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Satellite Terminology and Glossary
 

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 The process of optimising the orientation of a satellite antenna's main direction of sensitivitytowards the satellite to maximise the received signal level and to minimise the chance of receiving unwanted interference from other satellite systems. A commonly-used alternative expressions is "antenna pointing".

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.

Attenuation 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.

Availability The amount of time that the quality of a telecommunication service or communications link equals or exceeds a specified minimum value. For satellite communication links the availability is usually expressed as a percentage of the average year. See also Outage.

Azimuth 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.

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 IF.

Bandwidth 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.

Beacon A highly stable radio frequency signal, which is used by earth stations equipped with an automatically (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 (see Telemetry)

Beam 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.

Beamwidth 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).

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.

Boresight The direction of maximum antenna gain. For a receiving antenna, the boresight is aligned with the satellite as accurately as possible for maximum received signal strength.

Bouquet A collection of digital multimedia services marketed as a single package, often transmitted in a single data stream. See also Digital Multiplexing.

BSS 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.

C/I 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.

C/N 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.

C/(N+I) 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.

Capacity A proportion of the satellite's bandwidth and power which is used to establish one or more communication channel.

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).

Collective Reception See Community Reception.

Collocated Two or more satellites occupying approximately the same geostationary orbital position such that the angular separation between them is very small when viewed from the ground. The satellites appear to be exactly collocated to a small receiving antenna. In reality, the satellites are kept several kilometres apart in space to avoid collisions. An example of a collocated satellite system is the EUTELSAT HOT BIRD™ system located at the 13º East geostationary orbital position.

Community Reception The reception of satellite television and entertainment services for distribution to a group of the general public at one location (e.g. in a block of flats), or through a distribution system covering a limited area (e.g. a local cable network). The receiving system is usually more complex with a larger antenna than that used for individual (Direct-To-Home) reception.

Conditional Access (CA) A system for restricting 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.

Coverage 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).

Cross-Polar 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.

DVB 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.

DBS 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).

Downlink The part of a satellite communications link that involves signal (re-) transmission from the satellite and reception on the ground. See also Uplink.

Earth-Space Link Any communications link between an earth station and a satellite (uplink or downlink).

Elevation The angle measured in the local vertical plane between the satelliteand the local horizon. It is the vertical co-ordinate that is used to align a satellite antenna. See also Azimuth.

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.

Eclipse 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.

EIRP 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.

EPG 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.

EBU 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.

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. See also G/T.

Fixed Assignment The assignment of a 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.
Footprint 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.

FSS 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.

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.

Gain (Antenna) A measure of the amplifying or focussing power of an antenna when transmitting to, or receiving from, a particular direction in space. The gain of an antenna is the ratio of the power radiated (or received) per unit solid angle by the antenna in a given direction to the power radiated (or received) per unit solid angle by an isotropic antenna fed with the same power. The gain is usually expressed in dBi.

Geostationary 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.

Geostationary Satellite A satellite that appears to be located at a fixed point in space when viewed from the earth's surface.

Geostationary Orbit The orbit of a geosynchronous satellite whose orbit lies in the plane of the earth's equator.

Geosynchronous An object orbiting the earth at the earth's rotational speed and with the same direction of rotation. The object will appear at the same position in the sky at a particular time each day, but will not appear stationary if not orbiting in the equatorial plane.

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.
G/T See Figure of Merit.

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.

HPA 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.

Inclination 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

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.

IBO 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").

Interference 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.).

Intermodulation 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.

IRD 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.

Used to refer to a 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

Ku-Band Used to refer to a 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.

LHC(P) 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.

Margin 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".

[b]MCPC Multiple Channel Per Carrier.[/b] Refers to the multiplexing 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.

Multibeam 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.

Multicrypt DVB conditional access option based 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.

Noise 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.

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. Skyplex is a practical example of OBP.

Off-Axis 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 office more efficient satellite capacity utilisation at the expense of system complexity.

Orbit 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.

Outage 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.

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.

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.

Pay-Per-View
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.

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.

Polarisation 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.

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.

Radiation Pattern A three-dimensional representation of the gain of a transmit or receive antenna as a function of the direction or radiation or reception.

Rain Margin See Margin.

Receiver Noise Temperature 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.

Regenerative 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.

Repeater A device that amplifies or augments incoming electrical signals and re?transmits 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.

RHC(P) 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 propagation, rotates with time in a right-hand or clockwise direction.


S/N 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.

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.

Saturation The operation of a power amplifier, most often a satellite TWTA, at its maximum output power level ("saturated" power level).

SCPC 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".

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.

Sidelobe 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.

Simulcast Simultaneous transmission of an identical programme or service using two or more standards or transmission media. Used to refer to a technique pioneered by EUTELSAT for transmitting one broadcast analogue FM television carrier and one digital television carrier in a single satellite transponder that would normally only support the FM TV carrier.

Simulcrypt DVB conditional access option based on a commercial agreement between service providers, which allows access to a common population of proprietary IRDs. The IRD hardware is usually specific to the conditional access system and cannot be used to receive services delivered by service providers that are not party to the agreement.

Skyplex A system which multiplexes several digital television and entertainment services on-board the satellite, instead of on the ground, and retransmits them via a single, DVB-compliant carrier. This permits services to be delivered directly from different geographical locations to end-users located within a broadcast beam and equipped with standard DVB-compliant IRDs. The Skyplex system requires advanced on-board processing equipment for signal reception, multiplexing and retransmission.

SMATV Satellite Master Antenna TV. Collective television reception and distribution system serving a local population of users collocated in a block of flats, a hotel or other group-housing complex. SMATV systems use one or more high quality, centrally located antenna to receive the satellite signals, plus UHF and/or VHF antennas to receive local terrestrial broadcast services. The satellite and terrestrial signals are distributed to the end-users via a dedicated cable distribution network. Several different cable distribution architectures are possible.

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.

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.

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.

Telecommand (TC) The transmission of coded signals towards a satellite to initiate, modify or terminate equipment functions on-board the satellite.
Telemetry (™) 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.

Tracking 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.

Transparent 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).

Transponder 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.

TVRO 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.

Uplink The part of a satellite communications link that involves signal transmission from the ground and reception on-board the satellite. See also Downlink.

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.

X-Polarisation 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.

Y-Polarisation 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.