Optical satellites photograph sunlight reflected from the ground; radar satellites send out their own microwave pulses, so they can see at night and through clouds.
Earth-observation satellites come in two big families. NASA's Earthdata explains the difference in one line each. Passive instruments detect energy that comes naturally from the scene, such as reflected sunlight. Active instruments send out their own energy and measure what comes back.
An optical satellite works like a very powerful camera. India's Cartosat-3 is an example. ISRO calls it a third-generation agile satellite with high-resolution imaging capability. PSLV-C47 placed it in a 509 km orbit inclined at 97.5° on 27 November 2019.
Optical images are easy for people to read: they look like photographs. But they need daylight, and they cannot see through cloud. During India's monsoon, that is a serious problem.
A radar satellite sends microwave pulses and records the echoes. Earthdata says this lets it make high-resolution images night or day, regardless of weather. Microwaves pass through clouds and rain that would block an optical camera.
India's EOS-04 is a radar imaging satellite designed to give high-quality images under all weather conditions. PSLV-C52 placed it in a 529 km sun-synchronous orbit on 14 February 2022. ISRO lists its uses as agriculture, forestry and plantations, soil moisture and hydrology, and flood mapping.
Radar images look different from photos. Their brightness depends on the roughness and structure of the surface rather than its colour, so they take training to read.
Neither. Optical gives colour and fine detail on clear days. Radar is reliable in every weather. ISRO's National Remote Sensing Centre says microwave data are of the highest use during floods, exactly when clouds hide the ground from optical satellites. The two are most powerful when used together.
Search for Cartosat-3 and EOS-04: both fly low sun-synchronous orbits around 500 km, but one carries a camera and the other a radar. Find Cartosat-3 and EOS-04
The two families use wavelengths that differ by a factor of about a hundred thousand.
Visible light: around 0.5 μm = 5 × 10⁻⁷ m. Radar bands (Earthdata): X-band about 2.4–3.8 cm, C-band about 3.8–7.5 cm, S-band 7.5–15 cm, L-band 15–30 cm.
Take L-band at 24 cm (NISAR's L-band wavelength): 0.24 / 5 × 10⁻⁷ = 480,000 times longer than visible light. Cloud droplets are tiny compared with a radar wavelength, so they barely scatter the radar pulse; to visible light they are large, so a cloud is opaque.
Wavelength also controls what radar sees inside a forest. Earthdata notes that short X-band waves mostly bounce off leaves at the top of the canopy, while longer L-band and P-band waves reach branches and trunks.
The cost of being active is power. A radar's echo must travel to the ground and back, and the received power falls as 1/R⁴ (1/R² on the way down, 1/R² on the way back). Doubling the distance cuts the echo to 1/16. Power matters for radar satellites: EOS-04's solar arrays generate about 2,280 W.
A camera's echo problem does not exist: sunlight does the illuminating, and the satellite only receives. But it also cannot work without the Sun, which is why optical Earth-observation satellites use sun-synchronous orbits that pass over at the same convenient local time each day.
Optical cameras are passive (they use reflected sunlight); radar is active (it sends out its own microwave pulses).
Microwaves pass through clouds and rain and work at night, so radar can image flooded areas that clouds hide from optical cameras.
L-band, because its longer wavelength passes the leaves and reaches branches and trunks.