The concept of radar was formed at the beginning of the 20th century and developed rapidly around the Second World War. The principle of radar is that the device is a transmitting antenna, which reflects electromagnetic waves in the direction of encountering objects through an electromagnetic energy that is emitted in the space direction; After the radar antenna receives the reflected wave, it processes it to the receiving equipment and extracts some information related to the object (the distance between the target object and the radar, or the change rate of the distance, radial speed, position, height, etc.). ). Continuous wave radar and pulse radar are divided into two categories. Because radar is easy to achieve accurate pulse positioning and receive echoes, the receiving antenna and the transmitting antenna can use the same auxiliary antenna during static pulse transmission, so it is in the main position in radar development. Measuring distance is to measure the time difference between the actually transmitted pulse and the echo pulse. Because the electromagnetic wave propagates at the speed of light, it can be converted into an accurate distance from the target. The goal is to position the antenna to use sharp beam position measurement. Elevation angle measurement on narrow beam. The target height can be calculated according to the elevation and distance. When there is relative motion between the radar and the target, the target frequency received by the radar is different from the frequency emitted by the radar, which is called Doppler frequency difference. One of the main information that can be extracted from Doppler frequency is the range change rate between radar and target. When the interfering target and clutter appear in the radar unit with the same spatial resolution at the same time, the Doppler radar uses clutter interference with different frequencies to detect and track the target. The advantage of radar is that it can detect long-range targets day and night, and it is not blocked by fog, clouds and rain. It has all-weather and omni-directional characteristics and certain penetration. Therefore, it not only becomes an indispensable military electronic equipment, but also is widely used in social and economic development (such as weather forecast, resource exploration, environmental monitoring, etc.). ) and scientific research (research on celestial bodies, atmospheric physics, ionospheric structure, etc. ), and airborne synthetic aperture radar remote sensing has become a very important sensor. The spatial resolution can reach tens of meters, regardless of the distance. Radar has shown good application potential in flood monitoring, sea ice monitoring, soil moisture survey, forest resources inventory, geological survey and so on.

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