GAGAN is an Indian Space Based Augmentation System (SBAS). Airports Authority of India (AAI) and Indian Space Research Organization (ISRO) to provide the seamless navigation service for all the phases of fl ight over Indian airspace jointly undertake this project. The AAI’s efforts towards implementation of operational SBAS can be viewed as the first step towards introduction of modern CNS/ATM system over Indian airspace. This project involves establishing groundbased elements inclusive of eight Indian Reference Station (INRES), one Indian Master control center (INMCC), one Indian Navigation Land Uplink Station (INLUS).

To begin implementing an SBAS over the Indian airspace, Wide Area Augmentation System (WAAS) codes for L1 frequency and L5 frequency were obtained from US Air force (Department of Defense) on November 2001 and March 2005. The assignment of L1 and L5 codes is an indicator of the recognition of the need and seriousness of the Indian implementation of GAGAN as GPS augmentation system.

The implementation of the operational GAGAN will take into account the aspects of interoperability with other Satellite Based Augmentation System (SBAS) like WAAS, EGNOS, and MSAS etc. GAGAN planned to be implemented in three phases.
1. Technology Demonstration System (TDS)
2. Initial Experimental Phase (IEP)
3. Final Operation Phase (FOP)

The TDS is broadly defi ned to consist of eight numbers of INRES, one INMCC, one INLUS and a space segment. The ionospheric modeling, required communication links and the necessary software for navigation and communication are being developed as additional parallel effort. Eighteen GPS-TEC receivers have been installed at 5x5 degree grid and are downloading the GPS data for the last 18 months. To study the ionospheric behavior more effectively over entire Indian Airspace, Indian universities and R&D labs, which are involved in the development of regional based IONO-TROP model for GAGAN, have suggested nine more stations.

The scope of the TDS is planned to include the addition of a second frequency L5 both in the space and ground segments and procurement of State-of-the-art ground element. The IEP is being implemented concurrently with TDS Phase.

INRES has minimum two identical GPS receivers/ antenna subsystems to receive GPS signal (L1, L2 & L5) and GEO signals (L1&L5) form all the satellite in view. INRES is configured as multiple receiver chains for obtaining the measurement from the GPS and GEO satellites. During TDS phase eight INRES are planned and all have been installed at their respective stations. INRES is located at Delhi, Banglore, Ahmedabad, Kolkata, Jammu, Portblair, Guwahati and Trivendrum. Five stations are within airport operational area and three are outside the airport. These stations are so chosen to provide service coverage over Indian airspace.

INMCC is major subsystem of the GAGAN project and the function of the INMCC is correction and safety subsystem, operation of maintenance subsystem, service monitoring subsystem and data communication subsystems. Using suitable navigation software INMCC process the data received through communication link from the all the INRES. It also estimates the integrity and availability of the GPS satellites and uplinks these parameters via INLUS for broadcast to the user receiver through GEO. The INMCC for GAGAN TDS is located at Kundanhalli Banglore. The installation and integration of this major subsystem has been completed. Testing is in progress.

INLUS ground station will receive messages from INMCC and will transmit the correction to GSAT-4 for broadcast to the user platform. The INLUS also provides GEO satellites ranging information and corrections to GEO satellites clocks. The ground station has been establishes in June 2004 in Kundanhalli and it is collocated with INMCC at Banglore.

The up-linking station with the 11 meters antenna has been developed indigenously and has been installed. The RF equipment required at the up-linking station has been procured and installed indigenously and also
design is flexible enough to cater for subsequent phases of the program.

A geo-stationary navigation payload in C band and L1 and L5 frequencies (L band) will be carried on an Indian Geo-stationary satellite i.e. GSAT-4 placed at 82 Degrees East. The Indian payload will fly on GSAT-4 scheduled for launch by December 2006.

The locations were finalized after an elaborate study on multi-path, noise survey and obstruction clearance. A document was prepared for all the INRES and INMCC to check the compliance of the requirements.

At present, GAGAN ground segment is under integration test and data collection at INMCC from the INRES.