- G/T
- Receive figure of merit of an earth station — antenna gain divided by system noise temperature, expressed in dB/K. The key receiver parameter in satellite link budgets; higher is better.
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- G2 — Division/Corps Intelligence Staff
- The staff element responsible for intelligence at division level and above.
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- G2G — Government-to-Government Sale
- A procurement mechanism where one government sells directly to another, often through Foreign Military Sales (US) or equivalent national programmes. Bypasses commercial negotiation for strategic exports.
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- DiagramTry it: Interplanetary G2g Ops
- Galileo — Galileo Navigation Satellite System (EU)
- The European Union's global navigation satellite system, providing open service, commercial service, and a Public Regulated Service (PRS) for authorised government users. Nominally 30 satellites in MEO.
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- GaN — Gallium Nitride
- A wide-bandgap semiconductor material used in modern radar transmit/receive modules. Offers higher power density, efficiency, and bandwidth than legacy GaAs, enabling more capable phased-array radars like AN/SPY-6.
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- 2D animationTry it: Defense Spectrum Map
- GARS — Global Area Reference System
- A standardized cell-based system for dividing the Earth’s surface into named area units for battlespace referencing.
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- GBAD — Ground-Based Air Defense
- Air defense capabilities employing surface-based weapons and sensors to protect forces from air and missile threats.
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- GBAS — Ground-Based Augmentation System
- A local GNSS augmentation system at airports providing precision approach guidance with integrity monitoring. Differential corrections broadcast via VHF data link.
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- 2D animationTry it: Space Fundamentals Guide
- GBI — Ground-Based Interceptor
- US homeland defense missile based in Alaska and California. Intercepts ICBMs during the midcourse phase using a kinetic exoatmospheric kill vehicle.
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- GBS — Global Broadcast Service
- A high-bandwidth one-way military satellite broadcast for imagery and data to deployed users.
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- GBU — Guided Bomb Unit
- A US military designation for precision-guided bombs — a free-fall bomb body (MK-82, MK-84, BLU-109) fitted with a guidance kit (Paveway laser, JDAM GPS, StormBreaker dual mode).
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- GCC — Geographic Combatant Command
- A combatant command with a defined area of responsibility covering a region of the globe.
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- GCCS — Global Command and Control System
- The joint command-and-control system providing a fused common operational picture to commanders worldwide.
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- GCM — Galois/Counter Mode
- An authenticated encryption mode combining CTR-mode encryption with Galois-field multiplication for authentication. Provides both confidentiality and integrity in a single pass. The recommended mode for AES in modern systems.
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- 2D animationTry it: Encryption GuideTry it: Space Standards Guide
- GCP — Ground Control Point
- A surveyed point of known coordinates used to georeference and correct imagery or elevation data.
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- GCR — Galactic Cosmic Rays
- High-energy particles (primarily protons and heavy ions) originating outside the solar system. Constant background flux, anti-correlated with solar cycle. Primary driver of single event effects in electronics. Shielded most effectively by hydrogen-rich materials.
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- 2D animationTry it: Human Spaceflight GuideTry it: Space Environment Effects Guide
- GCS — Ground Control Station
- The ground-based facility from which an operator monitors and controls an unmanned aircraft.
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- GDOP — Geometric Dilution of Precision
- A dimensionless metric quantifying how satellite geometry amplifies ranging errors into position errors. Lower GDOP indicates better positioning geometry.
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- GDPR — General Data Protection Regulation
- European Union regulation governing the protection and privacy of personal data.
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- GDS — Ground Data System
- The ground software and hardware that command a spacecraft and process its returned telemetry and data.
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- GEO — Geostationary Earth Orbit
- A circular orbit at approximately 35,786 km altitude with 0-degree inclination and a period of exactly one sidereal day, causing the satellite to appear stationary over a fixed point on the equator.
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- GEO-belt — Geostationary Belt
- The ring of orbital slots at geostationary altitude where satellites appear fixed over the equator.
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- GEO-NEO — Geosynchronous Near-Equatorial Orbit
- Low-inclination geosynchronous orbit.
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- GEODSS — Ground-based Electro-Optical Deep Space Surveillance
- A network of optical telescopes that tracks objects in deep space and geostationary orbits.
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- GEOINT — Geospatial Intelligence
- Intelligence from exploiting and analyzing imagery, IMINT, and geospatial information to describe, assess, and visually depict geographic features and activities on Earth.
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- GEOINT_PED — GEOINT Processing, Exploitation, and Dissemination
- The processing and exploitation workflow specific to geospatial and imagery intelligence products.
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- Geolocation
- The process of estimating the geographic position of an emitter or asset, typically from RF signals. Common methods: TDOA (time difference of arrival), FDOA (frequency difference of arrival), AOA (angle of arrival), and combinations thereof.
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- GFE — Government-Furnished Equipment
- Equipment owned by the government and provided to a contractor for use in performing a contract.
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- GFP — Government-Furnished Property
- Property owned or leased by the government and provided to a contractor for contract performance.
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- GFSK — Gaussian Frequency-Shift Keying
- An FSK variant with a Gaussian filter that narrows bandwidth, used in Bluetooth.
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- GG — Gravity Gradient
- The differential gravitational force across a spacecraft that produces a stabilizing or disturbing attitude torque.
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- GIDEP — Government-Industry Data Exchange Program
- A cooperative program for sharing technical data on parts, reliability, and problem alerts.
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- GIE — Gridded Ion Engine
- An electric thruster that extracts and accelerates ions through electrostatic grids for high efficiency.
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- Gimbal
- Stabilised mechanical assembly (typically 2- or 3-axis) that isolates a sensor or weapon from carrier motion. Active gimbals use motors and IMU feedback; passive gimbals use spring-mass damping. The dominant payload form factor on small UAS.
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- Gimbaling
- Thrust vector control achieved by tilting an engine on a 2-axis pivot. Primary steering method for most liquid rockets during ascent; ±5–10° deflection provides pitch and yaw authority while roll is handled by differential throttling or ACS.
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- GIQE — General Image Quality Equation
- An empirical model predicting the NIIRS rating of imagery from resolution, sharpness, and noise parameters.
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- GIS — Geographic Information System
- A system for capturing, storing, analyzing, and displaying spatially referenced geographic data.
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- GLONASS — Global Navigation Satellite System (Russia)
- Russia's global navigation constellation of 24 operational satellites in three orbital planes at ~19,100 km altitude. Interoperable with GPS via multi-constellation receivers.
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- GLRT — Generalized Likelihood Ratio Test
- A detection test that estimates unknown parameters by maximum likelihood before applying a likelihood ratio.
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- GLS — GBAS Landing System
- A precision approach and landing system that uses a ground-based augmentation of GPS to guide aircraft to the runway.
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- GMDSS — Global Maritime Distress and Safety System
- An international system of communications for maritime distress alerting and safety.
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- GMLRS — Guided Multiple Launch Rocket System
- Precision-guided 227 mm artillery rocket with GPS/INS guidance and ~70 km range (ER variant: 150 km). Launched from M270 MLRS / M142 HIMARS. Operational workhorse for deep-fires strike since Iraq 2005; transformative in Ukraine 2022–.
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- GMSK — Gaussian Minimum-Shift Keying
- A continuous-phase modulation with a Gaussian pulse shaping filter, used in GSM.
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- GMST — Greenwich Mean Sidereal Time
- Earth’s rotation angle relative to the vernal equinox; converts ECI to ECEF.
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- GMTI — Ground Moving Target Indicator
- A radar mode that detects and tracks moving ground vehicles by rejecting zero-velocity clutter. Requires narrow Doppler bins and high PRF. Used by JSTARS, Global Hawk, and Sentinel aircraft.
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- GNC — Guidance, Navigation and Control
- Discipline determining where a vehicle is and steering it to where it should be.
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- GNC-launch — Guidance Navigation and Control (Launch)
- The launch vehicle system that steers the rocket along its ascent trajectory to the target orbit.
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- GNSS — Global Navigation Satellite System
- Satellite constellation systems providing position, navigation, and timing (PNT) services. Includes GPS (US), GLONASS (Russia), Galileo (EU), and BeiDou (China).
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- GOES — Geostationary Operational Environmental Satellite
- NOAA geostationary weather satellites (GOES-16/17/18/19). 5-min full-disk imaging, lightning mapper, space-weather instruments. Two operational positions: GOES-East (75.2°W) and GOES-West (137.2°W).
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- GP — Glide Path
- The component of an instrument landing system that provides vertical guidance along the approach descent path.
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- GPS — Global Positioning System
- US space-based positioning, navigation, and timing system: a constellation of ~31 MEO satellites at 20 200 km altitude broadcasting L1/L2/L5 signals. Civilian users get ~3-meter accuracy; the encrypted M-code provides anti-jam/anti-spoof for military users.
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- GPS III
- Third-generation GPS satellites with improved accuracy, more powerful M-code military signals for anti-jam/anti-spoof, and L1C civil signal interoperable with Galileo.
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- GPS Spoofing
- Transmission of counterfeit GPS signals to induce a target receiver to compute a false position or time. Increasingly prevalent in contested regions (Black Sea, Baltic). Defences include multi-antenna direction finding, multi-constellation cross-check, and cryptographic authentication (M-code, Galileo OSNMA).
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- GPS-Denied
- Operating in an environment where GPS signals are unavailable due to jamming, spoofing, terrain masking (urban canyon, forest, indoor), or jamming. Motivates alternative PNT strategies such as celestial, signals of opportunity, and vision-aided navigation.
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- DiagramTry it: Joint Ops GuideTry it: Tdl GuideTry it: Mil Ops Degraded Environments+1 more
- GPS-T — GPS Time
- A continuous atomic timescale used by the GPS system, offset from UTC by an integer number of leap seconds.
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- GPWS — Ground Proximity Warning System
- A system that warns pilots of imminent contact with terrain based on radio altimeter and other sensor data.
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- GQ — General Quarters
- The condition of maximum readiness in which all hands man battle stations.
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- GRA — Graveyard Orbit
- A disposal orbit above the GEO belt where retired geostationary satellites are boosted to free their slots.
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- Graceful Degradation
- System property of maintaining reduced but useful capability when components fail or are attacked. Contrasts with brittle failure where a single loss cripples the whole. Designed in via redundancy, federated architectures, and manual fallback modes.
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- Graveyard Orbit
- A disposal orbit at least 300 km above the geostationary belt where decommissioned GEO satellites are moved to avoid interference with operational spacecraft.
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- Gravity Assist
- A technique using a planet's gravitational field to alter a spacecraft's trajectory and velocity without expending propellant. Enables missions to outer planets with feasible delta-v budgets.
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- 2D animationTry it: Interplanetary LaunchTry it: Human Spaceflight Guide
- Gravity Gradient
- The differential gravitational pull across a non-spherical body due to its extent in the radial direction. Produces a restoring torque on elongated spacecraft that naturally aligns the long axis with the local vertical — used as passive attitude stabilisation.
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- 3D modelTry it: Space Fundamentals GuideTry it: Leop Commissioning Guide
- GRC — Governance, Risk, and Compliance
- Integrated approach to aligning security governance, risk management, and regulatory compliance.
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- Green Propellant
- Non-toxic alternatives to hydrazine (e.g., AF-M315E/ASCENT, LMP-103S). Reduce handling hazards and ground processing costs while offering comparable or superior performance.
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- Ground Segment
- All Earth-based elements of a space mission: ground stations (antennas, receivers, transmitters), mission control center, data processing facilities, and communication networks.
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- Ground Station
- A facility with antennas, receivers, transmitters, and processing equipment for communicating with spacecraft. Key parameters: antenna gain, G/T, EIRP, and geographic location.
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- Ground Track
- The path traced on Earth's surface by the sub-satellite point (the point directly below the satellite). Shape depends on orbital inclination, altitude, and eccentricity.
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- GROW Model
- A coaching framework: Goal, Reality, Options, Will. Used by leaders and coaches to structure development conversations by having the coachee articulate each stage rather than being told the answer.
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- DiagramTry it: Team Leadership Guide
- GS — Ground Speed
- The speed of an aircraft relative to the ground, equal to true airspeed adjusted for wind.
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- GSA — General Services Administration
- The U.S. agency that manages federal procurement, real estate, and shared support services.
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- GSD — Ground Sample Distance
- The distance between centres of adjacent pixels measured on the ground. The fundamental measure of spatial resolution for optical Earth observation sensors.
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- GSE — Ground Support Equipment
- Fixed/mobile equipment used to service a vehicle before launch.
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- GSM — Global System for Mobile Communications
- Widely deployed cellular standard frequently targeted for communications interception and analysis.
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- GSO — Geosynchronous Orbit
- An orbit with a period equal to one sidereal day, so the spacecraft returns to the same ground point each day.
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- GSSAP — Geosynchronous Space Situational Awareness Program
- US Space Force constellation of neighbourhood-watch satellites in drift orbits near the GEO belt, observing and characterising space objects. Four operational satellites; declassified 2014. Provides the authoritative US catalogue inputs for GEO.
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- 3D modelTry it: Ssa GuideTry it: Multiint Fusion GuideTry it: Constellation Guide
- GSW — Gunshot Wound
- A traumatic injury caused by a projectile fired from a firearm penetrating the body.
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- GT — Gross Tonnage
- A nondimensional measure of a ship’s total internal volume used for registration and regulation.
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- GTL — Gun-Target Line
- An imaginary straight line from a weapon to its target used in fire support coordination.
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- GTO — Geostationary Transfer Orbit
- Elliptical orbit used to raise a spacecraft from LEO to GEO via an apogee burn.
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