1. Radar and Radar Remote Sensing
RADAR stands for Radio Detection and Ranging. It is an active remote sensing system that transmits microwave energy toward the Earth's surface and records the energy that is returned to the sensor as an echo or backscatter.
Unlike passive optical remote sensing, radar does not depend on sunlight. Therefore, it can operate day and night and, at suitable wavelengths, can acquire data through clouds, haze and light rain. Hence, radar is widely described as an all-weather, day-and-night remote sensing technology.
Basic principle
Microwave pulse → transmission → interaction with surface → backscatter/echo → antenna receives signal → signal processing → radar image
2. Microwave Energy
Radar systems use microwave electromagnetic radiation, generally in wavelength ranges from approximately 1 mm to 1 m.
Important radar bands include:
| Band | Approx. wavelength | Common applications |
|---|---|---|
| X-band | ~2.4–3.8 cm | Fine surface and urban details |
| C-band | ~3.8–7.5 cm | Vegetation, soil, flood mapping |
| L-band | ~15–30 cm | Forest structure, biomass, soil and deformation |
| P-band | ~30–100 cm | Deep vegetation/forest structure |
The wavelength and polarization strongly influence how microwave energy interacts with different surface materials.
3. Radar Pulse, Antenna and Echo
A radar sensor generates a short burst of microwave energy called a radar pulse. The pulse is transmitted toward the Earth's surface through an antenna.
The antenna performs two major functions:
Transmits microwave energy.
Receives the returned signal.
The returned energy is called an echo. The strength of the returned signal is called backscatter intensity or backscatter coefficient.
Echo strength ≈ information about how strongly the target reflects/scatters microwave energy back toward the radar.
Different materials produce different backscatter because their roughness, moisture, dielectric properties, geometry, structure and orientation differ.
4. Backscatter
Backscatter is the portion of the transmitted microwave energy that is scattered back toward the radar antenna.
For example:
Smooth water surface → usually very low backscatter → dark in many SAR images.
Rough urban surfaces → often strong backscatter → bright.
Vegetation → variable backscatter depending on wavelength, moisture and canopy structure.
Wet soil → generally stronger radar response than very dry soil under comparable conditions.
Thus, different materials and surface conditions produce different radar signatures.
Important concept: Dielectric constant
The dielectric properties of a material, particularly those influenced by water content, strongly affect microwave interaction. This is why radar is particularly useful for soil moisture, wetlands and flood mapping.
5. Slant Range and Ground Range
Slant range is the direct distance between the radar antenna and a target on the Earth's surface.
Ground range is the horizontal/ground distance between the nadir point and the target.
Because radar observes the surface obliquely, the distinction between slant-range geometry and ground-range geometry is fundamental to radar image interpretation.
6. Side-Looking Radar
Most imaging radar systems use a side-looking configuration.
The radar looks sideways rather than directly downward. This creates a characteristic viewing geometry:
Radar → → → Surface targets
The side-looking geometry provides spatial information along two principal directions:
Range direction – distance from the radar.
Azimuth direction – direction along the flight path.
7. Synthetic Aperture Radar (SAR)
Synthetic Aperture Radar (SAR) is an advanced imaging radar technique used to produce high-resolution images.
A conventional radar would require a physically very large antenna to obtain extremely fine azimuth resolution. SAR overcomes this limitation by using the motion of the aircraft or satellite to synthesize a very long antenna aperture.
SAR principle
As the sensor moves along its flight path, it observes the same ground target from multiple positions. The recorded signals are combined using coherent signal processing to produce a high-resolution image.
Physical antenna + sensor movement + signal processing → Synthetic aperture → High-resolution SAR image
SAR is therefore a major technology used in modern Earth observation.
8. Speckle and Speckle Pattern
One of the most important characteristics of SAR imagery is speckle.
Speckle is a granular, salt-and-pepper-like pattern produced because radar signals reflected from many small scattering elements interfere with one another.
It is a consequence of the coherent nature of radar imaging, rather than simply random sensor noise.
Speckle pattern
The spatial distribution of bright and dark pixels caused by this interference is called the speckle pattern.
Speckle can reduce visual interpretability and affect classification. Common approaches to reduce it include:
Spatial filtering
Lee filter
Frost filter
Gamma MAP filter
Multi-looking
Temporal averaging
However, excessive filtering can remove useful spatial information.
9. Two-Way Attenuation
Radar energy experiences attenuation while travelling:
Radar → target → radar
Therefore, radar measurements involve a two-way propagation path.
Attenuation can result from:
Atmospheric absorption
Scattering
Rain
Water vapour
Interaction with vegetation
Surface and subsurface materials
The importance of atmospheric attenuation depends strongly on the radar wavelength and environmental conditions.
10. Radar Layover
Layover is a geometric distortion that occurs mainly in areas with steep slopes or tall structures.
Because radar measures targets according to their range distance, the top of a tall object or mountain slope may appear closer to the radar than its base.
Consequently, the top can appear to be displaced toward the radar-facing side.
Radar-facing slope → top returns before base → image displacement → layover
Layover is particularly important in mountainous terrain and urban areas.
11. Radar Shadow
A related geometric effect is radar shadow.
When a terrain feature blocks the radar signal, the area behind it may receive little or no microwave illumination.
Therefore:
Tall/steep object → blocked microwave illumination → low/no return → radar shadow
Layover and shadow are important considerations during SAR image interpretation and terrain correction.
12. Radar Image Geometry and Map Projection
Radar images initially have a geometry related to the sensor's observation system rather than conventional map geometry.
Important terms include:
Slant-range geometry
Ground-range geometry
Azimuth
Incidence angle
Look angle
Nadir
Orthorectification
Geocoding
Terrain correction
Radar map projection
Radar data are usually converted into a geographically meaningful map coordinate system through geocoding and terrain correction.
A Digital Elevation Model (DEM) is commonly used to correct terrain-related geometric distortions.
13. Voids and Overlaps
Radar geometry can produce areas with:
Voids – locations with missing or inadequate observations.
Overlaps – locations where radar observations or geometrically projected surfaces overlap.
These problems can be associated with terrain occlusion, layover, shadow, acquisition geometry and image mosaicking.
Careful geometric correction and appropriate acquisition planning are therefore important.
14. Corner Reflector
A corner reflector is an artificial radar target designed to produce a very strong and predictable radar return.
Its geometry causes microwave energy to be reflected back toward the radar.
Corner reflectors are widely used for:
SAR calibration
Geometric validation
Radiometric calibration
InSAR deformation monitoring
They normally appear as very bright targets in SAR imagery.
15. Cardinal Effect
The cardinal effect refers to strong directional variation in radar backscatter caused by the orientation of surface features relative to the radar's look direction.
Objects such as:
Buildings
Rows of crops
Ridges
Furrows
Linear structures
may produce substantially different radar responses depending on their orientation.
Therefore, radar backscatter is not determined only by the material; target geometry and orientation are also important.
16. Polarization
Radar systems can transmit and receive microwave energy with different polarization orientations.
The four fundamental polarization combinations are:
HH – Horizontal transmit, Horizontal receive
HV – Horizontal transmit, Vertical receive
VH – Vertical transmit, Horizontal receive
VV – Vertical transmit, Vertical receive
This is known as polarimetry.
Multipolarization
Multipolarization or multipolarimetric SAR acquires information using multiple polarization combinations.
It improves discrimination of:
Vegetation
Water
Soil
Urban surfaces
Forest structure
Agricultural crops
Polarization flexibility
Modern SAR systems may provide different combinations of polarization, sometimes described as polarization flexibility or polarization agility, depending on the sensor.
17. C-band and L-band
C-band
C-band has a relatively short microwave wavelength, approximately 5–6 cm for many Earth-observation systems.
It is useful for:
Flood mapping
Agriculture
Surface moisture
Vegetation monitoring
Sea-ice monitoring
Land-cover mapping
L-band
L-band has a longer wavelength, approximately 23–24 cm for many Earth-observation systems.
Its longer wavelength can penetrate vegetation more effectively than C-band and is particularly useful for:
Forest monitoring
Biomass studies
Soil and surface studies
Ground deformation
Vegetation structure
Polarimetric applications
18. PALSAR
PALSAR stands for Phased Array type L-band Synthetic Aperture Radar.
It is an L-band SAR instrument developed for Japan's Earth-observation satellite missions, notably ALOS (Advanced Land Observing Satellite).
PALSAR has been important for applications such as:
Forest mapping
Biomass estimation
Land-cover mapping
Soil and surface studies
Disaster monitoring
Interferometric applications
Polarimetric SAR research
PALSAR → L-band → SAR → phased-array antenna → Earth observation
19. Radar Interferometry (InSAR)
Radar Interferometry, commonly called InSAR (Interferometric Synthetic Aperture Radar), uses the phase difference between SAR observations acquired from different positions or at different times.
The basic principle is:
SAR image 1 + SAR image 2 → phase comparison → interferogram → surface information
InSAR can be used to measure or detect:
Ground deformation
Subsidence
Uplift
Earthquake-related deformation
Volcanic deformation
Landslide movement
Glacier movement
Differential InSAR (D-InSAR)
Differential InSAR removes or reduces the contribution of topography using an appropriate DEM and focuses on surface deformation.
20. Chaotic Echo
A radar return may contain signals from numerous small scattering elements with different:
Locations
Orientations
Reflectivity
Phases
Their signals can interfere constructively or destructively, producing a complex or seemingly chaotic/irregular echo pattern. This phenomenon contributes to the granular appearance of SAR imagery and is closely associated with speckle and coherent scattering.
21. Important Radar Scattering Mechanisms
Understanding scattering mechanisms is essential for interpreting radar images.
1. Surface scattering
Occurs mainly from the surface itself.
2. Volume scattering
Occurs within a volume containing many scattering elements, such as vegetation canopies.
3. Double-bounce scattering
Occurs when microwave energy is reflected between two approximately perpendicular surfaces.
Typical example:
Vertical tree trunk + flooded horizontal ground → strong double-bounce return
4. Corner reflection
Occurs when geometry strongly redirects the radar energy toward the sensor.
These mechanisms help explain why different land-cover types produce different backscatter signatures.
22. Key Factors Controlling Radar Backscatter
Radar backscatter depends on several interacting factors:
Backscatter = f(material properties + surface roughness + moisture + wavelength + polarization + incidence angle + target geometry + orientation)
Important factors include:
Dielectric properties
Surface roughness
Soil moisture
Vegetation structure
Target geometry
Incidence angle
Radar wavelength
Polarization
Orientation
Surface slope
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