block diagrams to allow engineers to build up models. The models can be dynamic models built from predefined blocks in Simulink. In addition to a time-based engine, Simulink has an acausal solver that allows engineers to create mech- anical, electrical, or hydraulic linkages without having to first solve the closed form differential equations. This means the solver figures out where the equilib- rium point is and then starts time moving forward. Simulink has a finite state engine in it so engineers can model logic. For example, a logic state might include how a spacecraft needs to behave dif- ferently when in ascent mode or in station-keeping mode. The algorithm focuses on mode change, not how long the spacecraft is in a particular mode.
There’s also a discrete event engine inside of Simulink that allows queuing models such as communication proto- cols or air traffic control strategies to be constructed. The platform has these different engines all communicating with each other so the engine can sort of move time forward and backward in the simulation environment to let the logic
engine, discrete event simulation engine, or acausal engine work in parallel.
Briefly remind our readers what MATLAB is, please.
FRIEDMAN: MATLAB is our technical computing environment. The best way to describe MATLAB is that it is a com- bination of a programming language and a powerful data analysis and visualiza- tion environment. Engineers will both build programs in MATLAB and deploy them. There’s a MATLAB complier for deploying outside of MATLAB, and there’s a lot of analysis that gets done inside MATLAB. And so engineers can use MATLAB to essentially pull data in, fit a model to it, test and analyze the model, and then sometimes deploy that model into Simulink as part of the modeling process.
Also, for verification tests, engineers can take all the data into MATLAB, write a script in MATLAB to analyze the data, and identify any anomalies or passing of the test. Engineers also write signal- processing algorithms sometimes inside
of MATLAB because of its array process- ing capabilities.
Let’s switch gears and zero in on your recent announcement, the real purpose for today’s discussion.
FRIEDMAN: Sure. The announcement is about what the Swedish Space Corporation was able to achieve with the Prisma project, a civil mission. The important thing I would say about the Prisma project is that it’s actually the second project. Their first project, another civil mission, was the SMART-1 satellite of the European Space Agency, which they had also developed using Model-Based Design. When they went about developing SMART-1, they saved a tremendous amount of time and they were very happy with the results. And so they reused a lot of the models from SMART-1 in the Prisma project because both systems are general- purpose geostationary platforms.
How did the company use Model-Based Design for the satellites, technically speaking?
MILITARY EMBEDDED SYSTEMS March/April 2011 25
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