Synthetic aperture radar sends long-wavelength microwaves that pass through clouds, and combines echoes along its path to act like a giant antenna.
During the monsoon, clouds often hide large parts of India for days at a time. A camera sees nothing but white. A radar satellite sees the ground underneath, because microwaves pass through cloud and rain, and because the radar brings its own signal, so it works at night too.
To see small details with radio waves you need an enormous antenna, far bigger than anything you could launch. NASA's Earthdata explains how SAR gets round this. As the satellite flies along, it sends pulse after pulse and records every echo. A computer then combines this whole sequence, as if all those positions were parts of one very long antenna. That imaginary long antenna is the synthetic aperture.
NISAR, the NASA–ISRO Synthetic Aperture Radar, was launched on 30 July 2025 from Sriharikota on ISRO's GSLV-F16. It weighs about 2,400 kg and flies a sun-synchronous orbit at 747 km.
It carries two radars. NASA built the L-band radar (wavelength 24 cm) and ISRO built the S-band radar (9.4 cm). Both share a 12 m reflector on a 9 m boom. NISAR maps the world's land and ice every 12 days at 3–10 m resolution. ISRO's NISAR page announced the start of its science phase on 28 November 2025.
India's own EOS-04 is another radar imaging satellite, built for all-weather images used in agriculture, forestry, soil moisture and flood mapping.
SAR can do something no camera can. By comparing the exact timing (phase) of echoes from two passes, a technique called InSAR, scientists can measure how much the ground has moved between them. Earthdata says these measurements can be accurate to the centimetre level, useful for watching volcanoes, earthquakes, landslides and sinking ground.
Follow NISAR's path over India: it passes at the same local times every day, imaging through cloud whatever the weather. Track NISAR
Why is a synthetic aperture necessary? A real antenna of length L at wavelength λ forms a beam about λ / L radians wide. At range R, its footprint along the flight path is roughly:
resolution ≈ λ × R / L
For NISAR's L-band (λ = 0.24 m) at R ≈ 747 km = 747,000 m with L = 12 m:
0.24 × 747,000 / 12 ≈ 14,900 m ≈ 15 km
That is useless for mapping fields. To get 10 m directly, the antenna would need to be 0.24 × 747,000 / 10 ≈ 17,900 m long, nearly 18 km.
SAR solves this in software. A point on the ground stays inside the real beam for the whole time the satellite flies across a 15 km stretch. During that time the radar records many echoes from it, each with a slightly different distance. Combining them with the right phase corrections gives the sharpness of an antenna as long as that stretch. NASA quotes NISAR's resolution as 3–10 m, roughly a thousand times finer than the real-antenna figure.
InSAR sensitivity: the radar wave travels to the ground and back, so a line-of-sight movement of d changes the path by 2d. One full phase cycle (2π) therefore equals d = λ / 2. For L-band, that is 24 / 2 = 12 cm per cycle; for S-band, 9.4 / 2 = 4.7 cm. Since phase can be measured to a small fraction of a cycle, movements of a centimetre or less become visible.
The radar combines echoes recorded along its flight path so that a small antenna behaves like a very long one, giving fine resolution.
NASA built the L-band radar and ISRO built the S-band radar.
The signal goes to the ground and back, so the path changes by twice the ground movement.