Asia Satellite | Technical

Glossary

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

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

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

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

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

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

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

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

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

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

L-band	The frequency range between 0.39-1.55 GHz, also known as the 1.5 GHz band. One use is for Digital Audio Broadcasting (DAB).
Lineariser	A microwave device that compensates for the non-linear characteristics of a travelling wave tube near saturation, allowing more efficient use of transponder power.
LNA	See Low Noise Amplifier.
LNB	See Low Noise Block down-converter.
Low Noise Amplifier	This is the preamplifier between the antenna and the earth station receiver. For maximum effectiveness, it must be located as near the antenna as possible, and is usually attached directly to the antenna receive port. The LNA is especially designed to contribute the least amount of thermal noise to the received signal.