Requirements analysis for large scale systems

I suggest you read this article in JOT.
The objectives of this paper are to clarify the terminology of software requirement models by suggesting a more consistent usage, to describe the different versions of requirement models, and to illustrate how to effectively address requirements analysis for large scale systems.

Quote:
Requirements are the things that you should discover before starting to build your product. Discovering the requirements during construction, or worse, when you client starts using your product, is so expensive and so inefficient, that we will assume that no right-thinking person would do it, and will not mention it again.
–Suzanne and James Robertson