Satellite Communications Glossary

Below is a list of commonly used terms and their definitions

Available Bit Rate/ Adaptive Bit Rate
Adaptive Coding And Modulation (ACM)
ACM or Adaptive Coding and Modulation is a technology which can automatically change the forward error correction and modulation of a link to compensate for changes in link conditions. Commonly these changes are due to weather, e.g. rain fade, but can also come from other sources such as RF level changes or interference. Learn More!
Asset Distribution Interface In connection with CableLabs
Asymmetrical Digital Subscriber Line
American National Standards Institute
Application Programming Interface
Automatic Protection Switching
ASI ( Asynchronous serial interface )
Asynchronous Serial Interface, or ASI, is a streaming data format which often carries an MPEG Transport Stream, Generally, the ASI signal is the final product of video compression, either MPEG2 or MPEG4, ready for transmission to a transmitter or microwave system or other device.
Technical Planning and Standards Development Organization
ATIS IPTV Interoperability Forum
ATM ( Asynchronous Transfer Mode )
ATM or Asynchronous Transfer Mode is, according to the ATM Forum, "a telecommunications concept defined by ANSI and ITU (formerly CCITT) standards for carriage of a complete range of user traffic, including voice, data, and video signals". ATM was developed to meet the needs of the Broadband Integrated Services Digital Network, as defined in the late 1980s, and designed to unify telecommunication and computer networks. It was designed for a network that must handle both traditional high-throughput data traffic (e.g., file transfers), and real-time, low-latency content such as voice and video.
Advanced Video Codec
Average Revenue Per User
A / B Switch
A switch in a satellite receive earth station, which has two inputs and a single output. Each switch input is connected to the output of an LNB. The single switch output is usually connected to a satellite receiver (set-top box). The switch permits selection of one of the two LNB inputs (A or B) for routing it to the common output (to a receiver for example), whilst providing adequate isolation between the signals produced by each LNB.
Analogue-to-Digital Converter. A device for converting an analogue voltage waveform into a series of digital numbers so that the signal can be manipulated numerically ("digitally processed").
Amplitude Modulation (AM)
One of the methods for transmitting information using radio waves by superimposing the information signal onto a radio frequency carrier wave. The amplitude of the carrier wave is varied in accordance with the time-varying amplitude of the input signal. The frequency of the carrier wave remains unchanged.
A device used to increase the level of a signal. Baseband The band of frequencies containing the information, prior to modulation (and subsequent to demodulation). dB (decibel) A term that expresses the ratio of power levels used to indicate gains or losses of signals. Decibels relative to one watt, milliwatt and millivolt are abbreviated as dBW, dBm and dBmV, respectively.
A device for transmitting or receiving radio waves. Also known as an aerial. In satellite communication systems the antenna usually consists of a parabolic reflector and a feedhorn. In a receiving system the reflector focuses radio waves onto the feedhorn for detection and conversion into electrical signals. In transmitting systems the reflector concentrates the radio waves emitted by the feedhorn into a narrow beam aimed towards the satellite.
Antenna Aperture
The total reflective area of a parabolic antenna (dish) over which radio waves are captured or radiated. The effective aperture is smaller than the physical aperture and is related to it by the Antenna Efficiency.
Antenna Efficiency
The ratio of the signal strength transmitted towards or received from a particular direction in space by a real antenna to the signal strength that would be obtained with a theoretically perfect antenna of the same physical size. This ratio is usually expressed as a percentage. Antenna Efficency Calculator
Antenna Illumination
The radiation of electromagnetic energy from the feedhorn to the surface of the parabolic reflector of a transmit antenna, or the focusing of electromagnetic energy captured by the reflector of a receiving antenna towards the feedhorn. With perfect illumination no signal energy is lost to the surrounding terrain. In practice there is always some loss.
Aspect Ratio (Television)
The ratio of the width of a television picture or television screen to its height. The ratio for conventional television systems is 4:3. In advanced television systems (e.g. widescreen) the ratio is usually 14:9 or 16:9, which better approximate the aspect ratio used in cinema.
A system in which one continuously-varying physical quantity (e.g. the intensity of a sound wave) is represented directly by another (e.g. the voltage of an electrical signal) as faithfully as possible.
Analogue Modulation
The process of Modulation, where the modulating wave or signal is analogue and the amplitude, frequency or phase of the carrier wave or signal is varied continuously according to the content of the modulating signal.
Automatic Repeat Request. An error detection and correction technique based on the transmission of data in discrete packets. A decoder in the receiver detects errors but cannot correct them. Instead it sends a retransmission request to the transmitter which then repeats the transmission.
Audio Subcarrier (Television)
A carrier signal modulated by a sound signal, where the carrier frequency is slightly higher than the maximum frequency encountered in a video signal. This signal is combined with a video signal and the combination is used to modulate a radio frequency carrier for subsequent transmission over a satellite link. The signal is referred to as a subcarrier because it is itself modulated onto a carrier.
Adjacent Channel Interference
Unwanted electrical interference from signals that are immediately adjacent in frequency to the desired signal. This can arise due to imperfections in the transmission channel and/or equipment.
Antenna Alignment
Unwanted electrical interference from signals that are immediately adjacent in frequency to the desired signal. This can arise due to imperfections in the transmission channel and/or equipment.
Antenna Noise Temperature
A receiving antenna collects noise from radiating bodies falling within its radiation pattern. For an on-ground receiving antenna, this includes contributions from the sky and from the surroundings (the earth). The combined affect of these noise sources is modelled by an equivalent noise temperature for the antenna, which varies with elevation angle and antenna size.
The measure of the weakening of a signal (loss) that occurs as it travels through a device or transmission medium (e.g. radio waves through the atmosphere, an electrical signal through a cable). Attenuation is usually measured in decibels.
The pointing direction of an antenna measured in the local horizontal plane in a clockwise direction from north. It is the horizontal co-ordinate that is used to align a satellite antenna. See also Elevation.
The range of frequencies used for a particular radio transmission (e.g. 36 MHz). It is the difference between the lowest and highest transmission frequencies used by a signal.
Band Switching
The process of selecting one of two frequency bands (the "low band" or the "high band") for reception of satellite signals. Frequency band switching is implemented in dual-band LNBs by changing the frequency of the local oscillator reference signal that is used to downconvert the received signals to an intermediate frequency (IF).
A highly stable radio frequency signal, which is used by earth stations equipped with an automatic (satellite) tracking system. Beacons can be generated on-board the satellite, or transmitted from the ground and relayed through the satellite. When generated on-board the satellite, they are also known as satellite or on-board beacons and sometimes carry telemetry signals.
A unidirectional flow of radio waves concentrated in a particular direction. A term commonly used to refer to an antenna's radiation pattern by analogy with a light beam. It is most often used to describe the radiation pattern of satellite antennas. The intersection of a satellite beam with the earth's surface is referred to as the (beam's) footprint.
A measure of the ability of an antenna to focus signal energy towards a particular direction in space (e.g. towards the satellite for a ground-based transmitting antenna), or to collect signal energy from a particular direction in space (e.g. from the satellite for a ground-based receiving antenna). 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 points).
A unit of information representing the physical state of a system having one of two values, such as on or off.
Bit Rate
In telecommunications and computing, bit rate (sometimes written bitrate or as a variable R) is the number of bits that are conveyed or processed per unit of time. The bit rate is quantified using the bits per second (bit/s or bps) unit, often in conjunction with an SI prefix such as kilo- (kbit/s or kbps), mega- (Mbit/s or Mbps), giga- (Gbit/s or Gbps) or tera- (Tbit/s or Tbps).
Number of bits of information transferred or transmitted every second.
Alternative expression for bit/s.
A sequence of bits processed as one unit of information. A byte is a digital "word" normally consisting of eight bits.
Bit Error Rate (BER)
An overall measure of the quality of a received digital bit stream. It is the ratio of the number of information bits that are received in error to the total number of bits received, averaged over a period of time.
A collection of digital multimedia services marketed as a single package, often transmitted in a single data stream. See also Digital Multiplexing.
Broadcasting Satellite Service. Typically used to refer to a range of frequencies intended for direct reception of satellite television and entertainment services. These frequencies are subject to internationally-agreed regulations that govern their use and are designed to ensure that all countries are able to offer services of this nature. In Europe, the BSS downlink frequency range is 11.7 - 12.5 GHz.
Bandpass Filter
A circuit or device that allows only a specified range of frequencies to pass from input to output, rejecting all signals at lower or higher frequencies.
BNC Connector
A twist-lock coaxial connector that is commonly used on commercial video equipment and on some brands of satellite receiver.
Block Up-Converter: Earth station transmitter combining signal up-conversion and power amplification in a single unit, normally located directly at the antenna input, or close to it.
The range of frequencies occupied by the source electrical signal that is to be transmitted over a radiocommunications link. It is the frequency band occupied by an analogue or data signal prior to modulation and frequency conversion, or after frequency conversion and demodulation. For example, the baseband of a video signal extends from 0 Hz to about 5 MHz.
Block Downconversion
The process of converting the frequency of an entire block of radio frequency signals received from the satellite to a lower intermediate frequency (of around 1 GHz). This permits subsequent processing of the signals within a satellite receiver, including selection of the desired signal from the available block of signals.
A pure frequency signal that is used to convey information through a transmission channel. The key characteristics of the carrier signal (frequency, amplitude or phase) are varied according to the content of the information. These variations are detected at the receiver and are used to reconstruct the original signal.
The microwave frequency band between approximately 4 GHz to 8 GHz, used mainly for long-distance radio telecommunications.
Carrier-to-Noise-plus-Interference-Ratio. A measure of the quality of a signal at the receiver input. It is the ratio of the power of the carrier to the combined power of noise and man-made interference, measured within a specified bandwidth (usually the modulated carrier's bandwidth). It is usually expressed in decibels. The higher the ratio, the better quality of the received signal.
Carrier-to-Interference-Ratio. A measure of the quality of a signal at the receiver input. It is the ratio of the power of the carrier to the power of interference arising from man-made sources, measured within a specified bandwidth (usually the modulated carrier's bandwidth). It is usually expressed in decibels. The higher the ratio, the better quality of the received signal.
Carrier-to-Noise-Ratio. A measure of the quality of a modulated carrier at the receiver input. It is the ratio of the power of the carrier to the power of the noise introduced in the transmission medium, measured within a specified bandwidth (usually the modulated carrier's bandwidth). It is usually expressed in decibels. The higher the ratio, the better quality of the received carrier.
A proportion of the satellite's bandwidth and power which is used to establish one or more communication channel.
A band of radio frequencies assigned for a particular purpose, usually for the establishment of one complete communication link, or a path for an electrical signal. This term is often used interchangeably with Transponder, but in general the channel bandwidth is less than the transponder bandwidth.
Circular Orbit
A satellite orbit in which the distance between the centres of mass of the satellite and of the primary body (the earth) is constant.
Circular Polarisation
A circularly-polarised wave, in which the electric field vector, observed in any fixed plane normal to the direction of propagation, rotates with time and traces a circle in the plane of observation. Unlike linear polarisation, circular polarisation does not require alignment of earth station and satellite antennas with the polarisation of the radio waves.
Clarke Belt
The circular orbit at approximately 35,800 km above the equator, where the satellites travel at the same speed as the earth's rotation (Geostationary Orbit) and thus appear to be stationary to an observer on Earth. Named after Arthur C. Clarke who first postulated the idea of geostationary communication satellites.
Clear Sky
A term describing the weather conditions encountered at the terrestrial end of an earth-space path of a satellite communication link. It is used to describe the condition where the attenuation of radio waves caused by precipitation (rain, snow, sleet, dew, etc.) is lowest (i.e. cloud-free sky and good visibility).
Contribution Link
A Satellite contribution link or service is a means to transport video programming by a satellite link from a remote source (such as an outside broadcast unit) to a broadcaster's studio or from the studio to a satellite TV uplink centre (for onward distribution by DTH, cable etc).
The geographical area in which satellite signals can be transmitted or received with sufficient quality when using appropriately sized earth stations. Satellite coverages are usually communicated in the form of footprints displaying satellite G/T, EIRP or other quantity, such as the antenna size required for good quality reception of a particular service.
Cross Modulation
Interference caused by the modulation of one carrier affecting another signal. It is usually due to nonlinear device operation, which can be caused by overloading an amplifier, and is worsened by signal power level imbalances (e.g. at the receiver input in the head-end of a cable distribution network).
Used to refer to a signal that has the opposite (orthogonal) polarisation to a given signal.
Cross-Polar Discrimination
(XPD). The ratio of the signal power received (or transmitted) by an antenna on one polarisation (the polarisation of the desired signal) to the signal power received (transmitted) on the opposite polarisation. This ratio is usually expressed in decibels. It is a measure of the ability of the antenna to detect (emit) signals on one polarisation and to reject signals at the same frequency having the opposite polarisation.
Cross-Polar Isolation
(XPI). The ratio of the signal power received (or transmitted) by an earth station on one polarisation (the desired signal) to the signal power received (transmitted) on the same polarisation but originating from a cross-polar signal. This ratio is usually expressed in decibels. It is a measure of interference from cross-polar signals into the desired signal, which occurs in all practical systems that exploit both orthogonal polarisation. Strictly speaking, the terms "cross-polar isolation" and "cross-polar discrimination" have different meanings but are often used interchangeably.
Coaxial (Screened) Cable
A cable consisting of an inner insulated core of stranded or solid wire surrounded by an outer insulated flexible wire braid. Used principally as a transmission line for radio frequency signals with low loss. Commonly shortened to Coax. Sometimes referred to as screened cable because the outer braid screens the inner conductor from electrical interference.
Code Division Multiple Access. A technique allowing multiple users to simultaneously share a common transmission bandwidth. Each user transmits continuously, generating a controlled level of interference into other users. Each transmitter is assigned a unique signature, or code, which is combined with the useful information at the transmitter. The receiver is able to recover the desired information and reject unwanted information by means of this unique code.
Channel Encoding (Coding)
The process of deliberately adding redundant information to a message at the transmit end of a transmission link so that errors can be detected and corrected at the reception point. The term "channel" is used to indicate that the encoding is specifically related to the transmission channel and to distinguish it from any other encoding used in the system (e.g. for digital image compression).
Chrominance (Television)
The colour information of a television picture. It is also used to refer to the modulated colour component of a PAL, SECAM or NTSC television signal.
Code Rate
The ratio of the number of bits in a data stream that carry useful information to the total number of bits, including those added for error correction purposes. For example, a code rate of ¾ indicates that ¾ of the bits carry useful information and ¼ of the bits are used to detect and correct errors in the receiver, after which time they are discarded.
Colour Bars (Television)
A television picture consisting of several coloured vertical bars, which is used for testing the performance of colour television equipment and transmission paths. There are several variants of the colour bar signal in use worldwide.
Colour Difference (Television)
A signal obtained by subtracting the brightness (luminance) information of a television picture from the primary colour information (red or blue). Two colour difference signals (red and blue) are conveyed in a PAL, SECAM or NTSC picture. The third (green) can be deduced in the television receiver from these two colour difference signals and the brightness information.
Compression (Coding)
A digital technique for reducing the information needed to represent a still image, a moving image or an audio signal without undue impact on the subjective quality of the processed material. The most important information is retained, whilst repeated or unnecessary information ("redundant" information) is discarded. Such techniques are used to reduce the capacity needed to store and/or transmit photographic, video and audio information.
Contrast (Television)
The extent to which adjacent light and dark areas of a television picture differ in brightness.
Interference received in one communication channel from signals conveyed by other communication channels.
CVBS (Television)
Composite Video Blanking & Synchs. A baseband television signal containing the picture information (luminance and chrominance) plus all the synchronization signals necessary to display a fully-locked television picture.
DHCP (Dynamic Host Configuration Protocol)
Dynamic Host Configuration Protocol (DHCP) is a network protocol that enables a server to automatically assign an IP address to a computer from a defined range of numbers (i.e., a scope) configured for a given network. DHCP assigns an IP address when a system is started.
A device that takes a block of frequency and converts it to a lower frequency.The lower the noise temperature, the better the performance.
The equipment used to receive the signals from a satellite.
DNS (Domain Name System)
DNS is an abbreviation for Domain Name System, a system for naming computers and network services that is organized into a hierarchy of domains. DNS naming is used in TCP/IP networks, such as the Internet, to locate computers and services through user-friendly names.
DRO (Dielectric Resonator or Dielectrically Stabilized Oscillator)
Highly stable oscillator circuit employed by LNBs.
Units used to express the Figure of Merit or G/T of an earth station, with the dimensions of 1/kelvin, expressed on the decibel logarithmic scale.
The relative gain of an antenna with respect to an equivalent isotropic antenna, expressed on the decibel logarithmic scale.
The equivalent noise temperature of a device in kelvin, expressed on the decibel logarithmic scale.
The power of a signal in watts, expressed on the decibel logarithmic scale.
Power of a radio wave incident on a surface area of one square metre, measured in watts and expressed on the decibel logarithmic scale
decibel (dB)
A unit for comparing two currents, voltages or power levels based on a logarithmic scale. It is used particularly for expressing the difference between very large and very small values (expression: R = 10*log10(r), where r is the linear ratio and R is the ratio in dB).
Direct Broadcast Satellite. A general term that is commonly used to describe satellites and satellite systems that broadcast information directly to individual end-users.
Direct-to-Home (DTH)
The process of delivering satellite signals directly to individual households, or receiving satellite signals directly in the home via an individual reception system (dish).
The part of a satellite communications link that involves signal (re-) transmission from the satellite and reception on the ground. See also Uplink.
Digital Video Broadcasting. A coherent set of European standards for transmission and reception of digital television signals via satellite, cable or terrestrial means, developed under the auspices of the Digital Video Broadcasting project and formalised by the European Telecommunication Standards Institute (ETSI). Although European, the standards have been adopted in many countries worldwide. There are many standards within the DVB family, including specifications for satellite (DVB-S), cable (DVB-C) and terrestrial (DVB-T) transmission and reception.
Digital Video Broadcasting - Satellite - Second Generation (DVB-S2) is a digital television broadcast standard that has been designed as a successor for the popular DVB-S system. It was developed in 2003 by the DVB Project, an international industry consortium, and ratified by ETSI (EN 302307) in March 2005. The standard is based on, and improves upon DVB-S and the electronic news-gathering (or Digital Satellite News Gathering) system, used by mobile units for sending sounds and images from remote locations world-wide back to their home television stations. Click here to see our full range of DVB-S2 satellite modems!
A circuit or device that restores a coded signal to its original form based on knowledge of the process used to code the signal.
A device that recovers the original signal from a modulated carrier signal, such that the characteristics of the original are faithfully reproduced. Implements the process of Demodulation.
A device that recovers the original signal from one that has been rendered unintelligible by Scrambling.
Digital-to-Analogue Converter
A device that converts a digital signal (a series of numbers or other characters) into its equivalent analogue form (a continuously-varying signal voltage).
A device that splits a collection of signals into two groups according to the frequency range in which they are located, or combines two groups of signals, each occupying a separate frequency range, into a single collection of signals.
Digital Satellite Equipment Control. A standardised method for two-way communication between devices in satellite reception systems. Information is exchanged between devices interconnected by standard coaxial cable by means of a modulated 22 kHz tone. DiSEqC™ is a trademark of Eutelsat.
Parabolic microwave antenna used for transmitting and/or receiving satellite signals. The term is derived from the shape of the reflector surface, but is taken to mean the whole of the antenna subsystem, including the feedhorn and the antenna structure.
A device for converting the frequency of a signal to a lower frequency. See also Downconversion and Frequency Conversion.
Dual Feed
An antenna system consisting of a reflector, a support structure and two LNBs, each equipped with a separate feedhorn or sharing an integrated feed assembly. The focal point of each feed is set so that the antenna system can receive from two different satellite orbital positions simultaneously. The angular separation between satellite positions is usually around 6 degrees, although other angles are possible.
Dual-Band Feed (horn)
A feedhorn that can simultaneously receive signals in two different frequency bands, for instance the C-band (4 GHz) and the Ku-band (11/12 GHz).
Discrete Cosine Transform. Used principally in digital video compression systems such as MPEG-2, which are designed to remove unimportant or irrelevant ("redundant") information from television pictures, thus reducing the amount of data to be conveyed to the receiver. Redundant information could be, for example, the static background of a scene in which only a single person or object is moving, which only needs to be sent to the receiver once. The DCT is one mathematical technique for identifying and removing this redundant data without unduly degrading the picture quality.
A reduction in the amplitude of the higher frequency portions of a frequency modulated signal (e.g. analogue television) and its accompanying noise after transmission via a radio link. De-emphasis is used in conjunction with a complementary Pre-Emphasis device in the transmitter so that their combined effect on the signal is neutral. The pre-/de-emphasis process improves the signal-to-noise ratio for high frequency signal components and thus the overall quality of the received signal. See also Pre-Emphasis.
The process of restoring a coded signal to its original form based on knowledge of the encoding process.
The act or process by which an output wave or signal is obtained from a carrier wave or signal, where the recovered wave or signal has the characteristics of the original modulating wave or signal. The reverse process of Modulation.
The extraction of multiple distinct messages or signals from a single composite signal ("multiplex").
A system or device in which discrete signals are used to represent continuous signals in the form of numbers or other characters. Information is represented by electrical "on / off", "high / low" or "1 / 0" pulses, instead of being represented by a continuously-varying quantity (e.g. signal voltage) as is the case in an Analogue system or device.
Digital Modulation
The process of Modulation, where the modulating wave or signal is digital and the amplitude, frequency or phase of the carrier wave or signal is varied in discrete steps according to the content of the modulating signal.
The transformation of a continuously varying quantity (e.g. signal voltage) into a series of discrete signals in the form of numbers or other characters.
The process of converting the frequency of a signal to a lower frequency. Downconversion is performed at the reception point to permit the recovery of the original signal. See also Upconversion and Frequency Conversion.
External reference
The signal that is supplied to an LNB via the center conductor of the IF cable. The reference frequency is normally 10 MHz and is used to accurately phase lock the conversion oscillator in the LNB to the wanted frequency.
Earth Station
An installation (antenna and associated equipment) located on the earth's surface and intended for communication with one or more satellites. The term is usually understood to refer to the ensemble of equipment that is needed to effect communications via satellite.
Earth-Space Link
Any communications link between an earth station and a satellite (uplink or downlink).
European Broadcasting Union. An organisation that brings together the main European broadcasters and, amongst other things, works on new standards which then require ETSI (European Telecommunications Standards Institute) approval.
The total or partial obscuring of one celestial body by another. The events that most affect satellites are eclipses of the Sun by the Earth or the Moon, which deprive the satellite of its usual source of power (solar energy) and cause it to cool down rapidly because it is no longer heated by the Sun. The satellite is designed to cope with such extreme events. Normally, there is no effect on the communications services provided by the satellite during eclipse.
Effective Isotropic Radiated Power. A measure of the signal strength that a satellite transmits towards the earth, or an earth station towards a satellite, expressed in dBW.
The angle measured in the local vertical plane between the satellite and the local horizon. It is the vertical co-ordinate that is used to align a satellite antenna. See also Azimuth.
Electronic Programme Guide. A graphical user interface generated by a digital satellite receiver and displayed on the user's television screen. It provides information on the timing and content of television programmes, which is conveyed in the digital signals received from the satellite. Its primary purpose is to help the user to rapidly identify and select programmes of interest, but it may also support other interactive services.
Radiation produced, or the production of radiation, by a radio transmitting station, which can be an earth station or a satellite.
Encoding (Coding)
The process of converting a message into a code that is designed to achieve a particular purpose (e.g. error detection and correction, bit rate reduction).
The process of "locking" a signal using secret information so that it can only be deciphered by an authorised recipient who is in possession of the appropriate secret "key". This process is used in Conditional Access systems as a mechanism for controlling and managing subscribers to a particular service or range of services.
Energy Dispersal
The process of modifying a signal before it is modulated onto a carrier wave so that the energy of the modulated carrier signal is spread as evenly as possible over its bandwidth. The purpose of this process is to reduce the potential of the signal to interfere with other radio frequency signals.
Error Correction
The process of reconstructing digital information that has been corrupted in the data transmission process. There are two basic variants of error correction: FEC and ARQ. Error correction requires the detection of erroneous data based on observation of the received data (see Error Detection).
Error Detection
The process of detecting erroneous digital information after data recovery in the receiver. Erroneous information usually results from transmission errors. Error detection exploits the properties of a code applied to the data in the transmitter. See also Error Correction.
Conditional access system used mainly with the D2-MAC television transmission standard.
iDirect’s next‐generation product line of routers, line cards, and iDX software, all built on the DVB‐S2 standard with Adaptive Coding and Modulation (ACM).
The number of times that an electrical or electromagnetic signal repeats itself in a specified time. It is usually expressed in cycles per second, or hertz (Hz). Satellite transmission frequencies are usually expressed in gigahertz (GHz).
frequency band
A range of frequencies used for transmission or reception of radio waves (e.g. 10.7 GHz to 12.75 GHz).
frequency spectrum
A particular distribution of electromagnetic radiation with frequency, which is characteristic of the source of the emission. In satellite communication systems, the spectrum of a transmitted signal depends upon the modulation scheme employed. The term "frequency spectrum" is also used more generally to refer to a continuous range of frequencies.
Figure of Merit
The ratio of the maximum gain of a receiving antenna to the receiving system's equivalent noise temperature. This value is usually expressed in dB/K. It is a measure of the ability of an earth station to receive a satellite signal with good quality (high carrier-to-noise ratio). In general, the G/T increases with increasing antenna diameter.
Fixed Assignment
The assignment of fixed amounts of satellite capacity to particular earth stations regardless of their traffic requirements, which may fluctuate over a period of time. See also On-Demand Assignment.
The geographic area over which a satellite antenna receives or directs its signals. There is often a collection of concentric footprints, each representing a particular satellite EIRP or G/T. These quantities can be related to the size of the antenna that is needed on the ground to receive or transmit a particular service respectively.
Frequency Reuse
A technique for utilising a specified range of frequencies more than once within the same satellite system so that the total capacity of the system is increased without increasing its allocated bandwidth. Frequency reuse schemes require sufficient isolation between the signals that use the same frequencies so that mutual interference between them is controlled to an acceptable level. Frequency reuse is achieved by using orthogonal polarisation states (horizontal/vertical for linear, or LHC/RHC for circular) for transmission and/or by using satellite antenna (spot) beams that serve separate, non-overlapping geographic regions.
Fixed Satellite Service. In general, this refers to any satellite communication service between earth stations located at fixed geographic positions. However, this term is often used to refer to the "unplanned" frequency bands that are not subject to the internationally-agreed regulations that govern the use of the BSS frequencies. The downlink FSS frequencies in Europe are 10.7 - 11.7 GHz and 12.5 - 12.75 GHz.
F / D Ratio
The ratio of an antenna's focal length to its diameter. It describes the basic geometric architecture of the antenna, which affects its physical size, its design and its electrical performance.
A standard, low-cost RF connector used to terminate the coaxial cables that interconnect satellite reception equipment (e.g. for connecting the LNB output to the satellite receiver's input).
Feedhorn (Feed)
A device resembling a horn that emits radio waves in a concentrated beam or collects and focuses radio waves that are incident on its aperture. In a receiving system it collects microwave signals reflected from the surface of the antenna. In a transmitting system it directs microwave signals onto the reflector surface for focussing into a narrow beam aimed at the satellite. The feed is mounted at the focus of the parabolic reflector. It is usually designed to match a particular antenna geometry (F/D ratio).
A device that blocks signals or radiation of certain frequencies while allowing others to pass unaltered.
Focal Length
The distance from the reflective surface of an antenna to its focal point, usually measured in the horizontal plane. Incoming satellite signals are directed to the Feedhorn which is normally located at the focal point. See also f/D ratio.
Frequency Division Multiplex. A system in which signals are each allocated a unique portion of a shared frequency range. Each individual signal is modulated and translated in frequency so that it occupies the correct frequency segment of the composite signal spectrum and does not interfere with the other signals sharing the same band of frequencies. Individual signals are recovered from the composite signal by filtering. FDM is used, for example, to convey multiple television signals in a cable distribution system.
Frequency Division Multiple Access. A method allowing multiple carriers to share a single satellite transponder or range of frequencies. The transponder bandwidth is divided into sub-channels, each of which is allocated to a particular earth station (carrier). The earth stations transmit continuously and the transponder conveys several carriers simultaneously at different frequencies.
FEC ( Forward Error Correction )
Forward Error Correction. An error detection and correction technique based on the addition of a code to the signal at the transmitter. A decoder in the receiver detects and corrects errors making use of the properties of this code. The amount of coding information added to the original signal is quantified by the Code Rate.
Frequency Modulation. One of the principal methods for transmitting information using radio waves by superimposing the information signal onto a radio frequency carrier wave. The frequency of the carrier wave is varied in accordance with the time-varying amplitude of the input signal. The amplitude of the carrier wave remains unchanged.
Frame (Television)
One complete TV picture, composed of two fields and a total of 525 and 625 scanning lines in NTSC and PAL systems, respectively.
Frequency Conversion
The process of altering the frequency of a signal so that it is suitable for transmission or other processing. See also Upconversion and Downconversion.
Frequency Translation
See Frequency Conversion.
The amount of amplification of input to output power often expressed as a multiplicative factor or in decibels.
GHz (GigaHertz)
Unit of frequency equal to 1,000,000,000 Hz.
The ratio of the maximum gain of a receiving antenna to the receiving system's equivalent noise temperature. This value is usually expressed in dB/K. It is a measure of the ability of an earth station to receive a satellite signal with good quality (high carrier-to-noise ratio). In general, the G/T increases with increasing antenna diameter.
An object orbiting the earth at such speed that it appears to remain stationary with respect to the earth's surface. See also Clarke Belt.
GEO ( Geostationary Orbit )
GEO satellites orbit at 35,786 km (22,282 mi) above the equator in the same direction and speed as the earth rotates on its axis. This makes it appear to the earth station as fixed in the sky. The majority of commercial communications satellites operate in this orbit; however, due to the distance from the earth there is a longer latency.
Geostationary Satellite
A satellite that appears to be located at a fixed point in space when viewed from the earth's surface.
Ground Segment
The ground segment consists of all the earth stations that are operating within a particular satellite system or network. These can be connected to the end-user's equipment directly or via a terrestrial network.
Ground Station
An alternative expression for Earth Station.
High Definition TV has a screen ratio of 16:9, and is normally provided with 5.1 independent channels of CD-quality stereo surround sound (also referred to as AC-3). The video signal enables an increased image resolution by offering up to 1080 active lines and 1920 pixels per line, instead of the 576 lines and 720 pixels per line provided by the PAL TV standard. The increased resolution at progressive scan instead of interlaced makes the picture extremely sharp.
The SI unit of frequency, equivalent to one cycle per second. The frequency of a periodic phenomenon that has a periodic time of one second.
High-definiton television
High-definition television (HDTV) provides a resolution that is substantially higher than that of standard-definition television. HDTV may be transmitted in various formats:
  • 1080p - 1920×1080p: 2,073,600 pixels (~2.07 megapixels [Mpx]) per frame
  • 1080i - typically either:
    • 1920×1080i: 1,036,800 pixels (~1.04 Mpx) per field or 2,073,600 pixels (~2.07 Mpx) per frame
    • 1440×1080i: 777,600 pixels (~0.78 Mpx) per field or 1,555,200 pixels (~1.56 Mpx) per frame
  • 720p - 1280×720p: 921,600 pixels (~0.92 Mpx) per frame
The letter "p" here stands for progressive scan while "i" indicates interlaced. When transmitted at two megapixels per frame, HDTV provides about five times as many pixels as SD (standard-definition television).
High Band
The upper part of the Ku-band downlink frequency range, from 11.7 GHz to 12.75 GHz.
Horizontal Polarisation
Type of linear polarisation where the electric field is approximately aligned with the local horizontal plane at an on-ground transmission or reception point. See also frequency reuse.
High Power Amplifier. A device that accepts a relatively weak input signal and boosts it to a power level that is suitable for transmission over an earth-space link.
HTTP (Hypertext Transfer Protocol)
The Hypertext Transfer Protocol is an application protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web.
IP (Internet Protocol)
The Internet Protocol is the principal communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
Input Back-Off. The ratio of the signal power measured at the input to a high power amplifier to the input signal power that produces the maximum signal power at the amplifier's output. The input backoff is expressed in decibels as either a positive or negative quantity. It can be applied to a single carrier at the input to the HPA ("carrier IBO"), or to the ensemble of input signals ("total IBO").
The angle between the plane of the orbit of a satellite and the Equatorial plane. A orbit of a perfectly-geostationary satellite has an inclination of 0 degrees.
Inclined Orbit
An orbit that approximates the geostationary orbit but whose plane is tilted slightly with respect to the Equatorial plane, with the consequence that the satellite appears to move about its nominal position in a daily "figure-of-eight" motion when viewed from the ground. Satellites are often allowed to drift into an inclined orbit near the end of their nominal lifetime in order to conserve fuel on-board the satellite, which would otherwise be used to correct this natural drift caused by the gravitational pull of the Sun and the Moon.
Individual Reception
The direct reception of satellite signals by simple domestic installations, in particular those equipped with a small antenna. See also Direct-to-Home and Community Reception.
Any undesired signal that tends to interfere with the reception of radio waves. It can be caused by transmissions within the same satellite system, by transmissions within other satellite systems that use the same frequencies, or from ground-based sources (e.g. point-to-point radio links, car ignition noise, etc.).
Mutual interference between signals spaced apart in frequency after non-linear amplification by a common amplifier. In satellite communication systems the phenomenon of intermodulation is usually only significant after the high power amplifier in an earth station or a satellite transponder. It is controlled by means of the IBO/OBO of the amplifier.
iDirect’s product line of routers and line cards, built on iDirect’s proprietary implementation of the TDM protocol.
Integrated Receiver-Decoder. A indoor device accepting signals from at least one LNB, which recovers the original signal from the signal delivered by the LNB. It includes a built-in decoder for reception of services that are protected by a Conditional Access system, subject to authorisation from the service provider. A plug-in "smart card" is often used for authorisation purposes.
Isotropic Antenna
A theoretical device that radiates energy or receives energy equally from all directions.
Intermediate Frequency. In radio communication systems, frequency conversion from baseband to the transmission frequency, and from the reception frequency to baseband, is usually carried out in two or more stages. Any frequency obtained after a frequency conversion that does not correspond to the baseband, the transmission frequency or the reception frequency is known as an intermediate frequency. In satellite reception systems the term IF is often used to refer to the frequency range in which the LNB delivers the signals it receives from the satellite (950 - 2150 MHz) to the IRD.
Interlaced Scanning(TV)
The process of scanning a single image ("frame") of a moving picture sequence in two sequential stages, where each stage produces a scanned image ("field") comprising one-half of the total number of horizontal lines used in the scanning process. The lines of each field alternate. It is a technique that is used in conventional television systems (e.g. PAL) to reduce the transmission bandwidth by exploiting the properties of the human eye.
Joint Photographic Experts Group. A group established by the International Organization for Standardization (ISO) that developed a widely-used international standard for the coding of still pictures. The term "JPEG" is often used to refer to the coding method itself, which reduces the information needed to represent the picture with good quality.
JPEG 2000
JPEG 2000 is an image compression standard and coding system. It was created by the Joint Photographic Experts Group committee in 2000 with the intention of superseding their original discrete cosine transform-based JPEG standard (created in 1992) with a newly designed, wavelet-based method. The standardized filename extension is .jp2 for ISO/IEC 15444-1 conforming files and .jpx for the extended part-2 specifications, published as ISO/IEC 15444-2. The registered MIME types are defined in RFC 3745. For ISO/IEC 15444-1 it is image/jp2.
Information transfer rate, equal to 1,000 bit/s.
Kelvin (K)
Basic SI unit of thermodynamic temperature.
Unit of frequency equal to 1,000 Hz
The range of frequencies that are available for use by satellite communication systems at around 30 GHz for the uplink and 20 GHz for the downlink. The Ka-band is used in particular for satellite internet services and regional broadcasting.
The range of frequencies attributed to satellite communication systems, around 14 GHz or 18 GHz for the uplink, and 11 to 12 GHz for the downlink. The Ku-band is often used for television services via satellite and for VSAT networks.
LEO ( Low Earth Orbit )
LEO satellites orbit from 160-2000km above the earth, take approximately 1.5 hrs for a full orbit and only cover a portion of the earth’s surface, therefore requiring a network or constellation of satellites to provide global, continual coverage. Due to the proximity to Earth, LEO satellites have a lower latency (latency is the time between the moment a packet is transmitted and the moment it reaches its destination) and require less amplification for transmission.
Network latency in a packet-switched network is measured either one-way (the time from the source sending a packet to the destination receiving it), or round-trip delay time (the one-way latency from source to destination plus the one-way latency from the destination back to the source). Round-trip latency is more often quoted, because it can be measured from a single point. Note that round trip latency excludes the amount of time that a destination system spends processing the packet. Many software platforms provide a service called ping that can be used to measure round-trip latency. Ping performs no packet processing; it merely sends a response back when it receives a packet (i.e. performs a no-op), thus it is a first rough way of measuring latency. Ping cannot perform accurate measurements,[2] principally because it uses the ICMP protocol that is used only for diagnostic or control purposes, and differs from real communication protocols such as TCP. Furthermore routers and ISP's might apply different traffic shaping policies to different protocols.
layer 7 (Aplication layer)
In computer network programming, the application layer is an abstraction layer reserved for communications protocols and methods designed for process-to-process communications across an Internet Protocol (IP) computer network. Application layer protocols use the underlying transport layer protocols to establish host-to-host connections.
L.O. Stability (Local Oscillator Frequency Stability)
The variance in the frequency of the Local oscillator with time, temperature, voltage, humidity or vibration.
LNB (Low Noise Block Downconverter)
An LNA and block downconverter enclosed in a single housing. The output of an LNB is an IF signal at L-Band [nominally 950 to 1450 MHz].
L.O. Frequency (Local Oscillator Frequency)
A single-frequency reference signal of high purity which is used by a mixer to convert a communications signal to a higher or lower frequency band.
The microwave frequency band between approximately 900 MHz to 2000 MHz.
Left-hand polarised wave. An elliptically- or circularly-polarised wave, in which the electric field vector, observed in any fixed plane normal to the direction of propagation, whilst looking in the direction of propagation, rotates with time in a left-hand or anticlockwise direction.
Linear Polarisation
Describes a wave in which the electric field vector, observed in any fixed plane normal to the direction of propagation, maintains a constant direction with time. With linear polarisation, the earth station and satellite antennas of a particular earth-space link must be precisely aligned so that their reference polarisation directions coincide, in order to obtain maximum reception quality.
Low Band
The lower part of the Ku-band downlink frequency range, from 10.7 GHz to 11.7 GHz.
Line Amplifier
An amplifier in a (long) transmission line that boosts the strength of a signal to an exploitable level.
Line Splitter
An active or passive device that divides a signal into two or more signals containing all the original information. A passive splitter feeds an attenuated version of the input signal to the output ports. An active splitter amplifies the input signal to overcome the splitter loss.
Luminance (Television)
The brightness information of a television picture. It is also used to refer to the brightness component (Y signal) of a PAL, SECAM or NTSC television signal.
MHz (MegaHertz)
Unit of frequency equal to 1,000,000 Hz.
MEO ( Medium Earth Orbit )
MEO satellites are located above LEO and below GEO satellites and typically travel in an elliptical orbit over the North and South Pole or in an equatorial orbit. These satellites are traditionally used for GPS navigation systems and are sometimes used by satellite operators for voice and data communications. MEO satellites require a constellation of satellites to provide continuous coverage. Tracking antennas are needed to maintain the link as satellites move in and out of the antenna range.
Mean Time Between Failure estimated lifetime of unit.
MF-TDMA ( Multiple‐Frequency Time Division Multiple‐Access )
MF-TDMA ( Multiple‐Frequency Time Division Multiple‐Access) is a broadband access method where different data streams are put into different slots that are separated by both frequency and time
Information transfer rate, equal to 1,000,000 bit/s.
Mesh Network Topology
A mesh network topology allows one remote VSAT location to communicate with another remote location without routing through the hub. This type of connection minimizes delay and often is used for very high quality voice and video conferencing applications. With this topology, larger antennas are required and more power is needed to transmit, thereby increasing cost.

star topology image
Unit of data transmission rate for a radio link, equal to 1,000,000 symbol/s. Can be directly related to the bandwidth required to establish the transmission link (e.g. 33 MHz for a transmission rate of 27.5 Msymbol/s).
The difference in decibels between the C/(N+I) achieved at the receiver input under clear sky conditions to the minimum C/(N+I) required for just acceptable transmission quality. Also referred to as the "Rain Margin".
Multiple Channel Per Carrier. Refers to the multiplexing of a number of digital channels (video programmes, audio programmes and data services) into a common digital bit stream, which is then used to modulate a single carrier that conveys all of the services to the end user. The single carrier supports multiple communication channels, hence the phase "multiple channel per carrier". The term MCPC is frequency used in the context of DVB systems, where the composite digital signal is referred to as a Transport Stream.
Generally refers to the use of multiple antenna beams on board the satellite to cover a contiguous geographical area, instead of a single wide-area beam. Multibeam architectures are often considered for satellites operating in the Ka-band, which is characterised by narrower beamwidths with respect to the Ku-band. Single, wide-area beams predominate in the latter.
DVB conditional access option based on a detachable Conditional Access (CA) module, which is supplied by the service provider to each subscriber. The CA module is connected to the subscriber's IRD via a standardised interface (the DVB Common Interface). Multicrypt has the advantage that the same IRD can be used to receive services from providers using different and incompatible conditional access systems.
A device in which two or more input signals are combined to give a single output signal. In satellite communication systems, it is a non-linear device used to generate a replica of an input signal at a higher or lower frequency by multiplying the input signal by a pure tone of a different frequency (the "local oscillator" signal). Usually part of a frequency conversion process. For example, an LNB local oscillator signal at 10.6 GHz mixed with incoming signal at 12 GHz would convert the input signal to an IF frequency of 12 -10.6 = 1.4 GHz.
A device which superimposes the amplitude, frequency or phase of a wave or signal onto another wave or signal (a carrier), which is then used to convey the original signal via a transmission medium (e.g. satellite link).
The frequency range from approximately 1 to 300 GHz, covering the frequency range suitable for satellite communications.
To superimpose the amplitude, frequency or phase of a wave or signal onto another wave or signal, which is then used to convey the original signal via a transmission medium (e.g. satellite link).
Motion Pictures Experts Group. A group established by the International Organization for Standardization (ISO) that establishes international standards for compression coding of moving pictures and audio programmes. The MPEG-2 standard is widely used for compressing video material (e.g. in the DVB standard).
MPEG-2 is widely used as the format of digital television signals that are broadcast by terrestrial (over-the-air), cable, and direct broadcast satellite TV systems. It also specifies the format of movies and other programs that are distributed on DVD and similar discs. TV stations, TV receivers, DVD players, and other equipment are often designed to this standard. MPEG-2 was the second of several standards developed by the Moving Pictures Expert Group (MPEG) and is an international standard (ISO/IEC 13818). Parts 1 and 2 of MPEG-2 were developed in a collaboration with ITU-T, and they have a respective catalog number in the ITU-T Recommendation Series. While MPEG-2 is the core of most digital television and DVD formats, it does not completely specify them. Regional institutions can adapt it to their needs by restricting and augmenting aspects of the standard. /dd>
MPEG-4 is a method of defining compression of audio and visual (AV) digital data. It was introduced in late 1998 and designated a standard for a group of audio and video coding formats and related technology agreed upon by the ISO/IEC Moving Picture Experts Group (MPEG) (ISO/IEC JTC1/SC29/WG11) under the formal standard ISO/IEC 14496 – Coding of audio-visual objects. Uses of MPEG-4 include compression of AV data for web (streaming media) and CD distribution, voice (telephone, videophone) and broadcast television applications. /dd>
MPEG-4 AVC ( Advanaced Video Coding )
H.264/MPEG-4 Part 10 or AVC (Advanced Video Coding) is a video compression format that is currently one of the most commonly used formats for the recording, compression, and distribution of video content. The final drafting work on the first version of the standard was completed in May 2003, and various extensions of its capabilities have been added in subsequent editions.

H.264 is perhaps best known as being one of the video encoding standards for Blu-ray Discs. It is also widely used by streaming internet sources, such as videos from Vimeo, YouTube, and the iTunes Store, web software such as the Adobe Flash Player and Microsoft Silverlight, and also various HDTV broadcasts over terrestrial (ATSC, ISDB-T, DVB-T or DVB-T2), cable (DVB-C), and satellite (DVB-S and DVB-S2).
Multiple Access
The simultaneous sharing of a common transmission bandwidth by multiple users. In satellite communications, it usually refers to the shared use of one or more transponders by multiple earth stations.
A signal that comprises multiple distinct signals or messages, usually for the purposes of transmission via a common communications channel.
The use of a common communications channel for sending two or more messages or signals (e.g. multiple digital television programmes on a single digital carrier, or "multiplex"). Multiplexing is the process of combining multiple signals into a composite signal that is suitable for transmission via the common communications channel.
NAT( Network Address Translation or Network Address Translator )
NAT (Network Address Translation or Network Address Translator) is the translation of an Internet Protocol address (IP address) used within one network to a different IP address known within another network. One network is designated the inside network and the other is the outside.
Any undesired electrical disturbance in a circuit or communication channel. When combined with a received signal, it affects the receiver's ability to correctly reproduce the original signal. Also known as Thermal Noise.
Noise Figure
A method for quantifying the electrical noise generated by a practical device. The noise figure is the ratio of the noise power at the output of a device to the noise power at the input to the device, where the input noise temperature is equal to the reference temperature (290 K). The noise figure is usually expressed in decibels.
Noise Temperature
A mathematical convenience for predicting the influence of noise in a communications system. It is a measure of the noise power generated by a practical device, expressed as the equivalent temperature of a resistor which, when placed at the input of a perfect noise-free device, generates the same amount of output noise. The noise temperature is usually expressed in kelvin or dBK.
Refers to a device or process in which the output is not directly proportional to the input. Often used in the satellite communication context to refer to the (undesirable) characteristics of practical high power amplifiers.
NTSC (Television)
The National Television Standards Committee, which created the North American conventional television broadcasting standard. The standard itself is also referred to as NTSC.
OSI (Open System Interconnection)
Short for Open System Interconnection, an ISO standard for worldwide communications that defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.
OBO (Output Back-Off)
The ratio of the signal power measured at the output of a high power amplifier to the maximum output signal power. The output backoff is expressed in decibels as either a positive or negative quantity. It can be applied to a single carrier at the output to the HPA ("carrier OBO"), or to the ensemble of output signals ("total OBO").
OBP (On-board Processing)
A general term that refers to signal processing functions implemented on-board the satellite that go beyond the amplification and frequency conversion performed in conventional, transparent satellite systems. On-board processing is usually but not necessarily implemented digitally, and may or may not include signal regeneration.
Any direction in space that does not correspond to an antenna's boresight direction.
On-Demand Assignment
The assignment of variable amounts of satellite capacity to particular earth stations according to their fluctuating traffic requirements (according to demand). See also Fixed Assignment. On-demand assignment offers more efficient satellite capacity utilisation at the expense of system complexity.
The path described by the centre of mass of a satellite in space, subjected to natural forces, principally gravitational attraction, and occasional low-energy corrective forces exerted by a propulsive device in order to achieve and maintain the desired path.
Orbital Plane
The plane containing the centre of mass of the earth and the velocity vector (direction of motion) of a satellite.
Out-of-Band Emission
Any emission on a frequency or frequencies outside the bandwidth of a signal which results from the modulation process. Out-of-band emissions are a potential source of interference to other services and need to be carefully controlled.
An outage is said to occur when the quality of a telecommunication service or communications link falls below a specified minimum value for acceptable communications performance. See also Availability.
PLL (Phase Locked Loop)
A type of oscillator that uses digital circuits and a precision reference signal to accurately control the frequency of the conversion oscillator. An LNB that incorporates a PLL oscillator has the characteristics of very low levels of phase noise and high levels of frequency stability.
PPM (Parts Per Million)
A unit of measure used to express the frequency stability of an oscillator. In Ku-band 1ppm represents a frequency change of 10 kHz when the conversion oscillator is 10.0 GHz.
The purchasing of programmes and services by a television viewer or service user on an individual basis (e.g. televised coverage of a sports event). Access to purchased material is controlled by means of a Conditional Access system.
Payload (Satellite)
Refers to all equipment on-board a satellite that is dedicated to the reception, frequency conversion, processing and retransmission of communication signals, including the satellite antennas, but excluding support equipment such as the platform (physical structure), power supplies and thermal control equipment.
Pointing Angles
The elevation and azimuth angles which specify the direction of a satellite from a point on the earth's surface.
Pointing Error (Antenna)
A value which quantifies the amount by which an antenna is misaligned with the satellite's position in space (see Alignment). This is either expressed as an angular error, or as a loss in signal strength with respect to the maximum that would be achieved with a perfectly aligned antenna.
Polar Mount
A mechanical support structure for an earth station antenna that permits all satellites in the geosynchronous arc to be scanned with movement of only one axis.
The phenomenon in which radio waves are restricted to certain directions of electrical and magnetic field variations, where these directions are perpendicular to the direction of wave travel. By convention, the polarisation of a radio wave is defined by the direction of the electric field vector. Four senses of polarisation are used in satellite transmissions: horizontal (X) linear polarisation, vertical (Y) linear polarisation, right-hand circular polarisation and left-hand circular polarisation.
Polarisation Alignment
The process of aligning the reference polarisation plane of an linearly-polarised antenna with a particular reference direction. For individual and collective systems receiving linearly-polarised signals, this consists of rotating the LNB about the feed axis so that its radio wave detector is aligned with the electric field vector of the incoming signal (to achieve detected signal strength).
Polarisation Switching
The process of selecting one of two orthogonal polarisations (e.g. linear horizontal or linear vertical) for reception of satellite signals. Polarisation switching is implemented in the LNB or, more rarely, in a separate device inserted between the feedhorn and the LNA/LNB or integrated with the feedhorn.
QOS (Quality of Service)
QOS refers to a broad collection of networking technologies and techniques. The goal of QoS is to provide guarantees on the ability of a network to deliver predictable results. Elements of network performance within the scope of QoS often include availability (uptime), bandwidth (throughput), latency (delay), and error rate. QoS can be improved with traffic shaping techniques such as packet prioritization, application classification and queuing at congestion points.
Quad / Quattro LNB
An LNB providing four outputs simultaneously. It is used to deliver signals received on both linear polarisations (horizontal and vertical) and in both frequency bands (high and low). Each output delivers the signals received on one polarisation and in one frequency band. It is used in collective reception systems to ensure that all services are available to all subscribers, regardless of the service (TV programme) selected by any individual.
Radiation Pattern
A three-dimensional representation of the gain of a transmit or receive antenna as a function of the direction of radiation or reception.
Receiver Noise
The equivalent noise temperature of a complete receiving system, excluding contributions from the antenna and the physical connection to the antenna, referred to the receiver input.
A term used to describe satellite systems/transponders that recover the original signals from the modulated signals received from the ground, process them in some way, then use them to modulate carriers for retransmission at the downlink frequencies, possibly with a different format. Regenerative repeaters are complex are often feature in the designs of future, advanced satellite systems.
A device that amplifies or augments incoming electrical signals and retransmits them towards the earth station(s) at a different frequency. In the satellite context, the term "repeater" usually refers to all Payload equipment, with the exception of the satellite antennas.
Right-hand polarised wave. An elliptically- or circularly-polarised wave, in which the electric field vector, observed in any fixed plane normal to the direction of propagation, whilst looking in the direction of propa
The equipment that receives incoming electrical signals or modulated radio waves and converts them into the original audio, video or data signals.
The outward flow of energy from any source in the form of radio waves.
The use of electromagnetic waves, lying in the radio frequency range, for communications purposes.
Radio Waves
Electromagnetic waves lying in the radio frequency range, propagated in space without artificial guide.
Radio-Frequency Links
Communication links established by means of radio waves.
Telecommunication by means of radio waves.
Radio Frequency. The 10 kHz to 300 GHz frequency range that can be used for wireless communication. The term RF is usually used to distinguish signals transmitted to and from the satellite from signals processed at other frequencies within the same communication system (e.g. intermediate frequencies).
Red Green Blue. Primary colours that, when suitably combined, produce the same visual effect as almost any other colour. These primary colours are used in colour television systems, which reproduce colour images by controlling the intensity of red, green and blue light sources on the television screen.
SCADA (supervisory control and data acquisition)
SCADA (supervisory control and data acquisition) is a system operating with coded signals over communication channels so as to provide control of remote equipment (using typically one communication channel per remote station).
SNMP ( Simple Network Management Protocol )
Simple Network Management Protocol (SNMP) is an "Internet-standard protocol for managing devices on IP networks". Devices that typically support SNMP include routers, switches, servers, workstations, printers, modem racks and more. It is used mostly in network management systems to monitor network-attached devices for conditions that warrant administrative attention. SNMP is a component of the Internet Protocol Suite as defined by the Internet Engineering Task Force (IETF). It consists of a set of standards for network management, including an application layer protocol, a database schema, and a set of data objects.
SCPC (Single Channel Per Carrier)
Single Channel Per Carrier. In SCPC systems, each communication signal is individually modulated onto its own carrier which is used to convey that signal to the end user. A number of similar carriers share a common satellite transponder and use a unique portion of its bandwidth. Each carrier supports a single communication channel only (e.g. one-half of a voice circuit), hence the phrase "single channel per carrier".
A unique signal state of a modulation scheme used on a transmission link, which conveys one or more information bits to the receiver.
The number of symbols transmitted every second.
Signal-to-Noise Ratio. A measure of the quality of an electrical signal, usually at the receiver output. It is the ratio of the signal level to the noise level, measured within a specified bandwidth (typically the bandwidth of the signal). It is usually expressed in decibels. The higher the ratio, the better quality of the signal. See also C/N.
Standard-definition television
Standard-definition television (SDTV) is a television system that uses a resolution that is not considered to be either high-definition television (HDTV 720p, 1080i, and 1080p) or enhanced-definition television (EDTV 480p). The two common SDTV signal types are 576i, with 576 interlaced lines of resolution, derived from the European-developed PAL and SECAM systems; and 480i based on the American National Television System Committee NTSC system.
Satellite Link
A radio link between a transmitting earth station and a receiving earth station through a communications satellite. A satellite link comprises one uplink and one downlink.
Satellite Network
One or more communications satellites and the cooperating earth stations.
Satellite System
A space system using one or more artificial satellites orbiting the earth.
The operation of a power amplifier, most often a satellite TWTA, at its maximum output power level ("saturated" power level).
SMPTE ( Society of Motion Picture and Television Engineers)
The Society of Motion Picture and Television Engineers (SMPTE), founded in 1916 as the Society of Motion Picture Engineers or SMPE, is an international professional association, based in the United States of America, of engineers working in the motion imaging industries. An internationally recognized standards organizations, SMPTE has more than 600 Standards, Recommended Practices and Engineering Guidelines for television production, filmmaking, digital cinema, audio recording, information technology, and medical imaging. In addition to development and publication of technical standard documents, SMPTE publishes the SMPTE Motion Imaging Journal, provides networking opportunities for its members, produces academic conferences and exhibitions, and performs other industry-related functions.
Shaped Beam
The radiation pattern of a satellite antenna that has been designed so that its footprint follows the boundary of a specified geographical area (the area of service provision) as closely as possible. Shaped beams maximise the antenna gain over the service area and reduce the likelihood of interference into systems serving other geographical areas.
Part of an antenna's radiation pattern which can detect or radiate signals in an unwanted direction (i.e. off-axis), which can produce interference into other systems or susceptibility to interference from other systems. The larger the side lobes, the more noise and interference an antenna can detect. Sidelobe levels are determined by the design of the antenna.
Space Segment
Commonly used to refer to the satellites of a particular satellite communication system.
Spot Beam
An antenna radiation pattern designed to serve a relatively small or isolated geographic area, usually with high gain. The radio frequency equivalent of a spotlight.
Spurious Emission
Any emission on a frequency or frequencies outside the bandwidth of a signal including harmonic emissions, parasitic emissions, intermodulation products and frequency conversion products. Spurious emissions are a potential source of interference to other services and need to be carefully controlled.
Star Topology ( Hub & Spoke )
In a star network topology the hub connects to the remote, where all communications are passed back through the hub. Virtually an unlimited number of remotes can be connected to the hub in this topology. Smaller, lower powered BUCs can be used at the remote end since they are only connecting back to the larger hub antenna.

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Steerable Beam
An antenna beam that can be repointed by mechanical and/or electrical means. Usually used to refer to relatively narrow satellite beams that can be steered over a part or the whole of the portion of the earth's surface that is visible from the satellite's orbital position.
Sun Outage
Sun-Satellite Conjunction can cause a large increase in the noise received by an earth station that is pointed towards the satellite, which degrades the signal quality and can even cause the signal to be lost for a short time. Whilst this is an unavoidable physical phenomenon, it does not affect the relatively small antennas that are used for individual and collective reception of broadcast television and entertainment services. See Sun Outage Predictor
Sun-Satellite Conjunction
The alignment of the Sun with the satellite as seen from an earth station, which takes place twice a year for several minutes around local midday. This event can affect the performance of receiving earth stations. See Sun Outage.
System Noise Temperature
The equivalent noise temperature of a complete receiving system, taking into account contributions from the antenna, the receiver and the transmission line that interconnects them, referred to the receiver input.
Satellite Receiver
A receiver designed for satellite reception system, which receives modulated signals from an LNA or LNB and converts them into their original form suitable for direct presentation to the user. See also IRD.
A device that takes an input signal and splits it into two or more identical output signals, each a replica of the input signal but with a different amplitude (typically).
SSPA (Solid State Power Amplifier)
A high power amplifier using solid state technology (i.e. transistors). Used for low and medium power applications.
The process of moving the electron beam in a television camera tube simultaneously in the horizontal and vertical directions so that an image is scanned from left to right and top to bottom. The electrical signal generated by this process is converted into an image on the television screen using the same scanning sequence.
Any signal carrying information that is transmitted within the bandwidth of another signal which itself modulates a carrier. Used in analogue TV transmission systems, for example, to convey colour and audio information.
Transmitter (BUC Block upconverter)
A device that converts L-band signal (950 to 1450 Mhz) into a RF transmit power signal via a waveguide interface.
TCP (Transmission Control Protocol)
The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite (IP), and is so common that the entire suite is often called TCP/IP.
TCXO (Temperature Compensated Crystal Oscillator)
An oscillator circuit, such as would be used as a local oscillator or a reference in a PLL LNB, that has been designed so that the frequency change with temperature is as small as possible.
Coded radiocommunication from the satellite to the ground for the transmission of data relating to the functioning and configuration of the satellite.
Thermal Noise
Any undesired electrical disturbance in a circuit or communication channel. The term "thermal" refers to the fact that the magnitude of the noise generated by an object is dependent upon the object's physical temperature.
The process of continuously adjusting the orientation of an antenna so that its boresight follows the movements of the satellite about its nominal position. Used in earth stations equipped with large antennas and earth stations operating to satellites in inclined orbit.
A term used to describe satellite systems or satellite transponders that do not alter the basic format of the signals they receive before retransmitting them. A transparent transponder simply converts signals to a lower frequency and amplifies them prior to retransmission, as opposed to regenerative transponders or on-board processing (see Regenerative and OBP respectively).
A transmitter-receiver device that transmits signals automatically when it receives pre-determined signals. The term "satellite transponder" refers to a transmitter-receiver subsystem on-board the satellite that uses a single high power amplification chain and processes a particular range of frequencies (the "transponder bandwidth"). There are many transponders on a typical satellite, each capable of supporting one or more communication channels.
Television Receive-Only. An earth station incapable of transmitting to the satellite and intended for the individual or collective reception of television (multimedia) services from the satellite.
TWTA (Travelling Wave Tube Amplifier)
A high power amplifier based on tube technology. Normally employed when high output power levels are required.
TDM (Time Division Multiplexing)
A system in which the bits of more than one digital signal are interleaved in time to form a single digital bit stream that carriers all of the information contained in the original signals.
TDMA (Time Division Multiple Access)
A method allowing multiple carriers to share a single satellite transponder or range of frequencies. The earth stations transmit sequentially in unique time slots at the same carrier frequency, so that only a single station transmits at any given time.
The science and technology of communication by artificial means (radio, television, telephony, etc.).
Terrestrial Interference
Interference between a satellite system and entirely earth-based microwave communication systems.
UDP (User Datagram Protocol )
The User Datagram Protocol (UDP) is one of the core members of the Internet protocol suite (the set of network protocols used for the Internet). With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol (IP) network without prior communications to set up special transmission channels or data paths.
UTC (Coordinated Universal Time)
A time-scale which forms the basis of a coordinated dissemination of standard frequencies and time signals throughout the world.
The part of a satellite communications link that involves signal transmission from the ground and reception on-board the satellite. See also Downlink.
Universal LNB
An LNB that is capable of receiving a signal transmitted on any linear polarisation and at any frequency within the range 10.7 - 12.75 GHz, usually by means of band and polarisation switching.
Unwanted Emissions
Any undesired emission resulting from the radiocommunication process, which could potentially interfere with other systems. Formal definition: the combination of spurious emissions and out-of-band emissions.
A device for converting the frequency of a signal into a higher frequency. See also Upconversion and Frequency Conversion.
UHF (Ultrahigh Frequency)
The frequency range from 300 MHz to 3 GHz.
The process of converting the frequency of a signal to a higher intermediate frequency or to the transmission frequency. Upconversion is performed at the transmission point to prepare the signal for transmission over the satellite link. See also Downconversion and Frequency Conversion.
VSAT (Very Small Aperture Terminal)
This is normally a two way satellite transmission system.
VSWR (Voltage Standing Wave Ratio)
Can best be described as waves coming into the L.O. Frequency shore from the ocean. Desirable VSWR waves come in smoothly and the energy used in the LNB that is used as the bases for the down conversion to the L-Band is dissipated into the sand. This desirable VSWR is called “primary incident” and results in a low VSWR
Vertical Polarisation
Type of linear polarisation where the electric field is approximately aligned with the local vertical plane at an on-ground transmission or reception point. See also frequency reuse.
Voice over Internet Protocol (VoIP) is a methodology and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet.
VPN is a network that is constructed by using public wires — usually the Internet — to connect to a private network, such as a company's internal network. There are a number of systems that enable you to create networks using the Internet as the medium for transporting data.
VHF (Very High Frequency)
The frequency range from 30 MHz to 300 MHz.
watt (W)
SI unit of power, equal to 1 joule/second.
A television picture or television screen that is wider than a conventional (4 by 3) television picture or screen, usually with an aspect ratio of 16 by 9.
A system of material boundaries in the form of a solid dielectric rod or dielectric-filled tubular conductor capable of guiding high-frequency electromagnetic waves.
A more precise definition of horizontal linear polarisation. X-polarisation is defined with respect to a particular direction from the satellite towards the earth, allowing precise calculation of the polarisation alignment angle for any given geographic location.
A more precise definition of vertical linear polarisation. Y-polarisation is defined with respect to a particular direction from the satellite towards the earth, allowing precise calculation of the polarisation alignment angle for any given geographic location.
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