NASA Technical Reports Server (NTRS) 19930015490: The systems engineering overview and process (from the Systems Engineering Management Guide, 1990)

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SYSTEMS ENGINEERING OVERVIEW AND PROCESS
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THE SYSTEMS ENGINEERING OVERVIEW AND PROCESS
(FROM THE SYSTEMS ENGINEERING MANAGEMENT GUIDE [1990])
Defense Systems Management College f)J 7
The past several decades have seen the rise In its simplest terms, systems engineer-
of large, highly interactive systems that are ing is both a technical process and a manage-
on the forward edge of technology. As a re- ment process. To successfully complete the
sult of this growth and the increased usage of development of a system, both aspects must
digital systems (computers and software), be applied throughout the system life cycle.
the concept of systems engineering has From a government's program management
gained increasing attention. Some of this at- point of view, the Defense Systems Manage-
tention is no doubt due to large program fail- ment College favors the management ap-
ures which possibly could have been avoided, proach and defines systems engineering as
or at least mitigated, through the use of sys- follows:
tems engineering principles. The complexity Systems engineering is the manage-
of modern day weapon systems requires con- ment function which controls the total
scious application of systems engineering system development effort for the pur-
concepts to ensure producible, operable and pose of achieving an optimum balance
supportable systems that satisfy mission of all system elements. It is a process
requirements. which transforms an operational need
Although many authors have traced the into a description of system parameters
roots of systems engineering to earlier dates, and integrates those parameters to op-
the initial formalization of the systems engi- timize the overall system effectiveness.
neering process for military development A system life cycle begins with the user's
began to surface in the mid-1950s on the bal- needs, expressed as constraints, and the
listic missile programs. These early ballistic capability requirements needed to satisfy
missile development programs marked the mission objectives. Systems engineering is
emergence of engineering discipline "special- essential in the earliest planning period, in
ists" which has since continued to grow. conceiving the system concept and defining
Each of these specialties not only has a need system requirements.
to take data from the overall development As the detailed design is being done, sys-
process, but also to supply data, in the form tems engineers: 1) assure balanced influence
of requirements and analysis results, to the of all required design specialties, 2) resolve
process. interface problems, 3) conduct design re-
A number of technical instructions, mili- views, 4) perform trade-off analyses, and
tary standards and specifications, and man- 5) assist in verifying system performance.
uals were developed as a result of these During the production phase, systems en-
development programs. In particular, MIL- gineering is concerned with: 1) verifying sys-
STD-499 was issued in 1969 to assist both tem capability, 2)maintaining the system
government and contractor personnel in baseline, and 3)forming an analytical
defining the systems engineering effort in framework for producibility analysis.
support of defense acquisition programs. During the operation and support (O/S)
This standard was updated to MIL-STD- phase, systems engineering: 1) evaluates
499A in 1974, and formed the foundation for proposed changes to the systems, 2) estab-
current application of systems engineering lishes their effectiveness, and 3) facilitates
principles to military development pro- the effective incorporation of changes, modi-
grams. fications and updates.
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READINGS INSYSTEMS ENGINEERING
Iterative Trade-Offs
Functional
i Requirements ! Analysis Synthesis __ EvDaelcuaianstidioonn _)'_1 DeEsclSreyimpstteieonmnts of
• What • How • Solutions
• Why
Figure 1 The Systems Engineering Process
THE SYSTEMS ENGINEERING PROCESS use as performance, design, interface,
support, production and test criteria.
Although programs differ in underlying • Provide source data for development of
requirements, there is a consistent, logical technical plans and contract work state-
process for best accomplishing system design ments.
tasks. Figure 1 illustrates the activities of • Provide a systems framework for logistic
the basic systems engineering process. analysis, integrated logistic support
The systems engineering process is itera- (ILS), trade studies and logistic documen-
tively applied. It consists primarily of four tation.
activities: functional analysis, synthesis, • Provide a systems framework for produc-
evaluation and decision, and a description of tion engineering analysis, producibility
systems elements. The product element trade studies, and production and manu-
descriptions become more detailed with each facturing documentation.
application and support the subsequent • Ensure that life cycle cost considerations
systems engineering design cycle. The final and requirements are fully considered in
product is production-ready documentation all phases of the design process.
of all system elements.
Since the requirement to implement a Successful application of systems engi-
systems engineering process may cause neering requires mutual understanding and
major budgetary commitments and impact support between the military and contractor
upfront development schedules, it is impor- program managers. They must be willing to
tant to understand the inherent objectives: make the systems engineering process the
backbone of the overall development
• Ensure that system definition and design program. They must understand the need to
reflect requirements for all system ele- define and communicate among the
ments: equipment, software, personnel, engineering specialty programs. They must
facilities and data. recognize the role of formal technical reviews
• Integrate technical efforts of the design and audits, including the value, objectives
team specialists to produce an optimally and uniqueness of each formal review and
balanced design. audit. They must also know the objectives of
• Provide a comprehensive indentured the program and possess a thorough inter-
framework of system requirements for pretation of the user's requirements.
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SYSTEMS ENGINEERING OVERVIEW AND PROC ESS
The output of the systems engineering • Producibility Plan
process is documentation. This is the means • Functional Flow Block Diagrams (FFBD)
by which it controls the evolutionary devel- • Requirements Allocation Sheets (RAS)
opment of the system. Systems engineering • Audit Reports
prepares a number of technical management • EMI/EMC Control Plan
and engineering specialty plans that define • Human Engineering Plan
how each phase of the acquisition cycle will • Trade Study Reports
be conducted. Draft plans are usually sub-
mitted with the proposal and final plans are The systems engineering process is an
delivered in accordance with the Contract iterative process applied throughout the ac-
Data Requirements List (CDRL). These quisition life cycle. The process itself leads to
plans are used by the government to ensure a well defined, completely documented and
compliance with the contract and used by the optimally balanced system. It does not pro-
contractor to develop detailed schedules and duce the actual system itself, but rather, it
allocation of resources. Specifications are produces the complete set of documentation,
submitted that form the basis for the design tailored to the needs of a specific program,
and development effort. Top-level specifica- which fully describes the system to be devel-
tions are incorporated into the statement of oped and produced. Each program's systems
work (SOW) and provided to the developer. engineering process, developed through
The developer will allocate these top-level tailoring and/or adding supplemental re-
requirements to lower level system compo- quirements, must meet certain general crite-
nents (hardware and software) and submit ria. Although not complete, the following
the associated specifications and design doc- guidelines should be considered in approach-
uments to the government for approval. The ing the basic process:
status of system development progress is
tracked and documented in the form of tech- • System and subsystem (configuration
nical review data packages, technical perfor- item) requirements shall be consistent,
mance measurement (TPM) reports, analysis correlatable, and traceable both within
and simulation rePorts and other technical data produced as basic documentation
documentation pertinent to the program. In (e.g., Functional Flow Block Diagram,
summary, this documentation may include: Requirements Allocation Sheet, and
Time Line Sheet) and as related docu-
• Systems Engineering Management Plan mentation (e.g., work breakdown struc-
(SEMP) ture and Logistic Support Analysis
• Specifications (system, segment, develop- Record).
ment, product, process, material) • The concept of minimum documentation
• Design Documentation shall be evident.
• Interface Control Documents (ICDs) • Acquisition and ownership cost shall be
• Risk Analysis Management Plan an integral part of the evaluation and de-
• Survivability/Vulnerability (S/V) Hard- cision process.
ness Plan • Baselines shall be established progres-
• Mission Analysis Report sively as an integral part of the systems
• Reliability Plan engineering process.
• Maintainability Plan • The systems engineering process shall
• Integrated Logistics Support Plan (ILSP) result in a design that is complete, at a
• Software Development Plan (SDP) given level of detail, from a total system
• Test and Evaluation Master Plan element viewpoint.
(TEMP)
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READINGS INSYSTEMS ENGINEERING
• The process shall provide for the timely Significant engineering decisions shall be
and appropriate integration of main- traceable to the systems engineering ac-
stream engineering with engineering tivities and associated documentation
specialties such as reliability, maintain- upon which they were based.
ability, human factors engineering,
safety, integrated logistic support, envi- Figure 2 is an overview of the four basic
ronmental assessments and producibility steps of the systems engineering process.
to ensure their influence on system
design. FUNCTIONAL ANALYSIS
• The process shall provide for continuing
prediction and demonstration of the an- Every engineering effort must begin with a
ticipated or actual achievement of the statement of a perceived need. At the
primary technical objectives of the sys- beginning of a DOD acquisition effort, this
tem. Problems and risk areas shall be statement will be in the form of a system
identified in a timely manner. requirement document, usually developed
• Formal technical reviews and audits through a Mission Area Analysis of antici-
shall be an integral part of the systems pated threats.
engineering process. Once the purpose of the system is known,
• The systems engineering process shall be the functional analysis activity identifies
responsive to change. The impact of what essential functions the system must
changes to system and/or program re- perform. In order to accomplish this, func-
quirements must be traceable to the low- tional analysis is composed of two primary
est level of related documentation in a process segments: functional identification
timely manner. and requirements identification and
G
_ . . _ ._fWil!._ I Evaluation J
Analysis Synthesis ______<.Alternat_ves_N_---_ Iand Decision [
I - _v,_ [(Trade-off) ]
• Mission Objectives
• Mission Environments
Input Requirements Functional _[
• Mission Constraints
• Measurements of ®
Effectiveness @
Technology Selection Factors
• Hardware
• Software
I Description of System Elements J
• Reliability
• Maintainability
• Personnel/Human Factors
• Equipment
• Survivability • Personnel
• Security • Facilities
• Safety
• Computer Software
• Standardization
• Technical Data
• IntegrateLdogisticSupport
• EMC
• System Mass Properties
• Producibility
• Transportability
• Electronic Warfare
• Computer Resources
Figure2 The SystemsEngineeringProcess
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SYSTEMS ENGINEERING OVERVIEW AND PROCESS
allocation (functional performance require- allocation of the functional performance
ments analysis). It answers the "what" and requirements to individual system elements
"why" questions relative to system design. or a combination of elements; and 3) follow-
The basic analytical tool for functional ing evaluation and decision, the RAS
identification is the Functional Flow Block provides the functionally oriented data re-
Diagram (FFBD), showing logical sequences quired in the description of the system
and relationships of operational and support elements.
functions at the system level. Specific func- The Time Line Sheet (TLS) is used to
tions will vary from system to system and perform and record the analysis of time-
will be traceable to mission requirements critical functions and functional sequences
and objectives. Maintenance flow diagrams In performing time requirements analysis
depicting general maintenance and support for complex functional sequences, additional
concepts will lead to analysis of require- tools, such as mathematical models or
ments on an end item/equipment basis. At computer simulations, may be needed. Time
this level, since functions are more standard- requirements analysis is performed in any or
ized, functional identification is often accom- all of the functional cycles of the process to
plished using the End Item Maintenance determine whether time is a critical factor.
Sheet (EIMS) or Logistic Support Analysis The TLS complements the FFBD in its
Record (LSAR). Similarly, detailed test ability to show a lower level of detail, as well
requirements are identified using the Test as to illustrate the impact of concurrent
Requirements Sheet (TRS), and productivity functions within a given sequence. TLSs are
requirements are identified using the used to support the development of design
Production Sheet (PS). requirements for the operation, test and
It should be kept in mind that the sys- maintenance functions. They identify time-
tems engineering process is always iterative. critical functions and depict the concurrency,
Each acquisition phase will involve function- overlap and sequential relationship of
al analysis to progressively more detail. For functions and related tasks. Time-critical
example, during the Concept Explora- functions are those that affect reaction time,
tion/Definition (C/E) phase, analysis of downtime or availability.
support functions will concentrate on Main-
tenance FFBDs, which will support the SYNTHESIS
establishment of gross maintenance con-
cepts. During Full Scale Development (FSD), Synthesis supplies the "how" answers to the
emphasis will shift to detailed analysis of the "what" outputs of functional analysis.
maintenance requirements of specific equip- Two documentation tools accomplish and
ment using the EIMS or LSAR. record the synthesis of design approaches or
The Requirements Allocation Sheet alternative approaches. The Concept
(RAS) is used as the primary analytical tool Description Sheet (CDS) is used to collect the
for requirements identification and alloca- performance requirements and constraints,
tion, or functional performance require- as delineated by functional analysis, that
ments analysis as it often is referred to, in apply to an individual subsystem or end
conjunction with FFBDs and special purpose item. The CDS also describes at the gross
documents such as EIMSs, TRSs, and PSs. level a design approach for meeting the
The RAS serves three purposes in document- requirements. The Schematic Block Dia-
ing the systems engineering process: 1) ini- gram (SBD) is used to develop and portray
tially, it is used to record the performance the conceptual schematic arrangement of
requirements established for each function; system elements to meet system and/or
2) during synthesis, it is used to show the subsystem requirements. The CDS and SBD
1:3
READINGS IN SYSTEMS ENGINEERING
are both applicable to all acquisition phases and constraints that establish the selection
and provide the basis for development of the criteria for a specific trade study area. The
descriptions of system elements. report also documents the rationale used in
the decision process and should present risk
EVALUATION AND DECISION assessment and risk avoidance consider-
ations. Other tools, such as analytical or
Since program risk and cost are dependent mathematical models or computer simula-
on practical trade-offs between stated oper- tions, may be needed and used in accomplish-
ating requirements and engineering design, ing the evaluation and decision process.
continual evaluations and decisions must be
made not only at the beginning of the DESCRIPTION OF SYSTEM ELEMENTS
program but throughout the design and
development activity. All systems can be defined by a set of inter-
The Trade Study Report (TSR) is used to acting system elements which fall into five
summarize and correlate characteristics of categories: equipment (hardware), software,
alternative solutions to the requirements facilities, personnel, and procedural data.
Functional Analysis
Fun iIoinRequirem[ents
& Decision System
ments i-i,, Identification JJ_ Identification&AllocatJioi[n_t_J Synthesis Elements
L............. _ ............. _J
l Functional Flow Requirements Concept Trade Design
i BlockDiagrams AllocationSheets Description Sheets Study Reports Sheets
I _FFBD) (RAS) (CDS) (TSR) (DS)
IIdentify and se- Define therequire- Constrain the de- Select, evaluate and Define,describeand
quence functions ments and constraints isigner to stop at a optimize promising specifyperformance.
that must be accom- foreach ofthe func- point in the cycle and or attractive con- design and testcri-
Basic Iplished to achieve tionsand relateeach create at the gross cepts. Document the teriaforthesystem
Documen- --d_ system or project ob- requirement tothesys- level adesign or syn- trade-off and su.p- elements
tation jectives. Develop the tem elements of thesis meeting the porting rationale. a. Equipment
binasgisintfeorrsysetsetmablish- •. FEaqcuiiplmietnites FreFqBuiDre,menRtsAS, TaLnSd Csooluntsiiodnesr awlilthpinossibthlee bc.. FPaecrislointnieels
functionalinterfaces c. Personnel constraints. framework of re- d. Procedural data
and identify system d. Procedural data quirements. e. Computer soft-
relationships. e. Computer software Schematic wRre
Block Diagrams
Time Line Sheets (SBD)
(TLS)
Develop and portray
Present critical func- schematic arrange-
tions against a time ment ofsystem ele-
base in the required se- ments tosatisfysys-
quence of accomplish- tem requirements.
ment.
............ ._m ..............
End Item
FacilityInterface
Maintenance Sheet Sheets
(EIMS)/Test Reqmt
{FIS)
Sheet (TRS)/
Special Production Sheets Identifyenviron-
Purpose {PS)/Logistic Sup- mental and physical
Documen- -'_ interfacesbetween
port Analysis
tation Record (LSAR) equipment and fa-
Identify mainten- cilitieosn an end
item basis.
iance, test and pro-
Iduction functions on
jaspecific end item,
ieuImssembly, and
Icomponent basis.
Indenture iscarriedtothelevelrequired forthe selectedlevelofengineering toidentify,describeand specify.
Figure3 BasicandSpecialPurposeDocumentationforSystemsEngineering
14
SYSTEMS ENGINEERING OVERVIEW AND PROCESS
Two documentation forms are used to specifications are prepared as part of the-
describe these system elements: the Design systems engineering process to form the
Sheet (DS) and the FacilityInterface Sheet basis for the design and development effort.
(FIS). The DS is used to establish and The top-level specification (system or seg-
describe the performance, design and test ment) is normally approved and draft lower
requirements forequipment end items,criti- level specifications (configuration items) are
cal components and computer software developed reflecting allocated system re-
programs. The FIS is used to identify the quirements to lower level components or sub-
environmental requirements and interface systems, which designers and subcontractors
design requirements imposed upon facilities translate into hardware and software pro-
by the functional and design characteristics duction plans.
of equipment end items. The DS and FIS In order to provide a continuing assess-
provide the basis for the formal identifica- ment of the system's capability to meet
tionrequired forconfiguration management. performance requirements, the systems
The systems engineering process pro- engineering organization prepares technical
duces the basic and special purpose docu- review data packages, technical performance
mentation that controls the evolutionary measurement (TPM) reports, analysis and
development of the system. Figure 3 simulation reports, and other documenta-
correlates the particular documentation tion.
associated with each step of the systems The systems engineering process is one
engineering process. approach to providing disciplined engineer-
The systems engineering process itself ing during all acquisition phases. Although
does not actually produce the system, but current application of the process has focused
produces the documentation necessary to de- on C/E, D/V, and FSD, systems engineering
fine,design, develop and testthe system. As process techniques and principles are equal-
such, a variety of engineering and planning ly applicable to the analysis and definition of
documentation is required throughout the production requirements.
acquisitioncycle,and systems engineering is The systems engineering process also pro-
the vehicle used to produce that documenta- vides the logic and timing for a disciplined
tion.
approach, with certain internal assurances
Numerous plans are prepared to define of technical integrity such as traceability.
which technical activitieswillbe conducted. Technical integrity ensures that the design
They address the integration of engineering requirements for the system elements reflect
specialties requirements, "design-for" re- the functional performance requirements,
quirements and organizational resource that all functional performance require-
requirements, and discuss how progress ments are satisfied by the combined system
toward system-level goals willbe measured. elements, and that such requirements are
The Systems Engineering Management Plan optimized with respect to system perfor-
isthe key planning document that reflects mance requirements and constraints.
these requirements. Contractor compliance
with these plans ismonitored by government The DSMC Systems Engineering Man-
organizations to ensure that standard poli- agement Guide may be purchased from the
ciesand procedures in the area of systems U.S. Government Printing Office (1991-306-
engineering are employed. Additionally, 417-QL 3).
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