# Ten Years of TerraSAR-X Operations
## Extension of the TerraSAR-X Ground Segment  
### Common TerraSAR-X/TanDEM-X Ground Segment  
The TerraSAR-X ground segment underwent major extensions when compared to its starting point, as described in [4]. One essential upgrade driver has been the TanDEM-X mission, the TerraSAR-X Add-On for Digital Elevation Measurements, which added a second, mostly identical satellite TDX in 2010. Flying TDX in a close helix formation around the TSX satellite (the one being kept in a reference orbital tube) enables a bistatic acquisition of SAR data, as needed for global DEM generation. Both TSX and TDX are used together for such acquisitions where only one satellite is actively transmitting SAR pulses and both satellites are receiving the echoes. To counterbalance this interferometric usage of the TSX satellite for TanDEM-X, the TerraSAR-X monostatic data acquisitions have to be performed by either TSX or TDX since then. Consequently, essential ground segment subsystems had to be upgraded from their \"one mission one satellite\" behavior into a combined \"two missions two satellites\" one even to fulfill their TerraSAR-X mission purpose only, foremost the flight dynamics and the mission planning subsystems. Their operational integration and commissioning had been achieved without disturbance of the on-going operational TerraSAR-X mission. In October 2010, the joint TerraSAR-X/TanDEM-X ground segment had been released for the TerraSAR-X operations based on two satellites that are flying in a close formation [6; 7]. Maintaining not only a reference orbit for TSX, but also the challenging helix formation of TDX relative to TSX in an extremely short distance posed new needs for spacecraft navigation and control, e.g., a common maneuver planning and execution. The reference orbit control, as described in Section 7, details on the formation flight are given in [8; 9]. Verification of the upgraded flight dynamic system components and specifically validation of their interaction with the operational TSX system was performed for about one year before launch using the in-orbit TSX satellite and on-ground simulated TDX GPS navigation data [10]. Conflict-free timelines for both satellites are obtained only if the acquisitions for both missions are jointly planned inside the mission planning system. TerraSAR-X data takes are assigned by mission planning to either one of the satellites, for TanDEM-X data takes the transmitting satellite has to be chosen. The resulting load distribution has to balance the usage between the two missions. Irreparable damages that are caused by a mutual illumination of the instruments have to be prevented. The operational qualification of the new \"two missions, two satellites\" version of the mission planning system under realistic conditions within the TDX commissioning phase had to be performed, while the former \"one mission one satellite\" version was still supporting the on-going TerraSAR-X operations. Operational roll-out, i.e., final replacement of the former version, was mastered within a short maintenance period of a few days only, services were continued with the same robustness and reliability as before [11]. On the payload data handling side, a new production chain for the reception and processing of TanDEM-X data has been established. DLR's existing German Antarctica Receiving Station (GARS) O'Higgins was complemented in 2010 by a new satellite receiving station, the Inuvik Satellite Station Facility (ISSF) in Canada. Both of the stations form the backbone of the TanDEM-X Ground Station Network, which was further supported in the years 2010-2015 by the partner ground station SSC Esrange in Kiruna, Sweden, to handle the TanDEM-X global DEM data amount. Like the main TerraSAR-X ground station at DLR Neustrelitz (NSG), also these stations not only support X-band data reception, but also S-band up- and downlink for telemetry, tracking, and commanding (TT&C) for both the TSX and TDX satellites and thus can be used for both missions, TerraSAR-X and TanDEM-X.