Glossary - P

Payload
part of a satellite which performs the primary function for which the satellite was designed; in a communications satellite the payload consists of the communications equipment which receives, processes and retransmits the required channels
(spacecraft technology)

Periapsis
the nearest point on an orbit to the body being orbited
(orbits)

Perichron
the nearest point on the orbit of a body or spacecraft orbiting Saturn from Saturn
(orbits)

Perigee
the nearest point on the orbit of a body or spacecraft orbiting the Earth from the Earth
(orbits)

Perihelion
the nearest point on the orbit of a body or spacecraft orbiting the Sun from the Sun
(orbits)

Perijove
the nearest point on the orbit of a body or spacecraft orbiting Jupiter from Jupiter
(orbits)

Perilune
the nearest point on the orbit of a body or spacecraft orbiting the Moon from the Moon
(orbits)

Periselene
the nearest point on the orbit of a body or spacecraft orbiting the Moon from the Moon
(orbits)

Period
the time taken by an object (such as a satellite) to complete one orbit around a larger body (such as the Earth)
(orbits)

Phase
the angular distance between the peaks or troughs of two waveforms of the same frequency
(communications)

Phase
the proportion of a celestial body illuminated by the Sun, as in the phases of the Moon (crescent, half, gibbous, full)
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Phase
one of several predefined periods in a mission or activity
(technology)

Phasing orbit
a temporary orbit used prior to the injection of a satellite into its final orbit
(orbits)

Photosphere
the lowest layer of a star’s atmosphere that emits visible light from the star and which constitutes the visible disk of a star (such as the Sun) when observed in visible light
(solar physics)

Piracy
1: unauthorised access to controlled transmissions
(broadcasting)

Piracy
2: the unauthorised use of transmission systems (ie by unlicensed broadcasters)
(communications)

Planetary defence
the protection of the Earth from asteroids and comets which may collide with the Earth in the future; consists of the identification of potential impactors, and then the deflection or destruction of them before they hit the Earth
(planetary science)

Planetary nebula
an expanding shell of luminous ionised gas expelled from the surface of red giant stars towards the end of their lives; with a small telescope planetary nebula have a similar appearance to planets
(astronomy, astrophysics)

Plasma
electrically conductive state of matter consisting of ions and electrons
(physics, astrophysics)

Plunge
when the tracking motion of an AZ-EL or ALT-Z antenna (usually on a radio telescope) exceeds 90° in elevation and then continues tracking as the elevation decreases on the other side without rotating around in azimuth
(astronomy)

Point to point link
one or two-way connection between two nodes
(communications)

Polar Orbit
an orbit with inclination i=90° or thereabouts
(orbits)

Polarisation plane
plane which contains the E-wave of an electromagnetic wave
(physics)

Polarisation rotator
a device which can select one of two orthogonal polarisations, operation may be manual or may be controlled electronically
(technology)

Positioning
technique of determining the location of an object or vehicle by triangulation from ground or satellite based beacons
(technology)

Pre-assigned multiple access
a multiple access system in which earth stations are permanently assigned transmission channels
(communications)

Prime focus
the point where a lens, mirror or dish brings waves to a focus
(technology)

Prograde
1: an orbit around the Sun in which the orbiting body orbits in the same direction as the planets in the Solar System
(orbits)

Prograde
2: an orbit around a celestial body in which the orbiting body orbits in the same direction as the celestial body rotates
(orbits)

Prograde burn
a thruster burn which increases the velocity of a spacecraft in the direction of its orbital motion around a celestial body
(orbital dynamics)

Prominence
a large structure consisting of plasma and magnetic fields which extends out from the Sun’s photosphere into the corona, often forming a loop
(solar physics)

Propagation delay
time taken for a signal to travel from the sender to the receiver
(communications)

Propellant
the fuel or oxidant used by a satellite or launch vehicle; propellants can be solid or liquid
(rocketry)

Protected use transponder
a transponder on which users are guaranteed that their traffic will be switched to another transponder, should the transponder fail
(satellite communications)

Pulsar
a neutron star with a high magnetic field which emits beams of electromagnetic radiation from its magnetic poles; as the pulsar rotates the beams are swept across the sky appearing as pulses of electromagnetic radiation to a fixed observer
(astronomy, astrophysics)