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Access The method of channel sharing or utilisation of a single frequency or time slot by several locations.
Adaptive Coding and Modulation ACM is a technology which can adjust the modulation and forward error correction or FEC of a link automatically to compensate the changes in link conditions (e.g. rain fade, level changes or interference). This technology is adopted in DVB-S2.
Amplifier A device used to increase the strength of electrical signals.
Analogue A form of storing, processing or transmitting information through a continuous variable (rather than pulsed) signal.
Antenna Control Unit The unit used for control and tracking the antenna pointing to the desired satellite
Antenna Device which picks up and delivers satellite signals to a receiver, most commonly a dish.
Aperture A cross sectional area of the antenna which is exposed to the satellite signal.
Apogee The point in a satellite's orbit when it is at its furthest distance from the Earth.
Asymmetric Transmission Transmission where the required bit rate for the forward path and the return path is different e.g. high forward path and low return path data rate.
Attenuation A decrease in the power of a received signal due to loss through lines, equipment or other transmission devices. Usually measured in dB.
Attitude Control Maintenance of the satellite's orientation with respect to the Earth and the sun.
Automatic Gain Control A circuit which automatically controls the gain of an amplifier so that the output signal level is virtually constant for varying input signal levels.
Automatic Level Control Also known as AGC
Availability Reliability of a communications link, typically expressed by the percentage of time e.g. 99.99% equals to a downtime of 9 hours per year.
Azimuth The horizontal angle for the antenna to point to the satellite. It is in clockwise direction from the true north.


Bandwidth A range of frequencies, expressed in Hz occupied by a modulated carrier on the range of frequencies which can be transmitted through a communications system. Bandwidth is one measure of the information carrying capacity of a transponder. The wider the bandwidth, the more information which can be transmitted. The bandwidth of the communications system must be at least as wide as the signal being transmitted.
Baseband The frequency band which contains the basic, low frequency information before modulation and after demodulation.
Beacon Low-power carrier transmitted by a satellite which supplies the controlling engineers on the ground with a means of monitoring telemetry data, tracking the satellite or conducting propagation experiments.
Beam The directed electromagnetic rays emanating from the spacecraft. Typically refers to aggregates of these rays such as China (coverage) beam or global (coverage) beam.
Beamwidth A measure of the ability of an antenna to focus signal energy towards a particular direction in space. The beamwidth is measured in a plane containing the direction of maximum signal strength. It is usually expressed as the angular separation between the two directions in which the signal strength is reduced to one-half of the maximum value (the -3 dB half power point)
BER See Bit Error Rate.
Bit A single unit of information. Often referred to as a 1 or 0 in the binary system and as an "on" or "off" state in computer operations.
Bit Error Rate The fraction of a sequence of message bits that are in error. It denotes the quality of a received demodulated digital signal. The lower the rate, the better the signal e.g. a BER of 10-4 means one error in every 10,000 bits.
Bit Rate Amount of digital information transmitted in a certain period of time, expressed in bits per second (bps).
BOL Beginning of Life of a satellite.
bps Bits Per Second. See Bit Rate.
BPSK Binary Phase Shift Keying.
Broadband A term used to refer to high-speed communications networks that are designed to handle bandwidth-intensive applications.
Broadcast Satellite Service The satellite service at designated frequencies designed to bring primarily video entertainment directly to consumers via high-power satellites and small user antennas. More commonly referred to as DBS (Direct Broadcast Satellite).
Broadcasting To transmit the same information to multiple receivers simultaneously over a satellite system e.g. radio, television or data broadcasting.
BSS See Broadcast Satellite Service.
Bus The section of the satellite with components to support tracking, telemetry and control (TT&C), power systems, propulsion and control of the spacecraft.


C/N See Carrier to Noise Ratio.
Cable Television Operator A service operator that receives transmissions from programme sources and distributes them to users (usually homes) via coaxial cable, usually for a fee.
Cache A place to store information temporarily. Web pages you request are stored in your browser's cache directory on your hard disk. When you return to a page you have recently viewed, the browser gets it from the cache rather than the original server, saving you time and the network additional traffic. You can usually vary the size of your cache, depending on your particular browser.
Carrier 1) A long distance telephone company that operates fiber/satellite/microwave networks to carry voice and data traffic. A local exchange carrier (LEC) is a local phone company and an inter-exchange carrier (IEC or IXC) carries long-distance calls.
2) A continuous radio frequency (RF) signal used to carry an information signal.
Carrier Frequency Frequency of the carrier wave that is modulated to transmit signals.
Carrier Identiifcation (ID) CID is a signal embedded into a video or data transmission path. It allows satellite operators and end users to identify the source of an interfering carrier. This technology is adopted by DVB and the related standards can be obtained via the link in the category of Transmission.
Carrier-In-Carrier Carrier-in-Carrier is a technology allows a full duplex satellite link to be operates both the forward and return carrier on top of each other with the same frequency as a single carrier by using Adaptive Cancellation method
Carrier Monitoring System A monitoring system on the ground that measures uplink and downlink signal performance of satellites.
Carrier to Noise Ratio The ratio of the received carrier power and the noise power in a given bandwidth, expressed in dB. This figure is directly related to G/T and S/N; and in a video signal the higher the C/N, the better the received picture.
Cassegrain Antenna The antenna principle that utilises a subreflector at the focal point which reflects energy to or from a feed located at the apex of the main reflector.
C-band The frequency range between 3.4-4.2, 4.5-4.8 and 5.85-7.075 GHz, also known as the 4/6 GHz band. Typically used for television broadcast and telecommunications services.
Channel Path for electrical communications between two facilities. Also called a circuit, link or path.
Circuit 1) Means of two-way communications, voice or data, between two or more points.
2) A group of electrical/electronic components connected to perform a specific function.
Circular Polarisation The polarisation of the electromagnetic wave varies in time with the electrical component (e-vector) tracing out a circle. Used in the old Intelsat system. Rare in domestic or regional fixed satellite services.
Clarke Belt Named after its founder Arthur C. Clarke, the Clarke Belt is an orbit used by satellites at a height of about 36000 km, in which satellites make an orbit in 24 hours yet remain in a fixed position relative to the Earth's surface.
CMS See Carrier Monitoring System.
Co-ax Co-axial cable and connectors that are commonly used as aerial and antenna cables.
Codec Coder/Decoder, a device used to convert analogue voice signals to digital signals.
Co-location Ability of multiple satellites to share the same geostationary orbital position due to the fact that different frequency bands or coverages are used.
Co-Pol The polarization of the desired signal transmission
Conditional Access A system to control the access to a particular service to authorised users only (e.g. subscribers to a particular digital bouquet, purchasers of individual pay-per-view events), by means of encryption and authorised decoding
Common Carrier A provider of telecommunications services of facilities to the general public on a non-discriminatory basis.
Conditional Access The system which allows the control of a user's access to pay services and services protected for copyright reasons.
Cross-Pol The opposite polarization of the desired signal transmission


DAMA Demand Assigned Multiple Access. A bandwidth-sharing scheme allowing multiple users to share a pool of frequencies or channels on demand. A central hub manages the usage of the bandwidth. This technology is mainly used for rural telephony.
dB Decibel. A relative unit of measurement used frequently in electronic communications to describe power gain or loss. It is used to specify measured and calculated values in audio systems, microwave system gain calculations, satellite system link-budget analysis, antenna power gain and in many other communications system measurements.
dB/K Unit of G/T ratio, decibels per Kelvin.
dBi The dB power relative to an isotropic source.
DBS See Direct Broadcast Satellite.
DBS Band BSS band in ITU terminology. Signal frequency range between 11.70-12.20 GHz intended for direct TV broadcast of satellite channels. This band is designated for use in ITU Region 3 (Asia).
dBW Decibel relative to one Watt. A logarithmic measure of the satellite's power e.g. 50 dBW is twice as powerful as 47 dBW.
Decoder Unit that is connected to a satellite receiver in order to unscramble a service that is protected by encryption. In the case of digital reception, the decoder is integrated in the receiver, which is called IRD. See Integrated Receiver/Decoder.
Delay The time it takes for a signal to go from the sending station through the satellite to the receiving station. This transmission delay for a single hop satellite connection is very close to one-quarter of a second.
Demodulation The process of extracting the original signal from a modulated carrier.
Demodulator Section of a satellite receiver designed to extract the audio and video information from an incoming signal.
Digital A form of storing, processing or transmitting information through a pulsed (rather than continuous variable) signal.
Digital Compression The reduction of the data needed to be broadcast (video, audio or data) giving minimum loss of received quality so as to make maximum use of the available transmission capacity. Thus, several digitally compressed TV channels can be transmitted in the space required for a single uncompressed analogue TV channel. The main way that compression works is by eliminating some of the redundant data in the signal.
Digitise To convert analogue signals into digital signals.
Direct Broadcast Satellite Satellites powerful enough to transmit a signal directly to a medium to small receiving dish (antenna). Dish sizes 50 to 75 cm. are common in this service. DBS does not require reception and distribution by an intermediate broadcasting facility, but transmits directly to the end user.
Dish See Antenna.
Double Hop A satellite communications link which passes the signal through two satellites. This most commonly occurs when transmitting a television signal between continents.
Downlink The receiving portion of a satellite circuit extending from the satellite to the Earth. Compare to Uplink.
Downconverter Downconverter is used to convert the signal from RF frequency band to the desired IF frequency
DTH Direct-to-Home. A satellite service that delivers television programming directly to consumer homes using a small antenna and related equipment. Satellites that provide DTH services are specialised to operate in a different frequency to allow for very small dishes.
Duplex Transmission in both directions of a telecommunications channel. Simultaneous two-way operation is known as "full duplex". Operating in only one direction at a time is known as "half duplex or simplex".
DVB Digital Video Broadcasting standard. A group of over 200 organisations from 23 countries which developed system specifications for the transmission of MPEG-2 digital signals by satellite, cable and terrestrial links. These specifications were passed to the European Telecommunications Standards Institute to form an ETSI standard. See also MPEG.
DVB Satellite Standard It is the specification ratified by ETSI for satellite video broadcasting. There are several generations, namely DVB-S, DVB-S2 and DVB-S2x. Each generation improves with newer coding, newer features and higher efficiency. The standard can be accessed via in the category of Transmission.


Earth Segment The ground-based facilities of a satellite communications system.
Earth Station The antennas, receivers, transmitters and other equipments needed on the ground to transmit and receive satellite communications signals.
Eclipse Occurs when the satellite's solar arrays are in the Earth's shadow.
Edge of Coverage Limit of a satellite's defined service area. A satellite's beam may be shaped for specific coverage or the service area may be geographically limited by the need to have a minimum elevation angle to the satellite.
EIRP Effective Isotropic Radiated Power. Measures the effect of focusing the satellite's energy in a particular area as compared to transmitting uniformly in all directions.
Electromagnetic Spectrum Entire range of wavelengths or frequencies of electromagnetic radiation extending from gamma rays to the longest radio wave including visible light. See also Radio Frequency.
Elevation The vertical angle for which the antenna moves upward from the horizon, pointing to the satellite.
Encryption Systematic modification of a signal to prevent unauthorised use.
EOL End of Life of a satellite.
Equalisation The technique of compensating for differences in attenuation of a signal at different frequencies.
Equalization The technique of compensating for differences in attenuation and groupdelay of a signal at different frequencies


F/D Ratio of antenna focal length to antenna diameter. A higher ratio means a shallower dish.
Fading A phenomenon of microwave or radio transmission where atmospheric effects cause a signal to be reduced in strength.
FCA Flux Control Attenuator
FDMA Frequency Division Multiple Access. A technique for allowing many users to share a transmission bandwidth by assigning each of them a share of the bandwidth such that the sum of all such user bandwidths plus necessary guardbands equals the allowed bandwidth.
FEC Forward Error Correction. A method of coding which inserts additional bits in the transmission that is used to detect and correct transmission errors.
Feed This term has at least two key meanings within the field of satellite communications. It is used to describe the transmission of video programming from a distribution centre. It is also used to describe the feed system of an antenna. The feed system may consist of a subreflector plus a feedhorn or a feedhorn only.
Feedhorn Device which collects signals at the focus of the antenna and channels them to the LNB.
Fixed Satellite Service The telecommunications service between non-moving earth stations (but the antennas may be movable, just not in motion at the time of use).
Fixed-dish System Satellite system in which the antenna is targeted at one particular satellite.
FM See Frequency Modulation.
Focal Length Distance from the centre feed to the centre of the dish.
Focal Point The area toward which the primary reflector directs and concentrates the signal received.
Footprint The geographic area covered by a satellite. The outer edge of which is generally defined as the area beyond which the quality of communications degrades below an acceptable commercial level.
Free-to-air Services Services which do not require any payments or any special decoders to receive.
Frequency Number of cycles in a given time. Typically refers to the rate of variation of the carrier wave or modulating signal. The RF signals of communications satellites are typically in the GHz frequency range.
Frequency Coordination A consultative process under ITU Radio Regulations where satellite system operators or their Administrations work together to minimise the potential for interference between systems.
Frequency Modulation A technique whereby a carrier wave is made to carry information by changing its frequency in proportion to variations in strength of a lower frequency signal.
FSS See Fixed Satellite Service.
FTA Free-to-air. See Free-to-air Services.


G/T A figure of merit of an antenna and low noise amplifier combination expressed in dB/K. "G" is the net gain of the system and "T" is the noise temperature of the system. The higher the number, the better the system.
Gain A measure of amplification expressed in dB.
Gregorian Antenna A parbolic antenna similar to Cassegrain design except that the secondary reflector is concave in shape
Geostationary Orbit An orbit at 35,786 km directly over the Earth's equator in which the orbital inclination and eccentricity are both near zero such that the satellite appears to hover over a specific portion of the Earth's equator. See also Clarke Belt.
Geostationary Satellite A satellite orbiting along the geostationary orbit. From the Earth, a geostationary satellite always appears to be in the same location because it finishes one rotation around the Earth in 24 hours.
Geosynchronous Orbit An orbit 35,786 km above the Earth's surface where satellites circle at the same rate as the Earth's rotation.
GHz Gigahertz. One billion cycles per second.
Global Beam An antenna downlink pattern used by satellites, which effectively covers about one-third of the globe.
GPS Global Positioning System. A satellite system that provides precise reference to the location of a point on the Earth. GPS satellite systems employ lower orbiting satellites than geostationary satellite networks.
Guard Band Television channels are separated in the frequency spectrum by spacing them several megahertz apart. This unused space serves to prevent the adjacent television channels from interfering with each other.


HDTV High Definition Television. Offers approximately twice the vertical and horizontal resolution of current NTSC analogue television broadcasting and supports sound quality approaching that of a CD.
Head-end The control centre for a cable system where signals are received or originated, processed and sent for distribution down the cable.
Hertz Unit of frequency, equal to one cycle per second (after Heinrich Hertz).
Hops The number of receive and transmit points in transmitting from one location to another.
HPA High Power Amplifier.
Hub The point on a network where circuits are connected or a network operations centre for VSAT operations.
Hz See Hertz.


IDU See Indoor Unit.
IF Intermediate Frequency. The intermediate step between a baseband and RF carrier frequency. For example, the intermediate frequency range between 950-2,150 MHz used for the distribution of satellite signals from the LNB at the dish to the user's satellite receiver. It is always used in direct-to-home systems and is the most suitable for distribution of digital signals in communal systems.
IFL See Interfacility Link.
Inclination The angle between the orbital plane of a satellite and the equatorial plane of the Earth.
Inclined Orbit A condition that occurs when a satellite is no longer corrected in velocity along the north-south direction. A satellite operator might do so to extend the life of a satellite because fuel will only be used to perform the velocity change in the east-west direction. The inclination happens gradually over time.
Indoor Unit The electronic equipment which is located inside the customer premises and provides the digital processing for access to the VSAT network.
Integrated Receiver/Decoder Set-top-box used for the reception and descrambling of signals. In the case of digital reception, the decoder is integrated with the satellite receiver.
Interfacility Link The cables which connect the indoor unit and the outdoor unit of a VSAT, or the transmission line between the RF equipment and the antenna at the hub station.
Inbound Inbound represents the transmission from the remote station towards a hub via satellite
In-Orbit-Test The functional test required for a satellite after launch and before the service commissioning
Interference Unwanted radio frequency energy that tends to interfere with the reception of the desired signals. May come from adjacent channels, adjacent networks or sources local to the earth station.
IP Internet Protocol. The method by which data is sent from one computer to another on the Internet. Each computer on the Internet has at least one IP address that uniquely identifies it from all other computers on the Internet. When you send or receive data e.g. an e-mail note or a Web page, the message gets divided into little chunks called packets. Each of these packets contains both the sender's Internet address and the receiver's address. See also TCP/IP.
IP Multicast Sending out data to distributed servers on Multicast Backbone. For large amounts of data, IP multicast is more efficient than normal Internet transmissions because the server can broadcast a message to many recipients simultaneously. Unlike traditional Internet traffic that requires separate connections for each source-destination pair, IP multicasting allows many recipients to share the same source. This means that just one set of packets is transmitted to all the destinations.
IRD See Integrated Receiver/Decoder.
ITU International Telecommunication Union. The United Nation's specialised agency for telecommunications regulation.


K Kelvin, a temperature scale based upon absolute zero used as a specification for noise performance of telecommunications devices.
Ka-band The frequency range between 17.7-20.2 and 27.5-30.0 GHz, also known as the 20/30 GHz band. Planned to be used for HDTV.
kHz Kilohertz. Unit of frequency, equal to 1000 cycles per second.
Klystron A type of high-power amplifier which uses a special beam tube.
Ku-band The frequency range between 10.7-13.25 and 14.0-14.5 GHz, also known as the 11/14 and 12/14 GHz band. Multiple uses in various regions as designated by the ITU.


Left-Hand Circular Polarization It represents the polarization which an elliptically- or circularly-polarised wave travel in with the electric field vector rotates with time in a left-hand or anticlockwise direction when observed in any fixed plane normal to the direction of propagation
L-band The frequency range between 0.39-1.55 GHz, also known as the 1.5 GHz band. One use is for Digital Audio Broadcasting (DAB).
Lineariser A microwave device that compensates for the non-linear characteristics of a travelling wave tube near saturation, allowing more efficient use of transponder power.
LNA See Low Noise Amplifier.
LNB See Low Noise Block down-converter.
LNBF An LNB with an integrated feedhorn
Low Noise Amplifier This is the preamplifier between the antenna and the earth station receiver. For maximum effectiveness, it must be located as near the antenna as possible, and is usually attached directly to the antenna receive port. The LNA is especially designed to contribute the least amount of thermal noise to the received signal.
Low Noise Block down-converter Amplifies received signals and converts them from microwave to lower intermediate frequency signals which can be sent via a cable to the receiver.


Mbps Mega bits per seconds. Millions of bits per second.
MCPC See Multiple Channels Per Carrier.
MHz Megahertz. Unit of frequency, equal to 1 million cycles per second.
Microwave RF carrier waves with wavelengths of less than one metre and frequency above 300 MHz. Typically used to refer to frequencies above 1 GHz.
MMDS Multichannel Microwave Distribution System. A system whereby microwaves carry television signal from a central transmitter to viewers in a reception area.
Mobile Satellite Service Services transmitted via satellites to provide mobile telephone, paging, messaging, facsimile, data and position location services directly to users.
Modem Modulator/Demodulator. The device used to translate digital signals to analogue signals for transmission over an analogue carrier.
Modulation The process by which the characteristics of a carrier wave are varied in accordance with a message signal (voice, data or video). Analogue satellite transmission commonly uses FM modulation. A digital satellite transmission commonly uses QPSK modulation.
MPEG Moving Pictures Experts Group. An International Organisation for Standard section sub-group which develops standards (MPEG-1, 2, 4 etc) for the digital compression and multiplexing of video and audio signals.
MSO Multiple System Operator. A company that owns and/or operates more than one cable system.
MSS See Mobile Satellite Service.
Multicast The broadcast of messages to a selected group of workstations on a LAN, WAN or the Internet. Multicast is communication between a single device and multiple members of a device group.
Multipaction Multipaction is an effect that occurs when electrons accelerated by Electric fields are self-sustained in a vacuum via an electron avalanche break down caused by the secondary electron emission
Multiple Channels Per Carrier A signal comprised of multiple digital streams that are multiplexed into a single stream, which is then transmitted on a single carrier. This is typically used by combining multiple compressed digital video signals into one.
Multiplex Verb: To combine two or more independent signals into one transmission channel. Noun: The combined digital signals transmitted on one satellite transponder.
Multiplexer A device that allows multiple logical signals to be transmitted simultaneously across a single physical channel.
Multipoint Pertaining or referring to a communications line to which three or more stations are connected. It implies that the line physically extends from one station to another until all are connected.
Multipoint-to-Multipoint A network configuration in which each node is connected with another. Also called a mesh network. Compare to Point-to-Point.


Network A method of inter-connecting many points or locations in a telecommunications or data communications system.
Noise Unwanted energy that degrades a signal. Noise is always present to some extent within any signal.
Noise Figure Measure of the performance (noise contribution) of an LNB in dB. The lower the figure, the better.
NTSC National Television Standard Committee. 525 line TV system established by US National Television Standards. Predominantly used in North America, Japan and the Philippines.
NTP Network Time Protocol is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks
NVOD Near-Video-on-Demand. Single programme/schedule with multiple daily start times.


ODU See Outdoor Unit.
OPBO Output Backoff. The amount a signal is below the saturated output of the amplifier.
Operational Life The expected in-orbit operational life of a satellite is based on the period during which the satellite's on-board fuel permits proper station keeping manoeuvres for the satellite.
Orbital Position Specific location of a satellite in the geostationary arc, specified in degrees, east or west longitude.
Orbital Slot See Orbital Position.
Outdoor Unit The RF electronics located with the antenna of the VSAT.
Outbound Outbound represents the transmission from the hub towards the remote stations via satellite


PAL Phased Alternate Line. TV system used by both British terrestrial and satellite broadcasters. Analogue standard for television transmission (mainly Europe), frame 4:3, 625 lines.
Parabolic Antenna The most frequently found satellite TV antenna. It takes its name from the shape of the dish described mathematically as a parabola. In receiving the microwave signal the function of the parabolic shape is to focus the weak microwave signal hitting the surface of the dish to a focal point in front of the dish. It is at this point that the feedhorn is located.
Passive Intermodulation Passive intermodulation occurs in passive devices that are subjected to two or more high power tones. The PIM product is the result of the two (or more) high power tones mixing at device nonlinearities such as junctions of dissimilar metals, metal-oxide junctions and loose connectors
Payload Supports the primary mission of the satellite, the receipt and transmission of signals, and comprises systems that include receivers, multiplexers, high power amplifiers and signal processing.
Pay-per-View Programming services which are paid for by subscribers on the basis of the number of hours or programmes watched rather than through a straight subscription fee.
Perigee The lowest point in a satellite's orbit. The point in the orbit of a satellite when it is closest to the object about which it revolves.
Permanent Circuit A permanent connection between any two nodes within the VSAT network.
Phase-Locked Loop A type of electronic circuit used to demodulate satellite signals.
PLL See Phase-Locked Loop.
Point-to-Multipoint A network configuration in which a major hub is connected to many subsidiary nodes. Also called a star network.
Point-to-Point A network in which two nodes are connected by a single dedicated line not used by any other nodes. Compare to Multipoint-to-Multipoint.
Polarisation A technique used by the satellite designer to increase the capacity of the satellite transmission channels by reusing the satellite transponder frequencies. In linear cross polarisation schemes, half of the transponders beam their signals to the Earth in a vertically polarised mode, the other half are horizontally polarised. Although the two sets of frequencies overlap, the polarisation separation is sufficient to ensure they do not interfere with each other. To successfully receive and decode these signals on the Earth, the earth station must be outfitted with a properly polarised feedhorn to select the vertically or horizontally polarised signals as desired.
Polariser A device to convert circular polarised signals to linear or vice versa.
Protocol A set of rules or conventions which governs a data communications system.
Pull To request data from a source such as a computer. The World Wide Web is based on pull technologies, where a page is not delivered until a browser requests it. The opposite of pull is push, where data is sent without a request being made. Increasingly, information services are harnessing the Internet to broadcast information using push technologies. The terms push and pull are used frequently to describe data sent over the Internet. See also Push.
Push In client/server applications, to send data to a client without the client requesting it. The World Wide Web is based on pull technologies where the client browser must request a Web page before it is sent. Broadcast media, on the other hand, are push technologies because they send information out regardless of whether anyone is tuned in. Increasingly, companies are using the Internet to deliver information push-style. See also Pull.


QAM Quadrature Amplitude Modulation. A method of combining two Amplitude Modulated (AM) signals - each having the same frequency, but differing in phase by 90 degrees, into a single channel, thereby doubling the effective bandwidth.
QoS QoS is the overall performance of a network to quantitatively measure quality of service, several related aspects of the network service are often considered, such as error rates, bandwidth, throughput, transmission delay, availability, jitter, etc.
QPSK Quadrature Phase Shift Keying. A digital modulation technique in which the carrier phase takes on one of four possible values.


Radio Frequency A frequency that is higher than the audio frequency but below the infrared frequencies. See also Electromagnetic Spectrum.
Radiocommunication Bureau A branch of the ITU responsible for the co-ordination of radio frequencies. Formerly the International Frequency Registration Board.
Rain Outage Loss of signal at Ku or Ka band frequencies due to absorption and increased sky-noise temperature caused by heavy rainfall.
Receiver Unit which takes signals from an antenna and converts them so they can appear on TV.
Right-Hand Circular Polarization It represents the polarization which an elliptically- or circularly-polarised wave travel in with the electric field vector rotates with time in a right-hand or clockwise direction when observed in any fixed plane normal to the direction of propagation
RF See Radio Frequency.


S/N See Signal to Noise Ratio.
Satellite Dish See Antenna.
S-band The frequency range between 1.55-2.5 GHz, also known as the 2.5 GHz band. Typically used for DBS (community reception). Not in common use.
Scintillation Signal fading due to atmospheric effects that focuses and defocuses the radio waves. Can last for tens of seconds or occasionally minutes. Highly variable with time of year and elevation angle of the earth station.
SCPC Single Channel Per Carrier. A scheme in which only one signal is loaded on a carrier.
Scrambling See Encryption.
SECAM Sequential Colour and Memory System. Colour TV standard developed in France and used predominantly in France, Eastern Europe, parts of the Middle East and Africa.
Set-Top Box See Receiver.
Shared Hub A satellite communications operations centre that is shared among a number of separate network users, often used for VSAT operations.
Signal A physical, time-dependent energy value used for the purpose of conveying information through a transmission link.
Signal to Noise Ratio The ratio of the signal power and noise power. A video S/N of 54 to 56 dB is considered to be an excellent S/N, that is, of broadcast quality. A video S/N of 48 to 52 dB is considered to be a good S/N at the head-end for cable TV.
Single Event Upset It describes a change of state caused by ions or electro-magnetic radiation striking to a sensitive component in a micro-electronic device, such as in a microprocessor, semiconductor memory, or power transistors
Single Hop A communications link which transmits the signal through a single satellite.
Slant Range The length of the path between a communications satellite and an associated earth station.
Smart Card Personalised credit card for use in satellite receivers with integrated decoders, for authorisation of subscribed pay TV channels.
SMATV Satellite Master Antenna Television system. A communal distribution system for satellite and terrestrial TV and radio signals, usually installed in a block of flats for shared use.
SNG Satellite News Gathering.
SOHO Small Office/Home Office. So-called SOHO products are specifically designed to meet the needs of professionals who work at home or in small offices.
Solar Outage See Sun Outage.
Solar Panel A device on satellites that converts solar energy into electrical energy using solar cells.
Space Segment The portion of a satellite communications link which is the transmission medium between the spacecraft and the Earth.
Spectrum The range of electromagnetic radio frequencies used in transmission of voice, data and television.
Spin Stabilisation A form of satellite stabilisation and attitude control which is achieved through spinning the exterior of the spacecraft about its axis at a fixed rate.
Spot Beam A focused antenna pattern sent to a limited geographical area. Spot beams are used by domestic satellites to deliver certain transponder signals to geographically well defined areas.
Star Topology A network configuration in which all nodes connect to a single central hub.
Stationkeeping Minor orbital adjustments that are conducted to maintain the satellite's orbital assignment within the allocated "box" within the geostationary arc.
Steerable Beam It is an antenna beam which it can be adjusted either mechanically or electrically. It usually refers to a regional satellite beam that can be steered over the earth's surface which can be visible from the satellite's orbital slot
Streaming A technique for transferring data such that it can be processed as a steady and continuous stream. Streaming technologies are becoming increasingly important with the growth of the Internet because most users do not have fast enough access to download large multimedia files quickly. With streaming, the client browser or plug-in can start displaying the data before the entire file has been transmitted.
Subcarrier A second signal "piggybacked" onto a main signal to carry additional information. In analogue satellite television transmission, the video picture is transmitted over the main carrier. The corresponding audio is sent via an FM subcarrier.
Sun Outage Sun Outages occur when an antenna is looking at a satellite, and the sun passes behind the satellite and within the field of view of the antenna, the sun's energy momentarily interferes with the satellite signals. Occurs for two periods each year during the spring and fall equinox.
Switch A device that opens or closes circuits or selects the paths or circuits to be used for transmission of information; switching is the process of interconnecting circuits to form a transmission path between users.
Switched Circuit A connection at one node which can be switched between several other nodes within the VSAT network.


TCP/IP Transmission Control Protocol/Internet Protocol. The suite of communications protocols used to connect hosts on the Internet. TCP/IP uses several protocols, the two main ones being TCP and IP. TCP/IP is built into the UNIX operating system and is used by the Internet, making it the de facto standard for transmitting data over networks. Even network operating systems that have their own protocols, such as Netware, support TCP/IP.
TDMA Time Division Multiple Access. A form of a multiple access scheme where a single carrier is time-shared by many users. Signals from earth stations reaching the satellite consecutively are processed in time segments without overlapping.
Terrestrial Broadcasters Broadcasters who transmit through the airwaves from one earth-bound aerial to another.
Threshold Most commonly referred to on the received signal, threshold is the signal to noise level that defines acceptable and unacceptable performance of the link.
Tracking An earth station feature that allows for following the motion of inclined satellites.
Transfer Orbit An intermediate elliptical orbit used to reach geosynchronous orbit, where the apogee is the same altitude as the final operating orbit.
Transponder Equipment in a satellite which receives a single uplinked channel from a satellite earth station; amplifies it, converts the frequency and changes the polarisation; then transmits it back to the Earth in a given power (EIRP).
Travelling-Wave-Tube Amplifier The main transmitter (microwave repeaters) on a satellite used to amplify the signal before it is rebroadcast back to the Earth. One of these is associated with each transponder and determines the available radio frequency communications power.
TT&C Tracking, Telemetry and Control. Refers to satellite control station used to monitor onboard satellite operations and to direct satellite electronics and propulsion equipment.
TVRO Television Receive Only. Usually a receive antenna attached to a building serving a single household.
TWTA See Travelling-Wave-Tube Amplifier.


UHF Ultra High Frequency. The band in the 300 to 3000 MHz range.
Uplink In satellite communications, the signal from the earth station to the satellite.
Uplink Power Control ULPC is a control unit to adjust the signal transmit power automatically to compensate the change in weather conditions according to the downlink beacon signal level variation
Upconverter Upconverter is used to convert the signal from IF frequncy to the desired RF frequency
Uninterruptible Power Supply UPS is a system to provide emergency power to maintain the power loading without interruption when the primary input power source fails


V-band Also known as the 40 GHz band. Reserved for future use.
VHF Very High Frequency. The band in the 30 to 300 MHz range.
Video Compression Data reduction and compression of an analogue television signals into a digital stream.
VOD Video-On-Demand. System that allows the viewer to see or choose a programme at the time the viewer specifies.
Voltage Standing Wave Ratio Voltage Standing Wave Ratio is a measure of the efficiency of a RF power transmitted from a source into a load
VSAT Very Small Aperture Terminal. Refers to small earth stations, usually 1.8 metre to 4 metre in diameter in C-band. A VSAT system is a satellite communications system that is typically used by corporate private networks or in rural and remote areas. A VSAT system consists of an antenna and the associated electronics. VSAT networks are widely used by retail business for verifying credit cards. VSAT systems can be used for voice, data and video. Small aperture terminals under 0.5 metre are sometimes referred to Ultra Small Aperture Terminals (USATs).


Waveguide A metallic microwave conductor, commonly rectangular in shape, used to carry microwave signals into and out of microwave antennas.
Webcasting Using the Internet, and the World Wide Web in particular, to broadcast information. Unlike typical surfing, which relies on a pull method of transferring Web pages, webcasting uses push technologies.


X-band The frequency range between 7.25-7.75 and 7.9-8.4 GHz, also known as the 7/8 GHz band. Typically used for military satellite communications.
XIPS Xenon Ion Propulsion System. A propulsion system on satellites that uses charged particles and electromagnetism to generate thrust for satellite stationkeeping. XIPS is significantly more efficient than other propulsion systems and lessens the weight of the satellite, thus reducing launch costs.