ROSESAT -- A GRAPHICAL SPACECRAFT SIMULATOR FOR RAPID PROTOTYPING

Transcription

1 ROSESAT -- A GRAPHICAL SPACECRAFT SIMULATOR FOR RAPID PROTOTYPING Xavier Cyril Space Systems Engineering, CAE Electronics Ltd Cote de Liesse, Saint Laurent, Quebec, Canada H4T 1G6 FAX: (514) , cyril@cae.ca ABSTRACT In the early system definition phase of any spacecraft development project there are few known parameters and many assumptions. Finding the optimum solution, from the wide variety of available technologies and techniques, is an onerous task for the development team who, for the sake of efficiency, depend on available analysis and simulation tools. While providing good support in the specific domains to which they are focused, these tools are generally poor in providing a total system perspective, in particular an end-to-end simulation. This paper discusses the use of a rapid prototyping tool, ROSESAT, that provides quick access to the realization of the proposed system through simulation. Together with the generic object libraries provided, ROSESAT enables the development team to quickly configure end-to-end simulations of desired spacecraft missions. The paper begins with an introduction followed by a step-by-step discussion of the rapid prototyping exercise. Examples are enumerated through representative spacecraft system schematics. The ability to quickly reconfigure various subsystem components to evaluate the system performance is demonstrated. This exercise culminates in a baseline simulation of the spacecraft together with a matching system specification and schematic representations of each of the spacecraft sub-systems. It will be shown that the use of this baseline simulator supports design verification by helping eliminate the ambiguities associated with a text based description of the system requirements. These simulators provide a comprehensive working model of the spacecraft into which high fidelity models can quickly be developed, inserted and validated as the development cycle progresses. It will be described how the same simulator can support the entire development life cycle of the spacecraft and eventually become the mission operations support simulator. The paper concludes with a section on the experience CAE has gained on rapid prototyping using ROSESAT and the lessons learned in this process. Keywords: Graphical Spacecraft Simulation, Rapid Prototyping, System Definition, System Performance, System Specification, Simulation Cost, Verification and Validation. 1. INTRODUCTION In the new millennium an increasing number of spacecraft will be launched, both for scientific exploration and earth applications. The ensuing highly competitive environment will require that high quality space systems be developed faster and cheaper than in the past. This will translate to technical and operational requirements being considered at the earliest stage of the development life cycle. This is possible through end-to-end system simulation of the spacecraft. The application of such a simulation will enable spacecraft systems to be conceptually designed before any resources are committed for the project. Once the spacecraft to be developed is realized through simulation, the 1

2 functional, performance and operational requirements can be optimized to meet the cost and schedule budgets. At the end of this exercise the project team will have a system requirements specification and a preliminary system architecture supported through a fully functional spacecraft simulator. At this point the project team can seek the budget appropriation and remain confident that the cost and schedule targets are realistic. Spacecraft simulators should be an integral part of the development life-cycle. However, developing an end-to-end spacecraft simulator has traditionally been an expensive and time consuming endeavor. Hence, it is usually developed only for the purposes of system verification, training and mission support, which happens much later in the development life cycle. CAE has developed a tool called ROSE (Real-time Object-oriented Simulation Environment), which, along with its associated application-specific object libraries, provides the rapid prototyping environment ideally suited to support the development life cycle. ROSESAT (Real-time Object-oriented Simulation Environment for Spacecraft Analysis and Testing) is the application-specific implementation of the ROSE. In this article, the rapid prototyping of spacecraft design using ROSESAT is presented, followed by a description of some of its other uses in the development life cycle. The article concludes with a summary of our experience in using ROSESAT on various space projects. 2. RAPID PROTOTYPING USING ROSESAT ROSESAT is an advanced interactive visually assisted modeling and simulation environment that can help users rapidly design spacecraft systems through scaleable concurrent simulation. Users can perform what-if analyses, navigate through the hierarchical structure for details, browse between schematics representing different subsystems, tune parameters and visualize results. Users can build, operate and test the design before committing any substantial resources or materials to the project. ROSESAT is an application-specific implementation of CAE ROSE. It consists of a graphical model development environment and a real-time simulation and testing environment. The graphical model development environment supports the building of an integrated spacecraft simulation consisting of all the subsystems, namely, power, thermal, propulsion, guidance, navigation and control, telemetry and command, and data handling. It allows users to describe a system or a subsystem by drawing schematics using components found in the object libraries. It allows a bottom-up design approach, whereby the systems are built using small building blocks called objects. Each object is a graphical representation of a mathematical function, a physical component or a complete spacecraft subsystem. The objects contain associated pseudo code describing the object functionality, interface information, associated descriptive text and parameters with default values. All of the objects are scaleable, such that the user may, via the calibration process, customize each object for its intended use, taking into account the particular design parameters of the specific system being modeled. While some objects are more generic than others, most can be adapted to account for design differences typically encountered, without the need to create new objects. Users can also reuse their existing simulation model code and encapsulate it in an object. Once in the library, this object can then be instantiated as many times as desired in the user s schematics. This is a very effective way of reusing validated legacy code. To define a model, the user drags and drops the objects available in the library into a schematic and connects them together. For example, as represented in (Figure 1) the spacecraft propulsion model is created by simply dragging and dropping the propulsion objects from the library window into the subsystem model window. Each object may have one or more connect points representing its external interface. Data compatibility is verified when two objects are being connected and incompatible connections are not permitted. Objects available in the libraries are validated objects and hence the 2

3 user can build the model with confidence. Several such models, each representing a subsystem can then be interconnected by simply drawing a line between the connecting objects on two or more schematics. Propulsion Library Propulsion Subsystem Figure 1 Rapid Model Building Environment The currently available libraries for spacecraft subsystem simulation are: Mathematics - contains mathematical functions needed to simulate control systems and to build more complex models. Dynamics - contains objects needed to simulate spacecraft dynamics, including perturbation calculations. Thermal - contains objects needed to simulate the thermal behavior of the spacecraft, taking into account the heat generated by the components, the heat flow due to internal radiation and conduction and external radiation. Propulsion - contains objects needed to simulate the spacecraft propulsion subsystem, including fuel storage, distribution and consumption. It supports both mono and bi-propellant applications. Power - contains objects needed to simulate the spacecraft electrical power generation and distribution subsystem. TM/TC - contains objects needed to simulate the command and data handling subsystem of the spacecraft. 3

4 On-board equipment - contains objects needed to simulate the equipment behavior, such as, actuators, sensors, GPS elements, etc. Failure modes (malfunctions and biases) are modeled and can be activated by the user. On-board software - contains objects needed to simulate the spacecraft control systems, such as, attitude and orbit control system. Each library is associated with a code generator, which produces optimized code for real-time execution. The two types of code generators available are sequential and network. The sequential code generator translates discrete model blocks from pseudo code to a target high-level language. The network code generators optimize and solve the distribution network model (thermal, hydraulic, electric) using the admittance matrix method. Power Subsystem Communication Attitude Control Subsystem Figure 2 Integrated Real Time Simulation Environment Once the models are defined, the simulation code for a defined set of schematics is automatically generated. The user can specify graphically the rates at which the models are to be executed. The resulting configuration can then be tested and the system design verified within the real-time simulation environment. The schematics representing the subsystem models come alive within this environment. In addition to display of the schematics, at run-time the objects can be animated to provide instant feedback to the user on the status of the model and/or the object. During simulation the user can navigate between schematics representing various subsystems (Figure 2) and by clicking on an object can investigate and plot the simulation variables associated with that particular object. 4

5 This feature allows the user to analyze the design variables in real-time. If the user chooses, he is also able to run batch simulations and gather data for later analysis using the tools provided or any other third party tools. The simulation environment provides the capability to carry out what-if scenarios through quick changes to the object characteristics and/or trading off objects representing different equipment. Making changes to the simulation configuration is rapid and the simulation can be rebuilt in a matter of minutes. An added feature is that while the simulation is running, parameters can be altered to see their impact without having to reload the simulation. The integrated multidisciplinary simulations of ROSESAT promote team work and help resolve interface issues early in the development life cycle, which otherwise could become a major cost driver during the integration of the various subsystems. Within this environment engineers of different design disciplines can work concurrently, and quickly determine the impact of their changes on the design of other subsystems. Subsystem optimization and tradeoffs between subsystems is facilitated within this environment, which are not fully exploited in the traditional ways of doing design, since they are very labor intensive. In addition to the standard plotting capability, the user can visualize in 3D the performance of the spacecraft (Figure 3). The user can specify several store points in a given simulation which he can later restore to analyze a specific segment of the mission without having to run the entire simulation. Space Station ATV Figure 3 Interactive Testing Environment 5

6 At this stage the simulator serves the purpose as a conceptual design tool and a requirements definition tool. Once the requirements are satisfactorily defined the project kicks off and the simulator becomes a tool to aid in the development effort. During the requirements analysis stage of the project the ROSESAT simulator can be used to validate the requirements; in parallel, the simulator can be enhanced based on the feedback obtained from this phase. In essence, the development effort is aided through the use of the simulator and the simulator in turn gets more mature with higher fidelity models as the various phases of the project get underway. In the design phase of the project the detailed design of the spacecraft can be verified with the same tool, where the user may spend less time on the simulation development and more time focusing on the design issues (that is, running simulations of various scenarios under nominal conditions and failure conditions). 3. OTHER APPLICATIONS OF ROSESAT As discussed in the previous section ROSESAT is a powerful tool for prototyping the spacecraft design through simulation. However, it can support several other functions in the development effort, such as, design verification and subsystem integration through hardware-in-the-loop (HITL) simulations (Figure 4). At the end of the design cycle the simulator provides a virtual spacecraft environment where real-world and simulated components may be mixed together to carry out functional testing at the subsystem and system level. Interface Electronics Unit HITL Gating Schematic Figure 4 Hardware-In-The-Loop Simulation 6

7 The last two decades have seen the extensive use of spacecraft simulators for mission preparation, training and mission rehearsal. During the mission preparation phase ROSESAT can be used to develop, test and validate spacecraft operations procedures. To support this phase the simulator has the capability to cover all mission phases and modes of operation, including contingency situations, where the simulation reacts to tele-commands as the real spacecraft would. Also, during this phase spacecraft operators need to be trained on characteristics of the spacecraft, its mission and the operations. ROSESAT has all the features needed to support this activity, in fact, its development originates from the nuclear power plant training simulators. Months before the launch of any spacecraft an end-to-end simulation is conducted to rehearse the launch sequence and mission operations. This again can be carried out using ROSESAT. Spacecraft operation involves intense data transfer between the onboard software and ground station. Consequently, telemetry interpretation (for anomaly isolation) and command verification are important tasks requiring robust real time simulation capability[1]. At this point in the system life cycle, the simulator reflects the as built system and includes all the nominal and off nominal characteristics of the system. It is now ready for mission support and system maintenance. 4. SUMMARY ROSESAT has been successfully used on several spacecraft projects spanning all phases of the system life cycle; a description of some of these projects and the lessons learned is presented in this section. For the European Space Agency, CAE has developed a generic space vehicle simulator[2] and demonstrated its application for prototyping the Automated Transfer Vehicle (ATV) design. The rapid prototyping capability of ROSESAT was first demonstrated within this project, whereby the entire ATV was functionally simulated with 3D visualization within a couple of months. This was largely possible because of the validated object libraries and the ease with which the tool allows one to put together a complete simulation. Within the Canadian Small Satellite Program the Hardware-In-The-Loop (HITL) simulation capability of ROSESAT was demonstrated. It was used to demonstrate the critical and enabling technologies[3] needed for small satellites. This HITL simulation system can incorporate into the simulation early hardware prototypes, actual spacecraft components, and eventually the complete spacecraft. It consists of ROSESAT running on an SGI workstation, an Interface Electronics Unit that provides the necessary hardware interfaces for HITL simulation, and a Test Bed (Bristol Aerospace s Power Subsystem) that allows for testing the hardware prototypes and flight software on the embedded controller. From start to finish the project took 8 months. Carleton University students in Canada are using ROSESAT for designing a student satellite[4]. This satellite is in the preliminary design stage and simulation is the primary mode for verifying the concepts brought forth by the students. Typically, the students have to complete their design project within 4 months. Vega Space Systems in Germany used ROSESAT to configure a real-time simulator for operations purposes within 3 months. The objective of this exercise was to demonstrate the power of the graphical simulation environment in the development of operations simulators[5], which traditionally tends to be a lengthy and expensive process. They were also able to effectively demonstrate within this project, that simulation models developed and validated previously can be reused within the ROSESAT environment. Vega is currently using this tool to build a spacecraft system verification facility. 7

8 Lessons learned in using a graphical simulator for spacecraft development: Availability of validated objects makes spacecraft design prototyping a relatively easy exercise. The ability to encapsulate existing simulation models shortens simulator development time. Ability to functionally simulate all aspects of the spacecraft in a unified and consistent manner facilitates system design. Combination of graphical modeling and auto code generation provides large savings in simulator development cost. Visual interactive simulations combined with quick reconfiguration capability allows the possibility to test a large set of design scenarios, which is generally not carried out in the traditional methodology. The virtual elimination of software coding provides more time to the designer to concentrate on the design issues. Potential to investigate the effects of multiple failures provides the means to conduct FMECA. 5. ACKNOWLEDGEMENTS The work reported here is made possible because of the dedication and hard work of many CAE engineers and developers. I wish to acknowledge their contribution to the development of the tool set. I would also like to thank CAE for permitting the publication of this article. REFERENCES 1. Begin M., and Cote D., Graphical Spacecraft Simulation for Mission Operations Support, CASI Symposium, St. Hubert, Canada, St-Pierre J., Cyril X., and Royle A., An Engineering Simulator for Manned Space Vehicles, Third Workshop on Simulators for European Space Programmes, Noordwijk, Netherlands, Tafazoli S., St-Pierre J., and Tyc G., Attitude Control System Design Using CAE ROSE, Fourth Workshop on Simulators for European Space Programmes, Noordwijk, Netherlands, Saraf S., Heinemann M., Habtemichael W., Staley D., and Cyril X., Sila - A Carlton University Student Satellite, To appear in the CASI Journal, Foweraker R., O Gorman A., and Walsh A., Simulating Spacecraft. How to do it Faster, Better and Cheaper with Graphical Modelling, Space Ops96, Munich, Germany, ROSESAT and ROSE are trademarks of CAE Electronics Ltd. 8