Skip to main content

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).


Comments

Post a Comment

Popular posts from this blog

Platforms in Remote Sensing

In remote sensing, a platform is the physical structure or vehicle that carries a sensor (camera, scanner, radar, etc.) to observe and collect information about the Earth's surface. Platforms are classified mainly by their altitude and mobility : Ground-Based Platforms Definition : Sensors mounted on the Earth's surface or very close to it. Examples : Tripods, towers, ground vehicles, handheld instruments. Applications : Calibration and validation of satellite data Detailed local studies (e.g., soil properties, vegetation health, air quality) Strength : High spatial detail but limited coverage. Airborne Platforms Definition : Sensors carried by aircraft, balloons, or drones (UAVs). Altitude : A few hundred meters to ~20 km. Examples : Airplanes with multispectral scanners UAVs with high-resolution cameras or LiDAR High-altitude balloons (stratospheric platforms) Applications : Local-to-regional mapping ...
Post a Comment
Read more

Optical Sensors in Remote Sensing

1. What Are Optical Sensors? Optical sensors are remote sensing instruments that detect solar radiation reflected or emitted from the Earth's surface in specific portions of the electromagnetic spectrum (EMS) . They mainly work in: Visible region (0.4–0.7 µm) Near-Infrared – NIR (0.7–1.3 µm) Shortwave Infrared – SWIR (1.3–3.0 µm) Thermal Infrared – TIR (8–14 µm) — emitted energy, not reflected Optical sensors capture spectral signatures of surface features. Each object reflects/absorbs energy differently, creating a unique spectral response pattern . a) Electromagnetic Spectrum (EMS) The continuous range of wavelengths. Optical sensing uses solar reflective bands and sometimes thermal bands . b) Spectral Signature The unique pattern of reflectance or absorbance of an object across wavelengths. Example: Vegetation reflects strongly in NIR Water absorbs strongly in NIR and SWIR (appears dark) c) Radiance and Reflectance Radi...
Post a Comment
Read more

Types of Remote Sensing

Remote Sensing means collecting information about the Earth's surface without touching it , usually using satellites, aircraft, or drones . There are different types of remote sensing based on the energy source and the wavelength region used. 🛰️ 1. Active Remote Sensing 📘 Concept: In active remote sensing , the sensor sends out its own energy (like a signal or pulse) to the Earth's surface. The sensor then records the reflected or backscattered energy that comes back from the surface. ⚙️ Key Terminology: Transmitter: sends energy (like a radar pulse or laser beam). Receiver: detects the energy that bounces back. Backscatter: energy that is reflected back to the sensor. 📊 Examples of Active Sensors: RADAR (Radio Detection and Ranging): Uses microwave signals to detect surface roughness, soil moisture, or ocean waves. LiDAR (Light Detection and Ranging): Uses laser light (near-infrared) to measure elevation, vegetation...
Post a Comment
Read more

Resolution of Sensors in Remote Sensing

Spatial Resolution 🗺️ Definition : The smallest size of an object on the ground that a sensor can detect. Measured as : The size of a pixel on the ground (in meters). Example : Landsat → 30 m (each pixel = 30 × 30 m on Earth). WorldView-3 → 0.31 m (very detailed, you can see cars). Fact : Higher spatial resolution = finer details, but smaller coverage. Spectral Resolution 🌈 Definition : The ability of a sensor to capture information in different parts (bands) of the electromagnetic spectrum . Measured as : The number and width of spectral bands. Types : Panchromatic (1 broad band, e.g., black & white image). Multispectral (several broad bands, e.g., Landsat with 7–13 bands). Hyperspectral (hundreds of very narrow bands, e.g., AVIRIS). Fact : Higher spectral resolution = better identification of materials (e.g., minerals, vegetation types). Radiometric Resolution 📊 Definition : The ability of a sensor to ...
Post a Comment
Read more

Radar Sensors in Remote Sensing

Radar sensors are active remote sensing instruments that use microwave radiation to detect and measure Earth's surface features. They transmit their own energy (radio waves) toward the Earth and record the backscattered signal that returns to the sensor. Since they do not depend on sunlight, radar systems can collect data: day or night through clouds, fog, smoke, and rain in all weather conditions This makes radar extremely useful for Earth observation. 1. Active Sensor A radar sensor produces and transmits its own microwaves. This is different from optical and thermal sensors, which depend on sunlight or emitted heat. 2. Microwave Region Radar operates in the microwave region of the electromagnetic spectrum , typically from 1 mm to 1 m wavelength. Common radar frequency bands: P-band (70 cm) L-band (23 cm) S-band (9 cm) C-band (5.6 cm) X-band (3 cm) Each band penetrates and interacts with surfaces differently: Lo...
Post a Comment
Read more