The advantage of radar is that it can detect long-distance targets day and night, and is not blocked by fog, cloud and rain. It has the characteristics of all-weather and all-time, and has certain penetration ability. Therefore, it has not only become an indispensable electronic equipment in military, but also widely used in social and economic development (such as weather forecast, resource detection, environmental monitoring, etc.) and scientific research (celestial research, atmospheric physics, ionospheric structure, etc.). Spaceborne and airborne synthetic aperture radar has become a very important sensor in remote sensing. Radar targeting the ground can detect the precise shape of the ground. Its spatial resolution can reach several meters to tens of meters, and is independent of distance. Radar also shows good application potential in flood monitoring, sea ice monitoring, soil moisture survey, forest resources inventory, geological survey and so on.