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Satellite dictionary - part 2

SENTHIL KUMAR

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SATELLITE DICTIONARY - PART 2


L


L-band- The frequency range between 0.39-1.55 GHz, also known as the 1.5 GHz band. One use is for Digital Audio Broadcasting (DAB).
LNB See Low Noise Block down-converter.

M


Mbps - Mega bits per seconds. Millions of bits per second.

MHz -Megahertz. Unit of frequency, equal to 1 million cycles per second.

Microwave -RF carrier waves with wavelengths of less than one metre and frequency above 300 MHz. Typically used to refer to frequencies above 1 GHz.

Modem -Modulator/Demodulator. The device used to translate digital signals to analogue signals for transmission over an analogue carrier.

MPEG -Moving Pictures Experts Group. An International Organisation for Standard section sub-group which develops standards (MPEG-1, 2, 4 etc) for the digital compression and multiplexing of video and audio signals.

MSO -Multiple System Operator. A company that owns and/or operates more than one cable system.


N



Network - A method of inter-connecting many points or locations in a telecommunications or data communications system.

Noise - Unwanted energy that degrades a signal. Noise is always present to some extent within any signal.

Noise Figure- Measure of the performance (noise contribution) of an LNB in dB. The lower the figure, the better.

NTSC - National Television Standard Committee. 525 line TV system established by US National Television Standards. Predominantly used in North America, Japan and the Philippines.

NVOD -Near-Video-on-Demand. Single programme/schedule with multiple daily start times.

O


Operational Life- The expected in-orbit operational life of a satellite is based on the period during which the satellite's on-board fuel permits proper station keeping manoeuvres for the satellite.

Orbital Position -Specific location of a satellite in the geostationary arc, specified in degrees, east or west longitude.

Outdoor Unit- The RF electronics located with the antenna of the VSAT.

P


PAL -Phased Alternate Line. TV system used by both British terrestrial and satellite broadcasters. Analogue standard for television transmission (mainly Europe), frame 4:3, 625 lines.

Parabolic Antenna- The most frequently found satellite TV antenna. It takes its name from the shape of the dish described mathematically as a parabola. In receiving the microwave signal the function of the parabolic shape is to focus the weak microwave signal hitting the surface of the dish to a focal point in front of the dish. It is at this point that the feedhorn is located.

Payload- Supports the primary mission of the satellite, the receipt and transmission of signals, and comprises systems that include receivers, multiplexers, high power amplifiers and signal processing.

Pay-per-View- Programming services which are paid for by subscribers on the basis of the number of hours or programmes watched rather than through a straight subscription fee.

Point-to-Multipoint- A network configuration in which a major hub is connected to many subsidiary nodes. Also called a star network.

Polarisation - A technique used by the satellite designer to increase the capacity of the satellite transmission channels by reusing the satellite transponder frequencies. In linear cross polarisation schemes, half of the transponders beam their signals to the Earth in a vertically polarised mode, the other half are horizontally polarised. Although the two sets of frequencies overlap, the polarisation separation is sufficient to ensure they do not interfere with each other. To successfully receive and decode these signals on the Earth, the earth station must be outfitted with a properly polarised feedhorn to select the vertically or horizontally polarised signals as desired.

Protocol - A set of rules or conventions which governs a data communications system.

Q



QAM- Quadrature Amplitude Modulation. A method of combining two Amplitude Modulated (AM) signals - each having the same frequency, but differing in phase by 90 degrees, into a single channel, thereby doubling the effective bandwidth.

QPSK - Quadrature Phase Shift Keying. A digital modulation technique in which the carrier phase take s on one of four possible values.

R



Radio Frequency- A frequency that is higher than the audio frequency but below the infrared frequencies.

Rain Outage- Loss of signal at Ku or Ka band frequencies due to absorption and increased sky-noise temperature caused by heavy rainfall.

S


S-band - The frequency range between 1.55-2.5 GHz, also known as the 2.5 GHz band. Typically used for DBS (community reception). Not in common use.

Scintillation - Signal fading due to atmospheric effects that focuses and defocuses the radio waves. Can last for tens of seconds or occasionally minutes. Highly variable with time of year and elevation angle of the earth station.

Shared Hub - A satellite communications operations centre that is shared among a number of separate network users, often used for VSAT operations.

Signal -A physical, time-dependent energy value used for the purpose of conveying information through a transmission link.

Signal to Noise Ratio - The ratio of the signal power and noise power. A video S/N of 54 to 56 dB is considered to be an excellent S/N, that is, of broadcast quality. A video S/N of 48 to 52 dB is considered to be a good S/N at the head-end for cable TV.

Smart Card -Personalised credit card for use in satellite receivers with integrated decoders, for authorisation of subscribed pay TV channels.

Spectrum -The range of electromagnetic radio frequencies used in transmission of voice, data and television.

Spot Beam - A focused antenna pattern sent to a limited geographical area. Spot beams are used by domestic satellites to deliver certain transponder signals to geographically well defined areas.

Streaming - A technique for transferring data such that it can be processed as a steady and continuous stream. Streaming technologies are becoming increasingly important with the growth of the Internet because most users do not have fast enough access to download large multimedia files quickly. With streaming, the client browser or plug-in can start displaying the data before the entire file has been transmitted.


Sun Outage - Sun Outages occur when an antenna is looking at a satellite, and the sun passes behind the satellite and within the field of view of the antenna, the sun's energy momentarily interferes with the satellite signals. Occurs for two periods each year during the spring and fall equinox.

Switch -A device that opens or closes circuits or selects the paths or circuits to be used for transmission of information; switching is the process of interconnecting circuits to form a transmission path between users.

T



TCP/IP - Transmission Control Protocol/Internet Protocol. The suite of communications protocols used to connect hosts on the Internet. TCP/IP uses several protocols, the two main ones being TCP and IP. TCP/IP is built into the UNIX operating system and is used by the Internet, making it the de facto standard for transmitting data over networks. Even network operating systems that have their own protocols, such as Netware, support TCP/IP.

Terrestrial Broadcasters - Broadcasters who transmit through the airwaves from one earth-bound aerial to another.


Tracking - An earth station feature that allows for following the motion of inclined satellites.


Transponder - Equipment in a satellite which receives a single uplinked channel from a satellite earth station; amplifies it, converts the frequency and changes the polarisation; then transmits it back to the Earth in a given power (EIRP).


U


UHF -Ultra High Frequency. The band in the 300 to 3000 MHz range.

Uplink -In satellite communications, the signal from the earth station to the satellite.

V


V-band - Also known as the 40 GHz band. Reserved for future use.

VHF -Very High Frequency. The band in the 30 to 300 MHz range.

Video Compression - Data reduction and compression of an analogue television signals into a digital stream.

VOD - Video-On-Demand. System that allows the viewer to see or choose a programme at the time the viewer specifies.

VSAT -Very Small Aperture Terminal. Refers to small earth stations, usually 1.8 metre to 4 metre in diameter in C-band. A VSAT system is a satellite communications system that is typically used by corporate private networks or in rural and remote areas. A VSAT system consists of an antenna and the associated electronics. VSAT networks are widely used by retail business for verifying credit cards. VSAT systems can be used for voice, data and video. Small aperture terminals under 0.5 metre are sometimes referred to Ultra Small Aperture Terminals (USATs).

W


Web casting - A metallic microwave conductor, commonly rectangular in shape, used to carry microwave signals into and out of microwave antennas.
Webcasting Using the Internet, and the World Wide Web in particular, to broadcast information. Unlike typical surfing, which relies on a pull method of transferring Web pages, webcasting uses push technologies.

X


X-band - The frequency range between 7.25-7.75 and 7.9-8.4 GHz, also known as the 7/8 GHz band. Typically used for military satellite communications.
XIPS Xenon Ion Propulsion System. A propulsion system on satellites that uses charged particles and electromagnetism to generate thrust for satellite stationkeeping. XIPS is significantly more efficient than other propulsion systems and lessens the weight of the satellite, thus reducing launch costs.
 
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