## 1 Introduction  
In recent years, commercial low-orbit satellites were developed successfully. The number of low-orbit communication satellites, such as Starlink and Oneweb, will exceed ten thousand [1, 2, 3], and SAR-satellite constellations such as ICEYE and Capella SAR will exceed one thousand [4, 5, 6]. These satellites are distributed among low orbits whose height is from 300 km to 1100 km. For the safety of satellites, each satellite will be assembled with early-warning detection sensors, including microwave radars and cameras in the future. At present, space-borne SAR imaging has been successfully employed in earth observation to achieve all-time and all-weather imaging with high resolution [7]. By the reduction of antenna aperture and transmission power, the SAR also can be assembled on the satellite to make full-time imaging for surrounding space targets. Meanwhile, the mono-pulse radar has been successfully employed in searching and tracking space targets, obtaininghigh-precision dynamic trajectory of space targets, and estimating the speeds and positions for SAR imaging of space targets. Space targets are complex and diverse, such as space debris, decommissioned spacecraft, and satellites in orbit. Additionally, the orbit of low-orbit space debris is stable, the database of low-orbit space debris has been established [8; 9]. Therefore, this paper focuses on high-resolution imaging of space targets, such as spacecraft and satellites. Since the size of most space targets, such as satellites and spacecraft, is between 1 m and 50 m, the resolution of radar imaging needs to be between 1 cm and 10 cm to realize the identification and classification of space targets. Therefore, the bandwidth of SAR signals should be set between 1.5 GHz and 15 GHz. Higher frequency bands, such as X-band, Ka-band, and W-band, are generally designed for the imaging system to obtain more detailed information of space targets. At present, great progress has been made in detecting ground and sea targets using SAR data, but there are still many technical difficulties in using SAR data to detect air or space targets [10]. The main detection method for air targets is inverse synthetic aperture radar (ISAR) [11], and the main detection methods for space targets are optical systems and doppler radar [12; 13]. The key to high-resolution SAR imaging system is to generate and receive wideband radar signals. The frequency-doubling technology is used to generate wideband radar signals [14], but the reception of wideband radar signals still faces severe technical challenges, especially for space-borne radars. According to the [PERSON] sampling theorem, the sampling rate of ADC in the SAR receiver should be more than 7 gigabit samples per second (GSPS) for the radar signal with the bandwidth of 3.5 GHz, which brings great challenges to ADC sampling devices and greatly increases the hardware cost. At present, in addition to using ultra-high-speed ADC directly, a series of methods to sample wideband signals has been proposed [15; 16; 17; 18], which can be divided into the following three: Firstly, the de-chirp technology is able to convert wideband echoes into narrowband echoes, reducing the sampling rate of ADC [15]. However, the detection distance of de-chirp method is limited, and difficult to change. Thus, it is not suitable for high-resolution imaging of space targets at any distance; Secondly, the multi-channel receiving and stitching method in frequency domain is used to receive wideband signals, which utilizes multiple receiving channels to divide wideband echoes into multiple narrowband signals [16]. This method is complicated in hardware and stitching algorithms, so it is difficult to apply to a real-time space-borne SAR system.