Things are getting set for the launch of India's Radar Imaging Satellite (RISAT-1) from the spaceport at Sriharikota at 5.45 a.m. on Thursday. The four stages of the Polar Satellite Launch Vehicle (PSLV-XL) have been stacked up at the first launch-pad built on the beachside on the Sriharikota island and the satellite is married up with the vehicle. The 71-hour countdown for the lift-off will begin at 6.45 a.m. on Monday. The RISAT-1 is essentially a remote-sensing satellite.

The RISAT-1, which has been ten years in the making, will be the Indian Space Research Organisation's (ISRO) first radar-imaging satellite. An ISRO satellite specialist said: “Radar imaging is a very complex technology. It is technologically challenging to build such a satellite.” RISATs use synthetic aperture radars and the great advantage of RISATs is that they can take pictures of the earth at all times, through rain, sun, clouds, fog and cyclones.

According to ISRO spokesman S. Satish, “This is not only the first radar imaging satellite to be built by India but this is the heaviest remote-sensing satellite built by the ISRO. Besides, it is the heaviest satellite to be put in orbit by the PSLV. RISAT-1 weighs 1858 kg.” A powerful PSLV-XL is being used to put RISAT-1 in polar orbit at an altitude of 480 km. The satellite's propulsion system will then be used to take it to the final orbit at an altitude of 536 km, Mr. Satish said.

The rocket is called PSLV-XL (XL stands for extra-large) because it uses six more powerful strap-on motors than those used in the standard PSLV version. If the normal PSLV version's six strap-on motors each use nine tonnes of solid propellants, each of them in the PSLV-XL use 12 tonnes of propellants. This is the third time the ISRO is using a PSLV-XL version. It was first used in October 2008 to put Chandrayaan-1 in orbit and again in July 2011 to during the GSAT-12 launch.

Mr. Satish said pictures from RISAT-1 would be used to estimate the crop yield — especially to monitor paddy crop, assess its acreage and predict its health during the kharif season, when the sky is covered with clouds. The satellite's images can be used for disaster management during cyclones and floods, to assess how much area has been inundated and so on. RISAT-1's life-span is five years.

M. Annadurai is the Programme Director for RISAT-1. N. Valarmathi is its Project Director.

ISRO scientists said that unlike the normal, optical remote-sensing satellites, the RISATs use a synthetic aperture radar (SAR). This radar emits waves in a special way and collects part of the reflected radiation. From this reflected radiation, images of the earth can be built and these images have excellent clarity. Since it is difficult to carry a radar/antenna with a big aperture on a satellite, an SAR is used because it can synthesise (that is, artificially create) a larger aperture electronically. Hence it is called a synthetic aperture radar.

Although this is the first time the ISRO is launching its own RISAT, it has twice launched Israel's RISATs in orbit using the PSLVs from Sriharikota. The ISRO first put Israel's RISAT, Tecsar, in orbit in January, 2008, and the agency deployed Israel's RISAT-2 in orbit in April 2009. Both Tecsar and RISAT-2 are reconnaissance/surveillance satellites. While Israel uses the images from Tecsar, India uses the images from RISAT-2 for surveillance.

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