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Satellite Remote Sensing: Concepts and Imaging Systems

 

Satellite remote sensing relies on detectors (sensors) that measure reflected/emitted electromagnetic radiation from the Earth.
How sensors collect data depends on:

  • Spectral coverage → Multispectral (few bands), Hyperspectral (hundreds), Thermal, Microwave.

  • Detector type → Discrete detectors, linear arrays, or area arrays.

  • Scanning mechanism → Scanning mirrors (whiskbroom) vs. linear arrays (pushbroom).

Multispectral Imaging Using Discrete Detectors and Scanning Mirrors

(Whiskbroom scanners)

  • Principle:

    • A single detector (or a few detectors) measures radiation one pixel at a time.

    • A scanning mirror sweeps across-track (perpendicular to the satellite path) to build up the image line by line.

    • The forward motion of the satellite provides the along-track dimension.

    • Known as a whiskbroom scanner.

  • Characteristics:

    • Good calibration stability.

    • Narrow instantaneous field of view (IFOV).

    • Susceptible to mechanical wear (moving parts).

  • Examples:

    • Landsat Series

      • MSS (Multispectral Scanner System) – 4 bands (green, red, NIR), 80 m resolution.

      • TM (Thematic Mapper, Landsat 4/5) – 7 bands (VIS–SWIR–TIR), 30 m resolution.

      • ETM+ (Enhanced Thematic Mapper, Landsat 7) – Added panchromatic 15 m band.

    • NOAA (National Oceanic and Atmospheric Administration)

      • GOES (Geostationary Operational Environmental Satellites) – meteorology, cloud monitoring.

      • AVHRR (Advanced Very High Resolution Radiometer) – 1.1 km resolution, 5–6 bands (global vegetation, SST, clouds).

    • SeaWiFS (Sea-viewing Wide Field-of-view Sensor) – Ocean color, phytoplankton, climate studies.

Multispectral Imaging Using Linear Arrays

(Pushbroom scanners)

  • Principle:

    • A linear array of detectors (CCD line) records an entire line (row) of pixels simultaneously, without moving mirrors.

    • The satellite's motion provides subsequent lines → "pushbroom" method.

  • Characteristics:

    • Higher signal-to-noise ratio (SNR).

    • No moving scanning mirrors → more reliable.

    • Allows finer spatial resolution.

  • Examples:

    • SPOT Satellites

      • SPOT 1, 2, 3: HRV (High Resolution Visible) sensors – 10–20 m resolution.

      • SPOT 4, 5: HRVIR (High Resolution Visible and Infrared) – added SWIR bands.

    • IRS (Indian Remote Sensing Satellites)

      • LISS-III – 23.5 m, 4 bands.

      • LISS-IV – 5.8 m, 3 bands (high resolution).

    • ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer, Terra) – 14 bands (VNIR, SWIR, TIR).

    • QuickBird – High-resolution commercial sensor (0.61 m pan, 2.44 m multispectral).

Imaging Spectrometry Using Linear and Area Arrays

(Hyperspectral / advanced multispectral sensors)

  • Principle:

    • Linear array detectors → pushbroom hyperspectral (hundreds of narrow bands).

    • Area array detectors → capture 2D scenes directly (frame imagers).

    • Provide very fine spectral resolution (10–20 nm) across visible, NIR, SWIR, sometimes TIR.

  • Characteristics:

    • Enables detailed spectral analysis → vegetation stress, mineral composition, atmospheric studies.

    • Large data volumes.

    • Requires high radiometric calibration.

  • Examples:

    • AVIRIS (Airborne Visible/Infrared Imaging Spectrometer, NASA) – Hyperspectral (224 bands, 10 nm width, 400–2500 nm). Used for mineral mapping, vegetation studies.

    • MODIS (Moderate Resolution Imaging Spectroradiometer, Terra & Aqua) – 36 bands, global daily coverage, climate & vegetation indices.


Imaging TypeDetector/MethodSatellites & SensorsResolutionKey Use
Multispectral (discrete detectors + scanning mirrors)Whiskbroom (moving mirror + single detectors)Landsat (MSS, TM, ETM+), NOAA-AVHRR, GOES, SeaWiFS15–1000 mLand cover, climate, ocean color
Multispectral (linear arrays)Pushbroom (CCD line arrays)SPOT (HRV/HRVIR), IRS (LISS-III, LISS-IV), ASTER, QuickBird0.6–30 mAgriculture, urban, geology
Imaging spectrometry (linear/area arrays)Hyperspectral / frame sensorsAVIRIS, MODIS10–1000 mMineral mapping, vegetation, climate


  • Whiskbroom scanners (Landsat MSS/TM/ETM+, NOAA AVHRR) = older tech, discrete detectors + scanning mirrors.

  • Pushbroom scanners (SPOT, IRS, ASTER, QuickBird) = linear CCD arrays, better resolution.

  • Hyperspectral imagers (AVIRIS, MODIS) = linear/area arrays, hundreds of bands for advanced applications.

.. 

Landsat Series

  • MSS (Multispectral Scanner System)

    • Type: Multispectral, whiskbroom scanner (discrete detectors + scanning mirror).

    • Bands: 4 bands (Green, Red, NIR).

    • Resolution: 80 m.

    • Application: Early land cover, vegetation, water bodies.

  • TM (Thematic Mapper, Landsat 4/5)

    • Type: Multispectral, whiskbroom scanner.

    • Bands: 7 bands (VIS, NIR, SWIR, TIR).

    • Resolution: 30 m (VIS–SWIR), 120 m (TIR).

    • Application: Agriculture, forestry, geology.

  • ETM+ (Enhanced Thematic Mapper, Landsat 7)

    • Type: Multispectral with panchromatic band.

    • Bands: 8 (includes 15 m pan).

    • Resolution: 15–30 m, 60 m TIR.

    • Application: Urban mapping, detailed land cover.

NOAA Satellites

  • GOES (Geostationary Operational Environmental Satellite)

    • Type: Geostationary meteorological satellite.

    • Bands: Visible, IR, Water vapor.

    • Resolution: 0.5–4 km.

    • Application: Real-time weather monitoring, cloud movement, storms.

  • AVHRR (Advanced Very High Resolution Radiometer)

    • Type: Multispectral radiometer, whiskbroom scanner.

    • Bands: 5–6 broad spectral bands (VIS, NIR, TIR).

    • Resolution: 1.1 km.

    • Application: Global vegetation (NDVI), SST, snow/ice monitoring.

SeaWiFS (Sea-viewing Wide Field-of-View Sensor)

  • Type: Ocean color radiometer.

  • Bands: 8 bands (400–900 nm).

  • Resolution: 1.1 km (global), 4 km (reduced).

  • Application: Chlorophyll mapping, phytoplankton productivity, climate studies.

SPOT (Satellite Pour l'Observation de la Terre)

  • SPOT 1–3: HRV (High Resolution Visible)

    • Type: Pushbroom scanner (linear CCD arrays).

    • Bands: 4 (Green, Red, NIR, Panchromatic).

    • Resolution: 10–20 m multispectral, 10 m pan.

  • SPOT 4–5: HRVIR (High Resolution Visible and Infrared)

    • Type: Pushbroom with SWIR.

    • Bands: 4–5 bands (includes SWIR).

    • Resolution: 10–20 m multispectral, 2.5–5 m pan.

    • Application: Agriculture, forestry, urban monitoring.

IRS (Indian Remote Sensing Satellites)

  • LISS-III (Linear Imaging Self-Scanning Sensor-III)

    • Type: Pushbroom scanner.

    • Bands: 4 (Green, Red, NIR, SWIR).

    • Resolution: 23.5 m.

  • LISS-IV

    • Type: Pushbroom scanner.

    • Bands: 3 (Green, Red, NIR).

    • Resolution: 5.8 m.

    • Application: High-resolution mapping, agriculture, land use.

ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer)

  • Type: Multispectral (VNIR, SWIR, TIR).

  • Bands: 14 bands.

  • Resolution: 15 m (VNIR), 30 m (SWIR), 90 m (TIR).

  • Special Feature: Along-track stereo imaging for DEM generation.

  • Application: Geology, volcano monitoring, mineral exploration.

QuickBird (DigitalGlobe)

  • Type: High-resolution commercial satellite.

  • Bands: 4 multispectral + Panchromatic.

  • Resolution: 0.61 m (pan), 2.44 m (multispectral).

  • Application: Urban mapping, infrastructure, defense.

AVIRIS (Airborne Visible/Infrared Imaging Spectrometer)

  • Type: Hyperspectral imaging spectrometer.

  • Bands: 224 bands (10 nm bandwidth).

  • Range: 400–2500 nm (VNIR–SWIR).

  • Resolution: Airborne (spatial resolution depends on altitude, typically 20 m).

  • Application: Mineral mapping, vegetation stress, environmental studies.

MODIS (Moderate Resolution Imaging Spectroradiometer)

  • Type: Multispectral radiometer.

  • Bands: 36 spectral bands (VIS–NIR–SWIR–TIR).

  • Resolution: 250 m (red, NIR), 500 m, 1000 m (other bands).

  • Revisit: 1–2 days (Terra & Aqua satellites).

  • Application: Climate, vegetation indices (NDVI, EVI), fire monitoring, SST.


CategorySensorsDetector typeKey Concept
Whiskbroom (scanning mirrors)Landsat MSS/TM/ETM+, NOAA AVHRR, GOES, SeaWiFSDiscrete detectors + scanning mirrorOne pixel at a time
Pushbroom (linear CCD arrays)SPOT HRV/HRVIR, IRS LISS-III/LISS-IV, ASTER, QuickBirdLinear detector arrayLine-by-line imaging
Hyperspectral / Imaging SpectrometersAVIRIS, MODISLinear or area arraysMany narrow spectral bands


  • Landsat, NOAA, SeaWiFS → older whiskbroom scanners.

  • SPOT, IRS, ASTER, QuickBird → modern pushbroom scanners (CCD linear arrays).

  • AVIRIS, MODIS → imaging spectrometers (linear/area arrays, hyperspectral or multispectral).


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