In its forthcoming mission, scheduled for the second week of July, the Polar Satellite Launch Vehicle (PSLV) will, for the first time, be carrying a communications satellite, the 1,425-kg GSAT-12. The PSLV was originally developed to carry India's remote sensing satellites, which are typically placed in a polar orbit. In 2002, on its seventh mission, the rocket was used to launch the country's first dedicated meteorological satellite, Metsat, later renamed Kalpana-1 in memory of Kalpana Chawla, the Indian-born U.S. astronaut who died in the Space Shuttle Columbia disaster in 2003. Like communication satellites, this meteorological satellite was put into geostationary orbit. In this orbit, some 36,000 km above the equator, the spacecraft matches the earth's rotation and therefore appears stationary from the ground. From its vantage point in space over India, Kalpana-1 keeps constant watch over evolving weather systems. For communications satellites like the GSAT-12, it is good perch to relay telephone conversations, data, and televisions broadcasts.

Satellites headed for geostationary orbit are put by rockets that launch them into an elliptical temporary orbit. From there, rocket engines on the spacecraft are fired periodically to manoeuvre them into the final position over the equator. In 2008, the most powerful version of the PSLV, the PSLV-XL, was used to place the lunar probe Chandrayaan-1, weighing 1,380 kg into an elliptical orbit 255 km at its closest to earth and nearly 23,000 km at its farthest; the spacecraft's own engines then took it, in stages, to orbit around the Moon. Next month, another PSLV-XL will leave the GSAT-12 in an elliptical orbit similar to that of the Chandrayaan-1. Both equatorial and polar launches from Sriharikota are challenging because of the need to drop the rocket's spent stages with considerable precision in international waters. In the case of an equatorial launch of the sort that will be done with the GSAT-12, the PSLV's six strap-on motors as well its first and second stages, when their propellant is exhausted, will have to be safely discarded before the rocket crosses the Malay peninsula; the third stage is dropped into the Pacific Ocean, while the fourth stage goes on to take the spacecraft into orbit. For polar launches, the rocket first flies south-east and then turns southwards in mid-flight to avoid dropping spent stages near Sri Lanka. Without this complicated dog-leg manoeuvre, its payload capability for polar launches would go up by about 50 per cent. These are hurdles the PSLV takes in its stride to fulfil a wide range of missions with sturdy reliability, a tribute indeed to all those who designed and now operate this remarkable launch vehicle.


PSLV-C18 puts four satellites in orbitOctober 12, 2011

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