Glossary - A

Access
the technique by which terminals in a network determine when they can transmit data on a shared medium such as a satellite channel
(satellite communications)

Access
the ability of a user to make use of a network
(communications)

Activation
commissioning; registration and enabling of a satellite earth station on a network or of a satellite TV receiver with a broadcaster
(satellite communications)

Active Galactic Nucleus (AGN)
compact central region of a galaxy with a high luminosity that is too high to be produced only by stars; the excess luminosity is believed to result form the accretion of matter by a supermassive black hole
(astrophysics)

Actuator
motor and feedback system used to adjust the pointing or position of a movable device such as an antenna
(launcher technology, spacecraft technology)

Adaptive Differential Pulse Code Modulation (ADPCM)
a variant of PCM which encodes only the difference between the value of the current sample and the previous one
(communications)

Aerobraking
the use of the upper atmosphere of a planet to slow a spacecraft, allowing the spacecraft to reduce its velocity without the expenditure of fuel
(orbital dynamics)

Addressability
the ability of a network to individually address and control a user's receiver
(communications)

Aerosol
suspension of liquid droplets in a gas, as is found in a cloud or fog
(physics)

Affiliate
independently owned broadcast station that contracts with a broadcasting network to transmit that network's programming
(broadcasting)

Air time
actual time spent communicating over a satellite link
(satellite communications)

Air time
time at which an event is broadcast
(broadcasting)

Albedo
a measure of the proportion of incident light that is diffusely reflected by a body such as a planet, moon, asteroid or comet; usually expressed as a number between 0 and 1 or as a percentage with more reflective bodies having a higher albedo value
(planetary science)

Amplifier
a device to increase the strength of an electronic signal
(technology)

Amplitude
the magnitude of a signal
(communications)

Analogue
used to describe systems in which the transmitted electrical signal is proportional to a parameter in the source signal (such as air pressure in a telephony system)
(communications)

Antenna
a device used to radiate or collect radio or microwave signals; usually a dish, but horns and multi-element arrays are also used
(technology)

Aperture
the cross sectional area of an antenna or of the lens or mirror of a camera
(technology)

Apogee
the furthest point on a satellite's orbit from the Earth
(orbits)

Apogee Kick Motor (AKM)
a rocket motor used to take a satellite from geostationary transfer orbit to geostationary orbit; it is fired when the satellite is at the apogee of the GTO
(spacecraft technology)

Assigned frequency band
the frequency band in which the transmissions from a radio station is authorised
(communications, regulations)

Assignment
the authorisation by an administration for a radio station to use a particular radio channel under specified conditions
(communications, regulations)

Asteroid
a minor planet consisting primarily of rock, metal or a mixture of rock and metal, with a size between 1 metre and 1000 km in diameter
(planetary science)

Astrobiology
the scientific study of non-terrestrial life; also known as exobiology
(biology)

Atmosphere
gaseous layer surrounds a planet
(planetary science)

Attenuation
the loss in power of a radio or optical signal between transmission and reception
(communications)

Attitude Control
the maintenance of a satellite in a particular orientation with regard to the Earth, using, for example, thrusters, gyros, etc.
(spacecraft technology)

Audio distribution
transmission of audio signals (usually music or advertising) from a central location to many remote locations; used, for example, by supermarkets to provide background music
(satellite communications)

Aurora
a natural light display in the upper atmosphere above the polar regions of a planet caused when charged particles (usually electrons and protons) are displaced from the from the magnetosphere by changes in the solar wind and ionise gases in the upper atmosphere resulting in the emission of coloured light
(atmospheric physics)

Authentication
process of validating and identifying a caller as being a legitimate user in a mobile satellite system
(satellite communications)

Authorised power
the maximum signal strength allowed for a particular class of licensed transmitting station
(communications, regulations)

Automatic Frequency Control (AFC)
the technique by which a circuit automatically tracks slow changes in the frequency of a signal; a circuit which performs this function
(communications)

Automatic Gain Control (AFC)
a circuit which automatically adjusts the gain of an amplifier to maintain a constant output level
(communications)

Availability
the probability of achieving a specified level of performance and maintaining that level for a predetermined period of time
(technology)

Az/El Mount
a mount (for an antenna, optical receiver or optical tracker) that requires two separate adjustments (in azimuth and elevation) to move from one satellite to another
(earth station technology)

Azimuth
angular displacement of the satellite to the east or west from south when viewed from the earth station
(orbital geometry)