Amplitude Modulation (AM)
The baseband signal is caused to vary the amplitude or height of the carrier wave to create the desired information content.
Amplifier
A device used to boost the strength of an electronic signal.
Analog
A form of transmitting information characterized by continuously variable quantities, as opposed to digital transmission, which is characterized by discrete bits of information in numerical steps. An analog signal is responsive to changes in light, sound, heat and pressure.
Analog-to-Digital Conversion (ADC)
Process of converting analog signals to a digital representation. DAC represents the reverse translation.
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.
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.
Apogee Kick Motor (AKM)
Rocket motor fired to circulate orbit and deploy satellite into geostationary orbit.
Attenuation
The loss in power of electromagnetic signals between transmission and reception points.
Attitude Control
The orientation of the satellite in relationship to the earth and the sun.
Audio Subcarrler
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.
B-Mac
A method of transmitting and scrambling television signals. In such transmissions MAC (Multiplexed Analog Component) signals are time-multiplexed with a digital burst containing digitized sound, video synchronizing, authorization, and information.
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).
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 Belt
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.
C/T
Carrier-to-noise-temperature ratio.