NAS Systems Engineering Manual- Requirements Management

4.3.1 Introduction to Requirements Management
The Requirements Management process, an element of System Engineering (SE), is an activity that spans the program’s entire lifecycle. Requirements Management iteratively identifies and refines the top-level requirements to successively lower levels, in concert with functional baselines and architectures, and synthesis of solutions established for the system of interest. For the purposes of Requirements Management, a system or a product means any physical product or software being designed, developed, and/or produced, or any intangible product, such as a product describing a process or a service.

4.3.1.4 Requirements Management Process Flow
Requirements Management is an iterative process that works with Functional Analysis and Synthesis to produce requirements. The process begins with the identified need and repeats through successively more detailed layers until requirements are detailed enough for their intended purpose. Figure 4.3-2 illustrates the FAA Requirements Management process flow that starts with the National Airspace System (NAS) Concept of Operations (CONOPS) and ends with the System Specification that will be used for system acquisition. Starting from the NAS and NAS Enterprise Architecture, the initial Functional Analysis produces the System CONOPS. The functions described in the System CONOPS are the first inputs to the Identify and Capture requirements step of the Requirements Management process. These functions, along with the performance and nonfunctional requirements, are formed into the first system requirements and documented in the SLMN. At this point in the process, there is insufficient detail in the requirements to synthesize a physical architecture, so the synthesis step is not performed. After the SLMN is completed during the first pass though the requirements process, the System CONOPS is further decomposed using the Functional Analysis process, as constrained by the requirements defined in the SLMN. This level of functional analysis produces the first level of the functional architecture and is used to refine the SLMN-level requirements into the initial requirements that are documented in the pPR. The pPRs are used to define the first version of the physical architecture during the Synthesis process.
The process then repeats to produce the fPR. The functional architecture, which is constrained by the pPR requirements, is decomposed. The fPRs are then decomposed from the functional architecture, which is constrained by the pPR-level physical architecture. The pPR-level physical architecture, which is refined by the fPR requirements, is used to derive the physical architecture at the fPR level.