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Satellite dictionary - part 1

SENTHIL KUMAR

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SATELLITE DICTIONARY – PART 1



A


Access- The method of channel sharing or utilisation of a single frequency or time slot by several locations.

Amplifier- A device used to increase the strength of electrical signals.

Analogue -A form of storing, processing or transmitting information through a continuous variable (rather than pulsed).

Antenna- Device which picks up and delivers satellite signals to a receiver, most commonly a dish.

Aperture- A cross sectional area of the antenna which is exposed to the satellite signal.

Attenuation -A decrease in the power of a received signal due to loss through lines, equipment or other transmission devices. Usually measured in dB.

Attitude -Control Maintenance of the satellite's orientation with respect to the Earth and the sun.

Automatic Gain Control- A circuit which automatically controls the gain of an amplifier so that the output signal level is virtually constant for varying input signal levels.

Azimuth -The horizontal angle for the antenna to point to the satellite. It is in clockwise direction from the true north.

B


Bandwidth -A range of frequencies, expressed in Hz occupied by a modulated carrier on the range of frequencies which can be transmitted through a communications system. Bandwidth is one measure of the information carrying capacity of a transponder. The wider the bandwidth, the more information which can be transmitted. The bandwidth of the communications system must be at least as wide as the signal being transmitted.

Baseband -The frequency band which contains the basic, low frequency information before modulation and after demodulation.

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.

Beam -The directed electromagnetic rays emanating from the spacecraft. Typically refers to aggregates of these rays such as China (coverage) beam or global (coverage) beam.

Bit -A single unit of information. Often referred to as a 1 or 0 in the binary system and as an "on" or "off" state in computer operations.

Bit Error Rate- The fraction of a sequence of message bits that are in error. It denotes the quality of a received demodulated digital signal. The lower the rate, the better the signal e.g. a BER of 10-4 means one error in every 10,000 bits.

Bit Rate -Amount of digital information transmitted in a certain period of time, expressed in bits per second (bps).
bps Bits Per Second..

BPSK -Binary Phase Shift Keying.

Broadband -A term used to refer to high-speed communications networks that are designed to handle bandwidth-intensive applications.

Bus -The section of the satellite with components to support tracking, telemetry and control (TT&C), power systems, propulsion and control of the spacecraft.

C



Carrier Frequency- Frequency of the carrier wave that is modulated to transmit signals.

Carrier to Noise Ratio- 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.

C-band- The frequency range between 3.4-4.2, 4.5-4.8 and 5.85-7.075 GHz, also known as the 4/6 GHz band. Typically used for television broadcast and telecommunications services.

Circular Polarisation - The polarisation of the electromagnetic wave varies in time with the electrical component (e-vector) tracing out a circle. Used in the old Intelsat system. Rare in domestic or regional fixed satellite services.

Clarke Belt - Named after its founder Arthur C. Clarke, the Clarke Belt is an orbit used by satellites at a height of about 36000 km, in which satellites make an orbit in 24 hours yet remain in a fixed position relative to the Earth's surface.

Co-ax- Co-axial cable and connectors that are commonly used as aerial and antenna cables.

Codec -Coder/Decoder, a device used to convert analogue voice signals to digital signals.

Conditional Access- The system which allows the control of a user's access to pay services and services protected for copyright reasons.

D


dB -Decibel. A relative unit of measurement used frequently in electronic communications to describe power gain or loss. It is used to specify measured and calculated values in audio systems, microwave system gain calculations, satellite system link-budget analysis, antenna power gain and in many other communications system measurements.

dBi -The dB power relative to an isotropic source.


DBS Band - BSS band in ITU terminology. Signal frequency range between 11.70-12.20 GHz intended for direct TV broadcast of satellite channels. This band is designated for use in ITU Region 3 (Asia).

dBW - Decibel relative to one Watt. A logarithmic measure of the satellite's power e.g. 50 dBW is twice as powerful as 47 dBW.

Decoder - Unit that is connected to a satellite receiver in order to unscramble a service that is protected by encryption. In the case of digital reception, the decoder is integrated in the receiver, which is called IRD.
Demodulation The process of extracting the original signal from a modulated carrier.

Demodulator- Section of a satellite receiver designed to extract the audio and video information from an incoming signal.

DTH -Direct-to-Home. A satellite service that delivers television programming directly to consumer homes using a small antenna and related equipment. Satellites that provide DTH services are specialised to operate in a different frequency to allow for very small dishes.

DVB -Digital Video Broadcasting standard. A group of over 200 organisations from 23 countries which developed system specifications for the transmission of MPEG-2 digital signals by satellite, cable and terrestrial links. These specifications were passed to the European Telecommunications Standards Institute to form an ETSI standard.

E



Earth Station- The antennas, receivers, transmitters and other equipments needed on the ground to transmit and receive satellite communications signals.
Eclipse -Occurs when the satellite's solar arrays are in the Earth's shadow.

Edge of Coverage- Limit of a satellite's defined service area. A satellite's beam may be shaped for specific coverage or the service area may be geographically limited by the need to have a minimum elevation angle to the satellite.

EIRP -Effective Isotropic Radiated Power. Measures the effect of focusing the satellite's energy in a particular area as compared to transmitting uniformly in all directions.

elevation -The vertical angle for which the antenna moves upward from the horizon, pointing to the satellite.

Encryption- Systematic modification of a signal to prevent unauthorised use.

EOL -End of Life of a satellite.

Equalisation The technique of compensating for differences in attenuation of a signal at different frequencies.


F
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F/D- Ratio of antenna focal length to antenna diameter. A higher ratio means a shallower dish.

FEC -Forward Error Correction. A method of coding which inserts additional bits in the transmission that is used detect and correct transmission errors.

Feed -This term has at least two key meanings within the field of satellite communications. It is used to describe the transmission of video programming from a distribution centre. It is also used to describe the feed system of an antenna. The feed system may consist of a subreflector plus a feedhorn or a feedhorn only.

Feedhorn -Device which collects signals at the focus of the antenna and channels them to the LNB.

Focal Length- Distance from the centre feed to the centre of the dish.

Focal Point- The area toward which the primary reflector directs and concentrates the signal received.

Footprint- The geographic area covered by a satellite. The outer edge of which is generally defined as the area beyond which the quality of communications degrades below an acceptable commercial level.

Free-to-air Services- Services which do not require any payments or any special decoders to receive.

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G



Gain- A measure of amplification expressed in dB.

Geostationary Orbit- An orbit at 35,786 km directly over the Earth's equator in which the orbital inclination and eccentricity are both near zero such that the satellite appears to hover over a specific portion of the Earth's equator.
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Geostationary Satellite- A satellite orbiting along the geostationary orbit. From the Earth, a geostationary satellite always appears to be in the same location because it finishes one rotation around the Earth in 24 hours.
Geosynchronous Orbit An orbit 35,786 km above the Earth's surface where satellites circle at the same rate as the Earth's rotation.

GHz -Gigahertz. One billion cycles per second.

Global Beam- An antenna downlink pattern used by satellites, which effectively covers about one-third of the globe.

GPS -Global Positioning System. A satellite system that provides precise reference to the location of a point on the Earth. GPS satellite systems employ lower orbiting satellites than geostationary satellite networks.


H



HDTV -High Definition Television. Offers approximately twice the vertical and horizontal resolution of current NTSC analogue television broadcasting and supports sound quality approaching that of a CD.

Hertz -Unit of frequency, equal to one cycle per second (after Heinrich Hertz).

HPA- High Power Amplifier.

Hub -The point on a network where circuits are connected or a network operations centre for VSAT operations.


I



Inclination-The angle between the orbital plane of a satellite and the equatorial plane of the Earth.

Integrated Receiver/Decoder- Set-top-box used for the reception and descrambling of signals. In the case of digital reception, the decoder is integrated with the satellite receiver.

Interference -Unwanted radio frequency energy that tends to interfere with the reception of the desired signals. May come from adjacent channels, adjacent networks or sources local to the earth station.

IP -Internet Protocol. The method by which data is sent from one computer to another on the Internet. Each computer on the Internet has at least one IP address that uniquely identifies it from all other computers on the Internet. When you send or receive data e.g. an e-mail note or a Web page, the message gets divided into little chunks called packets. Each of these packets contains both the sender's Internet address and the receiver's address.

K



Ka-band- The frequency range between 17.7-20.2 and 27.5-30.0 GHz, also known as the 20/30 GHz band. Planned to be used for HDTV.

kHz -Kilohertz. Unit of frequency, equal to 1000 cycles per second.

Ku-band- The frequency range between 10.7-13.25 and 14.0-14.5 GHz, also known as the 11/14 and 12/14 GHz band. Multiple uses in various regions as designated by the ITU.
 
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