## 1 Introduction  
Bistatic forward-looking synthetic aperture radar (BFL-SAR) has drawn much more attention because of its high resolution in both range and the azimuth direction, which fills in the blanks of conventional monostatic SAR. The BFL-SAR has many advantages, such as low probability interception, and immunity to physical attacks and electronic countermeasures, which is why it has significant potential in airplane navigation, missile guidance, and target detection.  
Many studies have been made on BFL-SAR [1; 2; 3; 4]. The ground moving target (GMT) detection and imaging theories were discussed for BFL-SAR in [5; 6]. [PERSON] introduced the general situation and the method for BFL-SAR [7]. In addition, several experiments have been carried out and the results demonstrate the feasibility of BFL-SAR [8; 9]. Differing from monostatic SAR, the slant range in bistatic SAR has two hyperbolic functions, which are defined as double square root (DSR) terms [10], thus, the principle of the stationary phase (POSP) could not be directly used in BFL-SAR. To derive the 2-D frequency spectrum, some researchers transformed the DSR term into a single square root regarded as the monostatic configuration [11]. Raw data simulations for bistatic SAR were proposed in [12; 13], which are based on their 2-D frequency spectra. However, the platforms are moving horizontally without acceleration and vertical speed. [PERSON] introduced an advanced hyperbolic approximation method to accurately fit the cubic term of the range history [14]. Some researchers adopted hyperbolic equivalent methods with compensating variation to obtain the 2-D frequency spectrum [15; 16]. The equivalence error cannot be ignored, especially in a high-speed maneuveringplatform. [PERSON] proposed a new method for 2-D frequency spectrum derivation [17]. The limitation of [PERSON]'s formula is the reduced accuracy with a difference between two stationary points. In the application of missile terminal guidance and other maneuvering platforms, the platform moves with high velocities and accelerations [18]. The method of series reversion (MSR) can obtain the exact spectrum, theoretically [19; 20]. The accuracy mainly depends on the order of the expansion terms. [PERSON] et al. used a high-order range equation and phase compensation to improve the accuracy of the range history and 2-D spectrum [21].  
Based on previous research, this paper discusses an asymmetric bistatic configuration with a forward-looking receiver and a stationary transmitter, in which the slant range is kept up to its fourth-order Taylor series. Firstly, linear range walk correction (LRWC) operation is performed in the range frequency azimuth time domain. Secondly, the method of the series reversion and POSP are adopted in the derivation of 2-D frequency spectrum. Then the range compression, range cell migration (RCM), secondary range compression (SRC), and high-order range-azimuth coupling terms are compensated in the 2-D frequency domain. Finally, the azimuth compression is implemented by a matched filter in the range Doppler domain.  
This paper is organized as follows: In Section 2, the geometry and signal models are established. In Section 3, the 2-D frequency spectrum is derived by MSR. Section 4 explores the proposed imaging algorithm for BFL-SAR. Section 5 shows the simulation results and error analysis of proposed algorithm. The conclusion of this letter is given in Section 6.