Wietske S Brouwer, Ramon Hanssen 

Deformation phenomena on Earth are inherently three dimensional. With InSAR, in many practical situations the maximum number of observations is two (ascending and descending), resulting in an infinite number of possible displacement estimates. Here we propose a practical solution to this underdeterminancy problem in the form of the strapdown approach. With the strapdown approach it is possible to obtain 3D-global/2D-local solutions, using minimal and largely undisputed contextual information. It is a generic method that defines a local reference system with transversal, longitudinal, and normal (TLN) axes, with displacement occurring in the transversal-normal plane only. Since the orientation of the local frame is based on the physics of the problem at hand, the strapdown approach gives physically more relevant estimates compared to conventional approaches. Moreover, with uncertainty in the orientation of the local frame and error propagation it is possible to assess the precision of the final estimates. Appropriate cartographic visualization using vector map with confidence ellipses enables an improved interpretation of the results.