## 5 Conclusions  
For the traditional L1 norm compressive sensing solution used to solve layover separation problem due to small number of baseline (two to three) has a poor scattering point of separation ability and low estimated stability, the proposed layover separation method based on multi-look compressive sensing uses multi-look to increase the dimension of measurement data, reduce the noise level, and improve the noise estimation of the stability. Moreover, the results of the L1 norm are close to those of L0 norm solution, with the measurement matrix satisfying incoherent characteristics and constraint conditions of isometric, increasing the awareness of the compression sparse reconstruction ability. In this way, the separation ability of the scatterers in layover areas of InSAR images and the stability of height estimation are improved, and the effectiveness of the method is verified by simulation and measured data.  
At the same time, due to the high computation time of multi-look compressive sensing, the multi-look compressed sensing method based on separable approximate sparse reconstruction adopts the iterative threshold method to improve the speed of processing, and can effectively reduce the operation time while maintaining the resolution ability of scatterers in layover areas and the height direction estimation accuracy, and provides the possibility for large-scale data processing, The effectiveness of the method is verified by simulation and measured data.  
In this paper, multi-look uses a fixed box to run the processing, which delivers certain height evaluation errors because of the uncertainty of the homogeneous region, so in future work we will use the adaptive rectangle boxes or adaptive irregular shapes for homogeneous pixels recognition to obtain better results using the two processing methods [28]. In addition, the results of the separable approximate sparse reconstruction method have a certain main lobe, which will have poor performance in scatterer separation in layover areas if the peaks of the main lobes in the two scatterers are not separable, and thus seeking more effective method for layover separation is also a topic of research [29]. Likewise, the computation time and stability of the method provide a set of contradictions, and finding ways to make the results more stable and faster is another area of future research. Furthermore, due to the small number of baselines in airborne InSAR system, the multi-look concept has an influence on image resolution--that is, the more efficient layover separation the proposed methods have, the lower resolution the SAR images have, which will hinder real application and lose the high resolution of airborne InSAR images, especially millimeter wave InSAR images. If data for the repeated passes of airborne InSAR system are well registered and the antenna phase center has a good processing method in millimeter wave InSAR systems, the two methods have broader application prospects, representing another research topic of interest for the future.  
Conceptualization, S.L. and F.X.; methodology, B.Z.; validation, B.Z. and L.W. ([PERSON]); formal analysis, S.L.; investigation, L.W. ([PERSON]); data curation, L.W. ([PERSON]); writing--original draft preparation, B.Z.; writing--review and editing, F.X.; visualization, L.W. ([PERSON]); supervision, S.L.; project administration, F.X.; funding acquisition, L.W. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This research was funded by Key Projects of the Ministry of Science and Technology of China.  
Not applicable.  
The authors would like to thank the editors and the anonymous reviewers for their valuable suggestions.