telecommand

Telecommand is the transmission of commands to a remote system to perform actions. In spaceflight, telecommand (TC) refers to instructions from ground stations or mission control to a spacecraft or its subsystems, enabling operations such as attitude adjustments, mode changes, instrument activation, or safe-mode entry. Telemetry (TM) is the reverse data flow, from spacecraft to ground.

Telecommands are digital and typically packetized data sent over radio or optical links. The spacecraft decodes, validates, and executes commands, which may be performed immediately or as part of a planned sequence or stored procedure. Command sequencing enables automation of routine operations and coordinated actions across subsystems.

Standards and encoding: Telecommand structure is defined in space data system standards, notably the CCSDS Telecommand framework and related Packet Utilization Standards. These specifications cover how commands are packaged, transmitted, and verified, including frames and verification fields. Missions may augment these with mission-specific command dictionaries and validation rules.

Security and reliability: Command transmission includes error detection, often with CRCs or parity checks, and may employ redundancy to tolerate link issues. Security measures such as authentication and encryption are increasingly used to prevent unauthorized commands, especially for critical systems; older missions may rely on lighter protections.

Applications and scope: Telecommands are essential for spacecraft operations, payload control, and testing. The ground segment typically includes mission operations centers, uplink facilities, and command-processing software, all guided by safety and reliability requirements to ensure correct and timely actions.