| Access | the technique by which terminals in a network determine when they can transmit data on a shared medium such as a satellite channel; the ability of a user to make use of a network |
| Access line | a local loop; the physical connection between a user and a network |
| Access time | the time between the request for a data transfer and the reception of that data |
| ACK | an ASCII character usually used to confirm message reception; also a packet used for the same purpose; in some communications protocols used as a "ready" reply |
| Acknowledgement | a signal sent by a receiver to a transmitter to indicate that a message has been received correctly |
| Activation | commissioning; registration and enabling of a satellite earth station on a network or of a satellite TV receiver with a broadcaster |
| Actuator | motor and feedback system used to drive a movable antenna |
| Adaptive Differnetial Pulse Code Modulation (ADPCM) | a variant of PCM which encodes only the difference between the value of the current sample and the previous one |
| Address | the location of a piece of data in a storage device; the digital representation of a terminal's location on a network; the unique identifier of a terminal on a network |
| Address resolution protocol | Internet protocol for dynamically mapping Internet addresses to physical hardware addresses on LANs; limited to LANs that support hardware broadcast; required to permit computers to discover the physical address of other computers on the same network |
| Addressability | the ability of a network to individually address and control a user's receiver |
| Advanced Data Communication Control Procedure (ADCCP) | a bit oriented datalink protocol similar to HDLC |
| Aerosol | suspension of liquid droplets in a gas, as is found in a cloud or fog |
| Affiliate | independently owned broadcast station that contracts with a broadcasting network to transmit that network's programming |
| Aggregate input | the sum of the data rates of the terminals connected to a statistical multiplexer |
| Air time | actual time spent communicating over a satellite link |
| Air time | time at which an event is broadcast |
| Algorithm | a formula or procedure which defines the method to be used to process data to generate a particular output |
| Aloha | a random access multi-access protocol in which packets of data are transmitted as soon as they arrive at the uplink earth station; in the event of a collision, the data is retransmitted |
| Alphameric | any character which is either a letter or a numeral |
| Alternate routing | a communications path used if the primary path is not available |
| Amplifier | a device to increase the strength of an electronic signal |
| Amplitude | the magnitude of a signal |
| Amplitude Modulation (AM) | a form of modulation in which the baseband signal is used to vary the amplitude of a carrier wave |
| 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) |
| Analogue channel | a communications channel which carries an analogue signal |
| Analogue Circuit Multiplication Equipment (ACME) | equipment designed to allow several analogue speech circuits to share a single speech channel by exploiting the statistics of the periods of silence in speech. |
| Analogue to digital conversion | the process of converting an analogue signal to a digital representation |
| Answer back | a reply message from a terminal to verify that the terminal has been addressed and is operational |
| Antenna | a device used to radiate or collect radio or microwave signals; usually a dish, but horns and multi-element arrays are also used |
| Aperture | the cross sectional area of an antenna |
| Apogee | the furthest point on a satellite's orbit from the Earth |
| 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 |
| Application layer | layer 7 of the OSI Reference Model; implemented by various network applications including electronic mail, file transfer and terminal emulation |
| Ariane | type of launch vehicle |
| ARQ | an error correction procedure in which the receiver asks the transmitter to resend a character or block of data when errors are detected |
| Aspect ratio | the ratio of the width of a TV picture to its height; conventional pictures have a 4:3 aspect ratio; widescreen is usually 16:9 but 14:9 and 23:3 are also used |
| Assigned frequency band | the frequency band in which the transmissions from a radio station is authorised |
| Assignment | the authorisation by an administration for a radio station to use a particular radio channel under specified conditions |
| Asynch, Asynchronous transmission | a transmission technique in which the absolute timing of the data transmission is variable; asynchronous data transmission typically frames the bits in each byte with start and stop bits |
| Asynchronous Balanced Mode | communications mode used in HDLC that allows either of two workstations in a peer oriented point to point configuration to initiate a data transfer |
| Asynchronous Transfer Mode (ATM) | a form of data transmission based on fixed length packets called calls, which can carry data, voice and video |
| AT command set | the name of the commands used to control most personal computer modems, taken from the characters "AT" which precede most commands; originally developed by Hayes |
| ATM Adaptation Layer | one of three layers of the ATM protocol reference model. The AAL translates incoming data into ATM cell payloads and translates outgoing cells into a format readable by the higher layers. Five AALs have been defined:
- AAL1 and AAL2 which handle isochronous (constant bit rate) traffic such as voice and video
- AAL3/4 and AAL5 which apply to data communications through the segmentation and reassembly of packets
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| ATM Forum | a 700 member vendor and end user consortium created to promote standards and interoperability for ATM products |
| Atmosphere | layer of air (mainly oxygen and nitrogen, with some carbon dioxide and water vapour) which surrounds the Earth |
| Attenuation | the loss in power of a radio signal between transmission and reception |
| Attitude Control | the maintenance of a satellite in a particular orientation with regard to the Earth, using, for example, thrusters, gyros, etc. |
| 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 |
| Audio frequency | the frequencies of sound waves which can be perceived by the human ear, generally 30 Hz to 20,000 Hz |
| Audio subcarrier | a secondary carrier carrying an audio signal which is modulated onto a video carrier |
| Authentication | process of validating and identifying a caller as being a legitimate user in a mobile satellite system |
| Authorisation | process of determining whether a certain level of access can be provided to a terminal |
| Authorised power | the maximum signal strength allowed for a particular class of licensed transmitting station |
| 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 |
| Automatic Gain Control (AFC) | a circuit which automatically adjusts the gain of an amplifier to maintain a constant output level |
| Automatic Repeat Request (ARQ) | an error correction procedure in which the receiver asks the transmitter to resend a character or block of data when errors are detected |
| Availability | the probability of achieving a specified level of performance and maintaining that level for a predetermined period of time |
| Az/El Mount | an antenna mount that requires two separate adjustments (in azimuth and elevation) to move from one satellite to another |
| Azimuth | angular displacement of the satellite to the east or west from south when viewed from the earth station |
