Satellite Glossary
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H.M.S. Communications




AM Amplitude Modulation
AMSC American Mobile Satellite Corporation

Analog data Data represented by a physical quantity that is considered to be continuously variable and whose magnitude is made directly proportional to the data or to a suitable function of the data.
Asynchronous transmission A method of data transmission which allows characters to be sent at irregular intervals by preceding each character with a start bit, and following it with a stop bit.
Availability The ratio, expressed as a percentage, of the total time a functional unit or service is capable of being used or is available to be used during a given interval to the length of the interval; e.g., if the unit is not capable of being used for 20 minutes in a week, the availability is 99.8 percent (10080 - 20 minutes/10080 minutes X 100).


Backbone The primary connectivity mechanism of a hierarchical distributed system. All systems which have a connectivity to an intermediate system on the backbone are assured of connectivity to each other.
Backhaul Point-to-point transmission from a remote site back to a central site for further distribution.
Bandwidth The bandwidth of a device is the difference between the limiting frequencies within which performance with respect to some characteristic falls within specified limits.
Baud A unit measuring the rate of information flow, with five baud roughly equivalent to one alphanumeric character.
Binary Phase Shift Keying ( BPSK) A digital modulation scheme used in transmission communications.
Broadcasting Using the electronic media to reach a wide-area audience.
Byte A sequence of eight adjacent binary digits usually treated as a unit.


C-band A band of frequencies used for satellite and terrestrial communications. The range of frequencies from 4 to 6 gigaHertz (billion cycles per second) is used by most communications satellites.
Call (1) Any demand to set up a connection. (2) A unit of traffic measurement.
Carrier The basic radio signal that transfers the information signal. It occupies a single radio frequency (see also Common Carrier).
Circuit mode A circuit switched operational mode for transferring (transporting and switching) user information through a network. Contrast with packet switching mode.
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.
Compression A technique to reduce the amount of data to be transmitted that reduces the amount of bandwidth needed to transmit video or audio, thus increasing the capacity of a satellite transponder.
Conus Contiguous United States. In short, all the states in the U.S. except Hawaii and Alaska.



Data Representation of facts, concepts, or instructions in a formalized manner suitable for communication, interpretation, or processing by humans or by automatic means.
Data Encryption Standard (DES) A cryptographic algorithm for the protection of unclassified computer data, issued as Federal Information Processing Standard Publication 46-1.
dB Decibel; an analog unit of measure of signal strength, volume, or signal loss due to resistance as expressed in logarithmic form.
Delay The time it takes for a signal to go from the sending station through the satellite to the receiving station. This transmission delay for a single hop satellite connection is very close on one-quarter of a second.


Demand Assigned Multiple Access (DAMA) Refers to contention access schemes that allow multiple communications users to share a discrete portion of the bandwidth.
Demodulation The process for retrieving an information signal that has been modulated onto a carrier.

Digital data Data represented by discrete values or conditions (i.e., "0" or "1"), as opposed to analog data.
Digital voice Analog voice signals that have been converted into digital signals.
Direct Broadcast Satellite (DBS) Transmits TV signals directly to dishes at viewer's homes. Usually a high-powered satellite that requires only small dishes.

Downlink Transmissions from a satellite to a ground station; also, the dish used for reception.
DVB Digital Video Broadcast


Earth Station The term used to describe the combination or antenna, low-noise amplifier (LNA), down-converter, and receiver electronics. used to receive a signal transmitted by a satellite. Earth Station antennas vary in size from the.2 foot to 12 foot (65 centimeters to 3.7 meters) diameter size used for TV reception to as large as 100 feet (30 meters) in diameter sometimes used for international communications.
Echo suppression Refers to the cancellation of any reflected signal in a communications channel.
Edge of Coverage Limit of a satellite's defined service area. In many cases, the EOC is defined as being 3 dB down from the signal level at beam center. However, reception may still be possible beyond the -3dB point.
EIRP Effective Isotropic Radiated Power
Electronic access The capability to access information via on-line access (dedicated or dial-up), E-Mail, and FAX.
Electronic Data Interchange (EDI) The exchange of routine business transactions in a computer-processable format, covering such traditional applications as inquiries, planning, purchasing, acknowledgments, pricing, order status, scheduling, test results, shipping and receiving, invoices, payments, and financial reporting.
Elevation The upward tilt to a satellite antenna measured in degrees required to aim the antenna at the communications satellite. When. aimed at the horizon, the elevation angle is zero. If it were tilted to a point directly overhead, the satellite antenna would have an elevation of 90 degrees
Encrypt To convert plain text into an unintelligible form by means of a cryptosystem.
Equatorial Orbit An orbit with a plane parallel to the earth's equator.



FM Frequency Modulation
Frame Relay A recently developed switching interface which operates in packet mode. Generally regarded as the future replacement for X.25.
Frequency The number of times that an alternating current goes through its complete cycle in one second of time. One cycle per second is also referred to as one hertz; 1000 cycles per second, one kilohertz; 1,000,000 cycles per second, one megahertz: and 1,000,000,000 cycles per second, one gigahertz.
FTP File Transfer Protocol. The Internet protocol (and program) used to transfer files between hosts.
Full-duplex operation A mode of operation in which simultaneous communication in both directions may occur between two terminals. Contrast with half duplex or simplex operation in which communications occur in only one direction at a time.


Gain A measure of amplification expressed in dB.
GAN Global Area Network (Inmarsat M-4)
Gateway In a communication network, one of the network nodes equipped for interfacing with a network using different protocols. Note 1: A gateway may contain devices such as protocol translators, impedance matching devices, rate converters, fault isolation, or signal translators as necessary to provide system interoperability.
Geosynchronous Orbit (GSO) Satellite in geosynchronous orbit travel around the Earth in an area approximately 22,300 miles from the surface at the same rate that the Earth turns, therefore completing one revolution every 24 hours.see The Clarke circular orbit above the equator.
Geostationary Refers to a geosynchronous satellite angle with zero inclination.So the satellite appears to hover over one spot on the earth's equator.
gHz GigaHertz, equals one billion hertz
Global Beam An antenna down-link pattern used by the Intelsat satellites, which effectively covers one-third of the globe. Global beams are aimed at the center of the Atlantic, Pacific and Indian Oceans by the respective Intelsat satellites, enabling all nations on each side of the ocean to receive the signal.
GMDSS Global Maritime Distress and Safety System
GPS Global Positioning System


Half-duplex operation That mode of operation in which communication between two terminals occurs in either direction but in only one direction at a time. Contrast with duplex or simplex operation. Note: Half-duplex operation may occur on half-duplex circuits or on duplex circuits, but it may not occur on simplex circuits.

Hertz (Hz) Cycle per second; a measure of electromagnetic frequency that represents the number of complete electrical waves in a second. One kiloHertz (kHz) is one thousand cycles per second; one megaHertz (MHz) is one million; one gigaHertz (gHz) is one billion.

HGS Hughes Global Services, Inc.
High Power Amplifier (HPA). Equipment used to amplify signals.

HMSCOM Your Full Line Satellite Service Provider!

Hub A network operations center for very small aperture terminal (VSAT) operation (see also Shared Hub).
Hybrid satellite A satellite which carries two or more different communications payloads (i.e., C-band and Ku-band).


Inmarsat Established in 1979 to serve the maritime industry by developing satellite communications for ship management and distress and safety applications, Inmarsat currently operates a global satellite system which is used by independent service providers to offer an unparalleled range of voice and multimedia communications for customers on the move or in remote locations.
Inmarsat-A Inmarsat's original phone, fax and data system
Inmarsat-B Digital successor to Inmarsat-A
Inmarsat-B HSD 64 kbit/s high speed data option
Inmarsat C Store-and-forward data through briefcase terminals
Inmarsat D, D+ Global messaging and data broadcasts to pager-sized terminals
Inmarsat E Global alerting services via Inmarsat
Inmarsat mini-M Inmarsat's smallest satellite phones for voice, fax and data
Inmarsat Aero I Inmarsat's latest aeronautical satcoms service
Integrated Services Digital Network (ISDN) A network that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice services, to which users have access by a limited set of standard multi-purpose user network interfaces, as defined in the ITU-T1 series.
INTELSAT The International Telecommunications Satellite Organization operates a network of satellites for international transmissions.
Internetworking The process of interconnecting a number of individual networks to provide a path from a terminal or a host on one network to a terminal or a host on another network. The networks involved may be of the same type, or they may be of different types. However, each network is distinct, with its own addresses, internal protocols, access methods, and administration.
IRD Integrated Receiver/Decoder
IRIDIUM A LEO based satellite voice, messaging and paging system.

ISDN Integrated Services Digital Network


Ka-band A higher frequency than Ku-band, operating from 18 to 31 GHz.

k/bs Kilobit per second
Kilohertz (kHz) KiloHertz. Kilo meaning thousand and Hertz meaning cycles so that 1KHz is equivalent to 1 thousand cycles per second.
Ku-band The range of frequencies between 11 and 14 GHz used increasingly by communications satellites.


L-Band The frequency range from 0.5 to 1.5 GHz. Also used to refer to the 950 to 1450MHz used for mobile communications
LEO - Low Earth Orbit
LNA Low Noise Amplifier

Local Area Network (LAN). A data communications system that (a) lies within a limited spatial area, (b) has a specific user group, (c) has a specific topology, and (d) is not a public switched telecommunications network, but may be connected to one.


Megahertz (MHz) Refers to a frequency equal to one million Hertz, or cycles per second.
MEO Medium Earth Orbit
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.
Modem Acronym for MOdulator-DEModulator. A device that modulates and demodulates signals. Note: Modems are primarily used for converting digital signals into quasi-analog signals for transmission over analog communication channels and for reconverting the quasi-analog signals into digital signals.
Modulation The process of superimposing an information signal onto a carrier for transmission.
MPEG Motion Picture Experts Group. A proposed International Standards organization (IS) standard for digital video and audio compression for moving images.

MSS Mobile Satellite Service
Multiplex The output signal from an encoder that is divided into two phases.
Multiplexing A means of transmitting two or more signals over a single wire or carrier wave.


Narrowcasting Using the electronic media to reach a specific audience


On-line Electronic availability on demand from a computer-based system without mounting removable media such as magnetic tape or disks.
Outage A telecommunication service condition wherein a user is deprived of service due to a malfunction of the communication system.


Packet In data communication, a grouping of a sequence of binary digits, including data and control signals, that is transmitted and switched as a composite whole. The data, control signals, and possibly error control information, are arranged in a specific format. The packet can be of either fixed or variable length.
Packet mode A packet switched operational mode for transferring (transporting and switching) user information through a network without establishing a connection. The packets do not necessarily arrive at their destination in the order they were sent, unlike the circuit mode of transmission. See packet switching.

Protocol Any set of standard procedures that permit devices to intercommunicate.

Public Switched Telephone Network (PSTN) Any common carrier network that provides circuit switching among public users. Note: The term is usually applied to the public switched telephone network, but it could be applied more generally to other switched networks, e.g., packet switched public data networks.


Quadrature Phase Shift Keying (QPSK) Digital modulation scheme used in transmission communications that allows increased sending capacity.


Radio Frequency Interference (RFI) A term used to denote apparent disturbance or distortion to satellite signals.
Rain Outage Loss of signal at Ku or Ka Band frequencies due to absorption and increased sky-noise temperature caused by heavy rainfall.


Shared hub A satellite communications operations center which is shared among a number of separate network users; often used for VSAT operations.
Simplex operation That mode of operation in which communication between two points occurs in only one direction at a time. Contrast with half duplex or duplex operation.

Solid State Power Amplifier (SSPA) A high power amplifier using semiconductors to amplify the signal.
Spot beams The focusing of energy from a satellite to the ground to concentrate the limited available effective radiated power (EIRP).
Synchronous transmission Digital transmission in which the time interval between any two similar significant instants in the overall bit stream is always an integral number of unit intervals. Note: Isochronous and anisochronous are characteristics, while synchronous and asynchronous are relationships.


T1 TDM digital channel carrier (1.544 MBPS)
T3 Channel (DS-3) In North America, a digital channel which communicates at 45.304 Mbps.

Telemetry, Tracking and Control (TT&C) Means of accessing the control systems in a spacecraft in order to position and configure the spacecraft; or the facility which monitors and controls the health of a satellite.
Teleport Ground facility used to interface with a satellite during communications.

Television Receive Only (TVRO) A satellite receiving antenna, also known as a downlink or a backyard dish.
Terrestrial In communications parlance, this refers to Earth, land or ground-based telecommunications systems.
Time Division Multiple Access (TDMA) A method of accessing a satellite using time separation; sending simultaneous data streams interleaved using a clock to separate the various data packets.

Transceiver A combination transmitter and receiver.
Transponder 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.


Ultra High Frequency (UHF) The band in the 500 to 900 MHz range, including TV channels 14 through 83.
Uplink To transmit to a satellite for relay; also, the dish used to transmit.


Very High Frequency (VHF) The band in the 30 to 300 MHz range, including TV channels 2 through 13.
Very Small Aperture Terminal (VSAT) A means of narrowcast transmission of video, voice and data by SCPA means to a satellite, typically used in business applications.

Video Transmission Services (VTS) The transmission of compressed or wideband video signals.
Virtual Circuit (VC) A communication arrangement in which data from a source user may be passed to a destination user over various real circuit configurations during a single period of communication.
Virtual Private Network (VPN) A capability that allows users to create their own private information network.


WAN Wide Area Network


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Last modified: May 15, 2006