Come March 2025, a significant space event would unfold — the launch of the 2.8-tonne NISAR satellite by a GSLV Mk-II rocket of India’s space agency, ISRO. When this remarkable satellite settles in its orbit 750 km above earth, it will mark the completion of a unique ₹5,000-crore India-US collaborative project, which began in 2009. 

The NISAR is basically a ‘synthetic aperture radar’ (SAR) satellite, only with much higher capabilities than its predecessors. An SAR satellite works by sending down radio frequency signals and forming a picture of the earth below by capturing the bounce-backs — as opposed to an optical satellite, where the cameras capture what they see; moreover, an SAR satellite can look through darkness, clouds and vegetation. 

The US space body NASA had previously launched several SAR satellites, but wanted a device capable of sharper imaging. Initially NASA had wanted to collaborate with DLR, but the German aerospace centre was busy with its own projects. NASA’s Jet Propulsion Laboratory then decided to do it on its own, but the budget was slashed. In the meantime, ISRO had been proving its mettle with low-cost missions, such as Chandrayaan-1 in 2008 and RISAT-2 in 2009, also an SAR satellite. Taking due note, NASA turned to ISRO for collaboration. In 2014, the NASA-ISRO Synthetic Aperture Radar (NISAR) mission was born. 

Applications

NISAR is designed to measure surface motions on the earth, such as the movement of ice sheets and glaciers, and changes in vegetation, down to fractions of an inch. If a parcel of land has sunk an inch, called subsidence, NISAR will spot it. If a bridge, dam or levee develops a structural misalignment, NISAR will point it out. Since there are thousands of kilometres of these structures, it is hugely challenging to detect these faults otherwise. Endowed with such capabilities, NISAR is also useful to study the depredations of climate change. 

How NISAR does this is a marvel of technology. 

NISAR is designed for a ‘12-day revisit cycle’, which means it will pass over a location twice every 12 days, to detect changes in the location between the two passes. Since the satellite will circle the earth from pole to pole, its ‘eye’ will rove along the longitudes, observing ‘strips’ of the earth. The larger the width of these strips — called swath width — the fewer times a satellite would need to orbit to revisit a place. But a larger swath width means a sacrifice in resolution (sharpness). 

NISAR is tooled for a swath width of 240 km and resolution of 10 metres — perhaps the best balance between the two ever. NASA’s MODIS satellite, for instance, has a swath width of 2,330 km, but its resolution is moderate. NISAR’s optimum combination of swath width and resolution is thanks to two features onboard the satellite — ‘digital beam forming’ (DBF) and two radars for two different frequencies, in the L and S bands. 

NISAR has a bi-directional antenna, which can receive as well as send signals. (Incidentally, the NASA-built 12-m-long, 2.4-m-wide antenna arrived in Bengaluru in October, marking a milestone in the NISAR mission.) The parts of the antenna used to send and receive signals are referred to as ‘elements’. Each element sends and captures signals independently. The DBF feature enables each beam to be ‘steered’ into different directions, while the antenna itself does not change direction. This enables better coverage (swath width) as well as more sharpness. 

‘New frontier’

Former ISRO scientist Rakesh Bhan, who has worked on the NISAR project, tells Quantum that the DBF on NISAR is technically superior to “conventional” ones. “Conventional DBF involves digitally adjusting the phase of an active array antenna to steer a beam, while NISAR-DBF phase is equalised by digital multiplication and addition. This reduces data rate of 20 gigabits per second to less than 3.5 gigabits per second,” he says. The reduced data rate is more manageable with fewer equipment onboard and in ground stations.

Of the satellite’s two radars, the L-band (1.2 GHz), made by NASA, can penetrate better through vegetation, ice and so on; while the S-band (3.3 GHz), made by ISRO, gives higher resolution. The two bands complement each other, making for a better picture. (This is not the first time that a dual-frequency SAR is being employed — Chandrayaan-2 (2019) also had L- and S-band radars, but the frequencies were different.) 

NISAR will spew 26 terabits of data per day. With its comprehensive specifications and robust capabilities, NISAR will not only enhance our understanding of the earth’s dynamic processes but also aid in disaster management and climate research, says Bhan, who calls the NISAR mission “a new frontier”.