a-z of sat

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Satellite Glossary part 1
Written by Satellite Help moderater

A work in progress, it should help beginners get up to speed, and dig a few experts out of a hole once in a while...
A

Analogue: Traditional method of transmitting and recording sound and pictures where the signal varies smoothly and continuously over time. Non-digital.

Analogue-to-Digital Conversion (ADC): Process of converting analogue signals to a digital representation. DAC represents the reverse translation.

Antenna: See "Satellite Dish".

Aperture: A cross sectional area of the antenna which is exposed to the satellite signal.

Apogee: The point in an elliptical satellite orbit which is farthest from the surface of the earth. Geosynchronous satellites which maintain circular orbits around the earth are first launched into highly elliptical orbits with apogees of 22,237 miles. When the communication satellite reaches the appropriate apogee, a rocket motor is fired to place the satellite into its permanent circular orbit of 22,237 miles.

Artefact: The effects seen on a TV screen caused by errors in a digital signal.

Asymmetric transmission: Where the information or data transmission speeds for the forward and return channels differ (e.g. high speed for the forward channel and low for return).

Attenuation: The loss in power of electromagnetic signals between transmission and reception points.

Audio Programme Identifier (APID): The Audio PID identifies the sub-stream of the satellite signal containing the audio (TV or Radio) component. There are often several APID’s for each video stream, allowing the selection of different languages by the end-user. Only applies to digital transmissions.


AZ/EL Mount: Antenna mount that requires two separate adjustments to move from one satellite to another. Put simply, side-to-side and up/down.

Azimuth: Angle between true north and a particular satellite. Can be measured with a compass.

B

Bandpass filter: An electronic circuit that allows a selected band of frequencies to pass unattenuated. All frequencies that lie outside of the passband are severely attenuated.

Bandwidth: This is the range of signal frequencies that can be carried on a communications channel. The capacity of a channel is measured in cycles per second, or Hertz (Hz), between the highest and lowest frequencies.

Bit: Abbreviation for binary digit. A one or a zero.

Bit Error Rate (BER): Denotes the quality of a received demodulated digital signal. The lower the rate, the better the signal. Example: a BER of 10 -4 means one error in every 10,000 bits.

Bit Rate: Transmission speed of digital information, expressed in bits per second or bps.

Byte: Sequence or group of eight bits. 1 byte = 8 bits; 1 Kbyte = 1,024 bytes; 1Mbyte = 1,024 Kbytes; 1Gbyte = 1,024 Mbytes.

C

C Band: This is the band between 4 and 8 GHz with the 6 and 4 GHz band being used for satellite communications.

Carrier to Noise Ratio (C/N): 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 sub-reflector at the focal point which reflects energy to or from a feed located at the apex of the main reflector.

Channel: A frequency band in which a specific broadcast signal is transmitted.

Circular Polarisation: Unlike many domestic satellites which utilise vertical or horizontal polarisation, some transmit their signals in a rotating corkscrew-like pattern. On some satellites, both right-hand rotating and left-hand rotating signals can be transmitted simultaneously on the same frequency; thereby doubling the capacity of the satellite to carry communications channels.

Clarke Belt: The geostationary orbit, named after Arthur C Clarke who first said that such an orbit should exist. See “Geostationary Satellite Orbit”.

Cliff effect: Sudden breakdown or loss of digital signal reception when it contains more errors than the error correction system can cope with. Up to this point there is no degradation in reception quality; a significant advantage over analogue where there is a gradual deterioration.

Compression: In order to make more efficient use of transmission capacity, digital broadcast signals are reduced in size by digital compression. This has a minimal effect on the received signal quality but allows several compressed TV channels to be transmitted in the space required for one analogue channel.

Co-location: More than one satellite located at the same orbital position.

Conditional Access System (CAS): Ensures that broadcast services are only accessible to those entitled to receive them. Usually achieved by "coding" (scrambling or encrypting) the service to make it unintelligible to unauthorised viewers. A system which controls access to pay and copyright-protected services.

D

dBi: The dB power relative to an isotropic source.

dBW: The ratio of the power to one Watt expressed in decibels.

Decibel (dB): The standard unit used to express the ratio of two power levels. It is used in communications to express either a gain or loss in power between the input and output devices.

Declination: The offset angle of an antenna from the axis of its polar mount as measured in the meridian plane between the equatorial plane and the antenna main beam.

Decoder: Device (in conjunction with a smart card on many DBS satellite systems) which decodes a scrambled signal and allows access to the satellite service.

Downlink: The satellite to earth half of a 2 way telecommunications satellite link. Often used to describe the receive dish end of the link.

Digital: Transmission and recording system where the signal is a sequence of ones or zeros, or on-or-off states.

Digital compression: The reduction of the information 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.

Digital TV: More efficient means of transmitting television services which offers more channels and interactive services, as well as improved picture and sound quality.

Digitise: To convert analogue signals into digital ones.

Dish: See "Satellite Dish".

Dolby Digital: High-quality digital surround sound system for movies and TV programs. The successor to Dolby Pro-Logic. A Dolby Pro-Logic audio signal can be carried either over an analogue or a digital transmission.

Dolby Pro-Logic: High-quality surround sound system for movies and TV programs. A Dolby Pro-Logic audio signal can be carried either over an analogue or a digital transmission.

Downlink: Signal channel or path from the satellite to all types of reception sites on earth.

DTH: Direct To Home - reception of satellite programs with a dish in an individual home.

DTO: Direct To Office - reception of satellite programs with an individual dish or shared dish in an office.

Dual LNB: An LNB which provides all the transponders broadcast in KU-Band on two independent outputs. Also known as “Twin LNB”.

DVB: Digital Video Broadcasting group - over 300 organizations from 40 countries developed specifications for the transmission of MPEG-2 signals by satellite, cable and terrestrial links.

E

Earth station: Earth-based dishes, receivers and other equipment needed to receive satellite signals.

EIRP: Effective Isotropic Radiated Power - This term describes the strength of the signal leaving the satellite antenna or the transmitting earth station antenna, and is used in determining the C/N and S/N. The transmit power value in units of dBW is expressed by the product of the transponder output power and the gain of the satellite transmit antenna.

Elevation: The upward tilt to a satellite antenna measured in degrees required to aim the antenna at the communications satellite. When aimed at the horizon, the elevation angle is zero. If it were tilted to a point directly overhead, the satellite antenna would have an elevation of 90 degrees.

Elevation: Angle between the earth's surface and a particular satellite at a given reception point (0° for horizontal and 90° for vertical).

EPG: Electronic Program Guide - sophisticated on-screen TV listings guide which provides current and future program details. It boasts features such as subject or channel searches, program summaries, immediate access to a selected program, and reminder and parental control functions.

F

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F/D: Ratio of antenna focal length to antenna diameter. A higher ratio means a shallower dish.

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 sub-reflector plus a feedhorn or a feedhorn only.

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: A map of the signal strength showing the EIRP contours of equal signal strengths as they cover the earth's surface. Different satellite transponders on the same satellite will often have different footprints of the signal strength. The accuracy of EIRP footprints or contour data can improve with the operational age of the satellite. The actual EIRP levels of the satellite, however, tends to decrease slowly as the spacecraft ages.

Forward Error Correction (FEC): Some of the data present in a satellite signal is added at the source and has nothing to do with the audio or video components. It’s ‘repair data’ - extra data which can then be used at your receiver to correct any errors found in the main signal. It’s a pre-emptive measure, and in perfect reception conditions completely redundant. A FEC of 1/2 means that 1 Byte out of 2 is used for error correction, while a ratio of 7/8 means 7 Bytes are used for the actual signal, and only one for error correction. Only applies to digital transmissions. Also known as “Viterbi Rate”.


Frequency: The number of times that an alternating current goes through its complete cycle in one second of time. One cycle per second is also referred to as one hertz; 1000 cycles per second, one kilohertz; 1,000,000 cycles per second, one megahertz: and 1,000,000,000 cycles per second, one gigahertz.

Forward Error Correction (FEC): A technique for improving the robustness of data transmission. Excess bits are included in the outgoing data stream so that error-correction algorithms can be applied upon reception. For the satellite standard the Viterbi code combined with the Reed Solomon code is used. Commercial use of transponders makes 3/4 and 5/6, which means that three out of four or five out of six bits contain useable information.

Feedhorn: A section of waveguide into which signals from a parabolic dish reflector are aimed. In most parabolic antenna designs, the feedhorn is mounted at the reflector's focal point. The output from the feedhorn directly feeds the low noise block converter (LNB).

Forward path: The transmission path issued from the service provider and reaching the end user.

Footprint: Geographic area covered by a satellite transmission.

FTA: Free-To-Air - Free to everyone with no need for a special decoder to receive them.

FTV: Free-to-View - Encrypted, but can be viewed with a card which itself is either free, or available at nominal (administrative-only) cost.

Frequency spectrum: A range of frequencies.

G

Gain: A measure of amplification expressed in dB.

Gain to Noise Temperature (G/T): In the characterisation of antenna performance, a figure of merit, where G is the antenna gain in dB (decibels) at the receive frequency, and T is the equivalent noise temperature of the receiving system in Kelvin.

Geostationary Satellite Orbit: Geostationary Satellite Orbit, whose path is 22,500 miles in height above the equator, and where a satellite travels at the same angular speed as the rotating earth. See “Clarke Belt”.

H

Headend: Electronic control centre - generally located at the antenna site of a CATV system - usually including antennas, preamplifiers, frequency converters, demodulators and other related equipment which amplify, filter and convert incoming broadcast TV signals to cable system channels.

HDTV - High Definition TV: Advanced TV systems that offer far superior picture quality, a widescreen aspect radio and high-quality digital audio.

I

IF: Intermediate Frequency. The frequency range 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 commonly used for the distribution of DBS satellite signals in commercial systems.

Inclination: The angle between the orbital plane of a satellite and the equatorial plane of the earth.

Inclined Orbit: Term used for a GEO satellite that has turned off its North/South stabilisation. From Earth, such a satellite seems to draw an 8 in the sky; part of the day it is above the Equator, and then below.

Interactive TV: TV programming where consumers can initiate, respond to or participate in what is happening on screen.

IPG: Interactive Program Guide - sophisticated user customisable on-screen TV listings guide which provides current and future program details. It boasts features such as subject or channel searches, program summaries, immediate access to a selected program, and reminder and parental control functions.

IRD: Integrated Receiver Decoder - satellite receiver with a built-in decoder used for the reception and decoding of free and subscription satellite signals.

K

Ka Band: Frequency range of 18-31 GHz.

Ku Band: Frequency range of 10.7-18 GHz. TV stations and networks frequently use Ku Band to get the signal from their remote satellite vans back to the TV station. Also, commonly used throughout Europe to deliver high powered satellite service to subscribers.

L

L Band: The frequency range from 0.5 to 1.5 GHz.

Low Noise Amplifier (LNA): This is the preamplifier between the antenna and the earth station receiver. For maximum effectiveness, it must be located as near the antenna as possible, and is usually attached directly to the antenna receive port. The LNA is especially designed to contribute the least amount of thermal noise to the received signal.

Low Noise Block Downconverter (LNB): A combination Low Noise Amplifier and downconverter built into one device attached to the feed. A small device mounted at the focal point of a satellite dish which converts and amplifies high-frequency satellite signals into lower frequencies. The successor to LNA's.

Line amplifier: Amplifier used in the middle of a network to reamplify a signal.

M

Microwave: Line-of sight, point-to-point transmission of signals at high frequency. Many CATV systems receive some television signals from a distant antenna location with the antenna and the system connected by microwave relay. Microwaves are also used for data, voice, and indeed all types of information transmission. The growth of fibre optic networks has tended to curtail the growth and use of microwave relays.

Modulation: The alteration of a carrier wave in relation to the value of the data being transferred. Analogue satellite transmissions use FM modulation. Digital satellite transmissions use QPSK modulation.

Multicast: Multicast is a subset of broadcast that extends the broadcast concept of one to many by allowing "the sending of one transmission to many users in a defined group, but not necessarily to all users in that group."

MPEG: Moving Pictures Experts Group - international standards organisation which developed the MPEG standard for the digital compression and multiplexing of video and audio signals.

MPEG-2: The agreed standard covering the compression of data (coding and encoding) for digital television.

Multiswitch: Device used to combine and distribute off-air antenna/cable TV signals and satellite signals.

Multicast: Transmitting information to a select group. For example, sending an E-mail to a mailing list. Teleconferencing and videoconferencing also use multicasting.

Multiplex: To combine two or more independent signals into one transmission channel, or the combined digital signals transmitted on one satellite transponder.

N

Near-Video-on-Demand: Films provided for cable TV or satellite viewers from a menu of titles. The same film starts again after a defined time period, such as every 15 to 30 minutes.

Network Identifier (NID): The Network Identifier identifies a certain Network Provider. This enables the receiver to search for just those channels originating from this network provider. Only applies to digital transmissions.


Noise: Any unwanted and unmodulated energy that is always present to some extent within any signal.

NTSC - National Television Standards Committee: A video standard established by the United States (RCA/NBC} and adopted by numerous other countries. This is a 525-line video with 3.58-MHz chroma sub-carrier and 60 cycles per second.

O

Orbital Position: Position of geostationary satellites measured in degrees east or west from the Greenwich meridian.

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P

Packet Switching: Data transmission method that divides messages into standard-sized packets for greater efficiency of routing and transport through a network.

PAL - Phase Alternation System: The German developed TV standard based upon 50 cycles per second and 625 lines.

Parabolic Antenna: The most frequently found satellite TV antenna, it takes its name from the shape of the dish described mathematically as a parabola. The function of the parabolic shape is to focus the weak microwave signal hitting the surface of the dish into a single focal point in front of the dish. It is at this point that the feedhorn is usually located.

Pay TV: Encrypted or scrambled TV service that can only be accessed by paying a subscription fee.

Pay-Per-View (PPV): Program services that are paid for on the basis of the number of hours or programs watched rather than a straight subscription fee.

Perigee: The orbital point were the satellite is closest from the earth.

Phase-Locked Loop (PLL): A type of electronic circuit used to demodulate satellite signals.

Polar Mount: Antenna mechanism permitting steering in both elevation and azimuth through rotation about a single axis. While an astronomer's polar mount has its axis parallel to that of the earth, satellite earth stations utilize a modified polar mount geometry that incorporates a declination offset.


Polarisation: A technique used by the satellite designer to increase capacity by reusing the transponder frequencies. In most Ku band satellites, half of the transponders beam their signals to earth in a vertically polarised mode; the other half horizontally. Although the two sets of frequencies overlap, they are 90 degrees out of phase, and will not interfere with each other. To successfully receive and decode these signals on earth, the LNB must be fitted with a properly polarised feedhorn to select the vertically or horizontally polarised signals as desired. Some satellite transponders use a technique known as left-hand and right-hand circular polarisation.

Polarisation Offset Angle: The angle from vertical that the LNB needs to be set at for maximum reception efficiency. The actual angle depends on the geographical location of the reception site. Also known as “Skew”.


Polarisation Rotator: A device that can be manually or automatically adjusted to select one of two orthogonal, or opposite polarisations.


Programme Clock Reference (PCR): This is used to synchronise the video and audio components of the signal.

Programme Identifier (PID): The channel identifier.

PVR: Personal Video Recorder - similar to a VCR but instead of using video tape, uses a built-in hard drive to digitally record programs and offers the ability to "pause" live television. Most have upgradeable software and some models support the recording of two channels simultaneously.

Q

Quadrature Phase Shift Keying (QPSK): System of modulating a satellite signal.

Quad LNB: An LNB which provides all the transponders broadcast in Ku-Band on four independent outputs.

Quattro LNB: Typically used in a commercial installation to supply a distribution system with separate switches and more than 4 receivers. A Quattro LNB is a Ku-band converter with four independent outputs: low band with vertical polarisation, low band with horizontal polarisation, high band with vertical polarisation, and high band with horizontal polarisation.

R

Radio: A frequency that is higher than the audio frequencies but below the infrared frequencies, usually above 20 KHz.

Receiver: See “Satellite Receiver”.

Redundancy: TV signal compression is based on the fact that all TV pictures contain redundant information; once digitised, signal size can be reduced by removing this redundant information.

Return Path: The transmission path issued from the end user and reaching the service provider.

Radio Frequency Interference (RFI): A term used to denote apparent disturbance or distortion to satellite signals.

Rain Outage: Loss of signal at Ku or Ka Band frequencies due to absorption and increased sky-noise temperature caused by heavy rainfall.

S

Satellite Dish: The part of the reception equipment which reflects and concentrates a satellite signal at the focal point the dish.

Satellite Receiver: The part of the reception equipment used to tune into a single channel broadcast from a satellite.

Secam: A colour television system developed by the French and used in the USSR. Secam operates with 625 lines per picture frame and 50 cycles per second, but is incompatible in operation with the European PAL system or the U.S. NTSC system.

Service Identifier (SID): The Service Identifier is used by the receiver to identify a certain service (as defined by the provider and/or satellite operator) within a transmission. Only applies to digital transmissions.


Signal to Noise Ratio (S/N): The ratio of the signal power and noise power. A video S/N of 54 to 56 dB is considered to be an excellent S/N, that is, of broadcast quality. A video S/N of 48 to 52 dB is considered to be a good S/N at the headend for Cable TV.

Set-Top-Box (STB): Generally a VCR-sized add-on box for a TV, enabling it to receive cable TV, terrestrial digital TV or satellite services.

Skew: See “Polarisation Offset Angle”.

Smart Card: Credit card-sized card used in conjunction with a decoder to allow access to subscription TV services.

SMATV: Satellite Master Antenna Television system - commercial distribution system for satellite and off-air TV signals usually installed in a hotel or apartment building for shared used.

Spot beams: The focusing of energy from a satellite to the ground to concentrate the limited available effective radiated power (EIRP).

Sub-carrier: One or more signals, mainly audio, carried beside the main signal, such as television on a transponder or cable channel.

Symbol Rate (SR): The rate at which digital data is transmitted from a given satellite transponder, measured in Kilosymbols per second (ksym/sec) or Megasymbols per second (Msym/sec). Historically Msym/sec was referred to as MBaud. A typical symbol rate for satellite would be 27500 ksym/sec or 27.5 Msym/sec. Only applies to digital transmissions.

T

Terrestrial Interference - TI: Interference to satellite reception caused by nearby ground based microwave transmitting stations operating on or near the same frequencies. TI can be reduced by using a deeper satellite dish or eliminated by using a bandpass filter.

Terrestrial transmission: A transmission using earth-based transmitters.

Transponder: A combination receiver, frequency converter, and transmitter package, physically part of a communications satellite. Transponders have a typical output of five to ten watts, operate over a frequency band with a 36 to 72 megahertz bandwidth in the L, C, Ku, and sometimes Ka Bands or in effect typically in the microwave spectrum, except for mobile satellite communications.

Transponder Identifier (TID): The Transponder or Transport Identifier identifies a certain transponder and is unique for each transponder within a given network (see NID). This enables the receiver to search for only those channels on a particular transponder. Only applies to digital transmissions.


TVRO: Television Receive Only - earth-based dishes, receivers and other equipment needed to receive satellite signals. Typically home satellite systems.

U

Ultra High Frequency (UHF): The band in the 500 to 900 MHz range.

Uplink: Transmission from earth to a satellite.

V

Very High Frequency (VHF): The band in the 30 to 300 MHz range.

Video inversion: A type of signal scrambling in which the video signals are reversed in polarity. At the descrambling end, the received video signals are inverted once again, which results in a reproduction of the original unscrambled satellite broadcast.

Video Programme Identifier (VPID): The Video PID identifies the sub-stream of the satellite signal containing the video component. Radio streams, which obviously don't contain any video signal, have the VPID value set to 8191 (this means empty). Only applies to digital transmissions.

Viterbi Rate: See “Forward Error Correction”.

VSWR: Voltage Standing Wave Ratio - A measurement of mismatch in a cable, waveguide, or antenna system.

W

Waveguide: A completely enclosed metallic duct or tube that acts as a transmission line, usually for microwave frequencies. The dimensions of the waveguide are critical to a fraction of a centimetre in order to avoid transmission line loss.

Wavelength: The distance while travelling at the speed of light that a radio wave will travel during a single cycle.