RISAT

Also called: Radar Imaging Satellite · Topic: Remote Sensing, GIS and GNSS · NCERT: Beyond NCERT

Meaning

RISAT (Radar Imaging Satellite) is India's series of C-band SAR satellites. They send out their own microwave pulses and record the echo, so they can take pictures of the ground through cloud and at night. This matters because India gets most of its floods and kharif crops in the south-west monsoon (June–September), when thick cloud stops ordinary cameras from seeing the ground. RISAT keeps working in exactly these months.

Explanation

How radar imaging works

Optical (passive) sensors, like the cameras on Resourcesat and Cartosat, record sunlight that bounces off the earth.
They need daylight.
They need a clear sky.
Radar (active) sensors make their own energy.
The satellite sends microwave pulses down to the ground.
The ground sends back an echo, and the sensor records it.
Microwaves pass through cloud, so radar works in the monsoon and at night.
SAR (Synthetic Aperture Radar) is the kind of radar RISAT carries.
It is a side-looking radar on a moving satellite.
It joins the echoes from many points along the orbit, so together they act like one very long antenna.
Result: a sharp image from a small antenna.

Why C-band

Radar bands get their names from their wavelength (the length of one microwave wave):
C-band: about 5.6 cm. Good for crops, floods and sea ice. RISAT uses this band.
S-band: about 10 cm.
L-band: about 24 cm. It passes through tree canopy and dry soil.
Rule: the longer the wavelength, the deeper the wave goes into the surface.
C-band sits near the surface. That is why it suits floodwater and paddy fields, RISAT's two main jobs.

The satellites in the series

RISAT-1: launched in 2012.
EOS-04 (RISAT-1A): launched in 2022. In 2022 ISRO renamed its Earth observation satellites "EOS", so EOS-04 is the same satellite as RISAT-1A.
Orbit: RISAT is a polar, sun-synchronous satellite. A sun-synchronous polar orbit is a near-polar orbit about 500–900 km high, in which the satellite passes over each place at the same local time on every pass.
Good point: it shows fine detail of the ground.
Weak point: it comes back over the same place only every few days.
EOS-09: a failed radar launch
EOS-09 was a 1,696 kg radar satellite meant for a 505 km sun-synchronous polar orbit. It was to add to India's all-weather imaging for farming, disaster management and surveillance [2].
It flew on PSLV-C61, ISRO's 101st launch, on 18 May 2025.
The rocket worked normally through the 2nd stage. A problem in the 3rd stage meant the mission could not be completed [2].

Where it occurs

India: the monsoon gap
Thick cloud covers the country in the south-west monsoon (June–September), so optical satellites see little.
RISAT maps floodwater and paddy under this cloud.
India: uses on the ground
Floods: for example, flood mapping in Assam and Bihar.
Crops: especially kharif rice (rice sown at the start of the monsoon). RISAT data is used to estimate how much land is under paddy.
Border surveillance: radar works at night and in all weather, so it can watch India's borders at any time.
Run by: ISRO, India's space agency.
World comparison: the EU's Sentinel-1 (Copernicus programme, run with ESA, series started 2014) also carries C-band SAR and maps floods under monsoon cloud. Its data is free, and it is widely used in India.

Don't confuse with

Resourcesat / Cartosat (optical): these carry cameras that record reflected sunlight (LISS, AWiFS, PAN), so cloud blocks them. RISAT carries radar, so it sees through cloud and works at night.
NISAR: a NASA-ISRO satellite that carries L-band and S-band radar. It was the first satellite to carry both [1]. RISAT carries only C-band radar and is a purely Indian mission.
Microwave radiometer (Bhaskara-I and II): a passive sensor that only measures the natural microwave energy the surface gives off. SAR on RISAT is active: it sends out its own pulses.
INSAT-3DS: a geostationary weather satellite that watches the same area all the time but shows coarser detail. RISAT is in a polar, sun-synchronous orbit, shows fine detail and revisits only every few days.

Prelims Hooks

RISAT = C-band SAR. EOS-04 = RISAT-1A (2022). RISAT-1 was launched in 2012.
Active vs passive: SAR is an active sensor that works through cloud and at night. Optical sensors are passive.
Band and wavelength: C-band ≈ 5.6 cm (RISAT, Sentinel-1); S-band ≈ 10 cm; L-band ≈ 24 cm (goes through tree canopy).
Trap: both RISAT and Sentinel-1 are radar satellites, not optical ones.
PSLV-C61/EOS-09 (18 May 2025, ISRO's 101st launch): a 1,696 kg radar satellite lost because of a 3rd-stage problem [2].
Satellite and job: RISAT → floods, kharif rice, border surveillance. Cartosat → maps/DEM. Oceansat → potential fishing zones (PFZ). Resourcesat → crops.

Mains Points

Radar fills the monsoon gap (GS-III, disasters and agriculture):
Cloud covers India in the south-west monsoon, so optical images fail exactly when floods come and kharif crops are sown.
C-band RISAT and EOS-04 provide flood maps (for example, Assam and Bihar) and paddy area estimates. These help relief work and crop forecasts.
Together with L- and S-band NISAR [1], radar also helps watch landslides and sinking land in the Himalaya, as at Joshimath.
Backup and continuity (GS-III, infrastructure and security):
The EOS-09 loss in May 2025 [2] shows that India needs spare and follow-on satellites to keep its radar coverage running.
All-weather radar also matters for border surveillance, so gaps in coverage are a security risk as well as a farming and disaster risk.
Using radar and optical together (GS-I and III):
Optical satellites show colour and plant health. Radar sees through cloud.
Using both, along with free Sentinel-1 data, gives coverage in every season. This makes India's disaster and farm planning more reliable.

Related concepts

Earth observation satellites: the world and India

Sources

1ISRO — GSLV-F16/NISAR mission page and launch brochureisro.gov.in · tier 1
2PIB / ISRO — PSLV-C61/EOS-09 Missionpib.gov.in · tier 1