The principles and methods of Model Driven Architecture are applied to the problem of requirements traceability for a System of Systems (SoS). Model transformations of operational threads are used to reduce the complexity of modeling mission requirements and their flow into the architecture of the SoS. The allocation of requirements to operational mission threads (OMTs) rather than to individual systems reduces the complexity of the requirements tracing. Relational transformations provide a mathematically based formalism for model transformations that permit precise computation of the transformation of operational threads into threads of systems allocated from the SoS. Connectivity requirements for the SoS are also exposed in this way and the number of permissible system threads are seen to correspond directly to the number of permissible transformations. The principles and methods are illustrated by an elementary case study for sensor fusion.