# Ten Years of TerraSAR-X Operations
## 7 Orbit Control  
The TerraSAR-X mission design is relying on the reference orbit, which forms the basis for both orbit control and mission planning, including the scheduling of SAR acquisitions. In contrast to traditional design considerations for Sun-synchronous, frozen eccentricity repeat orbits, the TSX reference orbit must be a closed orbit with matching states at the beginning and end of each 11-day repeat cycle. Therefore, the reference orbit design was formulated as an optimization problem [30]. The implemented TSX reference orbit is expressed in an Earth-fixed frame, and it can be repeated in 11-day intervals throughout the entire mission. The high requirements on guidance, navigation, and control are mainly driven by SAR interferometry. In order to permit repeat-pass interferometry applications, like subsidence mapping or glacier monitoring, the cross-track distance between radar acquisitions in repeated orbits should be as small as possible. Depending on the availability of digital elevation models, which are used to compensate topographic effects, cross-track distances below 350 m are desirable. Hence, the TSX osculating orbit is controlled within a \"tube\" that is defined about the Earth-fixed reference orbit. In order to fulfill the requirements, the radius of the \"tube\" is set to 250 m, which corresponds to the maximum allowed deviation of TSX from the reference orbit in the plane perpendicular to the flight direction. The implemented TSX orbit control concept is described in [31]. Within ten years of TerraSAR-X operations, more than 1400 orbit control maneuvers have been performed. With increasing solar activity strong variations in solar flux and geomagnetic activity occurred, which significantly affected the air density, and hence the orbit decay. Figure 14 (top) depicts the F10.7 cm solar flux evolution, which serves as an indicator for the solar activity, over the years 2008 to 2017. The diagrams below summarize all of the in-plane maneuvers that were performed in the same period (middle) and the achieved cross-track deviation from the reference orbit (bottom). Obviously, during low solar activity in 2008 and 2009, the maneuvers were relatively small (\(\leq\)1 cm/s) and the typical period between two successive maneuvers were 10 to 14 days. In contrast, maneuvers with sizes of up to 5 cm/s and maneuver cycles of two to three days became necessary to precisely control the TSX orbit during periods of high solar activity, e.g., at the end of 2011 and in 2014. Figure 13: Pie charts of TSX and TDX in orbit hydrazine consumptions. Each slice in the “consumed” region represents an operations year for which the consumed fuel is reported. Approximately half of the hydrazine is still available for both satellites, see region “available”. At the end of 2015 a novel orbit control strategy called \"distributed formation flying\" was tested and it has been operationally implemented since February 2016. Since then, TSX has contributed to the formation maintenance by means of a small but daily orbit raising maneuver in the order of 3 to 6 mm/s, depending on the formation geometry. In this way, the formation flight for TanDEM-X can be fully supported although the cold-gas, which was previously used by TanDEM-X for formation maintenance, has been almost depleted. Furthermore, the TerraSAR-X orbit control accuracy is not negatively influenced, as can be seen in the bottom of Figure 14. The daily orbit raising partially balances the natural orbit decay. However, additional orbit lowering and eccentricity correction maneuvers are necessary in order to stay within the 250 m control tube. These maneuvers are in anti-flight direction and they can be seen in the middle of Figure 14 as negative spikes in 2016 and 2017. Note the negative in-plane maneuvers before 2011 have a different cause. The original satellite safe mode implementation made use of the thrusters for coarse attitude and rate control.