Satellite and NTN glossary

235 terms from satellite communications and 5G non-terrestrial networks, defined in plain English.

3GPP: 3rd Generation Partnership Project, the standards body defining LTE, 5G, and NTN specifications.
5G NR: 5G New Radio air interface used by 5G devices.
5G-Advanced: 3GPP's branding for the evolution of 5G through Releases 18, 19, and 20, bridging initial 5G (Rel-15/16/17) toward 6G. Key 5G-Advanced NTN milestones: Release 18 introduced FR2-NTN (Ka-band broadband) and enhanced IoT-NTN features; Release 19 (frozen December 2025) added regenerative payloads (gNB-sat, eNB-sat), store-and-forward for IoT, RedCap/eRedCap NTN support, new NTN bands including n252, gNB-sat Xn interface, and ISAC-NTN study items; Release 20 begins 6G architectural study work including GNSS-resilient NTN design.
5GC: 5G Core network architecture supporting 5G service and control functions.
A2G: Air-to-Ground — a cellular access technology where ground-based base stations use upward-pointing antennas to serve aircraft in flight, providing in-flight connectivity without relying on satellite links. Unlike satellite-based IFC, A2G uses terrestrial spectrum and infrastructure and is therefore limited to airspace above populated continental routes where ground coverage exists. 3GPP Release 19 includes a dedicated work item for "Enhancements for Air-to-Ground Network for NR," adding downlink carrier aggregation and MIMO for better throughput and coverage at aircraft altitude.
ACM: Adaptive Coding and Modulation, dynamically tuning waveform robustness and throughput.
AMF: Access and Mobility Management Function in 5G core control plane.
Antenna Gain: Measure of how strongly an antenna focuses energy in a given direction.
AOS: Acquisition of Signal, the moment a satellite link becomes visible to a ground station.
Apogee: Highest point in an Earth orbit.
AROS: Advanced Remote Observation System — BlackSky's next-generation broad-area Earth observation satellite program, developed under a U.S. National Reconnaissance Office (NRO) contract. AROS satellites are multispectral, wide-area imaging platforms designed for AI-driven country-scale change detection and 3D mapping. The constellation plans to use optical inter-satellite links (OISL) for fast data delivery and targets a first launch in 2027.
Availability: Measured service uptime over a defined period.
Azimuth: Horizontal pointing angle used to steer an antenna toward a satellite.
Backhaul: Transport segment carrying traffic from access nodes (for example RAN sites, Wi-Fi edges, or gateways) toward aggregation/core networks.
Bandwidth: Amount of spectrum allocated for a channel, affecting potential throughput.
Beam Hopping: Dynamic beam scheduling approach that allocates capacity across regions over time.
Beamforming: Technique that shapes and steers antenna radiation patterns to improve signal gain, interference control, and coverage targeting.
Bent-pipe: A satellite transponder architecture that simply receives, amplifies, and retransmits signals without onboard processing. Simpler and cheaper than regenerative payloads but less flexible.
BER: Bit Error Rate, the fraction of received bits that are decoded incorrectly over a link.
BLOS: Beyond Line of Sight communication through relay paths like satellites.
BOG: 5G Broadcast over Geosynchronous Satellite — a 3GPP Release 19 NTN capability standardizing efficient one-to-many content delivery via GEO satellite. Proposed by Astrum Mobile and partners, Release 19 introduced a new operating band 246 (1467–1492 MHz) for BOG services. BOG enables delivery of multimedia content, emergency alerts, firmware updates, and infotainment across vast geographic areas in a single satellite transmission, combining GEO's wide-area coverage with 5G broadcast efficiency. First demonstrated in Singapore in October 2024; standardized in Release 19 (frozen December 2025).
BSS: Broadcasting Satellite Service (satellite context), typically associated with direct broadcast distribution such as DTH television.
C-band: Satellite frequency band occupying 3.7–4.2 GHz downlink and 5.925–6.425 GHz uplink. One of the earliest ITU-allocated satellite bands, widely used for GEO broadcast, traditional VSAT, and backbone connectivity. More resilient to rain fade than Ku- or Ka-band, making it preferred in tropical regions. The FCC C-band transition (2020–2023) cleared 3.7–3.98 GHz for 5G in the US, shifting remaining satellite use to the upper portion and international markets.
C/N: Carrier-to-noise power ratio at the receiver, commonly expressed in dB and used to characterize link quality.
C/N0: Carrier-to-noise-density ratio (typically in dB-Hz) used in link and receiver performance analysis.
Conditional Handover: A 3GPP NTN mobility mechanism where the phone is pre-configured with the incoming satellite's radio parameters and autonomously initiates handover when conditions are met (timer, elevation angle, or location-based triggers). Enables make-before-break transitions between satellites.
Conjunction Analysis: Assessment of close approaches between spacecraft to estimate collision risk.
Constellation: Coordinated set of satellites working together to provide regional or global service.
Control Plane: Signaling path used to establish sessions, mobility state, and policy control.
CPE: Customer Premises Equipment — networking hardware located at the customer site, such as routers, switches, or satellite terminals that connect the customer to the service provider network.
CubeSat: Small satellite class built from standardized modular units.
D2D: Direct-to-Device — the most common shorthand for satellite direct-to-device connectivity in NTN contexts. Refers to the technology enabling standard handheld phones and IoT devices to communicate directly with satellites without terrestrial networks. See Direct-to-Device (D2D), D2D NTN, and D2D Messaging for the specific 3GPP-standardized variants.
D2D Messaging: Low-rate direct satellite messaging service for text, alerts, and emergency workflows.
D2D NTN: Direct-to-device non-terrestrial connectivity where user devices communicate with space assets.
DDoS: Distributed Denial of Service — a cyberattack that overwhelms a network or server with traffic from many sources simultaneously. Satellite ground stations implement DDoS protection at the network perimeter.
Deep Space Network (DSN): NASA's network of giant radio dish complexes at Goldstone (California), Madrid (Spain), and Canberra (Australia), spaced ~120° apart so at least one can always see any deep-space mission. Carries commands, telemetry, and science data for missions like JWST and Voyager, and provides precision tracking/navigation for others (e.g., the Roman Space Telescope, whose science data flows through the Near Space Network instead). Live antenna activity is publicly viewable on NASA's DSN Now.
Deorbit: Controlled or natural orbital decay process that ends a spacecraft mission.
Direct-to-Cell: Satellite connectivity model where standard cellular devices connect directly to satellite-enabled radio access.
Direct-to-Device (D2D): Service model enabling handheld or IoT devices to communicate through satellite without dedicated large terminals.
Discontinuous Coverage: An NTN deployment scenario (3GPP Rel-17/18) where LEO satellites do not provide continuous coverage — gaps exist between satellite passes. Devices use store-and-forward or wait for the next pass. Common with small IoT constellations that cannot afford full global coverage.
Doppler Pre-Compensation: Technique that offsets expected Doppler shift in advance to improve receiver tracking and waveform robustness.
Doppler Shift: The change in signal frequency caused by relative motion between a satellite and ground terminal. More pronounced in LEO due to high orbital velocity (~7.5 km/s), requiring compensation in receiver design.
Downlink: Signal path from satellite (or network) down to user or ground station.
DTH: Direct-to-Home satellite broadcasting to consumer receivers.
E-LAN: A MEF Carrier Ethernet service type providing multipoint-to-multipoint connectivity — all connected sites can communicate directly with each other, like a distributed LAN.
E-Tree: A MEF Carrier Ethernet service type providing point-to-multipoint connectivity — one root site communicates with multiple leaf sites, but leaves cannot communicate directly with each other.
EAGLE-1: The first European space-based Quantum Key Distribution (QKD) satellite, developed by a consortium of 20+ European companies led by SES Techcom under co-funding from ESA and the European Commission. Planned for launch in February 2027 on an Arianespace Vega-C rocket into LEO, EAGLE-1 will demonstrate satellite-based QKD — distributing cryptographic keys between ground stations with eavesdropping detection guaranteed by quantum mechanics. The mission delivers operational data for the European Quantum Communication Infrastructure (EuroQCI) and is a key pillar of the EU's digital sovereignty strategy, complementing IRIS² for secure governmental connectivity.
Earth Station: Ground radio station that communicates directly with satellites using uplink/downlink paths.
Earth-Fixed Beams: A beam management technique where LEO satellites use active beam steering (phased arrays) to keep their coverage beams locked to fixed areas on the ground as the satellite moves overhead. Reduces handovers from the phone perspective.
Earth-Moving Cell: A 3GPP Release 17/18 NTN coverage model where a LEO satellite's beam footprint moves across the Earth as the satellite orbits, rather than being steered to remain over a fixed ground location. In Earth-moving cell mode the tracking area identity moves with the satellite, simplifying onboard beam management but increasing handover frequency from the device perspective. Contrasts with Earth-Fixed Beams, where phased arrays actively lock the beam to a fixed ground area.
Eb/N0: Energy-per-bit to noise-density ratio (usually in dB), a core metric for digital link performance.
ECEF: Earth-Centered, Earth-Fixed coordinate frame that rotates with Earth and is commonly used for ground-referenced position calculations.
ECI: Earth-Centered Inertial coordinate frame used for orbital mechanics and satellite trajectory propagation.
EIRP: Effective Isotropic Radiated Power, representing transmit power with antenna gain.
Elevation: Vertical pointing angle from the local horizon to the satellite line-of-sight.
eMTC: Enhanced Machine-Type Communication — a 3GPP LTE-based IoT standard optimized for low-power, wide-area devices. Being adapted for NTN satellite connectivity alongside NB-IoT.
eNB: 4G LTE base station node in the RAN.
eNB-sat: eNB on Satellite — a 3GPP Release 19 term for a 4G LTE base station (eNB) physically hosted onboard a satellite as a regenerative payload, specifically enabling NB-IoT NTN store-and-forward operation. An eNB-sat performs full LTE baseband processing in orbit without requiring a continuous live feeder link to the ground, allowing IoT devices to store data during satellite passes and forward it when a gateway link is available. The 4G LTE complement to gNB-sat (the 5G NR equivalent), both standardized in Release 19 (frozen December 2025).
EO: Earth Observation — the collection of data about Earth's surface, atmosphere, and oceans via satellites, aircraft, or ground sensors. Satellite EO encompasses optical imaging, SAR (synthetic aperture radar), and multispectral sensing. Commercial EO constellations such as Planet Labs, Maxar Technologies, ICEYE, and Capella Space provide high-revisit imagery used for agriculture, environmental monitoring, maritime awareness, and defense intelligence.
EPC: Evolved Packet Core, the core architecture used in LTE networks.
EPFD: Equivalent Power Flux Density — a regulatory metric used by the ITU to limit interference from non-geostationary satellite constellations (NGSO) into geostationary (GSO) systems.
Ephemeris: Orbital position data describing where a satellite will be over time.
eRedCap: Enhanced Reduced Capability — a 3GPP Release 18 IoT device class even more constrained than RedCap. eRedCap devices support only 5 MHz channel bandwidth (vs. 20 MHz for standard RedCap) and a single receive antenna, further reducing chipset cost and power consumption to target the lowest-cost IoT segments such as asset trackers and environmental sensors. 3GPP Release 19 (frozen December 2025) extended NR NTN support to eRedCap devices, with HD-FDD operation standardized for eRedCap UEs connecting via FR1-NTN satellite links over LEO constellations.
EROSS: European Robotic Orbital Support Services — an EU Horizon Europe programme developing robotic technology for satellite on-orbit servicing. The EROSS IOD (In-Orbit Demonstration) mission, led by Thales Alenia Space and planned for 2027, will demonstrate autonomous rendezvous, capture, docking, refueling, and component replacement of a client satellite in orbit. EROSS represents a growing class of servicing missions aimed at extending satellite operational life and reducing end-of-life debris.
ESIM: Earth Station in Motion — a satellite terminal installed on a moving platform (aircraft, ship, or vehicle) that maintains a continuous satellite link while in motion. ESIMs operating in Ka-band (FR2-NTN) enable broadband connectivity for aviation IFC, maritime, and land-mobile use cases, and are regulated under ITU-R frameworks for both GSO and NGSO constellations.
Fade Margin: Extra link budget headroom reserved to maintain service through fading events such as rain attenuation.
FEC: Forward Error Correction that adds redundant coding information to improve reliability under noisy channel conditions.
Feeder Link: High-capacity satellite link between gateway and satellite payload.
Feeder Link Switchover: A 3GPP Release 19 capability enabling a satellite with a regenerative payload to switch its connection between ground gateways without interrupting service. Critical for store-and-forward operation and continuity when the primary gateway loses feeder link access due to weather or outages.
Flat-Panel Antenna: Electronically steered or low-profile phased-array style antenna often used for mobility and fast beam tracking.
Footprint: Geographic area on Earth covered by a satellite beam.
FR1-NTN: Frequency Range 1 for Non-Terrestrial Networks — the sub-7 GHz NTN spectrum (410 MHz–7125 MHz) defined by 3GPP for direct-to-handset satellite services. Key NTN bands include n254 (L-band uplink/S-band downlink FDD), n255 (L-band FDD), and n256 (S-band FDD), standardized in Rel-17/18 for both NR-NTN and IoT-NTN deployments.
FR2-NTN: Frequency Range 2 for Non-Terrestrial Networks — the 17.3–30 GHz (Ka-band range) NTN spectrum defined by 3GPP in Release 18 for broadband satellite access. 3GPP Release 19 introduced three specific Ka-band NR NTN operating bands: n510 (17.7–20.2 GHz DL / 27.5–28.35 GHz UL), n511 (17.7–20.2 GHz DL / 28.35–30.0 GHz UL), and n512 (17.7–20.2 GHz DL / 27.5–30.0 GHz UL) for VSAT UE service link operation. Used by fixed VSATs and Earth Stations in Motion (ESIM) on aircraft, vessels, and vehicles for high-capacity satellite connectivity.
Frequency reuse: A technique where the same frequency band is used in multiple non-overlapping spot beams, multiplying total system capacity. A key enabler of high-throughput satellite (HTS) architectures.
Fronthaul: Transport segment between distributed radio units and centralized baseband functions.
FSPL: Free-Space Path Loss — the signal attenuation caused purely by distance and frequency as a radio wave spreads through free space. Measured in decibels (dB). Higher frequency and longer distance both increase FSPL.
FSS: Fixed Satellite Service, an ITU service category for communication links between fixed earth stations and space stations.
G/T: Receiver figure of merit (gain-to-noise-temperature, typically in dB/K) used in link budget analysis.
Gateway: Ground node that connects satellite access segments to terrestrial network domains.
GEO: Geostationary Earth Orbit: circular equatorial orbit where satellites appear fixed relative to Earth's surface.
Geofencing: Rule-based geographic boundary used to trigger service or policy behavior.
gNB: 5G base station node in the RAN.
gNB-sat: gNB on Satellite — the 3GPP Release 19 term for a 5G base station (gNB) physically hosted on board a satellite as part of a regenerative payload. A gNB-sat performs full baseband signal processing in orbit, supports the 3GPP Xn interface over inter-satellite links for native gNB-to-gNB signaling, enables on-satellite packet switching, and is the prerequisite for store-and-forward and reduced round-trip-time random access. Standardized in Rel-19 (frozen December 2025).
GNSS: Global Navigation Satellite System used for timing, positioning, and navigation.
GNSS-Resilient NTN: A 3GPP Release 20 study area (TR 38.742) developing methods for NR-NTN to operate when GNSS signals are unavailable or denied. Current NTN access (Rel-17 through Rel-19) requires UEs to have a GNSS position fix to calculate timing advance and Doppler pre-compensation values before transmitting to a satellite. Release 20 studies alternative mechanisms — including network-assisted Doppler estimation from the satellite downlink, uplink-based positioning, and waveform modifications — to enable NTN access without GNSS, addressing GNSS-jammed or GNSS-denied scenarios relevant to military, critical infrastructure, and high-security deployments.
Ground Track: The path traced on the Earth's surface directly below a satellite as it orbits. LEO satellites trace fast-moving ground tracks that shift westward with each orbit due to Earth's rotation. Ground track patterns determine coverage revisit time and are critical for constellation design.
GSO: Geosynchronous Orbit with ~24-hour period matching Earth's rotation, but not necessarily geostationary.
GTO: Geostationary Transfer Orbit used to move payloads toward geostationary altitude.
Handover: Transfer of active service between cells, beams, satellites, or access nodes.
HAPS: High Altitude Platform Station, near-space platform used for wide-area coverage.
HARQ: Hybrid Automatic Repeat Request — an error correction mechanism that combines forward error correction with retransmission requests. In NTN, HARQ is modified to handle the increased latency of satellite links (50+ ms vs terrestrial ~1 ms).
HD-FDD: Half-Duplex Frequency-Division Duplex — an operating mode where a device alternates between transmit and receive rather than doing both simultaneously, reducing power consumption and hardware complexity at the cost of peak throughput. Standardized for NTN in 3GPP Release 19 specifically for RedCap and eRedCap IoT devices connecting via satellite, where battery life and cost constraints make full-duplex operation impractical.
HEO: Highly Elliptical Orbit, often used for long dwell over selected regions.
HIBLEO: High Inclination Broadband LEO — an ITU and FCC filing designation for Globalstar's ~780 km, 86.4° inclination LEO satellite network licensed across L-, S-, and C-band frequencies. HIBLEO-4 refers to Globalstar's second-generation replenishment constellation supporting direct-to-device and IoT messaging services globally.
HTS: High Throughput Satellite architecture using spot beams and frequency reuse.
Hub-and-Spoke: Network topology where remote sites connect through a central hub.
In-Flight Connectivity (IFC): Passenger and operational connectivity services for aircraft.
Inclination: Tilt angle of an orbit relative to Earth's equatorial plane.
Inter-Satellite Link: A communication link between two satellites in orbit, typically using optical laser. Allows traffic to route through space without touching the ground — critical for connectivity over oceans and remote areas. Also called ISL or crosslink (military term).
Interference: Unwanted signal energy that degrades communication quality.
IoT-NTN: 3GPP framework enabling NB-IoT and eMTC devices to connect directly via satellite. Standardized in Rel-17 and progressively enhanced through Rel-19 with store-and-forward operation, improved power-saving modes, GNSS-independent timing support, and regenerative payload architectures.
IPsec: Security protocol suite used to protect IP traffic via encryption and integrity controls.
Iridium NTN Direct: Iridium's 3GPP standards-based NB-IoT and direct-to-device (D2D) satellite service, operating over Iridium's 66-satellite cross-polar LEO constellation at 780 km altitude providing true global coverage including the poles. Designed to deliver NB-IoT connectivity to standard 3GPP-compatible IoT chipsets without custom satellite modems, the service enables IoT devices to roam seamlessly between terrestrial and satellite coverage. Beta testing began in early 2026 with MNO partners including Deutsche Telekom and Vodafone IoT, with commercial launch targeted in 2026. Widely cited as the world's first truly global 3GPP standards-based NTN service, operating in the L-band under Iridium's MSS license.
Iris for Aviation: ESA's Iris Programme — a satellite-based datalink communications system for European air traffic management (ATM), providing secure VHF Data Link (VDL) and ACARS-compatible connectivity between aircraft and ground-based air traffic controllers over the Single European Sky. Led operationally by Viasat (which acquired Inmarsat in 2023) and ESSP, Iris uses Inmarsat's SwiftBroadband-Safety (SB-S) L-band service to enable 4D trajectory-based operations — sharing precise route and intent data between pilots and Air Navigation Service Providers (ANSPs). The pre-commercial phase concluded in late 2025, with commercial rollout targeting 28 European ANSPs and 1,100+ equipped aircraft by 2032. Not to be confused with IRIS² (the EU sovereign broadband satellite constellation).
IRIS²: Infrastructure for Resilience, Interconnectivity and Security by Satellite — the European Union's sovereign multi-orbit satellite constellation (often abbreviated as IRIS in news). Originally planned for 264 LEO satellites at ~1,200 km and 18 MEO satellites at ~8,000 km, the constellation was expanded to 348 satellites following an implementation agreement signed by the European Commission and SpaceRISE consortium in August 2026, accelerating IRIS² from planning to full-scale deployment. Operated by the SpaceRISE consortium (SES, Eutelsat, Hispasat) under a €10.6 billion concession signed in December 2024, IRIS² is designed to deliver secure governmental connectivity and broadband internet to EU member states, with initial government services expected from 2030. Not to be confused with the ESA Iris for Aviation programme (see Iris for Aviation).
ISAC-NTN: Integrated Sensing and Communication for Non-Terrestrial Networks — a 3GPP Release 19/20 study item combining radar-style sensing with the communication payload functions of a satellite or HAPS in a single system. ISAC-NTN enables satellites to simultaneously deliver broadband services and perform tasks such as aircraft tracking, maritime surveillance, weather sensing, and terrain mapping using the same waveform and spectrum. Standardization of ISAC use cases for NTN (SA1 TR 22.837) was completed in 2025.
ISL: Inter-Satellite Link used for space-to-space data transport between satellites.
ITU: International Telecommunication Union, a key global body for spectrum and orbital coordination.
Jitter: Variation in packet delay over time, important for real-time services.
Ka-band: Higher-frequency satellite band often used for high-throughput broadband.
Ku-band: Satellite band commonly used for broadband, VSAT, and mobility services.
L-band: Lower-frequency satellite band often used for reliable, device-friendly links.
Land Mobility SATCOM: Satellite connectivity supporting moving land platforms and vehicles.
Latency: End-to-end delay experienced by packets across the network path.
LEO: Low Earth Orbit, typically used for lower-latency satellite services.
LEO-PNT: Low Earth Orbit Positioning, Navigation, and Timing — an emerging class of systems that use LEO satellite constellations (typically 500–1,200 km altitude) to provide position, navigation, and timing services as a complement or resilient backup to traditional GNSS. LEO signals have higher power flux density and are harder to jam or spoof than GNSS signals from ~20,000 km. Examples include ESA's Celeste (10-satellite in-orbit demonstrator at 510 km to test Galileo-supplement signals), Satelles's commercial STL service (using the Iridium LEO constellation to deliver PNT signals ~1,000× stronger than GPS), and Xona Space Systems' Pulsar constellation (targeting centimeter-level accuracy for aviation and autonomous vehicles).
Link Budget: End-to-end accounting of gains and losses along a communications link.
LNA: Low-Noise Amplifier placed near the receive front-end to boost weak signals with minimal added noise.
MACsec: Media Access Control Security — a Layer 2 encryption standard (IEEE 802.1AE) that provides hop-by-hop data confidentiality and integrity on Ethernet links. Used in satellite ground segments for secure traffic between routers.
Maritime SATCOM: Satellite connectivity services for vessels and offshore operations.
MBS: Multicast and Broadcast Services — a 3GPP feature introduced in Release 17 and enhanced in Rel-18/19 that enables efficient point-to-multipoint data delivery over 5G networks. In NTN contexts, MBS allows a satellite to simultaneously deliver content (firmware updates, maps, emergency alerts, or video) to many devices without consuming per-user unicast resources. Release 18 extended MBS support to UEs in RRC_INACTIVE state to handle congestion scenarios.
MCS: Modulation and Coding Scheme selection used to balance throughput and robustness based on channel conditions.
Mega-constellation: Industry term for a large-scale satellite constellation comprising hundreds to thousands of satellites in LEO or MEO, designed to provide continuous global or near-global coverage. Examples include SpaceX Starlink (~7,000+ operational), Amazon Kuiper (3,236 planned), and China's Guowang/Qianfan constellations. Mega-constellations drive spectrum coordination complexity, collision avoidance requirements, and de-orbit obligations under ITU and national regulatory frameworks.
MEO: Medium Earth Orbit, balancing latency and coverage footprint.
Mesh Topology: Network topology allowing direct or multi-path communication among multiple nodes.
Midhaul: Transport segment between distributed and centralized RAN functions.
Mission Life: Planned operational lifetime of a spacecraft before decommissioning.
Molniya Orbit: A 12-hour highly elliptical orbit profile for high-latitude regional visibility.
MSS: Mobile Satellite Services using allocated satellite spectrum for mobile connectivity.
MU2N: Multi-hop UE-to-Network Relay — a 3GPP Release 19 extension of the Rel-18 UE-to-Network (U2N) relay feature, allowing two additional relay hops between a remote UE and the network. In NTN contexts, MU2N enables IoT devices in shadowed or remote locations to route connectivity through one or more intermediate UEs before reaching a satellite service link — extending NTN coverage into buildings, dense terrain, or areas where even a single satellite hop is insufficient.
Multi-Orbit: An architecture that combines satellites in different orbits (e.g., LEO + MEO) to optimize latency, coverage, and ground station requirements. Traffic may route from a LEO access satellite via cross-orbit ISL to a MEO relay satellite. Examples: Lynk + SES, IRIS2.
N2: 5G control-plane interface between RAN and core (toward AMF).
n252: 5G NR frequency band n252 — the S-band NTN downlink/uplink pair (approximately 1980–2010 MHz uplink / 2170–2200 MHz downlink) standardized in 3GPP Release 19 for direct-to-device satellite connectivity. n252 is rapidly becoming the foundational band for commercial direct-to-cell LEO constellations. Keysight and Samsung completed the first live end-to-end n252 NR-NTN link validation in January 2026; AST SpaceMobile's BlueBird constellation operates on n252.
N3: 5G user-plane interface between RAN and UPF.
N3X: Kongsberg Defence & Aerospace's Norwegian maritime surveillance satellite constellation, operated primarily for the Norwegian Armed Forces under a five-year contract. N3X microsatellites — built by Kongsberg NanoAvionics (Lithuania) — carry AIS receivers and wide-area imaging sensors to track vessel activity in and around Norwegian waters and the Arctic. Three initial satellites launched in March 2025, with three additional spacecraft planned for early 2027 to double the fleet to six. N3X is a dual-use system also serving commercial customers, and represents a growing category of small-nation sovereign surveillance micro-constellations being developed outside the major space powers.
NavIC: Navigation with Indian Constellation — India's regional satellite navigation system operated by ISRO, providing positioning and timing services across India and approximately 1,500 km beyond its borders. NavIC uses a constellation of seven satellites (3 GEO + 4 GSO) broadcasting at Ka-band and S-band, with an L5 + S-band dual-frequency civil service. 3GPP Release 19 standardized Assisted NavIC L1 SPS (A-GNSS) support for NR and LTE devices — a work item proposed by TSDSI (India's 3GPP partner) — reducing Time to First Fix and improving positioning accuracy for devices operating across South Asia. NavIC is directly relevant to NTN IoT deployments serving the Indian subcontinent.
NB-IoT: Narrowband IoT, a low-power cellular standard for small data messages.
Network Slicing: A 5G architecture concept where a single physical network is partitioned into multiple virtual networks (slices), each optimized for different service requirements (latency, bandwidth, reliability). In NTN, slicing enables differentiated QoS — e.g., a low-latency slice for D2D voice and a high-throughput slice for maritime broadband — over the same satellite link.
NGSO: Non-geostationary orbit category including LEO, MEO, and other non-GEO trajectories such as HEO profiles.
NISAR: NASA-ISRO Synthetic Aperture Radar — a joint Earth observation satellite developed by NASA (USA) and ISRO (India), launched July 30, 2025 from the Satish Dhawan Space Centre aboard ISRO's GSLV-F16 rocket. NISAR is the world's first dual-frequency (L-band and S-band) SAR satellite, providing sub-meter resolution imaging for forest biomass monitoring, earthquake deformation measurement, glacier dynamics, and coastal change detection. The satellite images nearly all of Earth's land and ice surfaces twice every 12 days, producing the largest-ever global interferometric SAR (InSAR) time series dataset for scientific Earth monitoring.
NOC: Network Operations Center responsible for monitoring and managing service operations.
NR-NTN: 3GPP New Radio for Non-Terrestrial Networks, extending 5G NR procedures to satellite access.
NRO: National Reconnaissance Office — a U.S. government intelligence agency that designs, builds, and operates classified reconnaissance satellites for national security. The NRO operates one of the world's largest government satellite constellations, providing overhead imagery, signals intelligence (SIGINT), and communications relay capabilities. It also contracts with commercial operators — including BlackSky (AROS program), Planet Labs, and Maxar Technologies — to supplement government imagery capacity.
NTN: Non-Terrestrial Network: connectivity using satellites or high-altitude platforms.
NTN-PWS: Non-Terrestrial Network Public Warning System — a 3GPP Release 19 IoT NTN feature enabling satellite delivery of emergency public warning alerts (equivalent to Earthquake and Tsunami Warning System / CMAS cell-broadcast messages) to NB-IoT NTN devices in areas without terrestrial cellular coverage. NTN-PWS is part of the IoT NTN Phase 3 work item standardized in Release 19 (frozen December 2025), addressing a critical gap in disaster warning for remote and maritime populations served only by satellite.
NTN-TN Interworking: Interoperability mechanisms between non-terrestrial and terrestrial network domains.
O-RAN: Open Radio Access Network architecture using disaggregated interfaces and interoperable components in RAN deployments.
OAM: Operations, Administration, and Maintenance — the processes and tools used to monitor, manage, and troubleshoot network infrastructure. In NTN, centralized OAM manages remote satellite-connected sites.
OCC: Orthogonal Cover Codes — a 3GPP Release 19 uplink enhancement for FR1-NTN that layers multiple UE transmissions across orthogonal code sequences on top of DFT-s-OFDM PUSCH resources. Allows many UEs within the same satellite beam to share identical time-frequency resources without mutual interference, significantly increasing satellite uplink capacity without additional spectrum allocation.
OFDM: Orthogonal Frequency Division Multiplexing waveform used in modern broadband systems.
OISL: Optical Inter-Satellite Link — a laser-based communication link between satellites in a constellation, enabling high-bandwidth (100s of Gbps) mesh networking in space without routing traffic through ground gateways. Distinct from radio-frequency ISLs, OISLs use narrow laser beams requiring precise pointing, acquisition, and tracking (PAT) mechanisms. Increasingly standard in large LEO constellations (e.g., Starlink's laser ISL mesh) and planned for AROS and military systems (PWSA Tranche 1 Transport layer).
Onboard Processing: Signal/data processing performed inside the satellite payload.
Optical feeder link: A laser-based link between a satellite and a ground station used for high-capacity backbone traffic. Offers much higher throughput than RF feeder links but is sensitive to weather and clouds.
Orbital Plane: The geometric plane in which a satellite orbits, defined by its inclination and right ascension of ascending node (RAAN). LEO constellations distribute satellites across multiple orbital planes to achieve continuous global coverage — e.g., Starlink uses 72 planes at 53° inclination.
Packet Loss: Percentage of packets that fail to reach the destination.
Path Loss: The reduction in signal power as it travels from transmitter to receiver. In satellite links, dominated by free-space path loss (FSPL) which increases with distance and frequency. Ka-band at GEO distance has ~214 dB of path loss.
Payload: Mission equipment onboard a satellite, such as communications transponders or sensors.
PC5: 3GPP sidelink reference interface used for direct UE-to-UE communication in supported scenarios.
PDCCH: Physical Downlink Control Channel carrying scheduling and control information in LTE/NR systems.
PDSCH: Physical Downlink Shared Channel carrying user data in LTE/NR downlink.
Perigee: Lowest point in an Earth orbit.
PFD: Power Flux Density — the measure of signal power arriving at a surface per unit area. Used in satellite link budgets and regulatory limits to prevent harmful interference.
PNT: Positioning, Navigation, and Timing services, often provided by GNSS or complementary resilient architectures.
Polar Orbit: High-inclination orbit passing near both poles for global revisit over time.
Polarization: Orientation of an electromagnetic wave (for example linear or circular), used for interference management and capacity reuse.
POP: Point of Presence — a physical location where a service provider houses networking equipment to connect to customers or peer with other networks. Satellite operators maintain POPs at ground station sites.
Propagation Delay: Time required for a signal to travel between endpoints.
PUCCH: Physical Uplink Control Channel used by UE to send uplink control information.
PUSCH: Physical Uplink Shared Channel used to transport uplink user data in LTE/NR.
PWSA: Proliferated Warfighter Space Architecture — the U.S. Space Development Agency's layered military LEO satellite network providing secure encrypted broadband connectivity, missile warning, and tracking data to warfighters. Deployed in successive two-year "tranches," Tranche 1 includes 154 Transport-layer satellites with optical inter-satellite links and Ka-band RF ground links. Tranche 3 (72 Tracking-layer satellites, $3.5B awarded December 2025) targets a FY2029 launch.
Q-band: Satellite RF band in the 33-50 GHz range, used primarily for high-capacity feeder links between satellites and gateways. Offers very high bandwidth but requires advanced rain fade mitigation.
Q/V-band: Very high frequency satellite bands often used for feeder links and advanced capacity.
QKD: Quantum Key Distribution — a cryptographic method that uses quantum mechanics to distribute encryption keys with provably secure transmission: any eavesdropping attempt disturbs the quantum states and is immediately detectable. Satellite-based QKD — exemplified by ESA's EAGLE-1 (planned 2027) — enables long-distance key exchange between ground stations separated by thousands of kilometres, overcoming the distance limits of fiber-optic QKD. Considered a foundational technology for post-quantum communication security and sovereign critical infrastructure protection.
QoS: Quality of Service mechanisms used to prioritize and shape network traffic classes.
RAAN: Right Ascension of the Ascending Node, an orbital orientation parameter.
Rain Fade: Signal attenuation due to precipitation, especially at higher frequency bands.
RAN: Radio Access Network: the network portion that talks directly with devices over radio.
RedCap: Reduced Capability NR — a 3GPP 5G standard for mid-tier IoT devices (wearables, sensors) that need more bandwidth than NB-IoT but less than full 5G. NTN support for RedCap UEs operating in FR1-NTN was standardized in Release 19 (frozen December 2025).
Regenerative Payload: Payload architecture that demodulates/remodulates traffic onboard the satellite.
Roaming: Use of partner network service outside a subscriber's home network domain, with charging and policy interworking.
Round-Trip Time (RTT): The total time for a signal to travel from user to destination and back. In satellite, includes propagation delay (speed of light through space), processing overhead at gateways, and any ISL hop delays. LEO RTT is ~25-50 ms, MEO ~120-170 ms, GEO ~500-600 ms.
ROV: Remotely Operated Vehicle — an unmanned vehicle controlled from a distance, often used for surveillance, inspection, or hazardous operations. In NTN contexts, ROVs connect via P5G with satellite backhaul for remote control.
RRC: Radio Resource Control protocol governing connection setup, mobility, and radio state signaling.
RSRP: Reference Signal Received Power measurement used to estimate downlink signal strength.
RSRQ: Reference Signal Received Quality metric combining signal strength and interference/noise conditions.
RTT: Round-Trip Time — see Round-Trip Time (RTT).
S-band: Lower-mid frequency band used in multiple mobile and satellite applications.
S1-AP: LTE control-plane interface between eNB and EPC.
S1-U: LTE user-plane interface between eNB and EPC.
SAR: Synthetic Aperture Radar — a radar imaging technology used by Earth observation satellites that generates high-resolution imagery regardless of cloud cover or lighting conditions. By processing microwave signals reflected from the ground along the satellite's flight path, SAR synthesizes a large effective antenna aperture to achieve sub-meter resolution. Commercial SAR operators include ICEYE, Capella Space, and Umbra; the joint NASA-ISRO NISAR mission (launched July 2025) is the first dual-frequency SAR satellite.
SATCOM: Satellite communications systems and services.
Satellite Bus: Core spacecraft platform supporting payloads, power, thermal, and control systems.
SDA: Space Development Agency — a U.S. Department of Defense organization that develops and deploys the Proliferated Warfighter Space Architecture (PWSA). Uses a spiral development model to field new capability in two-year "tranches." In December 2025, SDA awarded ~$3.5 billion in contracts for 72 Tracking-layer satellites (Tranche 3) to L3Harris, Lockheed Martin, Northrop Grumman, and Rocket Lab.
Service Link: The radio link between a satellite and the end-user device or terminal on the ground. Distinct from the feeder link (satellite to gateway) and inter-satellite link (satellite to satellite). In 3GPP NTN, the service link uses the NR-Uu or NB-IoT air interface.
SIB19: System Information Block 19 — a 3GPP Release 17 broadcast message containing NTN-specific parameters including satellite ephemeris data, timing advance, and serving cell location. Used by phones to predict satellite handovers.
Sidelink: Direct radio communication mode between devices (UE-to-UE) defined in 3GPP standards.
SLA: Service-Level Agreement defining committed performance and availability objectives.
Slant Range: Actual line-of-sight distance between terminal and satellite, used in delay and link calculations.
SMF: Session Management Function in 5G core, managing PDU sessions.
Solar Array: Satellite power subsystem that converts sunlight into electrical energy.
Spectral Efficiency: Amount of useful data transmitted per unit of bandwidth (for example bits/s/Hz).
Spot Beam: Focused satellite coverage beam used to improve spectral efficiency and capacity reuse.
SRI: Satellite Radio Interface — the air interface used between the satellite and the ground station in transparent (bent-pipe) NTN architectures. Typically a proprietary protocol, distinct from the 3GPP NR-Uu interface used on the service link to the phone.
STL: Satellite Time and Location — a commercial LEO-based positioning, navigation, and timing (PNT) service from Satelles that uses the Iridium 66-satellite LEO constellation (780 km altitude, global coverage including poles). STL signals arrive at ground receivers approximately 1,000× more powerful than GPS signals at the same frequencies, enabling reliable PNT in GPS-challenged environments including indoors, urban canyons, underground facilities, and RF-jammed zones. Already commercially deployed for financial trading synchronization, critical infrastructure timing, and resilient navigation applications as a GNSS complement.
Store-and-forward: A communication method where a satellite stores data received from one ground location and forwards it when passing over the destination. Common in IoT and messaging satellites with non-continuous coverage.
Sun-Synchronous Orbit (SSO): Near-polar orbit maintaining consistent local solar time on each pass.
TAC: Tracking Area Code — an identifier used in mobile networks to group cells into tracking areas for paging and mobility management. In NTN, tracking areas are fixed to the ground but the TAC assignment changes as different LEO satellites sweep overhead.
TDMA: Time Division Multiple Access method sharing a channel via time slots.
Teleport: Ground infrastructure site hosting antennas and network interconnect for satellite services.
TerraSAR-X: German X-band synthetic aperture radar Earth observation satellite (also abbreviated TSX), jointly operated by the German Aerospace Center (DLR) and Airbus Defence & Space under a public-private partnership. Launched in 2007 into a sun-synchronous orbit at 514 km, TerraSAR-X operates at 9.65 GHz and delivers 1–16 m resolution SAR imagery in SpotLight, StripMap, and ScanSAR modes regardless of cloud cover or lighting. Used globally for disaster monitoring, oceanography, glacier dynamics, geological mapping, and maritime domain awareness. The companion satellite TanDEM-X (TDX) flies in close formation with TerraSAR-X to generate global digital elevation models. A next-generation successor, TSX-NG, is in development.
TETRA: Terrestrial Trunked Radio — a European digital radio standard widely used by police, fire, ambulance, and military for mission-critical voice communications. Being complemented and gradually replaced by MCPTT over LTE/5G.
Throughput: Effective data delivery rate achieved over a communications path.
Timing Advance: A mechanism where the UE transmits earlier than normal to compensate for the long propagation delay to a satellite. In NTN, timing advance values are much larger than terrestrial (up to ~40 ms for GEO) and are derived from GNSS position and satellite ephemeris data broadcast in SIB19.
TLE: Two-Line Element set, a compact orbital data format commonly used for satellite tracking and propagation.
Transparent Payload: A satellite payload that amplifies and frequency-converts signals without demodulating or processing them onboard — also called bent-pipe. The ground station performs all baseband processing. Simpler and cheaper than regenerative payloads but less flexible for routing and beamforming.
Transponder: Satellite payload element that receives, amplifies, and retransmits radio signals.
TT&C: Telemetry, Tracking, and Command functions for spacecraft operation and control.
Tundra Orbit: A 24-hour highly elliptical orbit profile with repeatable regional dwell.
UAS: Uncrewed Aerial Systems — drones and unmanned aircraft managed via cellular or satellite networks. 3GPP Release 19 includes dedicated UAS-NTN enhancements for drone connectivity at altitude, covering Command and Control (C2) links over NTN for beyond-visual-line-of-sight (BVLOS) operations and integration with Unmanned Traffic Management (UTM) systems. NTN connectivity is critical for BVLOS drone operations in areas without terrestrial network coverage, enabling long-range logistics, inspection, and surveillance missions.
UE: User Equipment, a 3GPP term for end devices such as phones, modems, and IoT modules.
UHF: Ultra High Frequency range used by selected satellite and tactical communication systems.
UPF: User Plane Function in 5G core handling user data forwarding.
Uplink: Signal path from user terminal or ground station up to the satellite.
User Link: Satellite link between user terminal/device and satellite access segment.
User Plane: Data path carrying application payload traffic after session setup.
User Terminal: Customer-side equipment that accesses satellite service links.
V-band: Millimeter-wave satellite spectrum region (roughly 40-75 GHz) used for high-capacity links with strong atmospheric sensitivity.
VHF: Very High Frequency range used in specialized narrowband communication scenarios.
VHTS: Very High Throughput Satellite, extending HTS capacity concepts.
VLEO: Very Low Earth Orbit — a subset of LEO defined by altitudes from roughly 100 to 450 km, where atmospheric drag is significantly higher than in conventional LEO above 500 km. VLEO enables lower propagation latency (RTT as low as ~2 ms at 250 km) and higher imaging resolution for Earth observation, but requires active propulsion to offset atmospheric drag and yields shorter satellite mission life. Of increasing commercial interest for communications with drag-compensating electric propulsion.
VSAT: Very Small Aperture Terminal used for satellite enterprise and remote-site connectivity.
Wi-Fi: Local wireless LAN technology used to connect devices to a nearby access point.
Wide beam: A satellite antenna pattern covering a large geographic area with a single beam. Provides broad coverage but lower capacity per user compared to spot beams.
WRC-27: World Radiocommunication Conference 2027 — the next ITU spectrum regulatory conference, scheduled in Shanghai in October 2027. WRC-27 carries several agenda items critical to satellite and NTN: Agenda Item 1.13 proposes new spectrum for space-based direct-to-device services in the 694–2700 MHz range; Agenda Item 1.11 addresses inter-satellite links between NGSO and GSO satellites at 1518–1544 MHz for low-latency applications; and Agenda Item 1.15 considers spectrum for lunar communications. Outcomes will shape the regulatory framework for next-generation LEO D2D constellations and 6G NTN architecture through the early 2030s.
X-band: Satellite RF band around 7-8 GHz, commonly used for mission-critical government and defense links.

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