## 1 Introduction  
Synthetic aperture radar (SAR) has proven to be an indispensable sensor both in civil and military applications, which has been extensively studied in recent decades [1, 2, 3, 4]. Among different types of SAR system, geosynchronous SAR (GEO-SAR) can provide wide-area remote-sensing images with short revisit cycle, which has promising potential for advanced Earth observation missions [5]. Approximately one third of the Earth's surface can be imaged with fine spatial resolution and one-day revisit cycle [6]. Over recent decades, various aspects of GEO-SAR have been extensively investigated, including system and performance analysis [7, 8, 9, 10], mission design [11, 12], imaging algorithms [13, 14, 15, 16] and ionosphere/atmospheric effects and compensation [17, 18, 19]. In recent years, extensive works have been devoted to bistatic SAR system with GEO-SAR illuminator [20, 21]. In [6], the GEO-BiSAR system with airborne receiver (GEO-SA-BiSAR) was investigated, where the imaging performance and mission design were analyzed in detail. In [22], the azimuth aliasing characteristics of GEO-SA-BiSAR were analyzed and multichannel reconstruction technique was applied to suppress the ambiguity. Thereafter, the imaging method [23; 24] and moving target imaging [25; 26] of GEO-SA-BiSAR were further studied. On the other hand, the concept of GEO-LEO bi/multistatic SAR was analyzed in [27; 28; 29; 30], where multiple LEO satellites received the backscattered signal of GEO-SAR and achieved bi/multistatic SAR imaging. Enhanced spatial resolution and radiometric performance was achieved by GEO-LEO BiSAR. The receiving system onboard the LEO satellite can be designed with low cost and light weight [31; 32], which is favorable for the deployment of the LEO satellite constellation. In [33], a statistical model was proposed to evaluate the impact of the signal backscattered by LEO-SAR with a geostationary receiver, which can be used when designing the acquisition plan of the system. In [32], a constellation of geostationary-LEO multistatic SAR systems with LEO satellites working in transponding mode, named ConGaLSAR, was established. A typical ConGaLSAR system consisting of one geostationary SAR illuminator and 24 LEO transponders was put forward, which can achieve 3 m spatial resolution with 92.4 min revisit cycle. The ambiguity suppression for the constellation of GEO-LEO multistatic SAR was then analyzed in [34]. In addition, the azimuth multichannel signal reconstruction for ground moving target indication in GEO-LEO BiSAR system was studied in [35; 36; 37]. Due to the special configuration and echo characteristic, several imaging methods have been proposed for GEO-BiSAR [23; 38; 39; 40]. In [41], a weighted back-projection algorithm was proposed to deal with the azimuth nonuniform sampling for azimuth multichannel GEO-LEO BiSAR imaging. However, this method can only be used in a multichannel receiving system, and the computational complexity is quite high, which limits its application in sliding-spotlight GEO-LEO BiSAR. In [42], a 'non-stop-and-go' range model (NSGRM) for GEO-LEO BiSAR was put forward, which employs second-order and fourth-order polynomial range models for GEO transmitter and LEO receiver, respectively. An imaging method based on multireceiving and compressed sensing was then proposed. In [24], the echo characteristic of GEO-SA-BiSAR was first analyzed, and a frequency-domain imaging method using a range model based on one stationary equivalence (RMOSE) was proposed. However, the current range models and imaging methods for GEO-BiSAR generally introduce significant error during long integration time, which cannot be used for GEO-LEO BiSAR imaging in sliding-spotlight mode. The sliding-spotlight mode provides a valuable trade-off between resolution and azimuth swath-width using beam steering compared with stripmap mode and staring spotlight mode [43; 44; 45]. Plenty of imaging algorithms have been developed for high-resolution sliding-spotlight SAR data processing [46; 47; 48; 49].