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In many radar applications extended targets appear together with point targets. An important problem in particular for tracking algorithms is to rapidly detect these extended targets and to determine their centroid and shape. We present a detector for extended targets and maximum-likelihood (ML) estimators for the centroid and the extension which are based only on the outputs of the generalized monopulse ratio. The generalized monopulse ratio is based on the sum and difference beams generated from the digital outputs of arbitrary subarrays of a planar array. It is therefore applicable to various kinds of modern array antennas. The extended target is characterized by an ellipse in the azimuth-elevation plane. The statistical performance of the centroid and the extension estimators is characterized by analytically their mean and variance. Numerical simulations indicate that these statistical characterizations are accurate and that small extended targets can be detected early. The statistical performance measures can therefore also be used for parametric system studies.