RADAR stands for Radio Detection and Ranging which uses longer wavelengths compared to optical remote sensing and they can be active; producing their own source of energy.
Slat Range: radar antenna and target’s distance
Ground Range: satellite ground target and target
Azimuth: along track direction or distance
Incidence angle: angle between the line of sight of radar and vertical to the terrain
Range resolution: depends on the length of the pulse
Azimuth resolution: determined by antenna beam width and distance to the target
Wavelength and frequency: between 0.5 cm to 100cm frequencies
Radar polarization: orientation of the electric field of the electromagnetic wave, the polarization can be like polarized (HH, VV), cross polarized (HV, VH), and compact polarized (transmitting right circular and receive H and V coherently)
SARs respond to two characteristics in the agriculture land: structure and moisture:
Roughness: caused by tillage, soil erosion and weathering in the land or planting, backscatter increase with increase in soil roughness and the rougher soils appear brighter in the SAR images. The criteria of the soil roughness depends upon the wavelength and the incidence angle.
Strong backscatter when the SAR looks direction perpendicular to the direction of the rows; not present in HV and VH.
Dielectric constant is when the dipolar molecules rotate response to the applied field. The presence of water molecules rotates to align with the field. More water in the target means higher backscatter with higher returns.
The vegetation effects depend on the structure of the plant (type of the crop and the growth stage of the crop) and the presence of the water in the canopy at the molecular effect. The scattering can however be complex: multiple scattering from the canopy structure itself, direct scattering from the soil, or from the canopy, or multiple scattering from the soil and the canopy.
The canopies can attenuate or scatter microwaves, and also depends on the canopy components such as the wavelength itself.
The best frequency depends on the soil moisture and canopy (how deeper we want to penetrate)
V-polarized: couples with vertical structured vegetation and energy attenuated
H-polarized: greater penetration through canopy to underlying soil
Cross polarization: sensitive to target volume and not affected by row effects
HV or VH is best for either crop identification or crop biophysical estimation,
Incident angles: should not be mixed for temporal change detection