It can be called a worldwide problem in the field of radar, and relevant research is in the ascendant It is a cutting-edge technology and has become a breakthrough direction to improve the performance of target recognition It has broad application prospects in earth observation, disaster reduction and prevention, precision guidance and so on Radar is a great invention of mankind in the 20th century. Since its advent in 1935, its related technology and equipment have developed rapidly and been widely used. The principle of radar is not complicated. It uses electromagnetic wave to detect the target, receives and processes its echo, so as to obtain various information of the target, so as to achieve a certain detection purpose. With the development of radar today, although its performance has been continuously improved, its models are diverse and its applications are more and more widely, it has always developed around two major themes: first, continuously improve the survival and working ability of radar in complex environment; Second, continuously expand the ability to obtain target information, so as to improve the ability to distinguish, identify and recognize targets. The latter is technically mysterious and endless. It should not only be visible and clear, but also require complete, intuitive and clear target information elements. In this regard, full polarization radar imaging technology has become one of the most potential research directions. The so-called "full polarization" refers to a polarization mode of radar electromagnetic wave and the resulting multiple polarization states. It has an extremely important impact on improving the ability of radar target recognition. Through the unremitting efforts of researchers, the world-wide problem of full polarization radar imaging technology has made a certain breakthrough. Information perception, from "black and white image" to "color image" Similar to "B-ultrasound" for patients in hospitals, traditional single polarization radar imaging can only obtain the "black-and-white image" of the target. Full polarization radar imaging is like "color ultrasound". The image information obtained is richer and more intuitive. There are not only color information, but also delicate physical information, and the information capacity is doubled. Why can full polarization radar imaging do this? The secret is that it has the function of accurately obtaining the electromagnetic scattering information of the target and performing fine resolution, translation and recognition processing. Because when the target is irradiated by electromagnetic wave, there will be "variable polarization effect", that is, the polarization state of scattered wave will change relative to the incident wave. The specific mapping and transformation relationship between them is closely related to the physical properties of the target, such as attitude, size, structure, material and so on. In radar target recognition, if we can effectively perceive and reveal the "variable polarization effect" of the target, we can extract the rich physical information contained in the target, and then improve the performance of radar anti-jamming, target detection, classification and recognition. As a unique dimensional information, full polarization can describe the track characteristics of the vector end of electromagnetic wave electric field varying with time on the propagation section. It is the physical basis for obtaining the "variable polarization effect" of the target. The advantage of full polarization radar imaging in information perception is that it can not only obtain the full polarization scattering information of the target and environment by regulating the polarization state of transmitted and received electromagnetic waves, but also obtain the high-resolution radar image of the target and environment through radar imaging technology. However, to obtain high-quality target full polarization "color image" and perform fine processing, it also involves the cutting-edge technologies in the radar field, such as polarization measurement waveform, motion compensation, polarization calibration, radiometric calibration, scattering modeling, fine interpretation and intelligent recognition, which still has a long way to go. Target recognition, from "visible" to "clear" and "clear" Target recognition is often known as the "pearl on the crown" in the radar field, which is a scientific goal pursued by many researchers. After decades of development, radar technology has made great progress. Due to the profound changes of target, natural environment and electromagnetic environment, high-value target recognition is still a major technical problem in the field of radar detection. Full polarization radar imaging technology has become the key of target recognition. People often compare radar to a "thousand mile eye", but the information seen by the eye is often polysemy. It can be said that "the peak is formed on the side of the mountain, and the distance is different". For the same target, the radar images obtained from different perspectives may be significantly different. In some special cases, the radar images of different targets may show similarity. This is the scattering diversity of radar targets, and it is also a major technical bottleneck for radar target recognition. Therefore, through in-depth study of the electromagnetic scattering characteristics of radar targets, researchers have carried out a series of basic research and key technology research. For example, by mining and utilizing the scattering diversity of radar targets, the scattering mechanism of multi attitude targets in full polarization radar imaging is revealed, and the polarization recognition of multi attitude targets is realized. Through interdisciplinary research, promote the integrated development of fully polarized radar imaging, electromagnetic scattering cognition, artificial intelligence and other technologies, and promote the gradual leap of fully polarized radar imaging and target recognition technology from "visible" to "clear" and "clear". Of course, to achieve accurate, automatic and intelligent target recognition, especially high-value man-made target recognition in the confrontation environment, it is still "a long way to go", and scientists still need the continuous innovation of "I will seek from top to bottom". Innovative application can realize all-weather detection in land, sea, air and space Since the 1980s, researchers have made rapid development of polarimetric radar imaging technology through the organic integration of radar polarization and imaging radar. By deploying it on land, sea, air and space, as well as ships, satellites, missiles, UAVs and other platforms, all-weather detection has gradually become a reality, in earth observation, disaster reduction and prevention, space monitoring Battlefield reconnaissance and precision strike show broad application prospects—— Earth observation. The deployment of polarimetric imaging radar on spacecraft provides mankind with an unprecedented new perspective to understand our planet. In 2000, the United States used the imaging radar system carried by the space shuttle to realize global topographic mapping and obtain global elevation information for the first time. Since 2010, Germany has improved the accuracy of global topographic mapping by an order of magnitude by using the polarization imaging radar system carried by two satellites. In addition, ESA plans to launch a satellite equipped with full polarization imaging radar in order to achieve the accurate measurement of global forest biomass and contribute to the global carbon dioxide emission reduction plan. Disaster reduction and prevention. In recent years, natural disasters such as earthquakes, floods and tsunamis with great destructive power have occurred frequently. All polarization imaging radar can observe the earth all day and all day. It is not limited by the ground conditions. It has the advantages of fast, flexible and wide area. It is one of the most effective technical ways to quickly and comprehensively grasp the disaster situation in the disaster area. Since 2013, researchers have realized the research results of accurate estimation of building damage rate in wide area by using full polarization radar imaging and recognition technology. In addition, the full polarization imaging radar is expected to play an important role in the identification and early warning of biological disasters such as locust migration, and the deformation monitoring and early warning of high-value infrastructure. Space surveillance. "Space supremacy" is the strategic commanding height of the competition among the world's military powers. Space attack and defense confrontation has increasingly become the key to the victory and defeat of modern wars. As early as the 1950s, the United States began to develop large-scale ground-based polarization radar, and successfully identified that the second man-made satellite launched by the Soviet Union "had an angular reflector structure" in 1958. In addition, ground-based and sea-based anti missile radars, which are at the core of the US missile defense system, have full polarization radar imaging and recognition modes, which improves the recognition ability of missile warhead targets. Precision guidance. Precision guided weapons are the protagonist of modern information warfare. Full polarization radar imaging technology is also very useful in precision guidance. If polarimetric radar imaging and recognition technology is integrated into the radar seeker of precision guided weapons, the ability of automatic target detection, recognition and attack point selection can be greatly improved. It is reported that the "sulfur stone" missile developed by Britain is equipped with a polarization imaging radar seeker and adopts a circular polarization transceiver system, which can identify important targets such as tanks with attack value. The polarization imaging radar seeker developed by Germany adopts variable polarization transmitting and dual polarization receiving system, and has the ability to automatically identify and attack targets under complex conditions such as bad weather, strong clutter and electronic interference. (outlook new era)