ISO 21351:2005

Overview

ISO 21351:2005 - Space systems - Functional and technical specifications - defines the purpose, content and development process for Functional Specifications (FS) and Technical Specifications (TS) used in space projects. Applicable to all types of space systems, product elements and projects, the standard positions the FS as the baseline for concept investigation and the TS as the contractual baseline for development or procurement. ISO 21351 supports clear allocation of requirements, traceability and effective project management across performance, cost, schedule and risk.

Key Topics

The standard covers a structured approach to writing and managing FS and TS documents, including:

• Purpose and relationship of FS and TS (FS for needs/functional baseline; TS for business/contract baseline).
• Process for establishing initial and final FS and the TS (model workflows and development steps).
• Technical requirements description and attribute identification, such as:
  • Functional, mission and interface requirements
  • Environmental, physical and operational requirements
  • Human factors, logistics and product assurance (PA) requirements
  • Configuration and design requirements
• Requirements and recommendations common to FS and TS:
  • Responsibility, organization and format
  • Unique technical references and configuration management
  • Severity classification, supplementary information and restrictions
• Characteristics of a good requirement: performance, justification, traceability, unambiguity, uniqueness, identifiability, completeness and prioritization.
• FS-specific characteristics: flexibility, severity and maturity.
• TS-specific characteristics: verification, attainability, tolerance and risk.
• Wording and format rules to ensure clear, testable requirements.

Normative references in ISO 21351 include EN 13701 (space glossary) and ISO 17666 (risk management).

Applications

ISO 21351 is practical for:

• System engineers and requirements engineers creating FS/TS for satellites, payloads, launch support and ground segments.
• Project managers and prime contractors who need clear baselines for concept selection and procurement.
• Procurement, product assurance, verification & validation and configuration management teams to ensure traceability and contractual clarity.
• Suppliers and subcontractors that must interpret FS/TS as contract deliverables.

Benefits include improved requirement clarity, reduced ambiguity, enhanced traceability, better verification planning and lower program risk.

Related Standards

• ISO 14300-1 (recommends FS and TS use)
• EN 13701 (Glossary of terms referenced)
• ISO 17666 (Space systems - Risk management)
• ECSS and EN references cited in the standard (e.g., ECSS-E-10 guidance)

Keywords: ISO 21351, functional specification, technical specification, space systems, requirements management, configuration management, traceability, verification, system engineering.