JOIN USWith its landmark achievements, ISRO is making rapid strides towards becoming a world leader in space technology. Sudhindra Haldodderi writes about one of its latest creations that promises to revolutionise mobile television.
Launching a satellite and positioning it in the right orbit is no big deal for ISRO. Space scientists of the country have mastered this art with a textbook perfection. The first launch successes in Chandrayaan-1 and Mars Orbiter Mission created world records and catapulted ISRO into the elite five-nation space league. Today, ISRO operates the largest constellation of communication satellites in the Asia-Pacific region, while its remote sensing satellites are largest in number in the world.
Ever-growing demand of communication needs of the country is largely met by the indigenous INSATs/GSATs, but still there is enough space for more number of satellites. About 10 years ago, ISRO planned to build an exclusive satellite for mobile television services with multimedia applications. Beaming digitally coded signals to moving automobiles and mobile phones was considered as a revolutionary idea.
Consequently, the satellite GSAT-6 was conceptualised with new technologies for data compression, on-board antenna (5-6 m diameter) and high power S-band communication system. The system was expected to broadcast more than 10 audio and video channels simultaneously. GSAT-6 was also designed to carry multiple transponders operating on the same frequency band to enhance the coverage.
Not an easy journeyThe greatest challenge envisaged by ISRO was the deployment of 6.5 metre antenna in a rocket envelope, which cannot accommodate anything greater than 2.5 metre in girth. It was then that the concept of folding the antenna like an umbrella was designed. The folded umbrella would be placed in the rocket and unfurled in the orbit after the satellite was in position. What more, building the antenna system with perimeter truss consisting of composite tubes to support the front and rear webs with fibre reinforcement was attempted for the first time.
Graphite or epoxy tubes connected by hinges that allow the furled antenna were to be stowed for launch to fit within the rocket fairing. Pyro-initiated release mechanisms were required to open launch restraints before the antenna gets deployed. This also involved development of highly accurate, synchronous and agile actuation system.
Another technological challenge for ISRO scientists was to raise the power onboard the satellite, four to five times the normal limits, to enable the omni-directional high-powered spot beam transmission and there was also the challenge of shrinking the bandwidth to a great extent.
When a satellite system is required to communicate with a very small earth receiver such as handheld mobile phone or a moving automobile-based unit, the flux density of the signal received should be of a high order. This provides a high quality Satellite Digital Multimedia Broadcasting (S-DMB) link. To achieve this, satellites usually operate on S-Band and provision is made for a combination of high power amplifiers and a large aperture reflector antenna on the satellite. A 6.5 metre projected aperture paraboloid reflector has been used on the GSAT-6 satellite to achieve high beam directivity over the desired coverage regions.
While GSAT-6 is designed to provide digital CD-quality audio, TV-quality video and data broadcasting services to the country, the radiation pattern was designed to be carefully controlled to avoid interferences with other Asiatic regions.
While conceiving this ingenious satellite, the scientists came across two major design challenges: one, the packaging of the stowed structure and two, controlling the reaction caused by the unfurled structure when operating in the orbit.
Extensive ground tests were carried out on the satellite system to ensure proper functioning. An acceptable process coupled with test results and analysis needs was developed to predict antenna performance, since direct measurement of the antenna radiation pattern of large aperture is impractical.
The Government of India approved the project of design, development and launch of GSAT-6/INSAT-4E to provide Satellite Digital Multimedia Broadcasting (S-DMB) at a total estimated cost of Rs. 269 crores with a foreign exchange component of Rs. 102 crores. The project provided a platform for technology demonstration of large unfurlable antenna in spacecraft, handheld ground terminals and large network management systems.
To encourage private participation, outside investment and to borrow the state-of-the-art technologies, Antrix, the commercial arm of ISRO signed a strategic agreement with DEVAS Multimedia Private Limited. The $300 million deal provided leasing of 90 per cent of the space signal capacity of two satellites for 12 years to DEVAS. The Cabinet also approved building of GSAT-6A satellite of similar capacity as GSAT-6 by ISRO.
Projects underwayISRO envisaged development of technology and business of S-DMB services including interconnectivity of 2.5G/3G network. This would have enabled ISRO to become a leader in the growing world market of S-DMB services for mobile vehicles and cellular phones. DEVAS was expected to build the network of ground infrastructure, content providers and distribution channels.
But sadly, the 2G-spectrum scan of 2010 cast a dark shadow on Antrix-DEVAS deal with controversies in the procedural lapses prior to signing the contract.
Subsequently, the Indian government cancelled the contract with DEVAS in February 2011, reserved satellite under development GSAT-6 for military use, thereby inviting a hefty arbitration case filed by DEVAS in the International Court at Hague. GSAT-6 project was shelved and GSAT-7 (Rukmini) was successfully launched as the first Indian military satellite in 2013.
But fortunately, with the clouds of controversy finally clearing up, GSAT-6 with five spatial transponders for multimedia services has completed all the ground tests including integration and thermovacuum tests. This 2100 kg satellite is expected to be launched on a GSLV rocket from Sriharikota in a month or two.
In addition to GSAT-6, ISRO shall be launching three more satellites in the Indian Regional Navigation Satellite System series and two satellites in the GSAT series this year. CARTOSAT-2C is also expected to be launched at the end of this year which possesses 62 cm spatial resolution camera on board. The astronomy satellite ‘Astrosat’ is also slated for its launch in 2015. But the ultimate feather in ISRO’s cap shall be the launching of five foreign satellites – three from Britain and two from Indonesia.
The dream launch vehicle of ISRO, GSLV-Mark III, which has an indigenous cryogenic engine and is capable of lifting 4000 kg , is still under readiness review. Hopefully, by end of 2016, this launcher can position ISRO into the heavy-lifter league.
(The author is a former scientist of DRDO.)