The global single-point revisit is only 4.8 hours, and continuous imaging has set a world record, which is too advanced for senior three.

At 07: 47 on April 7, 2022, Beijing time, the Long March 4 C-38 carrier rocket carrying the 03-star "Gaofen III" (GF-3C) was launched from Jiuquan launch site 940 1, and the satellite successfully entered the scheduled orbit.

The launched "Gaofen III" 03 satellite is not an ordinary satellite. It belongs to the "high-resolution Earth observation system" national major science and technology special civil project. Gaofen-3 series satellites are the first C-band, multipole and high-resolution synthetic aperture radar (SAR) satellites in China. They have outstanding advantages such as all-weather monitoring, high-precision measurement and multi-mode imaging, and their main technical indicators reach or exceed the level of similar international satellites. To put it simply, the "Senior Three" series represented by the 03-star is a very powerful satellite, but it seems hard to say exactly where it is. We might as well learn more about this "unclear" satellite through several key knowledge points.

According to the imaging principle, remote sensing satellites are mainly divided into optical remote sensing satellites, radar remote sensing satellites and infrared remote sensing satellites, among which radar remote sensing satellites are mainly synthetic aperture radar (SAR) satellites. What is synthetic aperture radar?

Synthetic aperture radar (SAR) is a high-resolution microwave imaging radar. Based on the principle of synthetic aperture, pulse compression technology and signal processing method, a radar system for high-resolution remote sensing imaging in range and azimuth is obtained by using a real small aperture antenna.

Synthetic aperture radar (SAR) uses the relative motion between the target and the radar to complete the spatial sampling by a single array element, and replaces the wavefront spatial sampling set obtained by the array antenna with the echo time sampling sequence received by a single array element at different relative spatial positions. As long as the target is illuminated by radar beam or within the beam width, the target will be sampled and imaged. Using the trajectory formed by the relative motion of the target and radar, a synthetic aperture is formed to replace the huge real array aperture.

Simply put, through synthetic aperture radar imaging, the detected object is always in the imaging process from the beginning of being irradiated by radar beam to leaving the irradiation range of radar beam. Through a continuous imaging process, the imaging effect is equivalent to that of super-large aperture radar.

Gaofen-3 satellite is a radar remote sensing satellite equipped with C-band SAR. Electromagnetic waves are arranged according to the wavelength from long to short, followed by radio waves, infrared rays, visible light, ultraviolet rays, roentgen rays (X rays) and gamma rays (γ rays), and the energy is from low to high. In radio waves, microwave is called "UHF electromagnetic wave", with a frequency of 300 MHz-300 GHz and a wavelength of1mm-1m. In the field of radar or satellite communication, the radio frequency band is divided, in which the C band belongs to the category of microwave, with a frequency of 4.0- 8.0GHz and a wavelength of 75-37.5mm (. As a kind of microwave, C-band has the characteristics of easy bunching, high directivity and linear propagation, and its basic characteristics are penetration, reflection and absorption. For glass, plastic and porcelain, it can pass almost without being absorbed, but it will be absorbed and heated for water and food, and it will be reflected for metal. C-band is usually used for communication satellites, and the low-frequency C-band will not have rain attenuation similar to KU-band in rainy weather.

Gaofen-3 satellite is equipped with a large antenna with a length of 15 meters, and the resolution can reach 1 meter. It is the highest resolution C-band multipole satellite in the world.

Generally speaking, SAR satellites have many imaging modes, including strip imaging, scanning imaging, spotlight imaging, bistatic imaging, inverse synthetic aperture radar imaging, interferometric imaging, etc. These satellites have multi-mode imaging capabilities. Among these imaging modes, strip imaging, scanning imaging and spotlight imaging belong to the traditional imaging modes.

Stripe map means that with the movement of the satellite, the direction of the radar antenna remains unchanged, and it sweeps across the ground at a uniform speed to obtain continuous images. This mode images a strip on the ground, the length of the strip depends on the moving distance of the radar, and the resolution in the azimuth direction depends on the length of the antenna. Scanning imaging (Scan) means that the antenna will scan many times in the distance direction within a synthetic aperture time, so as to obtain a wider mapping bandwidth at the expense of azimuth resolution. The best azimuth resolution of this mode is equal to the product of azimuth resolution of stripe mode and the number of scanned stripes. Beamforming imaging means that when the satellite flies over or near the target area, the radar antenna constantly adjusts the beam direction to make it always aim at the area for illumination, thus simulating a wide antenna beam in a short time. Compared with strip imaging, it can greatly improve the resolution, but the detection range is only a limited focus area.

The 0 1 satellite of Gaofen-3 has 12 imaging modes, such as wave imaging and global observation imaging, on the basis of traditional imaging modes, and has expanded 10 experimental modes for specific applications in the subsequent operation process, becoming the SAR satellite with the most imaging modes in the world at that time. [1]03 satellite has increased the number of imaging modes to 20, and added three experimental modes: high-resolution wide imaging, moving target detection and ocean current measurement imaging, further expanding the satellite's application ability. [2]

Polarization is one of the essential properties of electromagnetic waves, and it is also one-dimensional important information besides frequency, amplitude and phase. The propagation and scattering of electromagnetic waves are vector phenomena, so polarization is used to study the vector characteristics of electromagnetic waves. The electric field vector of energy pulse emitted by radar can be polarized in vertical or horizontal plane. No matter what wavelength, radar signals can emit horizontal (H) or vertical (V) electric field vectors and receive horizontal (H) or vertical (V) or both echo signals. Monopolarization refers to horizontal transmission and horizontal reception (HH) or vertical transmission and vertical reception (VV). Dual polarization means adding another polarization mode at the same time, such as HH plus HV. Full polarization (multipolarization) is the most difficult, including HH, HV, VV and VH.

The polarization of electromagnetic waves is sensitive to the dielectric constant, physical characteristics, geometric shape and orientation of the target, so polarization measurement can greatly improve the ability of imaging radar to obtain various information of the target. Simply put, in the process of SAR earth observation and imaging, polarization technology can greatly enhance the satellite's ability to distinguish the types, changes and characteristics of things, distinguish different crops from cultivated land, and can be used to estimate vegetation height and attenuation coefficient, species identification, soil surface roughness and humidity, snow humidity and other biomass.

At present, radar polarization has become a mature technology, and the most difficult multi-polarization technology has been applied in the "Gaofen-3" series of satellites, which is one of the key supports for its multi-target and multi-mode detection capability across land and sea.

Gaofen-3 was launched at 0 1 on 201August 6 10. It is the first C-band fully polarized synthetic aperture radar satellite in China. 202 1 1 121October 23rd-Satellite launch; Now, after the satellite 03 is put into orbit, it marks the initial establishment of the first high-resolution microwave remote sensing satellite constellation in China, and the all-weather and all-weather microwave imaging capability has begun to take shape.

The "Gaofen-3" series of satellites fly in formation and are evenly distributed on the same orbital plane. It takes 99 minutes for three satellites to orbit the earth once. The maximum revisit period of global observation was shortened from 84 hours for a single satellite to Samsung 12 hours, and the average revisit period was shortened from single satellite 12 hours to Samsung's 4.8 hours. The same area can be revisited at least twice a day, with an average of five times a day, so that high dynamic imaging can be performed in the global area.

In 20 16, the "Gaofen-3" 0 1 satellite set a world record for single continuous imaging of high-power remote sensing satellites in hours, and the single continuous imaging time of the global observation mode of continuous ocean exploration of 02 and 03 satellites doubled, and the working time of SAR payload of 03 satellite increased from 30 minutes to 100 minutes, setting a new world record. [2]

"Gaofen-3" is firstly a synthetic aperture radar remote sensing satellite. C-band, multi-polarization technology, resolution of 1 meter. It has the advantages of all-weather monitoring, high-precision measurement and multi-mode imaging. The maximum imaging width is 650km, and the imaging working modes can be extended to 20. It is the C-band SAR satellite with the highest comprehensive performance in the world!

Author: Dabai Gaoguo

reference data

[1] quoted from China Academy of Space Technology/s/fbaplzhomm2180 yghek2g.

[2] Some quotations were published by Institute of Aerospace Information /is/NGCaXB5/?