Aerospace and Electronic Systems Magazine April 2018 - 32

Model-Based Systems Engineering in Small Satellite Conceptual Design
view, inspection, test, or analysis. NSS-1 is composed of seven
subsystems. Each subsystem has a mass, and there is a total mass
budget for the satellite of 50 kg. The ACS (Attitude control subsystem), CDHS (Command and data handling subsystem), payload,
TCS (Thermal control subsystem), and RFCS (Radio frequency
communication subsystem) subsystems draw electrical power
from the EPS (Electrical power subsystem), which can supply a
maximum of 80 W. Given the masses of all subsystems and the
power requirements of five subsystems, we calculated the total
mass and power demand for the satellite and compare it against
the requirement.
To meet the mass and power constraints, the parametric diagram is created from the NSS-1 satellite block as shown in Figure
9. Each block in this diagram corresponds to a satellite subsystem
modeled in the BDD (Figure 6). The constraint blocks implement
the mass and power budget constraint formulas. After connecting
the parameter ports to respective inputs and outputs, we validated
the model using ParaMagic.
The following solvers are supported by ParaMagic for mathematical analysis: Mathematica, Modelica, and MATLAB satisfiability modulo theory (SMT). For our analysis, we used the local
MATLAB SMT solver to analyze and tune mass and power allocations to various subsystems. After assigning causalities and values
to all parameters, MATLAB SMT is invoked within the ParaMagic
to solve for unknown and target parameters. In our case, margins of
safety (MoSs) for power and mass are target (output) parameters,

whereas satellite mass is the unknown parameter to be computed
through the following trivial equation:
mSat = mtcs + mss + mpl + mcdhs + macs + meps + mrfcs
The power constraint on EPS subsystem is computed based on
power demanded by electronic subsystems. The following equation is specified in the constraint block to compute the power budget:
Peps = Ptcs + Ppl + Pcdhs + Pacs + Prfcs
A snapshot of the parameters settings and ongoing solution by the
MATLAB SMT solver is depicted in Figure 10, in which four parameters (mass, pPow, mass_MOS, and power_MOS) are being
calculated based on valued parameters listed in the right-most column, named values.
The values of these parameters can be given directly or may
be imported from Microsoft Excel worksheets. The mass budget
tool was earlier created and enhanced within the Microsoft Excelbased SE platform [20]. In the SysML model repository, the tool
has enhanced capabilities to trace down the requirements, which
in turn are useful for change impact and V&V of the requirements.
The resulting values of the parameters can be seen on the same
dialog after the solution is complete. The values of all parameters
after the calculation are updated to the SysML instance, which
appear as in Figure 11. For brevity, we
have only shown the analysis for four
subsystems.
We can see that mass and power
budget margins are calculated to be
positive (mass_MOS = 0.082 and power_MOS = 0.47), which verify the mass
and power requirements as given in the
requirement diagram (Figure 12). We
have demonstrated only mass and power
budgets in this article. In addition, the
NSS-1 model repository contains the
following analysis tools implemented in
the form of constraint blocks and parametric diagrams:
C

Sensor field-of-view analysis

Reliability budget

Orbital analysis

Tradeoff analysis for ACS configurations

INTERFACE WITH EXISTING SE TOOLS
One of the main objectives of this work
was to establish seamless interfaces between our existing SE design and analysis tools and the SysML repository. With
the help of MagicDraw plug-ins, we are

Figure 10.

Solving budget equations using SysML parametrics.

IEEE A&E SYSTEMS MAGAZINE

APRIL 2018

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