CONTEXT :

• RISAT-2B, the satellite due to be launched before dawn on May 22 from Sriharikota, will mark the resumption of a vital ring of Indian all-seeing radar imaging satellites after seven years.
• At least a half-dozen could be foreseen in the near future, mainly to add to the reconnaissance capability from about 500 km in space. A constellation of such space-based radars means a comprehensive vigil over the country.

BACKGROUND :

• Many RISATs were planned. RISAT-2B is to be followed by RISAT-2BR1, 2BR2, RISAT-1A, 1B, 2A and so on.
• If ISRO orbited its first two radar satellites in 2009 and 2012, it plans to deploy four or five of them in 2019 alone.
• When it is cloudy or dark, ‘regular’ remote-sensing or optical imaging satellites — which work like a light-dependent camera — cannot perceive hidden or surreptitious objects on the ground.
• Satellites that are equipped with an active sensor, the synthetic aperture radar (SAR), can sense or ‘observe’ Earth in a special way from space day and night, rain or cloud. This all-weather seeing feature is what makes them special for security forces and disaster relief agencies.
• “Radar imaging satellites are at a premium in the Earth observation scene.
• “Worldwide, quite a few of them are planned to be launched.
• There is also a large availability of radar images data in the market.”