Model-based software engineering for an optical navigation system for spacecraft

Abstract

The project ATON (Autonomous Terrain-based Optical Navigation) at the German Aerospace Center (DLR) is developing an optical navigation system for future landing missions on celestial bodies such as the Moon or asteroids. Image data obtained by optical sensors can be used for autonomous determination of the spacecraft's position and attitude. Camera-in-the-loop experiments in the TRON (Testbed for Robotic Optical Navigation) laboratory and flight campaigns with unmanned aerial vehicle (UAV) are performed to gather flight data for further development and to test the system in a closed-loop scenario. The software modules are executed in the C++ Tasking Framework that provides the means to concurrently run the modules in separated tasks, send messages between tasks, and schedule task execution based on events. Since the project is developed in collaboration with several institutes in different domains at DLR, clearly defined and well-documented interfaces are necessary. Preventing misconceptions caused by differences between various development philosophies and standards turned out to be challenging. After the first development cycles with manual Interface Control Documents (ICD) and manual implementation of the complex interactions between modules, we switched to a model-based approach. The ATON model covers a graphical description of the modules, their parameters and communication patterns. Type and consistency checks on this formal level help to reduce errors in the system. The model enables the generation of interfaces and unified data types as well as their documentation. Furthermore, the C++ code for the exchange of data between the modules and the scheduling of the software tasks is created automatically. With this approach, changing the data flow in the system or adding additional components (e.g. a second camera) have become trivial.

Model-Based Reconfiguration Planning for a Distributed On-board Computer

“…This includes task stubs, configuration files, decision graphs and some other files. They are generated based on templates filled with the data from the model (see [6]).…”

ATON (Autonomous Terrain-based Optical Navigation) for exploration missions: recent flight test results

“…It ensures that a module is only executed if all necessary inputs are available. The integration of the ATON software was conducted in a model-driven manner: an extended SysML/UML model was created to describe the processing modules with their interfaces and parameters, data types, priorities and the data flow between the modules [7]. Custom code generators create the source code for data types, communication, module interfaces, and serialization code for the telemetry.…”

“…-Use model-driven software development: Since the core of a navigation fusing optical and inertial sensor data is a complex software, a model-driven software development is recommended [7]. It allows to control and adapt the interfaces of the single modules in a consistent way.…”

Using an UAV for testing an autonomous terrain-based optical navigation system for lunar landing

“…For the development of the ATON software a model-driven software development methodology was used. An SysML/UML 2 model of the ATON software was used to automatically generate the source code for data types, communication, module interfaces, and serialization code for the telemetry [5].…”