RISAT-1: ISRO's Moment of Glory

India, within a week of successful testing of a long range missile, the Agni-V launched a Radar Imaging Satellite primarily meant for civilian use such as agriculture, forestry and to facilitate disaster management. Indian space program has certainly come of age.

India's Polar Satellite Launch Vehicle (PSLV) proved its might for the 20th time in a row on 26^th April 2012, with PSLV – C19 injecting India's first indigenous microwave all-weather Radar Imaging Satellite RISAT into orbit nearly 18 minutes after a perfect lift off at 5.47 am from Satish Dhawan Space Centre at Sriharikota. At 1858 kg, RISAT I is the heaviest payload carried by PSLV so far. Soon after the satellite separated from the rocket, the solar panel was deployed and the 6m synthetic radar antenna panel weighing 230kg opened. It will start sending images from 1^st May 2012. The images will facilitate agriculture and disaster management. 

RISAT-1 would be parked at its final orbit of 536 km altitude, has a mission life of five years and would make 14 orbits per day. 

India had in April 2009 launched a Radar Imaging Satellite (RISAT-2) with all weather capability but it was bought from Israel for $110 million largely for surveillance purposes. The launch of RISAT-1 came several years after that of RISAT-2, which carried an Israeli-built x-band radar. The RISAT-2 mission was prioritized over RISAT-1 following the attacks in Mumbai in November 2008, resulting in RISAT-1 being delayed by several years.

What is a Radar Satellite?

Optical remote sensing satellites rely on sunlight to illuminate the ground below, working much like an ordinary camera. They do not work in darkness and their view can be obstructed by clouds. Radar satellites send out radio signals and record the echoes that return. They can image the ground even at night and are unaffected by cloud cover.

 

The Indian Space Research Organisation (ISRO) has long been aware of the advantages of deploying radar in space, remarked Y.S. Rajan, who was its Scientific Secretary for many years from the late 1970s and participated in the decision-making process that shaped the remote sensing programme. 

The engineering challenges of putting a radar on a satellite are “enormous” and considerably greater than for building optical imaging satellites, he told The Hindu. The processing of radar data and interpretation of images are also vastly more complicated. In addition, there was pressure from the launch vehicle team to hold down the weight of satellites, a factor that again worked in favour of optical satellites. 

So while ISRO opted to go the optical route for India's early remote sensing satellites, it was also very clear that the technological capability to build and use space-based radars needed to be developed, he said. Led by O.P.N. Calla, a group at ISRO's Space Applications Centre at Ahmedabad built a “Side-Looking Airborne Radar” that was installed on a Dakota aircraft in 1980. It subsequently built a more sophisticated “Airborne Synthetic Aperture Radar.” The National Remote Sensing Centre at Hyderabad operates two aircraft that can carry such radars. 

Apart from learning to build the hardware, the space agency sought to develop the necessary expertise in using radar imageries for various applications. It did so by taking data from foreign radar satellites, starting with Europe's ERS-1 that was launched in 1991. 

Military use

 

Radar satellites, popularly, have a swashbuckling image that is often associated with covertly watching over other countries and tracking their military hardware. These satellites can certainly serve that sort of function. But such spacecraft also support a range of more humdrum but vital operations.

A radar satellite opens up avenues for watching over another country's military operations. Such satellites can pick out military vehicles, aircraft and ships. 

A radar satellite is “a very powerful instrument” for detecting naval movements, remarked Bhupendra Jasani of the Department of War Studies at King's College London. They could even pick up submarines moving below the surface.

Radar imageries could also aid in examining if a nuclear reactor was being used for plutonium production. In that case, there would be signs that the reactor was being shut down more frequently, he told this correspondent. 

Though RISAT-1 is not a spy satellite, it is likely to be used to keep a vigil on border areas irrespective of hostile weather conditions, which is often the case in the Indian subcontinent and will add to the ability of Indian monitors to track movements on the ground in areas considered sensitive from a defence and security viewpoint. ISRO officials said this satellite would not be used for defence applications. RISAT-2, primarily a spy spacecraft, is already doing that job.

One view which merits consideration is that “Radar satellites can look through clouds and work in darkness. However, Radar signals are highly attenuated by moisture and hence would not be effective during monsoon or in regions with high rainfall. Radar images are usually noisy and hence can easily miss smart targets. Radar images are vulnerable to terrain effects and the like, that can be used to the advantage by adversaries. Identifying missile launches are also difficult in Radar images. Camouflaging is difficult with optical satellites that use hyperspectral/multispectral images. However, that is not the case with Radar images. There are advantages and disadvantages of both the type of sensors.”

RISAT-I will be followed by a geo-imaging satellite (GISAT) that will provide "near real-time pictures of large areas". Risat I will transmit pictures only during passage over India, while GISAT will deliver data on "areas of interest...sector wise imaging every five minutes and entire Indian land surface every 30 minutes at a 50 metre resolution".