# Ten Years of Experience with Scientific TerraSAR-X Data Utilization
### The TerraSAR-X Mission  
Since the late 1970s, the German Aerospace Center (DLR) has supported a long and successful technology development line for high frequency X-band SAR systems. On 15 June 2007, a key milestone was achieved when Germany's first operational radar satellite TerraSAR-X was launched into space. Operational product ordering and delivery was opened to scientific and commercial users on 1 January 2008 after a six month commissioning phase. TanDEM-X, a twin-satellite to TerraSAR-X, was launched in June 2010. Both satellites fly in a close formation enabling single pass interferometry [21] and serving two purposes. The TerraSAR-X mission utilizes one of the satellites when just a single acquisition is needed, while the TanDEM-X mission simultaneously acquires two images from the same area on the ground using both satellites. The main goal of the latter has been the generation of global, consistent and high-quality digital elevation models (DEMs) [21]. The SAR sensors of TerraSAR-X and TanDEM-X operate at X-band with 9.65 GHz center frequency. Depending on the SAR imaging mode, the transmitted bandwidth is 55 to 100, 150, or 300 MHz [22]. The satellite was positioned in a polar sun-synchronous dawn-dusk orbit with an inclination of 97.44 degrees and an altitude of 515 km. The repeat cycle is 11 days, with ground track repeatability within \(\pm\)250 m for the TerraSAR-X satellite. One repeat period consists of 167 consecutive orbits. TerraSAR-X features different modes and polarizations. The SpotLight, StripMap, and ScanSAR-modes provide high-resolution SAR images for detailed analysis as well as wide swath capability whenever a larger coverage is required. Operational imaging is possible in single and dual polarization. Quad-polarized data are available on an experimental basis only. Beam steering enables observations at different incidence angles and double side access can be realized by satellite roll maneuvers. Within the StripMap and ScanSAR modes, a continuous data take of up to 1500 km until the flight direction can be realized. This is not possible in the Spotlight modes. Here, the sensor rather focusses on a predefined position on the ground during the overflight. This enhances the synthetic aperture and, thus, provides a high spatial resolution in azimuth. The range bandwidth resolution is 150 MHz for Spotlight mode. The High Resolution Spotlight and the Staring Spotlight are acquired with the 300 MHz variant. The mission's potential was upgraded in 2013 when two new modes were implemented. The Staring Spotlight capability increases the spatial resolution in azimuth to 24 cm. The new WideScanSAR mode (6 ScanSAR beams) extends the spatial coverage to 200 km. The characteristics of the operational ScanSAR and StripMap modes are provided in Table 1, the specifications of the Spotlight mode are displayed in Table 2.  
The TerraSAR-X Ground Segment [24] controls and operates the satellite platform as well as the SAR sensor [25], performs instrument calibration [26], acquisition and archiving of the SAR data and the generation of user-defined products on request [22]. The Mission Control Center is located at DLR in Oberpfaffenhofen, Germany. The central receiving station for the TerraSAR-X mission is at DLR in Neustrelitz, Germany. Additionally, commercial data requests are downloaded at direct-access stations. The demand for TerraSAR-X products increased over the years and data downlink became a limiting factor. Since 2015, extra downlink capacity has been purchased at other receiving stations. Near-real-time access was realized at the DLR stations in Neustrelitz and O'Higgins in Antarctica.