I am currently working on the definition of the documentation architecture of a system, from customers needs to software/hardware requirements. I encounter a big problem with the level of refinement of requirements.

The classic architecture is : PTS --> SSS --> SSDD --> SRS/HRS with PTS : Purshaser Technical Specification SSS : Supplier System Specification SSDD : System Segment Design Description SRS / HRS : Software / Hardware Requirement Specification.

Requirements from PTS are reworked in SSS, this document only expressed the needs (no design requirements are defined at this level). Then, the system design is described in SSDD : we allocate requirements from the SSS to functions from the design and functions are then allocated to component (Software or hardware) (we are still at the SSDD level). Finally, for each component, we write one SRS or one HRS. Requirements in SRS or HRS are refinement of requirements from SSS (and traceability matrix are made between these two levels).

My problem is the following one :

Our system is a complex one, and some of the requirements in the SSS needs to be refined twice to be at the right level in the SRS (means that software people can understand the requirement to make their coding). But, with this document architecture, I can only refine once the requirements from the SSS. The second problem is that only a part of the requirements from the SSS needs to be refined twice. The other part only need one refinement.

On the picture below, the green boxes are requirements at the right level for SRS or HRS. And purple boxes are intermediate requirements which can not be included in SSS since they are design requirements. Where can I put these purple requirements ??

Is there someone who has already encountered this problem ? Should I write two documents at SRS level ? Should I include intermediate requirements in SSDD ? Should I includes the two refinement levels (purple and green) in the same SRS document (not sure that's possible since a SRS is only for one component) ???

Thanks for your help and expertise ;-)