This paper proposes an analytical formulation of the instrumental errors in multilevel hierarchic SAR architecture equipped with active phased array antennas. The basis of the study is the derivation of the so called post-calibration errors, which remain as residual error contributions after the application of a dedicated internal calibration procedure. The limitations of the current internal calibration approaches for the-state-of-the-art space-borne SAR missions are analyzed and alternative internal global calibration strategies are proposed to reduce the impact of the post-calibration error. Numerical simulations of instrumental errors are presented to evaluate the different proposed calibration procedures.

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