It is well known that InSAR phase observations are only sensitive to the projection of the 3D displacement vector onto the radar line-of-sight (LoS) direction. We require at least three LoS observations to uniquely estimate the three displacement components, and the system of equations needs to have a full rank coefficient matrix. Unfortunately, in many practical situations, only two LoS observations are available at most (i.e., ascendingand descending), resulting in an underdetermined system

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... th an infinite amount of possible solutions. Yet, this has not prevented many authors from performing LoS decompositions that are fundamentally flawed. Starting with a mathematical framework based on linear algebra, we introduce the concept of the null line, the direction in which no displacements can be observed, and identify the strict criteria to perform decompositions and projections. We propose using a null-line aligned (NLA) referenceframe, which results in bias-free estimates. Based on a literature survey, we identify the most common flaws in handling 3D InSAR geometry, and develop a taxonomy to label different classes of fallacies. This work results in recommendations for a more optimal and uniform handling of InSAR geometry, in terms ofclaims and results.