There are a number of different types of requirements that system engineers will have to develop on an acquisition program through its life-cycle. These requirements range from very high-level concept-focused to very specific for a part. The main types of requirements are:
- Functional Requirements
- Performance Requirements
- System Technical Requirements
A functional requirement is simply a task (sometimes called action or activity) that must be accomplished to provide an operational capability (or satisfy an operational requirement). Some functional requirements that are associated with operations and support can be discerned from the needed operational capability (see Operational Requirements). Others often result only from diligent systems engineering. Experience in systems engineering has identified eight generic functions that most systems must complete over their life cycle: development, manufacturing, verification, deployment, training, operations, support, and disposal. These are known as the eight primary system functions. Each must usually be considered in identifying all the functional requirements for a system.
A performance requirement is a statement of the extent to which a function must be executed, generally measured in terms such as quantity, accuracy, coverage, timeliness, or readiness. The performance requirements for the operational function and sometimes a few others often correlate well with the statement of the needed operational capability as developed by the Joint Capabilities Integration and Development System (JCIDS) Process. The statement of other performance requirements usually requires thorough systems engineering.
System Technical Requirements
Result in both allocated and derived requirements.
- Allocated Requirements: flow directly from the system requirements down to the elements of the system.
- Derived Requirements: dependent on the design solution (and so are sometimes called design requirements). They include internal interface constraints between the elements of the system.
A specification is a detailed, exact statement of particulars, especially a statement prescribing materials, dimensions, and quality of work for something to be built, installed, or manufactured. The overall purpose of a specification is to provide a basis for obtaining a product or service that will satisfy a particular need at an economical cost and to invite maximum reasonable competition. By definition, a specification sets limits and thereby eliminates, or potentially eliminates, items that are outside the boundaries drawn. A good specification should do four (4) things:
- Identify minimum requirements
- List reproducible test methods to be used in testing for compliance with specifications
- Allow for a competitive bid
- Provide for an equitable award at the lowest possible cost.
– The document that defines the proper organization for all armed-forces specifications is MIL-STD-961E “Defense and Program-Unique Specification Format and Content”.
AcqLinks and References:
- Requirements Development Checklist
- DAU Systems Engineering Fundamentals Guide
- Space and Missile Systems Center (SMC) Systems Engineering Primer & Handbook
- NASA Systems Engineering Handbook (large 9M file)
- EIA-632 “Processes for Engineering a System” – 7 Jan 99