NISAR: Eyes wide open on Earth

The story of the world’s most advanced and expensive earth observation satellite began nearly 15 years ago when the Indian Space Research Organisation turned down an offer from the National Aeronautics and Space Administration to launch a radar satellite using an Indian rocket from Sriharikota spaceport. The US satellite was to use sweepSAR (sweep synthetic aperture radar) technology to provide wide area coverage and fine spatial resolution at the same time.

New Delhi proposed an alternative: housing two radars in a single satellite. Both parties agreed, and an agreement was inked. The L-band radar, associated systems, and the antenna were made at the Jet Propulsion Laboratory, California, while the S-band radar was built at the Space Application Centre, Ahmedabad.

From next week, the two synthetic aperture radars aboard NISAR will detect changes in the planet’s surface down to fractions of an inch.

The spacecraft will bounce microwave signals off Earth’s surface and receive the return signals on a radar antenna reflector measuring 12 meters across. The satellite’s ability to see through clouds and rain, day and night, will enable users to continuously monitor earthquake and landslide-prone areas and determine how quickly glaciers and ice sheets are changing.

“NISAR can detect even small changes in the Earth’s surface such as ground deformation, ice sheet movement and vegetation dynamics,” ISRO says.

Other applications include sea ice classification, ship detection, shoreline monitoring, storm characterisation, changes in soil moisture, mapping and monitoring of surface water resources and disaster response. “It’s a highly advanced satellite,” notes K Sivan, former ISRO chairman.

Earthquakes, volcanoes, and ageing infrastructure can pose risks to lives and property. Able to see subtle changes in Earth’s surface, NISAR can help with hazard-monitoring efforts and potentially give decision-makers more time to prepare for a disaster.

For earthquakes, it will provide insights into which parts of a fault slowly move without producing quakes and which are locked together and could potentially slip. For the farm sector, it will help monitor the growth of crops from planting to harvest, generating crucial insights on how to time plantings and adjust irrigation schedules. The mission will have the resolution to see even small plots of farmland.

The satellite’s development path was a challenging one. The ISRO-NASA pact was signed in October 2014 with a target to launch the satellite by 2020. But developing the radar and antenna took more time, and the US could transfer the payload to India only in 2023-24. Unfortunately, within months, it was sent to the USA for repair.

In March 2024, the radar antenna reflector of the satellite was transported back to a facility in California, where reflective tape was applied and other precautionary measures were taken to mitigate temperature increases that could have potentially affected the deployment of the reflector from its stowed configuration before beginning the science operations in space.

The component returned to ISRO’s clean room by October 2024, when the Indian space agency began preparation for a February launch, which didn’t materialise. Ironically, a GSLV with a cryogenic engine will be used to launch the India-US satellite on July 30, even though the USA left no stone unturned for India to acquire the cryogenic technology.

The mission’s extraordinary capabilities come from the synthetic aperture radar. As the radar travels, its antenna continuously transmits microwave pulses and receives echoes from the surface. When the pulses hit something — a volcanic cone, for example — they scatter. The antenna receives those signals that echo back to the instrument, which measures their strength, change in frequency, how long they took to return, and if they bounced off of multiple surfaces, such as buildings.

“This mission packs in a wide range of science toward a common goal of studying our changing planet and the impacts of natural hazards,” says Deepak Putrevu, co-lead of the ISRO science team at the Space Applications Centre in Ahmedabad.