The success of current spaceborne Synthetic Aperture Radar (SAR) is boosting the performance requirements of next generation systems. In order to cope with the evolution of SAR the design of the new systems will need to meet higher requirements for spacial resolutions and higher coverage. This tendency is recognized nearly independently of the application area and manifests itself through several study programs initiated by space agencies aiming at the design of future SAR systems. A review of several ongoing studies shows that a promising candidate for the next generation SAR systems is a multi-channel radar utilizing digital beam-forming (DBF) techniques, which allows to overcome the fundamental high resolution/wide swath limitations of the conventional SAR systems. In this context the Microwaves and Radar Institute of German Aerospace Center is conducting the research and development of the innovative concepts and advanced operational modes for the future spaceborne SAR systems. The new imaging techniques and technologies have the potential to considerably enhance the imaging performance of future systems compared to the state-of-the-art SAR sensors like TerraSAR-X/TanDEM-X, Radarsat-2 or Sentinel-1. In the framework of these activities a modular reconfigurable multi-channel DBF radar demonstrator operating at multiple frequency bands was developed in the Institute. This paper describes the architecture and calibration of the system. Furthermore, the first measurement results will be presented.