P

Source edition 1965. Please read the Introduction to find out about this dictionary and our plans for it. Caution, many entries have not been updated since the 1965 edition.
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package

Any assembly or apparatus, complete in itself or practically so, identifiable as a unit and readily available for use or installation. See power package.

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 spray.

paddlewheel satellite

A satellite, such as Explorer VI, that has solar vanes or similarly shaped objects attached.

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 positron pair.

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)

Frequency modulation of a carrier by pulses which are amplitude modulated by information.

PAM/FM/FM (abbr)

Frequency modulation of a carrier by subcarrier(s) which is (are) modulated by pulses which are amplitude modulated by information.

panchromatic

See color sensitive, note.

parabola

An open curve all points of which are equidistant from a fixed point, called the focus , and a straight line. See conic section.

The limiting case occurs when the point is on the line, in which case the parabola becomes a straight line.

parabolic

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 attracting body.

parabolic reflector

A reflecting surface having the cross section along the axis in the shape of a parabola. See corner reflector, radar reflector, scanner.

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 parallel.

paraboloid

A surface of revolution generated by revolving a section of a parabola about its major axis.

paraboloidal

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 vision.

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.

parallax

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 not coincide.

This error is a function of the distance of the target from the baseline, as well as its relative direction.

parallax in altitude

Geocentric 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.

parallel

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. See coordinate, table.

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 circle.

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 called parallel.

2. A circle of the celestial sphere, parallel to the ecliptic, and connecting points of equal celestial latitude. Also called circle of longitude.

paramagnetic

Having a magnetic permeability greater than unity.

parameter

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.

parameterization

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 constant.

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 physical interpretation.

parasitic element

A radiating element, not coupled directly to the feed line of the antenna, which materially affects the pattern of the antenna.

parcel = fluid parcel.

Pardop (abbr) = passive ranging Doppler system.

parent

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.

parity

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

part

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 D_xf and D_yf, or f_x and f_y.

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 eclipse, note.

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 eclipse.

particle

1. An elementary subatomic particle such as proton, electron, neutron, etc.

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 electrostatic accelerator.

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 separation.

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 law, note.

pass

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 acquisition station.

passive

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 homing.

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 no transponder is used in the missile.

Space position is computed from several loop ranges between the transmitter, missile, and receivers.

path

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 curve.

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.

payload

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 m_p to the initial vehicle mass m_o or ζ = m_p/m_o. Also called mass ratio.

P-band

A frequency band used in radar extending approximately from 225 to 390 megacycles per second. See frequency bands.

PCM (abbr) = pulse code modulation.

PCM/FM (abbr)

Frequency modulation of a carrier by pulse-code-modulated information.

PCM/FM/FM (abbr)

Frequency modulation of a carrier by subcarrier(s) which is (are) frequency modulated by pulse-code-modulated information.

PCM/PM (abbr)

Phase modulation of a carrier by pulse-code-modulated information.

P-display = plan position indicator.

PDM (abbr) = pulse duration modulation.

PDM/FM (abbr)

Frequency modulation of a carrier by pulses which are modulated in duration by information.

PDM/FM/FM (abbr)

Frequency modulation of a carrier by subcarrier(s) which is (are) frequency modulated by pulses which are modulated in duration by information.

PDM/PM (abbr)

Phase 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 instantaneous sound 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 sound pressure.

peak-to-peak value

Of an oscillating quantity, the algebraic difference between the extremes of the quantity.

Peclet number (symbol Np_e)

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 Np_e = U l/k where U is a characteristic velocity; l is a characteristic length; and k is the thermometric conductivity. Also, Np_e = N_ReN_Pr where N_Re is the Reynolds number and N_Pr is the Prandtl number.

Peg, Pegs

International Astronomical Union abbreviations for Pegasus. See constellation.

Pegasus (abbr Peg, Pegs)

See constellation.

Pegs

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 an antenna, having the form of a narrow conical beam.

pencil-beam antenna

A unidirectional antenna, so 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 field.

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 atoms.

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.

penumbra

See umbra.

penumbral eclipse

See lunar eclipse, note.

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 gas.

A perfect fluid is sometimes further characterized as homogeneous and continuous.

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. Compare perfect fluid.

The normal volume of a perfect gas is 2.24136 X 10E4 centimeters cubed per mole.

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 radiator.

When viewed from a distance, an incandescent perfectly diffuse radiator appears as a uniformly illuminated flat surface regardless of its actual shape or orientation.

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 with Lambert 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.

peri

A prefix meaning near as in perigee.

periapsis

The orbital point nearest the center of attraction. See orbit.

periastron

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.

pericynthian

That point in the trajectory of a vehicle which is closest to the moon.

perifocus

The point on an orbit nearest the dynamical center ( focus). The pericenter is at one end of the major axis of the orbital ellipse.

perigee

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.

perihelion

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.

period

1. The interval needed to complete a cycle.

2. = orbital period.

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 variable.

periodic terms

See secular terms, note.

period of moon's node

See nutation, note.

period scrams

Electronic safety circuits that automatically insert safety rods in a reactor when the reactor period decreases below the safe minimum limit.

periscope

An optical instrument which displaces the line of sight parallel to itself to permit a view which may otherwise be obstructed.

periscopic

Of or pertaining to a periscope, as in periscopic sextant.

permanent magnetism

Magnetism which is retained for long periods without appreciable reduction, unless the magnet is subjected to demagnetizing force. See induced magnetism.

Because of the slow dissipation of such magnetism, it is sometimes called subpermanent magnetism, but the expression permanent magnetism is considered preferable.

permanent memory

In computer terminology, storage of information which remains intact when the power is turned off. Also called nonvolatile storage.

permeability

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 fluids.

3. = permeability coefficient.

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 called permeability.

Usually expressed in volume or mass per unit time, per unit area of cross section, per unit thickness, per unit pressure differential across the barrier.

permeation

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 involves diffusion 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.

Pers

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.

perturbation

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 perturbation.

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 value.

PFM (abbr) = pulse frequency modulation.

phase

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 reference.

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 gas.

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 constant.

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 frequency.

The extent of deviation is proportional to the amplitude of the modulating signal.

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 detection.

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)

Angle 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 wave.

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 month.

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 function.

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 of propagation.

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 ionization gage.

Phobos

A satellite of Mars orbiting at a mean distance of 9,400 kilometers.

Phoe

International Astronomical Union abbreviation for Phoenix. See constellation.

Phoebe

A satellite of Saturn orbiting at a mean distance of 12,960,000 kilometers.

Phoenix (abbr Phe, Phoe)

See constellation.

phon

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. Compare sone.

phonometer

An instrument for measuring the intensity or frequency of sounds.

phosphor

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.

phosphorescence

Emission of light which continues after the exciting mechanism has ceased. See luminescence. Compare fluorescence.

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.

phot

A photometric unit of illuminance or illumination equal to 1 lumen per square centimeter. Compare foot-candle, lux.

photocathode

An electrode used for obtaining photoelectric emission.

photocell = photoelectric cell.

photochemical reaction

A chemical reaction which involves either the absorption or emission of radiation.

photoconductive cell

A photoelectric cell whose electrical resistance varies with the amount of illumination falling upon the sensitive area of the cell.

photodissocation

The dissociation (splitting) of a molecule by the absorption of a photon. The resulting components may be ionized in the process (photoionization).

photoelectric

1. Pertaining to the photoelectric effect.

2. Using a photoelectric cell, as a photoelectric photometer.

photoelectric cell

A transducer which converts electromagnetic radiation in the infrared, visible, and ultraviolet regions into electrical quantities such as voltage, current, or resistance. Also called photocell. See photoelectric effect.

photoelectric effect

The emission of an electron 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 effect.

photoelectric photometry

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.

photoelectron

An electron which has been ejected from its parent atom by interaction between that atom and a high-energy photon.

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.

photogrammetry

The art or science of obtaining reliable measurements by means of photography.

photographic magnitude (symbol m_pg)

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 (m_pg) and visual (m_v) magnitudes are the same for stars of class A0 of magnitudes between 5.5 and 6.5. Photovisual magnitudes (m_pv) are measured from plates exposed through filters which hold back blue and violet thus giving magnitudes in the plate which closely approximate visual magnitudes (m_v).

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 density.

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.

photoionization

The ionization of an atom or molecule by the collision of a high-energy photon with the particle. See photoelectron.

photology

The study of light.

photoluminescence = fluorescence, see luminescence.

photometer

An instrument for measuring the intensity of light or the relative intensity of a pair of lights. Also called illuminometer.

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 sensor.

photometry

The study of the measurement of the intensity of light.

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.

photon

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 vision.

photosphere

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.

photosynthesis

A process operating in green plants in which carbohydrates are formed under the influence of light with chlorophyll serving as a catalyst. See closed ecological system.

phototheodolite

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.

phototube

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 magnitude, note.

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 liquid.

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 the observer).

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 electricity, cloud physics, precipitation physics, atmospheric acoustics, and atmospheric optics.

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.

physiology

The science that treats of the functions of living organisms or their parts, as distinguished from morphology, anatomy, etc.

phytotron

A biotron designed especially for research on plant life.

Pic, Pict

International Astronomical Union abbreviations for Pictor. See constellation.

pickoff

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 pilot.

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 other devices.

pickup

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.

pico

A prefix meaning multiplied by 10 ^-12.

Pict

International Astronomical Union abbreviation for Pictor. See constellation.

Pictor (abbr Pic, Pict)

See constellation.

piezoelectricity

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 element.

pig discharge

See Penning discharge, note.

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.

pilot

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.

piloted

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 magnetic field.

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 is generated.

P-indicator = plan position indicator (PPI).

pip

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 temperature constant).

Pisc

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.

pitch

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 body axis 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 note.

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.

pitchover

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 head.

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.

pixel

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.

plages

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/(e^c2/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 sound wave.

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.

planet

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 tables.

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 precession of the equinoxes.

Planetary precession is approximately 0.1247 second of arc per year.

planetocentric

1. Of or pertaining to a planet's center of mass.

2. Of or pertaining to the planet as a center of a system.

planetographic

Referring to positions on a planet measured in latitude from the planet's equator and in longitude 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.

planform

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.

plasma

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 frequency.

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 velocities.

The plasma sheath affects transmission, reception, and diffraction of radio waves; thus it is important in operational problems of spacecraft.

plasticity

The tendency of a loaded body to assume a deformed state other than its original state when the load is removed.

plate

1. A planar body whose thickness is small compared with its other dimensions.

2. A common name for the principal anode in an electron tube.

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 ended.

Pluto

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.

pod

An enclosure, housing, or detachable container of some kind, as an engine pod.

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 discharge, spark discharge.

point of inflection

See inflection.

poise

The unit of viscosity in the cgs system equal to 1 dyne second per square centimeter.

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 flow.

poison

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 relaxation method.

2. The relationship between the temperature T and pressure p of a perfect gas undergoing an adiabatic process; given by T = constant X p^μ where μ is the Poisson constant.

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 coordinates.

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.

polarimeter

An instrument for determining the degree of polarization of electromagnetic radiation, specifically the polarization of light.

polariscope

An instrument for detecting polarized radiation and investigating its properties.

polarity

The sign of the electric discharge associated with a given object, as an electrode or an ion.

polarizability

A measure of the degree to which any given atom or ion undergoes polarization in the presence of an external electric field.

polarization

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 curve.

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 earth's atmosphere.

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 field.

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.

polarizer

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 Polar Year.

pole

1. The origin of a system of polar coordinates.

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 pole, celestial pole, elevated pole, depressed pole, ecliptic pole, fictitious pole.

3. A point of concentration of electric charge. See dipole.

4. A point of concentration of magnetic force. See magnetic pole.

pole of the Milky Way

The pole in the galactic 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 = c_p/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 piezotropy.

population

In statistical usage, any definite class of individuals or objects. Also called universe. Compare sample.

port

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.

position

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 position, surface of position.

positional notation

Any scheme for representing quantities characterized by the arrangement of digits in 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 acceleration.

positive feedback

Feedback which results in increasing the amplification.

positive g or positive G

See physiological acceleration.

positron

A subatomic particle which is identical to the electron in atomic mass, theoretical rest mass, and energy, but opposite in sign. Compare proton.

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 function, potential energy.

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 density, potential temperature.

3. Short for electric potential.

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 process.

potential energy

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 gradient.

potential index of refraction

An atmospheric index of refraction so formulated that it would have no height variation in an adiabatic 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 refraction.

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 expression is 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 c_p is the specific heat or dry air at constant pressure. See equivalent potential temperature, adiabatic process.

potentiometer

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, exactly.

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 2.

poundal

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 1- pound mass, will produce an acceleration of 32.1747 feet per second squared.

power

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 decibels.

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 loading.

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 a_nxⁿ = a₀ + a₁x + a₂x² . . . + a_nxⁿ

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 splitter.

PPI scope = plan position indicator.

PPM (abbr) = pulse position modulation.

PPM/AM (abbr)

Amplitude modulation of a carrier by pulses which are position modulated by information.

Prandtl number (symbol N_Pr, P_r)

(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 N_Pr = μc_p/k where μ is the viscosity coefficient; c_p 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 Peclet numbers.

preamplifier

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.

precession

Change in the direction of the axis of rotation of a spinning body, as a gyro, when acted upon by a torque. See apparent wander, precession 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. See cloud attenuation, range attenuation.

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.

precision

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 chamber.

predissociation

A process by which a molecule that has absorbed energy separates into constituents before it loses energy by radiation. See dissociation.

preliminary stage = prestage.

presentation

In electronics, the act or process of displaying radar echoes on a cathode-ray screen; the echo or images displayed on a cathode-ray screen.

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 real surface.

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 pressure.

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 aneroid, sense 1.

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.

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. Compare Doppler broadening.

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.

pressure transducer

A transducer which produces an output related to imparted pressure.

pressure wave

1. In meteorology, a short period oscillation of pressure such as that associated with the propagation of sound through the atmosphere; a type of longitudinal wave. See sound wave, compression 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 wave.

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.

pressurization

The process of producing pressures higher than ambient, as in a pressurized cabin.

pressurized

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 suit.

pressurizing gas

Specifically, a gas used to expel propellant from a fuel tank.

prestage

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 chamber.

2. The partial flow thus ignited. Also called preliminary stage.

primary

1. Short for primary body.

2. Short for primary cosmic ray.

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 falling.

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 of energy.

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 circle.

primary radar

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 purpose.

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 hydrodynamical analysis.

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 planet.

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 meridian.

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 e_p)

In statistics, that value e_p for which there exists and even probability (0.5) that the actual error exceeds e_p. The probable error e_p is 0.6745 times the standard deviation σ.

The probable error is not 'probable' in the normal sense of the word.

probability

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 = 1/2^1/2. 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 thus: Also called error function, erf.

probe

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 environment.

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 lapse rate.

profile

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.

program

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.

projectile

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.

prominence

A filamentlike protuberance from the chromosphere of the sun. See flocculi. Compare flare.

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, note.

propagation

The spreading abroad or sending forward, as of radiant energy.

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 error.

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 propagation velocity.

propellant

(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 m_p to the initial vehicle mass m₀ or ζ = m_p/m₀ Also called mass ratio, propellant mass ratio.

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 star's apparent position relative to other stars. Proper motion is expressed in angular units.

proportional control

Control of an aircraft, rocket, etc. in which control- surface deflection is proportional to the movement of the remote controls. Compare flicker control.

proportional navigation

The control of the angular rate of the velocity vector of a vehicle in proportion to the apparent relative angular 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. η_p = (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.

proton

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 cycle.

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 flare.

protoplanet

Any of the sun's planets as it emerged or existed in the formative period of the solar system.

protosun

The sun as it emerged in the formation of the solar system.

prototype

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 kilogram.

proving stand

A test stand for reaction engines, especially rocket engines. See test stand.

PsA, Psc A

International Astronomical Union abbreviations for Piscis Austrinus. See constellation.

Psc, Pisc

International Astronomical Union abbreviations for Pisces. See constellation.

Psc A

International Astronomical Union abbreviations for Piscis Austrinus. See constellation.

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 process.

Meteorologically, this process corresponds to rising air from which the moisture is precipitating. Descent of air so lifted becomes by definition a dry-adiabatic process.

pseudocode

An arbitrary code not directly understandable by a computer. Also called interpreter code.

pseudoequivalent temperature = equivalent temperature, sense 2.

pseudo-wet-bulb potential temperature = wet-bulb potential temperature.

psychology

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 controls.

psychophysical quantity

A physical measurement, as a threshold, dependent on human attributes or perception.

PTM (abbr) = pulse time modulation.

pulmonary

Pertaining to, or affecting, the lungs or any component of the lungs.

pulse

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 be assigned.

2. Radar, sense 2.

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 a pulse.

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 modulation.

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 code.

pulse code modulation

(abbr PCM). Any modulation which involves a pulse code.

This is a generic term, and additional specification is required for a specific purpose.

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 amplitude.

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 varied.

A more precise term for pulse frequency modulation would be pulse repetition rate modulation.

pulsejet

A pulsejet engine.

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 modulation.

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 modulation.

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 radar.

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 functions.

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 carrier.

pulse train

In radio, a sequence of pulses.

pulse width

The time interval during which a pulse exceeds a reference level.

For measuring pulse width, the reference level is generally taken at the half-power points.

pulse width modulation (abbr PWM) = pulse duration modulation.

Pup, Pupp

International Astronomical Union abbreviations for Puppis. See constellation.

Puppis (abbr Pup, Pupp)

See constellation.

purge

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 index, dark adaptation, photopic vision.

push-pull = balanced

See balanced amplifier, balanced circuit.

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.

pyranometer

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, solarimeter, Robitzsch actinograph, albedometer.

pyrgeometer

An actinometer which measures the effective terrestrial radiation. See Angstrom pyrgeometer.

pyrheliometer

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 pyrheliometer, water-flow pyrheliometer, Eppley pyrheliometer, spectropyrheliometer, Michaelson actinograph.

pyrheliometry

The science and study of pyrheliometric measurements. See pyrheliometer.

pyrolysis

Chemical decomposition by the action of heat.

pyrometer

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 source.

pyrometry

High-temperature thermometry, the technique of measurement of temperatures, generally above 600 degrees C, at a distance.

pyron

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 propellants.

Pyx, Pyxi

International Astronomical Union abbreviations for Pyxis (= Malus ). See constellation.

Pyxis (= Malus)

(abbr Pyx, Pyxi). See constellation.