Terminology

Satellite Physical units & Definitions

B  
bit A unit of information representing the physical
state of a system having one of two values, such as on
or off.
bit/s Number of bits of information transferred or transmitted
every second.
bps Alternative expression for bit/s.
kbit/s Information transfer rate, equal to 1,000 bit/s.
Mbit/s Information transfer rate, equal to 1,000,000 bit/s.
byte A sequence of bits processed as one unit of information.
A byte is a digital “word” normally consisting
of eight bits.
D  
dBi The relative gain of an antenna with respect to an equivalent
isotropic antenna, expressed on the decibel logarithmic
scale.
dBW The power of a signal in watts, expressed on the decibel
logarithmic scale.
dBW/m2 Power of a radio wave incident on a surface area of
one square metre, measured in watts and expressed on the
decibel logarithmic scale
dBK The equivalent noise temperature of a device in kelvin,
expressed on the decibel logarithmic scale.
dB/K 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.
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).
F  
frequency 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.
G  
gigahertz (GHz) Unit of frequency equal to 1,000,000,000 Hz.
H  
hertz (Hz) SI unit of frequency, equivalent to one cycle per second.
The frequency of a periodic phenomenon that has a periodic
time of one second.
K  
kelvin (K) Basic SI unit of thermodynamic temperature.
kilohertz (kHz) Unit of frequency equal to 1,000 Hz.
M  
megahertz (MHz) Unit of frequency equal to 1,000,000 Hz.
Msymbol/s 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).
S  
symbol A unique signal state of a modulation scheme used on
a transmission link, which conveys one or more information
bits to the receiver.
symbol/s The number of symbols transmitted every second.
U  
UTC Coordinated Universal Time. A time-scale which forms
the basis of a coordinated dissemination of standard frequencies
and time signals throughout the world.
W  
watt (W) SI unit of power, equal to 1 joule/second.

Satellite Internet Terminology

A  
Adjacent Channel Interference Signal impairment to one frequency due to presence of another signal on a nearby frequency.
Antenna Noise Temperature Temperature of a hypothetical resistor at the input of an ideal noise-free receiver that would generate the same output noise power per unit bandwidth as that at the antenna output at a specified frequency.
Attenuation Is the reduction in amplitude and intensity of a signal.
Azimuth The horizontal angular distance between the vertical plane containing a point in the sky and true south.
B  
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  
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.
D  
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.
E  
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.
F  
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.
G  
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.
H  
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.
I  
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 0.
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.
K  
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.
L  
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.
M  
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”.
MCPC Multiple Channel Per Carrier. 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.
N  
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.
O  
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.
P  
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.
R  
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  
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.
T  
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.
U  
Uplink The part of a satellite communications link that involves
signal transmission from the ground and reception on-board
the satellite. See also Downlink.
V  
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  
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  
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.

Satellite Devices / Hardware / Equipments

A  
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.
Aerial See Antenna.
Amplifier An electronic device used to increase the strength (power)
of a signal fed into it.
Antenna A device for transmitting or receiving radio waves.
Also known as 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 a transmitting systems the
reflector concentrates the radio waves emitted by the
feedhorn into a narrow beam aimed towards the satellite.
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 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.
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.
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.
Attenuator A passive device that weakens a signal that passes through
it. The amount by which the signal power is reduced is
usually expressed in decibels.
B  
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.
BUC 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.
C  
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.
D  
Decoder A circuit or device that restores a coded signal to
its original form based on knowledge of the process used
to code the signal.
Demodulator 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.
Descrambler A device that recovers the original signal from one
that has been rendered unintelligible by Scrambling.
DiSEqC™ 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.
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).
Diplexer 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.
Dish 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.
Downconverter A device for converting the frequency of a signal to
a lower frequency. See also Downconversion and Frequency
Conversion.
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).
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.
F  
F-Connector 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.
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.
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).
Filter 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.
H  
HPA High Power Amplifier
I  
Isotropic Antenna A theoretical device that radiates energy or receives
energy equally from all directions.
L  
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.
LNA Low Noise Amplifier. A device that receives and amplifies
weak satellite signals at the output of a feedhorn, whilst
introducing as little electrical noise as possible in
the process. The frequency of the incoming signal is unchanged
at the output of the device. The feedhorn is typically
a physically separate device.
LNB Low Noise Block Downconverter. A device that processes
weak satellite signals directed by an antenna reflector
into a feedhorn, whilst introducing as little electrical
noise as possible in the process. An LNB consists of a
microwave detector followed by a high gain, low noise
microwave amplifier and a frequency converter, which downconverts
a block of frequencies (group of satellite signals) to
a lower intermediate frequency range (typically 950 to
2150 MHz). The feedhorn is often integrated with LNB in
a single mechanical unit.
Local Oscillator (LO) 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.
M  
Mixer 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.
Modulator 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).
O  
Offset (Fed) Antenna An antenna having a feedhorn that is offset from the
centre of the reflector. It generally offers better performance
than a symmetrically-fed antenna because the feed system
does not block the main reflector aperture.
Offset Feed A LNB that is slightly displaced with respect to the
focal point of the reflector so that it receives signals
originating from a different direction to that obtained
with an LNB placed at the focal point. A technique used
in dual-feed reception systems, which receive signals
from satellites located at two different orbital locations.
P  
Preamplifier A device that amplifies a weak signal for subsequent
processing, which may include further signal amplification.
In an SMATV system, it is the amplifier mounted adjacent
to an antenna to increase the strength of the signal prior
to its processing at the headend.
Q  
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.
R  
Receiver The equipment that receives incoming electrical signals
or modulated radio waves and converts them into the original
audio, video or data signals.
S  
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.
SCART Also known as Euroconnector or Peritel. Twenty-one-pin
connector commonly used in Europe to interconnect satellite
receivers, television sets and other audiovisual equipment
(e.g. videocassette recorders).
Scrambler A device that renders a signal unintelligible and/or
randomises its content. See also Scrambling.
Splitter 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.
T  
TWTA Travelling Wave Tube Amplifier. A high power amplifier
based on tube technology. Normally employed when high
output power levels are required.
U  
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.
Upconverter A device for converting the frequency of a signal into
a higher frequency. See also Upconversion and Frequency
Conversion.

Satellite Signal Processing

A  
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.
Analogue 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.
ADC 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”).
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.
ARQ 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.
B  
Baseband 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.
Broadband 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.
C  
CDMA 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.
Crosstalk 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.
D  
Decoding The process of restoring a coded signal to its original
form based on knowledge of the encoding process.
De-Emphasis 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.
Demodulation 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.
Demultiplexing The extraction of multiple distinct messages or signals
from a single composite signal (“multiplex”).
Digital 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.
Digitisation 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.
DCT 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.
Downconversion 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.
E  
Emission 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).
Encryption 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.
Eurocrypt Conditional access system used mainly with the D2-MAC
television transmission standard.
F  
FDM 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.
FDMA 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. 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.
FM 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.
I  
IF 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.
J  
JPEG 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.
L  
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.
M  
Microwave The frequency range from approximately 1 to 300 GHz,
covering the frequency range suitable for satellite communications.
Modulation 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).
MPEG 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 A widely-used video compression standard. See MPEG.
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.
Multiplex A signal that comprises multiple distinct signals or
messages, usually for the purposes of transmission via
a common communications channel.
Multiplexing 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.
N  
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.
Non-Linear 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.
P  
PAL (Television) Phase Alternating Line. A European conventional colour
television standard, which evolved from the American NTSC
standard. The term “Phase Alternating Line”
refers to the technique used to overcome the colour variations
that can occur in the NTSC system.
Parabola A geometric shape formed by the intersection of a cone
by a plane parallel to its side.
Parabolic Shaped like a parabola or paraboloid.
Parabolic Antenna An antenna having a main reflector surface that is a
paraboloid or is shaped like a paraboloid. It has the
property of reflecting parallel incoming signals to a
single focal point.
Paraboloid A geometric surface whose sections parallel to two co-ordinate
planes are parabolic and whose sections parallel to the
third plane are either elliptical or hyperbolic.
Power The rate at which electrical energy is fed into or taken
from a device or system, expressed in watts or dBW. The
signal strength on the uplink or downlink of a satellite
communications system is quantified by the power of the
radio wave radiated by the transmit antenna.
Power Flux Density The signal power received over a surface area of one
square metre, expressed in dBW/m2. Used to quantify the
strength of a radio wave at the reception point of an
earth-space link.
Pre-Emphasis An artificial increase in the amplitude of the higher
frequency portions of a baseband signal prior to frequency
modulation and transmission via a radio link. Used in
conjunction with the proper amount of de-emphasis at the
receiver, this results in an improved signal-to-noise
ratio for the demodulated FM signal. See also De-Emphasis.
Progressive Scanning The process of scanning a single image (“frame”)
of a moving picture sequence progressively from top to
bottom, producing a scanned image containing all of the
horizontal lines used in the scanning process. Used in
PC monitors and some advanced television systems. See
also Interlace Scanning.
Propagation To transmit in the form of a wave.
R  
Radiation The outward flow of energy from any source in the form
of radio waves.
Radio The use of electromagnetic waves, lying in the radio
frequency range, for communications purposes.
Radiocommunication Telecommunication by means of radio waves.
RF 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).
Radio-Frequency
Links 		Communication links established by means of radio waves.
Radio Waves Electromagnetic waves lying in the radio frequency range,
propagated in space without artificial guide.
RGB 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.
S  
Scanning 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.
Scrambling A process that renders a signal unintelligible and/or
randomises its content. It is used either to protect the
content of the signal from unauthorised access, or for
Energy Dispersal purposes.
Sparklies (Television) The visual effect of impulsive noise arising at the
output of an FM demodulator due to a weak signal, slight
mistuning of the receiver or interference. The noise appears
as randomly-distributed, momentary black and white flecks
on the picture.
Subcarrier 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.
T  
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.
Telecommunications 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.
U  
UHF Ultrahigh Frequency. The frequency range from 300 MHz
to 3 GHz.
Upconversion 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.
V  
VHF Very High Frequency. The frequency range from 30 MHz
to 300 MHz.
Videocrypt Proprietary conditional access system.
Videoguard Proprietary conditional access system.
W  
Widescreen
 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.
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