Abstract: GNSS-based InBSAR system employs the GNSS satellites as transmitters of opportunity and a fixed receiver in the ground. Based on the principal of SAR imaging and repeat-pass interferometry, the system has the potential of both temporal and spatial consecutive monitoring of 3-dimensional deformation information of target area. This paper analyzed the physical ground of high degree of freedom on the performance improvement of multiangle imaging, multi-baseline interferomentry and topography for GNSS-based InBSAR. The simultaneously availability of multi-satellite and multi-constellation of inherited GNSS system enables the high degree of freedom for the system. The typical processing algorithms of multi-angle image fusion and 3D deformation retrieval as well as tomographic image formation were summarized and analyzed to account for the advantages of the system in the paper. The state of art of related latest research achievements of the system, as well as the future work, were also surveyed and concluded in the paper.