How ISRO-NASA’s NISAR satellite will revolutionise earth monitoring from space

This ability enables it to monitor even the smallest changes on Earth’s surface, such as natural disasters and land-use changes, with precision and continuity. NISAR will play a crucial role in detecting land-use changes, ecological transformations, and help during the natural calamities.

The razor sharp vision of the NISAR satellite comes from its Dual-Band Synthetic Aperture Radar (SAR). Just as our eyes see objects when light reflects off them, NISAR uses reflected signals to create detailed images; but there is a crucial difference.

ISRO’s Earth observation satellites, such as Cartosat and Resourcesat, also capture reflected sunlight to observe the Earth. However, obtaining such precise images requires specific conditions: the Sun must be directly overhead the region being observed, the satellite must be positioned between the Sun and that region, and there should be no obstructions, such as clouds, volcanic smoke, or dust. Only under these circumstances can clear and accurate images be obtained, making nighttime imaging impossible with conventional methods.

Also read: NISAR satellite launched by India, US in maiden space collaboration

NISAR is unique because, unlike regular cameras, it can ‘see’ using microwaves, much like how a torch lights up the darkness. NISAR sends microwaves to Earth and analyses the reflected signals to detect changes, such as the growth of crops, drought, melting ice, or landslides.

The significance of dual-band frequencies

Like how two eyes help us judge distance, NISAR uses radar data from two adjacent points in its orbit to detect the topography and compute the changes in land height, such as mountains and valleys, with great accuracy.

Sunlight appears white because it blends all colours. When this ‘white’ light falls on a ‘green’ leaf, it absorbs all colours, except green; and gives us an appearance of green. Same happens with the ‘red’ rose. If the Sun emitted only blue light, the leaves and flowers of a rose plant would absorb it, leaving no reflection. Leaves and flowers would appear black. Similarly, using only one frequency band would limit the satellite’s vision. NISAR uses dual-band frequencies to capture a more complete view.

NISAR transmits microwaves in two frequency bands — L-Band and S-Band. Each band serves a different purpose. L-Band has a long wavelength of about 24 cm. It penetrates vegetation, soil, and ice sheets. This makes it ideal for studying forests, groundwater, and Antarctic glaciers. The S-Band has a short wavelength of about 10 cm. It captures fine surface details such as crop growth, small seismic shifts, or landslide movements. By combining data from both bands, NISAR gives a highly accurate picture of how Earth’s surface changes.

The SweepSAR technology

NISAR also uses SweepSAR technology. Imagine scanning a large crowd before focusing on individuals. Likewise, NISAR first scans a wide 242 km area, then focuses on small sections, ~ 10 meters, to capture sharp images. Like a super-smart painter creating a detailed mural of Earth’s surface, it can wide brush (the radar’s 242-km swath) quickly sketching vast scene but also zoom in for fine details (3-10 meter resolution).

When NISAR sends out radar waves, they cover a huge area all at once, like the cone of the flash light falling on a surface. As the echoes bounce back, the satellite’s antenna “sweeps” across to catch them, focusing on small sections one by one, like the painter refining tiny details.

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This “scan-on-receive” method ensures no gaps in the picture, even as the satellite moves. By combining wide coverage with sharp details, SweepSAR lets NISAR map Earth’s changes, landslides, glaciers, or forests.

NISAR has a huge 12-meter-wide deployable radar antenna. It unfurls in space like an umbrella. This antenna is crucial for the satellite to "sweep" across wide areas and focus on very small details.

Three months before full operation

Now in orbit, NISAR will take about 90 days to become fully operational. Scientists will use this time to calibrate their instruments, check data accuracy, and validate their performance.

First, the satellite must be stabilised in its correct orbit. Then, the 12-metre folded antenna will be unfurled like a canopy. Next, it will capture test images of pre-selected regions and compare them with ground data. Researchers will identify which signals indicate healthy crops, which indicate plants suffering from drought and so on. Called ‘ground truthing’, this calibration of instruments is necessary before the observations can be commenced.

Once fully operational, NISAR will orbit Earth to monitor each region every 12 days. Its microwave-based imaging ensures observation night or day, rain or shine. It is not hindered by obstacles like clouds or dust. It will provide all-weather imaging of Earth.

NISAR’s eagle-eyed surveillance will change disaster management, agriculture, and environmental conservation. Tracking Earth’s changing conditions gives scientists and policymakers valuable insights. This paves the way for a safer and more sustainable future.