The TanDEM-X mission based on two satellites provides a radar interferometer in space with the goal to derive a global Digital Elevation Model (DEM) with never achieved quality for a global coverage: a global DEM with a relative height accuracy of 2m and 10m absolute. In order to achieve this mission goal, the distance between both satellites, the so called baseline has to be known extreme precisely. Only then, systematic baseline errors can be detected and compensated for, i.e. an accurate calibration of the global DEM can be ensured. The paper describes the procedure and the results of calibrating the baseline, verifying the outstanding accuracy of this calibration procedure.