## 1 Introduction  
Bistatic synthetic aperture radar (BiSAR) has received increased interest throughout the world in the past decades due to its broad application prospects. Many researchers from different countries have carried out experiments with different platform pairs [1; 2; 3; 4; 5; 6]. There appears to be a novel kind of bistatic remote sensing configuration whose transmitter is placed on geostationary satellite (GEO) with receivers placed on the low earth orbit satellite(LEO) [7; 8; 9; 10], named the GTLR-BiSAR system. Compared with the monostatic case, the separation characteristic of GTLR-BiSAR has benefits, such as lower military vulnerability, saved costs, anti-interference, more abundant scattering information, and real-time monitoring, etc. [11; 12]. These advantages make the GTLR-BiSAR system an appropriate spaceborne system for Earth observation missions with high-resolution, wide-swath, high flexibility, and shorter revisit cycle characteristics [13; 14]. However, the time-varying relative positions of the GEO transmitter and LEO receivers bring challenges to the traditional imaging processing. On the one hand, the GTLR-BiSAR system operating in the sliding spotlight (SS) or terrain observation by the progressive scan (TOPS) mode achieve high spatial resolution or excellent coverage performance at the cost of the azimuth spectrum aliasing problem. On the other hand, the time-varying relative positions of the transmitter and receivers make the GTLR-BiSAR system suffer from serious azimuth and range variance problems. Based on these two aspects, imaging for the GTLR-BiSAR operating in SS or TOPS modes is rather complicated. Although existing time-domain back-projection algorithms [15; 16; 17] can serve most kinds of BiSAR cases, they are too inefficient to be good choices. Therefore, we concentrated our attention on the characteristics of the GTLR-BiSAR system and propose efficient imaging strategies for SS and TOPS modes in the frequency domain. With a geostationary satellite as an illuminator located at about 35,786 km above the Earth's equator, the GTLR-BiSAR system is characterized with excellent coverage performance and continuous observation ability. Since the transmitter is stationary, it provides little Doppler contribution during the signal transmission. After completing the preliminary work of time, space, and phase synchronization, the LEO passive receivers obtain bistatic echo by the beam steering technique with less cost consumption [18]. The receiver is moving relative to the illuminated scene, so it provides most Doppler contribution for the bistatic echo. Since the Doppler contribution distribution of the GTLR-BiSAR system is different from that of the traditional monostatic case, the corresponding imaging algorithm of the GTLR-BiSAR system needs to be proposed. For the GTLR-BiSAR system operating in SS or TOPS mode, there is a virtual rotation center for the beam steering vector, which extends the synthetic aperture time for high spatial resolution or increases beam forward and backward steering angles for a larger coverage area. Nevertheless, the azimuth Doppler bandwidth increases with the adjustment of the beam steering vector during the data acquisition, which causes the azimuth signal aliasing in the Doppler domain. [PERSON] et al. [19] and [PERSON] et al. [20] proposed the azimuth pre-processing methods for the SS mode in monostatic SAR data processing. [PERSON] [21] further proposed a unified azimuth pre-processing method for both SS and TOPS modes in the monostatic SAR system. Their research studies provide great reference values for our algorithm design, but the azimuth pre-processing method for GTLR-BiSAR data should be revised because of the Doppler contribution distribution difference. In this paper, we present the geometry of the SS and TOPS modes in the GTLR-BiSAR system first. Then the geometrical and mathematical methods are introduced to calculate the azimuth frequency modulation rate used in the aliasing removal pre-processing.