Synthetic aperture radar

Also called: SAR · Topic: Remote Sensing, GIS and GNSS · NCERT: Beyond NCERT

Meaning

Synthetic aperture radar (SAR) is an active radar sensor on a moving satellite. It sends microwave pulses to the ground, records the echoes from many points along its path, and uses a computer to join them. This lets its small antenna work like a very long one and give sharp images.

SAR works through cloud, smoke and darkness. So it gives all-weather, day-and-night images. That makes it the main tool for mapping floods during the south-west monsoon, when optical satellites cannot see the ground. Its offshoot, InSAR, measures ground sinking down to millimetres.

Explanation

Why SAR is needed: the antenna problem

• Radar (radio detection and ranging) sends microwave pulses to the ground and records the echoes that bounce back.
• The problem:
• A sharp radar image needs a very long antenna, often kilometres long.

• No satellite can carry an antenna that big.

• The SAR solution:

• The satellite keeps moving along its orbit. Its small antenna sends pulses from many positions along the path.
• A computer joins all these echoes together.
• So the small antenna acts like a very long antenna. This "fake" long antenna is called the synthetic aperture (aperture = the opening or size of the antenna). The result is a sharp image.

How SAR sees through weather, and what the image shows

• SAR is an active sensor (it sends out its own energy and records what comes back). It does not need sunlight.
• Why cloud does not block it:
• Microwaves are much longer than cloud droplets and smoke particles.
• So they pass straight through cloud, smoke and darkness.

• Result: all-weather, day-and-night images.

• Passive optical sensors (cameras and multispectral scanners that record reflected sunlight) need daylight and clear skies. Monsoon cloud blinds them.

• Reading a SAR image:
• Smooth surfaces, such as calm water, bounce the pulse away from the satellite, so they look dark.
• Rough surfaces, buildings and wet soil send more energy back, so they look bright.
• So floodwater shows up as clear dark patches next to bright land.

Radar bands: wavelength decides depth

• Rule: a longer wavelength goes deeper.
• L-band (24 cm) goes deeper into vegetation [1].
• S-band (10 cm) is more sensitive to moisture, light vegetation and small changes [1].
• C-band sits between them. EOS-04 uses it at 5.35 GHz [2][3].

InSAR: measuring ground movement

• InSAR (SAR interferometry): two SAR images of the same place, taken at different times, are compared.
• Process:
• The returning waves of the two images differ very slightly.

• This tiny difference shows how much the ground has moved, down to millimetres.

• Uses: measuring land subsidence (ground sinking), movement in earthquakes, and landslides.

Where it occurs

• EOS-04 (formerly RISAT-1A):
• India's C-band SAR, launched on PSLV-C52 on 14 Feb 2022 from Sriharikota (Andhra Pradesh) [2][3].
• Used for agriculture, forestry, soil moisture, hydrology (the study of water on land) and flood mapping.

• Resolution 1-50 m, swath (width of the ground strip imaged) 10-225 km, in a sun-synchronous orbit (an orbit that passes over each place at the same local time) at about 529 km [2][3].

• NISAR (NASA-ISRO SAR):

• Launched on GSLV-F16 on 30 July 2025 from Sriharikota [1][4].
• The first joint NASA-ISRO Earth-observation satellite, and the first satellite with dual-frequency SAR [1][4].
• L-band built by NASA-JPL. S-band built by ISRO's SAC (Space Applications Centre), Ahmedabad [1][4].

• Sun-synchronous orbit at 743 km (98.4° inclination). Scans nearly all land and ice surfaces twice every 12 days [1][4].

• Flood mapping during the monsoon: Assam, Bihar and Kerala, where floods strike in the south-west monsoon (June-September), the very season when cloud blocks optical satellites.

• InSAR for subsidence and hazards:
• Sinking towns in the Himalaya, plus earthquakes, landslides and glaciers there [1].
• Coastal cities that draw out too much groundwater.
• Mining areas.

Don't confuse with

• Real-aperture radar (ordinary radar): its image is only as sharp as its physical antenna is long. SAR uses the satellite's motion and computer processing to create a long "synthetic" antenna, so it gets a sharp image from a small one.
• LiDAR: also active, but it fires laser (light) pulses, so thick cloud blocks it. SAR uses microwaves and passes through cloud. LiDAR is best for exact heights, DEMs (Digital Elevation Models: digital maps of ground height) and the ground under forest canopy.
• Passive optical sensors (cameras, multispectral scanners, HySIS): they record sunlight reflected or heat given off by the Earth, so optical images need daylight and clear skies. SAR sends its own energy and works day and night.
• InSAR vs a single SAR image: one SAR image maps surfaces, such as floodwater. InSAR compares two images taken at different times to measure millimetre-scale ground movement.

Prelims Hooks

• SAR is an active microwave sensor. It gives all-weather, day-and-night images. Radar, SAR and LiDAR are active. Cameras, multispectral scanners and HySIS are passive.
• Trap: only microwave radar/SAR sees through cloud. LiDAR is active too, but it uses laser light, so cloud blocks it.
• On a SAR image, calm water looks dark (smooth surface), while buildings, rough ground and wet soil look bright.
• EOS-04 (RISAT-1A): C-band SAR, PSLV-C52, 14 Feb 2022, Sriharikota [2][3].
• NISAR: first dual-frequency SAR. L-band from NASA, S-band from ISRO. GSLV-F16, 30 July 2025, 12-day repeat [1][4].
• InSAR measures ground sinking down to millimetres. Band order by wavelength: L (24 cm) > S (10 cm). The longer L-band goes deeper into vegetation [1].

Mains Points

• Disaster management (GS-III):
• The problem: optical satellites are blind under monsoon cloud, exactly when floods strike in Assam, Bihar and Kerala.
• The SAR answer: C-band SAR such as EOS-04 gives all-weather, day-night flood maps for relief planning [2][3].

• NISAR's 12-day repeat cycle helps track earthquakes, landslides and glaciers in the Himalaya [1].

• Subsidence and urban risk:

• InSAR measures millimetre-scale sinking in Himalayan towns, coastal cities that pump out too much groundwater, and mining areas.

• This gives early warning, so planners can control groundwater use, restrict building and zone risky land.

• Self-reliance in geospatial data:

• India-built SAR (EOS-04, and ISRO's S-band on NISAR) cuts India's dependence on foreign radar data for agriculture, soil moisture and forestry.
• This supports the open, home-grown geospatial data that the National Geospatial Policy 2022 aims for.

Related concepts

• Sensor
• Photographic sensor
• Multispectral scanner
• Swath
• Whiskbroom scanner
• Instantaneous field of view
• Pushbroom scanner
• Passive and active remote sensing
• Radar
• LiDAR

Read more

• Sensors: cameras, scanners, radar and LiDAR