented. Unfortunately, in the equatorial ionosphere the spatial and temporal variability is much greater even during quite magnetic conditions. Therefore, the current SBAS Ionospheric Grid Concept may not be applicable in the equatorial region as such. There is a need to collect a new set of data continuously over the equatorial region for year together and develop the necessary Ionospheric and Tropospheric models for GAGAN.

Ionospheric & Tropospheric modeling and scintillation studies in the L-band will be carried out over the entire Indian airspace as an integral part in the TDS Phase. Twenty total electron content (TEC) GPS receivers have been installed at the Center of the 5 deg x 5 deg. ionospheric grid points (IGP) grids over the Indian region. The data from these receivers is being logged into a personal computer and the logged data is being stored in ISRO space application center, Ahmedabad for last one year. This data is disseminated to the universities and Research & Development laboratory for development of the appropriate Ionospheric, Tropospheric and Scintillation model.

This segment of the project consists of the following elements:
(i) Indian Land Uplink Station (INLUS) located at Banglore
(ii) Indian Reference Stations
(INRESs), located at Jammu, Delhi, Tiruvananthapuram, Bangalore, Kolkatta, Guwahati, Port Blair and Ahmedabad
(iii)Master Control Centre (MCC) located at Bangalore
(iv)128 Kbps Fiber or VSAT Link between INRES and MCC The installation and testing of this segment will start in the end of the year 2005

India is playing major role to establish the SBAS service over Indian airspace to provide seamless and universalisation services for user as well as countries adjacent to the Indian boundary. This is being executed by AAI and ISRO jointly. It will also provide the service to any number and type of users with integrity signal over Indian air space. In addition the availability of technical, operational expertise in this area will benefit India. It will provide India the leading role in the Asia Pacific region as SBAS service provider.