## 1 Introduction  
Synthetic Aperture Radar (SAR) imaging aims to characterize the interaction of the transmitted electromagnetic waves with scatterers in the imaged scene. In practice, the recorded SAR raw data do describe these interactions but also include a multitude of effects that are due to the SAR sensor hardware and the propagation of signals within it. These systematic, sensor induced effects are not related to the contents of the imaged scene and hinder a meaningful interpretation of the resulting imagery. Such issues are especially relevant in the case of advanced SAR sensors, with analog or even digital beam-forming (DBF), that employ antenna arrays: systematic errors at the level of the individual antenna array element will impair or even prohibit image formation itself, as the effective antenna illumination patterns will deviate from the expectation and lead to errors in SAR focusing. The continued interest in scientific and operational exploitation of SAR in a diverse set of remote sensing applications has lead to ever more stringent requirements regarding the calibration quality of SAR data delivered by modern sensors. At the same time, SAR sensors are becoming more complex and consequently more difficult to calibrate. This is especially true for new SAR imaging modes that involve simultaneous data acquisition in multiple channels for polarimetric imaging, single-pass interferometry or DBF. The task of SAR sensor calibration is therefore becoming simultaneously more important from an application point of view and more difficult in view of the underlying sensor complexity. This is especially true for airborne SAR sensors for research, which play an important role in the process of SAR sensor development and are often technological precursors of the next generation of satellite SAR sensors. The DLR's Microwaves and Radar Institute currently operates several airborne SAR sensors that cover a wide range of innovative imaging techniques, including polarimetric imaging, single-pass polarimetric multi-baseline interferometry and multi-channel along-track interferometry. This paper details a general calibration approach that was developed to meet the diverse requirements implied by these multi-channel SAR imaging modes. As such, the approach necessarily entails multiple steps that address calibration issues related to imaging geometry, radiometry and the polarimetric/interferometric inter-channel phase difference. SAR sensor calibration generally involves both internal calibration and external calibration [1]. Internal calibration is based on measurements performed internally by the SAR sensor. These can later be used to correct for distortions and transient drifts when radar pulses are transmitted or received. The discussion in this paper takes the internal calibration as a given and is mainly concerned with aspects of SAR sensor calibration that are outside the scope of internal measurements. These aspects make up the external sensor calibration, in which actual SAR measurements of reference targets with known properties are analyzed to refine an existing sensor model. These refinements are primarily concerned with the multi-aperture antenna array of the multi-channel SAR sensor under consideration. The relevant properties of reference targets, such as position, size and orientation, among others, must be well known such that the targets' expected radar return can be predicted with a high degree of accuracy for any given imaging geometry. External sensor calibration is then the process of estimating corrections to eliminate systematic deviations of the measured target response from the a priori expectation. The calibration technique discussed in this paper differs from existing techniques in that it is based entirely on range compressed raw data, instead of relying on the analysis of focused SAR imagery.