U

Source edition 1965. Please read the Introduction to find out about this dictionary and our plans for it. Caution, many entries have not been updated since the 1965 edition.
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Udop (abbr) = UHF Dovap.

U-figure = U-index.

UHF Dovap (abbr Udop)

A Dovap type trajectory measuring system operating in the UHF band. See Doppler, velocity, and position.

U-index

The difference between consecutive daily mean values of the horizontal component of the geomagnetic field. 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.

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.

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 bands.

ultrahigh-speed motion-picture photography

Picture taking at a frequency range above 10,000 pictures per second.

ultrahigh vacuum

See vacuum.

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 frequency

A frequency lying above the audiofrequency range. The term is commonly applied to elastic waves propagated in gases, liquids, or solids. See sound.

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.

ultrasonic wave

See sound wave.

ultraviolet (abbr UV)

Pertaining to or same as ultraviolet radiation.

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.

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.

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.

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.

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.

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.

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.

universe

In statistical terminology, = population.

UV (abbr) = ultraviolet radiation.

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.

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 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.

Uranus

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 time.