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This paper focuses on the bistatic synthetic aperture radar (BiSAR) signal processing in the spaceborne/airborne configuration. Due to the extreme differences in platform velocities and slant ranges, the airborne system operates in the inverse sliding spotlight mode, while the spaceborne system works in the sliding spotlight mode to achieve a tradeoff between the azimuth scene size and azimuth resolution. Such a mode is generally called double sliding spotlight mode. In this configuration, the echoed signal has two characteristics. Firstly, both transmitter and receiver have very short synthetic aperture times. Secondly, the airborne platform operates with wide squint difference, while the spaceborne platform works in the small squint case. According to these two features, we use different Taylor expansions to address the slant range histories of transmitter and receiver. Based on the presented model, a two-dimensional space-variant bistatic point target reference spectrum (BPTRS) is derived. Furthermore, we linearize the BPTRS to derive the transfer function of the baseband scene. From the transfer function, the signal features of the spaceborne/airborne configuration become very clear. Using the transfer function, the two-dimensional inverse scaled Fourier transform (ISFT) is used to focus the bistatic signal in the spaceborne/airborne configuration.