Sensors: cameras, scanners, radar and LiDAR

Detail

1. What a sensor is

• Sensor: a device that receives EMR (electromagnetic radiation: energy that travels as waves, such as light, heat and radio waves). It turns this energy into a signal and shows it as a photographic or digital image.
• Sensors come in two families:
• Photographic (analogue): cameras that record on film.
• Non-photographic: scanners that build a digital image piece by piece.

2. Photographic sensors (cameras)

• A photographic sensor is an analogue camera. It records the whole scene on film in one exposure (one instant).
• It works only in the optical region, 0.3-0.9 µm (µm = micrometre, one-millionth of a metre). This covers near-ultraviolet, visible light and near-infrared.
• It was the main tool of the aerial photography era, before satellites.
• Limits:
• Film cannot record longer wavelengths such as thermal infrared or microwave.
• The output is a picture, not numbers, so a computer cannot analyse it directly.

3. Non-photographic sensors: the multispectral scanner

• A multispectral scanner (MSS) builds the image bit by bit, line by line, as it sweeps across its view.
• How it works:
• A mirror collects energy from the ground.
• Detectors measure the energy in several separate wavelength bands. "Multispectral" means many bands.

• Each reading is stored as a digital number (DN): a whole number that shows how bright that ground cell is.

• Swath: the width of the ground strip imaged in one pass. The sensor's angular field of view sets it. A wider angle gives a wider swath.

• Scanners are of two kinds: whiskbroom and pushbroom.

4. Whiskbroom scanner (across-track scanner)

• Design: a rotating mirror with a single detector.
• Process:
• The mirror swings sideways, across the satellite's path, through 90°-120°.
• This full sweep is the total field of view (TFOV), and it sets the swath.

• The satellite moves forward, so the next sweep images the next line. The image grows like a broom sweeping a floor.

• Instantaneous field of view (IFOV): the small patch of ground the detector sees at one moment. It fixes the ground cell size, which is the spatial resolution.

• Weakness: one detector must cover the whole swath very quickly. It spends very little time on each cell, and the moving mirror can wear out.

5. Pushbroom scanner (along-track scanner)

• Design: a straight line (linear array) of detectors, with one detector for each ground cell across the swath.
• Process: a whole line across the swath is recorded at once. The satellite's forward motion "pushes" this line along the ground, like a push broom.
• NCERT worked example — SPOT HRV:
• Swath = 60 km = 60,000 m; ground cell = 20 m.

• Detectors needed = 60,000 ÷ 20 = 3,000 detectors.

• Advantages: no moving mirror. Each detector looks at its cell for longer, so the signal is stronger and the image sharper.

Feature	Photographic camera	Whiskbroom scanner	Pushbroom scanner
Output	Film (analogue)	Digital numbers	Digital numbers
How the image forms	Whole scene in one exposure	Mirror sweeps one cell at a time, line by line	Whole line recorded at once
Key part	Lens + film	Rotating mirror + single detector	Linear array of detectors
Wavelengths	0.3-0.9 µm only	Visible to thermal IR	Mainly visible and near-IR
Moving parts	Shutter	Yes (mirror)	None

6. Passive and active remote sensing

• Passive sensors record energy that comes from outside the sensor:
• reflected sunlight, for visible and near-IR images, or
• emitted heat, for thermal IR images.

• So optical sensors need daylight and clear skies. Cloud blocks them. This is a big problem over India during the south-west monsoon (June-September), the very time floods happen.

• Active sensors send out their own energy and record what comes back. So they work at night and through cloud.

	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
Examples	Cameras, multispectral scanners, HySIS	Radar, SAR, LiDAR

7. Radar

• Radar (radio detection and ranging) sends microwave pulses to the ground and records the echoes that bounce back.
• Why it sees through weather:
• Microwaves are much longer than the droplets in cloud and the particles in smoke.
• So they pass through cloud, smoke and darkness.

• Result: all-weather, day-and-night images. This makes radar ideal for mapping monsoon floods.

• What the echo shows:

• Smooth surfaces, such as calm water, bounce the pulse away, so they look dark.
• Rough surfaces, buildings and wet soil send more energy back, so they look bright.
• This is why floodwater shows up clearly on radar images.

8. Synthetic Aperture Radar (SAR)

• Problem: a sharp radar image needs a very long antenna, often kilometres long. No satellite can carry one.
• Solution — SAR:
• As the satellite moves, its small antenna sends pulses from many positions along its path.
• A computer joins all the echoes together.

• So the small antenna acts like a very long antenna (a "synthetic aperture"), and the image becomes sharp.

• InSAR (SAR interferometry): two SAR images of the same place, taken at different times, are compared. The tiny difference in the returning waves shows ground movement down to millimetres. It is used to measure land subsidence (ground sinking), earthquake movement and landslides.

• Radar bands: a longer wavelength goes deeper.
• 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, at 5.35 GHz on EOS-04 [5][6].

Indian and joint SAR missions

Mission	Launch	Radar	Key facts
EOS-04 (formerly RISAT-1A)	PSLV-C52, 14 Feb 2022, Sriharikota (Andhra Pradesh)	C-band SAR	All-weather, day-night imaging for agriculture, forestry, soil moisture, hydrology and flood mapping. Resolution 1-50 m, swath 10-225 km, sun-synchronous orbit at about 529 km [5][6]
NISAR (NASA-ISRO SAR)	GSLV-F16, 30 July 2025, Sriharikota	L-band (NASA-JPL) + S-band (ISRO's SAC, Ahmedabad)	First joint NASA-ISRO Earth-observation satellite. First satellite with dual-frequency SAR. Sun-synchronous orbit at 743 km (98.4° inclination). Scans nearly all land and ice surfaces twice every 12 days [4][7]

9. LiDAR

• LiDAR (Light Detection and Ranging) fires laser pulses and times how long they take to return.
• Process:
• Time taken × speed of light ÷ 2 = distance to the surface.
• Millions of pulses give a dense "point cloud" of heights.

• Some pulses slip through gaps in leaves and reach the ground. So LiDAR can map the ground under forest cover.

• Uses:

• DEMs (Digital Elevation Models: digital maps of ground height).
• 3-D city models for urban planning.

• Forest height and biomass, and flood-plain mapping.

• Usually flown on aircraft or drones. Like radar, it is active, but it uses light, not microwaves, so thick cloud blocks it.

	Radar / SAR	LiDAR
Energy	Microwave pulses	Laser (light) pulses
Cloud	Passes through	Blocked
Best for	Floods, soil moisture, subsidence (InSAR)	Exact heights, DEMs, ground under canopy, 3-D cities

10. Hyperspectral remote sensing

• Hyperspectral remote sensing records hundreds of narrow bands that sit next to each other (contiguous bands). A multispectral sensor records only a few broad bands.
• Why it matters:
• Every material reflects light in its own pattern, called its spectral signature.
• With hundreds of bands, each pixel gives an almost continuous curve.

• So each pixel can be matched to a specific mineral, crop or pollutant.

• ISRO's HySIS (Hyperspectral Imaging Satellite):

• Launched by PSLV-C43 on 29 Nov 2018 from Satish Dhawan Space Centre, Sriharikota [2][3].
• Two payloads [2][3]:
  • VNIR (visible and near-infrared): 0.4-0.95 µm, 60 contiguous bands.
  • SWIR (shortwave infrared): 0.85-2.4 µm, 256 contiguous bands.

• About 380 kg, built on ISRO's Mini Satellite-2 (IMS-2) bus. 30 m resolution, 30 km swath, from a 630 km sun-synchronous orbit [2][3].

• Pixxel's Firefly satellites (2025): a commercial hyperspectral constellation from the Bengaluru start-up Pixxel. It is part of India's growing private space sector.

Prelims Hooks

• Photographic sensors work only in the optical region, 0.3-0.9 µm. They cannot record thermal IR or microwave.
• Whiskbroom = rotating mirror + single detector, sweeping 90°-120° (TFOV). Pushbroom = linear array of detectors, no moving mirror.
• The IFOV sets the ground cell (spatial resolution). The angular field of view sets the swath.
• SPOT HRV: 60 km swath ÷ 20 m cells = 3,000 detectors.
• Radar, SAR and LiDAR are active. Cameras, multispectral scanners and HySIS are passive.
• Trap: LiDAR uses laser light, so cloud blocks it. Only microwave radar/SAR sees through cloud.
• EOS-04 (RISAT-1A): C-band SAR, PSLV-C52, 14 Feb 2022 [5][6].
• NISAR: first dual-frequency SAR. L-band from NASA, S-band from ISRO. Launched on GSLV-F16, 30 July 2025 [4][7].
• HySIS: PSLV-C43, 29 Nov 2018. VNIR has 60 bands, SWIR has 256 bands [2][3].
• InSAR measures ground sinking to millimetres. LiDAR maps ground height under forest canopy.

Mains Points

• Disaster management (GS-III): optical satellites are blind under monsoon cloud, exactly when floods strike in Assam, Bihar and Kerala.
• 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: InSAR can measure millimetre-scale sinking.

• Uses: sinking towns in the Himalaya, coastal cities that draw out too much groundwater, and mining areas.

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

• Resources and agriculture: hyperspectral data (HySIS, 316 bands across VNIR and SWIR) can tell apart minerals, crop stress, soil types and water pollution pixel by pixel [2][3]. This supports mineral exploration, crop insurance and river-pollution checks.

• Policy: together, India-built SAR (EOS-04, ISRO's S-band on NISAR), hyperspectral satellites (HySIS) and private constellations (Pixxel) support the open, home-grown geospatial data the National Geospatial Policy 2022 aims for.