Context:

Operational scenarios describe how a system is expected to behave in real-world use cases and environments. They include details about inputs, outputs, interactions, and functional workflows for various operational contexts. Well-defined operational scenarios are crucial because they inform the definition, allocation, and validation of system and software requirements, ensuring all mission goals are addressed.

When operational scenarios are incomplete, ambiguous, or missing, it becomes challenging to derive and allocate accurate and testable software requirements. This can lead to inadequate software development practices, integration issues, testing gaps, and operational inefficiencies. For NASA programs, where software is often mission-critical, this creates significant risks to safety, performance, and mission success.

Key Risks of Undefined or Ambiguous Operational Scenarios

1. Misaligned or Incomplete System and Software Requirements

• Issue: Lack of detailed operational scenarios can lead to misaligned requirements that fail to match actual operational needs. Alternatively, some scenarios may go unconsidered, leaving gaps in system functionality.
• Risk to Program:
  • Systems may fail to perform critical tasks in real-world environments.
  • Additional costs and schedule overruns result from late-stage requirement changes or rework.

2. Inability to Validate System and Software Performance

• Issue: Without operational scenarios, it becomes challenging to define performance benchmarks, functional workflows, or realistic operational conditions under which the system is tested.
• Risk to Program:
  • Verification and validation efforts are unfocused or insufficient, failing to test edge cases or mission-critical workflows.
  • Undetected performance issues emerge during operations, jeopardizing mission execution.

3. Poor Allocation of Requirements to Subsystems

• Issue: Ambiguous operational scenarios make it difficult to allocate system-level requirements to specific hardware, software, and interface subsystems.
• Risk to Program:
  • Misallocation creates inconsistencies during integration.
  • Subsystems may overlap responsibilities or leave gaps, reducing efficiency and functionality.

4. Misaligned Interfaces or Subsystem Interactions

• Issue: Operational scenarios define how different system components (software, hardware, communication protocols) interact during use. Ambiguity leads to poorly defined or mismatched interfaces.
• Risk to Program:
  • Integration issues create failures in functionality or communication between subsystems.
  • Unresolved interface mismatches delay integration testing or require significant rework.

5. Safety Gaps in Critical Systems

• Issue: Operational scenarios are essential for identifying hazards, failure modes, and safety mitigations. Poorly defined scenarios cause safety-related workflows or risks to be overlooked.
• Risk to Program:
  • Safety-critical paths remain unidentified, leading to accidents in human-rated or high-risk systems.
  • Systems lack proper fail-safes to handle emergency conditions or anomalies.

6. Scope Creep or Requirement Volatility

• Issue: Undefined operational scenarios leave gaps in understanding for stakeholders, leading to conflicting or continuously evolving expectations for functionality.
• Risk to Program:
  • Teams face frequent changes to requirements during development, which increases costs, lengthens schedules, and introduces defects.
  • Projects risk exceeding original budgets and deadlines due to scope creep.

7. Ineffective Risk Management

• Issue: Operational scenarios create the foundation for identifying and mitigating operational risks (e.g., environmental variables, human actions, anomaly handling). Without them, risk assessment becomes reactive rather than proactive.
• Risk to Program:
  • Latent risks remain undetected until late in the lifecycle, increasing the chances of failures during operations.
  • Inefficient risk mitigation strategies increase the likelihood of mission-critical incidents.

8. Delayed System Integration Testing

• Issue: The test cases for system integration depend on realistic operational scenarios. Without these scenarios, test coverage is incomplete or misaligned with actual mission conditions.
• Risk to Program:
  • Integration efforts fail to detect critical issues until systems are deployed.
  • Testing efforts are delayed, pressuring schedules and compressing time for defect resolution.

9. Reduced Stakeholder Confidence

• Issue: Stakeholders (sponsors, partners, operational teams) rely on operational scenarios to ensure that mission objectives align with system designs. Poorly defined scenarios reduce confidence in the program’s clarity and direction.
• Risk to Program:
  • Stakeholder mistrust leads to increased oversight or additional reviews, creating delays.
  • Reduced transparency damages long-term relationships with funding entities and stakeholders.

10. Inability to Handle "Off-Nominal" Situations

• Issue: Well-crafted operational scenarios often include off-nominal conditions (e.g., anomalies, edge cases, degraded performance). Missing or incomplete scenarios fail to prepare for these unexpected situations.
• Risk to Program:
  • Missions fail under off-nominal scenarios, such as hardware malfunctions or communication outages.
  • Systems lack robustness to respond to anomalies, causing mission aborts or catastrophic failures.

Root Causes of Undefined Operational Scenarios

1. Incomplete Stakeholder Input:
   • Mission stakeholders (e.g., operators, scientists) fail to provide enough clarity about mission goals, workflows, or end-user needs during the planning phase.
2. Lack of System Engineering Rigor:
   • Programs deprioritize operational scenario development during early lifecycle phases, focusing instead on individual system specifications.
3. Complexity of Missions:
   • For multi-faceted missions, defining all operational workflows can be challenging due to unknowns, ambiguity, or evolving mission parameters.
4. Inadequate Collaboration:
   • Poor communication between systems engineers, software teams, and domain experts results in incomplete or vague operational workflow descriptions.
5. High-Level Generalizations:
   • Scenarios are described using overly general terms, neglecting specific use cases, altered environmental conditions, and edge cases.
6. Challenges in Legacy System Integration:
   • When integrating legacy systems, programs often skip redefining operational workflows, assuming legacy scenarios still apply.
7. Time Constraints During Planning:
   • Programs may rush early lifecycle phases, deferring detailed scenario definitions to later stages of the project.

Mitigation Strategies

1. Conduct Operational Concept Definition Reviews

• Develop and review Operational Concept Documentation (OpsCon) early in the project lifecycle:
  • Define detailed normal and off-nominal scenarios for key mission phases.
  • Engage all stakeholders to identify mission goals, constraints, and success criteria.
• Use the OpsCon as the primary input for system and software requirements.

2. Use Scenario-Based Requirements Engineering

• For every defined scenario:
  • Trace mission workflows to system and software requirements.
  • Align requirements with specific environmental conditions, inputs, and outputs.
• Verify that every unique scenario maps to testable and measurable requirements, improving traceability.

3. Engage Cross-Disciplinary Experts

• Collaborate with domain experts, operators, engineers, and end users to define operational workflows clearly.
  • For space missions, include astronauts, payload scientists, ground control teams, and environmental experts.
• Use joint application development (JAD) workshops to capture detailed user interactions and operational goals.

4. Develop Logical and Physical Models

• Use model-based systems engineering (MBSE) approaches, such as SysML diagrams:
  • Develop activity, sequence, and state diagrams for operational events.
  • Model alternative workflows for anomalous, degraded, and recovery scenarios.
• Align these models with the systems architecture for improved clarity.

5. Simulate and Prototype Operational Scenarios

• Use simulation tools or prototyping environments to test operational scenarios:
  • Verify feasibility of operational workflows early in the software lifecycle.
  • Identify missing requirements by running scenarios in representative environments.
• Example tools: MATLAB/Simulink, NASA’s CoreSim, or real-time simulators.

6. Define External Environmental Constraints

• Conduct environmental impact analysis to define operational challenges:
  • Include extreme conditions (temperature, radiation, noise) that impact system operations.
  • Define workflows for launch, in-mission, and post-mission environmental states.

7. Prioritize Normal and Off-Nominal Scenarios

• Document scenarios for routine operations (e.g., data collection, routine maneuvers) alongside exceptions (e.g., communication loss, emergency recovery).
• Perform a Failure Modes and Effects Analysis (FMEA) to address safety-critical off-nominal situations.

8. Use Risk Assessments to Prioritize Scenarios

• Identify critical operational scenarios that pose the highest risks:
  • Evaluate impacts of missing scenarios using probability-consequence analysis.
• Prioritize the development of high-risk, high-value scenarios to mitigate consequences.

9. Make Scenario Development an Incremental Process

• Develop operational scenarios iteratively:
  • Draft initial scenarios early (e.g., during Concept and Requirements Definition).
  • Refine and validate scenarios as requirements, design, and risks evolve.

10. Establish Formal Scenario Review Milestones

• Include operational scenario reviews as key development milestones:
  • At System Requirements Review (SRR): Validate accuracy and completeness of scenarios.
  • At Preliminary Design Review (PDR) and Critical Design Review (CDR): Assess alignment of scenarios with designs.

Consequences of Undefined Operational Scenarios

1. Increased Rework:
   • Incomplete or ambiguous requirements force costly corrections in later development stages.
2. Delays in Testing and Integration:
   • Poorly defined workflows delay test case development, slowing integration timelines.
3. Mission Failures:
   • Software may fail to perform expected functions under real-world operational conditions.
4. Reduced Flexibility for Anomalies:
   • Systems lack robustness to respond to unexpected use cases, endangering mission safety.
5. Loss of Stakeholder Satisfaction:
   • Program credibility erodes as stakeholders identify overlooked functionality.

Conclusion:

Operational scenarios are the foundation for deriving and allocating accurate system and software requirements. Undefined or incomplete scenarios lead to misaligned requirements, integration challenges, and increased risks during operation. By prioritizing rigorous scenario development, leveraging modeling tools, fostering domain collaboration, and integrating scenario reviews into program milestones, NASA programs can ensure both mission success and stakeholder confidence.

3. Resources

3.1 References