Influencing Factors of Image Tone of Imaging Radar

The tone of radar image reflects the intensity of backscattered echoes of ground objects, with strong echoes being bright and weak echoes being dark (Figure 3-58). The intensity of echo depends on the interaction between ground objects and radar waves and the characteristics of imaging radar system, specifically, it is related to the working wavelength, depression angle and polarization mode, surface roughness, physical and electrical characteristics (dielectric constant, etc.) of imaging radar. ) and the corner reflector effect caused by the geometry of ground objects.

(1) Working wavelength of imaging radar

Figure 3-58 Schematic Diagram of Tone and Echo Intensity of Radar Image

See Table 2-3 for the working wavelength of imaging radar. The echo of the same object at different wavelengths is very different. Generally, the wavelength is short, the image resolution is high, but the penetration ability is poor; It has a long wavelength and a certain penetration ability, but the image resolution is worse than the former. In order to give consideration to the advantages of both, X-band (λ=3cm) and L-band (λ=23.5cm) radars are widely used at present. The working wavelength of radar also directly affects the surface roughness and dielectric constant of ground objects, thus affecting the echo intensity.

(2) Illumination depression angle of imaging radar

The relationship between radar echo intensity and depression angle is shown in Figure 3-59(c). With the increase of depression angle, the echo intensity increases. For smooth surfaces, the echo intensity is strong when the depression angle is large, and decreases rapidly with the decrease of depression angle. For rough surfaces, the echo intensity changes gently with the increase of depression angle.

(3) Polarization mode of imaging radar

Polarization refers to the electric field vector direction of electromagnetic wave at a given point in space in an oscillation period. Electromagnetic waves whose electric field vector direction does not change with time are called linearly polarized waves, which are decomposed into two directions: horizontal polarization (H) and vertical polarization (V). As shown in Figure 3-60, the horizontally polarized electric field vector is perpendicular to the incident plane (the plane containing the incident radar beam and perpendicular to the illumination plane), and the vertically polarized electric field vector is parallel to the incident plane.

Imaging radar has four polarization combination modes. If the polarization properties of radar transmission and reception are the same, it is called co-polarization mode, including HH and VV. If it is different, it is called cross polarization mode, which has two kinds: HV and VH. Different polarization combination methods lead to different echo intensities from the same object and different radar image tones. For example, HH echo of basalt is larger than HV echo, and HV image is darker. Multi-polarization remote sensing with controllable polarization imaging radar can greatly increase the ground information of radar images. Most ground objects can produce strong echo to horizontal polarization, and the resource remote sensing imaging radar mostly adopts HH mode, which produces the strongest echo signal. Most ground objects have the same polarization characteristics as incident waves, and a few ground objects will change the polarization characteristics of incident waves and produce echoes with another polarization characteristics. This phenomenon is called depolarization. It is precisely because some ground objects have depolarization that there will be cross polarization. Unclear rock boundary on HH image may be clearly displayed on HV image. There is no obvious difference in backscattering echo intensity and image difference between homopolarized HH and VV images in rough ground area, but the information of smooth ground HH image is not as rich as that of VV image.

Table 3-8 lists the parameters of four imaging radar systems.

Figure 3-59 Radar Echo of Smooth Surface and Rough Surface with Depression Angle

Figure 3-60 Horizontal Polarization and Vertical Polarization

Table 3-8 Imaging Radar Parameter Table

(According to Guo Huadong 199 1)

(4) Surface roughness of ground objects

The influence of surface roughness on radar echo is also obvious. As shown in Figure 3-59(a) and (b), the general rule is that glossy surface reflects all incident waves, the reflection angle is equal to the incident angle, and the direction is opposite, forming specular reflection without backscattering, and the image is black tone; The rough surface produces scattering (diffuse reflection) in all directions, and the backscattering echo is strong, and the image is light tone; The medium rough surface produces mixed reflection, backscatters the echo but is weak, and the image is gray tone with different shades. The surface roughness of ground objects can represent the scattering degree of the irradiated area, which is a relative quantity related to the working wavelength and incident angle of radar. According to Rayleigh standard, the ground is classified according to roughness, as shown in Table 39-.

Table 3-9 Classification Standard of Ground Roughness (φ = 45)

It can be seen that the surface roughness reflects the micro-fluctuation of the surface, which belongs to two different scales and two different concepts with the topographic fluctuation.

(5) Complex dielectric constant of ground objects

Complex dielectric constant is a complex constant that describes the electrical properties of the surface of an object, and its real part is expressed by dielectric constant. Ground objects have high dielectric constant, high reflectivity, strong echo, and light color tone, and vice versa. Generally, the dielectric constant of solid materials is less than 10, such as marble is 8; The dielectric constant of most anhydrous objects is less than 6, such as asphalt is 2.7, dry sand and soil is 2.0-5.0, and the dielectric constant of water is as high as 60-80. Therefore, the main factor affecting the dielectric constant of ground objects is the water content of ground objects, and the dielectric constant changes linearly with the liquid water content per unit volume. High water content, high dielectric constant, high reflectivity, strong echo and light image color. When soil and plants contain more water, the dielectric constant increases rapidly and becomes a good microwave scatterer. For example, the dielectric constants of fresh wheat seedlings, oilseeds and poplar branches reached 37.78, 45.46 and 19.2 1 respectively, indicating that plants have strong echoes. The measurement results of rocks show that the dielectric constant of all kinds of rocks has little change, and the rocks rich in iron and magnesium are higher. For example, the average granite is 4.77, gneiss is 5.22, sandstone is 4.80, diorite is 6.22, dolomite is 6.8-8.0, lava is 5.3-5.6, and hematite 14.2. The dielectric constant of mineralized rocks is between 10.73-27.99, which is obviously higher than that of surrounding rocks, and the radar echo is strong. However, from hills to mountainous areas, surface roughness is the factor controlling echo, and dielectric constant is difficult to become the dominant factor in lithology identification in a large range.

Dielectric constant is also related to the conductivity of ground objects and radar wavelength, which affects the penetration ability of radar waves to ground objects. Metals and ground objects with high water content have high conductivity and high reflectivity, and are the best scatterers. Cement and stone are medium scatterers; Dry wood is the worst scatterer because of its poor conductivity and low reflectivity. The shorter the radar wavelength, the shallower the effective penetration; The higher the soil water content, the shallower the penetration depth of radar wave and the greater the penetration depth of dry sand.

(6) corner reflector effect

The geometric shape of ground objects and its geometric relationship with radar beams also directly affect the echo intensity.

The dihedral angle and trihedral angle formed by building wall and ground, tree row and ground, etc. Form a natural corner reflector (Figure 3-6 1). When the symmetry axis of the corner reflector is consistent with the direction of the incident radar beam, almost all the incident radar waves can return to the radar system, and the echo is strong, and the image is very bright and light, such as bright spots or spots in towns and villages on the radar image, which is mainly the result of the corner reflector effect.

Linear objects, such as railways, highways, river banks, valleys, mountains and linear structures, will also produce strong echoes when the direction is orthogonal to the incident radar beam. Linear objects perpendicular to the radar view appear as striking bright lines on the radar image, while linear objects parallel to the radar view are not obvious on the radar image. When the normal direction of a plane object is consistent with the incident radar wave (φ=0), a strong echo is generated to form a bright tone. When the angle between them becomes larger, the echo weakens, the timbre darkens and even no echo is generated. This phenomenon is called directional effect (Figure 3-55). Due to the directional effect and the perspective contraction of the front slope, the hillside with small effective incident angle will produce strong echo, and the image color can range from light to white.

Imaging radar can image in all weather and all day, and it has certain penetration to clouds, light rain, vegetation and dry ground objects. The stereoscopic effect of its imaging enhances the terrain information, and has a certain enhancement effect on the linear structure and the ring structure. Spaceborne imaging radar can quickly image a large area with less geometric distortion. The preliminary geological application of imaging radar has made people discover many geological phenomena that have not been revealed by geology and other remote sensing means. The 1990s will be the golden age of radar remote sensing.

Fig. 3-6 1 schematic diagram of corner reflector