Model-Based Systems Engineering (MBSE) promises rigor, consistency, and early validation, while NASA’s core Flight System (cFS) delivers a proven, modular framework for embedded flight software. Bridging these two worlds requires more than diagram-to-code translation; it demands a disciplined mapping of abstract SysML semantics onto cFS’s message-centric runtime and a governance approach that sustains traceability under flight-quality constraints. This article presents a practical mapping from SysML artifacts to cFS components and outlines a traceability strategy that keeps requirements, models, interfaces, and C code aligned from architecture through operations.
Mapping SysML Artifacts to cFS Components
A pragmatic starting point is to treat SysML Blocks as cFS applications or services and to explicitly model the cFE infrastructure as a reusable library. Stereotyping helps: cfs::Application for executable flight apps, cfs::Service for cFE-provided capabilities (SB, EVS, ES, TIME, TABLE, SCH), cfs::Table for configurable parameters, and cfs::Message for command/telemetry definitions. SysML ports or interface blocks map to Software Bus topics; connectors capture SB subscriptions, pipes, and publish/subscribe relationships. Allocation relationships then bind Blocks to deployment constraints (processor, partition) and nonfunctional properties (CPU, memory) that cFS must satisfy.
Behavioral models guide how these elements execute under cFS. Activity and sequence diagrams capture command and telemetry flows—e.g., a Scheduler (SCH) wakeup message leading to housekeeping (HK) collection, SB publishing, and Event Service notifications. State machines define application modes (Init, Standby, Operate, Safe) and transition guards tied to health thresholds or fault responses; these modes map to the app’s main loop and command handlers. Parametric diagrams express constraints that drive Table contents (limits, calibration constants, scheduling periods), making the link from analysis models to cFS TABLE service explicit.
Data definitions and ICDs close the loop between SysML and the C data types required on the Software Bus. Interface Blocks with typed value properties map to message payloads; these can be transformed into cFS Electronic Data Sheet (EDS) entries or directly into C struct definitions and message IDs. Allocation of message fields to CCSDS primary/secondary headers and versioning policies can be captured at the model level to standardize packing, endian, and unit semantics. The result is a single source of truth for commands, telemetry, and tables that feeds both the ICD and the codebase.
Ensuring Traceability from Models to Flight Code
Traceability begins by assigning stable identifiers and establishing link types across artifacts: requirements to SysML elements, SysML elements to cFS components, and cFS components to code and tests. Requirements (shall statements) trace to Blocks, State Machines, and Interface Blocks; these, in turn, allocate to cFS apps, messages, and tables. Each message and table receives a unique, versioned ID carried from the model into EDS entries, C headers, and documentation, ensuring consistent naming and preventing SB ID collisions.
Automation cements these links. A model-to-artifact toolchain exports EDS or IDL, generates C headers for message payloads, scaffolds command/telemetry handlers, and emits ICD tables. The continuous integration pipeline builds cFS, executes unit tests (e.g., with UT-Assert), and runs integration tests on a simulation bench (such as NOS3) using model-derived test vectors. The pipeline produces a verification matrix that correlates requirement IDs with test results and code coverage, enabling rapid impact analysis when a requirement or interface changes.
Governance policies keep the system aligned as it evolves. Model baselines and interface control working groups enforce “interface freeze” and semantic versioning of messages and tables; any change request must update SysML, regenerate artifacts, and pass regression tests before merge. Code reviews include model conformance checks: are message IDs, field types, event IDs, and table schemas consistent with the model? Metrics such as model-to-code drift, test coverage per requirement, and message usage coverage on the SB provide objective evidence of continued alignment from model through flight code.
Integrating MBSE with cFS is less about replacing the craftsmanship of flight software and more about systematizing it: SysML formalizes intent, cFS operationalizes it, and automation preserves the threads between them. By mapping Blocks, behaviors, and data definitions directly onto cFS applications, messages, and tables—and by institutionalizing traceability in the toolchain and governance—teams can accelerate development without sacrificing rigor. The payoff is a flight codebase that remains consistent with its model, resilient to change, and auditable from requirement to executable binary.
