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- Any assembly or apparatus, complete in itself or practically so,
identifiable as a unit and readily available for use or installation. See power
- pad = launch pad.
- pad deluge
- Water sprayed upon certain launch pads
during the launch of a rocket so as to reduce the temperatures of critical
parts of the pad or the rocket. See underdeck
- paddlewheel satellite
- A satellite, such as Explorer VI, that has solar vanes or similarly shaped
- pair production
- An absorption process for X-ray and gamma ray
radiation in which the incident photon is
annihilated in the vicinity of the nucleus of the absorbing atom, with
subsequent production of an electron and
- This reaction does not occur for incident radiation energies of less
than 1.02 million electron volts.
- PAM (abbr)= pulse amplitude modulation.
- PAM/FM (abbr)
modulation of a carrier by pulses which are
modulated by information.
- PAM/FM/FM (abbr)
modulation of a carrier by subcarrier(s)
which is (are) modulated by pulses which are
modulated by information.
- See color
- An open curve all points of which are equidistant from a fixed point,
called the focus , and a straight line. See conic
- The limiting case occurs when the point is on the line, in which case
the parabola becomes a straight line.
- Pertaining to, or shaped like, a parabola.
- parabolic orbit
- An orbit
shaped like a parabola; the
orbit representing the least eccentricity
(that of 1) for escape from an
- parabolic reflector
- A reflecting surface having the cross section along the axis in the shape
of a parabola. See
- Parallel rays striking the reflector are brought to a focus at a point,
or if the source of the rays is placed at the focus, the reflected rays are
- A surface of revolution generated by revolving a section of a parabola about
- Pertaining to, or shaped like, a paraboloid.
- parabrake = deceleration parachute.
- parafoveal vision
- Vision in which the eye is so oriented toward the pertinent light source
as to have the light fall upon some portion of the retina surrounding the fovea. Also
called scotopic vision. See foveal
- The portion of the retina used in this type of vision contains
receptors known as rods. Although these rods do not permit the sort of
color-sensing vision possible with the cones in the central or foveal region
of the retina, they have the useful property of responding to very low
illuminance, particularly after dark adaptation is complete. Nighttime vision
is performed primarily with the rods.
- parallactic angle
- The angle between a body's hour circle
and its vertical
circle. Also called position angle.
- parallactic inequality
- A secondary effect in the solar perturbations
in the moon's longitude due to the ellipticity of the earth's orbit.
- The difference in the apparent direction or position of an object when
viewed from different points expressed as an angle.
- For bodies of the solar system, parallax is measured from the surface
of the earth and its center and is called geocentric parallax, varying with
the body's altitude and distance form the earth. The geocentric parallax when
a body is in the horizon is called horizontal parallax and is the angular
semidiameter of the earth as seen from the body. Parallax of the moon is
called lunar parallex. For stars, parallax is measured from the earth and the
sun, and is called annual, heliocentric, or stellar parallax. Compare aberration.
- parallax error
- The error in measurement between two pairs of antenna caused
by the fact that the center of the two baselines do
- This error is a function of the distance of the target from the
baseline, as well as its relative direction.
- parallax in altitude
parallax of a body at any altitude.
- The expression is used to distinguish the parallax at the given
altitude from the horizontal parallax.
- parallax second
- See parsec.
- A circle on the surface of the earth, parallel to the plane of the equator and
connecting all points of equal latitude, or a
circle parallel to the primary
great circle of a sphere or spheroid; also a closed curve approximating
such a circle. Also called parallel of latitude, circle of longitude.
- An astronomical parallel is a line connecting points having the same
astronomical latitude. A geodetic parallel is a line connecting points of
equal geodetic latitude. Geodetic and sometimes astronomical parallels are
also called geographic parallels. Geodetic parallels are shown on charts. A
standard parallel is one along which the scale of a chart is as stated. A
fictitious, grid, transverse, incerse, or oblique parallel is parallel to a
fictitious, grid, transverse, inverse, or oblique equator, respectively. A
magnetic parallel is a line connecting points of equal magnetic dip.
- parallel of altitude
- A circle of the celestial
sphere parallel to the horizon
connecting all points of equal altitude. Also called altitude circle,
almucantar. See circle of equal altitude.
- parallel of declination
- A circle of the celestial
sphere parallel to the celestial
equator. Also called circle of equal declination. See diurnal
- parallel of latitude
- 1. A circle (or approximation of a circle) on the surface of the earth,
parallel to the equator, and
connecting points of equal latitude. Also
- 2. A circle of the celestial
sphere, parallel to the ecliptic, and
connecting points of equal celestial
latitude. Also called circle of longitude.
- Having a magnetic permeability
greater than unity.
- 1. In general, any quantity of a problem that is not an independent
variable. More specifically, the term is often used to distinguish, from
dependent variables, quantities which may be assigned more or less arbitrary
values for purposes of the problem at hand.
- 2. In statistical terminology, any numerical constant derived from a
population or a probability distribution. Specifically, it is an arbitrary
constant in the mathematical expression of a probability distribution. For
example, in the distribution given by f(z) = αe-αx the constant α
is a parameter.
- 3. In celestial mechanics , the semi-latus rectum.
- The representation, in a mathematical model, of physical effects in terms
of admittedly oversimplified parameters,
rather than realistically requiring such effects to be consequences of the
dynamics of the system.
- Parameterization is often used in system analysis to determine the effect
on the system of changing one parameter while holding other parameters
- parametric equations
- A set of equations in which the independent
variables or coordinates are each expressed in terms of a parameter.
- For example, instead of investigating y = f(x) or F(x,y) = 0 it is
often advantageous to express both x and y in terms of a parameter u: x =
g(u); y = G(u). The parameter may or may not have a useful geometric or
- parasitic element
- A radiating element, not coupled directly to the feed line of the
which materially affects the pattern of the antenna.
- parcel = fluid parcel.
- Pardop (abbr) = passive ranging Doppler system.
- A radionuclide
that upon disintegration yields a specified nuclide, the
daughter, either directly or as a later member of a radioactive series.
- Thus, U238 is the parent of all members of the uranium
series, including the end product, Pb206.
- A symmetry property of a wave function.
- The parity is 1 (or even) if the wave function is unchanged by an
inversion (reflection in the origin) of the coordinate system; it is -1 (or
odd) if the wave function is changed only in sign.
- parity bit
- A bit added
to a binary code group which is used to indicate whether the number of
recorded 1 or 0 is even or odd.
- parking orbit
- An orbit
of a spacecraft
around a celestial body, used for assembly of components or to wait for
conditions favorable for departure from the orbit.
- parsec (abbr pc)
- A unit of length equal to the distance from the sun to a point having a
heliocentric parallax of 1 second (1"), used as a measure of stellar distance.
- The name parsec is derived from the words parallax second. 1 parsec =
pc = 3.084 X 10E13 kilometers = 206,265 astronomical units = 3.262 light years
- 1. One of the constituents into which a thing may be divided. Applicable
to a major assembly, subassembly,
or the smallest individual piece in a given thing.
- 2. Restrictive. The least subdivision of a thing; a piece that functions
in interaction with other elements of a thing, but it itself not ordinarily
subject to disassembly.
- partial-admission turbine
- A type of turbine in
which the working substance is directed only through part of the annular area
swept by the rotating turbine blades.
- partial correlation
- The correlation
between the residuals of two random variables with respect to common
regressors. Denoting the regression function of two variates y and z
with respect to a common set of regressors x1, x2, ...xn by Y
and Z ; the coefficient of partial correlation between y and
z is defined as the coefficient of simple, linear correlation between (
y - Y ) and ( z - Z ). See regression.
- partial derivative
- The ordinary derivative
of a function of two or more variables with respect to one of the variables,
the others being considered constants. If the variables are x and y
, the partial derivatives of f(x, y) are written Δf/Δx
and Δf/Δy, or Dxf and Dyf, or fx
- The partial derivative of a variable with respect to time is known as
the local derivative.
- partial node
- A point, line, or surface in a standing
wave system where some characteristic of the wave field has a minimum
amplitude differing from zero.
- The appropriate modifier should be used with the words partial node to
signify the type that is intended; e.g., displacement partial node, velocity
partial node, pressure partial node.
- partial lunar eclipse
- See lunar
- partial pressure
- The pressure
exerted by a designated component or components of a gaseous mixture.
- This may be separately measured in some cases by suitable selection of
gases, traps, or analytical trains. When the percentage composition of the
mixture is known, the partial pressure may be calculated from the total
pressure by Dalton
law of partial pressures.
- partial pressure suit
- A skintight suit which does not completely enclose the body but which is
capable of exerting pressure on the major portion of the body in order to
counteract an increased oxygen pressure in the lungs.
- partial solar eclipse
- See solar
- 1. An elementary subatomic particle such as proton, electron, neutron,
- 2. A very small piece of matter.
- 3. In celestial mechanics, a hypothetical entity which responds to
gravitational forces but which exerts no appreciable gravitational force on
other bodies, thus simplifying orbital computations.
- particle accelerator
- Specifically a device for imparting large kinetic
energy to charged particles, such as electrons, protons, deuterons,
and helium ions.
- Common types of accelerators are the cyclotron, synchrotron,
synchrocyclotron, betatron, linear accelerator, and Van de Graaff
- Paschen law
- A theoretical relationship for the direct-current breakdown voltage of two
parallel-plane electrodes immersed in a gas as a function of the gas pressure
and electrode separation. This relationship predicts the occurrence of a
minimum breakdown voltage for a certain product of the pressure times the
- The phenomenon is well verified experimentally and is referred to as
the Paschen minimum. This minimum voltage is on the order of 300 to 500 volts
and, for a gas pressure of 1 millimeter of mercury, occurs at an electrode
separation of 0.2 to 1 centimeter depending on the gas.
- Paschen minimum
- See Paschen
- 1. A single circuit of the earth by a satellite. Passes start at the time
the satellite crosses the equator from the southern hemisphere into the
northern hemisphere (the ascending node). See orbit.
- 2. The period of time the satellite is within telemetry range of a data
- Containing no power sources to augment output power, e.g., passive
electrical network, passive reflector (as in the Echo satellite). Applied
to a device that draws all its power from the input
signal. Compare active.
- passive homing
- The homing of an
aircraft or spacecraft wherein the craft directs itself toward the target by
means of energy waves transmitted or radiated by the target. See active
- passive homing guidance
- Guidance in
which a craft or missile is directed toward a destination by means of natural
radiations from the destination.
- passive ranging Doppler system
- (abbr Pardop). A trajectory-measuring
system similar to Dovap except that
is used in the missile.
- Space position is computed from several loop ranges
between the transmitter, missile, and receivers.
- 1. Of a satellite, the projection of the orbital plane on the earth's
surface, the locus of the satellite subpoint.
- Since the earth is turning under the satellite, the path of a single
orbital pass will not be a closed curve. Path and track are used
interchangeably. On a cylindrical map projection, the path is a sine-shaped
- 2. Of a meteor, the projection of the trajectory
on the celestial
sphere, as seen by the observer.
- 3. = flightpath.
- Pav, Pavo
- International Astronomical Union abbreviations for Pavo. See constellation.
- Pavo (abbr Pav, Pavo)
- See constellation.
- 1. Originally, the revenue-producing portion of an aircraft's load, e.g.,
passengers, cargo, mail, etc.
- 2. By extension, that which an aircraft, rocket, or the like carries over
and above what is necessary for the operation of the vehicle for its flight.
- payload mass ratio (symbol ζ)
- Of a rocket, the
ratio of the effective propellant mass mp to the initial vehicle mass mo
or ζ = mp/mo. Also called mass ratio.
- A frequency band used in radar extending
approximately from 225 to 390 megacycles per second. See frequency
- PCM (abbr) = pulse code modulation.
- PCM/FM (abbr)
modulation of a carrier by pulse-code-modulated
- PCM/FM/FM (abbr)
modulation of a carrier by subcarrier(s)
which is (are) frequency modulated by pulse-code-modulated
- PCM/PM (abbr)
modulation of a carrier by pulse-code-modulated
- P-display = plan position indicator.
- PDM (abbr) = pulse duration modulation.
- PDM/FM (abbr)
modulation of a carrier by pulses which are
modulated in duration by information.
- PDM/FM/FM (abbr)
modulation of a carrier by subcarrier(s)
which is (are) frequency modulated by pulses which are
modulated in duration by information.
- PDM/PM (abbr)
modulation of a carrier by pulses which are
modulated in duration by information.
- peak sound pressure
- For any specified time interval, the maximum absolute value of the
pressure in that interval.
- In the case of a periodic wave, if the time interval considered is a
complete period, the peak sound pressure becomes identical with the maximum
- peak-to-peak value
- Of an oscillating quantity, the algebraic difference between the extremes
of the quantity.
- Peclet number (symbol Npe)
- A nondimensional number arising in problems of heat transfer in fluids. It is
the ratio of heat advection to heat diffusion and may be written Npe = U l/k where U is a characteristic
velocity; l is a characteristic length; and k is the
thermometric conductivity. Also, Npe = NReNPr where NRe
is the Reynolds
number and NPr is the Prandtl
- Peg, Pegs
- International Astronomical Union abbreviations for Pegasus. See constellation.
- Pegasus (abbr Peg, Pegs)
- See constellation.
- International Astronomical Union abbreviations for Pegasus. See constellation.
- Peltier effect
- The production or absorption of heat at the junction of two metals on the
passage of an electrical current.
- Heat generated by current flowing in one direction will absorbed if the
current is reversed. This effect is presently being extensively studied as a
possible energy conversion method for space vehicles.
- pencil beam
- Emission, from
having the form of a narrow conical
- pencil-beam antenna
- A unidirectional
designed that cross section of the major lobe
by planes perpendicular to the direction of maximum radiation are
approximately circular, and having a very small angular cross section.
- Penning discharge
- A direct-current discharge
where electrons are forced to oscillate between two opposed cathodes and are
restrained from going to the surrounding anode by the presence of a magnetic
- It is sometimes referred to as a pig discharge since the device was
originally used as an ionization gage (Penning ionization gage). It is used as
a plasma-beam source by permitting the plasma to stream
out along the magnetic field through a hole in one of the cathodes.
- Penning effect
- An increase in the effective ionization
rate of a gas due to the presence of a small number of foreign metastable
- For instance, a neon atom has a metastable level at 16.6 volts and if
there are a few neon atoms in a gas of argon which has an ionization potential
of 15.7 volts, a collision between the neon metastable atom with an argon atom
may lead to ionization of the argon. Thus, the energy which is stored in the
metastable atom can be used to increase the ionization rate. Other gases where
this effect is used are helium, with a metastable level at 19.8 volts, and
mercury, with an ionization level at 10.4 volts.
- Penning gage
- See cold-cathode
ionization gage, note.
- See umbra.
- penumbral eclipse
- See lunar
- Per, Pers
- International Astronomical Union abbreviations for Perseus. See constellation.
- perfect fluid
- In simplifying assumptions, a fluid chiefly
characterized by lack of viscosity and, usually, by incompressibility. Also
called an ideal fluid, inviscid fluid. See perfect
- A perfect fluid is sometimes further characterized as homogeneous and
- perfect gas
- A gas which has the following characteristics: (a) it obeys the Boyle-Mariotte
law and the Charles-Gay-Lussac
law; thus satisfying the equation of state for perfect gases; (b) it has
internal energy as a function of temperature alone; and (c) it has specific
heats with values independent of temperature. Also called ideal gas.
- The normal volume of a perfect gas is 2.24136 X 10E4 centimeters cubed
- perfect gas laws = gas laws.
- perfectly diffuse radiator
- A body that emits radiant
energy in accordance with Lambert
law. The radiant
intensity emitted in any direction from a unit area of such a radiator
varies as the cosine of the angle between the normal to the surface and the
direction of the radiation. Compare diffuse radiation, isotropic
- When viewed from a distance, an incandescent perfectly diffuse radiator
appears as a uniformly illuminated flat surface regardless of its actual shape
- perfectly diffuse reflector
- A body that reflects radiant energy in such a manner that the reflected
energy may be treated as if it were being emitted (radiated) in accordance
law. The radiant intensity reflected in any direction from a unit area of
such a reflector varies as the cosine of the angle between the normal to the
surface and the direction of the reflected radiant energy.
- perfect radiator = black body.
- perfect vacuum = absolute vacuum.
- A prefix meaning near as in perigee.
- The orbital point nearest the center of attraction. See orbit.
- That point of the orbit of one
member of a binary star
system at which the stars are nearest to each other.
- That point at which they are farthest apart is called apastron.
- That point in the trajectory
of a vehicle which is closest to the moon.
- The point on an orbit nearest the dynamical center ( focus). The
pericenter is at one end of the major axis
of the orbital ellipse.
- That orbital point nearest the earth when the earth is the center of
attraction. See orbit.
- That orbital point farthest from the earth is called apogee. Perigee
and apogee are used by some writers in referring to orbits of satellites,
especially artificial satellites, around any planet or satellite, thus
avoiding coinage of new terms for each planet and moon.
- perigee propulsion
- A programmed-thrust technique for escape from a
planet, which uses intermittent applications of thrust at perigee (when
vehicle velocity is high) and coasting periods.
- perigee speed
- The speed of an orbiting body when at perigee.
- perigee-to-perigee period = anomalistic period.
- That point in a solar orbit which is
nearest the sun.
- That orbital point farthest from the sun is called aphelion. The term
perihelion should not be confused with parhelion, a form of halo.
- 1. The interval needed to complete a cycle.
- 2. = orbital
- 3. Specifically, the interval between passages at a fixed point of a given
phase of a
simple harmonic wave;
the reciprocal of frequency.
- 4. The time interval during which the power level (flux) of a reactor changes
by a factor of e (2.718, the base of natural logarithms).
- periodic quantity
- In mathematics, an oscillating quantity whose values recur for certain
increments of the independent
- periodic terms
- See secular
- period of moon's node
- See nutation,
- period scrams
- Electronic safety circuits that automatically insert safety rods in a reactor when
the reactor period decreases
below the safe minimum limit.
- An optical instrument which displaces the line of
sight parallel to itself to permit a view which may otherwise be
- Of or pertaining to a periscope, as in periscopic sextant.
- permanent magnetism
which is retained for long periods without appreciable reduction, unless the
magnet is subjected to demagnetizing force. See induced
- Because of the slow dissipation of such magnetism, it is sometimes
called subpermanent magnetism, but the expression permanent magnetism is
- permanent memory
- In computer terminology, storage of
information which remains intact when the power is turned off. Also called
- 1. Of a magnetic material, the ratio of the magnetic
induction to the magnetic
field intensity in the same region.
- 2. The ability to permit penetration or passage. In this sense the term is
applied particularly to substances which permit penetration or passage of
- 3. = permeability
- permeability coefficient
- The steady-state rate of a flow of gas through unit area and thickness of
a solid barrier per unit pressure differential at a given temperature. Also
- Usually expressed in volume or mass per unit time, per unit area of
cross section, per unit thickness, per unit pressure differential across the
- As applied to gas flow through solids, the passage of gas into, through,
and out of a solid barrier having no holes large enough to permit more than a
small fraction of the gas to pass through any one hole. The process always
through the solid and may involve various surface phenomena, such as sorption, dissociation,
migration, and desorption
of the gas molecules.
- permissible dose
- The amount of radiation
which may be received by an individual within a specified period with
expectation of no harmful result to himself.
- International Astronomical Union abbreviation for Perseus. See constellation.
- Perseus (abbr Per, Pers)
- See constellation.
- persistent train
- A meteor train which endures for an appreciable length of time.
- 1. Any departure introduced into an assumed steady
state of a system, or a small departure from a nominal path such as a
desired trajectory. Usually used as equivalent to small
- 2. Specifically, a disturbance in the regular motion of a celestial
body, the result of a force additional to that which causes the regular
motion, specifically, a gravitational force.
- perturbation method = method
of small perturbation.
- perturbation quantity
- Any parameter of a system, e.g, velocity components or temperature, which
may or may not have been assumed to be small perturbations for a mean or steady-state
- PFM (abbr) = pulse
- 1. Of a periodic
quantity, for a particular value of the independent variable, the
fractional part of a period through
which the independent variable has advanced, measured from an arbitrary
- The arbitrary reference is generally so chosen that the fraction is
less than unity. In case of a simple
harmonic quantity, the reference is often taken as the last previous
passage through zero from the negative to positive direction. Thus, if two
wave crest one-fourth cycle apart, they are said to be 90 degrees apart in
phase, or 90 degrees out of phase. The moon is said to be at first quarter
when it has completed one-fourth of its cycle from new moon.
- 2. The stage of aggregation of a substance, for example solid, liquid, or
- 3. The extent to which the disk of the moon or the planet, as seen from
the earth, is illuminated or not illuminated by the sun.
- 4. In astronomy = configuration.
- phase angle
- 1. The phase difference
of two periodically recurring phenomena of the same frequency,
expressed in angular measure.
- 2. The angle at a celestial
body between the sun and earth.
- phase constant
- See propagation
- phase detector
- A device that continuously compares the phase of two
signals and provides an output proportional to their difference in phase.
- phase deviation
- The peak difference between the instantaneous phase of the modulated
wave and the carrier
- The extent of deviation is proportional to the amplitude of the
- phase front
- A surface of constant phase (or phase
angle) of a propagating wave disturbance.
Also called wave front.
- Generally, phase fronts spread out spherically from their source; but
in cases where energy is assumed to travel in parallel rays (as in many
radiation problems), phase fronts may be approximated as plane surfaces
oriented perpendicularly to the rays.
- phase lock
- The technique of making the phase of an oscillator
signal follow exactly the phase of a reference signal by comparing the phases
between the two signals and using the resultant difference signal to adjust
the frequency of the reference oscillator. See correlation
- phase-lock loop
- An electronic servo system
incorporating phase lock
and used either as a tracking filter or as a frequency discriminator.
- phase modulation (abbr PM)
modulation in which the angle of a sine-wave carrier is caused to depart
from the carrier angle
by an amount proportional to the instantaneous value of the modulating
- Combinations of phase and frequency modulation are commonly referred to
as frequency modulation.
- phase-shaped antenna = shaped-beam antenna.
- phases of the moon
- The various appearances of the moon during different parts of the synodical
- The cycle begins with new moon or change of the moon at conjunction.
The visible part of the waxing moon increases in size during the first half of
the cycle until full moon appears at opposition,
after which the visible part of the waning moon decreases for the remainder of
the cycle. First quarter occurs when the waxing moon is at east quadrature;
last quarter when the waning moon is at west quadrature. From last quarter to
new and from new to first quarter, the moon is crescent; from first quarter to
full and from full to last quarter, it is gibbous. The
elapsed time, usually expressed in days since the last new moon, is called age
of the moon.
- phase space
- The sum of the three dimensions of ordinary space and the three dimensions
of velocity space. See distribution
- phase speed = phase velocity.
- phase velocity
- Of a traveling
plane wave at a single frequency,
the velocity of an equiphase surface along the wave normal. Also called
phase speed, wave speed, wave velocity.
- Thus, the component sin ( 2π/λ)(x - ct) represents a wavelength λ traveling in the positive x- direction with
phase velocity c. This concept is to be distinguished from signal velocity,
group velocity, and the velocity of fluid parcels. See velocity
- Phe, Phoe
- International Astronomical Union abbreviations for Phoenix. See constellation.
- Philips gage
- A cold-cathode type of vacuum gage
wherein an electrical discharge is maintained in the presence of a superposed
magnetic field in order to increase the ionization current. See cold-cathode
- A satellite of
Mars orbiting at a mean distance of 9,400 kilometers.
- International Astronomical Union abbreviation for Phoenix. See constellation.
- A satellite of
Saturn orbiting at a mean distance of 12,960,000 kilometers.
- Phoenix (abbr Phe, Phoe)
- See constellation.
- The unit of loudness level of sound,
numerically equal to the sound
pressure level in decibels, relative to 0.0002 mircobar, of a simple 1000
cycle per second tone judged by listeners to be equivalent in loudness.
- An instrument for measuring the intensity or
- A phosphorescent substance, such as zinc sulfide, which emits light when
excited by radiation, as on the scope of a cathode-ray tube. See phosphorescence.
- Emission of
light which continues after the exciting mechanism has ceased. See luminescence.
- An example of phosphorescence is the glowing of an oscilloscope screen
after the exciting beam of electrons has moved to another part of the screen.
- A photometric unit of illuminance
equal to 1 lumen per square centimeter. Compare foot-candle,
- An electrode
used for obtaining photoelectric
- photocell = photoelectric cell.
- photochemical reaction
- A chemical reaction which involves either the absorption
- photoconductive cell
- A photoelectric
cell whose electrical resistance varies with the amount of illumination
falling upon the sensitive area of the cell.
- The dissociation (splitting) of a molecule by the absorption of a photon. The
resulting components may be ionized in the
- 1. Pertaining to the photoelectric
- 2. Using a photoelectric
cell, as a photoelectric photometer.
- photoelectric cell
- A transducer
which converts electromagnetic
in the infrared, visible, and ultraviolet regions into electrical quantities
such as voltage, current, or resistance. Also called photocell. See photoelectric
- photoelectric effect
- The emission of an
from a surface as the surface absorbs a photon of
electromagnetic radiation. Electrons so emitted are termed photoelectrons.
- The effectiveness of the process depends upon the surface metal
concerned and the wavelength of the radiant energy to which it is expressed.
Cesium, for example, will emit electrons when exposed to visible radiation.
The energy of the electron produced is equal to the energy of the incident
photon minus the amount of work needed to raise the electron to a sufficient
energy level to free it from the surface. The resulting energy of the
electron, therefore, is proportional to the frequency (i.e., inversely
proportional to the wavelength) of the incident radiation.
- photoelectric emission
- See photoelectric
- photoelectric photometry
in which a photoelectric
cell is used as the sensing element.
- photoelectric transducer
- A transducer
which converts changes in light energy to changes in electrical energy.
- An electron which
has been ejected from its parent atom by interaction between that atom and a
- Photoelectrons are produced when electromagnetic radiation of
sufficiently short wavelength is incident upon metallic or other solid
surfaces (photoelectric effect) or when radiation passes through a gas.
- The art or science of obtaining reliable measurements by means of
- photographic magnitude (symbol mpg)
- Stellar magnitudes
measured from a photographic plate exposed without filters.
- Photographic plates are more sensitive to short wavelengths than the
human eye. The zero point of the photographic magnitude scale is such that
photographic (mpg) and visual (mv) magnitudes are the same for stars of class A0
of magnitudes between 5.5 and 6.5. Photovisual magnitudes (mpv) are measured from plates exposed through
filters which hold back blue and violet thus giving magnitudes in the plate
which closely approximate visual magnitudes (mv).
- photographic meteor
- A meteor
of brightness sufficient to be detected by photography.
- photographic transmission density
- The common logarithm of opacity. Hence,
film transmitting 100 percent of the light has a density of zero, transmitting
10 percent, a density of 1, etc. Density may be diffuse, specular, or
intermediate. Conditions must be specified. Also called optical
- Diffuse transmission density is the value of the photographic
transmission density obtained when the light flux impinges normally on the
sample and all the transmitted flux is collected and measured. Specular
transmission density is the value of the photographic density obtained when
the light flux impinges normally on the sample and only the normal component
of the transmitted flux is collected and measured.
- The ionization
of an atom or molecule by the collision of a high-energy photon with the
particle. See photoelectron.
- The study of light.
- photoluminescence = fluorescence, see luminescence.
- An instrument for measuring the intensity of
light or the relative intensity of a pair of lights. Also called
- If the instrument is designed to measure the intensity of light as a
function of wavelength, it is called a spectrophotometer. Photometers may be
divided into two classes: photoelectric photometers in which a photoelectric
cell is used to compare electrically the intensity of an unknown light with
that of a standard light, and visual photometers in which the human eye is the
- The study of the measurement of the intensity of
- At one time photometry referred only to the measurement of luminous
intensity, intensity of light in the wavelength to which the eye is
sensitive. This restriction has proved difficult to maintain in practice.
- photomultiplier = multiplier phototube.
- According to the quantum
theory of radiation,
the elementary quantity, or quantum, of radiant energy. It is regarded as a
discrete quantity having a momentum equal to hv/c , where h is
Planck constant, v is the frequency of the radiation, and c is
the speed of light in a vacuum. The photon is never at rest, has no electric
charge and no magnetic moment, but does have a spin moment. The energy of a
photon (the unit quantum of energy) is equal to hv.
- photon engine
- A projected type of reaction
engine in which thrust would be obtained from a stream of electromagnetic
radiation. Compare ion engine.
- Although the thrust of this engine would be minute, it may be possible
to apply it for extended periods of time. Theoretically, in space, where no
resistance is offered by air particles, very high speeds may be built up.
- photon gas
- A radiation field.
- photon rocket
- A photon
engine; a rocket vehicle powered by a photon engine.
- photopic vision
- Vision associated with levels of illumination 0.01 foot-lambert or higher,
characterized by the ability to distinguish colors and small detail. Also
called foveal vision. Compare scotopic
- The intensely bright portion of the sun visible to the unaided eye.
- The photosphere is that portion of the sun's atmosphere which emits the
continuum radiation upon which the Fraunhofer
lines are superimposed. In one sun model, the photosphere is thought to be
below the reversing layer in which Fraunhofer absorption takes place. In
another model, all strata are considered equally effective in producing
continuous emissions and line absorption.
- A process operating in green plants in which carbohydrates are formed
under the influence of light with chlorophyll serving as a catalyst. See closed
- An instrument or device incorporating one or more cameras for taking and
recording angular measurements.
- The phototheodolite, sometimes in conjunction with radar equipment, is
used to track rockets and to measure and record attitude, altitude, azimuth
and elevation angles, etc.
- An electron
tube that contains a photocathode
and has an output depending on the total photoelectric emission from the
irradiated area of the photocathode.
- photovisual magnitude
- See photographic
- photovoltaic cell
- A transducer
which converts electromagnetic
radiation into electric current.
- The solar cells used on satellites and space probes are photovoltaic
cells employing a semiconductor such as silicon which releases electrons when
bombarded by photons from solar radiation.
- phugoid oscillation
- In a flightpath, a long period longitudinal oscillation
consisting of shallow climbing and diving motions about a median flightpath
and involving little or no change in angle of attack.
- physical constant
- An abstract number or physically dimensional quantity having a fixed or
approximately fixed value; a universal and permanent value, as the constant of
gravitation; a characteristic of a substance, as the refractive index of
- A new, consistent set of values for physical constants, which has been
recommended by the National Academy of Sciences-National Research Council in
1963, is presented in tables VIII, IX, and X.
- physical double star
- Two stars in nearly the same line of sight and at approximately the same
distance from the observer, as distinguished from an optical double star (two
stars in nearly the same line of sight but differing greatly in distance from
- If the stars revolve about their common center of mass, they are called
a binary star.
- physical equation = equation of piezotropy.
- physical meteorology
- That branch of meteorology which deals with optical, electrical,
acoustical, and thermodynamic phenomena of atmospheres, their chemical
composition, the laws of radiation, and the explanation of clouds and
precipitation. As generally accepted, it does not include mathematical theory
of the motions of the atmosphere and the forces responsible therefore (which
matters fall in the field of dynamic meteorology).
Also called atmospheric physics.
- Subdivisions of physical meteorology include atmospheric
physics, precipitation physics, atmospheric acoustics, and atmospheric
- physical system = cgs system.
- physiological acceleration
- The acceleration
experienced by a human or an animal test subject in an accelerating vehicle.
See table XI.
- Several different terminologies have been used to describe
physiological acceleration. Since the terminology may be based either on the
action of the accelerating vehicle or the reaction of the passenger, the terms
used are often confusing to a reader without prior knowledge of the system of
terminology used. Probably the most easily understood system is the eyeballs
in, eyeballs out, eyeballs down, eyeballs up, etc., terminology used by test
pilots, which refers to the sensations experienced by the person being
accelerated. Thus, the acceleration experienced in an aircraft pullout or
inside loop is eyeballs down. Note that, in the NASA vehicle (center of
gravity displacement) terminology, this is -az acceleration. Some
physiological-acceleration terminologies designate accelerations in terms of
the equivalent displacement acceleration of the subject as if he were starting
from rest. In such terminologies a man standing up or sitting down on the
surface of the earth is experiencing 1 g of headward acceleration because of
gravity. Other descriptive terms used in this way are footward, forward (the
acceleration experienced by a man pressed into the seat back by an
accelerating vehicle), rearward, leftward, rightward, spineward, sternumward,
and tailward. One terminology based on reaction uses the terms head-to-foot
(the acceleration generated by a pullout in an aircraft), chest-to-back,
foot-to-head, and back-to-chest.
- The science that treats of the functions of living organisms or their
parts, as distinguished from morphology, anatomy, etc.
- A biotron
designed especially for research on plant life.
- Pic, Pict
- International Astronomical Union abbreviations for Pictor. See constellation.
- A sensing device that responds to angular movement to create a signal or
to effect some type of control, as a pickoff on a gyro in an automatic
- A pickoff may be a potentiometer, a photoelectric device, a kind of
value controlling the fluid flows and pressures in a system, or one of various
- 1. A device that converts a sound, scene, or other form of intelligence
into corresponding electric signals (e.g.,
a microphone, a television camera, or a phonograph pickup).
- 2. The minimum current, voltage, power, or other value at which a relay will
complete its intended function.
- 3. Interference
from a nearby circuit or electrical system.
- A prefix meaning multiplied by 10 -12.
- International Astronomical Union abbreviation for Pictor. See constellation.
- Pictor (abbr Pic, Pict)
- See constellation.
- The property exhibited by some asymmetrical crystalline materials which
when subjected to strain in suitable directions develop electric polarization
proportional to the strain.
- Inverse piezoelectricity is the effect in which mechanical strain is
produced in certain asymmetrical crystalline materials when subjected to an
external electric field; the strain is proportional to the electric field.
- piezoelectric transducer
- A transducer
utilizing a piezoelectric
- pig discharge
- See Penning
- pile = nuclear reactor
- The term pile comes from the first nuclear reactor, which was made by
piling up graphite blocks and pieces of uranium and uranium oxide. The term
reactor is now more commonly used.
- pillbox antenna
- A cylindrical parabolic
reflector enclosed by two plates perpendicular to the cylinder, so spaced
as to permit the propagation of only one mode in the desired direction of polarization.
- 1. A person who handles the controls of an
aircraft or spacecraft from within the craft, and in so doing, guides or
controls it in three-dimensional flight.
- 2. A mechanical system designed to exercise control
functions in an aircraft or spacecraft.
- 3. To operate, control, or guide an aircraft or spacecraft from within the
vehicle so as to move in three-dimensional flight through the air or space.
- Of an aircraft or spacecraft, under, or subject to, continuous control by a
person inside the vehicle.
- This term is more specific than the term manned.
- pinch effect
- 1. The result of an electromechanical force that constricts, and sometimes
momentarily ruptures, a molten conductor
carrying current at high density.
- 2. The self-contraction of a plasma column
carrying large currents due to the interaction of this current with its own
- The current required for such an effect is the order of 10E5 amperes.
If the current is pulsed on for a short time, a radially imploding shock wave
- P-indicator = plan position indicator (PPI).
- Signal indication on the oscilloscope
screen of an electronic instrument, produced by a short, sharply peaked pulse
of voltage. Also called blip.
- Pirani gage
- A thermal
conductivity vacuum gage in which an increase of pressure from the zero
point causes a decrease in the temperature of a heated filament of material
having a large temperature coefficient of resistance, thus unbalancing a
Wheatstone bridge circuit (or the circuit is adjusted to maintain the filament
- International Astronomical Union abbreviation for Pisces. See constellation.
- Pisces (abbr Psc, Pisc)
- See constellation.
- Piscis Australis = Piscis Austrinus.
- Piscis Austrinus (abbr PsA, Psc A)
- See constellation.
- 1. Of a vehicle, an angular displacement about an axis parallel to the lateral
axis of the vehicle.
- 2. In acoustics,
that attribute of auditory sensation in terms of which sounds may be ordered
on a scale extending from low to high.
- Pitch depends primarily upon the frequency of the sound stimulus, but
it also depends upon the sound pressure and waveform of the stimulus. The
pitch of a sound may be described by the frequency or frequency level of that
simple tone having a specified sound pressure level which is judged by
listeners to produce the same pitch.
- pitch attitude
- The attitude of an
aircraft, rocket, etc., referred to the relationship between the longitudinal
and a chosen reference line or plane as seen from the side.
- pitch axis
- A lateral axis through an aircraft, missile, or similar body, about which
the body pitches. It may be a body, wind , or stability axis.
Also called a pitching axis. See axis, sense 2 and
- pitching axis = pitch axis.
- pitching moment
- A moment
about a lateral
axis of an aircraft, rocket, airfoil, etc.
- This moment is positive when it tends to increase the angle of attack
or to nose the body upward.
- 1. The programmed turn from the vertical that a rocket takes as it
describes an arc and points in a direction other than vertical.
- 2. The point-in-space of this action.
- pitot-static head = pitot-static tube.
- pitot-static tube
- A device consisting essentially of a unit combination of a pitot tube
and a static
tube arranged coaxially or otherwise parallel to one another, used
principally in measuring impact and static
pressures; also called pitot-static
- The difference between impact and static pressure is used to measure
the velocity of flow past the tube by means of a differential-pressure gage.
The static pressure from a pitot-static tube may, in addition, be used in the
operation of an altimeter and similar instruments.
- pitot tube
- (Pronounced pee-toe. After Henri Pitot, 1695-1771, French scientist.) An
open-ended tube or tube arrangement which, when immersed in a moving fluid with its
mouth pointed upstream, may be used to measure the stagnation
pressure of the fluid for subsonic flow; or the stagnation pressure behind
the tube's normal
shock wave for supersonic flow.
- Neologism from picture element, thhe smallest homogenous area from a
recorded image. The luminence of each pixel is the mean of the luminence of
each part of the points inside the area of the target pixel.
- Clouds of calcium or hydrogen vapor that show up as bright patches on the
surface of the photosphere
of the sun.
- Planck constant (symbol h)
- A constant equal to 6.6256 X 10E-27 erg second. It scales the energy of
electromagnetic radiation of frequency v so that the radiation appears
only in quanta nhv , n being an integer.
- Planck distribution law = Planck law.
- Planck law
- An expression for the variation of monochromatic radiant flux per unit
area of source as a function of wavelength of black-body radiation at a given
temperature; it is the most fundamental of the radiation
laws. Mathematically, Planck law is dw = [c1 * λ -5/(ec2/Tλ - 1)]dλ where dw is the radiant flux from a black body
in the wavelength interval dλ, centered around wavelength λ per unit area of black-body surface at
temperature T; c1 and c2 are radiation
constants. This law was derived theoretically by M. Planck in 1901.
- plane polarized sound wave
- At a point in an elastic medium, a transverse
wave in which the displacements at all times lie in a fixed plane which is
parallel to the direction of propagation. Also called linearly polarized
- The above definition is equivalent to stating that, in a plane
polarized sound wave, the displacement vector at any point lies in a fixed
straight line passing through the point.
- A celestial
body of the solar
system, revolving around the sun in a nearly circular orbit, or a similar
body revolving around a star. See table XII. See also astronomical
constant, tables II and III, noting that some values differ in the three
- The larger of such bodies are sometimes called principal planets to
distinguish them from asteroids, planetoids, or minor planets, which are
comparatively very small. The larger planets are accompanied by satellites,
such as the moon. An inferior planet has an orbit smaller than that of the
earth. The four planets nearest the sun are called inner planets; the others,
outer planets. The four largest planets are called major planets. The four
planets commonly used for celestial observations are called navigational
planets. The word planet is of Greek origin, meaning, literally, wanderer,
applied because the planets appear to move relative to the stars.
- planetary aberration
- A displacement in the apparent position of a planet in the celestial
sphere due to the relative movement of the observer and the planet. See aberration.
- planetary boundary layer
- That layer of the atmosphere from a planet's surface to the geostrophic
wind level including, therefore, the surface boundary
layer and the Ekman
layer. Above this layer lies the free
atmosphere. Also called friction layer, atmospheric boundary layer.
- planetary circulation
- 1. The system of large-scale disturbances in a planet's troposphere
when viewed on a hemispheric or world-wide scale.
- 2. The mean or time-averaged hemispheric circulation of a planetary atmosphere;
also called general circulation.
- planetary configurations
- Apparent positions of the planets
relative to each other and to other bodies of the solar
system, as seen from the earth.
- planetary precession
- That component of general
precession caused by the effect of other planets on the
equatorial protuberance of the earth, producing an eastward motion of the equinoxes
along the ecliptic. See
of the equinoxes.
- Planetary precession is approximately 0.1247 second of arc per year.
- 1. Of or pertaining to a planet's center of mass.
- 2. Of or pertaining to the planet as a
center of a system.
- Referring to positions on a planet measured
from the planet's equator and in
from a reference meridian.
- planetoid = asteroid.
- See planet.
- plane wave
- A wave in
which the wave fronts
are everywhere parallel planes normal to the direction of propagation.
- The shape or form of an object, such as an airfoil , as seen from
above, as in a plan view.
- plan position indicator (abbr PPI)
- 1. A cathode-ray
indicator in which a signal appears on a radial line. Distance is
indicated radially and bearing as an angle.
- 2. In radar technique, a cathode-ray indicator on which blips produced by
signals from reflecting objects and transponders
are shown in plan position, thus forming a maplike display. Also called
P-indicator, P-scan, P-scope.
- A north-upward plan position indicator has north at the top of the
indicator regardless of the heading; a heading-upward plan position indicator
has the heading of the craft maintained at the top of the indicator. On a
delayed plan position indicator the start of the sweep is delayed so that the
center represents a selected range. This allows distant targets to be
displayed on a short range scale, thus providing larger scale presentation. An
open-center plan position indicator has no signal displayed within a set
distance from the center. An off-center plan position indicator is one
modified so that the center about which the trace rotates can be moved from
the center of the screen to provide a larger scale for distant targets. A
master plan position indicator controls remote indicators or repeaters.
- An electrical conductive gas comprised of
neutral particles, ionized
particles, and free
electrons but which, taken as a whole, is electrically neutral.
- A plasma is further characterized by relatively large intermolecular
distances, large amounts of energy stored in the internal energy levels of the
particles, and the presence of a plasma
sheath at all boundaries of the plasma. Plasmas are sometimes referred to
as a fourth state of matter.
- plasma cloud
- Specifically, a mass of ionized gas
flowing out of the sun.
- plasma engine
- A reaction
engine using magnetically accelerated plasma as propellant.
- A plasma engine is a type of electrical engine.
- plasma frequency
- The natural
frequency for motion of electrons in
a plasma. The
plasma frequency where e is charge on the electron; m
is mass of the electron; and N is number of electrons per cubic
centimeter. See critical
- plasma generator
- 1. A machine, such as an electric-arc chamber, that will generate very
high heat fluxes to convert neutral gases into plasma.
- 2. A device which uses the interaction of a plasma and electrical field to
generate a current.
- plasma length = Debye length.
- plasma physics
- The study of the properties of plasmas.
- plasma rocket
- A rocket using a plasma
engine. Also called electromagnetic rocket.
- plasma sheath
- 1. The boundary layer of charged particles between a plasma and its
surrounding walls, electrodes, or other plasmas.
- The sheath is generated by the interaction of the plasma with the
boundary material. Current flow may be in only one direction across the sheath
(single sheath), in both directions across the sheath (double sheath), or when
the plasma is immersed in a magnetic field, it may flow along the sheath
surface at right angles to the magnetic field (magnetic current sheath).
- 2. An envelope of ionized gas
that surrounds a body moving through an atmosphere at hypersonic
- The plasma sheath affects transmission, reception, and diffraction of
radio waves; thus it is important in operational problems of spacecraft.
- The tendency of a loaded body to assume a deformed state other than its
original state when the load is removed.
- 1. A planar body whose thickness is small compared with its other
- 2. A common name for the principal anode in an electron
- Platonic year = great year.
- plus count
- In the launch of a rocket, a count
in seconds (plus 1, plus 2, etc.) that immediately follows T-time, used to
check on the sequence of events after the action of the countdown has
- See planet, table.
- PM (abbr) = phase modulation.
- PMR (abbr) = Pacific Missile Range.
- pneumatic-probe pyrometer
- A thermometer
for high-temperature gases, in which the gas is sucked through a nozzle and
then cooled. Reliance is place principally on knowledge of the law of gas
expansion through the nozzle and on measurement of pressure and mass flow rate
of the gas.
- An enclosure, housing, or detachable container of some kind, as an
- point discharge
- A silent, nonluminous, gaseous
electrical discharge from a pointed conductor maintained at a potential
which differs from that of the surrounding gas. Compare corona
- point of inflection
- See inflection.
- The unit of viscosity in the cgs system equal to 1 dyne second per square
- Poiseuille flow
- The steady laminar flow of a fluid through a
narrow horizontal circular cylinder according to the relation
u = (1/4μ) (p/x) (a2 - r2
where u is the fluid velocity along the
cylinder's axis at a distance r from the cylinder's axis; μ is the dynamic viscosity of the fluid; a
is the cylinder radius; and p/x is the pressure gradient along the axis of the
cylinder. The velocity profile across the cylinder is seen to be parabolic,
and this relation affords a convenient experimental means of determining a
fluid's viscosity. Also called Hagen-Poiseuille flow. Compare Couette
- In a nuclear reactor, those atoms (of such elements as boron) other than
fuel that have
large capture cross
section for thermal neutrons. In capturing thermal neutrons
unproductively, these atoms decrease the number available to cause fission.
- Poisson constant (symbol μ)
- The ratio of the gas
constant to the specific
heat of a gas at constant pressure. See Poisson
equation, sense 2.
- Poisson distribution
- A one-parameter discrete frequency distribution giving the probability the
n points (or events) will be (or occur) in an interval (or time) x
, provided that these points are individually independent and that the
number occurring in a subinterval does not influence the number occurring in
any other nonoverlapping subinterval. It has the form
f(n,x) = e-σx(σx)n/n!
The mean and variance are both σx , and σ is the average
density (or rate) with which the events occur. When σx is large, the Poisson distribution approaches
the normal distribution. The binomial distribution approaches the Poisson when
the number of events n becomes large and the probability of success
P becomes small in such a way that nP aproaches σx.
- Poisson equation
- 1. The partial differential equation
2 = F
where 2 is the Laplacian operator;
a scalar function of position; and F
is a given function of the independent space variables. For the special
case F = 0, the Poisson equation reduces to the Laplace equation. See
- 2. The relationship between the temperature T and pressure p
of a perfect gas undergoing an adiabatic
process; given by T = constant X pμ
where μ is the Poisson
- This equation defines a family of process lines, called isentropes or
dry adiabats, each of which represents the changes of state possible in a
fluid with a constant value of entropy.
- polar blackout = blackout.
- polar coordinates
- 1. In a plane, a system of curvilinear
coordinates in which a point is located by its distance r from the
origin (or pole) and by the angle θ which a line (radius vector) joining the given
point and the origin makes with a fixed reference line, called the polar axis.
The relations between rectangular Cartesian coordinates and polar coordinates
are x = r cos θ, y = r sin θ, r2 = x2 +
y2 where the origins of the two systems coincide and the
polar axis coincides with the X-axis.
- 2. In three dimensions, short for space
- polar distance
- Angular distance from a celestial
pole; the arc of an hour circle
between a celestial pole, usually the elevated pole, and a point on the celestial
sphere, measured from the celestial pole through 180 degrees.
- If the declination, d, and the celestial pole are of the same name, the
polar distance is 90 degrees - d, but if of contrary name, it is 90 degrees +
d. See codeclination.
- An instrument for determining the degree of polarization
specifically the polarization of light.
- An instrument for detecting polarized
radiation and investigating its properties.
- The sign of the electric discharge associated with a given object, as an
or an ion.
- A measure of the degree to which any given atom or ion undergoes polarization
in the presence of an external electric field.
- 1. The state of electromagnetic
radiation when transverse vibrations take place in some regular manner,
e.g., all in one plane, in a circle, in an ellipse, or in some other definite
- Radiation may become polarized because of the nature of its emitting
source, as is the case with many types of radar antennas, or because of some
processes to which it is subjected after leaving its source, as that which
results from the scattering of solar radiation as it passes through the
- 2. With respect to particles in an electric
field, the displacement of the charge centers within a particle in
response to the electric force acting thereon. See polarizability.
- 3. The response of the molecules of a paramagnetic
medium (such as iron) when subjected to a magnetic
- A right-handed polarized wave is defined as one receding from the
observer and radiated by an electric vector rotating clockwise in a fixed
plane that is in front of the observer and at right angles to the direction of
propagation of the wave in question. Left-handed polarization is the rotation
in a counter-clockwise manner. This recommended definition of circular (or
elliptical) polarization sense is according to that of the Institute of Radio
Engineers. The definition of classical physics is exactly the opposite.
- A device for polarizing
radiant energy. See polarization.
- polar orbit
- The orbit
of an earth satellite
that passes over or near the earth's poles.
- Polar Year
- See International
- 1. The origin of a system of polar
- 2. For any circle on the surface of a sphere, the point of intersection of
the surface of the sphere and the normal line through the center of the
circle. See geographical
- 3. A point of concentration of electric charge. See dipole.
- 4. A point of concentration of magnetic force. See magnetic
- pole of the Milky Way
- The pole in
system of coordinates.
- polytropic atmosphere
- A model
atmosphere in hydrostatic
equilibrium with a constant nonzero lapse rate. The vertical distribution
of pressure and temperature is given by
p/p0 = (T/T0)g/Rγ
where p is the pressure; T is the
Kelvin temperature; g is the acceleration of gravity; R is the
gas constant for the gases concerned; and γ is the environmental lapse rate. The subscript 0
denotes values at the planet's surface.
- polytropic process
- A thermodynamic
process in which changes of pressure p and density ρ are related according to the formula
pρ-λ = p0ρ0-λ
where λ is a constant and the subscript 0 denotes
initial values of the variables. Therefore pressure and temperature are
similarly related: p/po = ( T/To ) k where k is the
coefficient of polytropy. For isobaric processes, k = 0; for isosteric
process, k = 1; for adiabatic processes k = cp/R , where c is the specific heat
at constant pressure and R is the gas constant.
- In meteorology this formula is applied to individual gas parcels and
should be distinguished from that for a polytropic atmosphere, which describes
a distribution of pressure and temperature in space. See equation of
- In statistical usage, any definite class of individuals or objects. Also
called universe. Compare sample.
- 1. A place of access to a system where energy may be
supplied or withdrawn or where system variables may be observed or measured.
- In any particular case, the ports are determined by the way in which
the system is used, and not by the structure alone. A designated pair of
terminals is an example of a port.
- 2. An opening, as the port in a solid rocket.
- posigrade rocket
- An auxiliary rocket which
fires in the direction in which the vehicle is
pointed, used, for example, in separating two stages of vehicle.
- 1. A point in space.
- 2. A point defined by stated or implied coordinates,
particularly one on the surface of the earth.
- 3. = attitude.
- 4. A crew member's station aboard an aircraft or spacecraft. See line of
position, band of
- positional notation
- Any scheme for representing quantities characterized by the arrangement of
sequence with the understanding that successive digits are to be interpreted
as coefficients of successive powers of an
integer called the base or radix.
- The base determines the name of the notation, as, binary (base 2),
decimal (base 10), or duodecimal (base 12).
- position angle = parallactic angle.
- position vector
- See vector.
- positive acceleration
- 1. Acceleration such that speed increases.
- 2. Accelerating force in an upward sense or direction, e.g., from bottom
to top, seat to head, etc.; acceleration in the direction that this force is
applied. See physiological
- positive feedback
- Feedback which
results in increasing the amplification.
- positive g or positive G
- See physiological
- A subatomic
particle which is identical to the electron in
atomic mass, theoretical rest mass, and energy, but opposite in sign. Compare
- The positron is short lived and can exist only when in motion. When it
combines with an electron, both particles are annihilated and two photons
result, equal in energy to the combined masses of the annihilated particles.
Production of positrons can occur only in pair
formation with the electron, the inverse of the annihilation process.
- posthypoxia paradox
- An abrupt convulsive incident which may occur when a marked oxygen
deficiency is relieved by sufficient oxygen; this is in contrast to the normal
rapid recovery from lack of oxygen. Also called oxygen paradox.
- potential (symbol φ)
- 1. A function of space, the gradient of
which is equal to a force. In
symbols, F = -φ, where F is the force;
is the del-operator; and φ
is the potential. A force which may be so
expressed is said to be conservative, and the work done against it in motion
from one given equipotential surface to another is independent of the path of
the motion. See Gibbs
- In celestial mechanics and geodesy, the negative of the potential,
sometimes called force function, is usually employed.
- 2. Applied to the value that an atmospheric thermodynamic variable would
attain if processed adiabatically from its initial pressure to the standard
pressure of 1000 millibars. See potential
- 3. Short for electric
- potential density
- The density a parcel of air would attain if compressed adiabatically by
descent to the standard pressure of 1000 millibars. The potential density
p' is most easily defined in relation to the potential
temperature θ as p' = p/Rθwhere p is a pressure of 10000
millibars and R is the gas constant, in appropriate units. See adiabatic
- potential energy
possessed by a body by virtue of its position in a gravity field
in contrast with kinetic
energy, that possessed by virtue of its motion.
- potential gradient
- In general, the local space rate of change of any potential, as
the gravitational potential gradient or the velocity potential
- potential index of refraction
- An atmospheric index
of refraction so formulated that it would have no height variation in an
atmosphere. Also called potential refractive index. Compare modified
index of refraction.
- The potential index of refraction is usually expressed in terms of B-units.
- potential refractive index = potential index of
- potential temperature
- The temperature
a parcel of
dry air would have if brought adiabatically from its initial state to the
(arbitrarily selected) standard pressure of 1000 millibars. Its mathematical
θ = T(1000/p)R/cp
where θ is the potential temperature; T is the
Kelvin temperature; p is pressure, millibars; R is the gas
constant for dry air; and cp is the specific heat or dry air at constant
pressure. See equivalent
potential temperature, adiabatic
- 1. An instrument for measuring differences in electric
potential by balancing the unknown voltage against a variable know
voltage. If the balancing is accomplished automatically, the instrument is
called a self-balancing potentiometer.
- 2. A variable electric resistor.
- potentiometric transducer
- A transducer
in which the displacement of the force summing members is transmitted to the
slider in a potentiometer,
thus changing the ratio of output resistance to total resistance.
- pound (abbr lb)
- 1. A unit of mass equal in the United States to 0.45359237 kilogram,
- 2. Specifically, a unit of measurement of the thrust or force of a
reaction engine representing the weight the engine can move, as an engine
with 100,000 pounds of thrust. See poundal, pound mass.
- 3. The force exerted on 1 pound mass
by the standard acceleration
of gravity. See gravity, sense
- A unit of force; that unbalanced force which, acting on a body of 1 pound
mass, produces an acceleration
of 1 foot per second squared. See pound, pound mass.
- pound force = pound, sense 3.
- pound mass
- 1. A mass
equal to 0.45359237 kilogram.
- 2. A unit of measure of the inertial property equal to the mass of a body
weighing 1 pound at the standard acceleration of gravity
(980.665 centimeters per second squared).
- pound weight
- A force
equal to the earth's attraction for a mass of 1 pound. This force, acting on a
mass, will produce an acceleration of 32.1747 feet per second squared.
- 1. (symbol P). Rate of doing work.
- 2. Luminous intensity.
- 3. The number of times an object is magnified by an optical system, such
as a telescope. Usually called magnifying power.
- 4. The result of multiplying a number by itself a given number of times,
as the third power of a number is its cube ; the superscript which
indicates this process as in 23 = 2 X 2 X 2.
- power density
- The rated power of a reactor or
isotopic power source per unit volume. Power density is often stated in
kilowatts per cubic centimeter of core volume.
- power gain
- 1. The ratio of the power that a transducer
delivers to a specified load, under
specified operating conditions, to the power absorbed by its input circuit.
- If the input and/or output power consist of more than one component,
such as multifrequency signal or noise, then the particular components used
and their weighting must be specified. This gain is usually expressed in
- 2. Of an antenna, in a given direction, 4pi times the ratio of the
radiation intensity in that direction to the total power delivered to the antenna.
- power loading
- The ratio of the gross weight of a propeller-driven aircraft to
its power, usually expressed as the gross weight of the aircraft divided by
the rated horsepower of the power plant corrected for air of standard density.
With turboprop engines, the equivalent shaft horsepower is used. Compare thrust
- power package
- An engine, especially a reciprocating
engine, together with its accessories, lines, cowling, etc., ready for
quick installation on an aircraft.
- power plant
- 1. The complete assemblage or installation of engine or
engines with accessories (induction system, cooling system, ignition system,
etc.) that generates the motive power for a self-propelled vehicle or vessel
such as an aircraft, rocket, etc.
- 2. An engine or engine installation regarded as a source of power.
- power series
- An infinite series of increasing power of the variable, of the form
a0 + a1x + a2x2 . . . + anxn
- Both the variable and the coefficients may take on complex values. The
totality of values of x for which a power series is convergent is called the
interval of convergence of the series.
- power spectrum
- The square of the amplitude of the (complex) Fourier
coefficient of a given periodic function. Thus if f(t) is periodic
with period T , its Fourier coefficients are
where ω = 2π/T and the power spectrum of f(t) is |F(n)|2. Here n take integral values and the
spectrum is discrete. The total energy of the periodic function is infinite,
but the power, or energy per unit period, is finite.
- Poynting-Robertson effect
- The gradual decrease in orbital
velocity of a small particle such as a micrometeorite in orbit about the
sun due to the absorption
and reemission of radiant
energy by the particle.
- PPI (abbr) = plan position indicator.
- PPI reflectoscope
- See beam
- PPI scope = plan position indicator.
- PPM (abbr) = pulse position modulation.
- PPM/AM (abbr)
modulation of a carrier by pulses which are
position modulated by information.
- Prandtl number (symbol NPr, Pr)
- (After Ludwig Prandtl, 1875-1953, German scientist). A dimensionless
number representing the ratio of momentum
transport to heat transport in
a flow, defined by the equation
NPr = μcp/k
where μ is the viscosity coefficient; cp is the specific heat at constant pressure; and
k is the coefficient of thermal conductivity.
- The Prandtl number may also be defined as the product of the Reynolds and
- 1. An amplifier,
the primary function of which is to raise the output of a low-level source to an
intermediate level so that the signal may be
further processed without appreciable degradation in the signal-to-noise ratio
of the system.
- A preamplifier may include provision for equalizing and/or mixing.
- 2. In radar an amplifier separated from the remainder of the receiver and
located so as to provide the shortest possible input circuit path from the
antenna so as to avoid deterioration of the signal-to-noise ratio.
- Change in the direction of the axis of rotation of a spinning body, as a
gyro, when acted upon by a torque. See apparent
of the equinoxes.
- The direction of motion of the axis is such that it causes the
direction of spin of the gyro to tend to coincide with that of the impressed
torque. The horizontal component of precession is called drift, and the
vertical component is called topple.
- precession in declination
- The component of general
precession along a celestial
meridian, amounting to about 20 seconds of arc per year.
- precession in right ascension
- The component of general
precession along the celestial
equator, amounting to about 46.1 seconds of arc per year.
- precession of the equinoxes
- The conical motion of the earth's axis about the
normal to the plane of the ecliptic,
caused by the attractive force of the sun, moon, and other planets on the
equatorial protuberance of the earth.
- The effect of the sun and moon, called lunisolar precession, is to
produce a westward motion of the equinoxes along the ecliptic. The effect of
other planets, called planetary precession, tends to produce a much smaller
motion eastward along the ecliptic. The resultant motion, called general
precession, is westward along the ecliptic at the rate of about 50.3 seconds
of arc per year. The component of general precession along the celestial
equator, called precession in right ascension, is about 46.1 seconds of arc
per year; and the component along a celestial meridian, called precession in
declination, is about 20.0 seconds of arc per year.
- precipitation attenuation
- The loss of radio energy due to
the passage through a volume of the atmosphere containing precipitation. Part
of the energy is lost by scattering
and part by absorption.
- Radars operating at wavelengths of 10 centimeters and higher are
generally unaffected, whereas even the smallest precipitation rates will
seriously attenuate radar energy of wavelengths less than 1 centimeter. For
rain and snow diameter-to-wavelength ratios less than 0.07, the loss is due
primarily to absorption. Scattering becomes important for ratios near 0.1 and
greater. Attenuation by dry snow is small for most radar wavelengths.
- The quality of being exactly or sharply defined or stated. A measure of
the precision of a representation is the number of distinguishable
alternatives from which it was selected, which is sometimes indicated by the
number of significant digits it contains. Compare accuracy.
- precombustion chamber
- In a rocket, a
chamber in which the propellants
are ignited and from which the burning mixture expands torchlike to ignite the
mixture in the main combustion
- A process by which a molecule that has absorbed energy separates into
constituents before it loses energy by radiation.
- preliminary stage = prestage.
- In electronics, the act or process of displaying radar echoes on a
cathode-ray screen; the echo or images
displayed on a cathode-ray
- preset guidance
- A type of guidance in
which devices in the aircraft or spacecraft, adjusted before launching, control the
path of the missile.
- pressure (symbol p )
- 1. In a gas, the net rate of transfer of momentum in the direction of the
positive normal to an imaginary plane surface of specified area located in a
specified position in the gas by molecules crossing the surface in both
directions, momentum transmitted in the opposite direction being counted as
negative, divided by the area of the surface.
- In general, it is assumed that the area of the imaginary plane surface
is small enough so that the pressure with respect to any part of the surface
is equal (within narrow limits) to the pressure based on the whole surface.
Different kinds of pressure (static, dynamic, partial, total, vapor, etc.)
are distinguished by the orientation of the surface with respect to mass-flow
velocity vectors or by the restriction to a specified set of molecular species
crossing the imaginary surface.
- 2. On a boundary surface, the force applied per unit area and equal to the
pressure in the gas as determined by molecules crossing an imaginary surface
located at a fixed distance of molecular magnitude in front of the real
surface, the positive normal being drawn from the imaginary surface toward the
- The term pressure when used alone can be assumed to refer to the total
pressure in a gas at rest or else to refer to the static pressure in a gas
flowing under steady-state conditions.
- 3. = atmospheric
- 4. As measured in a vacuum system, the quantity measured at a specified
time by a so-called vacuum
gage, whose sensing element is located in a cavity (gage tube) with an
opening oriented in a specified direction at a specified point within the
system, assuming a specified calibration factor.
- The sensitivity of the sensing element is, in general, not the same for
all molecular species, but the gage reading is frequently reported using the
calibration factor for air regardless of the composition of the gas. The
opening to the gage tube is often carelessly oriented with respect to
mass-flow vectors in the gas (which is seldom at rest), and errors due to
variations in wall temperatures of tube and system are frequently neglected.
The actual total pressure in a high-vacuum system cannot usually be measured
by a single gage, but in vacuum technology the term total pressure is
sometimes used to refer to the reading of a single untrapped gage which
responds to condensable vapors as well as permanent gases.
- pressure altimeter
- An altimeter
that utilizes the change of atmospheric
pressure with height to measure altitude. It
is commonly an aneroid altimeter. Also called barometric altimeter. See
- pressure altitude
- 1. Altitude in the earth's atmosphere above the standard datum plane,
standard sea level pressure, measured by a pressure altimeter.
- 2. The altitude in a standard atmosphere corresponding to atmospheric
pressure encountered in a real atmosphere.
- 3. The stimulated altitude created in an altitude chamber.
- pressure amplitude = maximum sound pressure.
- pressure breathing
- The breathing of oxygen or of a suitable mixture of gases at a pressure
higher than the surrounding pressure. See continuous pressure breathing, intermittent
- pressure-breathing system
- An oxygen system in which oxygen is injected inside the respiratory ducts
through a pressure higher than the surrounding pressure.
- pressure broadening
- The process in which the width of the lines in an emission
spectrum or absorption
spectrum of a gaseous radiative medium is increased due to perturbations
of the energy states by collisions of the molecules or atoms within the gas.
The extent of this line-broadening effect is directly proportional to the
number of impacts experienced by the emitter or absorber per unit time, and
hence is proportional to the pressure.
- pressure-demand oxygen system
- A demand
oxygen system that furnishes oxygen at a pressure
higher than atmospheric pressure above a certain altitude.
- pressure height = pressure altitude.
- pressure microphone
- A microphone
in which the electric output substantially corresponds to the instantaneous sound
pressure of the impressed sound wave.
- pressure stabilized
- Referring to membrane-type structures that require internal pressure for
maintenance of a stable structure; for example, the Atlas missile structure.
- pressure suit
- A garment designed to provide pressure upon the body so that respiratory
and circulatory functions may continue normally, or nearly so, under
low-pressure conditions, such as occur at high altitudes or in space without
benefit of a pressurized cabin.
- A pressure suit is distinguished from a pressurized suit, which
inflates, although it may be fitted with inflating parts that tighten the
garment as ambient pressure decreases. Compare g-suit.
- pressure thrust
- In rocketry, the product of the cross-sectional area of the exhaust jet leaving the nozzle exit
and the difference between the exhaust pressure and the ambient
- pressure transducer
- A transducer
which produces an output related to imparted pressure.
- pressure wave
- 1. In meteorology, a short period oscillation
such as that associated with the propagation of sound through the
atmosphere; a type of longitudinal
wave. See sound wave,
- These waves are usually recorded on sensitive microbarographs capable
of measuring pressure changes of amounts down to 10E-4 millibar. Typical
values for the period and wavelength of pressure waves are 1/2 to 5 seconds
and 100 to 1500 meters, respectively. Pressure waves produced by explosions in
the upper atmosphere are of value in determining the high-altitude
temperatures and winds.
- 2. A wave or periodicity which exists in the variation of atmospheric
pressure on any scale, usually excluding normal diurnal and seasonal
trends. See barometric
- Such waves can persist for an indefinite length of time only if they
coincide approximately with the free oscillations of the atmosphere. Waves of
a period longer than that associated with the passage of large-scale weather
disturbances are difficult to isolate, since they usually have such a small
amplitude that they can be extracted from the data and only by means of
precise statistical methods.
- The process of producing pressures
higher than ambient, as in a pressurized cabin.
- Containing air, or other gas, at a pressure
higher than ambient.
- pressurized suit
- A suit designed to be inflated so as to provide pressure directly upon the
body, not to air surrounding the body. Compare pressure
- pressurizing gas
- Specifically, a gas used to expel propellant from a fuel tank.
- 1. A step in the action of igniting a large liquid rocket taken prior to
the ignition of the full flow, and consisting of igniting a partial flow of propellants
into the thrust
- 2. The partial flow thus ignited. Also called preliminary stage.
- 1. Short for primary
- 2. Short for primary
- primary body
- The celestial
body or central
force field about which a satellite or
other body orbits, or from which it is escaping, or towards which it is
- The primary body of the moon is the earth; the primary body of the
earth is the sun.
- primary circle = primary great circle.
- primary circulation
- In meteorology, the prevailing fundamental atmospheric circulation on a
planetary scale which must exist in response to radiation differences with
latitude, to the rotation of the planet, and to the particular distribution of
land and oceans; and which is required from the viewpoint of conservation
- primary cosmic ray
- High-energy particles
originating outside the earth's atmosphere.
- Primary cosmic rays appear to come from all directions in space. Their
energy appears to range from 10E9 to more than 10E17 electron volts.
- primary great circle
- A great
circle used as the origin of measurement of a coordinate;
particularly, such a circle 90 degrees from the poles of a system of spherical
coordinates, as the equator. Also called primary circle, fundamental
- primary radar
using reflection only, in contrast with secondary radar which uses automatic
retransmission on the same or a different radio frequency.
- primary scattering
- Any scattering
process in which radiation is
received at a detector, such as the eye, after having been scattered just
once; to be distinguished from multiple scattering.
- primary standard
- A unit directly defined and established by some authority, against which
all secondary standards are calibrated, as the prototype kilogram.
- prime meridian
- 1. The meridian of
longitude 0 degrees, used as the origin for measurement of longitude.
The meridian of Greenwich, England, is almost universally used for this
- 2. Any meridian in any coordinate
system used as an origin for measurement of longitude.
- prime vertical
- The vertical
circle through the east and west points of the horizon. It may
be true, magnetic, compass , or grid depending upon which east
or west points are involved. Also called prime vertical circle.
- prime vertical circle = prime vertical.
- primitive atmosphere
- The atmosphere
of a celestial
body as it existed in the early stages of its formation; specifically, the
earth's atmosphere of 3 billion or more years ago, through to consist of water
vapor, carbon dioxide, methane, and ammonia gas.
- primitive equations
- The Eulerian equations
of motion of a fluid in which
the primary dependent variables are the fluid's velocity components. These
equations govern a wide variety of fluid motions and form the basis of most
- primitive period
- Of a periodic
quantity, the smallest increment of the independent
variable for which the function repeats itself.
- If no ambiguity is likely, the primitive period is simply called the
period of the function.
- principal planets
- The larger bodies revolving about the Sun in nearly circular orbits. See
- The known principal planets, in order of their distance from the Sun
are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
- principal stresses
- The normal stresses on three mutually perpendicular planes on which there
are no shear stresses.
- principal vertical circle
- The vertical
circle through the north and south points of the horizon,
coinciding with the celestial
- principle of reciprocity
- If an electromotive force at one point in a circuit
produces a current at a second point in the circuit, then the same voltage
acting at the second point will produce the same current at the first point.
- probable error (symbol ep)
- In statistics, that value ep for which there exists and even probability
(0.5) that the actual error exceeds ep. The probable error ep
is 0.6745 times the standard
- The probable error is not 'probable' in the normal sense of the word.
- The chance that a prescribed event will occur, represented as a pure
number P in the range 0<P<1. The probability of an impossible
event is zero and that of an inevitable event is unity.
- Probability is estimated empirically by relative frequency, that is,
the number of times the particular event occurs divided by the total count of
all events in the class considered.
- probability integral
- The classical form (still widely used in engineering work) of the definite
integral of the special normal
distribution for which the mean μ = 0 and standard
deviation σ = 1/21/2. Geometrically, the probability integral equals
the area under this density curve between -z and z , where z
is an arbitrary positive number. Often denoted by the symbol erf z
(read error function of z ) the probability integral is defined
Also called error function, erf.
- 1. Any device inserted in an environment
for the purpose of obtaining information about the environment.
- 2. In geophysics, a device used to make a sounding.
- 3. Specifically, an instrumented vehicle moving through the upper
atmosphere or space or landing
upon another celestial body in order to obtain information about the specific
- In sense 3, almost any instrumented spacecraft can be considered a
probe. However, earth satellites are not usually referred to as probes. Also,
almost any instrumented rocket can be considered a probe. In practice, rockets
which attain an altitude of less than 1 earth radius (400 miles) are called
sounding rockets, those which attain an altitude of more that 1 earth radius
are called probes or space probes. Spacecraft which enter into orbit around
the sun are called deep-space probes. Spacecraft designed to pass near or land
on another celestial body are often designated lunar probe, Martian probe,
Venus probe, etc.
- 4. Specifically, a slender device or apparatus projected into a moving fluid, as for
measurement purposes; a pitot tube.
- 5. Specifically, a slender projecting pipe on an aircraft which is thrust
into a drogue
to receive fuel in inflight refueling.
- process lapse rate
- The rate of decrease of the temperature
T of an air parcel as it is
lifted, -dT/dz , when z is altitude, or, occasionally, dT/dp
, where p is pressure.
- The concept may be applied to other atmospheric variables, e.g., the
process lapse rate of density. The process lapse rate is determined by the
character of the fluid processes and should be carefully distinguished from
the environmental lapse rate, which is determined by the distribution of
temperature in space. In the atmosphere the process lapse rate is usually
assumed to be either the dry-adiabatic lapse rate or the saturation-adiabatic
- 1. Of a variable, a curve representing corresponding values of two or more
variables which may occur.
- A profile accounts for the correlation from point to point on the curve
and has some possibility, not necessarily specified, of actual occurrence.
- 2. The contour or form of a body, especially in a cross section;
specifically, an airfoil profile.
- 3. Something likened to a profile (sense 1), such as a line on a graph, as
a flight profile.
- 1. In computer
operations, a plan for the solution of a problem.
- 2. To create a plan for the solution of a problem.
- A complete program includes plans for the transcription of data, coding
for the computer, and plans for the absorption of the results into the system.
The list of coded instructions, called a routine, plans a computation or
process from the asking of a question to the delivery of the result, including
the integration of the operation into an existing system. Thus, programming
consists of planning and coding, including numerical analysis, systems
analysis, specification of printing formats, and any other functions necessary
to the integration of a computer in a system.
- 1. Any object, especially a missile, fired,
thrown, launched, or otherwise projected in any manner, such as a bullet, a
guided rocket missile, a sounding rocket, a pilotless airplane, etc.
- 2. Originally, an object, such as a bullet or artillery shell, projected
by an applied external force.
- prolate spheroid
- An ellipsoid
of revolution, the longer axis of which is
the axis of revolution.
- An ellipsoid of revolution, the shorter axis of which is the axis of
revolution, is called an oblate spheroid.
- A filamentlike protuberance from the chromosphere
of the sun. See flocculi.
- Prominences can be observed visually (optically) whenever the sun's
disk is masked, as during an eclipse or by using a coronagraph;
and can be observed instrumentally by filtering in certain wavelengths, as
with a spectroheliograph.
A typical prominence is 6,000 to 12,000 kilometers thick, 60,000 kilometers
high, and 200,000 kilometers long.
- prompt neutrons
- In nuclear fission, those
neutrons released coincident with the fission process, as opposed to the
neutrons subsequently released.
- prompt radiation
- See radioactivity,
- The spreading abroad or sending forward, as of radiant
- propagation constant
- Of a traveling
plane wave at a given frequency,
the complex quantity whose real part is the attenuation
constant in nepers per unit length and whose imaginary part is the phase
constant in radians per unit length.
- propagation error
- For ranging
systems, the algebraic sum of propagation
velocity error and curved-path
- Except at long ranges and low angles, the curved-path component of
propagation error is generally negligible.
- propagation ratio
- For a wave
propagating from one point to another, the ratio of the complex electric
field strength at the second point to that at the first point.
- propagation velocity = velocity of propagation.
- propagation velocity error
- The difference between the effective value of propagation
velocity, over a ray path, and the assumed value. See effective
- (symbol p , used as a subscript). Any agent used for consumption or
combustion in a rocket and from
which the rocket derives its thrust, such as a fuel, oxidizer,
additive, catalyst, or any compound or mixture of these; specifically, a fuel,
oxidant, or a combination or mixture of fuel and oxidant used in propelling a
rocket. See fuel.
- Propellants are commonly in either liquid or solid form.
- propellant mass fraction (symbol ζ)
- Of a rocket, the
ratio of the effective propellant mass mp to the initial vehicle mass
ζ = mp/m0
Also called mass ratio, propellant mass
- propellant mass ratio = propellant mass fraction.
- proper motion
- That component of the space motion of a celestial
body perpendicular to the line of sight, resulting in the change of a
position relative to other stars. Proper motion is expressed in angular
- proportional control
of an aircraft, rocket, etc. in which control- surface deflection is
proportional to the movement of the remote controls.
- proportional navigation
- The control of the
angular rate of the velocity vector of a vehicle in proportion to the apparent
velocity of its moving target.
- proprioceptive stimulation
- Stimulation originating within the deeper structures of the body (muscles,
tendons, joints, etc.) for sense of body position and movement and by which
muscular movements can be adjusted with a great degree of accuracy and
equilibrium can be maintained.
- propulsive efficiency (symbol η)
- The efficiency with which energy available for propulsion is converted
into thrust by a rocket engine.
ηp = (2u/c)/[1 + (u/c)2]
where u is the absolute vehicle velocity,
and c is the effective exhaust velocity with respect to the vehicle.
Propulsive efficiency is a maximum when u = c.
- A positively charged subatomic
particle having a mass of 1.67252 X 10-24 gram, slightly less than that of a neutron but
about 1836 times greater than that of an electron.
- proton-proton reaction
- A thermonuclear
reaction in which two protons collide
at very high velocities and combine to form a deuteron. The
resultant deuteron may capture another proton to form tritium and the latter
may undergo proton capture to form helium. Compare carbon
- The proton-proton reaction is now believed to be the principal source
of energy within the sun and other stars of its class. A temperature of the
order of 5 million degrees Kelvin and high hydrogen (proton) concentrations
are required for this reaction to proceed at rates compatible with energy
emission by such stars.
- proton storm
- The flux of protons sent
into space by a solar
- Any of the sun's planets as it emerged or existed in the formative period
of the solar
- The sun as it emerged in the formation of the solar
- 1. Of any mechanical device, a production model suitable for complete
evaluation of mechanical and electrical form, design, and performance.
- 2. The first of a series of similar devices.
- 3. A physical standard to which replicas are compared, as the prototype
- proving stand
- A test stand for reaction
engines, especially rocket engines. See test stand.
- PsA, Psc A
- International Astronomical Union abbreviations for Piscis Austrinus.
- Psc, Pisc
- International Astronomical Union abbreviations for Pisces. See constellation.
- Psc A
- International Astronomical Union abbreviations for Piscis Austrinus.
- pseudoadiabatic expansion
- A saturation-adiabatic process in which the condensed water substance is
removed from the system, and therefore best treated by the thermodynamics of
open systems. See adiabatic
- Meteorologically, this process corresponds to rising air from which the
moisture is precipitating. Descent of air so lifted becomes by definition a
- An arbitrary code not directly understandable by a computer. Also
called interpreter code.
- pseudoequivalent temperature = equivalent temperature,
- pseudo-wet-bulb potential temperature = wet-bulb
- The science which studies the functions of the mind, such as sensation,
perception, memory, through, and, more broadly, the behavior of an organism in
relation to its environment.
- psychomotor ability
- Of or pertaining to muscular action ensuing directly from a mental
process, as in the coordinated manipulation of aircraft or spacecraft
- psychophysical quantity
- A physical measurement, as a threshold,
dependent on human attributes or perception.
- PTM (abbr) = pulse time modulation.
- Pertaining to, or affecting, the lungs or any component of the lungs.
- 1. A variation of a quantity whose value is normally constant; this
variation is characterized by a rise and a decay, and has a finite duration.
- The word pulse normally refers to a variation in time; when the
variation is in some other dimensions, it should be so specified, such as
space pulse. This definition is so broad that it covers almost any transient
phenomenon. The only features common to all pulses are rise, finite duration,
and decay. It is necessary that the rise, duration, and decay be of a quantity
that is constant (not necessarily zero) for sometime before the pulse and has
the same constant value for some time afterwards. The quantity has a normally
constant value and is perturbed during the pulse. No relative time scale can
- 2. Radar,
- 3. The intermittent change in the shape of an artery due to an increase in
the tension of its walls following the contraction of the heart. The pulse is
usually counted at the wrist (radial pulse), but may be taken over any artery
that can be felt.
- pulse amplitude
- A general term indicating the magnitude of
- For specific designation, adjectives such as average, instantaneous,
peak, root-mean-square (effective), etc., should be used to indicate the
particular meaning intended. Pulse amplitude is measured with respect to the
normally constant value unless otherwise stated.
- pulse amplitude modulation
- (abbr PAM). See pulse
- pulse code
- 1. A sequence of pulses so
modulated as to represent information.
- 2. Loosely, a code consisting of pulses, such as Morse code, binary
- pulse code modulation
- (abbr PCM). Any modulation which involves a pulse code.
- This is a generic term, and additional specification is required for a
- pulsed Doppler system
- A pulse
radar system which utilizes the Doppler
effect for obtaining information about the target (not
including simple resolution from fixed targets).
- pulse decay time
- The interval between the instants at which the instantaneous amplitude of
a pulse last reaches specified upper and lower limits, namely, 90 percent and
10 percent of the peak pulse
amplitude unless otherwise stated.
- pulsed radar = pulse radar.
- pulse duration
- The time interval between the first and last instants at which the
instantaneous amplitude reaches a stated fraction of the peak pulse
- pulse duration modulation
- A form of pulse
time modulation in which the duration of a pulse is varied.
- The terms pulse width modulation and pulse length modulation are also
used to designate this system of modulation but the term pulse duration
modulation is preferred.
- pulse frequency modulation
- (abbr PFM). A form of pulse
time modulation in which the pulse repetition rate is the characteristic
- A more precise term for pulse frequency modulation would be pulse
repetition rate modulation.
- A pulsejet
- pulsejet engine
- A type of compressorless jet engine
in which combustion takes place intermittently, producing thrust by a series
of explosions, commonly occurring at the approximate resonance
frequency of the engine. Often called a pulsejet.
- pulse length modulation = pulse duration modulation.
- pulse modulation
- 1. Modulation
of a carrier
by a pulse
train. Compare frequency
- In this sense, the term is used to describe that process of generating
carrier frequency pulses.
- 2. Modulation of one or more characteristics of a pulse carrier.
- In this sense, the term is used to describe methods of transmitting
information on a pulse carrier.
- pulse packet
- In radar,
the volume of space occupied by the radar pulse energy.
- pulse phase modulation (abbr PPM) = pulse position
- pulse position modulation
- (abbr PPM). A form of pulse
time modulation in which the position in time of a pulse is varied.
Also called pulse phase modulation.
- pulse radar
- A type of radar, designed
to facilitate range measurement, in which the transmitted energy is emitted in
periodic short pulses. Also called pulsed radar. Compare continuous-wave
- The distance to any target a detectable echo can be determined by
measuring one-half the time interval between transmitted pulse and received
echo and multiplying this number by the speed of light. This is by far the
most common type of radar.
- pulse repeater
- In a transponder,
a device used for receiving pulses from one
circuit and transmitting corresponding pulses into another circuit. It may
also change the frequency and wave forms of the pulses and perform other
- pulse spike
- An unwanted pulse of
relatively short duration superimposed on the main pulse.
- pulse time modulation
- (abbr PTM). Modulation
in which the values of instantaneous samples of the modulating
wave are caused to modulate the time of occurrence of some characteristic of a
- pulse train
- In radio, a sequence of pulses.
- pulse width
- The time interval during which a pulse exceeds a
- For measuring pulse width, the reference level is generally taken at
- pulse width modulation (abbr PWM) = pulse duration
- Pup, Pupp
- International Astronomical Union abbreviations for Puppis. See constellation.
- Puppis (abbr Pup, Pupp)
- See constellation.
- To rid a line or tank of residual fluid, especially of fuel or oxygen in
the tanks or lines of a rocket after a
test firing or simulated test firing.
- Purkinje effect
- The response of the human eye which makes it less sensitive to lights of
loner wavelengths under conditions of decreased illumination, e.g., red
appears darker at night than blue having the same brightness under photopic
conditions. See color
- push-pull = balanced
- See balanced
- push-pull amplifier = balanced amplifier.
- push-push circuit
- A circuit
employing two similar tubes with grids connected in phase opposition
and plates in parallel to a common load, and usually
used as a frequency multiplier to emphasize even-order harmonics.
- PWM (abbr) = pulse width modulation.
- An actinometer
which measures the combined intensity of incoming direct
solar radiation and diffuse
sky radiation. The pyranometer consist of a recorder and a radiation
sensing element which is mounted so that it views the entire sky. Sometimes
called solarimeter. See pyrheliometer,
- An actinometer which measures the effective
terrestrial radiation. See Angstrom
- An actinometer which measures the intensity of direct
solar radiation, consisting of a radiation sensing element enclosed in a
casing which is closed except for a small aperture, through which the direct
solar rays enter, and a recorder unit. See Angstrom
compensation pyrheliometer, silver-disk
- The science and study of pyrheliometric measurements. See pyrheliometer.
- Chemical decomposition by the action of heat.
- An instrument for the measurement of temperatures;
generally applied to instruments measuring temperatures above 600 degrees C.
- pyrometric photography
- The derivation of flame temperature
measurements by means of comparative photography with a calibrated light
- High-temperature thermometry,
the technique of measurement of temperatures, generally above 600 degrees C,
at a distance.
- A unit of radiant intensity of
electromagnetic radiation equal to 1 calorie per square centimeter per minute.
- pyrophoric fuel
- A fuel that
ignites spontaneously in air. Compare hypergolic
- Pyx, Pyxi
- International Astronomical Union abbreviations for Pyxis (=
Malus ). See constellation.
- Pyxis (= Malus)
- (abbr Pyx, Pyxi). See constellation.