## 1 Introduction  
The evolution of Synthetic Aperture Radar (SAR) technology is progressing from monostatic to bistatic configurations, driven by the need for more diverse remote sensing applications [1, 2]. In addition to providing navigation, positioning, and timing functions, the Global Navigation Satellite System (GNSS) also offers high-quality microwave sources with all-day coverage for most regions of the world, which have been considered illuminators of opportunity for passive radar applications [3, 4]. GNSS-based bistatic SAR (GNSS-BSAR) imaging is a research hotspot in the secondary utilization of GNSS signals [5, 6, 7, 8, 9]. This application can use numerous GNSS satellites as transmitters to perform radar function by receiving reflected echoes from the target area and obtain a two-dimensional radar image. Utilizing navigation satellites as the radar transmitters offers several advantages. The global coverage of the satellites ensures continuous illumination of any point on Earth's surface by GNSS signals. This capability is particularly useful for monitoring the safety of urban and suburban zones. In addition, the uniform distribution of GNSS satellites in space is conducive to selecting the optimal bistatic configuration to obtain appropriate radar resolution capabilities. Due to the timing function of navigation satellite systems, they have good time synchronization characteristics, which provides great potential for multi-static radar operations. The system offers cost-effectiveness as it requires only the design of the receiver, whose hardware is similar to that of standard navigation receiver chipsets. By modifying the signal processing algorithms within these chipsets, the device can be transformed into a low-cost radar receiver. The GNSS-BSAR system also has the characteristics of light weight, low power consumption, silent operation, and no electromagnetic pollution. This topic has been thoroughly investigated both theoretically and experimentally for many years, confirming its potential for radar imaging and its capability for area monitoring [10; 11; 12; 13; 14; 15; 16; 17; 18]. However, GNSS signals are not naturally suited for radar applications, presenting several challenges to be faced. The main problem is the extremely low landing power density of these signals, so in communication systems, the reflected signal of the target is usually considered as an undesirable multipath signal. GNSS-BSAR systems usually require long-time energy accumulation to obtain sufficient signal-to-noise ratio (SNR). However, under the long synthetic aperture time, the time-frequency error of the receiver and the influence of atmospheric effects are amplified and cannot be ignored. Because the GNSS signal is in the L-band, the phase error caused by atmospheric effects will be significant. The Earth's surface atmosphere is mainly divided into the troposphere and the ionosphere. The ionosphere, a plasma layer in the upper atmosphere filled with charged particles, significantly influences radio signal propagation [19; 20]. Ionospheric propagation introduces nonnegligible errors into the final SAR product, manifesting as phase dispersion, attenuation, Faraday rotation, and scintillation [21]. In bistatic SAR systems, azimuth imaging deterioration mainly results from aperture decorrelation, which is always introduced by scintillation phase errors [22]. Early successful examples of GNSS-BSAR imaging appeared in [10]. Scholars from the University of Birmingham studied signal synchronization and image formation algorithms for GNSS-BSAR imaging. The efficacy and applicability of the algorithms have been validated through experimental testing, where the receiver is moving or fixed on the ground. There are also researches on multi-static operation in GNSS-SAR. [PERSON] et al. [23] introduced a novel multi-static SAR configuration, where detailed system design, resolution analysis, and topology optimization were discussed. [PERSON] et al.