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Unformatted text preview: en more attractive than
those that were not selected.
Consequently, it is reasonable to expect the
system to be defined with increasingly better resolution
as time passes. This tendency is formalized at some
point (in Phase B) by defining a baseline system
definition. Usually, the goals, objectives, and constraints
are baselined as the requirements portion of the
baseline. The entire baseline is then subjected to
configuration control in an attempt to ensure that
successive changes are indeed improvements.
As the system is realized, its particulars become
clearer—but also harder to change. As stated above, the
purpose of systems engineering is to make sure that the
development process happens in a way that leads to the
most cost-effective final system. The basic idea is that
before those decisions that are hard to undo are made,
the alternatives should be carefully assessed.
The systems engineering process is applied
again and again as the system is developed. As the
system is realized, the issues addressed evolve and the
particulars of the activity change.
Most of the major system decisions (goals,
architecture, acceptable life-cycle cost, etc.) are made
during the early phases of the project, so the turns of the
spiral (that is, the successive refinements) do not
correspond precisely to the phases of the system life
cycle. Much of the system architecture can be ''seen"
even at the outset, so the turns of the spiral do not
correspond exactly to development of the architectural
hierarchy, either. Rather, they correspond to the
successively greater resolution by which the system is
Each of the steps in the systems engineering
process is discussed below. NASA Systems Engineering Handbook
Fundamentals of Systems Engineering
Recognize Need/Opportunity. This step is shown in
Figure 3 only once, as it is not really part of the spiral
but its first cause. It could be argued that recognition of
the need or opportunity for a new system is an
entrepreneurial activ ity, rather than an engineering one.
The end r...
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