Chapter of Youth·Wisdom Begets All Things｜Xidian “Radar Man”: Wear “High-definition Corrective Lenses” on Satellites
China Youth Network, Beijing, October 15 (Reporter Song Li) It can achieve all-weather and all-day observations from space about 36,000 kilometers above the earth. In February this year, the world’s first high-orbit SAR satellite, Land Exploration 4-01, officially completed its on-orbit test and entered the operational stage of operation.
Since it was successfully launched into space by the Long March 3B carrier rocket on August 13, 2023, this high-orbit SAR (Synthetic Aperture Radar, referred to as SAR) satellite has steadily served the application needs of disaster prevention and reduction, land and resources survey, ocean, water conservancy, meteorology, agriculture, environmental protection, forestry and other industries.
In the National Key Laboratory of Radar Signal Processing of Xi'an University of Electronic Science and Technology (hereinafter referred to as Xidian), there is a young scientific research team that has been deeply involved in the field of SAR theory for decades. Their research results are the key to the high-orbit satellite's ability to clearly observe the ground.
Breaking the problem: Adjusting the focus of the "Eye in the Sky" 36,000 kilometers away
"Our technology can be compared to a high-definition corrective lens. Wearing it, the satellite can see clearer and more accurately." Xidian Professor Sun Guangcai vividly compared the innovative breakthrough proposed by the team: spaceborne SAR multi-domain affine scale transformation imaging theory.
This theory solved the problem of satellite imaging deformation and won the second prize of the Natural Science Award of the China Electronics Society Science and Technology Award. The Land Exploration 4-01 satellite carries the engineering results of this theory. This theory has been successfully applied to many domestic SAR satellites, significantly improving my country's satellite Earth observation capabilities.
"Traditional spaceborne SAR imaging is based on the ideal assumptions of straight-line radar flight and a flat surface, which is like looking at the world through a set of fixed-parameter glasses," Sun Guangcai explained. In space of 36,000 kilometers, the curvature of the orbit, the curvature of the earth, and the signal delay all change by orders of magnitude. Traditional spaceborne SAR will fail, and the images will be blurred, distorted, and ghosted, just like the world in the eyes of a person with myopia and astigmatism. "If you want to see more clearly, you must adjust the 'myopia degree' more accurately."
In order to put this "corrective lens" on the high-orbit satellite, Sun Guangcai and his team overcame three hurdles one by one: building an accurate three-dimensional model, characterizing the modulation relationship of space-time coupling, and designing a focusing algorithm.
"Once the model goes wrong, there will be no way to deal with it later." Sun Guangcai knew that the model was the key. But the biggest fear in building a model is to form a formula. He emphasized, "The difficulty in breaking through the model is not in calculation, but in thinking." To this end, they faced the complex geometric reality of orbital curvature, and built a three-dimensional model bit by bit that can accurately depict the complex motion of high-orbit satellites and the true shape of the earth, laying a solid geometric foundation for ultra-large-width imaging.
If low-orbit SAR imaging is to calibrate the focus on a flat road, then high-orbit SAR is equivalent to completing focusing simultaneously on a violently bumpy roller coaster. The team deeply analyzed the precise spatiotemporal coupling relationship between the continuous high-speed flight of satellites and large-bandwidth signal propagation, and clarified the resulting strong nonlinear signal modulation characteristics.
The model is accurate and the modulation characteristics are clear. In the end, a powerful "brain" is needed to process it all. The team innovatively proposed the core theory of "multi-domain affine scale transformation", which will reunify and accurately "focus" signals that have been "defocused" due to complex motion and environmental interference, and invented a new imaging processing method accordingly.
Inheritance: Four generations work together for a clear photo
This breakthrough did not happen overnight. Behind it was decades of hard work by Xidian’s “radar people.”
Since the establishment of Xidian National Key Laboratory of Radar Signal Processing, founder Baozheng has set scientific research goals that are “top of mind”, aiming at the international frontier while also solving the urgent needs of the country. In the 1990s, this Chinese radar leader defied public opinion and pioneered the SAR research direction at Xidian. There was no experimental equipment at the time, so Professor Bao led his team to develop formulas and build models. In 2000, when Professor Xing Mengdao of the School of Electronic Engineering founded the spaceborne SAR team, related research in the domestic spaceborne field had just begun.
In 2007, 23-year-old Sun Guangcai was admitted to Xidian University for graduate school and studied under Professor Xing Mengdao. At that time, the first domestically produced SAR satellite was launched, but the world's top level had already achieved sub-meter resolution. “The goals of the older generation have guided us in the direction.” Sun Guangcai still adheres to his mentor’s teachings, “just keep exploring.” Under the influence of his predecessors' "standing upright" spirit, he developed the scientific research habit of "looking at problems with questions" and "starting from underlying principles". Sun Guangcai recalled that during a seminar, he had a sudden idea: whether the spatio-temporal coupling of electromagnetic waves and satellite motion could be the key to breaking through the one-step-stop model and the narrow-band signal assumption. This bold idea eventually became an important core issue.
The real challenge comes when the country proposes to develop high-orbit SAR and sub-meter ultra-high-resolution satellites. In more than ten years of overcoming difficulties, the most unforgettable thing for Sun Guangcai was the end of 2020: a key model of satellite was about to be launched, and the team's new technology needed to be verified in orbit on this satellite.
During the first two weeks after entering orbit, traditional methods failed and the image remained out of focus. "Very desperate. Looking at the empty seats, I even wondered if my research was going in the wrong direction." Frustrated, the team reorganized the modulation model, signal model and modulation method, and investigated the problems one by one. On the 21st day, the first high-definition image finally popped up on the screen. "The quality of the image can be described as unprecedented. The goal is extremely clear, and no description in any language can be exaggerated." By the 28th day, the debugging of the first five high-definition images was completed. "If these five images are not available, the core scientific objectives of this satellite will not be achieved." Sun Guangcai lamented.
Such a tough battle can never be won by one person.
Today, led by key teachers such as Sun Guangcai, Xidian's "radar people" have formed an excellent young scientific research echelon, including young teachers who are rooted in theoretical research, as well as doctoral and master's students who are active in the front line of engineering. In this team, young people can also play leading roles in key positions.
Liu Wenkang, a doctoral supervisor born in the 1990s at Xidian, is one of them. In 2015, he just started studying for a doctoral degree, which coincided with the world's first high-orbit SAR satellite entering the preliminary demonstration stage. At that time, there were no similar engineering cases or mature algorithm references in the world, and all technical paths needed to be explored independently. As the technical leader of the ground imaging processing and correction software module, he faced the triple dilemma of "no data, no precedent, and no standards" and deeply engaged in research on SAR imaging in mid- and high-orbit orbits. Through massive simulation demonstrations, he innovatively implemented algorithms that had never been applied in the spaceborne field into engineering solutions, solving the imaging problems caused by long synthetic aperture times in high-orbit orbits. From project demonstration, algorithm development, to on-orbit application after the satellite is launched in 2023, he has participated in the complete process of this satellite. "Every step of its realization and progress will give me confidence." Liu Wenkang said.
Lan Pengwei, a student of Sun Guangcai and a post-1995 doctoral student, solved a difficult problem hidden in the details. During the joint testing and debugging stage of a certain model mission, he was the first to discover image ghosting and data mismatch problems that no one noticed. After repeated investigations and verification, the root cause of the problem pointed to hidden dangers in the hardware system, which is a typical cross-professional engineering problem. Faced with the reminder that it was “not within his scope of duties” and the lazy voice that said “the transformation cost is high and there is no need to delve into it further,” Lan Pengwei was determined not to compromise. He sorted out the hardware operation logic from scratch, and through dozens of simulation experiments and precise mathematical deductions, he completely reproduced the fault phenomenon and identified the root cause of the problem. He used rigorous data demonstration to promote iterative optimization of the hardware system, completely eliminating hidden dangers that affected the quality of satellite imaging. "Every technological breakthrough is the result of countless trials and errors, perseverance and deep cultivation." Lan Pengwei said.
Zhiyuan: From the laboratory to the frontline of people’s livelihood
The continuous efforts of generations of Xidian "radar people" have turned spaceborne SAR from a paper concept into a practical experience in space. This technology has also been tested in disaster emergencies.
In September 2022, a strong earthquake occurred in Luding, Sichuan. The earthquake area was shrouded in thick clouds, and optical satellites were helpless. At the critical moment, the "Chaohu-1" and "Haisi-1" low-orbit SAR satellites equipped with the Xidian team's core imaging processing technology were dispatched at the critical moment. They penetrated the clouds and obtained high-quality images of the earthquake area, providing key support for disaster assessment and rescue guidance. During the 2023 Turkish earthquake, this technology also participated in international disaster monitoring.
In the summer of 2025, the Yangtze River Basin suffered floods. The Land Exploration 4-01 satellite captured high-definition imaging of the entire Yangtze River Basin. During the same period, it also conducted continuous monitoring of the water bodies of Dongting Lake and Poyang Lake, providing accurate data for water resource dispatch.
Liu Wenkang described it this way: "Other satellites may take a day or two to transfer, but high-orbit SAR can obtain images within a few hours. It can continuously observe the same place for more than 12 hours a day, which is suitable for continuous monitoring of disasters."
However, Liu Wenkang also said frankly: "my country's satellite hardware level has ranked among the top in the world, but the iterative application of technology and the improvement of the industrial chain still need to continue to advance."
Facing the next twenty years, Sun Guangcai always has a sense of mission to serve the country through scientific research: "There are many technical problems existing in the industry. As a practitioner, I have the responsibility to do my best to solve them." At the same time, the team will also regard the inclusiveness of technology as an important direction, "so that the results of aerospace remote sensing can truly benefit the national economy and people's livelihood."
