The constituent systems (CS) that together form a system of systems (SoS) have a continuous need to assess situations and make decisions. In addition to operating in the environment, they also need to decide upon their status in the system of systems and be aware of their relations to constellations in the system of systems. To be able to make the best possible decisions, the constituent systems need to have an accurate situation awareness, i.e., an understanding of the environment they are in, what other elements are present therein and how this will develop in the future. In this paper we present an analysis of the situation awareness needs of a constituent system in different stages of its life-cycle: ignorant of the SoS; prepared for joining the SoS; a passive CS; an active CS. A conclusion is that CS need to have world models that include information about other objects. Examples from the truck platooning application are given.