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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:

  • Longer wavelengths → penetrate vegetation and soil

  • Shorter wavelengths → sensitive to surface roughness

3. Backscatter (Radar Return Signal)

When radar waves hit a surface:

  • some energy is absorbed

  • some energy is transmitted

  • some energy is scattered back to the sensor → backscatter

Backscatter depends on:

  • surface roughness

  • moisture content

  • geometry / slope

  • structure (vegetation, buildings)

  • wavelength & polarization

4. Polarization

Radar signals can be polarized:

  • H (Horizontal)

  • V (Vertical)

Combinations:

  • HH (transmit H, receive H)

  • HV (transmit H, receive V)

  • VH

  • VV

Different polarizations help identify:

  • water

  • vegetation structure

  • urban features

5. Incidence Angle

The angle at which radar wave hits the surface.
It affects backscatter values:

  • Small angle → stronger return from smooth surfaces

  • Large angle → stronger return from rough surfaces

6. Speckle

Radar images contain a grainy noise called speckle, caused by coherent scattering.
Speckle reduction filters (Lee, Frost) are used.

7. SAR (Synthetic Aperture Radar)

SAR is the most widely used radar system.
It creates high-resolution images by combining multiple radar pulses as the satellite moves.

SAR enables:

  • high spatial resolution

  • mapping of large areas

  • detection of small surface changes (millimeters)

How Radar Sensors Work 

1. Transmit microwave pulse

The radar antenna sends pulses toward the Earth.

2. Interaction with the surface

Waves interact with objects:

  • smooth water → low backscatter

  • rough terrain → high backscatter

  • wet soil → strong backscatter

  • vegetation → volume scattering

3. Return signal is received

The sensor measures:

  • strength of backscatter

  • time delay (gives distance)

  • polarization

  • phase information

4. Image is formed

The backscatter intensity is converted into grayscale or colored radar images.

5. Advanced processing (optional)

  • Interferometry (InSAR) for measuring deformation

  • Polarimetric analysis (PolSAR)

  • Change detection

Advantages of Radar Sensors

  • All-weather capability

  • Works day and night

  • Penetrates cloud cover

  • Sensitive to surface roughness and moisture

  • Useful for topographic mapping

  • Can measure surface deformation (mm level)

Applications

🌧 Flood mapping

Radar penetrates clouds → detects water extent during storms.

🛰 Land deformation (InSAR)

Detects earthquakes, landslides, subsidence.

🌲 Forest monitoring

Estimates biomass, canopy structure.

🏙 Urban studies

Detects buildings, road networks, and urban expansion.

🌾 Agriculture

Monitors soil moisture and crop structure.

❄ Snow and ice

Maps glaciers and ice movements.

🌊 Sea and ocean

Monitors waves, oil spills, ships.

Radar Sensors

Spaceborne SAR Sensors

  • Sentinel-1 (C-band)

  • RADARSAT-1/2 (C-band)

  • RISAT-1/2 (C-band)

  • ALOS PALSAR (L-band)

  • TerraSAR-X (X-band)

  • COSMO-SkyMed (X-band)

Airborne Radar

  • AIRSAR

  • UAVSAR


Radar sensors are active remote sensing instruments that transmit microwave radiation and measure the backscattered signal from Earth's surface. They operate in the microwave region, work in all weather conditions, and use concepts like wavelength, polarization, incidence angle, and backscatter to produce images. Radar is widely used for flood mapping, land deformation (InSAR), vegetation studies, soil moisture, and urban analysis.


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