## 2 State of the Art  
A number of recent satellite SAR missions have involved extensive, well-documented and highly successful external calibration programs. Representative examples include the approaches developed for the TerraSAR-X [2] and TanDEM-X [3] missions, launched in 2007 and 2010, respectively, as well as the Cosmo-Skymed mission [4], with four satellites launched between 2007 and 2010, the Sentinel-1 mission [5], with satellites A and B launched in 2014 and 2016, respectively, and the Radarsat-2 mission [6]. Similar techniques have also been applied for the calibration of airborne SAR sensors, such the F-SAR sensor operated by the German Aerospace Center (DLR) [7; 8]. All of the above satellite missions feature highly complex radar instruments with phased-array antennas consisting of hundreds of individually controlled antenna elements to support advanced SAR imaging modes. Different imaging modes are facilitated by beam-forming techniques that combine signals from multiple antenna elements within the instrument. Here, amplitude and phase adjustments to individual signals change the effective antenna diagram while data are being acquired. This type of SAR sensor represents a considerable challenge in terms of calibration due to the large configuration space required to support a variety of imaging modes. In the case of the TerraSAR-X and TanDEM-X sensors, for instance, the operational imaging modes are defined in terms of more than 10,000 unique effective antenna patterns [9] (also known as 'beams'). Due to limited resources, in terms of time during the sensor commissioning phase and budget, it is neither possible nor desirable to attempt the individual characterization and calibration of these beams on the basis of SAR measurements acquired after the sensor has been deployed in orbit. Instead, large parts of the calibration effort are based on an antenna model [9; 10] that is able to accurately reproduce the effective antenna illumination pattern given the settings applied at the level of the individual antenna array element. This model can then be validated by comparing its predictions with actual SAR measurements for a comparatively tiny number of beams. Once validated, it is assumed to generalize to all other beams employed during operational data acquisition and may be relied upon to provide the detailed, propagation direction dependent antenna characterization that is used to compensate for systematic antenna-related effects during SAR data processing. A major focus of external calibration activities in the context of modern SAR missions is therefore the validation of the antenna model. Studies [9] and [11] outline the external validation of the antenna models used for the TerraSAR-X and Sentinel-1A SAR instruments, respectively. Similar calibration approaches have been used for the COSMO-SkyMed [12] and Radarsat-2 [13] instruments. These approaches are similar and comprise a set of techniques to assess the antenna diagrams in elevation and azimuth and estimate the antenna mispointing. Focused SAR imagery acquired over areas of homogeneous backscatters, typically the Amazon rain forest, are used to validate the elevation antenna diagram and estimate the elevation mispointing. The later estimate uses dedicated acquisitions employing a _notch_ antenna pattern. In azimuth, meanwhile, the validation of the diagram and the mispointing estimate are based on active calibration targets [14]. For polarimetric sensors, acquisitions with active targets can also be used for polarimetric cross-talk calibration [15]. In addition to antenna model validation, external calibration also entails the estimation calibration constants that account for constant offsets in gain, delay and phase that are mainly due to propagation effects within the instrument. These constants are estimated using reference targets with known properties, such that their expected response in SAR acquisitions can be predicted accurately.