What are the three different polarization modes of SAR?

1. What is SAR polarization?
Polarization: H horizontal polarization; V vertical polarization, that is, the vibration direction of the electromagnetic field. When the satellite transmits a signal to the ground, the vibration direction of the radio wave used can be in many ways. The currently used ones are:

Horizontal polarization (H-horizontal): Horizontal polarization means that when the satellite transmits a signal to the ground, the vibration direction of its radio wave is horizontal. Vertical polarization (V-vertical): Vertical polarization means that when the satellite transmits a signal to the ground, the vibration direction of its radio wave is vertical.

Electromagnetic wave transmission is divided into horizontal waves (H) and vertical waves (V), and reception is also divided into H and V. The radar system using H and V linear polarization uses a pair of symbols to represent the transmission and reception polarization, so it can have the following channels—HH, VV, HV, VH.

(1) HH - for horizontal transmission and horizontal reception

(2) VV - for vertical transmission and vertical reception

(3) HV - for horizontal transmission and vertical reception

(4) VH - for vertical transmission and horizontal reception

The first two of these polarization combinations are called similar polarizations because the transmit and receive polarizations are the same. The last two combinations are called cross polarizations because the transmit and receive polarizations are orthogonal to each other.

2. What are single polarization, dual polarization, and full polarization in SAR?

Single polarization refers to (HH) or (VV), which means (horizontal transmission and horizontal reception) or (vertical transmission and vertical reception) (if you are studying the field of meteorological radar, it is generally (HH).)

Dual polarization refers to adding another polarization mode to one polarization mode, such as (HH) horizontal transmission and horizontal reception + (HV) horizontal transmission and vertical reception.

Full polarization technology is the most difficult, requiring the simultaneous transmission of H and V, that is, the four polarization modes of (HH) (HV) (VV) (VH) exist at the same time.

Radar systems can have different levels of polarization complexity:

(1) Single polarization: HH; VV; HV; VH

(2) Dual polarization: HH+HV; VV+VH; HH+VV

(3) Four polarizations: HH+VV+HV+VH

Orthogonal polarization (i.e. full polarization) radars use these four polarizations and measure the phase difference between channels as well as the amplitude. Some dual-polarization radars also measure the phase difference between channels, as this phase plays an important role in polarization information extraction.

Radar satellite imagery In terms of polarization, different observed objects backscatter different polarization waves for different incident polarization waves. Therefore, space remote sensing can use multiple bands to increase information content, or use different polarizations to enhance and improve the accuracy of target identification.

3. How to choose the polarization mode of SAR radar satellite?

Experience shows that:

For marine applications, the HH polarization of the L band is more sensitive, while the VV polarization of the C band is better;

For low-scattering grass and roads, horizontal polarization makes the objects have greater differences, so the spaceborne SAR used for terrain mapping uses horizontal polarization; for land with roughness greater than the wavelength, there is no obvious change in HH or VV.

The echo strength of the same object under different polarizations is different, and the image tone is also different, which increases the information for identifying the object target. Comparing the information of the same polarization (HH, VV) and cross-polarization (HV, VH) can significantly increase the radar image information, and the information difference between the polarization echoes of vegetation and other different objects is more sensitive than the difference between different bands.
Therefore, in practical applications, the appropriate polarization mode can be selected according to different needs, and the comprehensive use of multiple polarization modes is conducive to improving the accuracy of object classification.