The cooperative nature of the TanDEM-X mission bistatic operation allows the periodically exchange of information between transmitter and receiver over the SAR acquisition. With the information gathered during the synchronization (sync) events it is possible to perform the phase and time referencing necessary for the calibration of the bistatic SAR image. However, given that transmitter and receiver are dedicated to the link during the sync events, the reception of SAR signal is periodically interrupted leading to gaps in the SAR raw data. Consequently, the processed SAR image will be corrupted, especially when acquiring close to the Nyquist rate. The paper addresses different approaches based on parametric and non-parametric spectral estimation to reconstruct the missing data and presents results with SAR data from TerraSAR-X and TanDEM-X.