# Ten Years of TerraSAR-X Operations
## 6 Satellite Operations  
For a period of three years, TSX was operated as a single satellite mission in 514 km altitude. In 2010, a transition to a dual satellite mission needed to be accomplished and even more in October 2010 the two spacecraft entered into a close formation with inter satellite distances in the order of few hundred meters. The close formation flight posed a major challenge for satellite operations. Concepts needed to be developed in order to cope with safety aspects. One major point of interest was the original safe mode concept of TSX. A satellite safe mode is typically entered by on-board automatic reactions in the case that a malfunction is detected. The original TSX safe mode used thrusters for attitude control in order to profit from the high reactivity of these actuators. Unfortunately, due to the thrusters' orientation attitude controls came along with the side effect of an unintended orbit change. Investigation showed that a safe mode drop might result in a potential collision risk of the spacecraft in orbit, which was not acceptable. As a consequence a multiple stage safe mode concept has been newly developed. The primary safe mode is now based on magnetorquers, which do not alter the relative orbit of TSX and TDX. In the case of an insufficient reactivity, further stages within the safe mode concept are applied, using thrusters in a more controlled way from the perspective of formation flight. The on-board safe mode has been implemented by the satellite manufacturer; however, this major concept change affected satellite operations in a profound way. Many flight procedures needed to be reworked, major on-board software updates became necessary, and in-orbit tests of the safe mode concepts before entering close formation have been performed. Another major challenge to be mastered is to ensure that the two satellites do not illuminate each other when transmitting a radar beam, as this could result in an irreparable damage of the partner satellite. A well-thought-out concept was established to comply with this requirement during nominal operation, but in particular, during on-ground or on-board caused contingency situations. For the latter one, the so-called Sync-Warnings were implemented. These are isochronal instrument activities on both satellites with a bidirectional exchange of sync pulses via one of six sync horns. In case one of the satellites gets no feedback from the other (i.e., the state/position of the other satellite is different to the predicated one), its instrument is switched to receive-only mode. In order to reduce the mutual illumination risk to a maximum extent during nominal operations, the so-called transmit exclusion window information is delivered to both mission planning and the mission control and monitoring system. This double layer check on ground prevents firstly planning, and secondly, the commanding of possible hazardous activities in forbidden areas. In addition to these ground based measures, the on-board exclusion zone mechanism was implemented, in order to prevent the execution of possible hazardous activities in such areas. See [29] for a detailed description of all TanDEM-X formation flight operation related aspects. Figure 12 illustrates acquisition activities, the on-board memory fill level, together with the downlink activities for the TerraSAR-X mission and the TanDEM-X mission for about one day. The combined mission planning system enables the concurrent support of the two missions by the two spacecrafts. In order to minimize the impact of the two missions on each other a balanced downlink planning strategy was implemented. The fact that the size of the on-board memory of the two spacecraft differs by the factor of two and the necessity to support the TerraSAR-X near real time feature with its prioritized downlink make the downlink planning even more challenging. The design of the TSX and TDX spacecraft targeted in both cases for an in-orbit lifetime of 5.5 years. These days, TSX has completed ten years and TDX seven years in orbit.