Welcome to the S+AS Limited Frequently Asked Questions and abbreviations page.
During the building of this page I have gone away from the typical FAQ format to a more explanatory system, for example, instead of asking "What is an Antenna ?", I have given a description of an Antenna under A below. To access the different sections you can click on the Alphabetical index , or by following the links set in each sub-section. Clicking "Back" on your browser will return you to the original section.
SatNews.com's Glossary of Satellite Terms section is well worth a visit if you are lost with satellite communication terminology.
Antenna - Common name for a Satellite dish. SatCom antenna come in various sizes, configurations and materials. Ranging from 0.9 meters to 33 meters the antenna is the most conspicuous and often the most impressive sub-system of an earth station. The main function of an antenna is a passive amplifier; be it transmitting or receiving, the gain of an antenna is calculated as a relative function of its size [aperture] and the wavelength of the signal to be amplified. The figure is expressed in dBi . Many factors govern the quality of an antenna, not least being the surface accuracy and rigidity of the main reflector and the placement of the feed. A SatCom antenna must remain pointed at the satellite under all environmental operational conditions and irrespective of the residual movements of the satellite. The larger the antenna the smaller the main transmit lobe [beamwidth] which requires KU Band antenna over 4.5m to be fitted with an automatic tracking system. The smaller antenna used in VSat systems have a beamwidth greater than the movement of a geostationary satellite and consequently, do not need tracking. An antenna system comprises the following parts;
The mechanical system which encompasses the reflector, back structure and pedastle.
The most frequently used types of antenna are parabolic [axisymmetric or offset] which include: Cassegrain. Gregorian, Offset front fed or Prime focus. Cassegrain and Gregorian make use of a dual reflector system fed by a primary radiator located at the focus, this dual reflector configuration is now being seen in a parabolic offset configuration. Typically antenna subsystems can achieve 66% efficiency, with the newer design, ellipsoidal Gregorian [dual-shaped] offset reflector antenna, attaining 82% or more.
Availability - In SatCom terms the link availability is expressed as a percentage of a year when the link will perform as per the required BER . ie. 99% availability states that the link will be unavailable for 87.6 Hours or put another way; if the link was running @ 64kbps more than 2.018 Million bits of data would be lost.
Axial Ratio - The Cross Polar Discrimination (XPD) figure of a cicularily polarized antennae/feed is refrered to in terms of its "Axial ratio". See X-Pol for further information.B
Baseband RF - A Radio Frequency signal generated by the Modem is either 70 [most common] or 140MHz and from -5dBm to -25dBm in power. This Modulated carrier is fed to the outdoor unit and is also known as the IF.
Bandwidth allocation - A transponder on a satellite can be/is divided, and sold, in smaller units to accommodate different users link requirements. The users data rate + FEC + modulation characteristics are calculated to indicate the carriers occupied bandwidth. The Bandwidth of a carrier is therefore directly proportional to its data rate [see the table below]. The satellite operator then increases this figure by a factor of 1.2 to 1.4 [to allow for separation of the carriers in the transponder] and "allocates bandwidth". This figure is then related to the power requirements [as calculated against the users BER request] and the user is charged for the greater of the two. Some satellite operators will increase the bandwidth, until the links power requirements are met. In this form of calculation a 64kBps carrier at R1/2 FEC, using QPSK modulation would have a bandwidth of 64kHz but would be allocated anywhere from 76.8kHz up. It should also be noted that QPSK modulated carriers suffer performance degradation if the carrier spacing is less than 1.3 times the symbol rate due to an effect known as, adjacent channel interference.
Bps - Bits per second. The users data rate of a satellite channel is expressed in Bits per second; Bps, Kilo[Thousand]bits per second; kBps, or Mega[Million]bits per second; Mbps. The rate of a satellite delivered data channel is measured in sps or symbols per second, see the table above for a comparison of Bps-to-Sps. An MCPC or Digital Video link would typically run at several Mbps, Video conferencing @ 384kBps, Audio @ 192 - 256 kBps and Data and Voice circuits @ 64kBps [or multiples thereof].
BER - Bit Error Rate. The figure of merit for a digital link is its BER, also called bit error probability. Mathematically this is the probability that a bit sent over the link will be received incorrectly [that a 1 will be read as a 0, for example] or alternatively, the fraction of a large number of transmitted bits that will be received incorrectly. This is expressed as a single number ie. 10 * 10E-4 or 0.0001 . Physically a bit error occurs because a symbol error has occurred, ie. at some point in the link noise has corrupted the transmitted symbol and the decision circuitry at the receiver cannot identify it correctly. Symbol errors arise from thermal noise, from external interference and from intersymbol interference.
Baseball switch - So called because when graphically identified it looks like the seam on an American `Baseball'. This device employs a high speed, DC powered electric motor, to alter the path of RF flowing through it, by rotating a specially shaped deflector. Two RF sources are shunted, simultaneously, from one port to another. In 1+1 redundant system the output of the secondary HPA is switched from a "dummy load" to the transmit port of the antenna at the same time as the primary source is switched away from the antenna into the load.
3dB Beamwidth - or 'half-power beamwidth' is the term (measured in degrees) applied across an antenna's peak gain envelope where the carrier power drops by 3dB. The smaller the antenna and the lower the frequency the larger the 3dB beamwidth becomes. For example if an antenna has a 3dB beamwidth of 1° (or ± 0.5°) then if the antenna is de-pointed by 0.5 degrees the received signal level will fall by 3dB. The 3dB point can also be used to calculate the antenna gain [Ga (see also dBi)] using the formula ;
Ga = 10log [31000 / (3dB Bw 2)]
A 60cm KU antenna with an RX gain of 36.12dBi will have a 3dB beamwidth of 2.75 degrees (ie easy to find the satellite) however an 11M KU band antenna which has an RX gain of 60.45dBi has a very narrow 0.167deg 3dB beamwidth which consequently needs an accurate tracking system to keep the reflector pointed within the peak gain envelope of the antenna. A C Band antenna has a fatter peak gain envelope than a similarly sized KU Band antenna primarily because the reflector cannot gather as much signal (ie it has a lower gain) due to the wavelength being longer in C Band than KU.
BUC - Block Up Convertor, a solid state device which acts as a fixed (in most cases) block and converts L Band to RF. The device is typically powered by a DC voltage presented on the center pin of the coaxial 'IFL' from the Modem. Medium (+8 Watt) and Higher power BUC's need a separate DC supply.
A BUC is essentially a two stage, fixed gain block, amplifier with a front end convertor and an SSPA with a gain that defines the BU's output power. The fixed gain, front end convertor in a BUC can be as high as 30dB or as low as 15dB depending upon the manufacturer which would mean a 4 Watt BUC could have 66dB of gain or as little as 51dB and still provide a 4 Watt (+36dB) output. Typically BUC's have no intelligence (called a 'dumb BUC'), Codan however manufacture a range of high quality medium and high power intelligent BUCs which have in-built monitor and control fuctions as well as variable gain.C
Clear-Sky - The Clear-sky value is taken to be the condition when the intrinsic atmospheric attenuation due to gases and water vapor (Rec. ITU-R PN.676-1) are applicable, without additional or excess attenuation due to tropospheric precipitation, such as rain and snow. Therefore the clear-sky value in a link budget would include a rain-fade margin and therefore represents the best achievable value of C/N or Eb/No.
CW - Clean Wave or Pure Carrier [PC] denotes a stable RF energy source with no modulation applied. The application of a CW to an amplifier is the standard method of defining its gain characteristics. The introduced carrier is increased as the amplifiers output is measured and at a given point the output RF will be disproportionate to the input level. When the output decreases, relative to the input, the amplifier is in saturation. The CW is also used to determine the intermodulation product produced when more than 1 carrier is present in the amplifier, due to its characteristics a CW is easily identified and accurate power level and noise introduction measurements are possible. Radyne/Comstream Modems provide one of the cleanest and most stable Pure Carriers I have seen, with a spectrum analyser set at 1Hz resolution bandwidth @ 100Hz span no noise, or power deviation, is evident on the peak. I have used this carrier during both Eutelsat and Intelsat antenna verification testing without problems on several occasions.
Co-Polar - In the same polarity, rather than X [cross]-polar.D
dB - deci [or 1/10th of a] Bel. Named in honour of the inventor of the telephone `Alexander Graham Bell'. The Bel used (in Satellite communications) to indicate the relative power of a signal, is defined as the common logarithm of the ratio of two power levels P1 and P2; Bels = log(P1/P2). A positive value for B would represent a power gain, a negative = power loss. A Bel is a rather large unit for use in electrical and RF engineering, so a smaller and more convenient unit is used. The decibel or dB has a magnitude of 1/10 of a bel. The calculation now appears dB = 10log (P1/P2).
Where D = the Diameter of the Antenna in meters and λ = the Wavelength in meters. Typically the Uplink or Downlink bands "center frequency" is used, the table below gives the common frequencies and values for λ;10log [9.9 * (D/λ) 2]
Therefore an antenna of 2.4 meters operating as a KU uplink would have a Gain of 10log [9.9 * (2.4/0.0211) 2], which is 10log [9.9 * (113.7440) 2], or 10log [9.9 * 12937.697] or 10log 128083.200 which equals 51.07dBi, however, this would only be true if the antenna was 100% efficient. Most antenna are only 66% efficient so a 2.4m KU-Band uplink antenna has a gain ,[Ga] of 49.14dBi. [3dB down would be 50% efficient !]. In S+AS's experience when calculating antenna gain it is always wise to deduct an extra 0.5dB, just to be on the safe side.
Downlink - The term Downlink is commonly used to describe the receiving earth station and also encompasses the satellite portion of a link. Typically a receive only [RO] downlink would consist of an Antenna, an LNB and a Data receiver / Demodulator. The downlink power from a satellite defines the carriers EIRP transmitted towards the earth and is identified on a footprint map.
Data Interface - A physical electrical connector/interface on the back of a Modem or computer. In communication terms the data 'port' passes data to the DTE or DCE connected to it. The DTE refers to the Data Terminal Equipment and is nominaly the equipment which uses the data and the DCE or Data Communication Equipment is part of the distribution system which passes the data to other DTE's. There are many different types of interfaces, see the list below and/or contact S+AS Limited tech_help for more information. Follow this link [W] to see an explanation of an RS232, 422 and V.35 interface.
Data Rate - The user data rate is the expected rate of flow of data over a satellite link ie data In(putted) to data Out(received) and is quoted in bits per second [Bps] and can also be called the throughput. The overall or aggregate data rate of a satellite link [expressed in Sps] includes any signaling or transport information required by the link as an addition to the user data rate and is further affected by the modulation index [see the PSK section for more detail].
Downconvertor - Downconvertors are used to translate the received satellite signals into an IF which is passed to the Demodulator for processing. This IF is typically LBand in Audio and Video reception with 70MHz being common in data systems. There are several different types of downconvertor the most common being the LNB or Low Noise Block downconvertor. This device powered by a DC bias [18 -24VDc] converts the received satellite signals KU or C Band, to an LBand by mixing this with another frequency and amplifying the sum. See LNB for more detail. Large earth stations and transceivers use a Low Noise Convertor [LNC] which provides a narrow band, high stability 70MHz output.
DeModulator - A DeModulator receives the IF and correlates the changes of phase in the carrier into an electrical energy. The resulting electrical differential is then passed through the FEC decoder and then out to the Data Interface. Typically DeModulators encompass several types of Decoders which translate and correct the resultant electrical pulses into a recognisable form of data. A common name for this system is a receiver or IRD [Integral or Integrated Receiver/Decoder].
Doppler shift - Although satellites are in a fixed "geostationary" orbit, they all drift slightly. This motion causes a delay or 'doppler shift' in the RF waveforms [satellite signal], resulting in periodic variation of data rate clocks at the receive sites which are slightly different than the data clock at the transmit site. As a result timing errors [periodic bit slips] are induced and require a doppler buffer to restore syncronisation. The buffer, operating in a First-In-First-Out mode, inputs data at the received clock rate [RT] and releases it in time with a second clock source, normally Station Timing [ST]. Doppler buffers should be sized to accommodate the clock slip which is calculated as being, at worst case, (LD)*(2*data rate). Typically the Link delay (LD) is 1.1ms, therefore if the data rate is 19.2kBps the maximum size of doppler-compensating receive buffer required is 1.1*2*19.2 = 42bits.E
E.I.R.P. - Effective Isotropic Radiated Power. Expressed in dBW the EIRP is the power radiated from an antenna. The calculation used is: EIRP = 10log Pa - Fl + Ga where Pa is the output power of the amplifier or SSPA in Watts, Fl is the Feed losses in dB, and Ga is the gain of the transmit antenna in dBi.
Note: This is the maximum EIRP but not the usable EIRP. A modulated carrier will produce spurii and other intermodulation products in the PA which will be transmitted to the satellite and cause interference with adjacent carriers, not to mention exceeding the satellite operators bandwidth allocation. To eliminate this effect the SSPA must be run with an Output Back Off [OBO] 1.5 to 2dB. therefore, the usable EIRP of a `2Watt PA/SF24 antenna combination' is 49.4 to 49.9dBW. Different PA's, even from the same manufacturer, have different characteristics with regard to in band interference product and therefore, it is wise to assume a 3dB OBO when calculating maximum usable EIRP for Single Carrier operation.
Eb/No - Energy per bit in a given Noise bandwidth. The digital communications equivalent of link quality, expressed as the ratio between the Carrier power to Noise power level [C/N] and the Energy per bit in a given Noise bandwidth [Eb/No]. For BPSK and QPSK the Eb/No = C/N * Bandwidth/Data Rate with 8PSK add 3dB. The higher the energy per bit the easier it is for the demodulator and decision circuitry to determine if the transmitted bit was a 1 or 0. The ability for the decoder to determine data polarity is also dependant upon the modulation index and the FEC used. The higher the modulation index and the deeper the code the more chance for missenterpretation exists and therefore the higher the required Eb/No to acheive a 10-7 BER.F
FEC - Forward error correction is used in satellite communications to overcome the effects of interference in a satellite link therefore making it more efficient. In encoding for forward error correction, redundant bits are added to an incoming bit stream so that errors in transmission may be detected and corrected at the far end. In some FEC systems the number of redundant bits is equal to the number of data bits, resulting in a doubling of data rate for a given channel transmission rate. The loss of communication capacity, bandwidth is traded for a guaranteed low error rate. [see; BER].Propagation disturbances [ie. attenuation caused by rain] result in a reduced carrier power to noise power ration [C/N] for a short period of time. Rather than designing the link with a very high clear sky C/N, it is economic to add FEC to the link to correct errors during periods of link degradation.
F/D - The ratio of the Focal distance of an antenna relative to its diameter. Axisymmetic prime focus antenna have an F/D of around 0.35 where as an offset design has an F/D in excess of 0.6 with most modern antenna using 0.8. the larger [or longer] the F/D becomes the flatter the reflector looks.
Footprint - A footprint is the term used to describe the area of the earth on which the satellite beams its signals. Each satellite has a different footprint and these days most satellites have several beams each with their own footprint or area of coverage. Most satellite operators provide this information either on their web site or in printed format. The footprint of a satellite will dictate its connectivity Eutelsat, for example, has a satellite at 13deg East which has a footprint which covers most of Europe, across the top of Africa into the Middle East and part of Russia. [Click here to view a pdf version of a footprint]. Having examined this footprint map it becomes obvious that you could not set up a link between Aberdeen and the southern desert region of Algeria. These maps also provide essential information to the system designer as they show the available downlink power. Using the example map the downlink antenna needed in Algiers needs to be larger than one installed in Paris as the available signal (from the satellite) is of lower power. Click here [W] to goto the footprint section of the Swedish Microwave Website. [W]G
Guided wave tube - Wave guide. A rectangular metallic tube used to transport RF from the output of an SSPA to the feed flange of a transmit antenna. Wave guide comes in all shapes and sizes depending on the frequency being carried and the type of deployment required, called wave guide because it "guides" the waves of RF energy to their destination. Typical losses for rigid waveguide at KU band TX = 0.1dB/M, RX = 0.08dB/M. Further details are available from S+AS Limited; see our contacts page.
Ground Segment - Another name for an earth station or uplink.
G/T - Gain over Temperature. The quality of a receiving antenna is expressed as the G/T or the ratio of the receive gain and the system noise temperature. This figure can be calculated using the formulae; G/T = gain in dBi - 10 log(system noise temperature) G/T can also be measured in the field using a calibrated spectrum analyser.
HPA - High Powered Amplifier. A generic term for an SSPA or other form of Amplifier used in Satellite transmission systems.
Intermediate frequency - The IF is typically a frequency which has been or is going to be converted in a downlink/uplink chain. The IF is used primarily as it is easier to transport between the indoor unit and the outdoor unit or vice-versa. [see LNB, Modulation or up-convertor].
Intermodulation product - Intermodulation arises from multiplication of signals due to non-linearity of the amplifier. This effect occurs naturally in all non-linear components and becomes increasingly present in multi-channel transmissions, when a number of signals are sharing a single amplifier. These spurii can be seen on a spectrum analyser as a multiple of the fundamental frequency, or frequencies. If these unwanted signals reach the satellite, the noise floor of the transponder would be increased and may also produce intermods which will lead to interference on the satellite. Over-driving the input of a transceiver will cause a single carrier to produce intermods that appear as "ears" or "shoulders". These are an effect of phase distortion created by the AM/PM conversion. This overdriven [or non-linear] state is decreased as the Output Back Off is increased. Some devices exhibit better intermodulation performance than others, see TWTA - v - SSPA for more detail.
IFL - The Inter-Facility Link is the cable/s used to transport the IF between the indoor baseband equipment or IDU and the outdoor transceiver or ODU of an earth station. Typically the IFL is a quadshielded Co-axial cable with an impedance or 50 or 75 Ohms, which has a high rejection and low attenuation characteristic. S+AS Limited carries stock of the Radyne/Comstream
recommended IFL cable which are specifically designed for use with modulated carriers.
Inclined orbit - A term used to describe a satellite in geosynchronous orbit which is deliberately being allowed to drift in a vertical excursion above and below the orbital plane. In a standard orbit the satellite would be kept "on station" in a 0.2° degree "box" using small gas jets. To save fuel and prolonging the life of the satellite, the expensive [in fuel terms] North/South control is relaxed and the satellite goes into an inclined orbit. The satellite transcribes an elongated figure of eight which grows by almost 1° degree per year. Earth stations which use the cheaper capacity offered on these satellites, must have a tracking system fitted so that they can physically follow the satellite throughout its 23hour 56min, figure of eight, cycle. Another way to maintain communication with an inclined orbit satellite, and avoid the necessity of a expensive tracking system is the "Comsat Maneuver" which involves tilting the satellite so that the downlink beam appears stationary.J
kBps - Kilo-bits-per-second see bps for more details.
LBand - The satellite frequency band known as "LBand" ranges from 600Mhz to 2150Mhz.
LNA - Low Noise Amplifier. An LNA will amplify the signals presented to it without conversion nominally confined to an "in band" 800MHz bandwidth. More modern LNAs are now becoming available in 2Ghz bandwidths. Typically this signal is fed into several separate Downconvertors where it is converted into the required, narrow band, IF.
LNB - Low Noise Block Downconvertor. Used in all forms of satellite communications systems the LNB is placed in the focal point of an antenna and converts the given signal into an L Band Intermediate frequency which is then fed into an L Band De-Modulator for processing. The key elements of an LNB are its gain, noise temperature and stability. The received signal is mixed with a Local Oscillator source typically 5.150Ghz for CBand and 10, 10.9, 11.3 and 11.475GHz for KU and then amplified. The most common type of LNB in use for digital data communication utilises a Phase Lock Loop circuitry [PLL] which produces a frequency and frequency and phase stable output of ±25kHz or better.
LNC - Low Noise Convertor. An LNC converts the received signal into a narrow band IF essentially this is an LNA and Downconvertor in a single ODU package. An LNC requires an externally referenced frequency source which is mixed with the origin to provide the wanted IF.
Link Budget - A link-budget is a calculation of the performance of a satellite link from end to end and shows the required EIRP to achieve a given Eb/No with a set of known parameters. These include, the antenna receive and transmit characteristics, a figure which represents the amount of attenuation derived from atmospheric losses and rain fade [both to and from the satellite], the distance to the satellite, the transmit and receive performance of the satellite and finally the link characteristics ie data rate, modulation and FEC employed. The production of a meaningful link-budget is a highly specialised job and can only be accurate if provided by the satellite operator. Without an accurate link calculation, which truly reflects the performance of the space and ground segment, the ability of a satellite delivered voice/data network to perform to expectations is at best intermittent.
Line of Sight - LOS. An antenna must have an unobstructed view of the satellite. Any obstruction will absorb the transmitted energy and reduce the effective aperture and Gain of the reflector. To determine the LOS the elevation and azimuth "look" angles must be calculated and a site survey performed to ensure that the proposed position of the earth station is clear. In the case of an uplink system extra care must be taken to ensure that the transmitted energy does not radiate within 5 degrees of any obstruction and that no person can interrupt the beam. Many programs exist to calculate the look angles, try our online version or alternatively click here to download an excellent example [smwlink3.zip - 1.59MB] provided by Swedish Microwave, visit their WWW site @ http://www.smw.se for more details.
Link Delay - The time taken for RF to travel from the transmitting earth station to the receive earth station and back. Typically the time variation encountered is 550ms each way which varies according to the distance from the earth station to the satellite. In a multi site VSat network requiring synchronous data transferal this variation has to be corrected with a plesiochronous or elastic doppler buffer to avoid data errors.M
The M to Z sections have been moved to a separate page as the FAQ was getting so large that I had complaints about the download/display time. Please click here to continue.
Last Revised: 5/January/2009