###### Abstract  
In the bistatic synthetic aperture radar (BiSAR) system, the unavoidable frequency deviation between the oscillators (USOs) will result in additional phase modulation in the demodulated radar signal, which significantly degrades the quality of the SAR image and digital elevation model (DEM) product. The innovative L-band spaceborne BiSAR system LuTan-1 (LT-1) employs a non-interrupted synchronization scheme to acquire the synchronization phase error. This advanced phase synchronization scheme avoids interrupting the normal BiSAR data acquisition and further increases the synchronization frequency. However, some non-ideal factors in the transmission link like attenuation, multipath effect, interference, etc., may cause the synchronization phase to be polluted by noise. A phase denoising approach based on compressive sensing (CS) is proposed to improve the accuracy of synchronization phase. The imaging phase with high signal-to-noise ratio (SNR) is input into the K-SVD algorithm to learn the prior information, and then the noise of the synchronization compensation phase is eliminated by maximum a posteriori (MAP) estimation. The data acquired from the ground validation system of the LT-1 synchronization module are adopted for the validation experiment. The proposed phase denoising method achieves higher phase synchronization accuracy compared with traditional ones. The processing results verify the effectiveness of the proposed method and demonstrate its potential for future on-orbit applications of the LT-1 mission. b 28 July 20228 July 20228 September 20228 September 20228 September 20228 July 20228  
As an important part of the medium- and long-term development plan for China's civil space infrastructure, LT-1 is an innovative spaceborne L-band BiSAR Earth observation mission designed to acquire highly accurate global DEM and monitor surface deformation [9; 14]. LT-1A and LT-1B satellites have been successfully launched on 26 January and 27 February 2022, respectively. Both satellites are equipped with an advanced full polarization SAR system and are able to acquire SAR image independently by a single satellite [9; 15]. In the LT-1 mission, two satellites operate in the flying around mode first, then the following up mode. In the former mode, LT-1A and LT-1B satellites fly in a close formation for highly accurate DEM generation through bistatic interferometry. In the latter mode, the two satellites are distributed with a phase difference of 180\({}^{\circ}\) in the same orbital plane in order to monitor the surface deformation through multi-flight differential interference technology [16]. The high precision DEM and surface deformation products produced by LT-1 will serve the research and applications including land and resources, earthquake, mapping, environment, disaster reduction, and forestry. Furthermore, the LT-1 mission will demonstrate some state-of-the-art SAR technologies on orbit, such as advanced nonlinear frequency modulation (NLFM) waveform and distributed SAR imaging [16; 17; 18]. However, the advantages of the BiSAR system brought by the separation of transmitter and receiver accompany new challenges, especially in synchronization [19]. The synchronization problem mainly consists of three parts: beam synchronization, time synchronization, and phase synchronization [20]. Beam synchronization means that the transmitting and the receiving antennas must point at the same ground footprint simultaneously. Time synchronization refers to that the receiving windows of two satellites must be strictly aligned. Since the pulse repetition frequency (PRF) timing is derived from different oscillators (USOs), any deviation of the clock source will lead to the desynchronization of the receiving windows. This time synchronization error accumulated over time risks reducing the range width and defocusing the image.