Aerospace and Electronic Systems Magazine April 2018 - 30

Model-Based Systems Engineering in Small Satellite Conceptual Design

Figure 7.

Internal block diagram of payload subsystem.

does, but it does not specify how it is internally performing its
functionality. Use-case diagrams depict the overall functional capability of the system. Users, project managers, analysts, developers, quality-assurance engineers, and anyone else interested in the
system as a whole can inspect these diagrams and understand what
the system is supposed to accomplish.
A use case is a description of a set of actions, called scenarios,
that a system performs to yield an observable result to an actor. An
actor is anything that interacts with the system and exchanges data
and information. It can be a user, external hardware, or another
system.
Figure 8 shows a high-level use-case diagram of serviceoriented onboard software architecture proposed for NSS-1. The
main actors involved in onboard software are various components
at the ground station. System-level scenarios of onboard software
include satellite telemetry processing, telecommand processing,

failure detection isolation and recovery, attitude control system
(ACS) algorithms, and payload management. These application
scenarios are represented as an application software package. The
application scenarios use packet utilization standard services [13]
and spacecraft onboard interface services (SOISs) represented as
a static library package. To support applications, a low-level application programming interface for device drivers and real-time
operating system are accessible to SOISs for providing abstraction
to application software.

SysML Parametrics
SysML parametric diagrams provide a way to integrate engineering analysis models described in mathematical equations and constraints. A parametric diagram is a restricted form of an internal
block diagram. The parametric diagram may contain constraint

Figure 8.

Top-level use-case diagram of NSS-1 onboard software.

30

IEEE A&E SYSTEMS MAGAZINE

APRIL 2018



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