## 1 Introduction
Finally, the reference function is utilized to complete the azimuth deramping operation similar to the methods in [19; 20; 21]. Apart from the azimuth aliasing problem, the spatial variances of GTLR-BiSAR data need to be solved. For the GTLR-BiSAR system, the time-varying relative positions of the GEO transmitter and LEO receivers bring about the following inconvenience: the targets with the same minimal bistatic ranges, but different azimuth positions, have different range cell migrations (RCMs) and different Doppler frequency modulation (FM) rates [11; 22]. This is because the targets confined in the same minimal range gate differ from each other in the minimal LEO range. As mentioned before, the moving LEO receivers provide most Doppler contributions and the RCMs are related to the moving station. Thus, the targets with the same RCMs and Doppler FM rates may spread in different bistatic range gates. That is the main reason for the image de-focus unless the traditional algorithm is improved. In order to solve the spatial variance problem in the BiSAR system, [PERSON] and [PERSON] [23] proposed the nonlinear chirp scaling (NLCS) method to identify the azimuth FM rates within the same range gate on condition that the bistatic angle was small. [PERSON] et al. [11] proposed a local fit method to improve the perturbation function and make the NLCS algorithm valid to the large bistatic angle case for the one-stationary BiSAR. Furthermore, [PERSON] carried out LEO-ground experiments [24] to take the squint angle of the transmitter into account, and the variety of the cubic phase term and FM rate in range was analyzed carefully. In fact, [PERSON] and [PERSON]'s BiSAR geometries were similar with the geometry of GTLR-BiSAR. However,their methods were both proposed for the stripmap working mode, the azimuth Doppler aliasing problem in SS and TOPS modes will make their methods invalid. The geometry and TOPS working mode in [25] are more relevant to our research, where Sentinel-1 serves as a non-cooperative by using the TOPSSAR technique and the software-defined radio (SDR) hardware works as a fixed receiver. Facing the azimuth Doppler aliasing problem, [PERSON] and [PERSON] [26] adopted the azimuth multichannel to recover the unambiguous signal. Their method was not suitable for the single-channel receiver operating in SS or TOPS modes. In brief, the existing methods were applied on condition that the signal was not aliasing in azimuth time and the frequency domain. Hence, in this paper, we put forward the azimuth variance correction strategy and modified the conventional chirp scaling function to solve the range variance problem in the range-Doppler (RD) domain based on their research. Then the azimuth compression was completed in the azimuth frequency domain with the idea of spectral analysis (SPECAN) processing. The remainder of this paper is organized as follows. Section 2 shows the imaging geometry and signal properties of the GTLR-BiSAR system; this is the basis of our proposed algorithm. Section 3 introduces the geometrical and mathematical methods to calculate the azimuth frequency modulation rate. The reference function is given to solve the azimuth Doppler aliasing problem. Section 4 describes our strategies to complete the azimuth and range variance corrections in the RD domain. Section 5 explicates the two-dimensional spectrum expression of the signal after the chirp scaling operation, and the azimuth compression is completed in the frequency domain. Section 6 shows the experiments of the simulated data. The correctness of our proposed processing algorithm has been verified. Furthermore, Section 7 introduces an equivalent real data experiment for demonstrating the proposed algorithm. Section 8 concludes the paper briefly.