TanDEM-X is a satellite formation consisting of two TerraSAR-X radar satellites designed to generate a high-accuracy global digital elevation model (DEM) meeting the HRTI-3 specification. The mission aims to provide a highly reconfigurable platform for demonstrating new radar imaging techniques. The system uses bistatic data acquisition with an innovative phase synchronization link and a novel satellite formation flying concept to collect bistatic data with short along-track baselines. The interferometric performance is analyzed, and an optimized DEM data acquisition plan is derived using multiple data takes with different baselines. Examples illustrate TanDEM-X's capabilities for developing new remote sensing applications. The mission includes four operational modes: bistatic, monostatic, alternating bistatic, and simultaneous transmit. The bistatic mode uses either TSX or TDX as a transmitter to illuminate a common radar footprint, with both satellites recording the scattered signal simultaneously. The pursuit monostatic mode avoids synchronization, while the alternating bistatic mode switches transmitters pulse-to-pulse. The simultaneous transmit mode splits the signal into two frequency bands to avoid RF interference. The mission's success depends on precise phase and time referencing, with synchronization pulses used to correct oscillator phase errors. The HELIX satellite formation enables stable interferometric mapping with a small number of formation settings. The orbit configuration and formation flying parameters are crucial for achieving the mission's objectives, with daily orbit maneuvers used to maintain a constant libration phase. The performance analysis considers coherence, SNR, quantization, and other error contributions to ensure high interferometric accuracy. The mission's success relies on maintaining a short along-track separation between the satellites to minimize temporal decorrelation and atmospheric disturbances. The system's design allows for flexible baseline selection and high-resolution SAR imaging, enabling the generation of a consistent global DEM with unprecedented accuracy.TanDEM-X is a satellite formation consisting of two TerraSAR-X radar satellites designed to generate a high-accuracy global digital elevation model (DEM) meeting the HRTI-3 specification. The mission aims to provide a highly reconfigurable platform for demonstrating new radar imaging techniques. The system uses bistatic data acquisition with an innovative phase synchronization link and a novel satellite formation flying concept to collect bistatic data with short along-track baselines. The interferometric performance is analyzed, and an optimized DEM data acquisition plan is derived using multiple data takes with different baselines. Examples illustrate TanDEM-X's capabilities for developing new remote sensing applications. The mission includes four operational modes: bistatic, monostatic, alternating bistatic, and simultaneous transmit. The bistatic mode uses either TSX or TDX as a transmitter to illuminate a common radar footprint, with both satellites recording the scattered signal simultaneously. The pursuit monostatic mode avoids synchronization, while the alternating bistatic mode switches transmitters pulse-to-pulse. The simultaneous transmit mode splits the signal into two frequency bands to avoid RF interference. The mission's success depends on precise phase and time referencing, with synchronization pulses used to correct oscillator phase errors. The HELIX satellite formation enables stable interferometric mapping with a small number of formation settings. The orbit configuration and formation flying parameters are crucial for achieving the mission's objectives, with daily orbit maneuvers used to maintain a constant libration phase. The performance analysis considers coherence, SNR, quantization, and other error contributions to ensure high interferometric accuracy. The mission's success relies on maintaining a short along-track separation between the satellites to minimize temporal decorrelation and atmospheric disturbances. The system's design allows for flexible baseline selection and high-resolution SAR imaging, enabling the generation of a consistent global DEM with unprecedented accuracy.