Aerospace and Electronic Systems Magazine April 2018 - 27

Waseem and Sadiq
Another important aspect of complexity is that the MBSE approach requires not only a language such as SysML to represent
the system but also a method that defines the activities and artifacts
and a tool to implement the modeling language and method. The
language, method, and tool each introduces its own concepts and
must be learned for the engineer to master MBSE. This skill must
then be applied to a particular domain, such as designing a spacecraft. Additional modeling challenges are associated with scaling
the modeling effort to larger projects and in the context of a diverse
development environment.
There are many types of models involved in the MBSE effort
beyond the SysML model, such as a multitude of analysis models, hardware models, and software models. Integration among the
models and tools, and other engineering artifacts, is another challenge associated with MBSE.

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Modeling elements for standard International System of
Units (SI) units have been identified and created in the NSS1 repository.
Mass, power, and reliability budgets have been incorporated.
State machine diagrams are used for the satellite's operational modes and their transitions at system and subsystem
levels.
A description of the satellite bus (controller area network
[CAN]) protocol is available via the SysML sequence diagram.

The NSS-1 model is developed using diverse SysML diagrams.
We describe each type of diagram used in the NSS-1 context in the
following paragraphs.

Package Diagram

SYSML MODEL OF THE NSS-1 SATELLITE
NSS-1 SATELLITE AND SYSTEM ENGINEERING
NSS-1 is a RS student satellite. The design philosophy of NSS-1
is conceived in such a way that all SE-related tasks are performed
by an independent systems engineering team, while design and development of various satellite units are undertaken by university
students. Throughout this process, the system engineering team
has to work iteratively in the system-level documents, design, and
analysis tools to tune and freeze the design parameters as early as
possible.
SE tasks have been performed using a conventional approach
and heritage knowledge-based tools and documents. This approach inherently involves a lot of manual procedures, which are
error prone because of parametric mismatch. We therefore decided to benefit from the model-based approach to address these
problems.

MODEL REPOSITORY FOR NSS-1
We reused some model elements from the FireSat SysML model, which is a case study provided in [8]. The NSS-1 modeling
group consists of experts from multidisciplinary areas of our
local aerospace industry setup and some principle investigators
from academia. The modeling repository uses SysML to capture
common design patterns of the typical nanosatellite class: managing values, describing scenarios, and describing functions,
parts, and subsystems, as well as the relationships among these
design patterns.
The repository illuminates a path to an integrated model-based
engineering environment, including interoperability with existing
SE models ready in existing analysis tools such as Microsoft Excel, MATLAB, and STK. This NSS-1 model repository demonstrates the possibility of a diverse set of analysis applications that
are provided with information about the space system from the system model to accomplish analysis driven by a formal description of
the mission, flight, and ground systems.
The following major modeling elements have been introduced
in the NSS-1 model that were not found in the FireSat model [8]:
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The SysML package diagram is used to organize the model elements contained in the model. Figure 3 shows the package diagram
for the NSS-1 satellite model, including packages for requirements, behavior, architecture, and parametrics. Package diagrams
also assist navigating through the model.

Requirement Diagram
The requirement diagram defines requirements, traceability, and
the test cases that verify each requirement. Figure 4 shows the requirements contained in the requirements package to specify the
NSS-1 system. For demonstration purposes, some requirements
are suppressed to save space.
The relationships among the requirements and other model
elements are important aspects of requirement diagrams. These
include relationships for defining a requirements hierarchy, deriving requirements, satisfying requirements, verifying requirements,
and refining requirements. Being able to ask a modeling tool to
generate requirement traceability and verification matrices, and
to perform an automated impact analysis when the requirements
change, is an important time-saving feature for systems engineering. SysML requirement diagrams in this context are quite helpful
to assess NSS-1 requirement specifications and their verification
and validation (V&V).

Figure 3.

NSS-1 model organization.

IEEE A&E SYSTEMS MAGAZINE

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