Satellite Broadcasting Terminology

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AKM - Apogee Kick Motor. The rocket motor used at the apogee of a Transfer Orbit to direct a satellite into its Drift Orbit.

ARQ - Automatic Retransmission Request. ARQ is an application layer error correction technique that identifies errors in frames of transmitted data and automatically requests retransmission of the erred frames.

Anik - The family name for Telesat Canada's satellites. Anik means brother in the Inuit dialect.

Antenna - A device for radiating and receiving radio waves, usually designed to focus the waves to or from one direction. The antenna is often referred to as a dish.

Attitude - The position of a satellite's axes measured against a set of reference axes. Usually measured as pitch, roll, and yaw.

BER - Bit Error Rate. The quality of a digital signal measured by the proportion of errors.

BTV - Business Television. The point-to-multipoint distribution of a private TV signal for corporate communications purposes, including employee information sharing, training and product introductions.

Backhaul - A terrestrial circuit that connects customer premises equipment to a central office, or to a central, shared earth station (e.g., Teleport).

Bandwidth - A measure of the amount of frequency spectrum occupied by a signal; usually measured in Hertz (Hz), Kilohertz (KHz) or Megahertz (MHz).

Baseband - The frequency range of the information signal (voice, data, or image) before it is modulated onto a satellite carrier. For instance, the voice baseband extends from 300 Hz to 3400 Hz.

C-band - One of two common satellite frequency bands (Ku-band is the other). C-band earth stations use the 6 GHz frequency band to transmit and the 4 GHz frequency band to receive.

CDMA - Code Division Multiple Access. An access technique in which every earth station may transmit at the same frequency and at the same time, but only at a given, limited power level. This technique requires that each earth station use a unique identifying code, which is modulated with the communications signal. CDMA is the access technique used with spread-spectrum data distribution and some VSAT and mobile satellite systems.

Carrier - A single-frequency radio wave of constant amplitude that can be modulated with an information carrying signal.

Channel - A path for electrical transmission between two points.

Channel Unit - A module containing equipment necessary to convert baseband information to a given form of modulation and vice versa.

dB - Logarithmic representation of a mathematical value. It can be used to represent power (dBW), audio level (dBm), gain of an antenna (dBi) and losses in a system (dB).

DAK - DirecPC Access Kit. This kit consists of a DirecPC computer card, cable, LNB and antenna.

DAMA - Demand Assigned Multiple Access. A technique used to permit a relatively larger number of earth stations to share a smaller pool of SCPC carriers on demand.

DBS - Direct Broadcast Satellite. A high-powered satellite used for direct broadcast of TV signals to home dishes.

DBW - Logarithmic representation of a power level reference to 1W of power.

DES - Digital Encryption Standard

DIU - Digital Interface Unit. See Indoor Unit.

DVC - Digital Video Compression. Digital processing of an analog TV signal, resulting in a fraction of the power and bandwidth of the original signal.

Demodulator - A device that extracts the baseband signal from a modulated carrier.

Digital System - A system of communication that uses a stream of bits or discreet quantities to represent the signal.

Downconverter - A device that lowers the frequency of the carrier, typically from Radio Frequency (RF) to Intermediate Frequency (IF).

Downlink - The transmission path between a satellite and the receiving earth station.

Drift Orbit - The orbit followed by a satellite, after the firing of the Apogee Kick Motor, as it drifts towards its allotted operational station.

EIRP - Equivalent Isotropic Radiated Power. A measure of the strength of the signal radiated by an antenna. On the downlink EIRP varies over the footprint.

Earth Station - The equipment needed to receive from and/or transmit to a satellite. Consists primarily of an antenna and ground communications equipment.

Elevation - The look angle of the earth station in degrees above the horizon. 0° indicates that the satellite is level with the horizon, and 90° indicates that it is directly overhead.

FDMA - Frequency Division Multiple Access. FDMA is a technique used to allow more than one earth station to share the bandwidth of a satellite transponder. Each earth station is assigned a specific carrier or set of carriers for transmission over a specific portion of the bandwidth.

FSS - Fixed Satellite Service. This is defined by the Interntional Telecommunications Union (ITU) as any of the various services provided by satellites to fixed earth stations (as opposed to mobile earth stations). A number of frequency bands are set aside for this, the current most common being the C-band and Ku-band assignments.

Feedhorn - A piece of antenna hardware, located at the focal point of the parabolic reflector, that radiates RF energy toward the antenna reflector and collects (received) RF energy from the antenna reflector.

Footprint - The area on the earth that a signal covers.

Frequency Hopping - A technique by which an earth station changes transmission frequency to access different satellite channels. This is required when the satellite has several spot beams and the transmission has to be directed to different areas.

GLACS - Ground Loop Attitude Control System. This Telesat developed system is now in use to control the Anik E2 satellite after the failure of both momentum wheels in 1994. Using two carriers looped through the satellite, the system measures data on the pointing of the satellite antenna and transmits commands to the satellite to make the appropriate corrections. Control is effected through magnetic torquing coils and through very short thruster bursts as required.

Gain/Temperature (G/T) - A measure of the receiving ability of an antenna/LNA combination.

Geostationary Satellite - A satellite orbiting the Earth 35,786 km above the equator. The orbiting velocity matches the Earth's rotational speed, thereby making the satellite appear stationary with respect to Earth. Satellite positions match the degrees of longitude on the Earth beneath the satellite.

HDLC - High Level Data Link Control. A communications line discipline defined by the International Standards Organization.

HPA - High Powered Amplifier. An earth station device that amplifies the modulated carrier for its voyage to the satellite.

Hub - Alternative name for the central earth station in a star network.

IF - Intermediate Frequency. A frequency translation from baseband or RF to more easily filter, distribute, modulate and demodulate the signal.

IFL - Interfacility Link. The link between an antenna and its associated ground communications equipment.

IRD - Integrated Receiver/Decoder. Serves as receiver and decoder for compressed digital TV signals, as well as authorization device.

Indoor Unit - The indoor unit (or DIU - Digital Interface Unit) contains the intelligence of the remote earth station and manages the data communications link.

Ka-band - (Pronounced "Kay-ay band") A satellite frequency band that uses 30 GHz signals to transmit and 20 GHz signals to receive.

Ku-band - (Pronounced "Kay-you band") One of two common satellite frequency bands (C-band is the other). Ku-band earth stations use the 14 GHz frequency band to transmit and the 12 GHz frequency band to receive.

LAN - Local Area Networks. A LAN is local network of computers that are all connected together with a single cable. Typical LAN's use the Ethernet or Token Ring protocol for communication among the computers.

LANAdvantage - This service name is a copyright of Hughes Network Systems. It specifies the LAN capable version of their Personal Earth Station VSAT product. LanAdvantage allows users to connect their Ethernet or Token Ring networks together via satellite.

LEO - Low Earth Orbit (up to 1,000 km).

LNA - Low Noise Amplifier. A device to amplify the received modulated carrier while adding little noise.

LNB - Low-Noise Block Downcoverter is usually located on a receive feed of an antenna. The LNB provides the L-band signal for the IRD.

MAC - Monitor, Alarm and Control. A system to provide information on the status of equipment and the means to change its status.

MPEG I - Original digital video compression (DVC) standard defined by the Moving Pictures Experts Group (MPEG) for compact disk operation at a bit rate of 1.5 Mbps.

MPEG II - Improved DVC standard defined by the MPEG group for broadcast television applications at bit rates between 4 Mbps and 15 Mbps.

MUX - The multiplexer combines the multiple digital outputs of an encoder before modulation.

Modulation - The process whereby some characteristic of a carrier is varied in accordance with some characteristic of the signal to be carried (e.g., the frequency of the carrier can be varied according to the amplitude of the signal).

Nimiq - Nimiq will be Canada's first Direct Broadcast Satellite. Nimiq is an Inuit word meaning any object or force that unites things or binds them together.

OPBO - Output (OP) Back Off (BO) is the representation of the receive power of a carrier with respect to the maximum receive power from a satellite channel.

Omni Directional Antenna - An antenna which radiates in all directions. Used on a satellite from launch until the communications antenna is in use.

Outdoor Unit - The Outdoor Unit (ODU) contains all electronics necessary for communicating with a satellite: an upconverter, a solid-state power amplifier, and a combination low noise amplifier and downconverter.

PKM - Perigee Kick Motor. The rocket motor used to inject a satellite into its transfer orbit after launch from the Orbiter.

Pitch - The movement of a satellite, about an axis through its center of gravity, which causes the antenna footprint to move East and West.

Polarization - The restriction of the transverse vibrations of an electromagnetic wave to a fixed plane (horizontal or vertical) or a rotating plane (left hand or right hand).

Propagation Delay - The time it takes for a signal to travel to the satellite and back to the earth. Satellite signals travel at the speed of light and complete the trip in 270 milliseconds.

Protocol - A formal set of conventions governing the formatting and relative timing of message exchange between two communicating systems.

Pure Aloha - An ethernet-like protocol that permits all earth stations in one network to share a pool of inbound bandwidth.

RF - Radio Frequency. A satellite radio frequency link establishes communications between a transmitter and a receiver using the satellite as a repeater.

RF Channel - Radio Frequency Channel. The unit of power and bandwidth that is used in Telesat's space segment tariff. Each RF channel can carry one standard (uncompressed) TV signal.

Roll - The movement of a satellite, about an axis through its center of gravity, which causes the antenna footprint to move North and South.

SCC - Satellite Control Centre. Telesat staff at the SCC, located in Gloucester, Ontario, control Telesat's satellites.

SCPC - Single Channel Per Carrier. A communications system in which each circuit has its own carrier frequency within a transponder.

SDLC - Synchronous Data Link Control. SDLC was invented by IBM to replace the older bisynchronous protocol for wide area connections between IBM equipment. A variation of the ISO HDLC protocol called HDLC Normal Response Mode (NRM) is essentially the same as SDLC.

SNOC - Satellite Network Operations Centre. Telesat staff at the SNOC, located in Allan Park, Ontario, detect service problems and coordinate their repair.

SWAN - Satellite Wide Area Network

Site Diversity - The use of two or more terminals, separated by some distance, to overcome the problems of fading due to rain.

Slotted Aloha - A means of sharing the inbound space capacity of a VSAT network. Slotted Aloha synchronizes the entire VSAT network into a number of fixed time slots. Each station may tansmit at any time at the beginning of a slot. Best used if message size is small, the network is large, and transactions are infrequent and predictable.

Spin Stabilized - The method of stabilizing a satellite by means of spin.

Spot Beam - A satellite antenna beam which provides service to only a part of the satellite's total coverage area.

Star Network - A network in which traffic flows to and/or from a central hub or master earth station; links between any other pair of earth stations are always via the hub.

Stream Access - A means of sharing the inbound space capacity of a VSAT network. A dedicated stream assignment allocates space segment on a port-to-port basis. Optimal when data arrives at a steady bit rate.

Sun Transit - Occurs when the main beam of an earth station receiving antenna is in direct line of sight with the sun introducing additional noise into the antenna. Sun Transit degradation, however, has been virtually eliminated with the widespread use of smaller, more efficient antennas.

TDM - Time Division Multiplexing. A communication system technique that interleaves information from several channel inputs onto a carrier in specific time slots.

TDMA - Time Division Multiple Access. TMDA is a technique used to provide more than one earth station with access to the same carrier by taking turns transmitting bursts.

TVRO - TV receive-only.

TWTA - Travelling Wave Tube Amplifier. A wideband microwave amplifying device in which power is added to a carrier as it propagates down a tube.

Teleport - A group of earth stations in one location providing various satellite services on a multi-customer basis; one version also includes commercial real estate development such as a multi-tenant office building or a business park.

Three Axis Stabilization - A system of satellite stabilization in which its axes remains fixed with respect to earth.

Thrusters - Devices in which propellant is burnt, or disintegrates, to produce a controlled force in a selected direction.

Transaction Reservation - A means of sharing the inbound space capacity of a VSAT network. Senders can reserve a slot before sending. transaction reservations works best if messages are large and relatively infrequent.

Transfer Orbit - The orbit of a satellite between the firing of the Perigee Kick Motor and the firing of the Apogee Kick Motor.

Transponder - Satellite equipment that receives signals on the uplink, translates them to the downlink frequency, and amplifies them for retransmission to earth.

Transponder Hopping - The same as frequency hopping.

Uplink - The signal path from an earth station to a satellite.

VDI - Voice, Data and Image.

VSAT - Very Small Aperature Terminal. VSAT usually refers to a transmit and receive earth station with an antenna diameter no larger than 2.4m.

Voice Activation - A communication system in which the carrier is not activated unless a signal is present.

YAW - The movement of a satellite, about an axis through its center of gravity, which causes the antenna footprint to rotate clockwise or anti-clockwise.

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