Command & Data Handling (C&DH)

Command and data handling is the avionics subsystem at the centre of every spacecraft: an onboard computer (or redundant pair) plus data buses, memory and interface electronics that together run the vehicle. Upward, it receives and validates commands from the ground — immediate actions, stored sequences to execute later, software updates — and distributes them to subsystems over standard buses. Downward, it gathers housekeeping telemetry from hundreds of sensors (temperatures, voltages, wheel speeds, tank pressures), packs it into frames, and hands it to the communications system alongside the payload's science or imaging data, buffered in solid-state recorders between ground contacts. It also hosts the flight software: the control loops, mode logic and — critically — fault management, the watchdog layer that detects anomalies and drops the spacecraft into a safe mode (Sun-pointed, minimal loads, waiting for the ground) when something looks wrong. Space computing's defining constraint is radiation: processors must tolerate bit-flips from charged particles, so C&DH design leans on hardened or error-corrected electronics, voting architectures and conservative margins — one reason flight computers trail consumer hardware by generations while far exceeding it in reliability.