NISAR: What NASA and ISRO's Radar Satellite Can See That No Optical Camera Ever Could

Every optical satellite, from Landsat to Cartosat, needs one thing the Indian subcontinent routinely withholds: a clear sky. For four to five months of the year, the monsoon drapes the country in cloud cover thick enough to blank out entire states. Optical sensors, which work exactly like a camera, record reflected sunlight. No sunlight reaching the sensor means no image. Radar works on an entirely different principle. NISAR, the NASA-ISRO Synthetic Aperture Radar satellite, transmits its own microwave pulses toward the ground and records what bounces back. Cloud water droplets are too small to scatter L-band and S-band microwaves, so the signal passes straight through. The satellite images a flooded rice paddy in West Bengal at 2 a.m. during a Category 2 monsoon depression with the same fidelity it would on a cloudless January morning over Rajasthan.

When a radar pulse hits the ground, the strength and phase of the returning signal encode information that no photograph can carry. Rough surfaces scatter the pulse in many directions; smooth water reflects it away from the sensor like a mirror, returning almost nothing. Vegetation canopy, depending on its moisture content and structure, scatters the pulse in characteristic ways. Soil moisture changes the dielectric constant of the ground surface, altering how much energy returns. NISAR carries two radar frequencies simultaneously: L-band at roughly 24 centimetres wavelength, which penetrates vegetation and reaches the soil beneath, and S-band at roughly 12 centimetres, which is more sensitive to the canopy itself. Running both at once lets scientists separate what is happening at the surface from what is happening a few centimetres below it. No optical camera, regardless of resolution, can see through a wheat field to measure the soil moisture underneath it.

The capability that separates SAR from every other Earth-observation technology is interferometry. When NISAR passes over the same patch of ground on two separate orbits, its onboard processor compares the phase of the returning signal between the two passes. A shift of even a few millimetres in the ground surface between those passes shows up as a measurable phase difference. This is how the satellite detects a hillside in Uttarakhand creeping downslope at 3 centimetres per year, a rate invisible to any camera but well within NISAR's detection threshold. The Chamoli disaster of February 2021, in which a rock-and-ice avalanche killed over 200 people, occurred in terrain that SAR data from European satellites later showed had been deforming for years before the collapse. NISAR will image every point on Earth's surface every 12 days. That repeat cycle is fast enough to catch the slow precursors that precede sudden failures: the swelling of a volcano, the subsidence of a coastal city, the progressive failure of a slope. The Gangetic delta, where Kolkata sits, is subsiding in places at rates of 10 to 25 millimetres per year due to groundwater extraction and sediment compaction. Optical images of the delta look the same year after year. NISAR's interferometric data will show exactly which neighbourhoods are sinking and how fast.

NISAR is a joint mission between NASA's Jet Propulsion Laboratory and ISRO, with JPL providing the L-band radar and ISRO providing the S-band radar and the spacecraft bus. The satellite will launch from Sriharikota aboard a GSLV rocket. Its swath width, the strip of ground it images on each pass, is 240 kilometres, wide enough to cover most of peninsular India in a single overpass. Over its three-year primary mission, it will generate the most detailed record of Earth surface change ever assembled: roughly 85 terabytes of data per day. For India specifically, the science priorities include monitoring Himalayan glaciers, which are thinning at rates that threaten the water security of hundreds of millions of people downstream; mapping crop types and growth stages across the Indo-Gangetic Plain without waiting for cloud-free optical windows; and tracking the slow movement of landslide-prone slopes in the Western Ghats and the northeastern states. ISRO's contribution of the S-band radar is not a junior partnership. The S-band frequency is particularly sensitive to surface roughness and vegetation water content, making it the instrument of choice for agricultural monitoring. India's farm sector, which spans roughly 140 million hectares of net sown area, will be one of the primary beneficiaries of the data stream.

Optical Earth observation has always been a photograph of a moment, contingent on weather and daylight, showing only what sits on the surface and reflects visible light. SAR observation is a measurement of change, indifferent to weather and darkness, sensitive to what lies just beneath the surface and to motion too slow for any human observer to register. The two technologies are not competing. Optical images are still essential for identifying what is on the ground, a SAR image of a city looks nothing like a photograph of one. But the questions that matter most for the next several decades, where is the ground moving, where is the ice thinning, where is the soil drying out, which slopes are failing, are questions that only radar can answer at the scale and frequency NISAR will provide. The satellite does not see the Earth more clearly. It hears a different frequency of what the Earth is saying.