SMAD

74 Requirements. Definition 4.1
4.1 Role of Requirements in System Development
To this point, the book has dealt with the mission analysis and concept development
process which ideally drives the system design. The mission objectives and system
concepts we have adopted have involved five basic measures: (1) required performance,.(2)
cost, (3) ~velopment and deployment schedule, (4) implicit and explicit
co~~ts, and (5) nsk. The same meas~ continue to apply during the entire system
englDeenng process, from concept to lDlplementation. Through this process, we
decompose and allocate the central system-derived requirements (sometimes
expressed as system specifications) to individual segments or system elements, interfaces
between these as well as·interfaces external to the system. To define the total
system, therefore, users, customers, system engineers, and segment developers must
co~tantly interact. Although we initiate the process in a "top-down" fashion, we
typically must continually reconcile system level requirements with technology and
lower-level design development.
A healthy tension often exists between the user and development communities.
~vel?~ may consider ~e user wedded to current operational approaches and
lDsensltive to how over-specified requirements constrain design. Users often believe
that developers favor new technology and ignore the practical needs associated with
operating a system and exploiting the mission data. Thus, the developer may establish
mission requirements without consulting the user, or the user may produce "nonnegotiable
stone tablets" and carry them down from the mountain too late or too overspecified
for actual use. Because both sides have valid concerns, however, they must
cooperate from the start in developing the mission's operational requirements. We
may implement this cooperation through so-called IPrs (Integrated Product Teams)
involving both users/customers and developers.
Typically, developers wanting to build as soon as possible drive prematurely
toward low-level detail. Sometimes they underemphasize the original mission drivers
-requirements which dominate performance, cost, and schedule risk. Customers
often constrain system development with overly specific requirements at levels below
the critical requirements that determine most of a program's cost and risk. While the
level of formality and detail may vary depending upon system maturity, complexity,
and size, critical requirements must remain in the forefront during design. development,
and validation of the system.
Overzealous requirements can also find their way into mission statements. For
example, a user may specify the scan rate and swath width under payload and coverage
~on.nan~e. Clearly,. these cons~ts on sensor design and constellation are inappro­
~nat~ lD this case,. pnor to establishing a system which meets the key requirements,
I.e., timely data WIth enough accuracy and resolution. Specifications on launch rate,
launch responsiveness, and spacecraft reliability are also common. But so long as a
system meets availability and maximum outage needs, the developer should be able to
allocate requirements for reliability, maintenance, and replacement. Mission requirements
concerning launch, operation, or maintenance may establish the design domain
but .not di~tate the design. On the other hand, the user must also be a party to the syste~
desIgn as It converges, to identify design characteristics likely to produce operational
problems.
Table 1-5 in Sec. 1.4 shows essential requirements for the FJreSat mission. These
req~ments ?either d!ctate nor impose needless constraints on design, but they do
Specify what IS essential to perform the mission and operate the system. The table
4.1 Role of Requirements. in System Development 75
contains enough information to derive the specific design characteristics with
sufficient controls on the user's essential requirements. Also, the table includes no
unverifiable terms or goals such as ''maximize,'' "sufficient," or "optimize," because
these words have no quantifiable interpretations. Requirements which we are asked to
implement only if no "impact" results, are in fact goals and we cannot treat them as
design dri,:ers. Every meanin~ requirement bears cost and will have an impact
Co~ts are those reqUIrements for a system which we cannot trade, usually
under any crrcumstances. Th~y may pertain to performance when levels of capability
of a sy~tem must have a ~ valu~ to be useful. One example is the necessity for a
resolution level of an optical or RF SIgnal, above which the desired information could
not be derived or would not be sufficiently better than existing systems to justify new
development. A related, fixed requirement could also be coverage and timeliness of
data, clearly a major consideration for FJreSat. Another might be cost-a constraint
increasingly important to the financial success of a new mission. Thus, if a cost ceiling
ofN millions could not be met for a new development, the feasibility, design attn'butes
or method of achieving a mission would 00 directly affected. The term "design to cost"
applies directly to a cost constraint. Schedule may also be a constraint, and many technically
worthwhile projects get scrubbed because developers could not solve some
problems soon enough to be competitive-this is often called a ''time to market" cons~t.
Others, but by no means all, include environmental and safety issues, legal and
political mandates, fixed asset usage, involvement of geographically distributed or
foreign offset contractors.
An alternative view of "goals" vs. ''requirements" is that the former represent
design margin. Any firm requirement must result in a level of margin in the design.
and we ~ regard the "goal" as specifying the desired margin. As the design matures.
the margm represents the trade-space available to decision-makers. The user must
ultimately decide whether the additional performance is worth its associated incremental
cost.
Designers often focus on performance areas, such as operating the payload and
distributing. the. ~sion data, and ~deremphasize the more mundane requirements.
such as availability and accommodation to the external environment. Yet these can be
cri?c~ . to cost and risk. F~r example, availability can demand increased component
~hab~ty and ~erefore raise development costs. It can drive maintenance concepts,
~cluding ~1.eD1Shment and on-o,rbit ~Pp?rt. It. can also affect production time, especIally
for cntical components. likewise, Ignormg external interfaces can produce a
system design without the external support needed to deploy and operate the mission.
When space systems perform more than one mission, planners must develoP
requirements which account for each mission. For example, the IR surveillance
pa>:load o,n FlreSat may serve o~er users with its performance in IR imaging and
radiometric measurement. If the mcreased cost and risk are acceptable, their requiremen~
could lead to more payload bands, added coverage, and added distribution
reqUIrements. That is why we must establish all valid missions early in requirements
definition, or we should incorporate accommodations for new missions in future
upgrades to a system's capabilities.
While we must address system requirements throughout all aspects of the
development cycle, the role and characteristics of requirements change in each development
phase. Consequently, we should use specific structure and language early in
the ~ss without premature detail. Table 4-1 shows how the requirements converge
dunng system development. Concept development must continue to reflect the driving