NISAR Sees Through the Coal Country Fog

In Guizhou's rainy season, Europe's workhorse radar satellite loses the thread. Sentinel-1's C-band coherence over the province is generally below 0.3, according to a paper published by Frontiersin, and at that level the radar returns from one pass barely match the next. Coherence is what lets an interferometric radar turn two images into a measurement of ground movement. Without it, the ground goes unmeasured.

That matters because Guizhou sits on coal. Panzhou City, the paper's study area, is described by its authors as a core coal-mining base in Southwest China, with proven reserves of 14.175 billion tons as of August 2025. Underneath the vegetation, the earth is being hollowed out, and the places where it sags are hard to see from orbit.

NISAR, the NASA-ISRO radar satellite, is built for exactly this problem. Its data is now free to anyone with a login, and a coal-field study published November 5 offers a preview of what that could mean for miners, regulators and anyone who owns a road or a pipeline near a working face.

Why wavelength decides

Dense canopy, karst terrain and persistent fog cause conventional C-band and X-band InSAR to suffer severe temporal decorrelation, the Frontiers authors write. Guizhou has vegetation coverage above 65% and annual rainfall of 1200 mm. The older alternatives are no better: GNSS stations and leveling surveys, the paper says, are constrained by high costs and limited spatial coverage.

Longer wavelengths pass through leaves and keep their coherence over time. NISAR carries an L-band radar with a 24 cm wavelength and an S-band instrument at 9.4 cm, according to NASA Science. JPL says the satellite can track changes across Earth's surface unhindered by the clouds or vegetation that impede optical sensors and higher-frequency radars. It is the first satellite to carry two SAR instruments at different wavelengths, and it monitors land and ice surfaces on a 12-day cycle, JPL said. NASA Science says it can measure surface change as small as a centimeter.

What L-band saw in Panzhou

The Frontiers study did not use NISAR. It used China's domestic LuTan-1 L-band satellite alongside Japan's ALOS-2, also L-band, combining cumulative D-InSAR with Stacking-InSAR. Over heavily vegetated karst mining zones, the LT-1 dataset produced 80.99% high-coherence pixels, defined as coherence above 0.6. Sentinel-1 managed generally less than 0.3 in the rainy season.

The deformation figures are large. LT-1 detected maximum cumulative subsidence of 0.796 m and a maximum rate of 0.654 m/yr, against 0.687 m and 0.632 m/yr from ALOS-2. The authors say the subsidence bowls migrate in step with the advance of underground working faces. They also say long-term intensive extraction has triggered widespread subsidence that threatens regional ecological security and the stability of linear transportation infrastructure.

The paper picks out one stretch of road, 1.245 km long, as a high-hazard segment. Its authors are careful about what that means: this is preliminary, deformation-oriented hazard zoning, not an absolute engineering risk grade. A satellite can show where the ground moves. It cannot say whether the pavement above it will fail.

A new dataset, still settling

NISAR launched on July 30, 2025. Its provisional L-band data was released on July 20, 2026, and NASA Science says those products are fully calibrated and have been validated at a limited number of sites around the world. The first release covers observations acquired on or after June 17, 2026. Calibrated L-band data from NASA and JPL is available through the Alaska Satellite Facility's archive, and ISRO has released S-band products through its Bhoonidhi portal.

For a mine surveyor or a pipeline operator, the access terms may matter as much as the physics. Tracking ground motion through cloud has so far meant commissioning commercial imagery or flying surveys. NISAR's archive is open to the public.

The satellite's first public showpiece shows both the promise and the caveats. Over Mexico City, home to some 20 million people and built atop an aquifer, JPL said NISAR "pinpointed areas of the city that are sinking by more than half an inch per month due in large part to groundwater pumping." By the 1990s and 2000s, parts of the metropolitan area were sinking around 14 inches a year, damaging infrastructure including the Metro.

That analysis rests on preliminary measurements taken between October 2025 and January 2026, during the city's dry season. JPL flags the yellow and red patches in its image as likely residual noise that should fade as NISAR collects more data. Craig Ferguson, deputy project manager at NASA Headquarters, said "Images like this confirm that NISAR's measurements align with expectations," of the early imagery. David Bekaert of the Flemish Institute for Technological Research and the NISAR science team went further, saying "we're going to see an influx of new discoveries from all over the world," and pointing to the satellite's consistent global coverage.

JPL also notes that NISAR's drum-shaped reflector is 39 feet across, "the largest radar antenna reflector NASA has ever sent into space."

The part no satellite supplies

Coal subsidence is a moving target by design. A working face advances, the bowl follows, and the hazard moves with it. Most of the monitoring that matters to a mine's neighbors, such as railway operators, road agencies and farmers, has been done where the instruments could see. Guizhou shows how much sits outside that zone.

NISAR's repeat passes should let analysts watch the bowls travel across cloudy, forested ground. Turning that into a decision about the 1.245 km of road above a moving face is still an engineer's job, and the satellite leaves it to them.