Aerospace and Electronic Systems Magazine April 2018 - 28

Model-Based Systems Engineering in Small Satellite Conceptual Design

Figure 4.

NSS-1 requirement diagram.

Block Definition Diagram
The BDD defines the physical architecture of the system. We have
displayed various model elements and relationships between NSS1 design elements on a BDD to express information about spacecraft architecture. We have also adopted design techniques for creating extensible system structures. This practice reduced the model
development time. In the future, this will also reduce the cost to
change the design as the stakeholders' needs evolve [2].
The top-level block diagram of the NSS-1 is depicted in Figure 5. SysML diagrams have underlying semantics for each of the
modeling elements. The diamond-ended arrow indicates the directed composition relationship between the blocks. The diagram
may be interpreted as "the NSS-1 system is composed of a space
segment, a ground segment, and associated launch services." The
BDD of the NSS-1 satellite is given in Figure 6, showing internal
parts of each subsystem as SysML blocks.

ment. They formally describe the qualitative flows between model
elements. Internal block diagrams are created for each block of a
BDD, which represents its internal parts in the form of instances
connected with one another and the environment through ports in a
specific context. As an example, Figure 7 shows the internal block
diagram of the payload subsystem, which consists of a narrowview camera (NVC), wide-view camera (WVC), an experimental
payload for lightening detection, a payload processing unit (PPU),
and the payload data transmitter (PLTx). The NVC and WVC send
the raw image data to the PPU via the LVDS (Low-voltage differential signaling) and I2C (Inter-integrated circuit) interface, respectively. The lighting detection unit sends lighting data to the PPU
through the RS-422 interface. The PPU processes, compresses, and
packetizes all payload data and sends them to the PLTx for onward
transmission to the ground station. The telemetry and telecommand control of the payload subsystem is carried out through the
CAN bus with the main OBC (On-board computer).

Internal Block Diagram

Use-Case Diagram

The internal block diagram shows the interconnection and external
interfaces between the system and its external user and environ-

SysML use-case diagrams are similar to standard UML use-case
diagrams. Use-case view shows what a system subsystem or unit

28

IEEE A&E SYSTEMS MAGAZINE

APRIL 2018



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