Half Transponder
A method of transmitting two TV signals through a single transponder through the reduction of each TV signal's deviation and power level.Half-transponder TV carriers each operate typically 4 dB to 7 dB below single-carrier saturation power.

Electronic control center - 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.

Heliosynchronous Orbit
At an altitude of 600 to 800 km and situated in a quasi-polar plane. The satellite is permanently visible

from that part of the Earth in sunlight. Heliosynchronous orbits are used for Earth observation or solar-study satellites.

Highly Elliptical Orbit. This is type of orbit used by the Russian Molniya Satellite system. It is also referred to as
Extremely Elliptical Orbit (EEO).

Hertz (Hz)
The name given to the basic measure of radio frequency characteristics. An electromagnetic wave completes a full oscillation from its positive to its negative pole and back again in what is known as a cycle. A single Hertz is thus equal to one cycle per second.

High Frequency (HF)
Radio frequencies within the range of 3,000 to 30,000 kilohertz. HF radio is known as


High-Power Satellite
Satellite with 100 watts or more of transponder RF power.

Hour Angle
Steering direction of a polar mount. An angle measured in the equatorial plane between the antenna beam and the meridian plane.

The master station through which all communications to, from and between micro terminals must flow. in the future satellites with on-board processing will allow hubs to be eliminated as MESH networks are able to connect all points in a network together.

Hughes Galaxy
A domestic U.S. satellite system which provides a range of telecommunications services.


INTELSAT Business Services.

International Frequency Registration Board of the ITU - International Telecommunications Union.  The IFRB regulates the allocation of satellite orbital locations.

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

The International Maritime Satellite Organization operates a network of satellites for international transmissions for all types of international mobile services including maritime, aeronautical, and land mobile.

The International Telecommunications Satellite Organization operates a network of satellites for international transmissions.

Energy which tends to interfere with the reception of the desired signals, such as fading from airline flights, RF interference from adjacent channels, or ghosting from reflecting objects such as mountains and buildings.

Inter Satellite Link - ISL
Radio or optical communications links between satellites.  They serve to interconnect constellations of satellites.

The international entity formed by the Soviet Union to provide international communications via a network of Sovie


An integrated receiver and decoder for reception of a transmission of voice, video and data.

Iridium Satellite System
This was a 66 satellite network designed for mobile telephone use and is now defunct.

ISDN - Integrated Services Digital Network.
A CCITT standard for integrated transmission of voice, video and data. Bandwidths include: Basic Rate Interface - BR (144 Kbps - 2 B & 1 D channel) and Primary Rate - PRI (1.544 and 2.048 Mbps).

International Standards Organization. Develops standards such as JPEG and MPEG. Closely allied with the CCITT.

Isotropic Antenna
A hypothetical omnidirectional point-source antenna that serves as an engineering reference for the measurement of antenna gain.

International Telecommunication Union.


Jammer -
An active electronic counter-measures (ECM) device designed to deny intelligence to unfriendly detectors or to disrupt communications.

ISO Joint Picture Expert Group standard for the compression of still pictures.


Ka Band
The frequency range from 18 to 31 GHz.

Kilobits per second. Refers to transmission speed of 1,000 bits per second.

Kelvin (K)
The temperature measurement scale used in the scientific community. Zero K represents absolute zero, and corresponds to minus 459 degrees Fahrenheit or minus 273 Celsius. Thermal noise characteristics of LNA are measured in Kelvins.

Kilohertz (kHz)
Refers to a unit of frequency equal to 1,000 Hertz.

A microwave tube which uses the interaction between an electron beam and the RF energy on microwave cavities to provide signal amplification. The klystron operates on principles of velocity modulation very similar to those in a TWT except that klystron interaction takes place at discrete locations along the electron beam. Common types of klystrons are the reflex klystron (an oscillator having only one cavity), two-cavity klystron amplifiers and oscillators, and multi-cavity klystron amplifiers.

Ku Band
The frequency range from 10.9 to 17 GHz.


The frequency range from 0.5 to 1.5 GHz. Also used to refer to the 950 to 1450MHz used for mobile communications.

Leased Line
A dedicated circuit typically supplied by the telephone company.

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.

Low Noise Converter (LNC)
A combination Low Noise Amplifier and down converter built into one antenna-mounted package.

Low Orbit
At an altitude of 200 to 300 km this orbit is used for certain types of scientific or observation satellites, which can

view a different part of the Earth beneath them on each orbit revolution, as they overfly both hemispheres.

Low-Power Satellite
Satellite with transmit RF power below 30 watts.


MAC (A, B, C, D2)
Multiplexed analog component color video transmission system. Subtypes refer to the various methods used to transmit audio and data signals.

The amount of signal in dB by which the satellite system exceeds the minimum levels required for operation.

Master Antenna Television (MATV)
An antenna system that serves a concentration of television sets such as in apartment buildings, hotels or motels.

Medium-Power Satellite
Satellite generating transmit power levels ranging from 30 to 100 watts.

Megahertz (MHz)
Refers to a frequency equal to one million Hertz, or cycles per second.

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 fiber optic networks have tended to curtail the growth and use of microwave relays.

Microwave Interference
Interference which occurs when an earth station aimed at a distant satellite picks up a second, often stronger signal, from a local telephone terrestrial microwave relay transmitter. Microwave interference can also be produced by nearby radar transmitters as well as the sun itself. Relocating the antenna by only several feet will often completely eliminate the microwave interference.

A communications device that modulates signals at the transmitting end and demodulates them at the receiving end.

The process of manipulating the frequency or amplitude of a carrier in relation to an incoming video, voice or data signal.

A device which modulates a carrier. Modulators are found as components in broadcasting transmitters and in satellite transponders. Modulators are also used by CATV companies to place a baseband video television signal onto a desired VHF or UHF channel. Home video tape recorders also have built-in modulators which enable the recorded video information to be played back using a television receiver tuned to VHF channel 3 or 4.


The Russian domestic satellite system which operated with highly elliptical satellites which overlooked the high latitudes of the territories of the USSR.


The Moving Pictures Experts Group, the television industry's informal standards group.

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

Main Provile at High Level - The agreed much higher bit-rate system adopted to provide high definition television in wide screen format.

Multiple Access
The ability of more than one user to have access to a transponder.

Multiple System Operator (MSO)
A company that operates more than one cable television system.

Multipoint Distribution System (MDS)
A common carrier licensed by the FCC to operate a broadcast-like omnidirectional microwave transmission facility within a given city typically carrying television signals

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."

Techniques that allow a number of simultaneous transmissions over a single circuit.

A Multiplexer. Combines several different signals (e.g. video, audio, data) onto a single communication channel for transmission. Demultiplexing separates each signal at the receiving end.


National Association of Broadcasters.

NASA (National Aeronautics and Space Administration)

The U.S. agency which administers the American space program, including the deployment of commercial and military satellites via a fleet of space shuttle vehicles.

National Space Development Agency of Japan.

National Cable Television Association.

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

Noise Figure (NF)
A term which is a figure of merit of a device, such as an LNA or receiver, expressed in dB, which compares the device with a perfect device.

The National Telecommunications and Information Administration is a unit of the Department of Commerce that address U.S. government telecommunications policy, standards setting and radio spectrum allocation.

Nutation Damping
The process of correcting the nutational effects of a spinning satellite which are similar in effect to a wobbling top. Active nutation controls use thruster jets.

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 subcarrier and 60 cycles per second.


The Office of Telecommunications of the United Kingdom government. This unit a part of the Department of Industries regulates telecommunications in the United Kingdom.

Orbital Period
The time that it takes a satellite to complete one circumnavigation of its orbit.


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.

PBS (Public Broadcasting System)

A domestic USA television and radio broadcast network.

The point in an elliptical satellite orbit which is closest to the surface of the earth.

Perigee Kick Motor (PKM)
Rocket motor fired to inject a satellite into a geostationary transfer orbit from a low earth orbit especially that of a STS or Shuttle-based orbit of 300 to 500 miles altitude.

The amount of time that a satellite takes to complete one revolution of its orbit.

Phase Alternation System (PAL)
A European color television system incompatible with the US NTSC television system.

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


A technique used by the satellite designer to increase the capacity of the satellite transmission channels by reusing the satellite transponder frequencies. In linear cross polarization schemes, half of the transponders beam their signals to earth in a vertically polarized mode; the other half horizontally polarize their down links. Although the two sets of frequencies overlap, they are 90 degree out of phase, and will not interfere with each other. To successfully receive and decode these signals on earth, the earth station must be outfitted with a properly polarized feedhorn to select the vertically or horizontally polarized signals as desired.

In some installations, the feedhorn has the capability of receiving the vertical and horizontal transponder signals simultaneously, and routing them into separate LNAs for delivery to two or more satellite television receivers. Unlike most domestic satellites, the Intelsat series use a technique known as left-hand and right-hand circular polarization.

Polarization Rotator
A device that can be manually or automatically adjusted to select one of two orthogonal polarizations.

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.

Polar Orbit
An orbit with its plane aligned in parallel with the polar axis of the earth

Protected-Use Transponder
A satellite transponder provided by the common carrier to a programmer with a built-in insurance policy. If the protected-use transponder fails, the common carrier guarantees the programmer that it will switch over to another transponder, sometimes pre-empting some other non-protected programmer from the other transponder.

PTT - Post Telephone and Telegraph Administration
Refers to operating agencies directly or indirectly controlled by governments in charge of telecommunications services in most countries of the world.

Pulse Code Modulation
A time division modulation technique in which analog signals are sampled and quantized at periodic intervals into digital signals. The values observed are typically represented by a coded arrangement of 8 bits of which one may be for parity.


Quadrature Phase Shift Keying is a digital modulation technique in which the carrier phase can have one of four

possible values of 0, 90, 180, 270 degrees on the equivalent of a 90 degrree rotation. There are even more advanced concepts based upon 8-phase (45 degree rotation), 16 phase (22.5 degree rotation) and so on to 32 phase, etc.


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

Receiver (Rx)
An electronic device which enables a particular satellite signal to be separated from all others being received by an earth station, and converts the signal format into a format for video, voice or data.

Receiver Sensitivity
Expressed in dBm this tells how much power the detector must receive to achieve a specific baseband performance, such as a specified bit error rate or signal to noise ratio.

RF Adaptor
An add-on modulator which interconnects the output of the satellite television receiver to the input (antenna  terminals) of the user's television set. The RF adaptor converts the baseband video signal coming from the satellite receiver to a radio frequency RF signal which can be tuned in by the television set on VHF channel 3 or 4.

Network layer device that determines the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information.


A sophisticated electronic communications relay station orbiting 22,237 miles above the equator moving in a fixed orbit at the same speed and direction of the earth (about 7,000 mph east to west).

Satellite Terminal
A receive-only satellite earth station consisting of an antenna reflector (typically parabolic in shape), a feedhorn, a low-noise amplifier (LNA), a down converter and a receiver.

SAW (Surface Acoustic Wave)
A type of steep-skirted filter used in the baseband or IF section of satellite reception and transmission equipment.

Scalar Feed
A type of horn antenna feed which uses a series of concentric rings to capture signals that have been reflected toward the focal point of a parabolic antenna.

A device used to electronically alter a signal so that it can only be viewed or heard on a receiver equipped with a special decoder.

A color 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.

SFD - Stauration Flux Density
The power required to achieve saturation of a single repeater channel on the satellite.

Off-axis response of an antenna.

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.

An organization formed in the mid 1980's to monitor frequency re-use.

Simplex Transmission
Capability for transmission in only one direction between sending station and receiving station.

Single-Channel-Per-Carrier (SCPC)
A method used to transmit a large number of signals over a single satellite transponder.

Single Sideband (SSB)
A form of amplitude modulation (AM) whereby one of the sidebands and the AM carrier are suppressed.

An adjustment that compensates for slight variance in angle between identical senses of polarity generated by two or more satellites.

Slant Range
The length of the path between a communications satellite and an associated earth station.

That longitudinal position in the geosynchronous orbit into which a communications satellite is "parked". Above the United States, communications satellites are typically positioned in slots which are based at two to three degree intervals.

SMATV (Satellite Master Antenna System)
The adding of an earth station to a MATV system to receive satellite programs.

Satellite news gathering usually with a transportable uplink truck.

A form of noise picked up by a television receiver caused by a weak signal. Snow is characterized by alternate dark and light dots appearing randomly on the picture tube. To eliminate snow, a more sensitive receive antenna must be used, or better amplification must be provided in the receiver (or both).

Solar Outage
Solar outages occur when an antenna is looking at a satellite, and the sun passes behind or near the satellite and within the field of view of the antenna. This field of view is usually wider than the beamwidth. Solar outages can be exactly predicted as to the timing for each site.

A form of satellite television "snow" caused by a weak signal. Unlike terrestrial VHF and UHF television snow which appears to have a softer texture, sparklies are sharper and more angular noise "blips". As with terrestrial reception, to eliminate sparklies, either the satellite antenna must be increased in size, or the low noise amplifier must be replaced with one which has a lower noise temperature.

The range of electromagnetic radio frequencies used in transmission of voice, data and television.

Satellite signal that falls on locations outside the beam pattern's defined edge of coverage.

Spin Stabilization
A form of satellite stabilization and attitude control which is achieved through spinning the exterior of the spacecraft about its axis at a fixed rate.

A passive device (one with no active electronic components) which distributes a television signal carried on a cable in two or more paths and sends it to a number of receivers simultaneously.

Spot Beam
A focused antenna pattern sent to a limited geographical area. Spot beams are used by domestic satellites to deliver certain transponder signals to geographically well defined areas such as Hawaii, Alaska and Puerto Rico.

Spread Spectrum
The transmission of a signal using a much wider bandwidth and power than would normally be required. Spread spectrum also involves the use of narrower signals that are frequency hopped through various parts of the transponder. Both techniques produce low levels of interference Between the users. They also provide security in that the signals appear as though they were random noise to unauthorized earth stations. Both military and civil satellite applications have developed for spread spectrum transmissions.

Spread spectrum multiple access. Refers to a frequency multiple access or multiplexing technique.

Solid state power amplifier. A VSLI solid state device that is gradually replacing Traveling Wave Tubes in satellite communications systems because they are lighter weight and are more reliable.

Minor orbital adjustments that are conducted to maintain the satellite's orbital assignment within the allocated "box" within the geostationary arc.

A second signal "piggybacked" onto a main signal to carry additional information. In satellite television transmission, the video picture is transmitted over the main carrier. The corresponding audio is sent via an FM subcarrier. Some satellite transponders carry as many as four special audio or data subcarriers whose signals may or may not be related to the main programming.

Subsatellite Point
The unique spot over the earth's equator assigned to each geostationary satellite.

The frequency band from 216 to 600 MHz, used for fixed and mobile radios and additional television channels on a cable system.

Synchronization (Sync)
The process of orienting the transmitter and receiver circuits in the proper manner in order that they can be synchronized . Home television sets are synchronized by an incoming sync signal with the television cameras in the studios 60 times per second. The horizontal and vertical hold controls on the television set are used to set the receiver circuits to the approximate sync frequencies of incoming television picture and the sync pulses in the signal then fine tune the circuits to the exact frequency and phase.


The transmission bit rate of 1.544 millions bits per second. This is also equivalent to the ISDN Primary Rate Interface for the U.S. The European T1 or E1 transmission rate is 2.048 million bits per second.

T3 Channel (DS-3)
In North America, a digital channel which communicates at 45.304 Mbps.

An electronic multilocation, multiperson conference using audio, computer, slow-scan, or full-rate video systems.


The name of the U.S. proposed  LEO satellite system that would deploy 840 satellites for global telecommunications services.


The AT&T Corporation has maintained its trademark for the Telstar name and currently operates its domestic satellite system under the Telstar name.

Terrestrial TV
Ordinary "over the air" VHF (very high frequency) and UHF (ultrahigh frequency) television transmissions which are usually limited to an effective range of 100 miles or less. Terrestrial tv transmitters operate at frequencies between 54 megahertz and 890 megahertz, far lower than the l4/l2 and 6/4 billion hertz (gigahertz) microwave frequencies used by satellite transponders.

Three-Axis Stabilization
Type of spacecraft stabilization in which the body maintains a fixed attitude relative to the
orbital track and the earth's surface. The reference axes are roll, pinch, and yaw, by nautical analogy.

Threshold Extension
A technique used by satellite television receivers to improve the signal-to noise ratio of the receiver by approximately 3 db (50%). When using small receive-only antennas, a especially equipped receiver with a threshold extension feature can make the difference between obtaining a decent picture or no picture at all.

A small axial jet used during routine stationkeeping activities. These are often fueled bydrazine or bi-propellant. In time ion-engines will probably replace such thrusters.

TI - Terrestrial Interference
Interference to satellite reception caused by ground based microwave transmitting stations.


Transfer Orbit
A highly elliptical orbit which is used as an intermediate stage for placing satellites into geostationary orbit.

An electronic device consisting of oscillator, modulator and other circuits which produce a radio or television electromagnetic wave signal for radiation into the atmosphere by an antenna.

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. Communications satellites typically have between 12 and 24 onboard transponders although the INTELSAT VI at the extreme end has 50.

Transponder Hopping
A single TDMA equipped earth station can extend its capacity by having access to several down-link beams by hopping from one transponder to another. In such a configuration the number of available transponders must be equivalent to the square of the number of beams that are interconnected or cross-strapped.

Telecommunications Standardization Sector. The world standards setting organization resulting from the combination of the CCITT (Consultative Committee on Telephony and Telegraphy) and the CCIR (Consultative Committee on International Radio).

Refers to a system that is supplied, installed and sometimes managed by one vendor or manufacturer.

Television Receive Only terminals that use antenna reflectors and associated electronic equipment to receive and process television and audio communications via satellite. Typically small home systems.

The process of adjusting an electronic receiver circuit to optimize its performance.

TWT (Traveling-wave tube)
A microwave tube of special design using a broadband circuit in which a beam of electrons interacts continuously with a guided electromagnetic field to amplify microwave frequencies.

TWTA (Traveling-wave-tube amplifier)
A combination of a power supply, a modulator (for pulsed systems), and a traveling-wave tube, often packaged in a common enclosure.


Ultra-high Frequency (UHF)
Officially the band of frequencies ranging from 300 to 3000 MHz. In television use, refers to the set of frequencies starting at 470 MHz, The UHF channels are designated as 14 through 70.

A unicast application transmits a copy of every packet to every receiver.

The earth station used to transmit signals to a satellite

Ultra Small Aperture Terminal. This refers to very small terminals for  DBS and other satellite applications where the terminal can be very small (under 50 cms).


ITU-T standard describing a synchronous, physical layer protocol used for communications between a network access device and a packet network. V.35 is most commonly used in the United States and in Europe, and is recommended for speeds up to 48 Kbit/s.

Van Allen radiation belts
These are two high level radiation belts discovered by an Explorer Satellite designed by Dr. Van Allen of Cal Tech. These belts which are highly destructive to communications satellites consists of two belts of highly charged particles and high energy neutrons.

Vertical blanking interval.

Vertical Interval Test Signal
A method whereby broadcasters add test signals to the blanked portion of the vertical interval. Normally placed on lines 17 through 21 in both field one and two.

Very High Frequencies (VHF)
The range of frequencies extending from 30 to 300 MHz; also television channels 2 through 13.

Very small aperture terminal. Refers to small earth stations, usually in the 1.2 to 2.4 meter range. Small aperture terminals under 0.5 meters are sometimes referred to Ultra Small Aperture Terminals (USAT's)

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


World Administrative Radio Conference sponsored by the ITU

A metallic microwave conductor, typically rectangular in shape, used to carry microwave signals into and out of microwave antennas.


The frequency band in the 7-8 GHz region which is used for military satellite communications

A set of packet switching standards published by the CCITT.

A set of CCITT standards for global messaging.


Zulu Time
This is the same a Greenwich Meridian Time (GMT). This is the time standard used in global satellite systems such as INTELSAT and INMARSAT in order to achieve global synchronization.


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