Passive and active remote sensing

Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 6 "Introduction to Remote Sensing"

Meaning

Passive remote sensing records energy that comes from an outside source: sunlight reflected by objects, or heat given off by the Earth. Active remote sensing sends out its own energy pulse and records the echo that comes back. This difference decides when a satellite can see the ground. Optical passive sensors need daylight and clear skies. Microwave active sensors such as radar and SAR work day and night and through cloud. This is why they are India's main tool for mapping floods during the monsoon.

Explanation

How passive sensing works

• Energy source: something outside the sensor, either the Sun or the Earth's own heat.
• Two kinds of signal:
• Reflected sunlight is used for visible and near-infrared (near-IR) images.

• Emitted heat is used for thermal infrared (thermal IR) images.

• Result: optical passive sensors need daylight and clear skies.

• With no sunlight, there is nothing to reflect, so they cannot work at night.
• Cloud blocks reflected light, so the ground is hidden.

• Exception: a thermal sensor records heat, not sunlight, so it can work at night.

• Examples:

• photographic cameras, which work only in the optical region, 0.3–0.9 µm (µm = micrometre, one-millionth of a metre)
• multispectral scanners, both whiskbroom and pushbroom
• hyperspectral sensors such as HySIS

How active sensing works

• Energy source: the sensor itself. It fires a pulse, the pulse hits the ground, and the sensor records what comes back.
• Result: it does not depend on the Sun, so it works day and night.
• Radar (radio detection and ranging) sends microwave pulses and records the echoes.
• Microwaves are much longer than cloud droplets and smoke particles.
• So they pass through cloud, smoke and darkness. This gives all-weather, day-and-night images.
• Smooth surfaces such as calm water bounce the pulse away from the sensor, so they look dark.

• Rough surfaces, buildings and wet soil send more energy back, so they look bright. This is why floodwater stands out clearly.

• SAR (Synthetic Aperture Radar): a sharp radar image needs an antenna that is often kilometres long. No satellite can carry one.

• The satellite's small antenna sends pulses from many points along its path.
• A computer joins all the echoes together.

• The small antenna then acts like a very long antenna.

• LiDAR (Light Detection and Ranging) fires laser pulses and times how long they take to return.

• Distance = time taken × speed of light ÷ 2.
• It is active, but it uses light, so thick cloud blocks it.

Side-by-side comparison

	Passive	Active
Energy source	Sun, or the Earth's own heat	The sensor itself
Day/night	Optical: daytime only (thermal can work at night)	Day and night
Cloud	Blocked	Microwaves pass through (but LiDAR light is blocked)
Examples	Cameras, multispectral scanners, HySIS	Radar, SAR, LiDAR

Radar bands (active microwave)

• A longer wavelength goes deeper into the surface.
• L-band (24 cm) goes deeper into vegetation [4].
• S-band (10 cm) is more sensitive to moisture, light vegetation and small changes [4].
• C-band sits in between. It is used at 5.35 GHz on EOS-04 [5][6].

Where it occurs

• Monsoon India: in the south-west monsoon (June–September), cloud covers the country at the very time floods hit Assam, Bihar and Kerala. Optical passive sensors are blind then, so radar is used instead.
• EOS-04 (formerly RISAT-1A): an active C-band SAR satellite.
• Launched by PSLV-C52 on 14 Feb 2022 from Sriharikota (Andhra Pradesh).
• It gives all-weather, day-night images for agriculture, forestry, soil moisture, hydrology and flood mapping.

• Resolution is 1–50 m and swath is 10–225 km, from a sun-synchronous orbit at about 529 km [5][6].

• NISAR (NASA-ISRO SAR): an active radar satellite.

• Launched by GSLV-F16 on 30 July 2025 from Sriharikota.
• It carries L-band (NASA-JPL) + S-band (ISRO's SAC, Ahmedabad).
• It is the first satellite with dual-frequency SAR.

• It scans nearly all land and ice surfaces twice every 12 days from a 743 km sun-synchronous orbit [4][7].

• HySIS: a passive hyperspectral satellite.

• Launched by PSLV-C43 on 29 Nov 2018 from Satish Dhawan Space Centre, Sriharikota.

• It has 60 VNIR bands and 256 SWIR bands, with 30 m resolution [2][3].

• Pixxel's Firefly satellites (2025): a passive hyperspectral constellation from a start-up based in Bengaluru.

• LiDAR: an active sensor, usually flown on aircraft or drones. It is used for DEMs (Digital Elevation Models, which are digital maps of ground height), 3-D city models and mapping the ground under forest cover.

Don't confuse with

• Active ≠ sees through cloud: LiDAR is active but uses laser light, so cloud blocks it. Only microwave radar/SAR passes through cloud.
• Passive ≠ daytime only: thermal infrared sensors are passive, but they record the Earth's own heat. So they can work at night. Only optical passive sensors need daylight.
• Multispectral vs hyperspectral: both are passive. Multispectral sensors record a few broad bands. Hyperspectral sensors such as HySIS record hundreds of narrow bands that sit next to each other (contiguous bands).
• Whiskbroom vs pushbroom: these are two designs of passive scanner, not a passive/active split. Whiskbroom uses a rotating mirror with a single detector. Pushbroom uses a linear array of detectors and has no moving mirror.

Prelims Hooks

• Active: Radar, SAR, LiDAR. Passive: cameras, multispectral scanners, HySIS.
• Trap: "All active sensors can image through cloud." This is false, because LiDAR uses laser light and cloud blocks it.
• Radar images: calm water looks dark and rough surfaces, buildings and wet soil look bright. This is why floodwater stands out.
• EOS-04 (RISAT-1A): C-band SAR, PSLV-C52, 14 Feb 2022 [5][6].
• NISAR: first dual-frequency SAR, with L-band from NASA and S-band from ISRO. Launched on GSLV-F16, 30 July 2025 [4][7].
• InSAR (comparing two SAR images of the same place taken at different times) measures ground sinking to millimetres.

Mains Points

• Disaster management (GS-III): optical passive satellites are blind under monsoon cloud, exactly when floods strike Assam, Bihar and Kerala.
• Active C-band SAR such as EOS-04 gives all-weather, day-night flood maps for relief planning [5][6].

• NISAR's 12-day repeat cycle helps track earthquakes, landslides and glaciers in the Himalaya [4].

• Subsidence and urban risk: active sensors measure changes in the ground that passive images cannot show.

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

• LiDAR DEMs and 3-D city models support smart-city planning and flood-plain zoning.

• Using both together: passive and active sensors do different jobs, so India needs both.

• Passive hyperspectral data (HySIS, 316 bands across VNIR and SWIR) can tell apart minerals, crop stress and water pollution pixel by pixel [2][3].
• Active SAR (EOS-04, ISRO's S-band on NISAR) covers the cloudy season.
• Together with private constellations such as Pixxel, these support the 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
• Radar
• Synthetic aperture radar
• LiDAR

Read more

• Sensors: cameras, scanners, radar and LiDAR