AAL

ATM Adaptation Layer

AOS

Acquisition of Signal. The time at which a radio signal is first heard from a satellite, usually just after it rises above the horizon.

ACSS

Access Control Sub-System

ADSL

Asymmetric Digital Subscriber Line

Amplifier

A device used to boost the strength of an electronic signal.

AM

Amplitude Modulation. The base-band signal is caused to vary the amplitude or height of the carrier wave to create the desired information content.

Analogue

Transmission method which uses a variable and continuous voltage to represent variations in the same quantity. Analogue is a form of transmitting information characterized by continuously variable quantities, as opposed to digital transmission, which is characterized by discrete bits of information in numerical steps. An analogue signal is responsive to changes in light, sound, heat and pressure.

ADC

Analogue-to-Digital Conversion. Process of converting analogue signals to a digital representation. DAC = Digital-to-Analogue Conversion represents the reverse translation.

Anomalistic Period

The elapsed time between two successive perigees of a satellite.

Antenna

A device that concentrates a beam of electromagnetic waves to a focal point so as to send/receive radio signals. Depending on their use and operating frequency, antennas can take the form of a single piece of wire, a di-pole a grid such as a yagi array, a horn, a helix, a sophisticated parabolic-shaped dish, or a phase array of active electronic elements of virtually any flat or convoluted surface.

Aperture

The effective cross sectional area of the antenna which is exposed to the satellite signal. The larger the aperture the stronger the signal the antenna receives or transmits.

Apogee

The point in an elliptical satellite orbit which is farthest from the surface of the earth. Geo-synchronous satellites which maintain circular orbits around the earth are first launched into highly elliptical orbits with apogees of 22,237 miles. When the communication satellite reaches the appropriate apogee, a rocket motor is fired to place the satellite into its permanent circular orbit at this distance.

Argument of Perigee

The polar angle that locates the perigee point of a satellite in the orbital plane; drawn between the ascending node, geo-centre, and perigee; and measured from the ascending node in the direction of satellite motion.

Ascending Node

The point on the ground track of a satellite orbit where the sub-satellite point(SSP) crosses the equator from the Southern Hemisphere into the Northern Hemisphere.

ASTRA

The provider of Satellites and capacity for TV distribution and data communications.

ASTRA 1H

19.2° East, other ASTRA satellites on 28.2, 24.2/23.5, 5.2° East

ATM

Asynchronous Transfer Mode

Attenuation

The loss in power of electromagnetic signals between transmission and reception points.

Attitude Control

The orientation of the satellite in relationship to the earth and the sun.

Audio Subcarrier

The carrier between 5 MHz and 8 MHz containing audio (or voice) information inside of a video carrier.

AZ/EL Mount

Antenna mount that requires two separate adjustments to move from one satellite to another;

Azimuth

The angle of rotation (horizontal) that a ground based parabolic antenna must be rotated through to point to a specific satellite in a geosynchronous orbit. The azimuth angle for any particular satellite can be determined for any point on the surface of the earth giver the latitude and longitude of that point. It is defined with respect to due north as a matter of easy convenience.

Backbone

The central part of a large network that links two or more subnetworks and is the primary path for data transmission for a large business or corporation. A network can have a wired backbone or a wireless backbone

Backhaul

A terrestrial communications channel linking an earth station to a local switching network or population centre.

Backup network

 Alternative secondary emergency network in case the main primary network fails. For example when a terrestrial network fails the VSAT network is automatically implemented as the backup network.

Bandwidth

The range of frequencies expressed in Hertz that can pass over a given transmission channel. This determines the rate at which information can be transmitted through that channel. For instance, a voice transmission by telephone requires a bandwidth of about 3000 cycles per second (3KHz). A TV channel occupies a bandwidth of 6 million cycles per second (6 MHz) in terrestrial Systems. In satellite based systems a larger bandwidth of 17.5 to 72 MHz is used to spread or "dither" the television signal in order to prevent interference.

Baud

The rate of data transmission based on the number of signal elements or symbols transmitted per second. Today most digital signals are characterized in bits per second.

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. This tracking beacon is usually a horn or omni antenna.

Beamwidth

The angular width of the main lobe of an antenna radiation pattern as measured between predetermined points on the lobe, usually the half-power (-3 dB) points. Large antennas have narrower beamwidths and can pinpoint satellites in space or dense traffic areas on the earth more precisely. Tighter beamwidths thus deliver higher levels of power and thus greater communications performance.

BER

Bit Error Rate.

Bird

Slang for a communications satellite located in geo-synchronous orbit.

Bit Error Rate (BER)

The fraction of a sequence of message bits that are in error. A bit error rate of 10-6 means that there is an average of one error per million bits.

Bit Rate

The speed of a digital transmission, measured in bits per second.

Block Down Converter

A device used to convert the 3.7 to 4.2 KHz signal down to UHF or lower frequencies (1 GHz and lower).

Broad beam

A single large circular beam that covers a large geographic area

Broadcast

The sending of one transmission to multiple users in a defined group (compare to unicast).  Transmissions through space utilizing pre-assigned radio frequencies typically for television radio or data.

BUC

Block Up-Converter. A BUC is used to transmit the signals from a VSAT back to the satellite.

C Band

This is the band between 4 and 8 GHz with the 6 and 4 GHz band being used for satellite communications. Specifically, the 3.7 to 4.2 GHz satellite communication band is used as the downlink frequencies in tandem with the 5.925 to 6,425 GHz band that serves as the uplink.

Capacity

 Maximum amount of traffic that a circuit or circuit group can handle.

Carrier

The basic radio, television, or telephony centre of frequency transmit signal. The carrier in an analogue signal is modulated by manipulating its amplitude (making it louder or softer) or its frequency (shifting it up or down) in relation to the incoming signal. Satellite carriers operating in the analogue mode are usually frequency modulated.

Carrier Frequency

The main frequency on which a voice, data, or video signal is sent. Microwave and satellite communications transmitters operate in the band from 1 to 14 GHz (a GHz is one billion cycles per second).

C/N

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.

Cassegrain Antenna

The antenna principle that utilizes a sub-reflector at the focal point which reflects energy to or from a feed located at the apex of the main reflector.

C-band

 Frequency band in the 4-to-6 GHz range. IT is susceptible to terrestrial microwave interference C which is a particular problem in urban areas of worldwide coverage C and is less susceptible to rain and weather than Ku-band.

CBR

Constant Bit Rate

CDMA

Code division multiple access. Refers to a multiple-access scheme where stations use spread-spectrum modulations and orthogonal codes to avoid interfering with one another.

CF-DAMA

Combined Free Demand Assignment Multiple Access. An access scheme for transmitting signals from a VSAT to the satellite.

Channel

A frequency band in which a specific broadcast signal is transmitted. Channel frequencies are specified in the United States by the Federal Communications Commission. Television signals require a 6 MHz frequency band to carry all the necessary picture detail.

Circular Polarization

A special method of radio energy emission where the electric and magnetic field vectors rotate about the central axis of radiation. As viewed along the radiation path, the rotation directions are considered to be right-hand-circular (RHCP) if the rotation is clockwise and left-hand-circular (LHCP) if the rotation is counter-clockwise. On some satellites, both right-hand rotating and left-hand rotating signals can be transmitted simultaneously on the same frequency; thereby doubling the capacity of the satellite to carry communications channels.

Clarke Orbit

That circular orbit in space 22,237 miles from the surface of the earth at which geo-synchronous 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 travelling 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 travelling around the earth. It is also referred to as "geo-stationary orbit".

Co-Location

Ability of multiple satellites to share the same approximate geo-stationary orbital assignment frequently due to the fact that different frequency bands are used.

Coverage

Coverage or Footprint is an area on the earths surface where effective transmission/reception to/from the satellite is possible. The contours of the footprint are usually specified by points at half the maximum power.

CRA

Collision Reduction Application (a proprietary optimisation of a VSAT inbound access scheme) or Continuous Rate Assignment.

Cross Modulation

A form of signal distortion in which modulation from one or more RF carrier(s) is imposed on another carrier.

DA

 Dedicated Access. An inbound transmission mode used when a remote VSAT requires a higher level of throughput for a continuous volume of traffic, such as in a file transfer.

DAMA

Demand-Assigned Multiple Access - An access scheme for transmitting signals from a VSAT to the satellite. DAMA is a highly efficient means of instantaneously assigning telephony channels in a transponder according to traffic as it allows many users to access the same channel on demand.

dBi

The dB power relative to an isotropic source.

DBS

Direct Broadcast Satellite. Refers to service that uses satellites to broadcast multiple channels of television programming directly to home mounted small-dish antennas.

dBW

The ratio of the power to one Watt expressed in decibels.

Decibel (dB)

The standard unit used to express the ratio of two power levels. It is used in communications to express either a gain or loss in power between the input and output devices.

Declination

The offset angle of an antenna from the axis of its polar mount as measured in the meridian plane between the equatorial plane and the antenna main beam.

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.

Demodulator

A satellite receiver circuit which extracts or "demodulates" the "wanted "signals from the received carrier.

Descending Node

The point on the ground track of the satellite orbit where the sub-satellite point (SSP) crosses the equator from the Northern Hemisphere into the Southern Hemisphere.

Deviation

The modulation level of an FM signal determined by the amount of frequency shift from the frequency of the main carrier.

DHCP

Dynamic Host Configuration Protocol. A means of automatically assigning an IP address to a device (eg, PC) on a network as it comes alive.

Digital

Conversion of information into bits (represented by 1 or 0) of data for transmission through wire, fibre optic cable, satellite, or over the air techniques. Digital transmission allows simultaneous communication of voice, data or video.

DAC

Digital-to-Analogue Conversion. Process of converting digital signals to analogue representation. ADC = Analogue-to-Digital Conversion is the reverse translation.

Dish

Common name for satellite antenna as they are often concave shaped like a dish.

Double hop

Transmission of data from terminal to terminal in two stages, for example, from one VSAT to another via a satellite link. This is the only form of communication between terminals in a star network.

Downlink

The satellite to earth half of a two-way telecommunications satellite link. Often used to describe the receive dish end of the link. Sometime used interchangeable with "Outbound" which describes the direction of the signal from the hub to the VSAT from the point of view of the hub.

Dual Spin

Spacecraft design whereby the main body of the satellite is spun to provide altitude stabilization, and the antenna assembly is de-spun by means of a motor and bearing system in order to continually direct the antenna earthward. This dual-spin configuration thus serves to create a spin-stabilized satellite.

Duplex Transmission

Capability for simultaneous data transmission between a sending station and a receiving station.

DVB

Digital Video Broadcast. The standard used for digital video transmission.

DVB-RCS

Digital Video Broadcast - Return Channel System. The standard used for the return channel of a two-way communication over satellite.

DVB-S

Digital Video Broadcast - Satellite. The standard used for outbound digital signal transmission (broadcast) over satellite.

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 65 cm to 3.7 m diameter size used for TV reception to as large as 30 meters in diameter sometimes used for international communications.

Eccentricity

The orbital parameter used to describe the geometric shape of an elliptical orbit; eccentricity values vary from (e = 0) to (e = 1), where (0) describes a circle and (1) describes a straight line

Echo Effect

A time-delayed electronic reflection of a speaker's voice. Echo is largely eliminated by modern digital echo cancellers.

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 centre. However, reception may still be possible beyond the -3dB point.

EIRP

Effective Isotropic Radiated Power - This term describes the strength of the signal leaving the satellite antenna or the transmitting earth station antenna, and is used in determining the C/N and S/N. The transmit power value in units of dBW is expressed by the product of the transponder output power and the gain of the satellite transmit antenna.

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.

Elliptical Orbit

Those orbits in which the satellite path describes an ellipse with the Earth at one focus.

Epoch

The reference time at which a particular set of parameters describing satellite motion (Keplerian elements) are defined.

Equatorial Orbit

An orbit with a plane parallel to the earth's equator.

F/D

Ratio of antenna focal length to antenna diameter. A higher ratio means a shallower dish.

Faraday Rotation

Rotation of the plane of polarization of an electromagnetic wave when travelling through a magnetic field. In space communications, this effect occurs when signals traverse the ionosphere.

FCA

Free Capacity Assignment. A capacity optimisation access scheme used on the inbound from a VSAT back to the hub.

FDMA

Frequency Division Multiple Access. Refers to the use of multiple carriers within the same transponder where each uplink has been assigned frequency slot and bandwidth. This is usually employed in conjunction with Frequency Modulation. Multiple access is achieved through frequency band discrimination; either in the ground receiver, in the case of a wide band frequency translating repeater, or in the satellite, in the case of a frequency channelled satellite.

FEC

Forward Error Correction

Feed

A device (feed horn) mounted at the focal point of antenna that emits signals into an antenna. It is also used to describe the feed system of an antenna.

FLSS

Forward Link Sub-System. A system of components within a hub.

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

A map of the signal strength showing the EIRP contours of equal signal strengths as they cover the earth's surface. Different satellite transponders on the same satellite will often have different footprints of the signal strength. The accuracy of EIRP footprints or contour data can improve with the operational age of the satellite. The actual EIRP levels of the satellite, however, tends to decrease slowly as the spacecraft ages.

Forward Error
Correction (FEC)

Adds unique codes to the digital signal at the source so errors can be detected and corrected at the receiver.

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.

FM

Frequency Modulation. A modulation method whereby the base-band signal varies the frequency of the carrier wave.

FTDMA

 Frequency and Time Division Multiple Access.

G/T

Antenna Gain to System Noise Temperature Ratio. A figure of merit of an antenna and low noise amplifier combination expressed in dB. "G" is the net gain of the system and "T" is the noise temperature of the system. The higher the number, the better the system.

Gain

A measure of amplification expressed in dB.

Geo-stationary

Refers to a geo-synchronous satellite angle with zero inclination. so the satellite appears to hover over one spot on the earth's equator. See Clarke Orbit.

Gigahertz (GHz)

One billion cycles per second. Signals operating above 3 Gigahertz are known as microwaves. above 30 GHz they are know as millimetre waves. As one moves above the millimetre waves signals begin to take on the characteristics of light-waves.

Gregorian

 A dual-reflector antenna system employing a parabolic main reflector and a concave ellipsoidal sub-reflector.

Ground Station

A radio station, on or near the surface of the Earth, designed to transmit or receive to/from a spacecraft.

Ground-track

The imaginary line traced on the surface of the Earth by the sub-satellite point (SSP).

Guard Channel

High frequency channels are separated in the frequency spectrum by spacing them several megahertz apart. This unused space serves to prevent the adjacent channels from interfering with each other.

Hertz (Hz)

The name given to the basic measure of radio frequency characteristics. An electromagnetic wave completes a full oscillation from its positive to its negative pole and back again in what is known as a cycle. A single Hertz is thus equal to one cycle per second.

HPA

High Power Amplifier

HSP

 Hub Satellite Processor.

Hub

The satellite central earth station through which all traffic is routed in a star configuration VSAT network. The hub is the master station through which all communications to, from and between VSATs must flow.

IB

Inbound. Refers to the path the signal takes as it is transmitted from the remote VSAT via the satellite to the hub. Inbound from the point of view of the hub.

IDU

 InDoor Unit. A satellite modem that is located at each remote VSAT site and connects the users PC or network to the hub via the satellite link.

IFL

InterFacility Link. The connection between the ODU (antenna, LNB and transmitter) and the IDU (satellite modem)

Inclination

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

Increment

The change in longitude of the ascending node between two successive passes of a specified satellite, measured in degrees West per orbit.

Interference

Energy which tends to interfere with the reception of the desired signals, such as fading from airline flights, RF interference from adjacent channels, or ghosting from reflecting objects such as mountains and buildings.

IPA

Internet Page Accelerator. The name given to a proprietary HTTP acceleration service.

IRD

An integrated receiver and decoder for reception of a transmission of voice, video and data.

Isotropic Antenna

A hypothetical omni-directional point-source antenna that serves as an engineering reference for the measurement of antenna gain.

Jammer

An active electronic counter-measures (ECM) device designed to deny intelligence to unfriendly detectors or to disrupt communications.

Ka Band

The frequency range from 18 to 31 GHz.

Keplerian Elements

The classical set of six orbital element numbers used to define and compute satellite orbital motions. The set is comprised of inclination, Right Ascension of the Ascending Node (RAAN), eccentricity, argument of perigee, mean anomaly, and mean motion, all specified at a particular epoch or reference year, day, and time. Additionally, a decay rate or drag factor is usually included to refine the computation.

Klystron

A microwave tube which uses the interaction between an electron beam and the RF energy on microwave cavities to provide signal amplification. The klystron operates on principles of velocity modulation very similar to those in a TWT except that klystron interaction takes place at discrete locations along the electron beam. Common types of klystrons are the reflex klystron (an oscillator having only one cavity), two-cavity klystron amplifiers and oscillators, and multi-cavity klystron amplifiers.

Ku Band

The frequency range from 10.9 to 17 GHz.

LAN

Local Area Network. A system of connecting PCs and other devices within the same physical proximity for sharing resources such as an Internet connections, printers, files and drives. When Wi-Fi is used to connect the devices, the system is known as a wireless LAN or WLAN.

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.

LHCP

Left Hand Circular Polarisation

LNB

Low Noise Block Converter

LNA

Low Noise Amplifier. This is the preamplifier between the antenna and the earth station receiver. For maximum effectiveness, it must be located as near the antenna as possible, and is usually attached directly to the antenna receive port. The LNA is especially designed to contribute the least amount of thermal noise to the received signal.

Low Noise Block
Down-converter

A combination Low Noise Amplifier and down-converter built into one device attached to the feed.

Margin

The amount of signal in dB by which the satellite system exceeds the minimum levels required for operation.

MCD

Multi Carrier Demodulator

Mean Anomaly

An angle that increases uniformly with time, starting at perigee. Used to indicate where a satellite is located along its orbit. Mean Anomaly is usually specified at the reference epoch time where the Keplerian Elements are defined.

Mean Motion

The Keplerian Element that indicates the complete number of orbits a satellite makes in a day.

Mesh network

A type of network where all points can directly communicate with each other in a single hop.

MF-TDMA

Multi-Frequency Time-Division Multiple Access

MGT

Multicast Group Table

Microwave

Line-of sight, point-to-point transmission of signals at high frequency. Microwaves are also used for data, voice, and indeed all types of information transmission.

Microwave Interference

Interference which occurs when an earth station aimed at a distant satellite picks up a second, often stronger signal, from a local telephone terrestrial microwave relay transmitter. Microwave interference can also be produced by nearby radar transmitters as well as the sun itself. Relocating the antenna by only several feet will often completely eliminate the microwave interference.

MMT

Multicast Mapping Table

Modem

 Modulator/Demodulator - accepts data from a computer or terminal device in the form of a digital signal and transforms the data into a form suitable for transmission over a communication facility.

Modulation

The process of manipulating the frequency or amplitude of a carrier in relation to an incoming video, voice or data signal.

Modulator

A device which modulates a carrier. Modulators are found as components in broadcasting transmitters and in satellite transponders.

Multicast

Multicast is a subset of broadcast that extends the broadcast concept of one to many by allowing the sending of one transmission to many users in a defined group, but not necessarily to all users in that group.

Multiple access

 Access to a transponder by more than one user. Transponders can be accessed in three ways, by frequency, time and code.

Multiplexing

A technique that combines many simultaneous transmissions over a single circuit in order to accommodate multiple users or applications.

NOC

Network Operation Centre. The centre at which the hub and satellite network is monitored, managed and controlled.

Nodal Period

The amount of time difference between two successive ascending nodes of a satellite orbit.

Noise

Any unwanted and un-modulated energy that is always present to some extent within any signal.

NF

Noise Figure. A term which is a figure of merit of a device, such as an LNA or receiver, expressed in dB, which compares the device with a perfect device.

OB

Outbound. Refers to the path the signal takes as it is transmitted from the hub via the satellite to the remote VSAT. Outbound from the point of view of the hub.

ODU

 Outdoor Unit. Comprises all components of a VSAT that are stationed outdoors. Includes antenna, mount, feed-arm, transmitter (HPC, SSPA or BUC), receiver (LNB) and cabling (IFL).

Orbital Period

The time that it takes a satellite to complete one circumnavigation of its orbit.

Orbital Plane

An imaginary plane, extending throughout space, that contains the satellite orbit.

orbital position

Position of geo-stationary satellites

Pass-band

The range of frequencies handled by a satellite translator or transponder.

PEP

Performance Enhancing Proxy. A PEP is used to optimise the TCP protocol to enhance its performance over the satellite link.

Perigee

The point in a satellite's orbit where it is closest to Earth.

Polar Mount

Antenna mechanism permitting steering in both elevation and azimuth through rotation about a single axis. While an astronomer's polar mount has its axis parallel to that of the earth, satellite earth stations utilize a modified polar mount geometry that incorporates a declination offset.

Polar Orbit

An orbit with its plane aligned in parallel with the polar axis of the earth

Polarization

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 polarization schemes, half of the transponders beam their signals to earth in a vertically polarized mode; the other half horizontally polarize their down links. Although the two sets of frequencies overlap, they are 90 degree out of phase, and will not interfere with each other. To successfully receive and decode these signals on earth, the earth station must be fitted with a properly polarized feed-horn to select the vertically or horizontally polarized signals as desired.

Polarization Rotator

A device that can be manually or automatically adjusted to select one of two orthogonal polarizations.

POS

 Point of Sale.

PPPoE

Point to Point Protocol over Ethernet

QOS

 Quality of Service - network traffic management technology for controlling throughput and priorities for specified applications C ports and customers.

QPSK

Quadrature Phase Shift Keying. A system of modulating a satellite signal. In this modulation four phases of the radio frequency carrier, each differing each other by 90° degrees, are employed to represent two bits of the data stream.

Rain Outage

Loss of signal due to absorption and increased sky-noise temperature caused by heavy rainfall.

Receiver (Rx)

An electronic device which enables a particular satellite signal to be separated from all others being received by an earth station, and converts the signal format into a format for video, voice or data.

Receiver Sensitivity

Expressed in dBm this tells how much power the detector must receive to achieve a specific base-band performance, such as a specified bit error rate or signal to noise ratio.

Repeater

Equipment which receives, amplifies, and retransmits signals in order to maintain adequate signal strength and intelligence characteristics.

RF  

Radio Frequency

RLSS

Return Link Subsystem. A system of components within a hub.

Router

Network layer device that determines the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information.

Satellite

A sophisticated electronic communications relay station moving in a fixed orbit at the same speed and direction of the earth that receives radio communication signals from earth stations and retransmits them to other earth stations at distant locations.

Satellite access

Allows sharing of bandwidths among multiple scheme users. Access methods increase the satellite's information-carrying capacity to reduce satellite idle time and spread costs among multiple users. Among the most commonly used access methods: FDMA, TDMA, CDMA and sophisticated versions of ALOHA.

Satellite broadband

A wireless high-speed Internet connection provided by satellites. Some satellite broadband connections (VSATs) are two-way up and down. Others are one-way, with the satellite providing a high-speed downlink and then using a dial-up telephone connection or other land-based system for the uplink to the Internet.

Satellite delay

 Time taken for transmissions to reach the satellite and return to earth.

Satellite hop

 Transmission of information from an earth station to a satellite and retransmission back to another earth station.

Satellite Pass

Segment of orbit during which the satellite "passes" nearby and in range of a particular ground station.

SCPC

Single Channel per Carrier

Sidelobe

Off-axis response of an antenna.

Sidereal Day

The amount of time required for the Earth to rotate exactly 360 degrees about its axis with respect to the "fixed" stars. The Sidereal Day contains 1436.07 minutes (see Solar Day).

Signal to Noise Ratio (S/N)

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.

Simplex Transmission

Capability for transmission in only one direction between sending station and receiving station.

Single hop

Transmission of information from terminal to terminal in one stage via a satellite link. For example, in a mesh network.

SIT

Satellite Interactive Terminal. Another name for an IDU or satellite modem.

Skew

An adjustment that compensates for slight variance in angle between identical senses of polarity generated by two or more satellites.

Slant Range

The length of the path between a communications satellite and an associated earth station.

Slot

That longitudinal position in the geo-synchronous orbit into which a communications satellite is "parked".

Solar Day

The Solar Day contains exactly 24 hours (1440 minutes). During the Solar Day, the Earth rotates slightly more than 360 degrees (see Sidereal Day).

Solar Outage

Solar outages occur when an antenna is looking at a satellite, and the sun passes behind or near the satellite and within the field of view of the antenna. This field of view is usually wider than the beamwidth. Solar outages can be exactly predicted as to the timing for each site.

Space segment

 Portion of the satellite bandwidth and transmission power assigned to the communication network.

Spectrum

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

Spill-over

Satellite signal that falls on locations outside the beam pattern's defined edge of coverage.

Spin Modulation

Periodic amplitude variations resulting from the rotation of a satellite's antennas about its axis, rotating the antenna peaks-and-nulls.

Spin Stabilization

A form of satellite stabilization and attitude control which is achieved through spinning the exterior of the spacecraft about its axis at a fixed rate.

Splitter

A passive device (one with no active electronic components) which distributes a television signal carried on a cable in two or more paths and sends it to a number of receivers simultaneously.

Spoofing

A common term to describe the action of the PEP when optimising the TCP data stream. Spoofing is an internal emulation of processes by acknowledging control and supervisory data frames locally rather than transmitting them over the satellite link.

Spot Beam

A focused antenna pattern sent to a limited geographical area.

Spread Spectrum

The transmission of a signal using a much wider bandwidth and power than would normally be required. Spread spectrum also involves the use of narrower signals that are frequency hopped through various parts of the transponder. Both techniques produce low levels of interference between the users. They also provide security in that the signals appear as though they were random noise to unauthorized earth stations. Both military and civil satellite applications have developed for spread spectrum transmissions.

SSPA

Solid state power amplifier. A VSLI solid state device that is replacing Travelling Wave Tubes in satellite communications systems because they are lighter weight and are more reliable.

Star network

 Network wherein all the terminals can communicate with each other only through the hub in a double hop.

Sub-carrier

A second signal "piggybacked" onto a main signal to carry additional information. In satellite television transmission, the video picture is transmitted over the main carrier. The corresponding audio is sent via an FM sub-carrier. Some satellite transponders carry as many as four special audio or data sub-carriers whose signals may or may not be related to the main programming.

SSP

 Sub-satellite Point. The point on the surface of the Earth directly between the satellite and the geo-centre