The global single-point revisit only 4.8 hours, continuous imaging set a world record, too advanced Gaofen3

Beijing time on April 7, 2022 07:47, carrying the "Gaofen-3" 03 star (GF-3C) of the "Long March 4 C" remote thirty-eight launch vehicle, in the Jiuquan Launch Complex 9401 station launch, the satellite successfully into the intended

The satellite was launched by the Long March 4C Remote 38 Launch Vehicle at 9401 station of Jiuquan Launch Complex.

The launch of the "Gaofen-3" 03 star is not a general satellite, which belongs to the "high-resolution earth observation system" national major scientific and technological projects in the civilian field, "Gaofen-3" series of satellites is the first of its kind in China. The "Gaofen-3" series of satellites is China's first C-band, multi-polarized, high-resolution synthetic aperture radar (SAR) satellites, with all-weather monitoring, high-precision measurements, multi-modal imaging and other outstanding advantages, the main technical indicators to meet or exceed the level of international satellites of the same type. Simply seems to 03 star as the representative of the "high three" series is a very powerful satellite, but where exactly powerful and seems to be difficult to say clearly, may wish to pass a few key knowledge points to further understand this type of "unknown" satellite.

In accordance with the imaging principle, remote sensing satellites are mainly divided into optical remote sensing satellites, radar remote sensing satellites, infrared remote sensing satellites and other three types, of which the radar remote sensing satellites to synthetic aperture radar (SAR) satellites. What is SAR?

Synthetic Aperture Radar (SAR) is a high-resolution microwave imaging radar, which utilizes the synthetic aperture principle, pulse compression technology and signal processing methods to obtain a real small aperture antenna to the distance and azimuth direction of the two-way high-resolution remote sensing imaging radar system.

Synthetic aperture radar utilizes the relative motion of the target and radar to complete the spatial sampling through a single array element, and replaces the wavefront spatial sampling set acquired by the array antenna with the echo time sampling sequences received by the single array element at different relative spatial positions. As long as the target is illuminated by the radar beam or lies within the beamwidth, the target will be sampled and imaged. A synthetic aperture is constructed using the trajectory formed by the target's relative motion to the radar to replace the large, true array aperture.

Simply put, through the synthetic aperture radar imaging, the detection of the object from the beginning of the radar beam irradiation to leave the radar beam irradiation range, the period has been in the imaging process, through a continuous imaging process to achieve equal to the same super-large aperture radar imaging effect.

"Gaofen-3" satellite is equipped with C-band SAR radar remote sensing satellite. Electromagnetic waves in accordance with the wavelength from long to short, in order of radio waves, infrared, visible light, ultraviolet, roentgen rays (X-rays), gamma rays (γ-rays), the energy from low to high. Among the radio waves, microwaves are called "ultra-high frequency electromagnetic waves", with a frequency of 300MHz-300GHz, and a wavelength of 1mm to 1 meter. In the field of radar or satellite communications, radio frequency bands are divided, of which the C-band belongs to the category of microwaves, with a frequency of 4.0-8.0GHz, and a wavelength of 75-37.5mm (centimeter wave). 37.5mm (centimeter wave). As a microwave, C-band has the characteristics of easy to gather into a beam, highly directional and straight line propagation, the basic nature of the performance of penetration, reflection, absorption, for glass, plastics and porcelain, almost through without being absorbed, for water and food, etc. will be absorbed by the heat, and for the metal will be reflected. C-band is usually used in communication satellites, low-frequency C-band in cloudy and rainy weather will not be similar to KU band rain fade phenomenon.

Gaofen-3 is equipped with a 15-meter-long antenna with a resolution of up to 1 meter, making it the world's highest-resolution C-band, multipolarized satellite.

In general, SAR satellites have a variety of imaging modes, including strip imaging, scanning imaging, cluster beam imaging, dual-station imaging, inverse synthetic aperture radar (SAR) imaging, interferometric imaging and other types of satellites, which are equipped with multi-mode imaging capabilities. Among these imaging modes, strip imaging, scanning imaging and cluster beam imaging are traditional imaging modes.

Stripmap (Stripmap) refers to the movement of the satellite, the radar antenna pointing to remain unchanged, by sweeping through the ground at a constant speed to get a continuous image of the mode of a strip of the ground imaging, the length of the strip depends on the radar moving distance, azimuthal resolution depends on the length of the antenna. Scan refers to a synthetic aperture time, the antenna will be scanned multiple times along the distance direction, obtaining a wide mapping bandwidth at the expense of azimuthal resolution, the best azimuthal resolution of this mode is equal to the azimuthal resolution of the strip mode and the product of the number of scanned strips. Spotlight imaging (Spotlight) refers to the satellite in the flyover near the target area, the radar antenna by constantly adjusting the beam pointing so that it is always aimed at the region for irradiation, can be simulated in a short period of time a wider antenna beam, compared with the strip imaging can greatly improve the resolution, but the detection range is only limited to the focus area.

Based on traditional imaging modes such as strip imaging and scanning imaging, Gaofen 3 Star 01 has 12 imaging modes such as wave imaging and global observation imaging, and has expanded more than 10 experimental modes for specific applications during subsequent operation, making it the world's largest synthetic aperture radar (SAR) satellite in terms of imaging modes. satellite. The [1]03 star increased the imaging modes to 20, and added three new experimental modes, such as high-resolution wide-area imaging, moving target detection and ocean current measurement imaging, further expanding the satellite's application capabilities. [2]

Polarization is one of the intrinsic properties of electromagnetic wave, which is another important one-dimensional information in addition to frequency, amplitude and phase. The propagation and scattering of electromagnetic waves are vector phenomena, and polarization is used to study this vector characteristic of electromagnetic waves. The electric field vector of a pulse of energy emitted by a radar can be polarized either vertically or horizontally. Regardless of the wavelength, a radar signal can transmit either a horizontal (H) or vertical (V) electric field vector and receive a return signal that is either horizontal (horizontal,H) or vertical (vertical,V) or both. Unipolarized means horizontally transmitting horizontally to receive (HH) or vertically transmitting vertically to receive (VV). Dual polarization is the addition of one mode of polarization along with another, such as HH plus HV. full polarization (multi-polarization) is the most difficult and contains four modes, including HH, HV, VV, and VH.

The polarization of electromagnetic waves is more sensitive to the dielectric constant, physical properties, geometry and orientation of the target, and thus polarization measurements can greatly improve the imaging radar's ability to acquire various information about the target. Simply put, in the process of SAR imaging of earth observation, polarization technology can greatly enhance the ability of the satellite to distinguish between the types of things, changes, features, can distinguish between different crops cultivated land, can be used for vegetation height, attenuation coefficient and other biomass estimation and species identification, can be used for the estimation of the roughness of the soil surface and the humidity, snow humidity and so on.

At present, radar polarization has become a mature technology, and the "Gaofen-3" series of satellites have applied the most difficult multi-polarization technology, which is one of the key supports for its ability to detect multiple targets and modes across land and sea, and in multiple fields.

"Gaofen-3" 01 star launched on August 10, 2016, is China's first C-band fully polarized synthetic aperture radar satellite; November 23, 2021 02 star launched; now 03 star into orbit, marking China's first high-frequency microwave remote sensing satellite constellation initially built, all-weather, All-weather microwave imaging capability of the earth has taken shape.

"Gaofen-3" series of satellites using formation flight, uniformly distributed in the same orbital plane, three stars around the Earth for 99 minutes. The maximum revisit cycle of global observation is shortened from 84 hours for a single star to 12 hours for three stars, and the average revisit cycle is shortened from 12 hours for a single star to 4.8 hours for three stars, and the same region can be revisited a minimum of two times a day, and an average of five times a day, which allows high dynamic imaging in the global region.

In 2016, "Gaofen 3" 01 star was launched on the launch of a high-power remote sensing satellite single continuous imaging of nearly the order of hours of the world record, 02, 03 star ocean continuous detection of the global observation mode single continuous imaging time has been extended by a factor of two, 03 star of the SAR load single circle working time from 30 03 star SAR load single circle working time increased from 30 minutes to 100 minutes, once again set a new world record, can realize the global sea long time continuous monitoring. [2]

"Gaofen-3" is first of all a type of synthetic aperture radar remote sensing satellites, using C-band, with multi-polarization technology, resolution of 1 meter, with all-weather monitoring, high-precision measurements, multi-mode imaging and other advantages, the maximum imaging width of 650 kilometers, the imaging mode of operation can be expanded to 20

The C-band SAR satellite has the highest comprehensive performance in the world!

Author: Dabai Gao Guo

References

[1] Quoted from the China Academy of Space Technology (CAST) /s/FBaplZFHOmM2180yGhEk2g

[2] Partially quoted from the Institute of Space and Astronautical Information (ISAI) /is/NGCaXB5/?