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D

 
D region
   (AS&T Dictionary)
A daytime layer of the Earth's ionosphere about 50 to 100 km in altitude above sea level.
D region
   (SOHO Glossary - GSFC)
The lowest layer of the Earth's ionosphere. It is between about 50 and 95 kilometers above Earth's surface. This is the layer which reflects radio waves. Also called the D Layer.
D-display
   (NASA SP-7, 1965)
In radar, a C-display in which the blips extend vertically to give a rough estimate of distance. Also called D-indicator, D-scan, D-scope.
D-indicator
   (NASA SP-7, 1965)
= D-display.
D-layer
   (NASA SP-7, 1965)
See ionosphere.
D-region
   (NASA SP-7, 1965)
See ionosphere.
D-scan
   (NASA SP-7, 1965)
= D-display.
D-scope
   (NASA SP-7, 1965)
= D-display.
dabble
   (NASA SP-7, 1965)
See double-dabble.
dacite
   (Photoglossary of Volcanic Terms - USGS)
Dacite is a light gray rock with 63 to 68 percent silica (SiO2). It is one of the most common rock types associated with enormous Plinian eruptions. Common minerals include plagioclase feldspar, pyroxene, amphibole and iron oxide. Dacite magmas generally erupt at temperatures between 800 and 1000C.
Dalton
   (NASA SP-7, 1965)
= atomic mass unit.
Dalton law
   (NASA Thesaurus / NASA SP-7, 1965)
The empirical generalization that for many so-called perfect gases, a mixture of these gases will have a pressure equal to the sum of the partial pressures that each of the gases would have as sole component with the same volume and temperature, provided there is no chemical interaction.
Dalton law of partial pressure
   (NASA SP-7, 1965)
= Dalton law.
damage assessment
   (NASA Thesaurus)
Estimate of injury or loss to components, subsystems, or entire systems, as well as the cost of repairs or replacement to restore serviceability.
damp
   (NASA SP-7, 1965)
To suppress oscillations or disturbances.
damped natural frequency
   (NASA SP-7, 1965)
The frequency of free vibration of a damped linear system.
The oscillation of a damped system may be considered periodic in the limited sense that the time interval between zero crossings in the same direction is constant if the system is linear, even though successive amplitudes decrease progressively. The frequency of the oscillation is the reciprocal of this time interval. The damped natural frequency decreases as the damping increases, and approaches zero as the damping approaches critical damping.
damped wave
   (NASA SP-7, 1965)
Any wave whose amplitude decreases with time or whose total energy decreases by transfer to other portions of the wave spectrum.
damping
   (NASA Thesaurus / NASA SP-7, 1965)
The suppression of oscillations or disturbances; the dissipation of energy with time. See viscous damping.
damping factor
   (NASA SP-7, 1965)
The ratio of the amplitude of any one of a series of damped oscillations to that of the following one at the same phase.
damping ratio
   (NASA SP-7, 1965)
The ratio of actual damping to critical damping.
It may be expressed as the ratio of output under static conditions to twice the output at the lowest frequency where a 90° phase shift is observed.
dark adaptation
   (NASA Thesaurus / NASA SP-7, 1965)
The process by which the iris and retina of the eye adjust to allow maximum vision in dim illumination, following exposure of the eye to a relatively brighter illumination.
dark blips
   (NASA SP-7, 1965)
See dark trace tube.
dark terrain
   (Galileo Project Glossary - JPL)
A type of surface on Ganymede and Callisto which is darker than other regions on these moons (see also bright terrain). On Ganymede dark terrain is often associated with furrows.
dark trace tube
   (NASA SP-7, 1965)
A cathode-ray tube, on which the face is bright, and signals are displayed as dark traces or dark blips.
dart configuration
   (NASA SP-7, 1965)
A configuration of an aerodynamic vehicle in which the control surfaces are at the tail of the vehicle. Contrast canard.
DAST program
   (NASA Thesaurus)
A NASA program which uses the Firebee 2 target drone aircraft as a test bed for getting flight data on research wings. The drone is launched from the wing of a B52 and recovered by parachute. The program's purpose is the study of flight loads and load control. Used for drones for aerodynamic and struct test.
data base management systems
   (NASA Thesaurus)
Software products that control data structures containing interrelated data stored so as to optimize accessibility and control, minimize redundancy, and offer multiple views of the data to various applications programs.
data compression
   (NASA Thesaurus)
Any technique used to reduce the amount of storage required to store data.
data integration
   (NASA Thesaurus)
Taking data from multiple sources and merging the data into a single data file.
data link
   (NASA SP-7, 1965)
Any communications channel or circuit used to transmit data from a sensor to a computer, a readout device, or a storage device.
data point
   (NASA SP-7, 1965)
A unit of fundamental information obtained through the processing of raw data.
data processing
   (NASA Thesaurus / NASA SP-7, 1965)
Application of procedures, mechanical, electrical, computational, or other, whereby data are changed from one form into another.
data processing equipment
   (NASA Thesaurus)
Machines for handling information in a sequence of reasonable operations. Used for data processors.
data processor
   (NASA SP-7, 1965)
A machine for handling information in a sequence of reasonable operations.
data reduction
   (NASA Thesaurus / NASA SP-7, 1965)
Transformation of observed values into useful, ordered, or simplified information.
data simulation
   (NASA Thesaurus)
The use of statistical or physical models to produce synthetic data for testing purposes.
data smoothing
   (NASA Thesaurus / NASA SP-7, 1965)
The mathematical process of fitting a smooth curve to dispersed data points.
data structures
   (NASA Thesaurus)
The organization of computer memory used to represent information in a computer program or database.
data transfer (computers)
   (NASA Thesaurus)
The technique used by the hardware manufacturer to transmit data from computer to storage device or from storage device to computer, usually under specialized program control.
data-acquisition station
   (NASA SP-7, 1965)
A ground station at which various functions to control satellite operations and to obtain data from the satellite are performed.
datum
   (NASA SP-7, 1965)
Any numerical or geometrical quantity or set of such quantities which can serve as a reference or a base for measurement of other quantities.
For a group of statistical references, the plural form is data; as geographic data for a list of latitudes and longitudes. Where the concept is geometrical the plural form is datums; as in two geodetic datums have been used.
datum (elevation)
   (Glossary of Hydrologic Terms - NOAA)
The arbitrary "zero plane" from which all stage measurements are taken from. Usually set below the natural bottom of the channel so all stage height readings will be greater than zero.
datum line
   (NASA SP-7, 1965)
Any line which can serve as a reference or base for the measurement of other quantities.
datum plane
   (NASA SP-7, 1965)
A plane from which angular or linear measurements are reckoned. Also called reference plane.
datum point
   (NASA SP-7, 1965)
Any point which can serve as a reference or base for the measurement of other quantities.
daughter, daughter element
   (NASA SP-7, 1965)
= decay product.
dawsonite
   (NASA Thesaurus)
A mineral consisting of aluminum sodium carbonate.
day
   (NASA SP-7, 1965)
1. The duration of one rotation of the earth, or another celestial body, on its axis.
A day is measured by successive transits of a reference point on the celestial sphere over the meridian, and each type takes its name from the reference used. Thus, for a solar day the reference is the sun; a mean solar day if the mean sun; and an apparent solar day if the apparent sun. For a lunar day the reference is the moon; for sidereal day the vernal equinox; for a constituent day an astre fictif or fictitious star. The expression lunar day refers also to the duration of one rotation of the moon with respect to the sun. A Julian day is the consecutive number of each day, beginning with January 1, 4713 BC.
2. A period of 24 hours beginning at a specified time, as the civil day beginning at midnight, or the astronomical day beginning at noon.
daylight saving time
   (NASA SP-7, 1965)
See time.
daytime visual range
   (NASA SP-7, 1965)
= visual range.
DCRsi
   (AS&T Dictionary)
The Digital Cassette Recording system incremental (DCRsi) rack mount and modular ruggedized systems are one-inch, transverse scan, rotary digital recorders capable of recording and reproducing at any user data rate from 0 to 13.4 Mbytes/seconds (0-107 Mbits/seconds). DCRsi is a user friendly mass storage data peripheral with a total storage capacity of 48 gigabytes.
DDR--Data Descriptor Record
   (Global Land Information System Glossary - USGS)
A DDR is a file containing image information which may include: (1) number of lines, number of samples, number of bands, data type, and the system on which the data was created; (2) corner coordinates of the image and related projection information; (3) the minimum and maximum values for each band of an image; (4) information describing how and when each band of the image was acquired; and (5) miscellaneous information (e.g., the last date and time modifications were made to an image).
De Broglie wavelength
   (NASA SP-7, 1965)
For a particle of mass m and velocity v , the De Broglie wavelength, lambda = h/mv, where h is Planck constant.
de Broglie wavelengths
   (Imagine the Universe Dictionary - NASA GSFC)
According to quantum mechanics all particles also have wave characteristics, where the wavelength of a particle is inversely proportional to its momentum and the constant of proportionality is the Planck constant.
De Laval nozzle
   (From Stargazers to Starships Glossary - GSFC)
A device for efficiently converting the energy of a hot gas to kinetic energy of motion, originally used in some steam turbines and now used in practically all rockets. By constricting the outflow of the gas until it reaches the velocity of sound and then letting it expand again, an extremely fast jet is produced.
de Laval nozzle
   (NASA SP-7, 1965)
[After Dr. Carl Gustaf Patrik de Laval (1845-1913), Swedish engineer.] A converging-diverging nozzle used in certain rockets. Also called Laval nozzle.
dead band
   (NASA SP-7, 1965)
An arrangement incorporated in a guidance system which prevents an error from being corrected until that error exceeds a specified magnitude.
dead reckoning
   (NASA Thesaurus / NASA SP-7, 1965)
(abbr DR)
In navigation, determination of position by advancing a previous known position for courses and distances.
dead spot
   (NASA SP-7, 1965)
In a control system, a region centered about the neutral control position where small movements of the actuator do not produce any response in the system.
dead time
   (NASA SP-7, 1965)
In a radiation counter, the time interval, after the start of a count, during which the counter is insensitive to further ionizing events.
debris avalanche
   (Photoglossary of Volcanic Terms - USGS)
Debris avalanches are moving masses of rock, soil and snow that occur when the flank of a mountain or volcano collapses and slides downslope. As the moving debris rushes down a volcano and into river valleys, it incorporates water, snow, trees, bridges, buildings, and anything else in the way. Debris avalanches may travel several kilometers before coming to rest, or they may transform into more water-rich lahars, which travel many tens of kilometers downstream.
debris cloud
   (Glossary of Weather Terms for Storm Spotters - NOAA)
A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado.
debug
   (NASA SP-7, 1965)
1. To isolate and remove malfunctions from a device, or mistakes from a routine or program.
2. Specifically, in electronic manufacturing, to operate equipment under specified environmental and test conditions in order to eliminate early failures and to stabilize equipment prior to actual use. Also called burn-in.
Debye length
   (NASA Thesaurus / NASA SP-7, 1965)
A theoretical length which describes the maximum separation at which a given electron will be influenced by the electric field of a given positive ion. Sometimes referred to as the Debye shielding distance or plasma length.
It is well known that charged particles interact through their own electric fields. In addition, Debye has shown that the attractive force between an electron and ion which would otherwise exist for very large separations is indeed cut off for a critical separation due to the presence of other positive and negative charges in between. This critical separation or Debye length decreases for increased plasma density.
Debye shielding
   (Plasma Physics and Fusion Energy Glossary)
If a positive (or negative) charge is inserted into a plasma, it will change the local charge distribution by attracting (repelling) electrons. The net result is an additional negative (positive) charge density which cancels the effect of the initial charge at distances large compared to the Debye length.
Debye shielding distance
   (NASA SP-7, 1965)
= Debye length.
DEC
   (Space Flight Glossary - JPL)
Declination, the measure of a celestial body's apparent height above or below the celestial equator.
deca
   (NASA SP-7, 1965)
= deka.
decade
   (NASA SP-7, 1965)
1. The interval between any two quantities having the ratio of 10:1.
2. A group of series of 10.
decade counter
   (NASA SP-7, 1965)
A counter that counts to 10 in one column of decimal notation; a scale of 10 counter.
decametric wave
   (NASA SP-7, 1965)
See frequency band.
decay
   (NASA Thesaurus / NASA SP-7, 1965)
Decrease of a radioactive substance because of nuclear emission of alpha or beta particles, positrons, or gamma rays. See radioactivity. In beta decay, for example, the emission of a beta lower case-particle, i.e., an electron, causes radioactive change into a daughter element of the same atomic weight as the parent element but of atomic number higher by 1.
decay constant
   (NASA SP-7, 1965)
1. = attenuation constant.
2. (symbol lambda) A constant relating the instant rate of radioactive decay of a radioactive species to the number of atoms N present at a given time t. Thus,

-(N / del (lower case delta)t) = lambdaN

If No is the number of atoms present at time zero then

N  equals N sub zero e to the power minus lambda t

Decay modes
   (Plasma Physics and Fusion Energy Glossary)
Different pathways for decay of radioactive nuclei. The decay modes for a given unstable state can include beta emission (negative = electron, positive = positron), electron capture, alpha emission, fission, and gamma emission.
decay product
   (NASA SP-7, 1965)
A nuclide resulting from the radioactive disintegration of a radionuclide, being formed either directly or as the result of successive transformations in a radioactive series. Also called daughter, daughter element.
A decay product may be either radioactive or stable.
decay time
   (NASA SP-7, 1965)
1. In computer operations, the time required for a pulse to fall to one-tenth of its peak value. See rise time.
2. In charge-storage tubes, the time interval during which the magnitude of the stored charge decreases to a stated fraction of its initial value.
The fraction is usually 1/e where e is the base of natural logarithms.
3. Approximately the lifetime of an orbiting object in a nonstable orbit.
Decay time is usually applied only to objects with short orbit lifetimes caused by atmospheric drag.
decayed object
   (NASA SP-7, 1965)
An object once, but no longer, in orbit.
Decca
   (NASA SP-7, 1965)
A long-range, ambiguous, two-dimensional navigation system using continuous-wave transmission to provide hyperbolic lines of position through the radio frequency phase comparison techniques from four transmitters.
Frequency band, 68 to 150 kilocycles.
Decca navigation
   (NASA Thesaurus)
A long range, ambiguous, two dimensional navigation system using continuous wave transmission to provide hyperbolic lines of position through the radio frequency phase comparison techniques from four transmitters.
decelerate
   (NASA SP-7, 1965)
To cause to move slower; to decrease speed.
deceleration
   (NASA Thesaurus / NASA SP-7, 1965)
The act or process of moving, or of causing to move, with decreasing speed. Sometimes called negative acceleration.
deceleration parachute
   (NASA SP-7, 1965)
A parachute attached to a craft and deployed to slow the craft, especially during landing. Also called a brake parachute , drogue parachute, parabrake.
December solstice
   (NASA SP-7, 1965)
= winter solstice.
deci
   (NASA SP-7, 1965)
(abbr d)
A prefix meaning multiplied by 10-1; one-tenth.
decibel
   (NASA SP-7, 1965)
(abbr db)
1. A dimensionless measure of the ratio of two powers, equal to 10 times the logarithm to the base 10 of the ratio of two powers P1/P2.
2. One-tenth of a bel.
The power P2 may be some reference power; in electricity, the reference power is sometimes taken as 1 milliwatt.
decibel per second
   (NASA SP-7, 1965)
A unit used to measure the rate of decay of a sound.
decimal coefficient of absorption
   (NASA SP-7, 1965)
See absorption coefficient.
decimal digit
   (NASA SP-7, 1965)
1. One of the digits used in decimal notation, i.e., 1,2,3,4,5,6,7,8,9, or 0.
2. One of 10 possible conditions.
decimal notation
   (NASA SP-7, 1965)
A mathematical system in which each digit is the coefficient of some power of 10.
decimal point
   (NASA SP-7, 1965)
The base point in decimal notation.
decimal-to-binary conversion
   (NASA SP-7, 1965)
The mathematical process of converting a quantity from decimal notation to the equivalent binary notation. For example: 1 = 1; 7 = 111; 23 = 10111, etc. See binary notation.
decimetric wave
   (NASA SP-7, 1965)
See frequency band.
decimillimetric waves
   (NASA SP-7, 1965)
See frequency bands.
decision element
   (NASA SP-7, 1965)
In computer operations, any device which as the result of the input of data issues one of two or more possible instructions.
declination
   (NASA Thesaurus / NASA SP-7, 1965)
1. (symbol delta lower case) Angular distance north or south of the celestial equator; the arc of an hour circle between the celestial equator and a point on the celestial sphere, measured northward or southward from the celestial equator through 90 degrees, and labeled N or S to indicate the direction of measurement.
2. (symbol D) Magnetic declination. See equatorial system.
declination
   (Solar System Dynamics Glossary - JPL)
Declination is the angular distance on the celestial sphere north or south of the celestial equator. It is measured along the hour circle passing through the celestial object.
decoder
   (NASA SP-7, 1965)
1. A device for translating electrical signals into predetermined functions.
2. In computer operation, a network or device in which one of two or more possible outputs results from a prescribed combination of inputs. Also called many-to-few matrix.
decoders
   (NASA Thesaurus)
Devices for translating electrical signals into predetermined functions. In computer operations, networks or devices in which one of two or more possible outputs results from a prescribed combination of inputs.
decommissioning
   (NASA Thesaurus)
Disposal or deactivation of equipment or sites whose usefulness has diminished to a point where it is no longer required for its original purpose.
decommutator
   (NASA SP-7, 1965)
Equipment for separation, demodulation, or demultiplexing commutated signals. See commutator.
decompression
   (NASA SP-7, 1965)
The reduction of atmospheric pressure; particularly, various techniques for preventing caisson disease.
decompression sickness
   (NASA Thesaurus / NASA SP-7, 1965)
A disorder experienced by deep sea divers and aviators caused by reduced atmospheric pressure and evolved gas bubbles in the body, marked by pain in the extremities, pain in the chest (chokes), occasionally leading to serve central nervous symptoms and neurocirculatory collapse. See bends, dysbarism.
decontamination
   (NASA SP-7, 1965)
The act of removing chemical, biological, or radiological contamination from, or neutralizing it on, a person, item, or area.
deconvolution
   (Imagine the Universe Dictionary - NASA GSFC)
An image processing technique that removes features in an image that are caused by the telescope itself rather than from actual light coming from the sky.
decoupled
   (NASA SP-7, 1965)
Of circuits or devices, interconnected through any means which passes only the static characteristics of a signal.
decrement
   (NASA SP-7, 1965)
A decrease in the value of a variable. See increment.
decrement gage
   (NASA SP-7, 1965)
A gage in which pressure is measured by the rate of decay in amplitude of the oscillations of an element suspended in the gas and set into motion by external controls. Also known as decrement viscosity gage or viscosity manometer.
Various types of decrement gage are distinguished according to the design of the oscillating element.
decrement viscosity gage
   (NASA SP-7, 1965)
= decrement gage.
Deep Space 1 Mission
   (NASA Thesaurus)
First of several technology demonstration missions supporting the NASA New Millennium Program. Advanced technologies include an ion propulsion system, solar concentrator arrays, autonomous navigation and control systems, an integrated camera and imaging spectrometer, and several telecommunications and microelectronics devices. The mission plan includes a flyby of Asteroid 1992 KD.
Deep Space Network
   (NASA Thesaurus)
A communications network managed by the Jet Propulsion Laboratory for command and control of all planetary flights.
deep well injection (wastes)
   (NASA Thesaurus)
Storage of liquid wastes, particularly chlorohydrocarbons, by injection into subsurface geologic strata for long term isolation from the environment.
definition
   (NASA SP-7, 1965)
The clarity, fidelity, sharpness, resolution and brilliancy of an image, as a photographic image.
deflagration
   (NASA Thesaurus / NASA SP-7, 1965)
A sudden or rapid burning, as opposed to a detonation or explosion.
deflecting force
   (NASA SP-7, 1965)
= coriolis force.
deflection of the vertical
   (NASA SP-7, 1965)
The angular difference, at any place, between the direction of plumb line (the vertical) and the perpendicular (the normal) to the reference spheroid. This difference seldom exceeds 30 seconds of arc. Also called station error.
When measured at the earth's surface the deflection of the vertical is equal to the angle between the geoid and the reference spheroid.
deflection-modulated indicator
   (NASA SP-7, 1965)
= amplitude-modulated indicator.
deflector
   (NASA Thesaurus / NASA SP-7, 1965)
A plate, baffle, or the like that diverts something in its movement or flow; as: (a) a plate that projects into the airstream on the underside of an airfoil to divert the airflow, as into a slot-sometimes distinguished from a spoiler; (b) a conelike device placed or fastened beneath a rocket launched from the vertical position, to deflect the exhaust gases to the sides; (c) any of several different devices used on jet engines to reverse or divert the exhaust gases; (d) a baffle or the like to deflect and mingle fluids prior to combustion, as in certain jet engines.
deformation
   (NASA Thesaurus)
A change in the shape or size of a solid body.
degas
   (NASA SP-7, 1965)
To remove gas from a material, usually by heating under high vacuum. Compare get.
degassing
   (NASA Thesaurus / NASA SP-7, 1965)
The deliberate removal of gas from a material, usually by application of heat under high vacuum.
degauss
   (NASA SP-7, 1965)
Slang for demagnetize.
degenerate matter
   (NASA Thesaurus)
A state of matter found in white dwarf stars and other ultrahigh-density objects in which the electrons follow Fermi-Dirac statistics, i.e., the matter reaches a density high enough so that the pressure increases more and more rapidly to the point where it becomes independent of the temperature and is a function of the density only, thereby departing from the classical laws of physics.
degradation
   (NASA Thesaurus / NASA SP-7, 1965)
Gradual deterioration in performance.
degree of condensation (DC)
   (Comet Glossary - JPL)
DC is an indicator of how much the surface brightness of the coma increases toward the center of the coma. As the DC increases, the coma size usually decreses and becomes more sharply defined.
degree of freedom
   (NASA SP-7, 1965)
1. A mode of motion, either angular or linear, with respect to a coordinate system, independent of any other mode.
A body in motion has six possible degrees of freedom, three linear and three angular.
2. Specifically, of a gyro the number of orthogonal axes about which the spin axis is free to rotate.
3. In an unconstrained dynamic or other system, the number of independent variables required to specify completely the state of the system at a given moment.
If the system has constraints, i.e., kinematic or geometric relations between the variables, each such relation reduces by one the number of degrees of freedom of the system. In a continuous medium with given boundary conditions, the number of degrees of freedom is the number of normal modes of oscillation.
4. Of a mechanical system, the minimum number of independent generalized coordinates required to define completely the positions of all parts of the system at any instant of time.
In general, the number of degrees of freedom equals the number of independent generalized displacements that are possible.
degrees of freedom
   (NASA Thesaurus)
A mode of motion, either angular or linear, with respect to a coordinate system, independent of any other mode. A body in motion has six possible degrees of freedom, three linear and three angular.
dehumidification
   (NASA Thesaurus)
The reduction, by any process, of the quantity of water vapor within a given space.
Deimos
   (NASA Thesaurus / NASA SP-7, 1965)
A satellite of Mars orbiting at a mean distance of 23,500 kilometers.
deionization
   (NASA Thesaurus)
The removal of ions from a solution by ion exchange.
deka
   (NASA SP-7, 1965)
(abbr da)
A prefix meaning multiplied by 10. Sometimes spelled deca.
Del, Dlph
   (NASA SP-7, 1965)
International Astronomical Union abbreviations for Delphinus. See constellation.
del-operator
   (NASA SP-7, 1965)
(symbol nabla del (down pointing triangle) )
The operator used in vector calculus and defined in Cartesian coordinates as
nabla del equals I del over del x plus j del over del y plus k del over del z
delay
   (NASA SP-7, 1965)
The time (or equivalent distance) displacement of some characteristic of a wave relative to the same characteristic of a reference wave; that is, the difference in phase between the two waves. Compare lag.
In one-way radio propagation, for instance, the phase delay of the reflected wave over the direct wave is a measure of the extra distance traveled by the reflected wave in reaching the same receiver.
delay element
   (NASA SP-7, 1965)
A device for causing time delay of a signal. See delay line.
delay line
   (NASA SP-7, 1965)
In electronic computers, any device for producing a time delay of a signal.
delay lines (computer storage)
   (NASA Thesaurus)
In electronic computers, devices for producing a time delay of a signal.
delay-line storage
   (NASA SP-7, 1965)
A storage or memory device consisting of a delay line and means for regenerating and reinserting information into the delay line.
delayed neutron activation analysis
   (Global Land Information System Glossary - USGS)
This non-destructive analytical technique is used to determine concentrations of specific chemical elements. The procedure is based on artificially induced neutron capture and the radioactive decay constants of unstable radionuclides that are produced.
delayed neutrons
   (NASA SP-7, 1965)
Neutrons emitted by excited nuclei in a radioactive process, so called because they are emitted an appreciable time after the fission. Compare prompt neutrons.
delayed plan position indicator
   (NASA SP-7, 1965)
See plan position indicator.
delayer
   (NASA SP-7, 1965)
A substance mixed in with solid rocket propellants to decrease the rate of combustion.
Dellinger effect
   (NASA SP-7, 1965)
= fadeout.
Delphinus
   (NASA SP-7, 1965)
(abbr Del, Dlph)
See constellation.
delta ray
   (NASA SP-7, 1965)
1. An electron ejected by recoil when a rapidly moving alpha particle or other charged particle passes through matter.
2. By extension any secondary ionizing particle ejected by recoil when a primary particle passes through matter.
delta wing
   (NASA Thesaurus / NASA SP-7, 1965)
A triangularly shaped wing of an aircraft.
deluge collection pond
   (NASA SP-7, 1965)
A facility at a launch site into which used to cool the flame deflector is flushed as the rocket begins its ascent. Also called a skimmer basin.
DEM--Digital Elevation Models
   (Global Land Information System Glossary - USGS)
The U.S. Geological Survey produces five primary types of digital elevation model data. They are 1) 7.5-minute DEM, which provides coverage of the contiguous United States, Hawaii, and Puerto Rico. in 7.5- x 7.5-minute blocks. 2) 1-degree DEM, which provides coverage of the United States in 1- x 1-degree blocks. 3) 30-minute DEM (2- x 2-arc-second data spacing), consisting of four 15- x 15-minute DEM blocks that cover the contiguous United States and Hawaii. And finally 4 and 5) 15-minute Alaska DEM and 7.5-minute Alaska DEM, both of which provide coverage of Alaska, but with different spacing.
demagnetization
   (NASA Thesaurus)
The reduction of residual magnetism to an acceptable level.
demand assignment multiple access
   (NASA Thesaurus)
A technique of assigning communication resources on an "as needed basis" such as in satellite communications. Used for DAMA.
demand oxygen system
   (NASA SP-7, 1965)
= demand system.
demand system
   (NASA SP-7, 1965)
An oxygen system in which oxygen flows to the user during inspiration only.
demodulation
   (AS&T Dictionary)
The process of recovering a signal from a modulated (varied frequency) carrier wave.
demodulation
   (NASA SP-7, 1965)
The process of recovering the modulating wave from a modulated carrier.
demodulator
   (NASA SP-7, 1965)
An electronic device which operates on an input of a modulated carrier to recover the modulating wave as an output.
demodulators
   (NASA Thesaurus)
Electronic devices which operate on an input of a modulated carrier to recover the modulating wave as an output.
demography
   (NASA Thesaurus)
Statistical study of human populations, especially with reference to size, density, distribution, and vital data.
demultiplexing
   (NASA Thesaurus)
Separation of two or more signals that were previously combined by a compatible multiplexer and transmitted over a single channel.
dendrochronology
   (NASA Thesaurus)
The use of annual growth rings in plant tissue to determine the age of the plant or tree. Used for tree ring dating.
denitrogenation
   (NASA SP-7, 1965)
The removal of nitrogen dissolved in the blood and body tissues, usually by breathing of pure oxygen for an extended period of time in order to prevent aeroembolism at high altitudes.
densimeters
   (NASA Thesaurus)
Instruments for measuring the density or specific gravity of liquids, gases, or solids.
densitometer
   (NASA Thesaurus / NASA SP-7, 1965)
An instrument for the measurement of optical density (photographic transmission, photographic reflection, visual transmission, etc.) of a material, generally of a photographic image.
density
   (High Energy Astrophysics Dictionary- GSFC)
A property of matter measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0; iron is 7.9; lead is 11.3.
density (mass/volume)
   (NASA Thesaurus)
The mass per unit volume of a material at a specified temperature.
density function
   (NASA SP-7, 1965)
The number of particles per unit volume. See distribution function.
density specific impulse
   (NASA SP-7, 1965)
The product of the specific impulse of a propellant combination and the average specific gravity of the propellants.
deoxyribonucleic acid
   (NASA Thesaurus)
The molecule that encodes genetic informatiion - a double-stranded moleculeeld together by weak bonds betweeen base pairs of nucleotides. Used for DNA.
departure
   (NASA SP-7, 1965)
= deviation, sense 1.
dependent variable
   (NASA SP-7, 1965)
Any variable considered as a function of other variables, the latter being called independent. Compare parameter.
Whether a given quantity is best treated as a dependent or independent variable depends upon the particular problem.
dependent variables
   (NASA Thesaurus)
Variables considered as a function of other variables, the latter being called independent.
depletion layer
   (NASA SP-7, 1965)
In a semiconductor, a region in which the mobile carrier charge density is insufficient to neutralize the net fixed charge density of donors and acceptors. Also called barrier.
deploy
   (NASA SP-7, 1965)
Of a parachute, to release so as to let it fill out or to unfold and fill out.
depolarization
   (NASA Thesaurus)
A decrease in the polarization of an electrode at a specified current density. Used for depolarizers.
depressed pole
   (NASA SP-7, 1965)
The celestial pole below the horizon, of opposite name to the latitude.
The celestial pole above the horizon is called the elevated pole.
depression angle
   (NASA SP-7, 1965)
= angle of depression.
depth perception
   (NASA SP-7, 1965)
The ability to estimate depth or distance between points in the field of vision.
derivative data
   (NASA SP-7, 1965)
Data which have been derived from other data by mathematical techniques.
descending node
   (Space Flight Glossary - JPL)
The point at which an orbit crosses the ecliptic plane going south.
descending node
   (NASA SP-7, 1965)
That point at which a planet, planetoid, or comet crosses to the south side of the ecliptic; that point at which a satellite crosses to the south side of the equatorial plane of its primary. Also called southbound node. The opposite is ascending node or northbound node.
desertification
   (NASA Thesaurus)
The formation of a desert or the gradual expansion of a desertline into previously usable land, due to man-made or natural causes.
desiccants
   (NASA Thesaurus)
Chemicals used to absorb moisture.
design gross weight
   (NASA SP-7, 1965)
The gross weight at take-off that an aircraft, rocket, etc., is expected to have, used in design calculations.
design to cost
   (NASA Thesaurus)
A process whereby cost factors are determined and calculated for the life cycle of a product as an integral part of its design.
desorption
   (NASA Thesaurus / NASA SP-7, 1965)
The process of removing sorbed gas.
destruct
   (NASA SP-7, 1965)
The deliberate action of destroying a rocket vehicle after it has been launched, but before it has completed its course.
Destructs are executed when the rocket gets off its plotted course or functions in a way so as to become a hazard.
destruct line
   (NASA SP-7, 1965)
On a rocket test range, a boundary line on each side of the downrange course beyond which a rocket cannot fly without being destroyed under destruct procedures, or a line beyond which the impact point cannot pass. See impact line, command destruct.
desynchronization (biology)
   (NASA Thesaurus)
The loss of synchronization between two or more rhythms so that they show independent periods.
detached shock
   (NASA SP-7, 1965)
= detached shock wave.
detached shock wave
   (NASA SP-7, 1965)
A shock wave not in contact with the body which originates it. See bow wave. Also called detached shock.
detachment
   (NASA Thesaurus / NASA SP-7, 1965)
A particular state of isolation in which man is separated or detached from his accustomed behavioral environment by inordinate physical and psychological distances. This condition may compromise his performance.
detection
   (NASA SP-7, 1965)
See recognition, note.
detector
   (NASA SP-7, 1965)
1. = sensor, sense 1.
2. An instrument employing a sensor, sense 1, to detect the pressure of something in the surrounding environment.
detectors
   (NASA Thesaurus)
Sensors or instruments employing a sensor.
detonation
   (NASA Thesaurus / NASA SP-7, 1965)
A rapid chemical reaction which propagates at a supersonic velocity.
detonation wave
   (NASA SP-7, 1965)
A shock wave in a combustible mixture, which originates as a combustion wave.
detonation waves
   (NASA Thesaurus)
Shock waves that accompany detonation and have a shock front followed by a region of decreasing pressure in which the reaction occurs.
deuterium
   (NASA Thesaurus / NASA SP-7, 1965)
(symbol D, d)
A heavy isotope of hydrogen having one proton and one neutron in the nucleus.
The symbol D is often used to designate deuterium in compounds, as HDO for molecules of that composition. Official chemical nomenclature uses the designation d with a number which designates the carbon atom to which deuterium is bound; e.g. 2-d propane designates CH3CHDCH3.
deuterium fluorides
   (NASA Thesaurus)
Fluorides of deuterium, a heavy isotope of hydrogen. Used for DF.
deuteron
   (NASA SP-7, 1965)
The nucleus of a deuterium atom.
deuteron (differs from Thesaurus)
   (Plasma Physics and Fusion Energy Glossary)
Deuteron: A deuterium ion; nucleus consisting of a proton and a neutron.
deuterons
   (NASA Thesaurus)
The nuclei of deuterium atoms.
deviation
   (NASA Thesaurus)
The variation from a specified dimension or design requirement, usually defining upper and lower limits.
deviation
   (NASA SP-7, 1965)
1. In statistics, the difference between two numbers. Also called departure.
It is commonly applied to the difference of a variable from its mean, or to the difference of an observed value from a theoretical value.
2. = magnetic deviation.
3. In radio transmission, the apparent variation of frequency above and below the unmodulated or center frequency.
dew point
   (NASA Thesaurus / NASA SP-7, 1965)
The temperature to which a given parcel of air must be cooled at constant pressure and constant water-vapor content in order for saturation to occur; the temperature at which the saturation vapor pressure of the parcel is equal to the actual vapor pressure of the contained water vapor. Any further cooling usually results in the formation of dew or frost. Also called dewpoint temperature.
When this temperature is below 0° C, it is sometimes called the frost point.
dew-point temperature
   (NASA SP-7, 1965)
= dew point.
dewar
   (Plasma Physics and Fusion Energy Glossary)
Cryogenic storage container in which two concentric vessels are separated by an insulating vacuum which prevents conductive and convective heat losses. The vessels themselves are often silvered to reduce radiative heat losses.
dewatering
   (NASA Thesaurus)
Removal of water by draining, pumping, or other means.
DF (abbr)
   (NASA SP-7, 1965)
= direction finder.
See radio direction finder.
DF lasers
   (NASA Thesaurus)
Gas lasers in which the active material is deuterium fluoride. Used for deuterium fluoride lasers.
DGL optional format
   (Global Land Information System Glossary - USGS)
The USGS digital line graph (DLG-3) optional format, which was designed for data interchange, allows for the creation of a vector polygon data structure. The topological linkages are explicitly encoded for node, area, and line elements. The files are composed of 8-bit ASCII characters organized into fixed logical records of 80 bytes. Bytes 1 - 72 contain the data, bytes 73 - 77 are blank, and bytes 78 - 80 contain a record sequence number. The detailed description of the DLG-3 optional format is described in Digital Line Graphs from 1:2,000,000-Scale Maps, Data Users Guide 3 (1990).
DHC 2 aircraft
   (NASA Thesaurus)
De Havilland Canada STOL utility aircraft. Used for DHC Beaver aircraft.
diabatic process
   (NASA SP-7, 1965)
A process in a thermodynamic system in which there is a transfer of heat across the boundaries of the system.
Diabatic process is preferred to nonadiabatic process.
diameters
   (NASA Thesaurus)
Lengths of the longest straight lines through the centers of the largest cross sections.
diamonds
   (NASA Thesaurus)
(A) An isometric mineral, representing a naturally occurring crystalline form of carbon dimorphous with graphiteand being the hardest natural substance known. (B) Artificially produced crystallized carbon similar to the native form. (C) A crystalline mineral that resembles diamonds in brilliance.
diamonds
   (NASA SP-7, 1965)
The pattern of shock waves often visible in a rocket exhaust which resembles a series of diamond shapes placed end to end.
diaphragm (anatomy)
   (NASA Thesaurus)
Musculomembranous partition separating the abdominal and thoracic cavities.
diaphragm manometer
   (NASA SP-7, 1965)
A displacement manometer employing a flexible diaphragm as the movable partition.
diastolic blood pressure
   (NASA SP-7, 1965)
The pressure exerted by the blood during periods between cardiac contraction.
dichotomy
   (NASA SP-7, 1965)
In astronomy, a configuration of three bodies so that they form a right triangle; specifically, such a configuration in the solar system with the sun at the apex of the 90° angle.
didymium
   (NASA Thesaurus)
A mixture of rare earth elements that is freed from cerium. It was once regarded as an element but contains chiefly neodymium and praseodymium and is usually associated with lanthanum. It is used in coloring glass for optical filters.
dielectric
   (NASA SP-7, 1965)
A substance that contains few or no free charges and which can support electrostatic stresses.
In an electromagnetic field, the centers of the nonpolar molecules of a dielectric are displaced, and the polar molecules become oriented close to the field. The net effect is the appearance of charges at the boundaries of the dielectric. The frictional work done in orientation absorbs energy from the field which appears as heat. When the field is removed the orientation is lost by thermal agitation and so the energy is not regained. If free-charge carriers are present they too can absorb energy.
A good dielectric is one in which the absorption is a minimum. A vacuum is the only perfect dielectric. The quality of an imperfect dielectric is its
dielectric strength; and the accumulation of charges within an imperfect dielectric is termed dielectric absorption.
dielectric absorption
   (NASA SP-7, 1965)
See dielectric.
dielectric constant
   (NASA SP-7, 1965)
(symbol epsilon )
For a given substance, the ratio of the capacity of a condenser with that substance as dielectric to the capacity of the same condenser with a vacuum for dielectric. It is a measure, therefore, of the amount of electrical charge a given substance can withstand at a given electric-field strength; it should not be confused with dielectric strength.
The dielectric constant is a function of temperature and frequency and is written as a complex quantity
epsilon equals epsilon prime minus I eplison double prime
where ' is the part that determines the displacement current and '' the dielectric absorption (see dielectric). For a nonabsorbing, nonmagnetic material ' is equal to the square of the index of refraction and the relation holds only at the particular frequency where these conditions apply.
dielectric gradient
   (NASA SP-7, 1965)
The spatial variation of the dielectric constant in a substance or medium.
This term is used frequently in reference to the propagation of radio energy. The magnitude of these gradients and the distance over which they occur, relative to the wavelength of the incident radiation, determine the extent to which targets will reflect radar energy. Sufficiently intense dielectric gradients (or index of refraction gradients) are believed to be the cause of certain echoes known as angels.
dielectric strength
   (NASA SP-7, 1965)
A measure of the resistance of a dielectric to electrical breakdown under the influence of strong electric fields; usually expressed in volts per centimeter. Sometimes called breakdown potential.
dielectrics
   (NASA Thesaurus)
Substances that contain few or no free charges and which can support electrostatic stresses. Used for dielectric materials.
difference of latitude
   (NASA SP-7, 1965)
The shorter arc of any meridian between the parallels of two places, expressed in angular measure.
difference of longitude
   (NASA SP-7, 1965)
The smaller angle at the pole or the shorter arc of a parallel between the meridians of two places, expressed in angular measure.
differential analyzer
   (NASA Thesaurus / NASA SP-7, 1965)
An analog computer designed and used primarily for solving differential equations.
differential correction
   (NASA SP-7, 1965)
In celestial mechanics, a method for finding from the observed residuals minus the computed residuals (O - C) small corrections which, when applied to the orbital elements or constants, will reduce the deviations from the observed motion to a minimum.
differential manometer
   (NASA SP-7, 1965)
A manometer which indicates the pressure difference across two ports.
differential pressure
   (NASA SP-7, 1965)
The pressure difference between two systems or volumes.
differential pulse code modulation
   (NASA Thesaurus)
An efficient signal encoding method of reducing the transmission rate of digital signals. The basic principle of DPCM is to quantize code and transmit the difference between the actual sample and prediction value. Used for DPCM (modulation).
differential rotation
   (SOHO Glossary - GSFC)
The change in solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees per day) than do high latitudes (approx. 12 degrees per day).
differential thermal analysis
   (NASA SP-7, 1965)
The technique of detecting endothermic and exothermic phase changes and other processes within a heated material by the corresponding temperature changes.
differential transducer
   (NASA SP-7, 1965)
A device which is capable of measuring simultaneously two separate stimulus sources and which provides an output proportional to the difference between the stimuli. See transducer.
differentiator
   (NASA SP-7, 1965)
1. In computer operations, a device whose output is proportional to the derivative of an input signal.
2. In electronics, a transducer whose output waveform is the time derivative of its input waveform.
differentiators
   (NASA Thesaurus)
In computer operations, devices whose output is proportional to the derivative of an input signal. In electronics, a transducer whose output waveform is the time derivative of its input waveform.
diffracted wave
   (NASA SP-7, 1965)
A wave whose front has been changed in direction by an obstacle or other nonhomogeneity in a medium, other than by reflection or refraction.
diffraction
   (NASA Thesaurus / NASA SP-7, 1965)
The process by which the direction of radiation is changed so that it spreads into the geometric shadow region of an opaque or refractive object that lies in a radiation field.
Diffraction is an optical edge effect.
Reference to Huygens' principle is a common means of explaining diffraction. Analysis of the interference between individual Huygens wavelets which originate in the vicinity of the edge of an irradiated body reveals that detectable amounts of radiant energy must invade the nominal shadow zone of the object, and there, by interference, set up characteristic energy distributions known as diffraction patterns. The amount of diffractive
bending experienced by a ray is a function of wavelength; thus dispersion occurs, although dispersion is in the opposite sense to that produced by refraction.
diffraction grating
   (From Stargazers to Starships Glossary - GSFC)
A flat optical surface, transparent or reflecting, ruled with many parallel grooves at precisely spaced distances. The active parts are not the grooves but the flat sections left between them, which act like a large number of precisely spaced slits. The light waves passing those slits resonate with each other in a way which depends on wavelength, causing different wavelengths to be steered in different directions. The overall effect on light containing different wavelengths is like that of a glass prism: the intensity of the light deflected is much smaller than with a prism, but the ability to separate close colors is much better.
diffraction propagation
   (NASA Thesaurus / NASA SP-7, 1965)
Wave propagation around objects, or over the horizon, by diffraction.
Diffraction is due to the fact that from every point in a wave front a spherical front is generated which falls off in intensity away from the forward direction. A continuous series of such actions carries radiation around objects, or around the curvature of the earth, but with rapidly diminishing intensity.
diffraction radiation
   (NASA Thesaurus)
Electromagnetic radiation excited by an electron flux passing near a diffractive, periodic structure, such as a wiggler magnet in a free electron laser.
diffractive optics
   (NASA Thesaurus)
Optical elements that add up scattered light from a multiple of disturbances in amplitude or phase to generate a transformed wavefront.
diffuse aurora
   (Earth's Magnetosphere Glossary - GSFC)
A spread-out glow often covering much of the auroral oval. It is not seen by the eye but can be observed quite well by satellite cameras.
diffuse radiation
   (NASA Thesaurus / NASA SP-7, 1965)
Radiant energy propagating in many different directions through a given small volume of space; to be contrasted with parallel radiation.
The ideal form of diffuse radiation is isotropic radiation. Careful distinction should be made between this concept and that of a perfectly diffuse radiator.
diffuse reflection
   (NASA SP-7, 1965)
Any reflection process in which the reflected radiation is sent out in many directions usually bearing no simple relationship to the angle of incidence; the opposite of specular reflection. See diffuse reflector, perfectly diffuse reflector.
A term frequently applied to the process by which solar radiation is scattered by dust and other suspensoids in the atmosphere. See diffuse sky radiation.
diffuse reflector
   (NASA SP-7, 1965)
Any surface which reflects incident rays in a mutliplicity of directions, either because of irregularities in the surface or because the material is optically inhomogeneous, as a paint, although optically smooth; the opposite of a specular reflector. See perfectly diffuse reflector.
Ordinary writing papers are good examples of diffuse reflectors, where as mirrors or highly polished metal plates are examples of specular reflectors. Almost all terrestrial surfaces (except calm water) act as diffuse reflectors of incident solar radiation.
diffuse sky radiation
   (NASA SP-7, 1965)
Solar radiation reaching the earth's surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. Also called skylight, diffuse skylight, sky radiation.
Of the total light removed from the direct solar beam by scattering in the atmosphere (approximately 25 percent of the incident radiation), about two-thirds ultimately reaches the earth as diffuse sky radiation.
diffuse skylight
   (NASA SP-7, 1965)
= diffuse sky radiation.
diffuse sound
   (NASA SP-7, 1965)
Sound energy for which energy is uniform in the region considered and when all directions of energy flux at all parts of the region are equally probable.
diffuser
   (NASA SP-7, 1965)
A specially designed duct, chamber, or section, sometimes equipped with guide vanes, that decreases the velocity of a fluid, as air, and increases its pressure, as in a jet engine, a wind tunnel, etc. See supersonic diffuser.
diffusers
   (NASA Thesaurus)
Specially designed ducts, chambers, or sections, sometimes equipped with guide vanes, that decrease the velocity of a fluid, as air, and increases its pressure, as in jet engines, wind tunnels, etc. Used for shock diffusers.
diffusion
   (Plasma Physics and Fusion Energy Glossary)
The interpenetration of one substance into another as a result of thermal / random motion of the individual particles.
diffusion
   (NASA Thesaurus / NASA SP-7, 1965)
In an atmosphere, or in any gaseous system, the exchange of fluid parcels between regions, in apparently random motions of a scale too small to be treated by the equations of motion.
2. In materials, the movement of atoms of one material into the crystal lattice of an adjoining material, e.g.,penetration of the atoms in a ceramic coating into the lattice of the protected metal.
3. In ion engines, the migration of neutral atoms through a porous structure incident to ionization at the emitting surface.
diffusion coefficient
   (NASA Thesaurus / NASA SP-7, 1965)
The absolute value of the ratio of the molecular flux per unit area to the concentration gradient of a gas diffusing through a gas or a porous medium where the molecular flux is evaluated across a surface perpendicular to the direction of the concentration gradient. See diffusion, coefficient of mutual diffusion.
diffusion equation
   (NASA SP-7, 1965)
See diffusivity.
diffusion velocity
   (NASA SP-7, 1965)
1. The relative mean molecular velocity of a selected gas undergoing diffusion in a gaseous atmosphere, commonly taken as a nitrogen (N2) atmosphere.
The diffusion velocity is a molecular phenomenon and depends upon the gaseous concentration as well as upon the pressure and temperature gradients present.
2. The velocity or speed with which a turbulent diffusion process proceeds as evidenced by the motion of individual eddies.
diffusive equilibrium
   (NASA SP-7, 1965)
The steady state resulting from the diffusion process, primarily of interest when external forces or sources and sinks exist within the field. See isothermal equilibrium.
diffusivity
   (NASA Thesaurus / NASA SP-7, 1965)
A measure of the rate of diffusion of a substance, expressed as the diffusivity coefficient K. When K is constant, the diffusion equation is
del q over del t equals K nabla del squared q
where q is the substance diffused; nabla del (down pointing triangle)2 is the Laplacian operator; and t is time. The diffusivity has dimensions of a length times a velocity; it varies with the property diffused, and for any given property it may be considered a constant or a function of temperature, space, etc., depending on the context. Also called coefficient of diffusion. See conductivity, kinematic viscosity, exchange coefficients.
In the case of molecular diffusion the length dimension is the mean free path of the molecules. By analogy, in eddy diffusion, length becomes the mixing length. The coefficient is the called then eddy diffusivity, and is in general several orders of magnitude larger than the molecular diffusivity.
difluence
   (NASA SP-7, 1965)
The divergence vector of adjacent streamlines. The opposite of confluence.
dig pole
   (NASA SP-7, 1965)
= magnetic pole.
digit
   (NASA SP-7, 1965)
1. A single symbol or character representing an integral quantity.
2. Any one of the symbols used in positional notation as coefficients of each power, or order, of the base. See binary digit, decimal digit.
digital
   (NASA SP-7, 1965)
Using discrete expressions to represent variables.
digital computer
   (NASA Thesaurus / NASA SP-7, 1965)
A computer which operates with information, numerical or otherwise, represented in a digital form.
digital count
   (Global Land Information System Glossary - USGS)
Digital count is the total number of pixels occurring in an image for each possible data value.
digital data
   (Galileo Project Glossary - JPL)
Data in numerical form that can be stored and used in a computer.
digital electronics
   (NASA Thesaurus)
The use of circuits in which there are usually only two states possible at any point. The two states can represent any of a variety of binary digits (bits) of information. Used for digital circuits.
digital filters
   (NASA Thesaurus)
Computational means of attenuating undesired frequencies in sets of time-dependent data.
digital output
   (NASA SP-7, 1965)
Transducer output that represents the magnitude of the stimulus in the form of a series of discrete quantities coded to represent digits in a system of notation. Compare analog output.
digital signals
   (Galileo Project Glossary - JPL)
Data that is converted to numerical form so that it can be stored and used in a computer.
digital television
   (NASA Thesaurus)
Television in which picture redundancy is reduced or eliminated by transmitting only the data needed to define motion in the picture, as represented by changes in the areas of continuous white or black.
digitization
   (Global Land Information System Glossary - USGS)
Digitization of graphics typically involves recording the location of each point, line, and polygon on a map, along with their associated labels and attributes.
digitize
   (NASA SP-7, 1965)
To express an analog measurement of a variable in discrete units.
digitizer
   (NASA SP-7, 1965)
A device which converts analog data into numbers expressed in digits in a system of notation. Also called analog-to-digital converter.
dihedral angle
   (NASA SP-7, 1965)
The acute angle between two intersecting planes or between lines representative of planes.
dike
   (Photoglossary of Volcanic Terms - USGS)
Dikes are tabular or sheet-like bodies of magma that cut through and across the layering of adjacent rocks. They form when magma rises into an existing fracture, or creates a new crack by forcing its way through existing rock, and then solidifies. Hundreds of dikes can invad the cone and inner core of a volcano, sometimes preferentially along zones of structural weakness.
Dione
   (NASA Thesaurus / NASA SP-7, 1965)
A satellite of Saturn orbiting at a mean distance of 378,000 kilometers.
dioptric light
   (NASA SP-7, 1965)
A light concentrated into a collimated beam by means of refracting lenses or prisms.
One collimated by means of a reflector is a catoptric light.
dip
   (NASA SP-7, 1965)
1. = dip angle.
2. = magnetic dip.
dip angle
   (NASA SP-7, 1965)
(symbol delta lower case)
The vertical angle between the true horizon and the apparent horizon.
dip equator
   (NASA SP-7, 1965)
= aclinic line.
diplex transmission
   (NASA SP-7, 1965)
The simultaneous transmission of two signals using a common carrier wave. Compare duplex operation, multiplexing.
diplexer
   (NASA SP-7, 1965)
A device permitting an antenna system to be used simultaneously or separated by two transmitters. Compare duplexer.
dipole
   (NASA SP-7, 1965)
1. A system composed of two, separated, equal electric or magnetic charges of opposite sign.
2. = dipole antenna.
dipole antenna
   (NASA Thesaurus / NASA SP-7, 1965)
A straight radiator, usually fed in the center, and producing a maximum of radiation in the plane normal to its axis. The length specified is the overall length.
Common usage in microwave antennas considers a dipole to be a metal radiating structure which supports a line current distribution similar to that of a thin straight wire, a half wavelength long, so energized that the current has two nodes, one at each of the far ends.
dipoles
   (NASA Thesaurus)
Systems composed of two, separated, equal electric or magnetic charges of opposite sign.
direct air cycle
   (NASA SP-7, 1965)
A thermodynamic propulsion cycle involving a nuclear reactor and gas turbine or ramjet engine, in which air is the working fluid. Also called direct cycle.
Air is successively compressed in the compressor section, heated in the nuclear reactor, and expelled through the turbine-tailpipe section to obtain thrust.
direct broadcast satellites
   (NASA Thesaurus)
Domestic satellites used for direct TV transmission to home receivers. Used for DBS (satellites).
direct current
   (Plasma Physics and Fusion Energy Glossary)
Electric current which travels continuously in the same direction over a sustained period of time; contrast with AC (Alternating Current) which oscillates as a function of time.
direct cycle
   (NASA SP-7, 1965)
= direct air cycle.
direct motion
   (NASA SP-7, 1965)
Eastward or counterclockwise motion of a planet or other object as seen from the North Pole (motion in the direction of increasing right ascension).
direct product
   (NASA SP-7, 1965)
= scalar product.
direct solar radiation
   (NASA SP-7, 1965)
In actinometry, that portion of the radiant energy received at the instrument direct from the sun, as distinguished from diffuse sky radiation, effective terrestrial radiation, or radiation from any other source. See global radiation.
Direct solar radiation is measured by pyrheliometers.
direct wave
   (NASA SP-7, 1965)
A radio wave which travels directly from the transmitting antenna to the receiving antenna, in contrast with an indirect wave, which undergoes an abrupt change of direction by refraction or reflection.
direct-current discharge
   (NASA SP-7, 1965)
The conduction of direct current through two electrodes immersed in a gas. See Townsend discharge, glow discharge, arc discharge.
direction angle
   (NASA SP-7, 1965)
In tracking, the angle between the antenna base line and an imaginary line connecting the center of the baseline with the target.
direction cosine
   (NASA SP-7, 1965)
1. The cosine of the angle formed by the intersection of a line, as a line of sight to an orbiting body, with an axis of a rectangular coordinate system with the origin on the line.
Every line has three direction angles and three direction cosines: 1, m, n corresponding to capital psi, theta, lower case phi, the direction angles with the x, y, and z axes.
2. Specifically, in tracking, the cosine of the angle between a base line and the line connecting the center of the baseline with the target.
direction finder
   (NASA SP-7, 1965)
(abbr DF)
direction finding
   (NASA Thesaurus)
A procedure or process for locating or localizing the origin of radar, acoustical, or optical emissions.
direction of relative movement
   (NASA SP-7, 1965)
See relative movement.
directional antenna
   (NASA Thesaurus / NASA SP-7, 1965)
An antenna that radiates or receives radio signals more efficiently in some directions than in others. See Adcock antenna, loop antenna, sense antenna.
A group of antennas arranged for this purpose is called an antenna array.
directional array
   (NASA SP-7, 1965)
= antenna array.
directional emittance
   (NASA SP-7, 1965)
Emittance in a stated direction from a surface. The direction is usually specified as angle from the normal.
directional gyro
   (NASA SP-7, 1965)
1. A two-degree-of-freedom gyro with a provision for maintaining its spin axis approximately horizontal.
2. A flight instrument incorporating a gyro that holds its position in azimuth and thus can be used as a directional reference.
directional properties
   (NASA SP-7, 1965)
1. Of metals, properties whose magnitude varies depending on the relation of the test axis to a specific direction within the metal. The variation results from preferred orientation or from fibering of constituents or inclusions.
2. For thermal radiation properties, in a specified direction from the surface, usually measured as the angle from the normal.
directional solidification (crystals)
   (NASA Thesaurus)
Controlled solidification (crystal growth) of molten metal in a casting so as to provide feed metal to the solidifying front of the casting.
directional stability
   (NASA Thesaurus / NASA SP-7, 1965)
The property of an aircraft, rocket, etc., enabling it to restore itself from a yawing or sideslipping condition. Also called weathercock stability.
directivity
   (NASA Thesaurus / NASA SP-7, 1965)
The ability of an antenna to radiate or receive more energy in some directions than in others. See beam.
The directivity of an antenna implies a maximum value, and it is equal to the ratio of the maximum field intensity to the average field intensity at a given distance.
directories
   (NASA Thesaurus)
Alphabetical, geographical, or classified listings by field of persons, organizations, programs and/or objects such as instruments, devices, and products. Use of this term excludes directories in computers.
directrix
   (NASA SP-7, 1965)
An auxiliary line used in the geometrical construction of a conic.
disasters
   (NASA Thesaurus)
Large-scale drought, glacier movement, floods, fires, storms, etc.
discharge correction factor
   (NASA SP-7, 1965)
(symbol zetad)
Of a rocket nozzle, the ratio of the mass flow rate in the nozzle to that of an ideal nozzle which expands an identical working fluid from the same initial conditions to the same exit pressure.
discharge tube
   (NASA SP-7, 1965)
A form of cold-cathode ionization gage in which the color and form of a cold-cathode discharge, without the presence of a magnetic field, gives an indication of the pressure and the nature of the gas.
discone antenna
   (NASA SP-7, 1965)
An antenna formed of a disk and a cone whose apex approaches and becomes common with the outer conductor or the coaxial feed at its extremity.
The center conductor terminates at the center of the disk which is perpendicular to the axis of the cone.
discontinuity
   (NASA Thesaurus)
A break in sequence or continuity of anything.
discontinuity
   (NASA SP-7, 1965)
A break in sequence or continuity of anything.
Discos (satellite attitude control)
   (NASA Thesaurus)
A satellite orbit "DIsturbance COmpensation System" designed to maintain an object (proof object) in correct orbit by detecting forces and compensating for them by using thrusters.
discrete
   (NASA SP-7, 1965)
Composed of distinct or discontinuous elements.
discrete address beacon system
   (NASA Thesaurus)
Radar beacon system with discretely addressable transponders and a ground-air-ground data link for automated air traffic control (FAA).
discrete aurora
   (Earth's Magnetosphere Glossary - GSFC)
Typical ribbon-like structures of aurora observed from the ground. From space they may appear as brighter spots in the diffuse aurora.
discrete radio source
   (NASA SP-7, 1965)
A source of small angular extent of cosmic radio waves.
discrete spectrum
   (NASA SP-7, 1965)
A spectrum in which the component wavelengths (and wave numbers and frequencies) constitute a discrete sequence of values (finite or infinite in number) rather than a continuum of values. See continuous spectrum.
discrete variable
   (NASA SP-7, 1965)
A quantity that may assume any one of a number of individually distinct or separate values.
discriminant analysis (statistics)
   (NASA Thesaurus)
A linear combination of a set of N variables that will classify (into two different classes) the events or items for which the measurements of the N variables are available, with the smallest proportion of misclassifications. Used for discriminant functions.
discriminator
   (NASA Thesaurus / NASA SP-7, 1965)
In general, a circuit in which output depends upon the difference between an input signal and a reference signal.
dish
   (NASA SP-7, 1965)
A parabolic reflector type of radio or radar antenna.
disk
   (High Energy Astrophysics Dictionary- GSFC)
The visible surface of the Sun (or any heavenly body) projected against the sky.
disk galaxies
   (NASA Thesaurus)
Galaxies consisting of a central bulge of a spheroidal aggregation of stars and a surrounding disk of stars fanning outward in a thin layer.
disk operating system (DOS)
   (NASA Thesaurus)
A program with which the computer performs such mundane but useful tasks as storing, locating, and retrieving files on disk, reading the keyboard, and issuing display and print information.
dislocation
   (NASA SP-7, 1965)
In crystallography, a type of lattice imperfection whose existence in metals is postulated in order to account for the phenomenon of crystal growth and of slip, particularly for the low value of shear stress required to initiate slip.
One section of the crystal adjacent to the slip plane is assumed to contain one more atomic plane that the section on the opposite side of the slip plane. Motion of the dislocation results in displacement of one of the sections with respect to another.
dispersion
   (NASA SP-7, 1965)
1. In rocketry (a) deviation from a prescribed flight path, (b) specifically, circular dispersion.
2. A measure of the scatter of data points around a mean value or around a regression curve.
Usually expressed as a standard-deviation estimate, or as a standard error of estimate. Note that the scatter is not centered around the true value unless systematic errors are zero.
3. The process in which radiation is separated into its component wavelengths.
Dispersion results when an optical process, such as diffraction, refraction, or scattering, varies according to wavelength.
4. In spectroscopy, a measure of the resolving power of a spectroscope or spectrograph, usually expressed in angstroms per millimeter.
5. As applied to materials, a scattering of very fine particles (e.g., ceramics) within the body of a metallic material usually resulting in overall strengthening of the composite material.
dispersion equation
   (NASA SP-7, 1965)
= frequency equation.
dispersive medium
   (NASA SP-7, 1965)
A medium in which the phase velocity of a wave, either electromagnetic or hydromagnetic, is a function of the frequency.
A plasma is a dispersive medium whereas free space is not, since waves of all frequencies travel in free space with the velocity of light.
displacement
   (NASA Thesaurus / NASA SP-7, 1965)
A vector quantity that specifies the change of position of a body or particle usually measured from the mean position or position of rest.
Displacement can be represented by a rotation vector or translation vector or both.
displacement manometer
   (NASA SP-7, 1965)
A differential manometer which indicates the pressure difference, if any, across a solid or liquid partition which can be displaced against a restoring force.
display
   (NASA SP-7, 1965)
The graphic presentation of the output data of any device or system.
dissociation
   (NASA Thesaurus / NASA SP-7, 1965)
The separation of a complex molecule into constituents by collision with a second body, or by absorption of a photon.
The product of dissociation of a molecule is two ions, one positively charged and one negatively charged.
dissociative recombination:
   (Plasma Physics and Fusion Energy Glossary)
The combination of an electron with a positive molecular ion, followed by dissociation of the molecule in which the resulting atoms/molecules carry off the excess energy released in the recombination.
dissolved gases
   (NASA Thesaurus)
Gases in solution.
distance marker
   (NASA SP-7, 1965)
A reference marker indicating distance, particularly such a marker on a radar indicator, to indicate distance of a target from the radar antenna. On a plan position indicator it is usually one of a series of concentric circles. Also called range marker. See range ring.
distance measuring equipment
   (NASA Thesaurus / NASA SP-7, 1965)
(abbr DME)
A radio aid to navigation which provides distance information by measuring total round-trip time of transmission from an interrogator to a transponder and return.
distorted-angle fabric
   (NASA SP-7, 1965)
Material of a special, often basketlike weave suitable for pressure suits. Such fabrics permit a certain amount of flexibility when the suit is pressurized.
distortion
   (NASA Thesaurus / NASA SP-7, 1965)
1. An undesired change in waveform.
Noise and certain desired changes in waveform, such as those resulting from modulation or detection, are not usually classed as distortion.
2. In a system used for transmission or reproduction of sound, a failure by the system to transmit or reproduce a received waveform with exactness.
3. An undesired change in the dimensions or shape of a structure as, distortion of a fuel tank due to abnormal stresses or extreme temperature gradients.
distributed feedback lasers
   (NASA Thesaurus)
Lasers containing a periodic medium which provides the necessary feedback for laser action.
distributed processing
   (NASA Thesaurus)
Processing with multiple small computers that are capable of operating independently but can communicate over a network with each other and/or a central computer.
distribution function
   (NASA Thesaurus / NASA SP-7, 1965)
The density function or number of particles per unit volume of phase space. The distribution function is a function of the three space coordinates and the three velocity coordinates.
A point in phase space represents a given position in ordinary space and a given velocity in velocity space. Therefore, the distribution function evaluated at such a point is the number or average density of particles per cubic length and cubic velocity that have the position and the velocity which is represented by the point. Distribution function represents the average density over a reasonably long time, or the most probable distribution of particles at a particular time.
diurnal
   (NASA SP-7, 1965)
Having a period of, occurring in, or related to a day.
diurnal aberration
   (NASA SP-7, 1965)
Aberration caused by the rotation of the earth. The value of diurnal aberration varies with the latitude of the observer and ranges from zero at the poles to 0.31 second of arc.
diurnal circle
   (NASA SP-7, 1965)
The apparent daily path of a celestial body, approximating a parallel of declination.
diurnal motion
   (NASA SP-7, 1965)
The apparent daily motion of a celestial body as observed from a rotating body.
diurnal variations
   (Glossary of Weather Terms for Storm Spotters - NOAA)
Daily; actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and diurnal falls at night).
divergence
   (Glossary of Weather Terms for Storm Spotters - NOAA)
The expansion or spreading out of a vector field; usually said of horizontal winds. convergence. Divergence at upper levels of the atmosphere enhances upward motion, and hence the potential for thunderstorm development (if other factors also are favorable).
divergence
   (NASA Thesaurus / NASA SP-7, 1965)
1. The expansion or spreading out of a vector field; also a precise measure thereof.
In mathematical discussion divergence is considered to include convergence, i.e., negative divergence.
2. A static instability of a lifting surface or of a body on a vehicle wherein the aerodynamic loads tending to deform the surface or body are greater than the elastic restoring forces.
divergence theorem
   (NASA SP-7, 1965)
The statement that the volume integral of the divergence of a vector, such as the velocity U, over the volume V is equal to the surface integral of the normal component of U over the surface s of the volume, often called the export through the closed surface, provided U and its derivatives are continuous and single-valued throughout V and s. This may be written
The triple integral of v nabla del dot V, d V equals the double line integral of sub s V  dot n d s
where n is a unit vector normal to the element of surface ds, and the symbol divergence theorem indicates that the integration is to be carried out over a closed surface. This theorem is sometimes called Green's theorem in the plane for the case of two-dimensional flow, and Green's theorem in space for the three-dimensional case described above. Also called Gauss theorem.
The divergence theorem is used extensively in manipulating the meteorological equations of motion and aerodynamic equations of motion.
divertors (fusion reactors)
   (Plasma Physics and Fusion Energy Glossary)
Component of a toroidal fusion device that diverts charged particles on the outer edge of the plasma into a separate chamber where they strike a barrier and become neutralized. In a reactor, the divertor would incorporate a system for pumping out the neutralized particles as exhaust from the machine. A divertor, like a limiter, prevents the particles from striking and degrading the chamber walls and dislodging secondary particles that would cool and contaminate the plasma. Whereas a limiter is a material object used to limit the shape of the plasma, a divertor is a magnetic-field construction. The advantage of the divertor is that it allows the neutralization region to be removed from the main plasma.
DLG standard format
   (Global Land Information System Glossary - USGS)
The DLG Standard Format is no longer distributed. Designed to minimize data storage, the topological linkages of the USGS DLG standard format are only contained in the line elements. The files are comprised of standard 8-bit ASCII characters organized into fixed length records of 144 bytes. Nine distinct record types are defined in this format. Coordinates are expressed as integer mils (one unit = .001") in a Cartesian coordinate system. The origin is positioned at the center of the DLG cell. The coordinate domain is limited to the range -32768 and +32767. These values must be transformed using coefficients stored in the header record of the file to convert to the original Albers Equal-Area coordinates.
DLG--Digital Line Graph
   (Global Land Information System Glossary - USGS)
A DLG is line map information in digital form. The DLG data files include information about planimetric base categories, such as transportation, hydrography, and boundaries.
DME (abbr)
   (NASA SP-7, 1965)
= distance measuring equipment.
DMSP satellites
   (NASA Thesaurus)
Satellites of the defense meteorological satellite program, a program sponsored by the United States Air Force System Command's Space Division which provides timely global imagery and specialized meteorological data for supporting a variety of Department of Defense operations. Used for Defense Meteorological Satellite Program.
Dobson spectrophotometer
   (NASA SP-7, 1965)
A photoelectric spectrophotometer which is used in the determination of the ozone content of the atmosphere. The instrument compares the solar energy at two wavelengths in the absorption band of ozone by permitting the two radiations to fall alternately upon a photocell. The stronger radiation is then attenuated by an optical wedge until the photoelectric system of the photometer indicates equality of incident radiation. The ratio of radiation intensity is obtained by this process and the ozone content of the atmosphere is computed from the ratio.
docking
   (NASA SP-7, 1965)
The act of coupling two or more orbiting objects; the operation of mechanically connecting together, or in some manner bringing together, orbital payloads.
document markup languages
   (NASA Thesaurus)
Standardized nomenclatures that specify the organization of complex text (including technical and scientific notation, graphics, and images) and define the document type, data elements within the document, and the relationship between data elements for electronically prepared, stored, interchangd, and published documents.
documentation
   (NASA Thesaurus)
The assembling, coding, and disseminating of recorded knowledge.
dodging
   (Global Land Information System Glossary - USGS)
Dodging is a process used to lighten areas of a photographic print during the main exposure so that the areas which need lightening receive less than the regular exposure. This process, which generally provides more image detail and reduces scene contrast, is performed by a skilled technician using their hands or a paddle over the area in need of less exposure.
doghouse
   (NASA SP-7, 1965)
Slang for a protuberance or blister that houses an instrument or instruments on an otherwise smooth skin of a rocket.
doghouses (electronics)
   (NASA Thesaurus)
Small enclosures placed at the base of transmitting antenna towers to house antenna tuning equipment.
dogleg
   (NASA SP-7, 1965)
A directional turn made in the launch trajectory to produce a more favorable orbit inclination, as in Echo I was launched on a dogleg to achieve an orbit inclined 47° to the equator.
dolomite (mineral)
   (NASA Thesaurus)
A common rock-forming rhombohedral material consisting of calcium, magnesium, and carbonates. It is used for refractory products.
domestic satellite communications systems
   (Global Land Information System Glossary - USGS)
A system that utilizes a geosynchronous satellite to re-broadcast satellite data received at a central reception and preprocessing center
donor
   (NASA SP-7, 1965)
In transistors, the N-type semiconductor, the electrode containing impurities which increase the number of available electrons. Contrast acceptor.
dopa
   (NASA Thesaurus)
An intermediate organic compound produced by oxidation of tyrosine by tyramine; also, an intermediate product in the synthesis of both epinephrine and melanin. Used for dihydroxyphenylalanine.
doping
   (NASA SP-7, 1965)
Addition of impurities to a semiconductor or production of a deviation from stoichiometric composition to achieve a desired characteristic.
Doppler broadening
   (Plasma Physics and Fusion Energy Glossary)
Frequency spreading which causes broadening of single-frequency radiation (e.g., spectral lines) when the radiating bodies (atoms, molecules, etc.) have different velocities. Radiation from each individual radiating body has a different Doppler shift, and the collection of radiations at different frequencies broadens the peak of the line in an intensity-vs-frequency plot.
Doppler broadening
   (NASA SP-7, 1965)
The broadening of either an emission line or an absorption line due to random motions of molecules of the gas that is emitting or absorbing the radiant energy. See pressure broadening.
In the case of an emitting gas, for example, those molecules which are approaching the observer as they emit quanta of radiant energy will, because of the Doppler effect, appear to send out a train of waves of slightly shorter wavelength than that characteristic of a stationary molecule, while receding molecules will appear to emit slightly longer wavelengths. The net effect, averaged over many molecules, is to superimpose, on the natural line width, a bell-shaped broadening that is proportional to the square root of the absolute temperature of the gas.
Doppler effect
   (NASA Thesaurus / NASA SP-7, 1965)
The change in frequency with which energy reaches a receiver when the receiver and the energy source are in motion relative to each other. Also called Doppler shift.
In the case of sound, or any other wave motion where a real medium of propagation exists (excepting, therefore, light and other electromagnetic radiations) one must distinguish two principal cases: If the source is in motion with speed v relative to a medium which propagates the waves in question at speed c, then the resting observer receives waves emitted, with actual frequency f as if they had a frequency f' given by the Doppler equation
f ' = f/[1 ± (v/c)]
where the positive sign refers to the case of the source receding from the observer, and the vice versa for the negative sign. If, on the other hand, the source is at rest relative to the propagating medium while the observer moves with speed v relative to the source,
f ' = f '[1± (v/c)]
where the positive sign now refers to the case of observer approaching the source. For electromagnetic radiation, f/f ' = [1minus/plus (v/c)]/[1 ± (v/c)] where the top signs represent the source receding from the observer and the bottom signs, approaching the observer.
Doppler error
   (NASA SP-7, 1965)
In using Doppler radar, the error in the measurement of target radial velocities due to atmospheric refraction. Compare range error, azimuth error.
These errors result from (a) the assumption of a constant wave velocity for a nonhomogeneous atmosphere and (b) the refraction or bending of the rays such that the ray path does not coincide with the geometrical straight line between the target and the radar. Errors due to (a) are of no practical importance, and, as in the case of elevation-angle error, the effects due to (b) are negligible except for elevation angles near the horizontal.
Doppler navigation
   (NASA Thesaurus / NASA SP-7, 1965)
Dead reckoning performed automatically by a device which gives a continuous indication of position by integrating the speed derived from measurement of the Doppler effect of echoes from directed beams of radiant energy transmitted from the craft. See Doppler radar.
Doppler radar
   (NASA Thesaurus)
Radar which utilizes the Doppler effect to determine the radial component of velocities of relative radar targets or to select targets having particular radial velocities.
Doppler radar
   (NASA SP-7, 1965)
A radar which detects and interprets the Doppler effect in terms of the radial velocity of a target.
Doppler ranging
   (NASA SP-7, 1965)
(abbr Doran)
A continuous-wave trajectory measuring system which utilizes the Doppler effect to measure the distances between a transmitter, a rocket transponder, and several receiving stations.
From these measurements trajectory data are computed. In contrast to less sophisticated systems, Doran obviates the necessity of continuously recording the Doppler signal by making simultaneous distance measurements with four different frequencies.
Doppler shift
   (NASA SP-7, 1965)
1. = Doppler effect.
2. The magnitude of the Doppler effect, measured in cycles per second.
Doppler system
   (NASA SP-7, 1965)
In radar, any system utilizing the Doppler effect for obtaining information.
Doppler-Fizeau effect
   (NASA Thesaurus / NASA SP-7, 1965)
The Doppler effect applied to a source of light. When the distance between the observer and the source of light is diminishing, the lines of the spectrum are displaced toward the violet, and, when the distance is increasing, they are displace toward the red, the displacement being proportional to the relative velocity of approach or recession.
Dor, Dora
   (NASA SP-7, 1965)
International Astronomical Union abbreviations for Dorado. See constellation.
Dorado
   (NASA SP-7, 1965)
(abbr Dor, Dora)
See constellation.
Doran (abbr)
   (NASA SP-7, 1965)
= Doppler ranging.
dorsal
   (NASA SP-7, 1965)
Pertaining to the back.
dosimeter
   (NASA SP-7, 1965)
1. An instrument for measuring the ultraviolet in solar and sky radiation. Compare actinometer.
2. A device, worn by persons working around radioactive material, which indicates the dose of radiation to which they have been exposed.
dosimeters
   (NASA Thesaurus)
Instruments for measuring the ultraviolet in solar and sky radiation. Devices worn by persons working around radioactive material, which indicate the dose of radiation to which thay have been exposed. Used for dosimetry.
dot product
   (NASA SP-7, 1965)
= scalar product.
double amplitude
   (NASA SP-7, 1965)
In vibration terminology, the total, or peak-to-peak, dimensional displacement of a vibrating structure.
double base propellants
   (NASA Thesaurus)
Solid rocket propellants using two unstable compounds, such as nitrocellulose and nitroglycerin. The unstable compounds used in a double based propellant do not require a separate oxidizer. Used for cordite.
double local oscillator
   (NASA SP-7, 1965)

An oscillator mixing system which generates two radio-frequency signals accurately spaced a few hundred cycles apart and mixes these signals to give the difference frequency which is used as the reference.
This equipment is used in an interferometer system to obtain a detectable signal containing the phase information of an antenna pair and the reference signal to allow removal of the phase data for use.
double precision
   (NASA SP-7, 1965)
Of a computer, capable of processing twice as many columns as the number of digits in the quantities usually processed. See precision.
double sheath
   (NASA SP-7, 1965)
See plasma sheath.
double stars
   (NASA Thesaurus / NASA SP-7, 1965)
Stars which appear as single points of light to the eye but which can be resolved into two point by a telescope.
A double star is not necessarily a binary, a two-star system revolving about a common center, but may be an optical double, two unconnected stars in the same line of sight.
double-base propellant
   (NASA SP-7, 1965)
A solid rocket propellant using two unstable compounds, such as nitrocellulose and nitroglycerin.
The unstable compounds used in a double-base propellant do not require a separate oxidizer.
double-dabble
   (NASA SP-7, 1965)
A technique for binary to decimal conversion. Starting with the most significant bit, proceed, bit-by-bit, as follows: If the next bit is 0, double what you have (double); if the next bit is 1, double what you have and add 1 (dabble).
Thus, 111 (binary) = 7 (decimal)
10111 (binary) = 23 (decimal).
double-entry compressor
   (NASA SP-7, 1965)
A centrifugal compressor that takes in air or fluid on both sides of the impeller, with vanes on each side to accelerate the fluid into the diffuser. The double-entry compressor is not a multistage compressor.
double-integrating gyro
   (NASA SP-7, 1965)
A single-degree-of-freedom gyro having essentially no restraint of its spin axis about the output axis. In this gyro an output signal is produced by gimbal angular displacement, relative to the base, which is proportional to the double integral of the angular rate of the base about the input axis.
doublet
   (NASA SP-7, 1965)
1. In fluid mechanics, a source and a sink of equal strength whose distance apart is zero.
2. In spectroscopy, two lines resulting from transitions from the same state.
Dovap (abbr)
   (NASA SP-7, 1965)
= Doppler, velocity and position.
Dovap elsse
   (NASA SP-7, 1965)
An elsse which utilizes the Dovap transponder as a signal source. Also called beat-beat Dovap.
down range
   (NASA SP-7, 1965)
The airspace extending downstream on a given rocket test range.
downbursts
   (Glossary of Weather Terms for Storm Spotters - NOAA)
Strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder.
downdrafts
   (Glossary of Weather Terms for Storm Spotters - NOAA)
Small-scale columns of air that rapidly sink toward the ground, usually accompanied by precipitation as in a shower or thunderstorm.
downlink
   (Space Flight Glossary - JPL)
Signal received from a spacecraft.
downlinking
   (NASA Thesaurus)
The transmission of signals (data, information, etc.) from satellites to ground terminals.
downrange
   (NASA Thesaurus)
The airspace extending downstream on a given rocket test range.
downtime
   (NASA Thesaurus / NASA SP-7, 1965)
A period during which equipment is not operating correctly because of machine failure.
DR (abbr)
   (NASA SP-7, 1965)
= dead reckoning.
Dra, Drac
   (NASA SP-7, 1965)
International Astronomical Union abbreviation for Draco. See constellation.
Draco
   (NASA SP-7, 1965)
(abbr Dra, Drac)
See constellation.
dracontic month
   (NASA SP-7, 1965)
The average period of revolution of the moon about the earth with respect to the moon's ascending node, a period of 27 days 5 hours 5 minutes 35.8 seconds, or approximately 27¼ days. Also called nodical month.
drag
   (NASA Thesaurus / NASA SP-7, 1965)
(symbol D)
A retarding force acting upon a body in motion through a fluid, parallel to the direction of motion of the body. It is a component of the total fluid forces acting on the body. See aerodynamic force.
drag coefficient
   (NASA SP-7, 1965)
(symbol CD)
A coefficient representing the drag on a given airfoil or other body, or a coefficient representing a particular element of drag. See Rayleigh formula.
drag coefficients
   (NASA Thesaurus)
The ratios of drag to the products of dynamic pressures and reference areas.
drag force anemometers
   (NASA Thesaurus)
Instruments for measuring both the static and dynamic velocity head and flow in high frequency, unsteady flow.
drag parachute
   (NASA SP-7, 1965)
1. = drogue parachute.
2. Any of various types of parachutes attached to high-performance aircraft that can be deployed, usually during landings, to decrease speed and also, under certain flight conditions, to control and stabilize the aircraft.
drainage basin
   (Global Land Information System Glossary - USGS)
Geographic area or region containing one or more drainage areas that discharge run-off to a single point.
draperies
   (NASA SP-7, 1965)
(abbr D)
See aurora.
dredged materials
   (NASA Thesaurus)
Sand, mud, silt, gravel, etc., recovered from the bottoms of harbors, canals, etc., during dredging operations.
dredging
   (NASA Thesaurus)
Mechanical or hydraulic excavation of underwater material. Used in maintaining and building of channels and ports as well as underwater mining of sand, gravel, and minerals.
drift
   (Earth's Magnetosphere Glossary - GSFC)
A magnetically trapped ion or electron moves as if it were attached to a magnetic field line. Drift is one of the features of such motion, namely its slow shift from one guiding field line to its neighbor. In the Earth´s magnetic field, such drifts gradually move particles all the way around Earth. Viewed from far above the north magnetic pole, ions drift around the Earth clockwise, electrons counter-clockwise, resulting in an electric current circling the Earth, the ring current.
drift
   (NASA SP-7, 1965)
1. The lateral divergence from the prescribed flight path of an aircraft, a rocket, or the like, due primarily to the effect of a crosswind.
2. A slow movement in one direction of an instrument pointer or other marker.
3. A slow change in frequency of a radio transmitter.
4. The angular deviation of the spin axis of a gyro from a fixed reference in space.
5. In semiconductors, the movement of carriers in an electric field.
drift mobility
   (NASA SP-7, 1965)
In a semiconductor, the average drift velocity of carriers per unit electric field.
In general, the mobilities of holes and electrons are different.
drift motion
   (Plasma Physics and Fusion Energy Glossary)
Ordinarily particles placed in a magnetic field will simply orbit in circles, but if the magnetic field is not uniform, or curves, or there is an electrical field perpendicular to the magnetic field, or another force is applied perpendicular to the magnetic field, then the "guiding centers" of the particle orbits will drift (generally perpendicular to the magnetic field and to the applied force).
drift rate
   (NASA Thesaurus / NASA SP-7, 1965)
The amount of drift, in any of its several senses, per unit time.
Drift rate has many specific meanings in different fields. The type of drift rate should always be specified.
drift velocity
   (NASA SP-7, 1965)
The average velocity of a charged particle in a plasma in response to an applied electric field.
The motion of an individual particle is quite erratic as it bounces off other particles and has its direction continually changed. On the average, however, a particle will slowly work its way in the direction of the applied electric force. This velocity is usually much smaller than the random velocity of the particle between collisions.
drogue
   (NASA SP-7, 1965)
1. A device, usually shaped like a funnel or cone, dragged or towed behind something and used, e.g., as a sea anchor.
2. A funnel-shaped part at the end of the hose of a tanker aircraft, used in air refueling to drag the hose out and stabilize it and to receive the probe of the receiving aircraft.
3. = drogue parachute.
drogue parachute
   (NASA SP-7, 1965)
1. A type of parachute attached to a body, used to slow it down; also called deceleration parachute or drag parachute.
2. A parachute used specifically to pull something, usually a larger parachute, out of stowage, as, a drogue parachute deploys a drag parachute.
drogue recovery
   (NASA SP-7, 1965)
A type of recovery system for space vehicles or space capsules after initial reentry into the atmosphere using deployment of on ore more small parachutes to diminish speed, to reduce aerodynamic heating, and to stabilize the vehicle so that larger recovery parachutes can be safely deployed at lower altitudes without too great an opening shock.
drone
   (NASA SP-7, 1965)
A remotely controlled aircraft.
drone aircraft
   (NASA Thesaurus)
Remotely controlled aircraft. Used for drone helicopters.
drooped airfoils
   (NASA Thesaurus)
A baseline airfoil with an abrupt change in cross-section at about midspan from the fuselage. The outboard portion of the wing has a cross-section with a nearly flat bottom and a drooped (downward) leading edge in relation to the inboard baseline wing.
drop size
   (NASA Thesaurus)
The diameter of a drop if it is approximately spherical; otherwise, the approximate shape and appropriate dimensions must be described.
drop towers
   (NASA Thesaurus)
Large devices for low gravity processing of molten material which consist of either a capsule which is dropped, or a drop tube where containerless low gravity studies are conducted or both. Used for drop tubes.
dropout
   (NASA SP-7, 1965)
Any discrete variation in signal level during the reproduction of recorded data which results in a data-reduction error.
dropouts
   (NASA Thesaurus)
Discrete variations in signal levels during the reproduction of recorded data which result in data reduction errors.
drops (liquids)
   (NASA Thesaurus)
Small bodies of liquid held together primarily by surface tension. Used for liquid drops.
dropsonde
   (NASA Thesaurus / NASA SP-7, 1965)
A radiosonde equipped with a parachute, dropped from an aircraft to transmit measurements of atmospheric conditions as it descends.
dry adiabat
   (NASA SP-7, 1965)
A line of constant potential temperature on a thermodynamic diagram. In terms of pressure p , and specific volume v , the equation for a dry adiabat may be written
pvcp /cv = Constant
where cp and cv are the specific heats of dry air at constant pressure and volume, respectively.
Meteorologically, the dry adiabat is intended to represent the lifting of dry air in a dry-adiabatic process. Since this is also an isentropic process, a dry adiabat is an isentrope.
dry emplacement
   (NASA SP-7, 1965)
A launch emplacement that has no provision for water cooling during launch. Compare wet emplacement.
dry friction damping
   (NASA SP-7, 1965)
= Coulomb damping.
dry line
   (Glossary of Weather Terms for Storm Spotters - NOAA)
A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west). The dry line typically advances eastward during the afternoon and retreats westward at night. However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even further east, regardless of the time of day.
dry weight
   (NASA SP-7, 1965)
The weight of a rocket vehicle without its fuel. Compare take-off weight.
This term, appropriate especially for liquid rockets, is sometimes considered to include the payload.
dry-adiabatic atmosphere
   (NASA SP-7, 1965)
= adiabatic atmosphere.
dry-adiabatic lapse rate
   (NASA SP-7, 1965)
The rate of decrease of temperature with height of a parcel of dry air lifted adiabatically through the earth's atmosphere in hydrostatic equilibrium.
This lapse rate is g/cp, where g is the acceleration of gravity and cp is the specific heat of dry air at constant pressure; numerically equal to 9.767° C per kilometer or about 5.4° F per thousand feet.
Potential temperature is constant with height in an atmosphere with this lapse rate.
DSCC
   (Space Flight Glossary - JPL)
Deep Space Communications Complex, one of three DSN tracking sites at Goldstone, California; Madrid, Spain; and Canberra, Australia; spaced about equally around the Earth for continuous tracking of deep-space vehicles.
DSIF (abbr)
   (NASA SP-7, 1965)
Deep Space Instrumentation Facility. A worldwide network of tracking stations operated for the NASA by the Jet Propulsion Laboratory.
DSN
   (Space Flight Glossary - JPL)
Deep Space Network
DSS
   (Space Flight Glossary - JPL)
Deep Space Station, the antenna front-end equipment at DSCCs.
DTM--Digital Terrain Model
   (Global Land Information System Glossary - USGS)
A DTM is a land surface represented in digital form by an elevation grid or lists of three-dimensional coordinates.
dual modulation
   (NASA SP-7, 1965)
The process of modulating a single carrier wave or subcarrier by two different types of modulation (e.g., amplitude and frequency-modulation) each conveying separate information.
dual thrust
   (NASA SP-7, 1965)
A rocket thrust derived from two propellant grains using the same propulsion section of a missile.
The dual-thrust technique is considered to provide what is in effect a two-stage propulsion system without the disadvantages of jettisoning the booster unit and with the advantages of lower weight and shorter length.
dual wing configurations
   (NASA Thesaurus)
A configuration of two wings of nearly the same planform and area, one behind the other.
dual-thrust motor
   (NASA SP-7, 1965)
A solid propellant rocket engine built to obtain dual thrust.
In a single chamber unit the booster propellant grain may be bonded to the sustainer grain, with the thrust level regulated by mechanically changing the nozzle throat area or by using different grain compositions or configurations. In a dual-chamber unit, the separate chambers may be arranged in tandem, or concentrically.
duality principle
   (NASA Thesaurus)
Principle that for any theorem in electric circuit analysis there is a dual theorem in which quantities are replaced with dual quantities. Examples are current and voltage or impedance and admittance.
duality theorem
   (NASA Thesaurus)
Theorem which states that if either of two dual linear programming problems has a solution, then so does the other.
duct
   (NASA SP-7, 1965)
Specifically, a tube or passage that confines and conducts a fluid, as a passage for the flow of air to the compressor of a gas-turbine engine, a pipe leading air to a supercharger, etc.
duct geometry
   (NASA Thesaurus)
The shape and dimensions of ports or other openings designed for passage of fluids (gases, liquids, or mixtures) in or external to engines.
duct propulsion
   (NASA SP-7, 1965)
A means of propelling a vehicle by ducting a surrounding fluid through an engine, adding momentum by mechanical or thermal means, and ejecting the fluid to obtain a reactive force. Compare rocket propulsion.
ducted fan
   (NASA Thesaurus / NASA SP-7, 1965)
1. A fan enclosed in a duct.
2. = ducted-fan engine.
ducted fan engines
   (NASA Thesaurus)
Aircraft engines incorporating a fan or propeller enclosed in a duct; especially jet engines in which an enclosed fan or propeller is used to ingest ambient air to augment the gases of combustion in the jetstream.
ducted rocket
   (NASA SP-7, 1965)
= rocket ramjet.
ducted-fan engine
   (NASA SP-7, 1965)
An aircraft engine incorporating a fan or propeller enclosed in a duct; especially, a jet engine in which an enclosed fan or propellant is used to ingest ambient air to augment the gases of combustion in the jetstream.
The air may be taken in at the front of the engine and passed around the combustion section, or it may be taken in aft of the combustion chamber. In the former case the ducted fan may be considered a type of bypass engine.
ducting
   (NASA SP-7, 1965)
The trapping of an electromagnetic wave, in a waveguide action, between two layers of the atmosphere, or between a layer of the atmosphere and the earth's surface. See refractivity, total refraction.
Ducting is likely to occur where the gradient of the index of refraction exceeds 48 N-units per 1000 feet of altitude.
ducts
   (NASA Thesaurus)
Specifically, tubes or passages that confine and conduct fluids, as passages for the flow of air to compressors of gas turbine engines, or pipes leading air to superchargers.
dummy
   (NASA SP-7, 1965)
1. In computer operations, an artificial and intrinsically useless unit of information inserted solely to fulfill certain prescribed conditions such as word length or block length.
2. In rocketry, an inert stage, i.e., no propellant.
dummy antenna
   (NASA SP-7, 1965)
A device which has the necessary impedance characteristics of an antenna and the necessary power-handling capabilities, but which does not radiate or receive radio waves. Also called artificial antenna.
In receiver practice, that portion of the impedance not included in the signal generator is often called dummy antenna.
dump
   (NASA SP-7, 1965)
In computer operations, (a) to destroy intentionally or accidentally stored information, (b) to transfer all or part of the contents of one section of storage into another section.
dump combustors
   (NASA Thesaurus)
Combustors having a means of reducing flow velocity and forming recirculation zones through the sudden enlargement area between the inlet duct and the combustion chamber.
dunes
   (NASA Thesaurus)
Low mounds, ridges, banks, or hills of loose, windblown granular material, usually sand, capable of movement. Used for barchans, coastal dunes, and sand dunes.
duplex operation
   (NASA Thesaurus / NASA SP-7, 1965)
The operation of associated transmitting and receiving apparatus in which the processes of transmission and reception are concurrent. Compare diplex transmission.
duplexer
   (NASA Thesaurus / NASA SP-7, 1965)
A device which permits a single antenna system to be used for both transmitting and receiving.
Duplexer should not be confused with diplexer, a device permitting an antenna system to be used simultaneously or separately by two transmitters.
dust
   (NASA Thesaurus)
An imprecise term referring to particulates capable of temporary suspension in air or other gases - also particles smaller than an arbitrary selected size.
dust
   (NASA SP-7, 1965)
In meteor terminology, finely divided solid matter, with particle sizes in general smaller than micrometeorites, as meteoric dust, meteoritic dust.
duty factor
   (NASA SP-7, 1965)
1. In computer operations, the ratio of active time to total time.
2. In a pulse carrier composed of pulses that recur at regular intervals, the product of the pulse duration and the pulse repetition frequency.
duty ratio
   (NASA SP-7, 1965)
In a pulse radar or similar system the ratio of average to peak pulse power.
dwarf galaxies
   (NASA Thesaurus)
Galaxies with low luminosity.
dwarf novae
   (NASA Thesaurus)
Short period binary systems in which a red quasi-main sequence star fills its Roche lobe and transfers matter, via an accretion disk, onto a white dwarf.
dye lasers
   (Plasma Physics and Fusion Energy Glossary)
A type of laser in which the active material (the material which emits the laser light) is a dye. These lasers are tunable when the dye has very large molecules (such as acridine red or esculin) and the laser action takes place between the first excited and ground electronic states, because each of these states contains a broad continuum band of vibrational-rotational levels.
dye marker
   (NASA SP-7, 1965)
A substance which, when placed in water, spreads out and colors the water immediately surrounding so as to make a spot readily visible from the air.
dynamic balance
   (NASA SP-7, 1965)
The condition which exists in a rotating body when the axis about which it is forced to rotate, or to which reference is made, is parallel with a principal axis of inertia. No products of inertia about the center of gravity of the body exist in relation to the selected rotational axis.
Dynamic unbalance may be expressed in terms of slug-foot squared (or equivalent weight * length squared units) or in terms of inclination of the principal axis from the reference axis.
dynamic height
   (NASA SP-7, 1965)

The height of a point in the atmosphere expressed in a unit proportional to the geopotential at that point. Since the geopotential at altitude z is numerically equal to the work done when a particle of unit mass is lifted from sea level up to this height, the dimensions of dynamic height are those of potential energy per unit mass. Also called geodynamic height.
The standard unit of dynamic height Hd is the dynamic meter (or geodynamic meter), defined as 10 meters per second squared; it is related to the geopotential lower case phi, the geometric height z in meters, and the geopotential height Z in geopotential meters by
dlower case phi= 10d Hd = 9.8dZ = gdz
where g is the acceleration of gravity in meters per second squared. (Some sources prefer to give the constants 10 and 9.8 the units of meters per second squared so that the units of lower case phi and Z would be the same as those of the geometric height.) The dynamic meter is about 2 percent longer than the geometric meter and the geopotential meter. One of the practical advantages of the dynamic height over the geometric height is that when the former is introduced into the hydrostatic equation the variable acceleration of gravity is eliminated. In meteorological height calculations, geopotential height is more often used than dynamic height.
dynamic load
   (NASA Thesaurus / NASA SP-7, 1965)
A load imposed by dynamic action, as distinguished from a static load. Specifically, with respect to aircraft, rockets, or spacecraft, a load due to an acceleration of craft, as imposed by gusts, by maneuvering, by landing, by firing rockets, etc.
dynamic meteorology
   (NASA SP-7, 1965)
The study of atmospheric motions as solutions of the fundamental equations of hydrodynamics or other systems of equation appropriate to special situations.
dynamic meter
   (NASA SP-7, 1965)
The unit of measurement of dynamic height. Also called geodynamic meter.
dynamic model
   (NASA SP-7, 1965)
A model of an aircraft or other object having its linear dimensions and its weight and moments of inertia reproduced in scale in proportion to the original.
dynamic models
   (NASA Thesaurus)
Models of aircraft or other objects having their linear dimensions and weight and moments of inertia reproduced in scale in proportion to the original.
dynamic parallax
   (NASA SP-7, 1965)
A value for the parallax of a binary star computed from the observations of the period and angular dimensions of the orbit by assuming a value for the mass of the binary system. Also called hypothetical parallax.
dynamic pressure
   (NASA Thesaurus / NASA SP-7, 1965)
(symbol q)
The pressure of a fluid resulting from its motion, equal to one-half the fluid density times the fluid velocity squared (1/2pV2). In incompressible flow, dynamic pressure is the difference between total pressure and static pressure. Also called kinetic pressure. Compare impact pressure.
dynamic range
   (NASA Thesaurus)
The range of a signal detector or transmitter between the smallest and largest detectable signal levels which can be detected without inducing changes in its gain characteristics; usually expressed in decibels.
dynamic response
   (NASA SP-7, 1965)
= frequency response.
dynamic scale
   (NASA SP-7, 1965)
The scale of the flow about a model relative to a flow about its prototype.
If two such flows have the same Reynolds number, both flows are said to be at the same dynamic scale.
dynamic similarity
   (NASA SP-7, 1965)
The relationship existing between a model and its prototype when, by virtue of similarity between their geometric dimensions and mass distributions or elastic characteristics, the motion of the model in some respect (such as linear velocity, acceleration, vibration, flutter, etc.) is similar to the motion of the prototype; also, the similarity between the fluid flow about a scale model and its prototype when the flows have the same Reynolds number.
dynamic stability
   (NASA Thesaurus / NASA SP-7, 1965)
The characteristics of a body, such as an aircraft or rocket, that causes it, when disturbed from an original state of steady flight or motion, to damp the oscillations set up by restoring moments and gradually return to its original state; specifically, the aerodynamic characteristics.
dynamic storage
   (NASA SP-7, 1965)
In computer operations, information storage in which the information is continuously changing position, as, for example, delay-line storage, or magnetic-drum storage.
In computer programming, memory allocated and deallocated as needed on the fly, also dynamic memory allocation.
dynamic viscosity
   (NASA SP-7, 1965)
Of a fluid, the ratio of the shearing stress to the shear of the motion. It is independent of the velocity distribution, the dimensions of the system, etc., and for a gas it is independent of pressure except at very low pressures. Also called coefficient of molecular viscosity, coefficient of viscosity.
For the dynamic viscosity µ of a perfect gas, the kinetic theory of gases gives
mu equals one over three open parens rho c I bar close parens
where is the gas density, c is the average speed of the random heat motion of the gas molecules and is proportional to the square root of the temperature, and bar l is the mean free path. For dry air at 0° C, the dynamic viscosity is about 1.7 * 10-4 gram per centimeter per second.
Whereas the dynamic viscosity of most gases increases with increasing temperature, that of most liquids, including water, decreases rapidly with increasing temperature.
dynamical friction
   (NASA SP-7, 1965)
1. The drag force between electrons and ions drifting with respect to each other.
In a fully ionized plasma, collisions between electrons and ions are small angle Coulomb collisions and they produce a velocity-dependent drag force which is called dynamical friction.
2. Sliding friction, in contrast to static friction.
dynamical mean sun
   (NASA SP-7, 1965)
A fictitious sun conceived to move eastward along the ecliptic at the average rate of the apparent sun. See mean sun.
The dynamical mean sun and the apparent sun occupy the same position in January, when the earth is at perihelion.
dynamics
   (NASA Thesaurus)
Study of the motion of a system of material particles under the influence of forces, especially those which originate outside the system under consideration.
Dynamics Explorer 1 satellite
   (NASA Thesaurus)
A twin satellite of Dynamics Explorer 2 satellite designed to study the magnetosphere, ionosphere, and atmosphere coupling.
Dynamics Explorer 2 satellite
   (NASA Thesaurus)
A twin satellite of Dynamics Explorer 1 satellite designed to study the magnetosphere, ionosphere, and atmosphere coupling.
Dynamics Explorer satellites
   (NASA Thesaurus)
Two satellites that have been designed to occupy different orbits and supply comparative data for studying the boundary region between earth and space. Of the 24 goals of the program, one half require both satellite's data, one fourth one satellite's data and one fourth the other satellite's data. The satellites were launched together in August of 1981.
dynamo process
   (Earth's Magnetosphere Glossary - GSFC)
The generation of an electric currents by the flow of an electrically conducting fluid through a magnetic field. For instance, the magnetic field originating inside the Earth is believed to come from a dynamo process involving the flow of molten iron in the Earth´s hot core. The energy required to create the current is obtained from the motion of the flow.
dynamo theory
   (AS&T Dictionary)
A hypothesis which states that daily variations in the Earth's magnetic field are the result of the movement of ionized air in tides across that magnetic field and that produces currents in the lower ionosphere.
dynamo theory
   (NASA SP-7, 1965)
The hypothesis, first proposed by Balfour Stewart, which explains the regular daily variations in the earth's magnetic field in terms of electrical currents in the lower ionosphere, generated by tidal motions of ionized air across the earth's magnetic field.
dynamometer
   (NASA Thesaurus / NASA SP-7, 1965)
An instrument for measuring power or force; specifically, an instrument for measuring the power, torque, or thrust of an aircraft engine or rocket. See thrust meter.
dynamotor
   (NASA SP-7, 1965)
A machine combining motor and generator action in a single magnetic field, either with two armatures or with one armature having two separate windings.
dyne
   (Space Flight Glossary - JPL)
A unit of force equal to the force required to accelerate a 1-g mass 1 cm per second per second (1cm/sec). Compare with Newton.
dyne
   (NASA SP-7, 1965)
That unbalanced force which acting for 1 second on a body of 1 gram mass produces a velocity change of 1 centimeter per second.
The dyne is the unit of force in the CGS system.
dynode
   (NASA SP-7, 1965)
In an electron tube, an electrode which performs a useful function by means of secondary emission.
dysbarism
   (NASA SP-7, 1965)
A condition of the body resulting from the existence of a pressure differential between the total ambient pressure and the total pressure of dissolved and free gases within the body tissues, fluids, and cavities.
Characteristic symptoms, other than hypoxia, caused by decreased barometric pressure are bends and abdominal gas pains at altitudes above 25,000 to 30,000 feet. Increased barometric pressure, as in descent from high altitude is characterized by painful distention of the ear drums.
dyspnea
   (NASA Thesaurus / NASA SP-7, 1965)
Difficult or labored breathing.
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