Aerospace Science and Technonlogy Dictionary U Section

U-figure: = U-index.
U-index: The difference between consecutive daily mean values of the horizontal component of the geomagneticfield. Also called U-figure. See u-index.
Each value is derived from a 48-hour interval covering 2 Greenwich-mean-time days, and is assigned to the second day of the pair. The monthly U-index, the mean of the daily values, is the most frequently used.
u-index: The value u = U / sin A cos B where U is the U-index; A is the magnetic colatitude; and B is the angle between the magnetic meridian and the horizontal component of the magnetic field intensity.
The annual and longer period mean values of the u-index exhibit one of the strongest solar and terrestrial activity relationships known.
Udop (abbr): = UHF Dovap.
UHF Dovap: (abbr Udop)
A Dovap type trajectory measuring system operating in the UHF band. See Doppler, velocity, and position.
Uhuru satellite: NASA's first Small Astronomy Satellite, also known as SAS-1.
ullage: The amount that a container, such as a fuel tank, lacks of being full.
ullage rocket: A small rocket used in space to impart an acceleration to a tank system to insure that the liquid propellants collect in the tank in such a manner as to flow properly into the pumps or thrust chamber.
ULS: Ulysses spacecraft.
ultimate pressure: The limiting pressure approached in a vacuum system after sufficient pumping time to establish that further reductions in pressure will be negligible. Also called the ultimate vacuum.
The terms blank-off pressure or base pressure are also sometimes used in referring to a pump under test.
ultimate strength: The maximum conventional stress (tensile, compressive, or shear) that a material can withstand.
ultimate vacuum: = ultimate pressure.
ultrahigh frequency: (abbr UHF)
See frequency band.
ultrahigh vacuum: See vacuum.
ultrahigh-speed motion-picture photography: Picture taking at a frequency range above 10,000 pictures per second.
ultralight aircraft: An aircraft for one person weighing less than 254 pounds with a top speed of 55 knots and a maximum stalling speed of 24 knots.
ultrasonic: In acoustics, of or pertaining to frequencies above those that affect the human ear, i.e., more than 20,000 vibrations per second.
The term ultrasonic may be used as a modifier to indicate a device or system intended to operate at an ultrasonic frequency, as an ultrasonic vibrator. Supersonic was formerly used in acoustics synonymously with ultrasonic; this usage is now rare.
ultrasonic densimeters: Density measuring instruments utilizing ultrasonic devices (sensors).
ultrasonic frequency: A frequency lying above the audiofrequency range. The term is commonly applied to elastic waves propagated in gases, liquids, or solids. See sound.
ultrasonic processing: The use of ultrasonic radiation to synthesize a compound or material, or alter the structure, properties, or form of a material.
ultrasonic wave: See sound wave.
ultrasonics: The technology of sound at frequencies above the audio frequency range.
Supersonics was once used in acoustics synonymously with ultrasonics. This usage is now rare.
ultraviolet: (abbr UV)
Pertaining to or same as ultraviolet radiation.
ultraviolet astronomy: Use of special optical instruments for the observation of astronomical phenomena in the ultraviolet spectrum.
Ultraviolet Coronagraph Spectrometer: An ultraviolet spectrometer aboard SOHO.
ultraviolet radiation: Electromagnetic radiation of shorter wavelength than visible radiation; roughly, radiation in the wavelength interval from 100 to 4000 angstroms. Also called ultraviolet. See X-ray, note.
Ultraviolet radiation from the sun is responsible for many complex photochemical reactions characteristic of the upper atmosphere; e.g., the formation of the ozone layer through ultraviolet dissociation of oxygen molecules followed by recombination to form ozone.
ultraviolet telescopes: Optical telescopes designed to collect ultraviolet light (wavelengths not capable of passing through Earth's atmosphere) and as such must be used in space.
Ulysses mission: A proposed ESA/NASA mission using the STS for orbital launching of two spin-stabilized spacecraft equipped with instruments for solar and astrophysical observations. Used for International Solar Polar Mission.
Uma, U Maj: International Astronomical Union abbreviations for Ursa Major. See constellation.
umbilical: Short for umbilical cord. Often used in the plural, umbilicals.
umbilical cord: Any of the servicing electrical or fluid lines between the ground or a tower and an uprighted rocket vehicle before the launch. Often shortened to umbilical.
umbilical tower: A vertical structure supporting the umbilical cords running into a rocket in launching position.
umbra: 1. The darkest part of a shadow in which light is completely cut off by an intervening object. A lighter part surrounding the umbra, in which the light is only partly cut off, is called the penumbra.
2. The darker central portion of a sun spot, surrounded by the lighter penumbra.
Umbriel: A satellite of Uranus at a mean distance of 267,000 kilometers.
Umi, U Min: International Astronomical Union abbreviations for Ursa Minor. See constellation.
Umkehr effect: Due to the presence of the ozone layer, an anomaly of the relative zenith intensities of scattered sunlight at certain wavelengths in the ultraviolet as the sun approaches the horizon.
uncontrolled reentry (spacecraft): The descent into a denser atmosphere of a spacecraft in an elliptical orbit due to aerodynamic drag and other perturbation forces. The gradually increasing deceleration causes some kinetic energy to be converted into atmospheric heat. The centrifugal force decreases and gravity pulls the spacecraft further into the atmosphere. The spacecraft eventually burns.
uncoupled mode: A mode of vibration that can exist in a system concurrently with and independently of other modes.
undamped natural frequency: Of a mechanical system, the frequency of free vibration resulting from only elastic and inertial forces of the system.
under surface blowing: Use of jets blowing on the underside of airfoils for variations in pressure distribution.
underbreathing: = hypoventilation.
underdeck spray: That part of a pad deluge in which the water is directed upward from under the rocket.
underground acoustics: The sounding of subsoils, rocks, etc., for mineralogy and other exploratory purposes.
underground structures: Subterranean construction of tunnels, passageways, chambers, or excavations.
underwater physiology: The study of the bodily responses to the environmental stresses of the underwater milieu such as pressure, temperature and immersion effects.
underwater resources: Earth resources (minerals, petroleum, etc.) within or under the oceans.
unidirectional antenna: An antenna which has a single well-defined direction of maximum gain.
unified field theory: Any theory which attempts to express gravitational theory and electromagnetic theory within a single unified framework; usually, an attempt to generalize Einstein's general theory of gravitation alone to a theory of gravitation and classical electromagnetism.
unilateral transducer: A transducer that cannot be actuated as its output by waves in such a manner as to supply related waves at its inputs.
union: In Boolean algebra, the operation in which concepts are described by stating that they have the characteristics of one or more of the classes involved. Union is expressed as OR.
unipole: A hypothetical antenna radiating or receiving equally in all directions. Also called isotropic antenna.
A pulsating sphere is a unipole for sound waves. In the case of electromagnetic waves unipoles do not exist physically but represent convenient reference antennas for expressing directive properties of actual antennas.
unit hydrograph: The hydrograph of one inch of direct (surface plus interflow) runoff resulting from rainfall uniformly distributed over the drainage basin and of uniform intensity throughout a specified period of time.
units of measurement: Precisely stated quantities in terms of which the magnitudes of other quantities of the same kind can be stated. f shichrms of which the magnitude of other Precisely specified quantities in te
Unity connecting module: Component of the International Space Station providing six ports that serve as connecting points for other station modules and framework elements.
universal constant of gravitation: The constant of proportionality in Newton's law of universal gravitation and which plays an analogous role in A. Einstein's general relativity.
universal gas constant: See gas constant.
universal gravitational constant: See gravitation.
universal time: (abbr UT)
Time defined by the rotational motion of the earth and determined from the apparent diurnal motions which reflect this rotation; because of variations in the rate of rotation, universal time is not rigorously uniform. Also called Greenwich mean time. Compare ephemeris time.
In the years preceding 1960 the arguments of the ephemerides in the American Ephemeris and Nautical Almanac were designated as universal time.
universal transmission function: A mathematical relationship that attempts to describe quantitatively the complex infrared propagation (including absorption and reradiation) in the atmosphere.
universe: In statistical terminology, = population.
unsaturation (chemistry): A state in which the atomic bonds of an organic compound's chain or ring are not completely satisfied (not saturated); unsaturation usually results in a double bond (as for olefins) or a triple bond (as for the acetylens).
unstructured grids (mathematics): In computational fluid dynamics, grid systems where the flowfield is discretized into triangular-shaped elements for two-dimensional fields, and tetrahedral elements for three-dimensional fields. In this type of grid system the grid points cannot be associated with grid lines.
up-converters: Parametric amplifiers characterized by the output signal frequencies being greater than the frequencies of the input signals.
updrafts: Small-scale currents of rising air.
uplink: Signal sent to a spacecraft.
uplinking: The transmission of signals from ground terminals to satellites in telecommunication systems.
upper air: In synoptic meteorology and in weather observing, that portion of the atmosphere which is above the lower troposphere. Compare upper atmosphere.
No distinct lower limit is set but the term is generally applied to the levels above 850 millibars.
upper air observation: A measurement of atmospheric conditions aloft, above the effective range of a surface weather observation. Also called sounding, upper air sounding. See radiosonde.
This is a general term, but is usually applied to those observations which are used in the analysis of upper air charts (as opposed to measurements of upper atmospheric quantities primarily for research).
upper air sounding: = upper air observation.
upper atmosphere: The general term applied to the atmosphere above the troposphere. Compare upper air.
For subdivision of the upper atmosphere, see atmospheric shell.
upper branch: That half of a meridian or celestial meridian from pole to pole which passes through a place or its zenith.
upper culmination: = upper transit.
upper limb: That half of the outer edge of a celestial body, especially the moon, having the greatest altitude in contrast with the lower limb, that half having the least altitude.
upper stage: A second or later stage in a multistage rocket.
upper surface blowing: Use of jet blowing on the upper surface of airfoils to create variations in pressure distribution.
upper transit: Transit of the upper branch of the celestial meridian. Also called superior transit, upper culmination. Transit of the lower branch is called lower transit.
upslope flow: Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.
upstream: Toward the source of the flow, or located in the area from which the flow is coming.
upwelling water: The process by which water rises from a deeper to a shallower depth. Used for upwelling.
uranium: A radioactive metallic element whose isotope, uranium-235, is a nuclear fission fuel. Plutonium, another fission fuel, can be produced from the more plentiful isotope uranium-238.
Uranus: See planet, table.
Uranus (planet): Seventh planet from the sun, a gas giant or Jovian planet.
Uranus atmosphere: The atmosphere of the planet Uranus.
Uranus rings: Ring structures encircling the planet Uranus and similar to those of the planet Saturn.
Ursa Major: (abbr Uma, U Maj)
See constellation.
Ursa Minor: (abbr Umi, U Min)
See constellation.
URSI (abbr): International Scientific Radio Union. Usually pronounced as a word.
USNC-IGY (abbr): U.S. National Committee for the International Geophysical Year, National Academy of Sciences.
USO: Ultra Stable Oscillator, in a spacecraft telecommunications subsystem.
UT (abbr): = universal time.
UTC: Universal Time, Coordinated.
UV: Ultraviolet (meaning "above violet") radiation. Electromagnetic radiation in the neighborhood of 100 nanometers wavelength.
UV (abbr): = ultraviolet radiation.
UVCS: See Ultraviolet Coronagraph Spectrometer