## 1 Introduction  
Spaceborne synthetic aperture radar (SAR) is a kind of high-resolution microwave imaging technology which has many characteristics, such as all-time, all-weather, high-resolution and a long detection distance. It is a radar that uses the Doppler information generated by the relative motion between the radar platform and the detected target, and uses signal processing method to synthesize a larger antenna aperture [1, 2, 3]. As it can penetrate clouds, soil and vegetation, it has become more and more widely used in many important fields [4, 5, 6]. Recent publications have reviewed the applications of satellite remote sensing techniques for hazards manifested by solid earth processes, including earthquakes, volcanoes, floods, landslides, and coastal inundation [7, 8, 9]. The world's first SAR imaging satellite, SEASAT-1, was launched in 1978, proving the feasibility of microwave imaging radar in Earth observation [10]. Since then, many countries have scrambled to carry out spaceborne SAR research, and it has been used as an important means of Earth observation in recent years. For example, the Gaofen-3 satellite launched by China in 2016 is a C-band multi-polarization SAR satellite that can quickly detect and assess marine disasters, and it could improve China's disaster prevention and mitigation capabilities [11]. The Sentinel-B satellite launched by the European Space Agency (ESA) in 2016 and the previously launched Sentinel-A satellite belong to the Sentinel-1 SAR satellite mission [12]. Its purpose is to provide an independent operational capability for continuous radar mapping of the Earth with enhanced revisit frequency, coverage,timeliness and reliability for operational services and applications requiring long time series. In 2019, SpaceX successfully launched the Radar Satellite Constellation Mission (RCM) satellite with a Falcon-9 rocket at Vandenberg Air Force Base in the United States [13]. The satellite has a wide range of applications, including ice and iceberg monitoring, marine winds, oil pollution monitoring and response, and ship detection, etc. In recent years, natural disasters have occurred in various places, and many countries have put forward their requirements regarding the increased performance of SAR satellites. The future spaceborne SAR will develop in the direction of multi-band, high-resolution, and ultra-wide amplitude. The National Aeronautics and Space Administration (NASA) plans to launch the NASA--Indian Space Research Organization (ISRO) Synthetic Aperture Radar (NISAR) mission satellite in 2022 [14]. It will be the first satellite to use two different frequencies (L-band and S-band) to measure changes in the Earth's surface by less than 1 cm. The German Aerospace Agency plans to launch TerraSAR-NG satellites in 2025, which can achieve a resolution of 0.25 m [15,16]. Most of the above mentioned missions place high demands on the real-time performance of SAR data processing. The traditional processing method is that the satellite stores the raw data and sends them to the equipment on the ground for processing. With the continuous improvement of high-resolution and low-latency requirements for spaceborne SAR, traditional data processing methods cannot meet these needs. In recent years, the rapid development of large-scale integrated circuit technology has made on-board real-time processing of spaceborne SAR possible. This technology can greatly reduce the pressure of satellite-to-ground data transmission, improve the efficiency of information acquisition, and enable policymakers to plan and respond quickly. Spaceborne SAR real-time processing has become a research highlight in the field of aerospace remote sensing. In 2010, the ESA funded the National Aerospace Laboratory of the Netherlands to develop the next-generation multi-mode SAR On-board Payload Data Processor (OPDP) [17].