ISO 22671-2021 PDF

Full title and description

Space data and information transfer systems — Space link extension (SLE) — Forward communications link transmission unit (CLTU) service specification. ISO 22671:2021 defines the Forward CLTU (Command/Communications Link Transmission Unit) service as an abstract SLE transfer service that enables a mission user to send CLTUs to a spacecraft; it specifies the required operations, parameters and resulting behaviors without mandating particular implementations or ground/radio technologies.

Abstract

ISO 22671:2021 specifies the Forward CLTU service in an abstract, implementation‑neutral form. The document defines the operations that comprise the service, the parameter data associated with each operation, the expected behaviors when operations are invoked, and the valid sequences and relationships between operations and behaviors. It is intended to enable interoperable cross‑support between mission and ground systems while leaving specific implementation, communications and radiative technologies to the implementer.

General information

• Status: Published
• Publication date: June 2021 (Edition published 2021)
• Publisher: International Organization for Standardization (ISO)
• ICS / categories: 49.140 (Space data and information transfer systems)
• Edition / version: Edition 4 (2021)
• Number of pages: 143

Scope

This standard defines the Forward CLTU transfer service as part of the Space Link Extension (SLE) family. It covers the abstract service model including operations, request/response parameters, state machines and allowed operation sequences required to provide CLTU transfer to a spacecraft. The scope explicitly excludes specifying particular hardware or software implementations, radio transmission and reception techniques, ground station internal interfaces, or operational management and scheduling procedures.

Key topics and requirements

• Abstract service definition for Forward CLTU including service operations and expected behaviors.
• Parameter definitions for each operation (inputs, outputs, constraints and optional elements).
• Service state model and valid operation sequencing (state transitions, allowed/forbidden calls).
• Error reporting, fault handling and notification semantics relevant to CLTU transfer.
• Interoperability considerations with the SLE reference model and related SLE transfer services.
• Conformance requirements for service providers and users at the protocol/behavioral level (not implementation level).
• Compatibility and mapping guidance with established CCSDS SLE API and recommended practices.

Typical use and users

ISO 22671:2021 is used by space agencies, mission operations centres, ground segment integrators, satellite communications vendors, and software developers implementing SLE service providers or clients. Typical activities include designing and testing SLE interfaces for uplink CLTU delivery, verifying cross‑agency interoperability, developing ground station middleware that exposes the Forward CLTU service, and preparing procurement or interface requirement documents for multi‑agency support.

Related standards

ISO 22671:2021 is part of the Space Link Extension family and relates to other SLE and CCSDS documents such as the SLE Cross Support Reference Model and SLE transfer service specifications (for example, Return‑All‑Frames and Return OCF specifications), forward space packet service specifications, and API/recommended practice documents that describe SLE APIs and transport mappings. These companion standards together define the broader SLE framework for interoperable ground‑to‑space and cross‑support services.

Keywords

Space Link Extension; SLE; CLTU; Forward CLTU; command uplink; space data transfer; ground segment; interoperability; CCSDS; service specification; telecommand.

FAQ

Q: What is this standard?

A: ISO 22671:2021 is the international service specification that defines the Forward CLTU (command/communications link transmission unit) service within the Space Link Extension (SLE) framework.

Q: What does it cover?

A: It covers the abstract service model for sending CLTUs to a spacecraft — the operations, parameters, behaviors, state transitions and allowable sequences — while deliberately omitting implementation details and physical communications methods.

Q: Who typically uses it?

A: Space agencies, ground segment developers, SLE service implementers, system integrators, and vendors involved with telecommand uplink services and interoperability testing use this standard.

Q: Is it current or superseded?

A: The document is the 2021 edition (Edition 4). It supersedes earlier editions; users should check their national/organizational catalogues for any later confirmations or revisions beyond 2021 before relying on it for new procurements.

Q: Is it part of a series?

A: Yes. It is part of the SLE/space link extension set of standards and is intended to be used alongside related SLE service specifications, the SLE reference model, and relevant CCSDS recommended practices and API documents.

Q: What are the key keywords?

A: SLE, CLTU, forward CLTU, telecommand uplink, space link extension, interoperability, CCSDS, service specification, ground segment.