The act of switching to a different, redundant system upon the failure or abnormal behavior and/or termination of the currently-active system.

Formation Control, Swarms
Formation, swarms are constellations of identical or similar spacecrafts with identical/similar mission objectives being deployed to make it possible to achieve and/or enhance the mission goals in an cost efficient manner. The control of multiple spacecraft (swarms) having the same mission goals but are spatially separated is called formation control. Formation control requires sophisticated control systems to avoid duplication of efforts and facilitation of efficient operations.
With increasing use of smaller but multiple (identical) application satellites the intelligent co-ordination and control of the formation constituents becomes more difficult than operations of a single spacecraft. Use of magnetic fields to maintain the formation co-ordination is proposed (2006-5886). Large numbers of multiple agents within a swarm must be multiple failure tolerant and the agents must co-operate with each other for the achievements of their “high level” goals (2006-5555).
In 2010 a paper (AIAA 2010-1989) was presented to discuss the combined mission planning aspects of TerraSaAR-X and TenDEM-X. The combined TerraSAR-X / TanDEM-X mission planning system will handle the two satellites as well as two completely different missions with their different mission goals. As a consequence, the new combined TerraSAR-X / TanDEM-X mission planning system not only has to support two satellites with their mutual constraints, but also will handle two different missions at the same time: the long-term mapping approach of the TanDEM-X mission and the short-term on-demand approach of the TerraSAR-X mission. There are two critical issues regarding the operational safety of the formation flight: the close distance of the two satellites implies a significant collision risk in case of anomalies. Next, illuminating the other satellite with radar pulses can cause severe damage to the illuminated satelliteI