A satellite system is the complete set of elements needed to provide a space-based service or product to users on Earth. It includes the space segment, the ground segment, and the user segment, plus all the interfaces, operations, and logistics that interconnect them. From a systems-engineering perspective, a satellite system starts from the mission requirements (what information or service is needed, including performance, coverage, latency, reliability, cost, etc.) and translates them into an integrated architecture (system requirements) across these three segments. Key generic elements of a satellite system: • Mission objectives: e.g. broadband communication, earth observation and exploration, measuring sea surface temperature, providing accurate positioning, broadcasting TV, etc. • Performance requirements: accuracy, resolution, timeliness, availability, continuity, integrity, data rate, etc. • Architecture: number of satellites and orbits, ground stations, communication links, data processing chain, user equipment, orchestration software. • Operations: planning, monitoring, control, maintenance, calibration/validation, and end-of-life disposal. • Service and products: what is ultimately delivered to the users (images, geophysical variables, timing and positioning, voice/data connectivity, etc.). The specificities of this general concept adapted to each of the three families are as follows: EO satellite systems • Objective: measure geophysical variables (e.g. land cover, soil moisture, atmospheric composition, sea state) from space. • Space segment: satellites carry remote sensors (optical, SAR, TIR, radiometers, GNSS-R, etc.) in orbits optimized for coverage and illumination (typically LEO, often Sun-synchronous, but also GEO). • Ground segment: receiving stations on the Earth’s surface offering high-capacity data downlink, processing chains (from L0 to L2 and beyond), archives, and dissemination services. • User segment: scientists, agencies, companies, or general public that access images and derived products via portals, APIs, etc., with specialized devices and software for, visualization, analysis, and assimilation into models. • System focus: radiometric and geometric accuracy, calibration/validation, revisit time, spatial/temporal resolution, uncertainty quantification and traceability. Satellite navigation systems • Objective: provide Position, Navigation and Timing (PNT) services globally. • Space segment: constellations of medium-Earth orbit (MEO) satellites (GPS, Galileo, GLONASS, BeiDou), each broadcasting precise time-tagged signals on multiple frequencies. Sometimes these constellations are augmented by GEO or geo-synchronous satellites. There are plans to deploy LEO PNT constellations to augment these satellites systems. • Ground segment: worldwide monitoring networks and control centers estimating satellite orbits and clock states, uploading navigation messages, and performing integrity checks. • User segment: multitude of receivers in phones, cars, aircraft, ships, timing receivers in power grids and telecom networks, etc. • System focus: global coverage, high availability, high integrity, precise timekeeping, robust geometry (DOP), mitigation of ionospheric/tropospheric errors and multipath. Future requirements: indoor penetration Satellite communication systems • Objective: to relay information (voice, data, video, IoT messages) between locations on Earth (and sometimes between satellites). • Space segment: GEO, MEO, and LEO satellites carrying communication payloads including transponders, antennas phased arrays, on-board processors, inter-satellite links, etc. • Ground segment: gateways, teleports, network operation centers, and integration into terrestrial networks. • User segment: terminals of many kinds: TV dishes, VSATs, satellite phones, maritime/aviation terminals, IoT nodes, single low-power IoT receivers. • System focus: capacity (data rate), coverage, availability, quality of service, spectrum efficiency, latency, and cost per bit. Satellite Systems are the umbrella that ties together the three named segments to turn orbital infrastructure into useful services for EO, navigation, or communications.