The space segment is the part of the system that is physically in space: the satellites or platforms, payloads, inter-satellite links, etc. It encompasses: • Satellites / platforms: the bus (platform) plus the payload. • Bus subsystems: o Structure and mechanisms o Power (solar arrays, batteries, power conditioning) o Attitude and Orbit Control System (AOCS) – sensors and actuators to point and stabilize the satellite and control its orbit o Thermal control – passive and active thermal regulation o On-board data handling – computers, data storage, internal networks o Telemetry, Tracking & Command (TT&C) – housekeeping communications o Propulsion – for orbit insertion, station-keeping, collision avoidance, de-orbit • Payload: the mission-specific instruments or communication equipment. • Constellation architecture: number of satellites, orbital planes, orbital parameters, inter-satellite links, etc. Specifically, per system type: EO – Space segment • Orbits: Mostly LEO, often Sun-synchronous, with altitudes ~500–800 km to balance resolution and coverage. Some missions use non-SSO (e.g. inclined orbits) or highly elliptical orbits for special coverage. Some others are GEO (e.g. many meteorological satellites, such as MeteoSat, GOESS, Himawari…) • Payloads: Optical imagers, multispectral/hyperspectral sensors, SAR, TIR radiometers, microwave radiometers, altimeters, GNSS-R, etc. • Key features: o High pointing stability and knowledge to ensure geometric accuracy. o Agile platforms for off-nadir pointing, target tracking, or stereo imaging. o On-board mass memory and high-rate downlinks. o Calibration hardware (on-board lamps, blackbodies, lunar or sky views, etc.). • Constellations: single satellites vs. multi-sat constellations to reduce revisit time; sometimes formation flying for interferometry or tomography. NAV – Space segment • Orbits: Typically MEO (height ~20,000 km), near-circular, with multiple orbital planes to ensure at least 4–8 satellites are always visible anywhere on Earth. • Payloads: Very stable atomic clocks, navigation signal generators, RF chains and antennas broadcasting on standard frequency bands (e.g. L1, L2, L5 bands and one at S-band, as well). • Key features: o Very high reliability and redundancy (multiple clocks, redundant payload units). o Carefully controlled orbit and attitude to maintain predictable geometry. o Global coverage with high availability; satellites designed for long lifetimes (10–15+ years). • Constellations: global constellations (GPS, Galileo, Glonass, Beidou, etc.), plus regional (NavIC, QZSS) or augmentation systems (SBAS, GBAS, etc.). COM – Space segment • Orbits: o GEO (height ~35,786 km): apparently in a fixed position in the sky, which is ideal for broad coverage broadcast and fixed services. o MEO: e.g. some broadband or legacy systems. o LEO: large constellations for low latency broadband and IoT, in future PNT • Payloads: transponders (bent-pipe or regenerative), high-throughput multi-beam payloads, regenerative processors, analog or digital signal routing, beam-forming networks, small to large (deployable) single beam or multibeam antennas, sometimes often laser communication terminals for inter-sat links. • Key features: o Very high RF power and large antennas, especially in GEO. o Sophisticated on-board processing for routing, switching, beam hopping, etc. o Flexible resource allocation (digital payloads) to adapt to traffic patterns. • Constellations: from a few GEO satellites to hundreds or thousands of LEO satellites.