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- Udop (abbr) = UHF Dovap.
- U-figure = U-index.
- UHF Dovap (abbr Udop)
- A Dovap
measuring system operating in the UHF band. See Doppler, velocity, and
- The difference between consecutive daily mean values of the horizontal
component of the geomagnetic field.
Also called U-figure. See
- 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.
- 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
- The annual and longer period mean values of the u-index exhibit one of
the strongest solar and terrestrial activity relationships known.
- 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.
- 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
- ultrahigh frequency (abbr UHF)
- See frequency
- ultrahigh-speed motion-picture photography
- Picture taking at a frequency range above 10,000 pictures per second.
- ultrahigh vacuum
- See vacuum.
- In acoustics, of or pertaining to frequencies
above those that affect the human ear, i.e., more than 20,000 vibrations per
- 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 frequency
- A frequency lying above the audiofrequency
range. The term is commonly applied to elastic waves propagated in gases,
liquids, or solids. See sound.
- 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.
- ultrasonic wave
- See sound
- ultraviolet (abbr UV)
- Pertaining to or same as ultraviolet
- ultraviolet radiation
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.
- Uma, U Maj
- International Astronomical Union abbreviations for Ursa Major. See
- 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
- 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.
- A satellite of
Uranus at a mean distance of 267,000 kilometers.
- Umi, U Min
- International Astronomical Union abbreviations for Ursa Minor. See
- 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.
- uncoupled mode
- A mode of
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.
- underbreathing = hypoventilation.
- underdeck spray
- That part of a pad deluge
in which the water is directed upward from under the rocket.
- unidirectional antenna
- An antenna which
has a single well-defined direction of maximum gain.
- 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.
- 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.
- A hypothetical antenna
radiating or receiving equally in all directions. Also called isotropic
- 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
- In statistical terminology, = population.
- UV (abbr) = ultraviolet
- universal gas constant
- See gas
- universal gravitational constant
- See gravitation.
- universal time (abbr UT)
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
- 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
- upper air
- In synoptic
meteorology and in weather observing, that portion of the atmosphere
which is above the lower troposphere. Compare upper
- 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.
- 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
- upper atmosphere
- The general term applied to the atmosphere
above the troposphere.
- For subdivision of the upper atmosphere, see atmospheric
- 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
- 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
- upper transit
of the upper
branch of the celestial
meridian. Also called superior transit, upper culmination. Transit
of the lower branch is called lower transit.
- See planet, table.
- 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.
- UT (abbr) = universal