Recently Synthetic Aperture Radar Along Track Interferometric (ATI-SAR) has been successfully applied for traffic monitoring. In this work we treat the problem of detecting moving targets with unknown parameters (velocity and signal to clutter power ratio) from ATI-SAR images, exploiting a Generalized Likelihood Ratio Test (GLRT). We provide a closed form for the probability density function (pdf) of the log-likelihood ratio, for the threshold corresponding to an assigned probability of false alarm (PFA) and for the Receiver Operating Characteristic (ROC). The moving target detection capabilities are investigated on TerraSAR-X data provided by DLR.