Satellite Subsystems: The Anatomy of a Spacecraft

10 Essential Subsystems Working in Concert

A satellite is a system of systems. Each subsystem has a distinct function but relies on others for power, data, and thermal stability.

1. Structure & Mechanisms (Mechanical): Bus (primary structure), deployables (solar arrays, antennas, booms), separation system, vibration damping. Survives launch loads.
2. Electrical Power System (EPS): Solar arrays (multi-junction GaAs), batteries (Li-ion), PCDU (power conditioning & distribution), MPPT (max power point tracking). Generates, stores, regulates 28V/50V/100V bus.
3. Attitude Determination & Control System (ADCS / ACS): Sensors: star trackers, sun sensors, IMU (gyros/accelerometers), GPS, magnetometers. Actuators: reaction wheels, CMGs, magnetic torquers, thrusters. Maintains pointing (nadir, inertial, target).
4. Propulsion: Chemical (monopropellant hydrazine, bipropellant MMH/NTO) or Electric (Hall, ion, PPT). Orbit raising, station-keeping, momentum dumping, deorbit.
5. Thermal Control (TCS): Passive: MLI blankets, coatings (OSR, white paint), heat pipes, radiators, louvers. Active: heaters (thermostatic), fluid loops (large sats). Maintains -20°C to +50°C for components.
6. Telemetry, Tracking & Command (TT&C / Communications): S-band (command/telemetry), X/Ka-band (high-rate payload downlink), antennas (omni, medium-gain, high-gain deployable), transponders, encryption. Ground station network.
7. Command & Data Handling (C&DH / OBC): On-board computer (rad-hard CPU, e.g., LEON, PowerPC), mass memory (solid-state recorder), RTOS (VxWorks, RTEMS), software (FSW), data bus (MIL-STD-1553B, SpaceWire, CAN). Autonomous fault detection/recovery.
8. Guidance, Navigation & Control (GNC): Orbit determination (GPS, ground tracking, optical), maneuver planning, formation flying, rendezvous/proximity ops. Integrates ADCS + Propulsion + Navigation.
9. Payload: Mission-specific: Optical/SAR imagers, spectrometers, comm transponders, scientific instruments (particle detectors, magnetometers), technology demonstrators. The "raison d'être."
10. Flight Software (FSW) / Avionics Software: Not a hardware subsystem but critical. Mode management, autonomy, fault protection, scripting, patching. Runs on C&DH.

Cross-Cutting Concerns

• Radiation Hardening: TID (Total Ionizing Dose), SEE (Single Event Effects) mitigation for all electronics.
• EMC/EMI: Strict electromagnetic compatibility.
• Contamination Control: Molecular/particulate (optics, thermal surfaces).
• Planetary Protection: Sterilization (if landing/orbiting protected bodies).

Integration & Test (I&T) verifies subsystem interfaces. Launch loads, thermal vacuum, vibration, acoustic, EMC, functional. Then: launch, LEOP, commissioning, operations.