Simulating Spacecraft.  How to do it faster, cheaper and better with
graphical modelling

R.A.Foweraker,        A.Walsh
RFOWERAK@ESOC.ESA.DE  AWALSH@VEGA.FIBRONICS.DE
VEGA


Abstract:
Developing an accurate simulation of a spacecraft is a costly
and time-consuming task.  The developers must model the interaction
of many complex objects in different subsystems and keep track
of how these objects are connected.  As a result of tight budgets
and incomplete or poorly understood documentation, modelling
errors always occur.  The use of object oriented analysis and
design methodologies helps greatly, but a fundamental problem
is that it is not easy to identify these modelling errors in
textual source code, whatever language or environment is used.
A solution to this is graphical modelling.

This paper describes the experiences and lessons learned 'M
usmg a COTS graphical simulation tool, ROSEÕtmþ, to develop
a simulation of a fictional three-axis stabilised, sun-pointing
spacecraft called MiniSV.

A graphical modelling enviromnent represents objects with icons.
The attributes and behaviour of the individual objects are
defined and then familiar graphic icons are assigned to each.
These icons are connected together on schematics, produc'mg a
highly realistic image of the system being simulated. The
source code is generated directly from the schematics ensuring
model fidelity.  At run-time the model is viewed with the same
schematics used to create the model.  Individual object
attributes can be seen for each object.  Modelling errors can
often be seen directly from the way the objects are connected
on the schematic.

As a result of this work we believe that higher quality
spacecraft simulators can be developed and maintained for
less than one half of the current cost.  We describe the many
advantages of using graphical modelling techniques for
spacecraft simulation and how it changes the whole development
lifecycle.  We identify the features of the ROSE tool that are
responsible for the significant time savings experienced.
Finally, we propose new ideas on how spacecraft simulators
can be developed in the future.