More than two decades after India's first operational optical remote sensing satellite went into orbit, the country's space capabilities are poised for a big technological leap with the launch of an entirely indigenous radar imaging satellite, RISAT-1. The Indian Space Research Organisation (ISRO) is preparing to send the satellite into space aboard the Polar Satellite Launch Vehicle that is scheduled to lift-off from Sriharikota early on Thursday.
The RISAT-1's radar will be able to see through clouds and work in darkness, conditions that hamper optical satellites. Its images will be useful for a variety of applications, from crop forecasting and disaster management to addressing the country's strategic needs.
The RISAT-1 will, however, be the country's second radar imaging satellite. India already operates the Israeli-built RISAT-2, which was launched in April 2009 and appears to have been quickly procured to meet security requirements.
After the ISRO launched IRS-1A in 1988, it sent up well over a dozen earth-viewing satellites bearing a variety of optical imaging cameras. These satellites have created a large user community within the country. Their data is also being received and utilised in several countries.
An important reason for the ISRO's initial emphasis on optical imaging was the far greater complexity of a radar satellite, according to Pramod Kale, who was once director of the ISRO's Space Applications Centre at Ahmedabad that builds payloads carried on Indian satellites.
With RISAT-1, ISRO scientists and engineers demonstrate their mastery of that difficult and closely guarded technology. If the satellite works as its creators hope, it will match and perhaps in some respects even surpass Canada's second-generation RADARSAT-2 that is now operational.
The RISAT-1 uses the ‘synthetic aperture radar' technique. It carries out complex processing of the radar echoes received from the same place on the ground so as to simulate a much bigger antenna than it actually carries. Doing so greatly increases the image resolution that is possible.
Radar images from the satellite will have a resolution that can be varied from 50 metres down to 3 metres. However, as resolution increases, less of the ground can be imaged as the satellite passes overhead.
In a special ‘spotlight mode,' where the satellite will keep looking at a small region on the ground, it will be capable of providing one-metre resolution images. (The best resolution now possible on the ISRO's optical remote sensing satellites is believed to be about 0.8 metres.)
The satellite is equipped with an advanced ‘active phased array' antenna. Instead of a single device generating the microwave signals, the antenna has a large number of modules that collectively produce the radar beam. By suitably adjusting the signals generated by various modules, the beam can be electronically moved around. Even if a few modules fail, the satellite can continue to function albeit perhaps with some degradation in performance.
