**5. Discussion**

Taking the algorithm's efficiency, practicability, robustness, and precision into consideration, the simulation and experimental results have verified that the SSCF technique proposed in this paper is the most suitable method for image registration in THz InISAR imaging. Until now, there has been limited research on InISAR imaging with terahertz radars. This paper takes the lead in putting forward a THz InISAR imaging system, and it has carried out InISAR experiments in the laboratory environment. In the next stage, long-distance experiments that connect a traveling-wave tube amplifier will be carried out to verify the SSCF technique proposed in this paper.

#### **6. Conclusions**

In this paper, a translational motion parameter estimation method based on SSCF technique was proposed to achieve image registration in InISAR imaging under a low-SNR environment. The "angle glint" phenomenon and phase wrapping in motion measurement curves were solved, and the interference of noise was also removed with the rectangular filter operation in image domain. Based on this method, the estimation accuracy of translational motion parameters can fulfill image registration requirements when the SNR was larger than −30 dB. Both simulation and experimental results are presented to verify the validity of the proposed method. The method also can be extended to target with a more complicated motion feature. Until now, the experiments were carried out in a laboratory environment, but the work in this paper can provide support to the remote application of THz InISAR imaging systems in the future.

**Author Contributions:** Y.Z. was responsible for the theoretical work and performed the experiments. Y.Q. and H.W. provided the experimental environment and equipment. B.D. and Q.Y. revised the paper. The authors also wish to extend their sincere thanks to editors and reviewers for their careful reading and fruitful suggestions.

**Funding:** This work was supported by the National Nature Science Foundation of China (Grant No. 61871386). **Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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