India is all set to join a select band of countries that have satellites with Ka band transponders with the launch of the GSAT-4, with the launch scheduled for April 15. It will carry both communication and navigation payloads. The high Ka band transponders allows for miniaturisation of the satellite dish, for instance, as they are much more powerful.
GSLV-D3, is scheduled to launch the 2,220 kg GSAT-4, an experimental advanced technology communication satellite that carries communication and navigation payloads into a geosynchronous transfer orbit.
Ka band's applications will include wideband multimedia services, mobile information system, SPACE LAN, e-Commerce and high bandwidth Internet. Interconnectivity too will be possible with wide band channels between any of the beams or all the beams can be used together in broadcast mode. Another objective of the payload is to develop advanced digital signal processor-based subsystems, implement various interface protocols and verify interconnectivity of terminals between multiple beams.
Operating in the frequencies of 30 giga hertz (GHz) uplink and 20 GHz downlink, the K band of the microwave band of the electromagnetic spectrum. This allows for high resolution and close-range targeting radars. The payload on Gsat-4 will provide for eight beams covering the country. The payload will also have beacon transmitters in the 30 GHz to 20 GHz that facillitate propagation studies. Ka band payload also has the facility of RF tracking and antenna pointing.
According to ISRO Chairman K Radhakrishnan, “The advantages of using a regenerative transponder are many. It allows for the use of smaller ground terminals at the user end incorporating efficient processing on-board. The regenerative transponder on the satellite also increases flexibility by facilitating networik interconnection on-board the satellite without the use of a hub, which results in increased capacity, reduced errors and greater throughput.” A regenerative transponder is one where there is a provision for detection and demodulation process.
The second payload to be carried by GSAT-4 is GAGAN or the GPS Aided Geo Augmented Navigation, a navigational payload operating in the C, L1 and L5 bands. GAGAN forms the space segment of GAGAN Satellite Based Augmentation Systems (SBAS) developed by India. The technology demonstration phase of the GAGAN was completed in August, 2007. As part of the TDS, eight Indian Reference Stations (INRES) were installed at eight airports. They are linked to the Indian Master control Centre (INMCC) at Kundanhalli near Bangalore.
The cost of the launch of the Gsat-4 is set to cost around Rs 225 crore. The launch vehicle will cost around Rs 175 crore and the Gsat-4 Rs 150 crore.