Aerospace Science and Technology Dictionary G Page

Aerospace Science
&
Technology Dictionary

G

g: 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: A layer of free electrons in the ionosphere which is occasionally observed above the F2-layer. See ionosphere.
G mode: A wave mode generated by a gravity wave.
G-display: 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: An inertial force usually expressed in multiples of terrestrial gravity.
G-indicator: 1. G-display.
2. A display that shows the amount of inertial force acting on body.
G-meter: A meter that indicates acceleration.
G-region: See ionosphere.
G-scan: = G-display.
G-scope: = G-display.
g-suit: 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: 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: 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: Mixtures of gadolinium, a rare earth metal, with other metals.
gadolinium-gallium garnet: A semiconducting crystalline compound used primarily as a solid-state laser material.
gage pressure: In engineering literature, a term used to indicate the difference between atmospheric pressure and absolute pressure, as read from a differential manometer.
Gaia hypothesis: 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: 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: Gain coefficient is a measurement to denote an increase in signal power in transmission from one point to another
gain factor: See gain, sense 2(a).
gal: (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: 1. Pertaining to our galaxy, the Milky Way.
2. Pertaining to the galactic system of coordinates, as galactic latitude.
galactic circle: = galactic equator.
See galactic system of coordinates.
galactic cosmic rays: Energetic particles that come from outside the solar systems. They generally come from within our galaxy.
galactic equator: See galactic system of coordinates.
galactic halos: 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: The total amount of matter contained in a galaxy.
galactic pole: See galactic system of coordinates.
galactic radio waves: Radio waves emanating from our galaxy. See cosmic radio waves.
galactic system of coordinates: 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: A natural satellite of Saturn, orbiting at a mean distance of 62,000 kilometers.
galaxies: Vast assemblages of stars or nebulae, composing island universes separated from other such assemblages by great distances.
galaxy: 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: 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: The four largest and brightest satellites of Jupiter (Io, Europa, Ganymede, and Callisto).
Galileo probe: 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: 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: 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: Application of mathematics to a game, business situation, or other problem to maximize gain or minimize loss.
gamma: This is a unit of magnetic intensity.
Gamma Emission: 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: = gamma ray.
gamma radiation: = gamma ray.
gamma ray astronomy: 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: 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: 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: Stimulated emission devices producing coherent gamma radiation.
Gamma Ray Observatory: A late 1980s NASA mission to explore the gamma ray window to the universe from 0.06 MeV to 30 GeV.
gamma ray spectra: The energy distribution of gamma rays emitted by nuclei.
gamma ray spectrometers: Instruments for deriving the physical constants of materials by using induced gamma radiation as the emission source.
gamma ray spectroscopy: 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: Special telescopes for the observation (and recording) of astronomical phenomena in the gamma ray spectrum.
gamma rays: The highest energy, shortest wavelength electromagnetic radiation. Usually, they are thought of as any photons having energies greater than about 100 keV.
gamma rays: 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: A balloon-borne gamma-ray telescope made by a group at the California Institute of Technology. Abbreviated "GRIP".
gantry: 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: 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: Large cranes mounted on platforms that usually run back and forth on parallel tracks astride the work area. Used for gantries.
gantry scaffold: 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: A satellite of Jupiter orbiting at a mean distance of 1,071,000 kilometers. Also called Jupiter III.
Ganymede: A satellite of Jupiter orbiting at a mean distance of 1,071,000 kilometers. Also called Jupiter III.
gaps (geology): Ravines or gorges cut deeply through a mountain ridge, or between hills or mountains. Used for cols and passes.
garbage: Miscellaneous objects in orbit, usually material ejected or broken away from a launch vehicle or satellite.
garnets: Groups of minerals that are silicates of cubic crystalline form.
gas: 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: Atomization of fluids by high velocity gas jets.
gas cap: 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: 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: (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: = Boltzmann constant.
gas discharge tubes: 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: 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: The giant planets, Jupiter, Saturn, Uranus, and Neptune, of our solar system.
gas laws: 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: Mathematical process of determining overall engine performance, individual module performances and sensor performances from any specific set of engine related measurements.
gas scrubbing: The contacting of a gaseous mixture with a liquid for the purpose of removing gaseous contaminants or entrained liquids or solids.
gas turbine: 1. A turbine rotated by expanding gases, as in a turbojet engine or in a turbosupercharger.
2. A gas-turbine engine.
gas turbines: Turbines rotated by expanding gases, as in a turbojet engine or in a turbosupercharger.
gas-solid interactions: Effects of the impingement of gases (particles) on solid surfaces in various environments.
gas-turbine engine: 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: 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: = electric discharge.
gaseous electric discharge: = electric discharge.
gaseous electronics: 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: Preformed deformable devices designed to be placed between two adjoining parts to prevent the passage of liquid or gas between the parts.
gasohol (fuel): Synthetic fuel consisting of a mixture of gasoline and grain alcohol (ethanol).
gate: 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: 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: 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: 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: = divergence theorem.
Gauss-Seidel: 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: (symbol k )
Originally used in astronomical calculations as k = , 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: = normal distribution.
Gaussian elimination: A technique for solving linear equations by progressive differencing.
Gaussian gravitation constant: = Gaussian constant.
Gaussian noise: = random noise.
Gay-Lussac law: = Charles-Gay-Lussac law.
GCA (abbr): = ground-controlled approach.
GCF: Ground Communications Facilities, provides data and voice communications between JPL and the three DSCCs.
GCI (abbr): = ground-controlled intercept.
GCP--Ground Control Point: 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: Ground Data System, encompasses DSN, GCF, MCCC, and project data processing systems.
gee: A suffix meaning earth, as in perigee , apogee. See perigee, note.
gegenschein: 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: A mineral of the mellite group. It is isomorphous with akermenite. Used for verlardenite.
Geiger counter: 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: Instruments for detecting and measuring radioactivity. In full, Geiger-Mueller counter. Used for Geiger-Mueller tubes.
Geiger-Muller counter: Full term for Geiger counter.
gels: 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: International Astronomical Union abbreviations for Gemini. See constellation.
Gemini: (abbr Gem, Gemi)
See constellation.
gene expression: The process by which a gene's coded information is converted into the structures present and operating in the cell.
general circulation: = planetary circulation.
general perturbations: In orbital determinations, a method of calculating perturbative effects by expanding and integrating in series. See perturbation.
general precession: 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: 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: 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: 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: 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: Parameter search procedures loosely based on the mechanics of natural population genetics and the survival-of-the-fittest.
genetic code: 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: 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: The intentional production of new genes and alteration of genomes by the substitution or addition of new genetic material. Used for hybrids (biology).
geo: 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: Refers to a reference system centered at the Earth's center.
geocentric: Relative to the earth as a center; measured from the center of the earth.
geocentric diameter: The diameter of a celestial body measured in seconds of arc as viewed from the earth's center.
geocentric distance (delta): The comet's distance from the Earth in astronomical units.
geocentric latitude: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: 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: Of or pertaining to geodesy; geodesic.
geodetic accuracy: The degree to which point positions or boundaries indicated on maps or imagery correspond with true geodetic positions.
geodetic control network: 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: 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: 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: The great circle midway between the poles of revolution of the earth, connecting points of 0 degrees geodetic latitude. See astronomical equator.
geodetic latitude: 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: A geodesic line on the spheroidal earth. Also called geodesic. Compare geodesic line.
geodetic longitude: 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: A line connecting points of equal geodetic longitude. Also called geographic meridian. See astronomical meridian.
geodetic parallel: A line connecting points of equal geodetic latitude. Also called geographic parallel. See astronomical parallel.
geodetic position: 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: 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: Surveys which takes into account the size and shape of the Earth.
geodimeters: 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: = dynamic height.
geodynamic meter: = dynamic meter. See dynamic height.
geodynamics: Study of the dynamic forces or processes within the Earth. Used for crustal dynamics.
geoelectricity: The Earth's natural electric fields and phenomena. It is closely related to geomagnetism.
geographic coordinates: 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: Computer assisted systems that acquire, store, manipulate, and display geographic data. Some systems are not automated.
geographic latitude: = geodetic latitude.
geographic longitude: = geodetic longitude.
geographical coordinates: = geographic coordinates.
geographical mile: The length of 1 minute of arc of the equator, or 6087.08 feet.
geographical pole: 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: 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: 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: 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: 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: 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: The expression of the internal and external processes and events that affect a planetary body.
geologic time: The time extending from the end of the formative period of the Earth to the beginning of human history.
geologic unit: 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: 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: 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: Of or pertaining to geomagnetism.
geomagnetic coordinates: A system of spherical coordinates based on the best fit of a centered dipole to the actual magnetic field of the earth.
geomagnetic equator: 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: 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: 1. The meridional lines of a geomagnetic coordinate system.
2. = magnetic meridian.
geomagnetic pole: 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: A worldwide disturbance of the Earth's magnetic field, associated with solar activity.
geomagnetic storm: = magnetic storm.
geomagnetism: 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: The internal geometric fidelity of an imaging system.
geometric chord: See chord, note.
geometric dilution of precision: 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: Null
geometric mean: A measure of central position. The geometric mean of n quantities equals the n th root of the product of the quantities.
geometric position: = true position.
geometric rectification (imagery): The correction of image distortions due to sensor view angle, platform attitude, or target surface features.
geometrical acoustics: 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: See horizon.
geometrical optics: The geometry of paths of light rays and their imagery through optical systems. Used for ray optics.
geometrical theory of diffraction: A ray theory of diffraction process.
geomorphology: 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: Apparatus used in model experiments for deep solar convection and Jovian atmospheric circulation for Spacelab 1 and Spacelab 3.
geophysical fluids: 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: 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: 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: 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 at height Z is given mathematically by the expression,

where g is the acceleration of gravity.
geopotential height: 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

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: 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: 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: 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: Pressures that exceed the normal hydrostatic pressure of about 0.465 psi per foot of depth.
georef (abbr): = World Geographic Reference System. Pronounced as a word.
georegistered: 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: 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: 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: 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: A geosynchronous orbit in which the spacecraft is constrained to a constant latitude.
geostrophic wind: That horizontal wind velocity for which the coriolis acceleration exactly balances the horizontal pressure force. Compare gradient wind.
geostrophic wind level: The lowest level at which the wind becomes geostrophic in the theory of the Ekman spiral, proportional to

where is the kinematic eddy viscosity and 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: A direct, circular, low inclination orbit about the Earth having a period of 23 hours 56 minutes 4 seconds.
geosynclines: 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: 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: 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: Internal temperature of the planet Earth. Used for geothermometry.
geothermal energy extraction: The removal for storage and/or utilization of heat from natural sources within the Earth (hot springs, geysers, hot rocks, etc.)
geothermal energy utilization: Any application of energy derived from sources within the Earth.
geothermal gradient: 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: 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: The gamut of operations involved in the exploration, exploitation, and conversion of energy derived from geothermal sources.
German Infrared Laboratory: A proposed infrared telescope for Spacelab that was discontinued in 1985. It superseded the LIRTS (telescope).
get: To remove gas from a vacuum system by sorption.
Get Away Specials (STS): Low-cost, man-independent Space Shuttle experimental payloads.
getter: 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: Materials which are included in a vacuum system or device for removing gas by sorption.
geysers: 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): = Greenwich hour angle.
GHz: Gigahertz (10⁹ Hz).
giant molecular cloud: 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: The planets Jupiter, Saturn, Uranus, and Neptune. Also called Jovian planets.
gibbous: 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: = Gibbs function.
Gibbs function: 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: (abbr G)
A prefix meaning multiplied by 10⁹.
gimbal: 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: 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: 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: A rocket engine mounted on a gimbal.
gimbals: 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: The third Japanese X-ray mission, also known as ASTRO-C.
Giorgi system: = MKSA system.
Giotto mission: 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: 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: 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: 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: 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: 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: High power lasers used in laser fusion technology research.
glassy carbon: Form of carbon with unique properties and characteristics. Formed by carbonizing phenolic resins made by reacting phenols with cellulosics, aldehydes, and ketones.
glaze icing: 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: 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: = gliding angle.
glide path: 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: 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: The ratio of the horizontal distance traveled to the vertical distance descended in a glide. Also called gliding ratio.
glide scope: 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: 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: 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: The angle between the horizontal and the glide path of an aircraft. Also called glide angle or glide slope.
gliding ratio: = glide ratio.
Glimm method: Numerical technique for solving gas dynamics problems involving hyperbolic systems of conservation laws.
GLL: Galileo spacecraft.
Global Change Master Directory: 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: Helioseismology instrument aboard SOHO which analyzes the vibrational modes of the Sun.
Global Positioning System: 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: The total of direct solar radiation and diffuse sky radiation received by a unit horizontal surface.
Global radiation is measured by pyranometers.
global velocities: 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: = ball lightning.
globular clusters: 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: 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: 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: Adrenocortical steroid hormones that are involved in the metabolism of fats, proteins, and carbohydrates, and have anti-inflammatory properties.
gluons: 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: Greenwich Mean Time, similar to UTC but not updated with leap seconds.
GMT (abbr): = Greenwich mean time.
GMT--Greenwich Mean Time: GMT is the mean solar time of the meridian of Greenwich used as the prime basis of standard time throughout the world.
gneiss: 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: The part of a sundial which casts the shadow, usually a rod or fin pointed at the celestial pole.
gnomonic projection: 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: The study of germ-free animals.
Godunov method: 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: The first in a series of geostationary operational environmental satellites launched in October 1975. It ceased operation in June of 1977.
GOES 2: The second in a series of geostationary operational environmental satellites launched in June 1977. Used for Geostationary Operatl Environ Satellite B.
GOES 3: The third in a series of geostationary operational environmental satellites launched in June 1978.
GOES 4: The fourth in a series of geostationary operational environmental satellites launched in September 1980.
GOES 5: The fifth in a series of geostationary operational environmental satellites launched in May 1981.
GOES 6: The sixth in a series of geostationary operational environmental satellites launched in April 1983.
GOES satellites: Geostationary operational environmental satellites. Used for Geostationary Operational Environ Sats.
GOLF: See Global Oscillations at Low Frequencies
goniometer: An instrument for measuring angles.
goniometers: Instruments for measuring angles.
goodness of fit: 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: Gaseous oxygen.
Grad-Shafranov equation: 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: 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 -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: Optical systems with components whose refractive indexes vary continuously within the material used for the optical elements.
gradient wind: 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: Mathematical term for operators which determines the magnitude and direction of the greatest rate-of-change of a given function with position.
grain: 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: The standard of mass in the metric system.
gram-calorie: See calorie.
gram-centimeter: The CGS gravitation unit of work.
gram-molecule: The mass in grams of a substance numerically equal to its molecular weight.
grand unified theory: 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: Assets bestowed or transferred, such as money or land, for a particular purpose.
granule: 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: 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: A diagram indicating the relationship between two or more variables.
graph theory: The mathematical study of the structure of graphs and networks.
graphical user interface: 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: Structural materials composed of epoxy resins reinforced with graphite.
graphite-polyimide composites: Composite materials utilizing graphite reinforcing fibers in a resin matrix.
graphoepitaxy: The use of artificial surface relief structures to induce crystallographic orientation in thin films.
Grashof number: (symbol N_Gr, G_r).
A nondimensional parameter used in the theory of heat transfer, defined by

where l is a representative length; $T$ ₁ and $T$ ₂ are representative temperatures; g is the acceleration of gravity; and is the kinematic viscosity.
The Grashof number is associated with the Reynolds number and the Prandtl number in the study of convection.
grass: 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: 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: Coarse, granular aggregates, with pieces larger than sand grains, resulting from the natural erosion of rocks. Used for gravel deposits.
graviceptor: = gravireceptor.
gravimeters: 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: The measurement of gravity or gravitational acceleration, especially in geophysics and geodesy.
gravireceptors: 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: 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: See Newtonian universal constant of gravitation, Gaussian constant.
gravitational: Pertaining to gravitation.
gravitational collapse: 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: (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 4² by the choice of units for other parameters in the particular problem.
gravitational fields: Regions that give rise to forces of gravitational attraction.
gravitational force: 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: 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: 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: 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: See red shift, note.
gravitational tide: 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: = gravity wave.
gravitational wave antennas: Devices for receiving propagating gravitational fields produced by some change in the distribution of matter.
gravitational waves: Hypothetical waves that travel at the speed of light, by which gravitational attraction is propagated.
graviton: The hypothetical elementary unit of gravitation which is equivalent to the electron in electromagnetic theory.
gravitons: The hypothetical elementary units of gravitation which are equivalent in the electrons in electromagnetic theory.
gravity: 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: 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

Aerospace Science & Technology Dictionary

Aerospace Science
&
Technology Dictionary