The PSLV-C19, the newest in the series of polar satellite launch vehicles of the Indian Space Research Organisation (ISRO), burst off the launch-pads of Sriharikota in the wee hours of 26.04.2012 on its space mission of placing indigenously developed Radar Imaging Satellite the RISAT-1 in a polar circular orbit.
ISRO’s Satish Dhawan Space Centre in Sriharikota, at precisely 5.47 a.m., on 26.04.2012, launched the launch vehicle’s core stage igniters and set of six strap-on motors ignited within seconds of each to signal the successful lift-off of the PSLV-C19 with the RISAT -1 firmly docked inside its metal frames.
The RISAT-1 with a payload of 1858 kg, the heaviest satellite being launched yet by the PSLV, is a state-of-the-art Active Microwave Remote Sensing Satellite carrying a Synthetic Aperture Radar (SAR) payload that will operate in the C-band. In simpler terms, the RISAT-1 can beam back imaging of the earth surface features during day and night and under all imagined weather conditions. The SAR which gives the RISAT-1 its magic lens also makes it superior to the generation of optical remote sensing satellites in terms of clearer imaging at all times and under any condition.
Addressing the team of scientists and engineers, ISRO Chairman K. Radhakrishnan said he was happy to announce that the PSLV-C19 was a “grand success” and had injected into polar orbit India’s first Radar Imaging Satellite.
According to ISRO scientists, once the satellite onboard propulsion system will raise the orbital altitude to 536 km with orbital inclination of approximately 97 degrees to place the RISAT-1 into a polar sun-synchronous orbit, the satellite will begin its daily routine of 14 orbits with a of 25 days. During its mission life of five years, RISAT-1 will use its active microwave remote sensing capability for cloud penetration and day-night imaging of the earth surface and provide critical data inputs for a range of applications.
The satellite’s applications will range across agriculture — paddy monitoring in the kharif season — and management of natural disasters like flood and cyclone and could greatly assist food security planning in India.
The PSLV C-19 is the 21st flight in the PSLV series of satellite launches and the third to involve the high-end version (PSLV-XL) equipped with six extended strap-on motors, each carrying 12 tonnes of solid propellant. The two earlier flights of PSLV-XL were used to launch Chandrayaan-1 and GSAT-12 communication satellite.
It can spy and also do a hundred humdrum things
India’s space and security capabilities are poised for a big leap with the launch of an entirely indigenous radar imaging satellite, RISAT-1.
In the popular mind, radar satellites 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.
Optical satellites rely on sunlight to illuminate the ground below, working much like an ordinary camera does. Radar satellites, on the other hand, must send out pulses of radio waves and then pick up signals that bounce back.
Once the monsoon sets in over India, cloud cover often severely limits the useful images that satellites with optical cameras can supply. But radar can see through cloud and rain. Nor does darkness hamper its operation.
Optical or radar?
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. 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.
Flood mapping, agriculture
Satellite radar data, often from Canada’s RADARSAT satellites, is now routinely used during the monsoon to provide near real time flood mapping. In last year’s monsoon, for instance, radar data was drawn upon to identify affected areas when floods struck Assam, West Bengal, Orissa, Uttar Pradesh and Kerala.
Monitoring crops from space to derive timely and more accurate estimates of acreage and yield was identified as an important application early on in India’s remote sensing programme. But with small field sizes in the country, different crops being grown in the same area and variations in agricultural practices, establishing ‘signatures’ that can distinguish one crop from another has been difficult enough with optical remote sensing.
It becomes even more complicated with radar where a number of factors, such as soil characteristics, moisture levels in the soil and even the plant size and shape, influence the signals that return to the satellite.
Radar, however, opens up the possibility of monitoring crops grown during the monsoon when extensive cloud cover often hinders optical satellites. Data from Canada’s RADARSAT satellites is currently being used for operational rice crop inventory at the state and national levels, according to a journal paper published by a team of ISRO scientists. There has also been some success with jute.
RADARSAT data was costly, remarked one person who was involved with the Indian remote sensing programme. “With our own satellite, we will be able to carry out more extensive studies for establishing ways to monitor other crops with the required accuracy.”
Glacier study
Satellite-borne radar could prove useful in studying glaciers in the Himalayas, according to Anil V. Kulkarni who earlier worked at the Space Applications Centre and is now with the Divecha Centre for Climate Change at the Indian Institute of Science in Bangalore.
Radar techniques could potentially be employed to understand some key parameters of glaciers, including their rate of movement and area. It may also be possible to derive indications of whether they are gaining or losing mass from one year to the next. Such information could provide important insights into how climate change is affecting the glaciers.
Radar data could also be utilised to figure out how much snow was melting in summer. With suitable modelling, it should then be possible to estimate the run-off that flows into various rivers, he pointed out.
RISAT-1’s radar data is likely to find many more applications, including in geology, terrain mapping and forestry. Oceanography can benefit from information on winds and currents that such data can supply. Canada is reportedly using its RADARSAT satellites to manage shipping operations, including monitoring offshore fishing activities.
Military use
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 the wake of submarines moving below the surface. Radar imageries could also aid in examining if a nuclear reactor was being used for plutonium production.
RISAT-1’s radar can see through clouds and work in darkness
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.
PM congratulates scientists
Hailing the successful launch of PSLV carrying Radar Imaging Satellite (RISAT-1), Prime Minister Manmohan Singh on 26.04.2012 said it was an important milestone in India’s space programme and congratulated the ISRO scientists for displaying mastery of the complex launch vehicle technology. “I would like to warmly congratulate all scientists of the Indian Space Research Organisation (ISRO) for the successful launch today of the Polar Satellite Launch Vehicle (PSLV)-C19 carrying the Radar Imaging Satellite -1(RISAT -1), the heaviest satellite launched till date using PSLV, ” PM said. “The twentieth consecutive successful launch of the PSLV is an important milestone in our space programme and is testimony to ISRO’s mastery of the complex launch vehicle technology,” he said.
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