The TanDEM-X mission has the goal to deliver a digital elevation model (DEM) that fulfils HRTI-3 quality requirements on a global scale. The interferometric height is determined by the phase difference between the two acquired images and the spatial geometry. Baseline errors intrinsic of the bi-static SAR configuration combined with errors and drifts of the radar instrument introduce phase inaccuracies in the interferogram. Therefore, an accurate calibration of the interferometric system parameters and independent height references are required. In order to validate the DEM calibration concept with real interferometric data, a measurement campaign was carried out with the experimental airborne radar system (E-SAR) of the German Aerospace Center (DLR) in Oberpfaffenhofen. This paper will present some of the various results from these interferometric experiments, stressing on the quality assessment of the ICESat GLAS14 elevation data, which will be a key aspect in a successful TanDEM-X DEM generation.