# An FPGA-Based Four-Channel 128k-Point FFT Processor Suitable for Spaceborne SAR
## 1 Introduction  
Spaceborne synthetic aperture radar is an important part of the field of space Earth observation. Because of its all-time and all-weather working capabilities, it has been widely used in many fields of national defense and the national economy, such as resource exploration and disaster monitoring [1; 2; 3; 4; 5]. Since the Seasat satellite of the National Aeronautics Administration (NASA) first demonstrated the ability of spaceborne synthetic aperture radar (SAR) to acquire high-resolution images of the world in 1978, many countries have begun to launch satellites carrying SAR payloads and to study spaceborne SAR imaging [6]. South Korea deployed its first SAR satellite KOMPSAT-5 in 2013, which has been used for all-weather and all-day observations of the Korean Peninsula [7]. Spain launched its first SAR satellite, PAZ, in 2018, mainly to meet the defense needs of the Spanish government [8]. The Canadian Missile Defense Agency developed a radar satellite constellation program [9] and launched three SAR satellites, including RCM 1, 2, and 3 in 2019, for detecting the Canadian mainland and the surrounding Arctic, Pacific, and Atlantic waters. China has developed a series of Yaogan SAR satellites, and recently launched Yaogan 33 in 2020, mainly for scientific experiments and disaster prevention and mitigation [10]. The United States proposed a Capella constellation plan consisting of 36 SAR satellites; Capella 1 was launched in 2018, and Capella 2, Capella 3 and Capella 4 will be launched in 2021 [11; 12]. Most of the abovementioned tasks require real-time SAR data processing. The traditional processing method is to download the data from the satellite to the ground station for processing. However, under the observation modes of high resolution and large data granularity, the downloading time for the massive data diminishes the effects of improved ground processing capabilities. Obviously, real-time on-orbit processing to extract theinformation can effectively solve the above problems [13]. A high-performance on-board SAR real-time processing platform needs to be equipped with a processor that meets many constraints such as power consumption, performance, and capacity. At present, Central Processing Unit (CPU), Digital Signal Processing (DSP), Graphics Processing Unit (GPU), Application Specific Integrated Circuit (ASIC), and Field Programmable Gate Array (FPGA) processors have all demonstrated their superiority and shortcomings in real-time processing. The CPU processor undoubtedly has powerful processing capabilities and strong versatility, but its huge power consumption makes it unsuitable for the strong power constraints of on-orbit real-time processing [14]. DSPs have strong design flexibility and are easy to develop and maintain [15]; however, in the case of spaceborne SAR with massive data processing requirements, it is difficult for DSPs to provide a sufficient processing power/watt ratio (FLOPS/watt). The GPU processor has a high computing peak and high-speed memory interface, but once customized, the hardware resources cannot be changed. ASIC can provide sufficient processing capacity by virtue of its fully customized design, but a large-scale complex logic design requires more complete design verification and a longer development cycle. In recent years, FPGA has made significant progress in on-chip resource storage, computing resources, and software and hardware co-design. Therefore, FPGA can meet the needs of large throughput and strict real-time processing, and has the advantages of strong applicability, parallel computing, strong compatibility and flexible design. Due to these significant advantages, FPGA has gradually become the main technology used in real-time processing systems [16; 17]. With the continuous improvement of SAR satellite observation capabilities and observation accuracy requirements of various countries, the observation angle of SAR satellites continues to increase, the observation range continues to expand, and the imaging resolution continues to improve [18; 19].