Glossary
ANIK
The Canadian domestic satellite system that transmits Canadian
Broadcasting Corporation's (CSC) network feeds throughout the country.
This system also carries long distance voice and data services
throughout Canada as well as some transborder service to the U.S. and
Mexico.
Antenna
A device for transmitting and receiving radio waves. Depending on their
use and operating frequency, antennas can take the form of a single
piece of wire, a di-pole a grid such as a yagi array, a horn, a helix, a
sophisticated parabolic-shaped dish, or a phase array of active
electronic elements of virtually any flat or convoluted surface.
Audio Subcarrier
The carrier between 5 MHz and 8 MHz containing audio (or voice)
information inside of a video carrier.
Automatic
Frequency Control (AFC)
A circuit which automatically controls the frequency of a signal.
Automatic Gain
Control (AGC)
A circuit which automatically controls the gain of an amplifier so that
the output signal level is virtually constant for varying input signal
levels.
AZ/EL Mount
Antenna mount that requires two separate adjustments to move from one
satellite to another;
Azimuth
The angle of rotation (horizontal) that a ground based parabolic antenna
must be rotated through to point to a specific satellite in a
geosynchronous orbit. The azimuth angle for any particular satellite can
be determined for any point on the surface of the earth giver the
latitude and longitude of that point. It is defined with respect to due
north as a matter of easy convenience.
Backhaul
A terrestrial communications channel linking an earth station to a local
switching network or population center.
Backoff
The process of reducing the input and output power levels of a traveling
wave tube to obtain more linear operation.
Band Pass Filter
An active or passive circuit which allows signals within the desired
frequency band to pass through but impedes signals outside this pass
band from getting through.
Bandwidth
A measure of spectrum (frequency) use or capacity. For instance, a voice
transmission by telephone requires a bandwidth of about 3000 cycles per
second (3KHz). A TV channel occupies a bandwidth of 6 million cycles per
second (6 MHz) in terrestrial Systems. In satellite based systems a
larger bandwidth of 17.5 to 72 MHz is used to spread or "dither" the
television signal in order to prevent interference.
Baseband
The basic direct output signal in an intermediate frequency based
obtained directly from a television camera, satellite television
receiver, or video tape recorder. Baseband signals can be viewed only on
studio monitors. To display the baseband signal on a conventional
television set a "modulator" is required to convert the baseband signal
to one of the VHF or UHF television channels which the television set
can be tuned to receive.
Baud
The rate of data transmission based on the number of signal elements or
symbols transmitted per second. Today most digital signals are
characterized in bits per second.
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.
This tracking beacon is usually a horn or omni antenna.
Beamwidth
The angle or conical shape of the beam the antenna projects. Large
antennas have narrower beamwidths and can pinpoint satellites in space
or dense traffic areas on the earth more precisely. Tighter beamwidths
thus deliver higher levels of power and thus greater communications
performance.
Bird
Slang for a communications satellite located in geosynchronous orbit.
Bit
A single digital unit of information
Bit Error Rate
The fraction of a sequence of message bits that are in error. A bit
error rate of 10-6 means that there is an average of one error per
million bits.
Bit Rate
The speed of a digital transmission, measured in bits per second.
Blanking
An ordinary television signal consists of 30 separate still pictures or
frames sent every second. They occur so rapidly, the human eye blurs
them together to form an illusion of moving pictures. This is the basis
for television and motion picture systems. The blanking interval is that
portion of the television signal which occurs after one picture frame is
sent and before the next one is transmitted. During this period of time
special data signals can be sent which will not be picked up on an
ordinary television receiver.
Block Down
Converter
A device used to convert the 3.7 to 4.2 KHz signal down to UHF or lower
frequencies (1 GHz and lower).
Broad beam
A single large circular beam that covers a large geographic area
Broadcast
The sending of one transmission to multiple users in a defined group
(compare to unicast).
Business
Television
Corporate communications tool involving video transmissions of
information via satellite. Common uses of business television are for
meetings, product introductions and training.
C Band
This is the band between 4 and 8 GHz with the 6 and 4 GHz band being
used for satellite communications. Specifically, the 3.7 to 4.2 GHz
satellite communication band is used as the down link frequencies in
tandem with the 5.925 to 6,425 GHz band that serves as the uplink.
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.
Carrier
The basic radio, television, or telephony center of frequency transmit
signal. The carrier in an analog signal. is modulated by manipulating
its amplitude (making it louder or softer) or its frequency (shifting it
up or down) in relation to the incoming signal. Satellite carriers
operating in the analog mode are usually frequency modulated.
Carrier Frequency
The main frequency on which a voice, data, or video signal is sent.
Microwave and satellite communications transmitters operate in the band
from 1 to 14 GHz (a GHz is one billion cycles per second).
Cassegrain Antenna
The antenna principle that utilizes a subreflector at the focal point
which reflects energy to or from a feed located at the apex of the main
reflector.
CDMA
Code division multiple access. Refers to a multiple-access scheme where
stations use spread-spectrum modulations and orthogonal codes to avoid
interfering with one another.
Channel
A frequency band in which a specific broadcast signal is transmitted.
Channel frequencies are specified in the United States by the Federal
Communications Commission. Television signals require a 6 MHz frequency
band to carry all the necessary picture detail.
Circular
Polarization
Unlike many domestic satellites which utilize vertical or horizontal
polarization, the international Intelsat satellites transmit their
signals in a rotating corkscrew-like pattern as they are down-linked to
earth. 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.
Clamp
A video processing circuit that removes the energy dispersal signal
component from the video waveform.
Clarke Orbit
That circular orbit in space 22,237 miles from the surface of the earth
at which geosynchronous satellites are placed. This orbit was first
postulated by the science fiction writer Arthur C. Clarke in Wireless
World magazine in 1945. Satellites placed in these orbits, although
traveling around the earth at thousands of miles an hour, appear to be
stationary when viewed from a point on the earth, since the earth is
rotating upon its axis at the same angular rate that the satellite is
traveling around the earth.
C/No
Carrier-to-noise ratio measured either at the Radio Frequency (RF) or
Intermediate Frequency (IF)
Codec
Coder/decoder system for digital transmission.
Co-Location
Ability of multiple satellites to share the same approximate
geostationary orbital assignment frequently due to the fact that
different frequency bands are used.
Color Subcarrler
A subcarrier that is added to the main video signal to convey the color
information. In NTSC systems, the color subcarrier is centered on a
frequency of 3.579545 MHz, referenced to the main video carrier.
Common Carrier
Any organization which operates communications circuits used by other
people. Common carriers include the telephone companies as well as the
owners of the communications satellites, RCA, Comsat, Direct Net
Telecommunications, AT&T and others. Common carriers are required to
file fixed tariffs for specific services.
Companding
A noise-reduction technique that applies single compression at the
transmitter and complementary expansion at the receiver.
Composite Baseband
The unclamped and unfiltered output of the satellite receiver's
demodulator circuit, containg the video information as well as all
transmitted subcarriers.
Conus
Contiguous United States. In short, all the states in the U.S. except
Hawaii and Alaska.
Cross Modulation
A form of signal distortion in which modulation from one or more RF
carrier(s) is imposed on another carrier.
EOL
End of Life of a satellite.
Equatorial Orbit
An orbit with a plane parallel to the earth's equator.
INMARSAT
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.
INTELSAT
The International Telecommunications Satellite Organization operates a
network of satellites for international transmissions.
Interference
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.
IRD
An integrated receiver and decoder for reception of a transmission of
voice, video and data.
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).
Master Antenna
Television (MATV)
An antenna system that serves a concentration of television sets such as
in apartment buildings, hotels or motels.
Megahertz (MHz)
Refers to a frequency equal to one million Hertz, or cycles per second.
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 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.
Modulation
The process of manipulating the frequency or amplitude of a carrier in
relation to an incoming video, voice or data signal.
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.
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.
Router
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.
Satellite
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).
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.
Scrambler
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.
Secam
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.
Sidelobe
Off-axis response of an antenna.
T1
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.
TDMA
Time division multiple access. Refers to a form of multiple access where
a single carrier is the shared by many users. Signals from earth
stations reaching the satellite consecutively are processed in time
segments without overlapping.
TI - Terrestrial
Interference
Interference to satellite reception caused by ground based microwave
transmitting stations.
Transmitter
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.
VSWR
Voltage Standing Wave Ratio. A measurement of mismatch in a cable,
waveguide, or antenna system.
VSAT
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)
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