G
g
   (NASA SP-7, 1965)
An acceleration equal to the acceleration of gravity, 980.665 centimeter-second-squared, approximately 32.2 feet per second per second at sea level; used as a unit of stress measurement for bodies undergoing acceleration. See acceleration of gravity; gravity.
G layer
   (NASA SP-7, 1965)
A layer of free electrons in the ionosphere which is occasionally observed above the F2-layer. See ionosphere.
G mode
   (SOHO Glossary - GSFC)
A wave mode generated by a gravity wave.
G-display
   (NASA SP-7, 1965)
In radar, a rectangular display in which a target appears as a laterally centralized blip when the radar antenna is aimed at it in azimuth and wings appear to grow on the blip as the distance to the target is diminished. Horizontal and vertical aiming errors are respectively indicated by horizontal and vertical displacement of the blip. Also called G-scan, G-scope, G-indicator.
G-force
   (NASA SP-7, 1965)
An inertial force usually expressed in multiples of terrestrial gravity.
G-indicator
   (NASA SP-7, 1965)
1. G-display.
2. A display that shows the amount of inertial force acting on body.
G-meter
   (NASA SP-7, 1965)
A meter that indicates acceleration.
G-region
   (NASA SP-7, 1965)
See ionosphere.
G-scan
   (NASA SP-7, 1965)
= G-display.
G-scope
   (NASA SP-7, 1965)
= G-display.
g-suit
   (NASA SP-7, 1965)
A suit that exerts pressure on the abdomen and lower parts of the body to prevent or retard the collection of blood below the chest under positive acceleration. Compare pressure suit.
g-tolerance
   (NASA SP-7, 1965)
A tolerance in a person or other animal, or in a piece of equipment, to an acceleration of a particular value.
GAC--Global Area Coverage
   (Global Land Information System Glossary - USGS)
GAC data are derived from a sample averaging of the full resolution AVHRR data. Four out of every five samples along the scan line are used to compute one average value and the data from only every third scan line are processed, yielding 1.1 km by 4 km resolution at the subpoint.
gadolinium alloys
   (NASA Thesaurus)
Mixtures of gadolinium, a rare earth metal, with other metals.
gadolinium-gallium garnet
   (NASA Thesaurus)
A semiconducting crystalline compound used primarily as a solid-state laser material.
gage pressure
   (NASA SP-7, 1965)
In engineering literature, a term used to indicate the difference between atmospheric pressure and absolute pressure, as read from a differential manometer.
Gaia hypothesis
   (NASA Thesaurus)
The hypothesis that the Earth's atmosphere, biosphere, and its living organisms behave as a single system striving to maintain a stability conducive to the existance of life.
gain
   (NASA SP-7, 1965)
1. A general term used to denote an increase in signal power or signal strength in transmission from one point to another. Gain is usually expressed in decibels and is widely used to denote transducer gain.
2. An increase or amplification. In radar there are two general usages of the term: (a) antenna gain, or gain factor, is the ratio of the power transmitted along the beam axis to that of an isotropic radiator transmitting the same total power; (b) receiver gain, or video gain, is the amplification given a signal by the receiver. See height gain.
gain coefficient
   (Global Land Information System Glossary - USGS)
Gain coefficient is a measurement to denote an increase in signal power in transmission from one point to another
gain factor
   (NASA SP-7, 1965)
See gain, sense 2(a).
gal
   (NASA SP-7, 1965)
(From Galileo). A unit of acceleration equal to 1 centimeter per second per second, or 1000 milligals.
The gal and milligal are used in measuring the acceleration of gravity.
galactic
   (NASA SP-7, 1965)
1. Pertaining to our galaxy, the Milky Way.
2. Pertaining to the galactic system of coordinates, as galactic latitude.
galactic circle
   (NASA SP-7, 1965)
= galactic equator.
See galactic system of coordinates.
galactic cosmic rays
   (NASA Thesaurus)
Energetic particles that come from outside the solar systems. They generally come from within our galaxy.
galactic equator
   (NASA SP-7, 1965)
See galactic system of coordinates.
galactic halos
   (NASA Thesaurus)
The tenuous, spherical cloud surrounding spiral galaxies. It is the locus of old stars and globular clusters. Halos appear to be required, at least to some extent, for the stability of disk galaxies.
galactic mass
   (NASA Thesaurus)
The total amount of matter contained in a galaxy.
galactic pole
   (NASA SP-7, 1965)
See galactic system of coordinates.
galactic radio waves
   (NASA Thesaurus / NASA SP-7, 1965)
Radio waves emanating from our galaxy. See cosmic radio waves.
galactic system of coordinates
   (NASA SP-7, 1965)
An astronomical coordinate system using latitude measured north and south from the galactic equator and longitude measured in the sense of increasing right ascension from 0 to 360 degrees. See coordinate, table.
Galactic latitude is designated b, galactic longitude l. The reference points for galactic coordinates were changed by action of the International Astronomical Union in 1958. The new values are: the north galactic pole lies in the direction right ascension = 12 hours 49 minutes, declination = 27.4 degrees N (equinox 1950); the new zero of longitude is the great semicircle originating at the new north galactic pole at the position angle 0 = 123 degrees with respect to the equatorial pole for 1950.
Galatea
   (NASA Thesaurus)
A natural satellite of Saturn, orbiting at a mean distance of 62,000 kilometers.
galaxies
   (NASA Thesaurus)
Vast assemblages of stars or nebulae, composing island universes separated from other such assemblages by great distances.
galaxy
   (NASA SP-7, 1965)
A vast assemblage of stars, nebulae, etc., composing an island universe separated from other such assemblages by great distances.
The sun and its family of planets is part of a galaxy commonly called the Milky Way. The nearest galaxy to the Milky Way is the spiral galaxy Andromeda at a distance of approximately 800,000 light years.
Galilean Invariance
   (Plasma Physics and Fusion Energy Glossary)
Principle which states that the fundamental laws of physics are the same in all inertial (uniform-velocity) frames of reference. When applied to Newtonian mechanics and the laws of electricity and magnetism, one can derive the special theory of relativity.
Galilean satellites
   (NASA Thesaurus)
The four largest and brightest satellites of Jupiter (Io, Europa, Ganymede, and Callisto).
Galileo probe
   (NASA Thesaurus)
The NASA Jupiter atmospheric entry probe to be deployed from the Galileo spacecraft. The probe will make in situ measurements while descending from a parachute.
Galileo project
   (NASA Thesaurus)
A NASA program to probe Jupiter, its environment and natural satellites. The spacecraft was placed in Earth orbit by the Space Transportation System (STS) on October 18, 1989. Used for Galileo mission.
Galileo spacecraft
   (NASA Thesaurus)
A NASA orbiter spacecraft which will carry the Galileo probe and, following deployment at Jupiter, will become an orbiting platform for remote sensing of Jupiter and its satellites.
game theory
   (NASA Thesaurus)
Application of mathematics to a game, business situation, or other problem to maximize gain or minimize loss.
gamma
   (Global Land Information System Glossary - USGS)
This is a unit of magnetic intensity.
Gamma Emission
   (Plasma Physics and Fusion Energy Glossary)
Nuclear decay process whereby the nucleus goes from an excited state to a more stable state by emitting a gamma ray. (See entry for gamma ray.)
gamma photon
   (NASA SP-7, 1965)
= gamma ray.
gamma radiation
   (NASA SP-7, 1965)
= gamma ray.
gamma ray astronomy
   (NASA Thesaurus)
Astronomy based on the detection of gamma ray emission and interactions from supernova remnants, neutron stars, flare stars, galactic core and disc, black holes, etc.
gamma ray bursts
   (NASA Thesaurus)
Short (about 0.1 - 4 sec.) intense low-energy (about 0.1 - 1.2 MeV) bursts first recorded by the Vela satellite system in 1967. Their isotropic distribution suggests an extragalactic origin, but a galactic disk origin cannot be ruled out. Used for cosmic gamma ray bursts.
gamma ray bursts
   (High Energy Astrophysics Dictionary- GSFC)
Bursts of gamma-rays from space lasting from a fraction of a second to many minutes. There is no clear scientific consensus as to their cause or even their distance. Abbreviated "GRB".
gamma ray lasers
   (NASA Thesaurus)
Stimulated emission devices producing coherent gamma radiation.
Gamma Ray Observatory
   (NASA Thesaurus)
A late 1980s NASA mission to explore the gamma ray window to the universe from 0.06 MeV to 30 GeV.
gamma ray spectra
   (NASA Thesaurus)
The energy distribution of gamma rays emitted by nuclei.
gamma ray spectrometers
   (NASA Thesaurus)
Instruments for deriving the physical constants of materials by using induced gamma radiation as the emission source.
gamma ray spectroscopy
   (Global Land Information System Glossary - USGS)
This non-destructive analytical technique is used to determine concentrations of specific chemical elements such as potassium, thorium, and uranium. These elements have naturally occuring radioactive isotopes based on their normal radioactive decay and the associated emission of gamma-rays at specific wavelengths.
gamma ray telescopes
   (NASA Thesaurus)
Special telescopes for the observation (and recording) of astronomical phenomena in the gamma ray spectrum.
gamma rays
   (High Energy Astrophysics Dictionary- GSFC)
The highest energy, shortest wavelength electromagnetic radiation. Usually, they are thought of as any photons having energies greater than about 100 keV.
gamma rays
   (NASA Thesaurus / NASA SP-7, 1965)
A quantum of electromagnetic radiation emitted by a nucleus, each such photon being emitted as the result of a quantum transition between two energy levels of the nucleus. Gamma rays have energies usually between 10 thousand electron volts and 10 million electron volts with correspondingly short wavelengths and high frequencies. Also called gamma radiation.
X-rays occur in the same energy range as gamma rays but are of nonnuclear origin. In atmospheric electricity, gamma rays are of some importance in contributing to atmospheric ionization, along with alpha particles and beta particles. Gamma ray photons have much greater penetration ranges than do alpha and beta particles, often amounting to distances of the order of a hundred meters in air at sea level. These high-energy photons may initiate their ionizing action by ejecting photoelectrons from neutral atoms or molecules of the air, by ejecting electrons by the Compton effect, or (for gamma photons with energies above a few million electron volts) by pair production in which an electron and a positron are created.
Gamma-Ray Imaging Platform
   (High Energy Astrophysics Dictionary- GSFC)
A balloon-borne gamma-ray telescope made by a group at the California Institute of Technology. Abbreviated "GRIP".
gantry
   (NASA SP-7, 1965)
A frame structure that spans over something, as an elevated platform that runs astride a work area, supported by wheels on each side; short for gantry crane or gantry scaffold.
gantry crane
   (NASA SP-7, 1965)
A large crane mounted on a platform that usually runs back and forth on parallel tracks astride the work area. Often shortened to gantry.
gantry cranes
   (NASA Thesaurus)
Large cranes mounted on platforms that usually run back and forth on parallel tracks astride the work area. Used for gantries.
gantry scaffold
   (NASA SP-7, 1965)
A massive scaffolding structure mounted on a bridge or platform supported by a pair of towers or trestles that normally run back and forth on parallel tracks, used to assemble and service a large rocket as the rocket rests on its launching pad. Often shortened to gantry.
This structure is a latticed arrangement of girders, tubing, platforms, cranes, elevators, instruments, wiring, floodlights, cables, and ladders - all used to attend the rocket.
Ganymede
   (NASA Thesaurus)
A satellite of Jupiter orbiting at a mean distance of 1,071,000 kilometers. Also called Jupiter III.
Ganymede
   (NASA SP-7, 1965)
A satellite of Jupiter orbiting at a mean distance of 1,071,000 kilometers. Also called Jupiter III.
gaps (geology)
   (NASA Thesaurus)
Ravines or gorges cut deeply through a mountain ridge, or between hills or mountains. Used for cols and passes.
garbage
   (NASA SP-7, 1965)
Miscellaneous objects in orbit, usually material ejected or broken away from a launch vehicle or satellite.
garnets
   (NASA Thesaurus)
Groups of minerals that are silicates of cubic crystalline form.
gas
   (NASA SP-7, 1965)
The state of matter in which the molecules are practically unrestricted by intermolecular forces so that the molecules are free to occupy any space within an enclosure.
In vacuum technology the word gas has been loosely applied to the noncondensable gas and vapor within a vacuum system.
gas atomization
   (NASA Thesaurus)
Atomization of fluids by high velocity gas jets.
gas cap
   (NASA SP-7, 1965)
The gas immediately in front of a body as it travels through the atmosphere.
This gas is compressed and heated. If the speed is sufficiently high, the gas becomes incandescent; it is to this condition that the term is usually applied, as in the gas cap of a meteor.
gas chromatography
   (NASA Thesaurus)
A method of chemical analysis involving the separation of volatile constituents of a mixture by means of gas flow entrainment, vapor pressure differences, and affinity of specific compounds for various liquids or solid materials.
gas constant
   (NASA SP-7, 1965)
(symbol R, R*,R0)
The constant factor in the equation of state for perfect gases. The universal gas constant is

R0 = 8.3143 joules/degrees K-mol

The gas constant for a particular gas, specific gas constant, r = R/m where m is the molecular weight of the gas. See Boltzmann constant.
gas constant per molecule
   (NASA SP-7, 1965)
= Boltzmann constant.
gas discharge tubes
   (NASA Thesaurus)
Evacuated enclosures containing a gas at low pressure that permits the passage of electricity through the gas upon application of sufficient voltage. Note: The tubes are usually provided with metal electrodes, but one form permits an electrodeless discharge with induced voltage. Used for discharge tubes and gas discharge counters.
gas generators
   (NASA Thesaurus)
A device used to generate gases in the laboratory; a chemical plant for producing gas from coal, for example, water gas. Used for gas generator engines.
gas giant planets
   (NASA Thesaurus)
The giant planets, Jupiter, Saturn, Uranus, and Neptune, of our solar system.
gas laws
   (NASA SP-7, 1965)
The thermodynamic laws applying to perfect gases: Boyle-Mariotte law, Charles-Gay-Lussac law, Dalton law, equation of state. Also called perfect-gas laws, ideal-gas laws.
gas path analysis
   (NASA Thesaurus)
Mathematical process of determining overall engine performance, individual module performances and sensor performances from any specific set of engine related measurements.
gas scrubbing
   (NASA SP-7, 1965)
The contacting of a gaseous mixture with a liquid for the purpose of removing gaseous contaminants or entrained liquids or solids.
gas turbine
   (NASA SP-7, 1965)
1. A turbine rotated by expanding gases, as in a turbojet engine or in a turbosupercharger.
2. A gas-turbine engine.
gas turbines
   (NASA Thesaurus)
Turbines rotated by expanding gases, as in a turbojet engine or in a turbosupercharger.
gas-solid interactions
   (NASA Thesaurus)
Effects of the impingement of gases (particles) on solid surfaces in various environments.
gas-turbine engine
   (NASA SP-7, 1965)
An engine incorporating as its chief element a turbine rotated by expanding gases. In its most usual form, it consists essentially of a rotary air compressor with an air intake, one or more combustion chambers, a turbine, and an exhaust outlet.
gaseous diffusion
   (NASA Thesaurus)
A method of isotopic separation based on the fact that gas atoms or molecules with different masses will diffuse through a morous barrier (orm membrane) at different rates. The method is used to separate uranium 235 from uranium 238. It requires large gaseous diffusion plants and enormous amounts of electric power.
gaseous discharge
   (NASA SP-7, 1965)
= electric discharge.
gaseous electric discharge
   (NASA SP-7, 1965)
= electric discharge.
gaseous electronics
   (NASA SP-7, 1965)
The study of the conduction of electricity through gases, involving study of the Townsend, glow, and arc discharges, and all the collision phenomena on an atomic scale. Formerly called gaseous discharges.
gaskets
   (NASA Thesaurus)
Preformed deformable devices designed to be placed between two adjoining parts to prevent the passage of liquid or gas between the parts.
gasohol (fuel)
   (NASA Thesaurus)
Synthetic fuel consisting of a mixture of gasoline and grain alcohol (ethanol).
gate
   (NASA SP-7, 1965)
1. To control passage of a signal as in the circuits of a computer.
2. A circuit having an output and inputs so designed that the output is energized only when a definite set of input conditions are met. In computers, called AND-gate.
gating
   (NASA SP-7, 1965)
The process of selecting those portions of a wave which exist during one or more selected time intervals or which have magnitudes between selected limits.
gauge theory
   (NASA Thesaurus)
A field theory in which symmetrics of the theory are implemented locally in space and time. This leads to theories where forces are generally carried by vector bosons. Some gauge theories are electrodynamics, quantum chromodynamics, and Yang Mills theory.
gauss
   (NASA SP-7, 1965)
A unit of magnetic induction (or magnetic flux density) equal to 1 dyne per unit cgs magnetic pole.
Prior to 1932, the gauss was used both as a unit of magnetic induction and as a unit of magnetic field intensity, but the latter quantity is now measured in oersteds.
Gauss theorem
   (NASA SP-7, 1965)
= divergence theorem.
Gauss-Seidel
   (Global Land Information System Glossary - USGS)
The Gauss-Seidel method is a technique for interpolating irregularly spaced data points, such as spot elevations, onto a regular grid (e.g., Digital Elevation Models). Unlike simple interpolation methods which assume only correlation, the Gauss-Seidel method is used when some characteristics of the system are known, such as the local value of a derivative. This method, which must be solved iteratively, takes the form of an implicit equation. The Successive Over Relaxation (SOR) method, a refinement to the Gauss-Seidel method, causes the system to converge more rapidly so fewer iterations are required to achieve the same result.
Gaussian constant
   (NASA SP-7, 1965)
(symbol k )
Originally used in astronomical calculations as k = square root of G , where G is the constant of gravitation. k is now defined as exactly 0.01720209895 radians. Also called Gaussian gravitation constant. See astronomical unit.
Gaussian distribution
   (NASA SP-7, 1965)
= normal distribution.
Gaussian elimination
   (NASA Thesaurus)
A technique for solving linear equations by progressive differencing.
Gaussian gravitation constant
   (NASA SP-7, 1965)
= Gaussian constant.
Gaussian noise
   (NASA SP-7, 1965)
= random noise.
Gay-Lussac law
   (NASA SP-7, 1965)
= Charles-Gay-Lussac law.
GCA (abbr)
   (NASA SP-7, 1965)
= ground-controlled approach.
GCF
   (Space Flight Glossary - JPL)
Ground Communications Facilities, provides data and voice communications between JPL and the three DSCCs.
GCI (abbr)
   (NASA SP-7, 1965)
= ground-controlled intercept.
GCP--Ground Control Point
   (Global Land Information System Glossary - USGS)
GCPs are physical points on the ground whose positions are known with respect to some horizontal coordinate system and/or vertical datum. When mutually identifiable on the ground and on a map or photographic image, ground control points can be used to establish the exact spatial position and orientation of the image to the ground. Ground control points may be either horizontal control points, vertical control points, or both.
GDS
   (Space Flight Glossary - JPL)
Ground Data System, encompasses DSN, GCF, MCCC, and project data processing systems.
gee
   (NASA SP-7, 1965)
A suffix meaning earth, as in perigee , apogee. See perigee, note.
gegenschein
   (NASA Thesaurus / NASA SP-7, 1965)
A round or elongated spot of light in the sky at a point 180 degrees from the sun. Also called counterglow, zodiacal counterglow. Compare zodiacal light.
gehlenite
   (NASA Thesaurus)
A mineral of the mellite group. It is isomorphous with akermenite. Used for verlardenite.
Geiger counter
   (NASA SP-7, 1965)
An instrument for detecting and measuring radioactivity. In full, Geiger-Muller counter.
This counter, essentially a thin-walled gas-filled metallic tube with a needle electrode projected within, detects the radiating particle indirectly. The particle penetrates the thin wall and ionizes the gas; a current is momentarily set up, which is detectable and measurable. Compare scintillation counter.
Geiger counters
   (NASA Thesaurus)
Instruments for detecting and measuring radioactivity. In full, Geiger-Mueller counter. Used for Geiger-Mueller tubes.
Geiger-Muller counter
   (NASA SP-7, 1965)
Full term for Geiger counter.
gels
   (NASA Thesaurus)
Liquids containing colloidal structural networks that form continuous matrices and completely pervade the liquid phase. Gels deform elastically upon application of shear forces less than the yield stress. At shear forces above the yield stress, the flow properties are principally determined by the gel matrices.
Gem, Gemi
   (NASA SP-7, 1965)
International Astronomical Union abbreviations for Gemini. See constellation.
Gemini
   (NASA SP-7, 1965)
(abbr Gem, Gemi)
See constellation.
gene expression
   (NASA Thesaurus)
The process by which a gene's coded information is converted into the structures present and operating in the cell.
general circulation
   (NASA SP-7, 1965)
= planetary circulation.
general perturbations
   (NASA SP-7, 1965)
In orbital determinations, a method of calculating perturbative effects by expanding and integrating in series. See perturbation.
general precession
   (NASA SP-7, 1965)
The resultant motion of the components causing precession of the equinoxes. The general precession is westward along the ecliptic at the rate of about 50.3 seconds of arc per year.
The effect of the sun and moon, called lunisolar precession, is to produce a westward motion of the equinoxes along the ecliptic. The effect of other planets, called planetary precession, tends to produce a much smaller motion eastward along the ecliptic. The component of general precession along the celestial equator, called precession in right ascension, is about 46.1 seconds of arc per year; and the component along a celestial meridian, called precession in declination, is about 20.0 seconds of arc per year. See astronomical constants.
generalized coordinates
   (NASA SP-7, 1965)
Any set of coordinates specifying the state of the system under consideration. Usually employed in problems involving a finite number of degrees of freedom, the generalized coordinates are chosen so as to take advantage of the constraints of the system in reducing the total number of coordinates. Also called Lagrangian coordinates.
generalized transmission function
   (NASA SP-7, 1965)
In atmospheric-radiation theory, a set of values, variable with wavelength, each one of which represents an average transmission coefficient for a small wavelength interval and for a specified optical path through the absorbing gas in question. See universal transmission function.
generation
   (NASA SP-7, 1965)
In any technical or technological development, as of a missile, jet engine, or the like, a stage or period that is marked by features or performances not marked, or existent, in a previous period of development or production, as in the first generation of rockets using liquid propellants.
Genesis mission
   (NASA Thesaurus)
A space mission to collect solar wind samples from a halo orbit about the sun-Earth L1 point for two years, returning those samples to Earth in 2003 for analysis and examination. Analysis of the samples collected by the mission will contribute to an understanding of the origins of the solar system.
genetic algorithms
   (NASA Thesaurus)
Parameter search procedures loosely based on the mechanics of natural population genetics and the survival-of-the-fittest.
genetic code
   (NASA Thesaurus)
The sequence of nucleotides, coded in triplets along the messenger RNA that determines the sequence of amino acids in protein synthesis.
genetic effect of radiation
   (NASA SP-7, 1965)
Inheritable changes, chiefly mutations, produced in living organisms by the absorption of ionizing radiations. On the basis of present knowledge these effects are purely additive, and there is no recovery.
genetic engineering
   (NASA Thesaurus)
The intentional production of new genes and alteration of genomes by the substitution or addition of new genetic material. Used for hybrids (biology).
geo
   (NASA SP-7, 1965)
A prefix meaning earth, as in geology, geophysics.
Some writers use the established terms such as geology to refer to the same concept on other bodies of the solar system, as the geology of Mars, rather than areology or marsology, geology of the Moon, rather than selenology.
geocentri
   (Solar System Dynamics Glossary - JPL)
Refers to a reference system centered at the Earth's center.
geocentric
   (NASA SP-7, 1965)
Relative to the earth as a center; measured from the center of the earth.
geocentric diameter
   (NASA SP-7, 1965)
The diameter of a celestial body measured in seconds of arc as viewed from the earth's center.
geocentric distance (delta)
   (Comet Glossary - JPL)
The comet's distance from the Earth in astronomical units.
geocentric latitude
   (NASA SP-7, 1965)
Of a position in the earth's surface; the angle between a line to the center of the earth and the plane of the equator.
Because the earth is approximately an oblate spheroid, rather than a true sphere, this differs from geographic latitude, the maximum difference being 11.6 minutes of arc at latitude 45 degrees.
geocentric parallax
   (NASA SP-7, 1965)
The difference in the apparent direction or position of a celestial body measured in seconds of arc, as observed from the center of the earth and a point on its surface. See parallax.
geochemistry
   (NASA Thesaurus)
The study of the distribution of the amounts of the chemical elements in minerals, ores, rocks, soils, water, and the atmosphere. Also, the study of the circulation of the elements in nature, on the basis of the properties of the atom and ions. A major concern of geochemistry is the synoptic evaluation of the abundance of the elements of the Earth's crust and in major classes of rocks and minerals.
geochronology
   (NASA Thesaurus)
The study of time in relationship to the history of the Earth, especially by the absolute age determination and relative dating systems developed for this purpose.
geocorona
   (AS&T Dictionary)
An envelope or shell of ionized gases made up primarily of hydrogen, that surrounds the Earth to about 15 Earth radii, possibly including the metasphere and the protosphere, and which emits Lyman-alpha radiation in the presence of sunlight.
geocorona
   (NASA SP-7, 1965)
1. The shell of hydrogen surrounding the earth at the limit of the atmosphere.
In Shlovsky's system of nomenclature, the geocorona includes the metasphere, the outer, fully ionized zone, and the protosphere, the inner zone.
2. = Van Allen radiation belts.
The use of geocorona in sense 2 should be discouraged, since it conflicts with the relatively well-established usage in sense 1.
geodesic line
   (NASA SP-7, 1965)
The shortest line on a mathematically derived surface, between two points on the surface. Also called geodesic.
A geodesic line on the spheroidal earth is called a geodetic line.
geodesy
   (NASA Thesaurus)
The science which deals mathematically with the size and shape of the Earth, and the Earth's external gravity field, and with surveys of such precision that overall size and shape of the Earth must be taken into consideration. Used for Earth figure, Earth shape, and Iszak ellipsoid.
geodesy
   (NASA SP-7, 1965)
The science which deals mathematically with the size and shape of the earth, and the earth's external gravity field, and with surveys of such precision that overall size and shape of the earth must be taken into consideration.
geodetic
   (Solar System Dynamics Glossary - JPL)
Geodetic coordinates, latitude and longitude, specify a location on the Earth's oblate (non-spherical) surface. Latitude, unless otherwise specified, is generally the geodetic latitude. Geodetic latitude is defined as the angle between the equatorial plane and a line normal to the surface at that location. Geodetic longitude is the angular distance between the location's meridian and the Greenwich meridian.
geodetic
   (Global Land Information System Glossary - USGS)
Of or determined by geodesy; that part of applied mathematics that deals with the determination of the magnitude and figure either of the whole Earth or of a large portion of its surface. Also refers to the exact location points on the Earth's surface
geodetic
   (NASA SP-7, 1965)
Of or pertaining to geodesy; geodesic.
geodetic accuracy
   (NASA Thesaurus)
The degree to which point positions or boundaries indicated on maps or imagery correspond with true geodetic positions.
geodetic control network
   (Galileo Project Glossary - JPL)
An ellipsoid or sphere-shaped "net" that is used as a type of base-map or control to correlate co-ordinates on a flat image with their actual location on the spherical planetary body in order to obtain accurate measurements from images
geodetic coordinates
   (NASA Thesaurus / NASA SP-7, 1965)
Quantities which define the position of a point on the spheroid of reference with respect to the planes of the geodetic equator and of a reference meridian. Compare geographic coordinates.
geodetic datum
   (NASA SP-7, 1965)
A datum consisting of five quantities, the latitude and longitude and elevation above the reference spheroid of an initial point, a line from this point, and two constants which define the reference spheroid. Azimuth or orientation of the line, given the longitude, is determined by astronomic observations. Alternatively, the datum may be considered as three rectangular coordinates fixing the origin of a coordinate system whose orientation is determined by the fixed stars, and the reference spheroid is an arbitrary coordinate surface of an orbiting ellipsoidal coordinate system.
A geodetic datum forms the basis for the computation of horizontal control surveys in which the curvature of the earth is considered.
geodetic equator
   (NASA SP-7, 1965)
The great circle midway between the poles of revolution of the earth, connecting points of 0 degrees geodetic latitude. See astronomical equator.
geodetic latitude
   (NASA SP-7, 1965)
Angular distance between the plane of the equator and a normal to the spheroid. It is the astronomical latitude corrected for the meridional component of the deflection of the vertical. Also called geographic latitude, topographical latitude.
This is the latitude used for charts.
geodetic line
   (NASA SP-7, 1965)
A geodesic line on the spheroidal earth. Also called geodesic. Compare geodesic line.
geodetic longitude
   (NASA SP-7, 1965)
The angle between the plane of the reference meridian and the plane through the polar axis and the normal to the spheroid. It is the astronomical longitude corrected for the prime vertical component of the deflection of the vertical divided by the cosine of the latitude. Also called geographic longitude.
This is the longitude used for charts.
geodetic meridian
   (NASA SP-7, 1965)
A line connecting points of equal geodetic longitude. Also called geographic meridian. See astronomical meridian.
geodetic parallel
   (NASA SP-7, 1965)
A line connecting points of equal geodetic latitude. Also called geographic parallel. See astronomical parallel.
geodetic position
   (NASA SP-7, 1965)
A position of a point on the surface of the earth expressed in terms of geodetic latitude and geodetic longitude.
A geodetic position implies an adopted geodetic datum, which must be stated for a complete record of the position.
geodetic survey
   (NASA SP-7, 1965)
1. A survey which takes into account the size and shape of the earth.
2. An organization engaged in making geodetic surveys, sense 1.
geodetic surveys
   (NASA Thesaurus)
Surveys which takes into account the size and shape of the Earth.
geodimeters
   (NASA Thesaurus)
Trade name of electronic-optical devices that measure ground distances precisely by electronic timing and phase comparison of modulated light waves that travel from a master unit to a reflector and return to a light-sensitive tube where an electric current is set up. They are normally used at night and are effective with first-order accuracy up to distances of 5-40 km (3-25 miles). The term is derived from GEOdetic DIstance METER.
geodynamic height
   (NASA SP-7, 1965)
= dynamic height.
geodynamic meter
   (NASA SP-7, 1965)
= dynamic meter. See dynamic height.
geodynamics
   (NASA Thesaurus)
Study of the dynamic forces or processes within the Earth. Used for crustal dynamics.
geoelectricity
   (NASA Thesaurus)
The Earth's natural electric fields and phenomena. It is closely related to geomagnetism.
geographic coordinates
   (NASA SP-7, 1965)
Coordinates defining a point on the surface of the earth, usually latitude and longitude. Also called terrestrial coordinates, geographical coordinates. See coordinate, table, for relationship between geographic coordinates and celestial coordinates.
Geographic coordinates can refer to either astronomical or geodetic coordinates.
geographic information systems
   (NASA Thesaurus)
Computer assisted systems that acquire, store, manipulate, and display geographic data. Some systems are not automated.
geographic latitude
   (NASA SP-7, 1965)
= geodetic latitude.
geographic longitude
   (NASA SP-7, 1965)
= geodetic longitude.
geographical coordinates
   (NASA SP-7, 1965)
= geographic coordinates.
geographical mile
   (NASA SP-7, 1965)
The length of 1 minute of arc of the equator, or 6087.08 feet.
geographical pole
   (NASA SP-7, 1965)
Either of the two points of intersection of the surface of the earth with its axis or rotation where all meridians meet, labeled N or S to indicate whether the north geographical pole of the south geographical pole.
geographical position
   (NASA SP-7, 1965)
1. That point on the earth at which a given celestial body is in the zenith of a specified time.
The geographical position of the sun is also called the subsolar point, of the moon the sublunar point, and of a star the substellar or subastral point.
2. Any position on the earth defined by means of its geographic coordinates, either astronomical or geodetic.
geography
   (NASA Thesaurus)
The study of all aspects of the Earth's surface including its natural and political divisions, the distribution and differentiation of areas and, often, man in relationship to his environment.
geoid
   (NASA SP-7, 1965)
The figure of the earth as defined by the geopotential surface which most nearly coincides with mean sea level over the entire surface of the earth.
Because of variations in the direction of gravity, to which it is everywhere perpendicular, the geoid is not quite an ellipsoid of revolution, the sea-level surface being higher under mountainous areas. Compare equilibrium spheroid, geosphere.
geoidal horizon
   (NASA SP-7, 1965)
That circle of the celestial sphere formed by the intersection of the celestial sphere and a plane through a point on the geoid perpendicular to the zenith-nadir line. See horizon.
geoids
   (NASA Thesaurus)
The figure of the Earth as defined by the geopotential surface which most nearly coincides with mean sea level over the entire surface of the Earth.
geologic activity
   (Galileo Project Glossary - JPL)
The expression of the internal and external processes and events that affect a planetary body.
geologic time
   (Galileo Project Glossary - JPL)
The time extending from the end of the formative period of the Earth to the beginning of human history.
geologic unit
   (Galileo Project Glossary - JPL)
A body of rock (or ice, in Europa's case) that has a distinct origin and consists of dominant, unifying features that can be easily recognized and mapped.
geological faults
   (NASA Thesaurus)
A surface or zone of rock fracture along which there has been displacement, from a few centimeters to a few kilometers in scale. Used for closed faults, cross faults, echelon faults, geofractures, grabens, rifts, splits (geology), step faults, and thrust faults.
geology
   (NASA Thesaurus)
The study of the planet Earth--the materials of which it is made, the processes that act on these materials, the products formed, and the history of the planet and its life forms since its origin. Geology considers the physical forces that act on the Earth, the chemistry of its constituent materials, and the biology of its past inhabitants as revealed by fossils. Clues on the origin of the planet are sought in a study of the Moon and other extraterrestrial bodies. The knowledge thus obtained is placed in the service of man--to aid in the discovery of minerals and fuels of value in the Earth's crust, to identify geologically stable sites for major structures, and to provide foreknowledge of some of the dangers associated with the mobile forces in a dynamic Earth.
geomagnetic
   (NASA SP-7, 1965)
Of or pertaining to geomagnetism.
geomagnetic coordinates
   (NASA SP-7, 1965)
A system of spherical coordinates based on the best fit of a centered dipole to the actual magnetic field of the earth.
geomagnetic equator
   (NASA SP-7, 1965)
The terrestrial great circle everywhere 90 degrees from the geomagnetic poles.
Geomagnetic equator should not be confused with magnetic equator, the line connecting all points of zero magnetic dip. Compare aclinic line.
geomagnetic latitude
   (NASA Thesaurus / NASA SP-7, 1965)
Angular distance from the geomagnetic equator, measured northward or southward through 90 degrees and labeled N or S to indicate the direction of measurement. Also erroneously called magnetic latitude.
Geomagnetic latitude should not be confused with magnetic latitude, the magnetic dip. Phenomena closely related to the earth's magnetic field are often plotted according to geomagnetic latitude rather than geographic latitude.
geomagnetic meridian
   (NASA SP-7, 1965)
1. The meridional lines of a geomagnetic coordinate system.
2. = magnetic meridian.
geomagnetic pole
   (NASA SP-7, 1965)
Either of two antipodal points marking the intersection of the earth's surface with the extended axis of a dipole assumed to be located at the center of the earth and approximating the source of the actual magnetic field of the earth.
That pole in the Northern Hemisphere (latitude, 78 1/2 degrees N; longitude, 69 degrees W) is designated north geomagnetic pole, and that pole in the Southern Hemisphere (latitude, 78 1/2 degrees S, longitude, 111 degrees E) is designated south geomagnetic pole. The great circle midway between these poles is called geomagnetic equator. The expression geomagnetic pole should not be confused with magnetic pole, which relates to the actual magnetic field of the earth. See geomagnetic latitude.
geomagnetic storm
   (Solar Physics Glossary - NASA GSFC)
A worldwide disturbance of the Earth's magnetic field, associated with solar activity.
geomagnetic storm
   (NASA SP-7, 1965)
= magnetic storm.
geomagnetism
   (NASA SP-7, 1965)
1. The magnetic phenomena, collectively considered, exhibited by the earth and its atmosphere and by extension the magnetic phenomena in interplanetary space.
2. The study of the magnetic field of the earth. Also called terrestrial magnetism.
geometric accuracy
   (NASA Thesaurus)
The internal geometric fidelity of an imaging system.
geometric chord
   (NASA SP-7, 1965)
See chord, note.
geometric dilution of precision
   (NASA Thesaurus)
A navigation and positioning system performance index expressing the dilution of range measurement precision due to the geometric relationship between user and satellites. It is formulated as the square root of the sum of the variances of position estimates in the three orthogonal directions and can be employed to determine the optimal locations for network satellites and in the selection of optimal satellite signals sources. Used for GDOP.
geometric dilution of precision
   (NASA SP-7, 1965)
Null
geometric mean
   (NASA SP-7, 1965)
A measure of central position. The geometric mean of n quantities equals the n th root of the product of the quantities.
geometric position
   (NASA SP-7, 1965)
= true position.
geometric rectification (imagery)
   (NASA Thesaurus)
The correction of image distortions due to sensor view angle, platform attitude, or target surface features.
geometrical acoustics
   (NASA Thesaurus)
The study of the behavior of sound under the assumption that sound transversing a homogeneous medium travels along straight lines or rays. Used for ray acoustics.
geometrical horizon
   (NASA SP-7, 1965)
See horizon.
geometrical optics
   (NASA Thesaurus)
The geometry of paths of light rays and their imagery through optical systems. Used for ray optics.
geometrical theory of diffraction
   (NASA Thesaurus)
A ray theory of diffraction process.
geomorphology
   (NASA Thesaurus)
A science that deals with the land and submarine relief features of the Earth's surface and genetic interpretation of them through using the principles of physiography in its descriptive aspects and of dynamic and structural geology in its explanatory phases. Used for physiography.
geophysical fluid flow cells
   (NASA Thesaurus)
Apparatus used in model experiments for deep solar convection and Jovian atmospheric circulation for Spacelab 1 and Spacelab 3.
geophysical fluids
   (NASA Thesaurus)
General term for the liquids and gases on or in the Earth (from water in all forms, to petroleum and hydrocarbons in liquid and gaseous form, and molten rock material within the Earth).
geophysics
   (NASA Thesaurus)
The physics of the Earth and its environment, i.e., its solid earth, air, waters, and (by extension) space. Classically, geophysics is concerned with the nature of and physical occurrences at and below the surface of the Earth including, therefore, geology, oceanography, geodesy, seismology, and hydrology. The trend is to extend the scope of geophysics to include meteorology, geomagnetism, astrophysics, and other sciences concerned with the physical nature of the universe.
geophysics
   (NASA SP-7, 1965)
The physics of the earth and its environment, i.e., earth, air, and (by extension) space.
Classically, geophysics is concerned with the nature of and physical occurrences at and below the surface of the earth including, therefore, geology, oceanography, geodesy, seismology, hydrology, etc. The trend is to extend the scope of geophysics to include meteorology, geomagnetism, astrophysics, and other sciences concerned with the physical nature of the universe.
geopotential
   (NASA Thesaurus / NASA SP-7, 1965)
The potential energy of a unit mass relative to sea level, numerically equal to the work that would be done in lifting the unit mass from sea level to the height at which the mass is located; commonly expressed in terms of dynamic height or geopotential height. Compare gravitational potential.
The geopotential lower case phiat height Z is given mathematically by the expression,

theta equals the integral from zero to z, g d Z

where g is the acceleration of gravity.
geopotential height
   (NASA Thesaurus / NASA SP-7, 1965)
The height of a given point in the atmosphere in units proportional to the potential energy of unit mass ( geopotential) at this height, relative to sea level.
The relation, in the cgs system, between the geopotential height H and the geometric height Z is

H equals one over nine hundred eighty, the integral from zero to z, d g Z

where g is the acceleration of gravity, so that the two heights are numerically interchangeable for most meteorological purposes. Also, 1 geopotential meter is equal to 0.98 dynamic meter. See dynamic height. At the present time, the geopotential height unit is used for all aerological reports, by convention of the World Meteorological Organization.
geopotential meter
   (NASA SP-7, 1965)
A unit of length used in measuring geopotential height; 1 geopotential meter is equal to 0.98 dynamic meter. See dynamic height.
Geopotential Research Mission
   (NASA Thesaurus)
A NASA gravity field mapping mission utilizing the low-low satellite tracking concept to measure the Doppler shift between two coorbiting polar satellites. Used for Gravsat satellites.
geopotential surface
   (NASA SP-7, 1965)
A surface of constant geopotential, i.e., a surface along which a particle of matter could move without undergoing any changes in its potential energy. Also called equigeopotential surface, level surface.
Geopotential surfaces almost coincide with surfaces of constant geometric height. Because of the poleward increase of the acceleration of gravity along a constant geometric-height surface, a given geopotential surface has a small geometric-height over the poles than over the equator. See potential, geopotential height, dynamic height.
geopressure
   (NASA Thesaurus)
Pressures that exceed the normal hydrostatic pressure of about 0.465 psi per foot of depth.
georef (abbr)
   (NASA SP-7, 1965)
= World Geographic Reference System. Pronounced as a word.
georegistered
   (Global Land Information System Glossary - USGS)
An image that has been geographically referenced or rectified to an Earth model, usually to a map projection. Sometimes referred to as geocoded or geometric registration.
GEOS-D satellite
   (NASA Thesaurus)
Another in a series of the European Space Agency's geostationary scientific satellites launched by NASA for long-term cosmic radiation studies. Used for Geodynamic Experimental Ocean Satellite.
Geosari project
   (NASA Thesaurus)
Launch of GEOS on second development flight of Ariane launcher into a geostationary elliptical orbit in 1979. The name is derived from a combination of GEOS and ARIane.
geosphere
   (NASA SP-7, 1965)
The solid and liquid portions of the earth; the lithosphere plus the hydrosphere. Compare geoid, equilibrium spheroid.
Above the geosphere lies the atmosphere and at the interface between these two regions is found almost all of the biosphere, or zone of life.
geostationary orbit
   (Space Flight Glossary - JPL)
A geosynchronous orbit in which the spacecraft is constrained to a constant latitude.
geostrophic wind
   (NASA Thesaurus / NASA SP-7, 1965)
That horizontal wind velocity for which the coriolis acceleration exactly balances the horizontal pressure force. Compare gradient wind.
geostrophic wind level
   (NASA SP-7, 1965)

The lowest level at which the wind becomes geostrophic in the theory of the Ekman spiral, proportional to

square root of v over sin theta
where nu is the kinematic eddy viscosity and lower case phi the latitude. Also called gradient wind level.
In practice it is observed that the geostrophic wind level is between 1.2 and 1.6 kilometers, and it is assumed that this marks the upper limit of frictional influence of the earth's surface. The geostrophic wind level may be considered to be the top of the Ekman layer and planetary boundary layer , i.e., the base of the free atmosphere.
geosynchronous orbit
   (Space Flight Glossary - JPL)
A direct, circular, low inclination orbit about the Earth having a period of 23 hours 56 minutes 4 seconds.
geosynclines
   (NASA Thesaurus)
Mobile downwarpings of the crust of the Earth, either elongate or basinlike, measured in scores of kilometers, in which sedementary and igneous rocks accumulate to thicknesses of thousands of meters.
geotechnical engineering
   (NASA Thesaurus)
The science and practice of that part of civil engineering involving the inter-relationship between a geologic environment and the works of man.
geotechnical fabrics
   (NASA Thesaurus)
Generic term for a variety of artificial fiber products used in engineering construction of civil works such as embankments. Also called geofabrics, filter cloth, geotextiles and civil engineering fabrics. Used for geofabrics and geotextiles.
geotemperature
   (NASA Thesaurus)
Internal temperature of the planet Earth. Used for geothermometry.
geothermal energy extraction
   (NASA Thesaurus)
The removal for storage and/or utilization of heat from natural sources within the Earth (hot springs, geysers, hot rocks, etc.)
geothermal energy utilization
   (NASA Thesaurus)
Any application of energy derived from sources within the Earth.
geothermal gradient
   (AS&T Dictionary)
The gradual change in temperature of rock from the Earth's surface to its core; on the average of about +10 deg C per kilometer.
geothermal system
   (AS&T Dictionary)
A localized geological environment in which circulating steam or hot water carries some of the Earth's naturally generated heat from its core close enough to the surface to be used.
geothermal technology
   (NASA Thesaurus)
The gamut of operations involved in the exploration, exploitation, and conversion of energy derived from geothermal sources.
German Infrared Laboratory
   (NASA Thesaurus)
A proposed infrared telescope for Spacelab that was discontinued in 1985. It superseded the LIRTS (telescope).
get
   (NASA SP-7, 1965)
To remove gas from a vacuum system by sorption.
Get Away Specials (STS)
   (NASA Thesaurus)
Low-cost, man-independent Space Shuttle experimental payloads.
getter
   (NASA SP-7, 1965)
1. A material which is included in a vacuum system or device for removing gas by sorption.
2. To remove gas by sorption. Also called get.
getters
   (NASA Thesaurus)
Materials which are included in a vacuum system or device for removing gas by sorption.
geysers
   (Photoglossary of Volcanic Terms - USGS)
Most geysers are hot springs that episodically erupt fountains of scalding water and steam. Such eruptions occur as a consequence of groundwater being heated to its boiling temperature in a confined space (for example, a fracture or conduit). A slight decrease in pressure or an increase in temperature will cause some of the water to boil. The resulting steam forces overlying water up through the conduit and onto the ground. This loss of water further reduces pressure within the conduit system, and most of the remaining water suddenly converts to steam and erupts at the surface
GHA (abbr)
   (NASA SP-7, 1965)
= Greenwich hour angle.
GHz
   (Space Flight Glossary - JPL)
Gigahertz (109 Hz).
giant molecular cloud
   (High Energy Astrophysics Dictionary- GSFC)
Massive clouds of gas in interstellar space composed primarily of hydrogen molecules (two hydrogen atoms bound together), though also containing other molecules observable by radio telescopes. These clouds can contain enough mass to make several million stars like our Sun and are often the sites of star formation.
giant planets
   (NASA SP-7, 1965)
The planets Jupiter, Saturn, Uranus, and Neptune. Also called Jovian planets.
gibbous
   (NASA SP-7, 1965)
Bounded by convex curves.
The term is used particularly in reference to the moon when it is between first quarter and full or between full and last quarter, or to other celestial bodies when they present a similar appearance. See phases of the moon.
Gibbs free energy
   (NASA SP-7, 1965)
= Gibbs function.
Gibbs function
   (NASA SP-7, 1965)
A mathematically defined thermodynamic function of state, which is constant during a reversible isobaric-isothermal process. Also called Gibbs free energy, thermodynamic potential. Compare Helmholtz function.
In symbols the specific Gibbs function g is g = h - Ts where h is specific enthalpy; T is Kelvin temperature; and s is specific entropy. By use of the first law of thermodynamics for reversible processes, dg = -s dT + dp.
giga
   (NASA SP-7, 1965)
(abbr G)
A prefix meaning multiplied by 109.
gimbal
   (NASA SP-7, 1965)
1. A device with two mutually perpendicular and intersecting axes of rotation, thus giving free angular movement in two directions, on which an engine or other object may be mounted.
2. In a gyro, a support which provides the spin axis with a degree of freedom.
3. To move a reaction engine about on a gimbal so as to obtain pitching and yawing correction moments.
4. To mount something on a gimbal.
gimbal freedom
   (NASA SP-7, 1965)
In a gyro, the maximum angular displacement about the output axis of a gimbal.
It is expressed in degrees of output angle or in equivalent angular input.
gimbal lock
   (NASA SP-7, 1965)
A condition of a two-degree-of-freedom gyro wherein the alignment of the spin axis with an axis of freedom deprives the gyro of a degree of freedom, and therefore of its useful properties.
gimbaled motor
   (NASA SP-7, 1965)
A rocket engine mounted on a gimbal.
gimbals
   (NASA Thesaurus)
Devices with two mutually perpendicular and intersecting axes of rotation, thus giving free angular movement in two directions, on which engines or other objects may be mounted. In gyros, supports which provide the spin axes with degrees of freedom.
Ginga satellite
   (High Energy Astrophysics Dictionary- GSFC)
The third Japanese X-ray mission, also known as ASTRO-C.
Giorgi system
   (NASA SP-7, 1965)
= MKSA system.
Giotto mission
   (NASA Thesaurus)
The European Space Agency's mission to fly through the head of Halley's Comet in order to make in situ measurements of the composition and physical state as well as the structures of the head. Included in the onboard equipment are cameras to determine the structures, spectrometers to determine the composition, and a plasma detector and a magnetometer to measure the interactions with the solar wind. The time of encounter with the comet was during the second week of March 1986.
GISS--Goddard Institute of Space Studies
   (Global Land Information System Glossary - USGS)
Based out of New York, New York, GISS, one of NASA's Goddard Space Flight Center research facilities, institutes activities involving research and analysis in specialized subjects, including climate, biogeochemical cycles, remote sensing, and planetary atmospheres.
glacial drift
   (NASA Thesaurus)
A general term for drift transported by glaciers or icebergs and deposited directly on land or in seas. Used for drumlins, end moraines, eskers, glaciofluvial deposits, moraines, and Stoss-and-Lee topography.
glaciers
   (NASA Thesaurus)
Large masses of ice formed, at least in part, on land by the compaction and recrystallization of snow, moving slowly by creeping downslope or outward in all directions due to the stress of their own weight, and surviving from year to year. Included are small mountain glaciers as well as ice sheets continental in size, and ice shelves which float on oceans but are fed in part by ice formed on land. Used for active glaciers and advancing glaciers.
glaciology
   (NASA Thesaurus)
The study of all aspects of snow and ice; the science that treats quantitatively the whole range of processes associated with all forms of solid existing water.
glare
   (NASA Thesaurus)
A condition of vision in which there is disconfort or a reduction in ability to see details, objects, or both, caused by an unsuitable distribution or range of luminance, or by extreme conditions in space.
glass lasers
   (NASA Thesaurus)
High power lasers used in laser fusion technology research.
glassy carbon
   (NASA Thesaurus)
Form of carbon with unique properties and characteristics. Formed by carbonizing phenolic resins made by reacting phenols with cellulosics, aldehydes, and ketones.
glaze icing
   (NASA SP-7, 1965)
Glaze aircraft icing (also known as clear icing) is hard and transparent. It is formed by the relatively slow freezing of supercooled water droplets as they spread over the surface. It tends to acccumulate more rapidly than rime icing, and is often very hard and therefore more difficult to remove. Since it does not freeze instantly, the ice can form into shapes that cause significant aerdynamic penalties, and it therefore the most hazardous form of icing. However, it is the least frequent type of ice encountered, reponsible for about 10% of icing reports.

glide
   (NASA SP-7, 1965)
1. A controlled descent by a heavier-than-air aeronautical vehicle under little or no engine thrust in which forward motion is maintained by gravity and vertical descent is controlled by lift forces.
2. A descending flight path of a glide, sense 1, as, a shallow glide. 3. To descend in a glide, sense 1.
glide angle
   (NASA SP-7, 1965)
= gliding angle.
glide path
   (NASA SP-7, 1965)
1. The flight path of an aeronautical vehicle in a glide, seen from the side.
2. The path used by an aircraft or spacecraft in approach procedure and which is generated by an instrument-landing facility.
glide paths
   (NASA Thesaurus)
Flight paths of aeronautical vehicles in a glide, seen from the side. The paths used by aircraft or spacecraft in approach procedure and which are generated by instrument landing facilities. Used for glide angles and glide slopes.
glide ratio
   (NASA SP-7, 1965)
The ratio of the horizontal distance traveled to the vertical distance descended in a glide. Also called gliding ratio.
glide scope
   (NASA SP-7, 1965)
1. An inclined surface which includes a glide path and which is generated by an instrument-landing facility.
2. = slope angle.
3. = gliding angle.
glider
   (NASA SP-7, 1965)
1. A fixed-wing aircraft specially designed to glide, or to glide and soar. This kind of aircraft ordinarily has no powerplant.
2. See hypersonic glider.
gliders
   (NASA Thesaurus)
A heavier-than-air aircraft that is supported in flight by the dynamic reaction of the air against its lifting surface and whose free flight does not depend principally on an engine.
gliding angle
   (NASA SP-7, 1965)
The angle between the horizontal and the glide path of an aircraft. Also called glide angle or glide slope.
gliding ratio
   (NASA SP-7, 1965)
= glide ratio.
Glimm method
   (NASA Thesaurus)
Numerical technique for solving gas dynamics problems involving hyperbolic systems of conservation laws.
GLL
   (Space Flight Glossary - JPL)
Galileo spacecraft.
Global Change Master Directory
   (Global Land Information System Glossary - USGS)
The Master Directory (MD) is an online computer-based system designed to enable rapid identification and location of data of interest to earth and space science researchers. It provides brief, high level data set information from which the user should be able to identify data of potential interest. The Master Directory has remote linking capabilities to a number of other online data directories. The MD resides on a VAX at the National Space Science Data Center (NSSDC) and may be reached from GLIS by selecting the Remote Option (6) on the GLIS Main Menu screen.
Global Oscillations at Low Frequencies
   (SOHO Glossary - GSFC)
Helioseismology instrument aboard SOHO which analyzes the vibrational modes of the Sun.
Global Positioning System
   (NASA Thesaurus)
A satellite navigation system which will display many (up to 24) satellites in three sets of orbits by means of a precise time standard and three-dimensional information on position and velocity.
global radiation
   (NASA SP-7, 1965)
The total of direct solar radiation and diffuse sky radiation received by a unit horizontal surface.
Global radiation is measured by pyranometers.
global velocities
   (NASA SP-7, 1965)
The range of velocities, slightly less than circular velocity, that permit sustained flight once around the earth in equilibrium glide. Compare orbital velocity.
globe lightning
   (NASA SP-7, 1965)
= ball lightning.
globular clusters
   (High Energy Astrophysics Dictionary- GSFC)
Spherically symmetric star clusters, containing over 100,000 individual stars, which are in a roughly spherical distribution about the main hub of a galaxy. They form more or less a spherical halo around the main body of the galaxy.
glow discharge
   (Plasma Physics and Fusion Energy Glossary)
Low-density, low-temperature plasma discharge (such as in a fluorescent light) which, well, glows. Sputtering in glow discharges is useful in plasma processing of materials. The voltage applied to the plasma must be greater than the ionization potential of the gas used; most of the plasma voltage drop is near the cathode, where the majority of ionization occurs. Discharge is sustained by secondary electrons emitted when ions or recombination radiation impact on the cathode; electrons are accelerated away from the cathode and ionize neutral gas in the discharge.
glow discharges
   (NASA SP-7, 1965)
Electrical discharges which produce luminosity.
Thus corona discharge is a glow discharge, but point discharge is not. Relatively high electric field strengths are required for glow discharges, for the density of radiatively recombining gas atoms and molecules must be high. See gaseous electronics.
glucocorticoids
   (NASA Thesaurus)
Adrenocortical steroid hormones that are involved in the metabolism of fats, proteins, and carbohydrates, and have anti-inflammatory properties.
gluons
   (NASA Thesaurus)
The carriers of the strong force which holds atomic nuclei together (holding together groups of quarks making up stable particles, which in turn are bound together in the atomic nuclei).
GMT
   (Space Flight Glossary - JPL)
Greenwich Mean Time, similar to UTC but not updated with leap seconds.
GMT (abbr)
   (NASA SP-7, 1965)
= Greenwich mean time.
GMT--Greenwich Mean Time
   (Global Land Information System Glossary - USGS)
GMT is the mean solar time of the meridian of Greenwich used as the prime basis of standard time throughout the world.
gneiss
   (NASA Thesaurus)
A foliated rock formed by regional metamorphism, in which bands or lenticles of granular materials alternate with bands or lenticles in which minerals having flaky or elongate prismatic habits predominate. Generally less than 50 percent of the minerals show preferred parallel orientation. Although a gneiss is commonly feldspar- and quartz-rich, the mineral composition is not an essential factor in its definition.
gnomon
   (From Stargazers to Starships Glossary - GSFC)
The part of a sundial which casts the shadow, usually a rod or fin pointed at the celestial pole.
gnomonic projection
   (NASA Thesaurus)
A projection on a plane tangent to the surface of a sphere having the point of projection at the center of the sphere. Used in cartography and in crystallography.
gnotobiotics
   (NASA Thesaurus / NASA SP-7, 1965)
The study of germ-free animals.
Godunov method
   (NASA Thesaurus)
Non-oscillatory finite-volume scheme that incorporates the exact or approximate solution to the Riemann initial-value problem, or a generalization of it.
GOES 1
   (NASA Thesaurus)
The first in a series of geostationary operational environmental satellites launched in October 1975. It ceased operation in June of 1977.
GOES 2
   (NASA Thesaurus)
The second in a series of geostationary operational environmental satellites launched in June 1977. Used for Geostationary Operatl Environ Satellite B.
GOES 3
   (NASA Thesaurus)
The third in a series of geostationary operational environmental satellites launched in June 1978.
GOES 4
   (NASA Thesaurus)
The fourth in a series of geostationary operational environmental satellites launched in September 1980.
GOES 5
   (NASA Thesaurus)
The fifth in a series of geostationary operational environmental satellites launched in May 1981.
GOES 6
   (NASA Thesaurus)
The sixth in a series of geostationary operational environmental satellites launched in April 1983.
GOES satellites
   (NASA Thesaurus)
Geostationary operational environmental satellites. Used for Geostationary Operational Environ Sats.
GOLF
   (SOHO Glossary - GSFC)
See Global Oscillations at Low Frequencies
goniometer
   (NASA SP-7, 1965)
An instrument for measuring angles.
goniometers
   (NASA Thesaurus)
Instruments for measuring angles.
goodness of fit
   (NASA Thesaurus)
The degree to which the observed frequencies of occurrence of events in an experiment correspond to the probabilities in a model of the experiment.
gox
   (NASA SP-7, 1965)
Gaseous oxygen.
Grad-Shafranov equation
   (Plasma Physics and Fusion Energy Glossary)
Reduced MHD-equilibrium equation for an axisymmetric, toroidal plasma. (Similar reduced equations can be derived for the cases of helical symmetry and for the straight cylinder.) Analytic and numerical studies of these equations are important in exploring potential plasma configurations.
gradient
   (NASA SP-7, 1965)
1. The space rate of decrease of a function. Of a function in three space dimensions, the vector normal to surfaces of constant value of the function and directed toward decreasing values, with magnitude equal to the rate of decrease of the function in this direction. The gradient of a function f is denoted by -nabla del (down pointing triangle)f (without the minus sign in the older literature), and is itself a function of both space and time. The ascendant is the negative of the gradient. See del-operator.
2. Often loosely used to denote the magnitude of the gradient or ascendant. 3. Either the rate of change of a quantity (as temperature, pressure, etc.) or a diagram or curve representing this.
gradient index optics
   (NASA Thesaurus)
Optical systems with components whose refractive indexes vary continuously within the material used for the optical elements.
gradient wind
   (NASA SP-7, 1965)
Any horizontal wind velocity tangent to the contour line of a constant pressure surface or an isobar of a geopotential surface. At such points the coriolis acceleration and the centripetal acceleration together exactly balance the horizontal pressure force. Compare geostrophic wind.
gradients
   (Plasma Physics and Fusion Energy Glossary)
Mathematical term for operators which determines the magnitude and direction of the greatest rate-of-change of a given function with position.
grain
   (NASA SP-7, 1965)
1. An elongated molding or extrusion of solid propellant for a rocket, regardless of size.
2. In photography, a small particle of metallic silver remaining in a photographic emulsion after development and fixing. In the agglomerate, these grains form the dark area of a photographic image.
3. An individual crystal in a polycrystalline metal or alloy.
gram
   (NASA SP-7, 1965)
The standard of mass in the metric system.
gram-calorie
   (NASA SP-7, 1965)
See calorie.
gram-centimeter
   (NASA SP-7, 1965)
The CGS gravitation unit of work.
gram-molecule
   (NASA SP-7, 1965)
The mass in grams of a substance numerically equal to its molecular weight.
grand unified theory
   (NASA Thesaurus)
A theory describing the unification of gravity with the other elementary forces in physics, i.e., the weak force, the strong force and the electromagnetic force. Used for GUT.
grants
   (NASA Thesaurus)
Assets bestowed or transferred, such as money or land, for a particular purpose.
granule
   (SOHO Glossary - GSFC)
A roughly circular region on the Sun whose bright center indicates hot gases rising to the surface, and whose dark edges indicate cooled gases that are descending towards the interior. Individual granules appear and disappear on time scales of about 5 minutes and are typically about 1000 km.
granules
   (NASA SP-7, 1965)
Small bright features of the photosphere of the sun, covering 50 to 60 percent of the surface. They have been likened in appearance to rice grains.
graph
   (NASA SP-7, 1965)
A diagram indicating the relationship between two or more variables.
graph theory
   (NASA Thesaurus)
The mathematical study of the structure of graphs and networks.
graphical user interface
   (NASA Thesaurus)
A man-computer interface which relies on graphical and/or pictorial means for presenting the user with command options and their results. Input to a graphical user interface relies heavily on the use of point-and-click devices (such as a 'mouse'). Most graphical user interfaces are designed to facilitate multitasking through separate application windows (computer programs).
graphite-epoxy composites
   (NASA Thesaurus)
Structural materials composed of epoxy resins reinforced with graphite.
graphite-polyimide composites
   (NASA Thesaurus)
Composite materials utilizing graphite reinforcing fibers in a resin matrix.
graphoepitaxy
   (NASA Thesaurus)
The use of artificial surface relief structures to induce crystallographic orientation in thin films.
Grashof number
   (NASA Thesaurus / NASA SP-7, 1965)
(symbol NGr, Gr).
A nondimensional parameter used in the theory of heat transfer, defined by

N sub G r equals l to the third g open bracket open parens T sub one minus T sub zero close parens over v squared T sub zero close bracket

where l is a representative length; T1 and T2 are representative temperatures; g is the acceleration of gravity; and nu is the kinematic viscosity.
The Grashof number is associated with the Reynolds number and the Prandtl number in the study of convection.
grass
   (NASA SP-7, 1965)
1. Sharp, closely space discontinuities in the trace of a cathode-ray tube, produced by random interference; so named because of their resemblance to blades of lawn grass.
2. In radar, a descriptive colloquialism used to refer to the indication of noise on an 'A' or similar type of display. See noise.
graticule
   (NASA SP-7, 1965)
1. The network of lines representing parallels and meridians on a map, chart, or plotting sheet. See grid.
2. A scale at the focal plane of an optical instrument to aid in the measurement of objects. See reticle.
gravels
   (NASA Thesaurus)
Coarse, granular aggregates, with pieces larger than sand grains, resulting from the natural erosion of rocks. Used for gravel deposits.
graviceptor
   (NASA SP-7, 1965)
= gravireceptor.
gravimeters
   (NASA Thesaurus)
Instruments for measuring variations in the gravitational field, generally by registering differences in the weight of a constant mass as the gravimeter is moved from place to place. Used for gravity meters.
gravimetry
   (NASA Thesaurus)
The measurement of gravity or gravitational acceleration, especially in geophysics and geodesy.
gravireceptors
   (NASA Thesaurus / NASA SP-7, 1965)
Highly specialized nerve endings and receptor organs located in skeletal muscles, tendons, joints, and in the inner ear which furnish information to the brain with respect to body position, equilibrium, and the direction of gravitational forces. See gravitation.
gravitation
   (NASA Thesaurus / NASA SP-7, 1965)
The acceleration produced by the mutual attraction of two masses, directed along the line joining their centers of masses, and of magnitude inversely proportional to the square of the distance between the two centers of mass.
This acceleration on a unit mass has the magnitude G(m/r2), where m is the mass of the attracting body, r is the distance between the centers of mass, and G is the gravitational constant equal to 6.670 +/- 0.005 x 10-2 cm2/gram sec2. In the case of masses in the earth's gravitational field, m is the mass of the earth, equal to 5.975 x 1027 grams. However, the rotation of the earth and atmosphere modifies this field to produce the field of gravity.
gravitation constant
   (NASA SP-7, 1965)
See Newtonian universal constant of gravitation, Gaussian constant.
gravitational
   (NASA SP-7, 1965)
Pertaining to gravitation.
gravitational collapse
   (AS&T Dictionary)
The sudden shrinkage of a massive star's core at the end of nuclear buring, occurring when the outflow of energy is no longer balanced by the inward pull of the star's own gravity; the relatively rapid contraction of a cloud of interstellar dust or gas when the gravity of the cloud is stronger than the outward pressure of its internal radiation.
gravitational constant
   (NASA Thesaurus / NASA SP-7, 1965)
(symbol G)
The coefficient of proportionality in Newton law of gravitation. G = 6.670 +/- 0.005 x 10-8 dyne-cm2/g2. Also called constant of gravitation, Newtonian universal constant of gravitation.
In celestial mechanics, G may be used as a symbol with units unspecified or in a particular problem may be made equal to 1 or 4pi2 by the choice of units for other parameters in the particular problem.
gravitational fields
   (NASA Thesaurus)
Regions that give rise to forces of gravitational attraction.
gravitational force
   (Plasma Physics and Fusion Energy Glossary)
Force which attracts two bodies together based on the product of their masses and the reciprocal of the square of their distances. "Gravity" is the force field created by one massive body (like the earth) which another body (like you) will experience.
gravitational instability
   (AS&T Dictionary)
The unchecked and sudden shrinkage under gravity of gas or dust of a star or cloud; a process in which a system that is partially out of equilibrium is pulled farther out of equilibrium by gravity.
gravitational lenses
   (AS&T Dictionary)
The effect of focusing of a massive and mostly invisible object, such as a cluster of galaxies or a single galaxy, on light coming from a more distant object lying behind it; the effect can make single background sources appear as multiples.
gravitational potential
   (NASA SP-7, 1965)
1. The potential associated with the force of gravitation arising from the attraction between mass points, e.g., the earth's center and a particle in space.
The gravitational potential, associated with the force of gravitation, should not be confused with the geopotential associated with the force of gravity. The latter is equal to the former plus the centrifugal force due to the earth's rotation. The potentials of the three forces are related in the same manner.
2. At any point, the work needed to remove an object from that point to infinity.
gravitational red shift
   (NASA SP-7, 1965)
See red shift, note.
gravitational tide
   (NASA SP-7, 1965)
An atmospheric tide due to gravitational attraction of the sun or moon. See thermal tide.
The simidiurnal solar atmospheric tide is partly gravitational; the semidiurnal lunar atmospheric tide is totally gravitational.
gravitational wave
   (NASA SP-7, 1965)
= gravity wave.
gravitational wave antennas
   (NASA Thesaurus)
Devices for receiving propagating gravitational fields produced by some change in the distribution of matter.
gravitational waves
   (NASA Thesaurus)
Hypothetical waves that travel at the speed of light, by which gravitational attraction is propagated.
graviton
   (NASA SP-7, 1965)
The hypothetical elementary unit of gravitation which is equivalent to the electron in electromagnetic theory.
gravitons
   (NASA Thesaurus)
The hypothetical elementary units of gravitation which are equivalent in the electrons in electromagnetic theory.
gravity
   (Galileo Project Glossary - JPL)
The acceleration produced by the mutual attraction of two masses, and of magnitude inversely proportional to the square of the distance between the two centers of mass.
gravity
   (NASA SP-7, 1965)
1. Viewed from a frame of reference fixed in the earth, force imparted by the earth to a mass which is at rest relative to the earth. Since the earth is rotating, the force observed as gravity is the resultant of the force of gravitation and the centrifugal force arising from this rotation and the use of an earthbound rotating frame of reference. It is directed normal to sea level and to its geopotential surfaces. See virtual gravity, geopotential height, standard gravity.
The magnitude of the force of gravity at sea level decreases from the poles, where the centrifugal force is zero, to the equator, where the centrifugal force is a maximum but directed opposite to the force of gravitation. This difference is accentuated by the shape of the earth, which is nearly that of an oblate spheroid of revolution slightly depressed at the poles. Also, because of the asymmetric distribution of the mass of the earth, the force of gravity is not directed precisely toward the earth's center. The magnitude of the force of gravity is usually called either gravity, acceleration of gravity, or apparent gravity.
2. = acceleration of gravity.
3. By extension, the attraction of any heavenly body for any mass; as Martian gravity.
gravity anomalies
   (NASA Thesaurus)
The differences between the observed values of gravity at different points and the theoretical calculated value. They are based on a simple gravity model, usually modified in accordance with some generalized hypothesis of variation in subsurface density as related to surface topography.
gravity dam
   (Glossary of Hydrologic Terms - NOAA)
A concrete structure proportioned so that its own weight provides the major resistance to the forces exerted on it.
gravity gradient
   (Galileo Project Glossary - JPL)
Refers to the difference in the acceleration of gravity from one side of an object to another, because gravity is a force that changes with distance between two points.
Gravity Probe B
   (NASA Thesaurus)
An experiment designed to measure general relativistic induced torques on a gyroscope in orbit about the Earth.
gravity wave
   (NASA SP-7, 1965)
A wave disturbance in a fluid in which buoyancy (or reduced gravity) acts as the restoring force on fluid parcels displaced from hydrostatic equilibrium. Also called gravitational wave.
gravity waves
   (NASA Thesaurus)
Waves in an interface between fluids of different density in which the restoring force is gravity.
gravity well
   (NASA SP-7, 1965)
An analogy in which the gravitational field is considered as a deep pit out of which a space vehicle has to climb to escape from a planetary body.
gray body
   (NASA SP-7, 1965)
A hypothetical body which absorbs some constant fraction, between zero and one, of all electromagnetic radiation incident upon it, which fraction is the absorptivity and is independent of wavelength. As such, a gray body represent a surface of absorptive characteristics intermediate between those of white body and a blackbody. No such substance are known in nature. Also called grey body.
Gray code
   (NASA SP-7, 1965)
A binary code in which adjacent quantities differ in one place or column only. Often used in mechanical devices.
gray scale
   (NASA Thesaurus)
Images that are not colored or multispectral.
grayout
   (NASA SP-7, 1965)
A temporary condition in which vision is hazy, restricted, or otherwise impaired, owing to insufficient oxygen. Compare blackout.
grazing incidence
   (NASA Thesaurus)
Incidence at a small glancing angle.
great circle
   (NASA SP-7, 1965)
The intersection of a sphere and a plane through its center. Also called orthodrome.
The intersection of a sphere and a plane which does not pass through its center is called a small circle.
great circles
   (NASA Thesaurus)
Circles which intersect a sphere and a plane through its center.
Great Red Spot
   (NASA SP-7, 1965)
An oval feature in the visible cloud surface of Jupiter, at latitude 20 to 25 degrees S. It is about 25,000 miles long in the planet's east-west direction, and about 7000 miles wide in the north-south direction. It is often reddish in color, but may be white or grey, or nearly invisible compared to its surroundings. See South Tropical Disturbance.
The neighboring cloud matter seems to pass around it on the northern side, producing the so-called Red Spot Hollow, by which it may be detected even when the spot itself is invisible. Its rotation period averages 9 hours, 55 minutes, 38 seconds (very nearly the same as the rest of the planet), but varies enough so that through the years since its discovery in 1878 it has made more than one complete revolution with respect to the underlying planet.
great year
   (NASA SP-7, 1965)
The period of one complete cycle of the equinoxes around the ecliptic, about 25,800 years. Also called platonic year. See precession of the equinoxes.
greatest elongation
   (NASA SP-7, 1965)
The maximum angular distance of a body of the solar system from the sun, as observed from the earth. The direction of the body east or west of the sun is usually specified, as greatest elongation west.
green flash
   (NASA SP-7, 1965)
A brilliant green coloring of the upper edge of the sun as it appears at sunrise or disappears at sunset when there is a clear, distinct horizon.
The green flash is due to refraction by the atmosphere, which disperses the first (or last) spot of light into a spectrum. The green is bent more than red or yellow and hence is visible sooner at sunrise and later at sunset.
Green theorem
   (NASA SP-7, 1965)
See divergence theorem.
greenhouse effect
   (NASA Thesaurus)
The heating of the Earth's surface because outgoing long-wavelength terrestrial radiation is absorbed and re-emitted by the carbon dioxide and water vapor in the lower atmosphere and eventually returns to the surface.
greenhouse effect
   (NASA SP-7, 1965)
The heating effect exerted by the atmosphere upon the earth by virtue of the fact that the atmosphere (mainly, its water vapor) absorbs and remits infrared radiation. In detail: the shorter wavelengths of insolation are transmitted rather freely through the atmosphere to be absorbed at the earth's surface. The earth then reemits this as long-wave (infrared) terrestrial radiation, a portion of which is absorbed by the atmosphere and again emitted (see atmospheric radiation). Some of this is emitted downward back to the earth's surface ( counter-radiation).
The mean surface temperature for the entire world, 14 degrees C, is almost 40 degrees C higher than the mean temperature required for radiative equilibrium of a blackbody at the earth's mean distance from the sun. It is essential, in understanding the concept of the greenhouse effect, to note that the important additional warming is due to the counterradiation from the atmosphere. The glass panes of a greenhouse function in this manner, hence the name.
greenhouses
   (NASA Thesaurus)
Structures enclosed by glass or plastic devoted to the cultivation or protection of tender plants or to the production of plants out of season.
Greenwich apparent time
   (NASA SP-7, 1965)
(abbr GAT)
Local apparent time at the Greenwich meridian; the arc of the celestial equator, or the angle at the celestial pole, between the lower branch of the Greenwich celestial meridian and the hour circle of the apparent or true sun, measured westward from the lower branch of the Greenwich celestial meridian through 24 hours; Greenwich hour angle of the apparent or true sun, expressed in time units, plus 12 hours.
Greenwich civil time
   (NASA SP-7, 1965)
(abbr GCT)
= Greenwich mean time.
(United States terminology from 1925 through 1952).
Greenwich hour angle
   (NASA SP-7, 1965)
(abbr GHA)
Angular distance west of the Greenwich celestial meridian; the arc of the celestial equator, or the angle at the celestial pole, between the upper branch of the Greenwich celestial meridian and the hour circle of a point on the celestial sphere, measured westward from the Greenwich celestial meridian through 360 degrees; local hour angle at the Greenwich meridian.
Greenwich mean time
   (NASA SP-7, 1965)
(abbr GMT)
Local mean time at the Greenwich meridian; the arc of the celestial equator, or the angle at the celestial pole, between the lower branch of the Greenwich celestial meridian and the hour circle of the mean sun, measured westward from the lower branch of the Greenwich celestial meridian through 24 hours; Greenwich hour angle of the mean sun, expressed in time units, plus 12 hours. Called Greenwich civil time in U.S. terminology from 1925 through 1952. Also called universal time, Z-time.
Mean time reckoned from the upper branch of the Greenwich meridian is called Greenwich astronomical time.
Greenwich meridian
   (NASA SP-7, 1965)
The meridian through Greenwich, England, serving as the reference for Greenwich time.
The Greenwich meridian is accepted almost universally as the prime meridian, or the origin of measurement of longitude.
Greenwich sidereal time
   (NASA SP-7, 1965)
(abbr GST) Local sidereal time at the Greenwich meridian; the arc of the celestial equator, or the angle at the celestial pole, between the upper branch of the Greenwich celestial meridian and the hour circle of the vernal equinox, measured westward from the upper branch of the Greenwich celestial meridian through 24 hours; Greenwich hour angle of the vernal equinox, expressed in time units.
Gregorian calendar
   (From Stargazers to Starships Glossary - GSFC)
Introduced in 1582 by Pope Gregory the 13th, this calendar modifies the Julian calendar for greater precision, decreeing that century years such as 1900 are not leap years, except if the number of centuries is divisible by 4 (e.g. 2000).
Gregorian calendar
   (NASA SP-7, 1965)
The calendar now in common use, in which each year has 365 days except leap years. See calendar year.
grey body
   (NASA SP-7, 1965)
= gray body.
grid
   (NASA SP-7, 1965)
1. A series of lines, usually straight and parallel, superimposed on a chart or plotting sheet to serve as a directional reference for navigation. See graticule.
2. Two sets of mutually perpendicular lines dividing a map or chart into squares or rectangles to permit location of any point by a system of rectangular coordinates. See military grid, World Geographic Reference System.
3. An electrode with one or more openings to permit passage of electrons or ions. It usually consists of a wire mesh electrode placed between the anode and cathode of an electron tube to serve as a control of the current flowing between them.
4. Pertaining to or measured from a reference grid, as grid azimuth, grid latitude, grid meridian.
grid generation (mathematics)
   (NASA Thesaurus)
Numerical generation of curvilinearcoordinate systems for the numerical solution of partial differential equations.
grid variation
   (NASA SP-7, 1965)
See variation, note.
GRIST (telescope)
   (NASA Thesaurus)
An ESA Spacelab payload designed for grazing incidence solar phenomena. Used for grazing incidence solar telescope.
grivation
   (NASA SP-7, 1965)
= grid variation.
gross national product
   (NASA Thesaurus)
The total value of the goods and services produced in a nation during a specific period and also comprising the total expenditures by consumers and government plus gross private investment. Used for GNP.
gross thrust
   (NASA SP-7, 1965)
The total thrust of a jet engine without deduction of the drag due to the momentum of the incoming air (ram drag).
The gross thrust is equal to the product of the mass rate of fluid flow and the velocity of the fluid relative to the nozzle, plus the product of the nozzle exit area and the difference between the exhaust pressure and ambient pressure.
gross weight
   (NASA SP-7, 1965)
The total weight of an aircraft, rocket, etc., as loaded; specifically, the total weight with full crew, full tanks, payload, etc. Also called take-off weight. See design gross weight.
ground
   (NASA SP-7, 1965)
1. The earth's surface, especially the earth's land surface. Used in combination to form adjectives, as in ground-to-air, ground-to-ground , and air-to-ground. See surface.
2. The domain of nonflight operations that normally take place on the earth's surface or in a vehicle or on a platform that rests upon the surface, as in ground support.
3. = electrical ground.
ground clutter
   (Glossary of Weather Terms for Storm Spotters - NOAA)
A pattern of radar echoes from fixed ground targets (buildings, hills, etc.) near the radar. Ground clutter may hide or confuse precipitation echoes near the radar antenna.
ground clutter
   (NASA SP-7, 1965)
= ground return.
ground effect (aerodynamics)
   (NASA Thesaurus)
Increase in the lift of an aircraft operating close to the ground caused by reaction between high-velocity downwash from its wing or rotor and the ground.
ground effect (communications)
   (NASA Thesaurus)
The effect of ground conditions on radio communications.
ground environment
   (NASA SP-7, 1965)
1. The environment that surrounds and affects a system of piece of equipment while it operates on the ground.
2. That system or part of a system, as of a guidance system, that functions on the ground; the aggregate of equipment, conditions, facilities, and personnel that go to make up a system, or part of a system, functioning on the ground. See environment.
ground layer
   (NASA SP-7, 1965)
= surface boundary layer.
ground penetrating radar
   (NASA Thesaurus)
A radar imaging technique in which coherent background propagation of the received reflected wavefield forms a spatial image of the scattering interface within the region of interest.
ground resonance
   (NASA Thesaurus)
Mechanical phenomenom occurring when a rotor,operating within a certain speed range, experiences coupling between a rotor in-plane mode and a model support system mode, causing vibration in the system.
ground return
   (NASA SP-7, 1965)
Radar echoes reflected from the terrain. Also called ground clutter, land return.
Echoes from the sea are called sea clutter or sea return.
ground start
   (NASA SP-7, 1965)
An ignition sequence of a rocket's main stage, initiated and cycled through on the ground. Compare air start, in-flight start.
In large rockets, the ground start may be fueled from pressurized tanks external to the rocket, permitting takeoff with the rocket's own internal propellant load intact.
ground support equipment
   (NASA Thesaurus / NASA SP-7, 1965)
(abbr GSE) That equipment on the ground, including all implements, tools, and devices (mobile or fixed), required to inspect, test, adjust, calibrate, appraise, gage, measure, repair, overhaul, assemble, disassemble, transport, safeguard, record, store, or otherwise function in support of a rocket, space vehicle, or the like, either in the research and development phase or in an operational phase, or in support of the guidance system used with the missile, vehicle, or the like.
The GSE is not considered to include land or buildings; nor does it include the guidance-station equipment itself, but it does include the test and checkout equipment required for operation of the guidance-station equipment.
ground tracks
   (NASA Thesaurus)
The inclination of a satellite, together with its orbital altitude and period of its orbit, create a track defined by an imaginary line connecting the satellite and the Earth's center; the satellite's apparent path over the ground.
ground truth
   (NASA Thesaurus)
Data obtained on the ground concerning the significance of anomalies observed in remote sensing to help interpretation.
ground water
   (NASA Thesaurus)
That part of the subsurface water that is in the zone of saturation, including underground streams.
ground wave
   (NASA SP-7, 1965)
A radio wave that is propagated over the earth and is ordinarily affected by the presence of the earth's surface and the troposphere. The ground wave includes all components of a radio wave over the earth except ionospheric and tropospheric waves. Compare sky wave.
The ground wave is refracted because of variations in the dielectric constant of the troposphere including the condition known as a surface duct.
ground-controlled approach
   (NASA SP-7, 1965)
(abbr GCA) A ground radar system providing information by which aircraft approaches may be directed via radio communications.
ground-controlled intercept
   (NASA SP-7, 1965)
(abbr GCI) A radar system by means of which a controller may direct an aircraft to make an interception of another aircraft.
ground-effect machine
   (NASA SP-7, 1965)
A machine that hovers or moves just above the ground by creating a cushion of supporting air between it and the ground surface and varying the thrust vector and magnitude to regulate direction and rate of motion.
ground-handling equipment
   (NASA SP-7, 1965)
Equipment on the ground used to move, lift, or transport a space vehicle, a rocket, or component parts.
Such equipment includes the gantry, the transporter, and the forklift.
group velocity
   (NASA Thesaurus / NASA SP-7, 1965)
The velocity of a wave disturbance as a whole, i.e., of an entire group of component simple harmonic waves. The group velocity U is related to the phase speed c of the individual harmonic waves of length l by the frequency equation U = c - l ( dc / dl ). The phase speed c is thus equal to the group velocity only in the case of nondispersive waves, i.e., when dc/dl = 0. See velocity of propagation.
Gru, Grus
   (NASA SP-7, 1965)
International Astronomical Union abbreviations for Grus. See constellation.
Grus
   (NASA SP-7, 1965)
(abbr Gru, Grus)
See constellation.
GSE (abbr)
   (NASA SP-7, 1965)
=ground-support equipment.
GSFC--Goddard Space Flight Center
   (Global Land Information System Glossary - USGS)
he NASA GSFC was established in 1959 just northeast of Washington, D.C. Goddard's mission is to expand knowledge of the Earth, its environment, the solar system and the universe through the development and use of near-Earth orbiting spacecraft. The GSFC is responsible for supporting NASA's leadership role in space and Earth sciences; research and application of technology for sensors, instruments and information systems; planning and executing space flight projects for scientific research; and for tracking of manned and unmanned Earth satellites through worldwide ground and space communication systems.
GSSR
   (Space Flight Glossary - JPL)
Goldstone Solar System Radar, a technique which uses very high-power X and S-band transmitters at DSS 14 to illuminate solar system objects for imaging.
GTL
   (Space Flight Glossary - JPL)
Geotail spacecraft.
GTO
   (Space Flight Glossary - JPL)
Geostationary (or geosynchronous) Transfer Orbit.
guanosines
   (NASA Thesaurus)
Guanine riboside; a nucleoside composed of guanine and ribose. Used for vernine.
guayule
   (NASA Thesaurus)
A desert shrub native to southwestern United States and north Mexico that produces polymeric isoprene essentially identical to that made by Hevea rubber trees in southeast Asia.
guest star
   (Imagine the Universe Dictionary - NASA GSFC)
The ancient Chinese term for a star that newly appears in the night sky, and then later disappears. See also nova.

guidance
   (NASA SP-7, 1965)
The process of directing the movements of an aeronautical vehicle or space vehicle, with particular reference to the selection of a flight path. See control.
In preset guidance a predetermined path is set into the guidance mechanism and not altered, in inertial guidance accelerations are measured and integrated within the craft, in command guidance the craft responds to information received from an outside source. Beam-rider guidance utilizes a beam; terrestrial-reference guidance, some influence of the earth; celestial guidance, the celestial bodies and particularly the stars; and homing guidance the information is in response to transmissions from the craft, in semiactive homing guidance the transmissions are from a source other than the craft, and in passive homing guidance natural radiations from the destination are utilized. Midcourse guidance extends from the end of the launching phase to an arbitrary point enroute and terminal guidance extends from this point to the destination.
guidance (motion)
   (NASA Thesaurus)
The process of directing the movements of an aeronautical vehicle or space vehicle, with particular reference to the selection of a flight path.
guide vanes
   (NASA Thesaurus)
Control surfaces that may be moved into or against a rocket's jetstream, used to change the direction of the jet flow for thrust vector control. Used for jetavators.
guided missile
   (NASA SP-7, 1965)
1. Broadly, any missile that is subject to, or capable of, some degree of guidance or direction after having been launched, fired, or otherwise set in motion.
2. Specifically, an unmanned, self-propelled flying vehicle (such as a pilotless aircraft or rocket) carrying a destructive load and capable of being directed or of directing itself after launching or take-off, responding either to external direction or to direction originating from devices within the missile itself.
3. Loosely, by extension, any steerable projectile. See ballistic missile.
guiding center
   (Earth's Magnetosphere Glossary - GSFC)
An ion and electron in a magnetic field, of suitably low energy, is constrained to circle ("gyrate") around a local magnetic field line, while the center of its circular motion slides up or down along the line and also slowly shifts from one guiding field line to its neighbor, following certain rules. The center of that circle is known as the particle´s guiding center and the entire mode of motion is called guiding center motion.
guiding center
   (NASA SP-7, 1965)
The center point of the Larmor orbit of a charged particle gyrating in a magnetic field.
It is often convenient to separate the total motion of a particle into the Larmor orbit plus the motion of the guiding center. In addition to the unimpeded motion of the guiding center of a particle along the magnetic field, the presence of electric fields will displace the guiding center perpendicular to the magnetic field.
gulfs
   (NASA Thesaurus)
Relatively large parts of oceans or seas extending far into the land, partly enclosed by an extensive sweep of the coasts, and opened to the sea through straits. Gulfs are the largest of various forms of inlets of seas. They are usually larger, more enclosed, and more deeply indented than bays (topographic features).
gun launchers
   (NASA Thesaurus)
Ordnance devices for firing missiles and rockets with initial attitude control.
gust front
   (Glossary of Weather Terms for Storm Spotters - NOAA)
The leading edge of gusty surface winds from thunderstorm downdrafts.
gust tunnel
   (NASA SP-7, 1965)
A wind tunnel in which gusts are simulated. Specifically, a tunnel in which models are passed over a vertical jet or jets simulating gusts.
gypsum
   (NASA Thesaurus)
The mineral consisting primarily of fully hydrated calcium sulfate (calcium sulfate dihydrate).
gyration
   (Earth's Magnetosphere Glossary - GSFC)
A term used in plasma studies for the circular motion of an ion or electron around its guiding center.
gyres
   (NASA Thesaurus)
Closed circulatory systems in a body of water, larger than an eddy or a whirlpool. There is a circular motion of water in each of the major ocean basins, centered on a subtropical high-pressure region. These movements are generated by connective flow of warm surface water poleward, by the deflective effect of the Earth's rotation and by the effects of prevailing winds. The water within each gyre turns clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Acceleration causes sea level to fall along mainland coasts; deceleration leads to rise.
gyro
   (NASA SP-7, 1965)
1. A device which utilizes the angular momentum of a spinning mass (rotor) to sense angular motion of its base about one or two axes orthogonal to the spin axis. Also called gyroscope.
This definition does not include more complex systems such as stable platforms using gyros as components.
2. Short for directional gyro, gyrocompass , etc.
gyro horizon
   (NASA SP-7, 1965)
An artificial horizon or an attitude gyro.
gyro horizons
   (NASA Thesaurus)
Artificial horizons or attitude gyroscopes.
gyro pickoff
   (NASA SP-7, 1965)
A device which produces a signal, generally a voltage, as a function of the angle between two gyro gimbals or between a gimbal and the base.
gyrocompasses
   (NASA Thesaurus)
Compasses consisting of a continuously driven Foucault gyroscope whose supporting ring normally confines the spinning axis to a horizontal plane, so that the Earth's rotation causes the spinning axis to assume a position in a plane passing through the Earth's axis, and thus to point to true north.
gyrodampers
   (NASA Thesaurus)
Single-gimbal control moment gyros actively controlled to extract the structural vibratory energy through the local rotational deformations of a structure; used in large space structures.
Gyrodyne aircraft
   (NASA Thesaurus)
A rotorcraft whose rotors are not engine-driven except for initial starting, but are made to rotate by action of the air when the rotorcraft is moving; and whose means of propulsion, consisting usually of conventional propellers, is independent of the rotor system.
gyrofrequency
   (NASA Thesaurus / NASA SP-7, 1965)
The natural period of revolution of a free electron in the earth's magnetic field. See magnetoionic theory.
gyroscope
   (NASA SP-7, 1965)
= gyro.
gyroscopes
   (NASA Thesaurus)
Devices which utilize the angular momentum of a spinning mass (rotor) to sense angular motion of its base about one or two axes orthogonal to the spin axis. Used for gyros, gyroscopic drift, and gyrostats.
gyroscopic inertia
   (NASA SP-7, 1965)
The property of a rotor of resisting any force which tends to change its axis of rotation. See gyro.
gyrotron
   (Plasma Physics and Fusion Energy Glossary)
A device for producing microwave energy that utilizes a strong axial magnetic field in a cavity resonator to produce azimuthal bunching of an electron beam.