Glossary of Satellite TV Related Terms

Glossary

ACCELERATION Any change in velocity.

ACOUSTIC COUPLER An electronic device that fosters the efficient transmission of digital data over ordinary analog telephone circuits.

AMPLIFIER Any device used in boosting the strength of an electronic signal.

ANALOG COMPUTER (see also DIGITAL COMPUTER) A computer whose processing functions are based on measuring continuous variables rather than counting discrete entities.

ANALOG MODULATION (see also DIGITAL MODULATION) The process of manipulating the frequency or the amplitude of an electromagnetic carrier wave by continuously variable amounts, as opposed to digital modulation in which any variations occur in discrete numerical steps.

ANALOG TO DIGITAL CONVERSION (see also DIGITAL TO ANALOG CONVERSION) Converting continuously varying analog signals into their discrete digital counterparts.

ANTENNA A resonant device that picks up faint radio signals and feeds them into a receiver.

APERTURE The effective cross sectional area of an antenna designed to transmit or receive electromagnetic signals.

APOGEE (see also PERIGEE) The highest point along an elliptical orbit.

ASCENDING NODE The specific point along the equatorial plane at which a satellite crosses the equator as it moves from the southern hemisphere to the northern hemisphere.

ASTRODYNAMICS The practical application of celestial mechanics, propulsion theory, mathematics, and similar disciplines to the problem of planning and directing the trajectories of powered and coasting space vehicles.

ATTENUATION (1) Any reduction in the strength or quality of an electromagnetic Signal due to an intervening medium such as foliage, the ionosphere, or the atmosphere. The intervening medium distorts, reflects, and refracts the signal as it passes through. (2) The loss of power or signal quality of a modulated electromagnetic wave usually caused by intervening obstructions such as foliage or the charged particles in the ionosphere.

ATTITUDE The orientation of a spacecraft with respect to an arbitrary set of reference axes.

AVIONICS Any of the various electronic systems carried onboard an airplane.

BANDWIDTH An adjacent span of frequencies occupied by an electromagnetic signal.

BAUD The number of bits of information transmitted per second.

BENT PIPE COMMUNICATION SATELLITE (see also STORE AND FORWARD COMMUNICATION SATELLITE) A communication satellite that picks up signals from the ground and immediately rebroadcasts them on a different frequency toward a different geographical region.

BIPROPELLANT ROCKET A rocket powered by two separate liquids: fuel, and oxidizer. The propellants are pumped or fed under pressure into the combustion chamber where they are mixed and burned.

BIRD Industry slang for a communication satellite.

BIT A single binary 1 or binary 0.

BIT ERROR RATE The number of errors, expressed as a fraction of the total number of bits sent usually in a digitally modulated channel.

BROADCASTING The process of transmitting an electromagnetic signal to multiple, widely dispersed receivers.

BYTE An adjacent sequence of binary digits, usually, but not always, eight in number. A byte is used in representing a single letter, number, special symbol, or punctuation mark.

CARRIER WAVE A sinusoidal electromagnetic wave usually, but not always, modulated with useful information.

CELLULAR TELEPHONE SYSTEM Aground based mobile telephone system in which telephone calls are relayed through numerous low power transmitters arranged in a gridlike pattern within an urban area.

CHANNEL An adjacent band of frequencies that contain a specific broadcast signal.

CHIPPING RATE The rate at which an electronic circuit produces binary digits.

CIRCUIT A complete closed electrical pathway that controls the flow of electrons or other submicroscopic charged particles.

CIRCULAR POLARIZATION (see also LINEAR POLARIZATION) The process of transmitting electromagnetic signals in a rotating corkscrewlike pattern.

COAXIALCABLE A pair of conductors consisting of a central conductor surrounded by an outer conductor. Coaxial cables have relatively good immunity to interference and low power losses when carrying high frequency transmissions.

CODEC The abbreviation for coder/decoder, a device used in digital communication systems to convert analog signals to digital signals, and vice versa.

CODE DIVISION MULTIPLE ACCESS (see also FREQUENCY DIVISION MULTIPLE ACCESS and TIME DIVISION MULTIPLE ACCESS) A broadcast system in which each transponder spreads its modulated signal over the entire bandwidth assigned to all the common carriers. COMMUNICATION LINK Any physical or free space connection between one location and another used for the purpose of transmitting and receiving information using electromagnetic waves as the carrier medium.

COMMUNICATION SATELLITE An electronic relay station orbiting in space that picks up messages transmitted from the ground and retransmits them to a distant location, usually on a different frequency.

COMSAT A specific satellite communications organization that represents America's interests in the Intelsat and Inmarsat consortiums. Also industry slang for a communication satellite.

CONIC SECTIONS Circles, ellipses, parabolas, and hyperbolas created by passing a plane through a cone at various angles. To a first approximation, a spacecraft freely coasting in the gravitational field of a spherical celestial body such as the earth travels along one of the conic sections.

CONSTELLATION Any collection of similar satellites designed to provide multiple coverage or multiple redundancy. Also, a group of stars regarded to be grouped together into a recognizable pattern by the people living on earth.

DECIBEL A physical unit for expressing the ratio of two quantities of signal power. One decibel is equal to 10 times the common logarithm of this ratio.

DEORBIT The process of firing a rocket onboard an orbiting space vehicle to reduce its orbital energy so it will reenter the earth's atmosphere and come back to earth either gently or catastrophically.

DIGITAL COMPUTER (see also ANALOG COMPUTER) A computer whose processing functions are based on counting discrete entities rather than measuring continuous variables.

DIGITAL MODULATION (see also ANALOG MODULATION) The process of converting incoming voice, video, or data signals into a form suitable for transmission over electromagnetic channels toward distant receivers.

DIGITAL TO ANALOG CONVERSION (see also ANALOG TO DIGITAL CONVERSION) The process of converting discrete digital signals into their continuously varying digital counterparts.

DOPPLER SHIFT Any systematic change in the frequency of a carrier wave that results when transmitter and receiver are moving at different velocities with a component of that velocity toward or away from one another.

DOWNLINK (see also UPLINK) Any space to earth telecommunications pathway.

DRAG The friction between a vehicle moving through the earth's atmosphere opposing the vehicle's forward motion.

DUPLEX TRANSCEIVER (see also SIMPLEX TRANSCEIVER) A transceiver that can transmit and receive simultaneously so that the parties communicating can carry on a natural conversation.

EARTH STATION Any ground based communication facility capable of transmitting, receiving, and processing data relayed to and from orbiting satellites.

ECCENTRICITY The "oblateness" of a satellite's orbit. The eccentricity of any orbit equals the difference between its apogee radius and its perigee radius divided by their sum.

ECHO BALLOON One of the early spherical passive communication satellites 100 or 135 ft in diameter, constructed by coating the skin of a Mylar balloon with aluminum. ECHO SUPPRESSION The process of attenuating the echo effect in a satellite based communication channel.

ECLIPSE INTERVAL The period of time during which a satellite is shrouded in darkness within the earth's shadow.

EFFECTIVE RADIATED POWER (ERP) The power, expressed in watts, that is radiated in the direction of the maximum antenna gain.

ELECTROMAGNETIC WAVES Any sinusoidal carrier wave created by mutually orthogonal electric and magnetic fields that travels through a vacuum at an invariant speed of 186,000 mi/s.

ELECTRON A subatomic particle with a specific negative charge and a very small mass that revolves around the nucleus of an atom.

ELECTRONICS A specific branch of physics concerned primarily with the natural and controlled flow of electrons and other submicroscopic charged particles through various substances. ELEVATION ANGLE The angle between the local horizon and the line of sight vector pointing toward a satellite.

ELLIPSE A closed, oval shaped curve.

EPHEMERIS Any complete set of numbers that, taken together, specify the approximate orbit of a satellite.

FIBER OPTICS The technology of modulating information onto light beams transmitted through long, ultrathin strands of glass.

FOLIAGE ATTENUATION Any reduction in signal strength or signal quality resulting from the limbs and leaves of trees that happen to be situated along the signal's line ofsight path. FOOTPRINT A closed contour on the ground within which a communication satellite's beam provides sufficient signal strength to service customer needs.

FREQUENCY (see also WAVELENGTH) The number of complete sinusoidal cycles an electromagnetic carrier wave goes through in 1 second.

FREQUENCY DIVISION MULTIPLE ACCESS (see also CODE DIVISION MULTIPLE ACCESS and TIME DIVISION MULTIPLE ACCESS) Any broadcast system in which each transponder operates within its own assigned frequency slot or bandwidth.

FREQUENCY REUSE Any of several techniques for maximizing the capacity of a ground based or a space based communication system. Frequency reuse is accomplished by isolating or polarizing the signals so carrier waves oscillating at the same frequency can service more users than would otherwise be possible.

GEOCENTRIC Earth centered.

GEODETIC Pertaining to the earth.

GEOLOCATION (see also NAVIGATION) The process of fixing the position of a moving or stationary craft in longitude and latitude on the surface of the earth.

GEOSYNCHRONOUS ORBIT (GEOSTATIONARY ORBIT) A circular orbit with a 24hour period 19,300 nautical mi above the earth's equator. A satellite in a geostationary orbit appears to remain at a fixed location in the sky as seen by observers on the spinning earth.

GIGAHERTZ One billion cycles per second.

GRAVITY GRADIENT STABILIZATION A passive method of spacecraft stabilization and attitude control in which an elongated satellite (usually equipped with a telescoping boom) is oriented with its long axis pointing radially away from the earth so it can take advantage of the inverse square decrease in the earth's gravitational field to create torques that help keep it in its natural vertical orientation.

GROUNDTRACE The path a satellite traces out over the surface of the spinning earth as it coasts forward at its natural orbital speed around its circular or elliptical orbit.

HERTZ One cycle per second.

HOHMANN TRANSFER MANEUVER Atwo bum powered flight sequence that carries a satellite from one circular orbit to another along a specific transfer ellipse tangent to the two circular orbits at its perigee and apogee. The satellite traverses 180 degrees as it travels along the transfer ellipse between the two bums.

HUB STATION The master station through which all communications to and from the users must flow.

HYPERGOLIC PROPELLANTS Any fuel oxidizer propellant combination that bursts into flame spontaneously on contact.

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

INMARSAT A specific international organization devoted primarily to providing telecommunication services for ships at sea.

INTELSAT A specific international organization devoted primarily to providing landbased telecommunication services for a worldwide class of users.

INTERFERENCE Randomly varying electromagnetic energy that tends to distort and attenuate the desired signals being picked up.

IONOSPHERE Four roughly concentric layers of charged particles in the earth's upper atmosphere. The particles in the ionosphere bend, distort, and reflect electromagnetic waves.

Ka BAND The range of electromagnetic frequencies from 18 to 31 GHz. KILOHERTZ One thousand cycles per second.

Ku BAND The range of electromagnetic frequencies from 10.9 to 17 GHz. L BAND The range of electromagnetic frequencies from 950 to 1450 MHz. LIGHT Those electromagnetic waves to which the human eye is sensitive. LINEAR POLARIZATION (see also CIRCULAR POLARIZATION) A process of transmitting electromagnetic signals such that they are made to oscillate parallel to one another, usually in the horizontal or vertical directions.

MEGAHERTZ One million cycles per second. MICROSECOND One millionth of a second. MILLISECOND One thousandth of a second. MOBILE COMMUNICATION SATELLITES Any satellite that provides voice and data relay services between freely moving users on or near the ground below.

MODEM An electronic device that modulates outgoing electromagnetic signals at the transmission end of a communications channel and/or demodulates the incoming signals at the receiving end.

MODULATION (see also ANALOG MODULATION and DIGITAL MODULATION) (1) The process of manipulating the frequency or the amplitude of a carrier wave in response to an incoming voice, video, or data signal. (2) The process of encoding an electromagnetic carrier wave with useful information.

MOMENTUM WHEEL A heavy flywheel rigidly mounted to the inside of a satellite whose rotational speed can be adjusted with the power from onboard electric motors to help maintain and adjust the spacecraft's angular orientation in real time.

MONOPROPELLANT ROCKET A liquid rocket powered by a single liquid such as bydrazine. The monopropellant is sprayed over a catalyst to induce the desired chemical reaction, thus creating thrust.

MULTIPATH A specific distortion that results when a portion of an electromagmetic signal is reflected from nearby surfaces, thus smearing and distorting the main signal which arrives by a more direct route. The so called "ghosts" seen on a television screen are created by multipath reflections.

MULTIPLEXING A specific body of electronic processing techniques that allow several simultaneous messages to be sent over a single communication channel at the same time.

MULTISTAGE ROCKET An arrangement of rockets usually stacked one atop the other or mounted in parallel side by side. As the fuel in each stage is exhausted, the empty stage is jettisoned so the smaller rockets that remain will not have such a heavy load to push up toward orbital velocity.

NANOSECOND One billionth of a second.

NAVIGATION (see also GEOLOCATION) The process of fixing the position of a craft and directing that craft along a preferred route from one known location to another.

NOISE The unwanted and unmodulated electromagnetic energy that always corrupts, to some extent, the modulated signals being sent through a communication channel.

OMNIDIRECTIONAL ANTENNA An antenna that transmits or receives electromagnetic signals to or from all directions.

OPTICAL FIBERS Hair thin cylinders of glass through which optical pulse trains are transmitted toward distant recipients.

ORBIT The gravity induced path followed by a satellite as it coasts around the earth or any other celestial body.

ORBITAL MECHANICS A specific branch of physics and mathematics devoted to the analysis, prediction, construction, and modification of satellite orbits.

ORBITAL PERIOD The amount of time it takes an artificial satellite to make one complete circuit around the earth.

ORBITAL SLOT A longitudinal location along the geosynchronous arc specifically assigned to a particular satellite.

ORBITALVELOCITY The speed required for an artificial satellite to remain in orbit at a particular orbital altitude.

PACKET SWITCHING A specific message switching technique in which the messages traveling through a communication channel contain all the information necessary for routing, control, and self checking for any coding and transmission errors.

PARITY CHECKING An automatic computer based error detection procedure that uses extra checking bits that are carried along with the usual numerical bits being processed.

PARKING ORBIT Any temporary orbit, usually circular, within which a space vehicle awaits powered transfer to another orbit.

PASSIVE COMMUNICATION SATELLITE A simple satellite with no moving or electronic parts that reflects electromagnetic waves from one ground station to another.

PAYLOAD The electronic and mechanical devices carried onboard a space vehicle designed to help it perform its useful mission.

PERIGEE (see also APOGEE) The lowest point along an elliptical orbit.

PERTURBATION Any unwanted disturbance that distorts the regular orbital motion of a satellite.

PHOTON An elementary bundle of electromagnetic radiant energy.

PICOSECOND One thousandth of a nanosecond, i.e., one trillionth of a second.

POLAR ORBIT Any orbit that carries an orbiting satellite alternately over the North and South Poles.

PROPELLANT Any liquid, solid, or gaseous substance carried onboard a rocket or a satellite for the purpose of developing translational or attitude control thrust.

PROTON A subatomic particle with a positive charge, often located in the nucleus of an atom.

RECEIVER (see also TRANSMITTER) Any electronic. device that enables the desired modulated signal to be separated from all the other signals coming into the antenna.

REENTRY The return of a spacecraft into the earth's atmosphere.

REFRACTION The bending of an electromagnetic wave due to density variations in the medium (water, air, metal) through which that electromagnetic wave is passing.

ROAMING A cellular telephone's usage of any cellular telephone switch other than its home switch.

SATELLITE (1) Any artificial or natural body orbiting a larger body such as the earth. The satellite is held in its circular or elliptical trajectory by the combination of its tangential velocity and the gravitational force of the larger body pulling it radically inward toward the center of the gravitational field. (2) Any object that orbits the earth.

SCRAMBLER A device that electronically alters a modulated electromagnetic signal so only those recipients equipped with special decoders can pick up and interpret a pristine version of the original signal.

SEMICONDUCTOR Any substance such as silicon, germanium, or selenium that normally insulates against the flow of electricity, but that can, when infused with trace amounts of certain specific impurities, be made to conduct the flow.

SIGNAL TO NOISE RATIO The power of the modulated signal being received, divided by the random electromagnetic noise coming in on that same channel.

SIMPLEX TRANSCEIVER (see also DUPLEX TRANSCEIVER) A transceiver that is able to transmit or receive, but not both simultaneously. With a simplex transceiver the two parties communicating must talk and listen alternately.

SOLAR ARRAY An adjacent set of solar cells used in converting incoming solar energy into electricity.

SOLID STATE DEVICE An electronic valve or amplifier composed of solid, monolithic materials whose electrical properties are controlled by specific impurities purposely infused into its crystal lattice structure during manufacture.

SPECIFIC IMPULSE An abstract measure of the efficiency of a rocket propellant combination. The specific impulse equals the number of seconds a pound of the propellant will burn to produce a pound of thrust.

SPACECRAFT Any vehicle designed to operate in space.

SPECTRUM An adjacent set of electromagnetic waves.

SPIN ST ' ABILIZATION (see also THREE AXIS STABILIZATION) A specific form of satellite stabilization and altitude control in which the entire satellite spins at a fixed rate about a specific axis of spin.

SPOT BEAM A focused, conical shaped pattern of electromagnetic waves directed from the antennas of a communication satellite toward a specific geographical region on the ground.

SPREAD SPECTRUM SIGNAL Any modulated signal superimposed on an electromagnetic wave with the characteristic that the number of bits of useful information being transmitted is appreciably less than the bandwidth of the transmission. Spreadspectrum transmission techniques provide improved jamming immunity, multipath rejection, and ranging accuracy for broadcast systems.

STATIONKEEPING MANEUVERS On orbit rocket burns used in making small orbital adjustments to keep an orbiting satellite within an acceptable distance of its assigned orbital slot.

STORE AND FORWARD COMMUNICATION SATELLITE (see also BENT PIPE COMMUNICATION SATELLITE) A communication satellite that picks up signals from the ground, records them on electronic or magnetic storage media, then rebroadcasts them on command when it is over some distant geographical region.

SUBLIMATION The process by which a solid substance changes directly into a gas without passing through the intervening liquid state.

TELEPROCESSING The use of telephone lines or other communication channels to transmit useful information between remote locations.

THRUSTER A small onboard rocket used by a satellite for stationkeeping maneuvers, momentum dumping, and altitude control.

THREE AXIS STABILIZATION (see also SPIN STABILIZATION) A specific form of satellite stabilization and control in which the satellite's main body is made to maintain a fixed attitude relative to the earth's surface and its orbital track.

TIME DIVISION MULTIPLE ACCESS (see also FREQUENCY DIVISION MULTIPLE ACCESS and CODE DIVISION MULTIPLE ACCESS) A broadcast system in which each transponder operates within its own assigned discrete time slots.

TRANSCEIVER Any electronic device that can transmit and receive modulated electromagnetic waves.

TRANSFER ORBIT An intermediate elliptical orbit along which a satellite travels from its initial low altitude orbit out to a higher altitude orbit.

TRANSMITTER (see also RECEIVER) Any electronic device that broadcasts modulated electromagnetic signals toward one or more distant receivers. TRANSPONDER An electronic device carried onboard a communication satellite that picks up signals from the ground on one frequency and immediately rebroadcasts them on a different frequency.

ULTRAHIGH FREQUENCY (UHF) That specific band of frequencies ranging from 300 to 3000 MHz.

UNIVERSAL LAW OF GRAVITATION A specific physical law discovered by Isaac Newton which states that all material bodies in the universe attract one another with a force that is directly proportional to the product of their masses and inversely proportional to the distance between their centers.

UPLINK (see also DOWNLINK) Any earth to space telecommunications pathway.

VAN ALLEN RADIATION BELTS Two large donut shaped rings of high energy charged particles (electrons and protons) spiraling around the earth's magnetic lines of flux. VELOCITY The speed at which an object moves in a particular direction.

VERY HIGH FREQUENCY (VHF) That specific band of frequencies ranging from 30 to 300 MHz.

VOLATILE STORAGE A computer storage medium in which the information being stored vanishes if the power is turned off or temporarily interrupted.

WAVELENGTH The distance between two adjacent crests or troughs of a wave.