SysML is the "Systems Modeling Language" -- basically UML for systems engineering. It is implemented as a profile on top of UML. It uses some UML diagrams and adds others. SysML provides a way to model all aspects (e.g., mechanical, electrical, software systems) of a system in a single unified model.
SysML provides capabilities to model system requirements, architecture, behavior, and parametrics. It can support simulation for trade studies and so on. You can think of it as CAD for systems engineering.
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We are especially interested in using requirements for requirements management.
We are starting to use it in particular for capturing and allocating requirements because it offers the following benefits for that purpose:
1) The most important reason is because we can capture everything in a single model in a (standards-based) tool specifically designed for that purpose. This means that, for instance, if I define a set of common requirements in a package in one project file, I can retrieve the relevant package in another project file. I can add only the relevant requirements to the target diagram. (In a document-centric approach I have to decide whether to duplicate requirements or just reference an entire document. The first option creates a maintenance problem and the latter is very often problematic for the reader.)
2) Each requirement is a unique model element. I can link semantically meaningful items (e.g., <<rationale>>). (These are currently possible in requirements management database tools and you can make this work in a document.) I can create semantically meaningful links (e.g., trace, refine) between requirements. Moreover, I can create links (satisfy) to Test Cases and detail the test procedures, and I can display all of these visually on a diagram. The links help us ensure traceability across the enterprise.
Ultimately, this means we have a real chance to maintain the requirements, do real traceability, make sure people actually read them, and consequently, we hope, meet them.
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OMG's page on SysML offers a start (OMG)
The best book I have found on SysML (by far) is A Practical Guide to SysML.
Paul