This paper presents a novel approach in the instrumental error analysis of Synthetic Aperture Radars (SAR) based on active phased array antennas (APAA). A residual post-calibration error exists due to differences between the calibration and operational excitations of the transmit/receive modules (TRMs) for those source errors that are monitored by the internal calibration facility. Analytical expressions are derived for the residual post-calibration errors and validated through intensive simulations of some instrumental errors for a specific sensor architecture. Alternative internal global calibration strategies are proposed to reduce the impact of the post-calibration error. The simulations show that the proposed elevation dependent calibration can provide a substantial reduction of the post-calibration errors even in the case of relaxed tolerance of the TRM weight setting errors.