On the Limitations of InSAR Decomposition Imposed by Displacement Direction and Coordinate System

Satellite-based Interferometric Synthetic Aperture Radar (InSAR) is widely used to monitor ground deformation over large areas. However, each InSAR observation measures only the scalar projection of the three-dimensional displacement vector onto the satellite line of sight. Consequently, two independent ascending and descending line-of-sight observations can determine only two independent displacement components unless an additional physical constraint or external observation is introduced. This study examines the bias and noise sensitivity of two-geometry InSAR displacement decomposition when the additional constraint is expressed in different global and local coordinate systems. We formulate the decomposition in the East–North–Vertical and the slope-oriented Upwards–Transverse–Normal reference frames and analyze how the estimated components are affected by the true displacement direction, the orientation of the constrained axis, and its relationship with the null direction of the observation geometry. Synthetic numerical experiments are used to evaluate conventional zero-North, vertical–planimetric, slope-aligned, and null-direction-aligned decompositions. The results show that a constraint aligned with the null direction prevents the unobservable displacement component from biasing the two estimated components, whereas even small coordinate-system misalignments with respect to the true displacement direction may produce substantial errors when the constrained direction is poorly oriented with respect to the null direction. An interactive dashboard is provided to support geometry-specific sensitivity analysis and the selection of physically meaningful coordinate systems for practical InSAR applications.