# Ten Years of TerraSAR-X Operations
## 3 Evolution of SAR Product Portfolio
### Experimental Products  
The SAR instrument on both satellites TSX and TDX allows for the simultaneous reception of the transmitted SAR signals with the fore and aft segments of the antenna when using the redundant receiver chain in parallel with the primary one. The signals received by either antenna part are recorded in a dedicated channel and are then exploited accordingly on ground. Thus, data acquisition either in full polarization mode to obtain quad pol images or with separated phase centers to allow for along-track interferometry (ATI) is enabled in this dual-receive antenna (DRA) configuration mode. The way in which DRA data are recorded on-board necessitates complex signal demultiplexing operations on ground before any raw data processing can take place. The needed compensation of the relative instrument electronics phase drifts results in a challenging calibration approach. Due to this complexity, the DRA configuration is offered for Stripmap mode acquisitions only.  
In full polarization mode, the transmit polarization is toggled between V and H from pulse to pulse. The fore and aft antenna segments use different polarizations for the parallel receiving of the returned signals. The pulse repetition frequency (PRF) is doubled as for the dual polarization mode in the single-receive antenna configuration (SRA), leading to the same half range swath extent. Full (quad) polarization products are available in the same flavors as the SM basic products, i.e., as SSC and MGD, GEC, EEC SE/RE variants [14].  
In case of ATI acquisitions, the same polarization is used for transmitting and receiving. The separated phase centers of the fore and aft antenna segments generate a baseline in flight direction, which can be exploited e.g., for ground moving target indication (GMTI). Acquisitions may either use a high PRF setting, and thus a reduced swath width as for fully polarized acquisitions or a low PRF setting resulting in a range extent similar to the single polarization acquisitions. The high-PRF variant is more suitable for GMTI applications, such as traffic monitoring due to its wider unambiguous velocity measurement range. Thereby, the signal-to-clutter ratio is increased allowing for an improved tracking of moving objects in stationary clutter. The ATI products are available as SSC only. Besides the two separate images for the fore and aft antenna segments, the data are also processed to a nominal SRA SSC, and the delivered products thus contain three SSC channels, a SRA image, and the two DRA images.  
The effective along-track baseline between the two DRA phase centers of this mode is in the order of 1.2 m only. Thus, its unambiguous ATI phase cycle corresponds to up to 200 m/s Doppler velocity, and it is hence most suited for velocity measurements of fast moving objects. Besides being designed for traffic and vessel detection, it is still sensitive enough to also derive large scale ocean surface current velocity fields (see e.g., [21]). Experimental products from the DRA configuration are offered in DLR's Earth Observation Data Service EOWEB under the heading _TSX-1 Experimental Products_ in the collections _TSX-1.SAR.L1b-Stripmap-Quadpol_ (830 acquisitions) and _TSX-1.SAR.L1b-Stripmap-ATI_ (128 acquisitions), respectively.