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.
Our apologies if your specific question is not here. We are constantly adding to the page so please revisit the FAQ page in the near future. If you have a question, which you would like answered or included, or wish to submit a question and answer for inclusion in this document, please Email the FAQ desk at S+AS Limited.

Compiled by Mike Bartlett
EMail: mik@sasltd.com
Copyright © 1999/2008 S+AS Limited. All rights reserved.


[ A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z ]

Part 2   M

Modem - A piece of equipment containing a Modulator and De-Modulator. The Modem will also house the Data interface plus any other option cards. Generally there is also an M&C [Monitor and Control] interface which allows configuration parameters to be set and some monitor elements to be read. Normally this port is a standard RS232 data interface and any terminal will be able to effect communications. The Radyne/Comstream M&C interface can be accessed using "Terminal" [or similar] under Windows using a data rate of 1200 Baud with 7 stop bits, Odd parity and No flow control. many units also provide an ethernet 100baseT/LAN or SNMP interface. The Modem, heart of a digital uplink system, is connected to the transceiver, and directly controls the uplink power and frequency. Modern Modems operating with an L Band IF also supply 10 MHz reference and DC power to a BUC.

MCPC - Multiple Channels per Carrier. This mode of operation, commonly used in Telephony and Digital Video Broadcasting [DVB], combines or multiplexes many single data carriers or channels onto a common single frequency bearer or carrier. An 8Mbps MCPC, or multiplexed carrier, occupies less bandwidth than 4 * 2Mbps SCPC carriers and is an efficient form of data transmission.

Modulation - The act of transferring an electrical differential into a changed state RF. This task is carried out by the modulator.The most common method of modulation for satellite communications is digital phase modulation, called Phase Shift Keying, abbreviated as PSK. PSK shows good bit-error rate characteristics and makes multi-phase modulation available. An M-phase PSK modulator puts the phase of a carrier into one of M states according to the value of a modulated voltage. Two state or biphase PSK is called BPSK and 4 state or quadriphase is termed QPSK. Other combinations of phase modulation are possible, and used in terrestrial links, where a cleaner and more phase stable environment can be found. 8PSK or Octal-phase keying and 8QAM (Quadrature Amplitude Modulation) is now becoming common in commercial satellite links, even 16QAM has been seen, a method of modulation which up until recently was not available due to the instability of both ground and space segment equipment.
In 2008 6PSK modulation appeared. In 6PSK the phases would be at their maximum 60 degree separation (6 x 60 = 360) which would make the schema more robust than 8PSK and operate with a higher spectral efficiency than QPSK. This modulation teqnique was developed to allow the existing 40Gbps devices (as used in TCP/IP-Fiber transmission systems) to migrate to 100Gbps easily as the Modulation index enables a 2.5 bits per symbol assignment (QPSK is 2 and 8PSK is 3). The resultant symbol rate for 100Gbps is therefore 40Gsps. This modulation type is being used in the GEO-Mobile Radio system which is a satellite based extension of the GSM terrestrial cellular network.

Modulation index -
Is the number which defines the bits per phase used in a modulation schema. BPSK = 1, QPSK = 2, 6PSK = 2.5, 8QAM/PSK = 3, 16QAM= 4 and is used in the calulation to define the Symbol rate (Sps). Sps (ksps) = Data Rate (kbps) x FEC Code / Modulation Index = Occupied Bandwidth of carrier (kHz).

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Null - A point or stage at which RF energy is cancelled [or "nulled out"] by either an exact opposite phased energy, or physical elimination. A co-polar radiation pattern, produced when testing the off-axis performance of an antenna, is a series of peaks and nulls.

Noise - The addition of noise onto a received data carrier reduces the receivers ability to determine the purity of the data. Apart from external sources the earth stations downlink sub-system will also add an element of noise. Noise is inherent in all components of a downlink chain with the LNA (or LNB), antenna and waveguide losses being the main contributors. For example a system comprising of a 3.7M antenna at 30 degrees elevation with a 0.9dB LNB (with noise temperature 70K) will have a typical noise contribution of 120K when the additional noise of the antenna (35K) and the contribution of the waveguide losses (15K) are added. (See also G/T).

NF(dB) T  (K)  NF(dB) T  (K)  NF(dB) T  (K)  NF(dB) T  (K) 
0.1 7 1.1 84 2.1 180 3.1 302
0.2 14 1.2 92 2.2 191 3.2 316
0.3 21 1.3 101 2.3 202 3.3 330
0.4 28 1.4 110 2.4 214 3.4 344
0.5 35 1.5 120 2.5 226 3.5 359
0.6 43 1.6 129 2.6 238 3.6 374
0.7 51 1.7 139 2.7 250 3.7 390
0.8 59 1.8 149 2.8 263 3.8 406
0.9 67 1.9 159 2.9 275 3.9 422
1.0 75 2.0 170 3.0 289 4.0 438

This table can be used to convert LNA or LNB advertised noise figures (dB) to noise temperatures (T).

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OBO - Output Back Off. The OBO is essentially a reduction in power applied to an SSPA or TWTA to minimise the effect of any intermodulation product created by modulated carriers. The OBO is proportional to the number of carriers in operation and increases by 3dB as the number of carriers doubles. ie Nil [ie. In CW operation only] = 0dB, 1 or 2 [modulated carriers] = 3dB OBO, 4 = 6dB OBO, 8 = 9dB etc. Unfortunately this rule cannot be defined as absolute as amplifiers vary, however if this figure is used when calculating the EIRP requirements of an uplink earth station, satellite bandwidth allocation or spectral masking, should not be exceeded. Tube amplifiers are more prone to produce intermodulation product than a solid state due to their non-linear characteristics and a further 1 to 2 dB OBO should be applied.

ODU - The Out Door Unit, so called because it is normally mounted on the antenna, is the generic name for the transceiver and is the complement of the IDU.

OMT - Ortho Mode Transducer. The OMT sits directly behind the feedhorn and has several important functions relating to reception and transmission of satellite signals. The main function of the OMT is to transfer RF to individual ports (for example transmit and receive) and to provide an isolation between these two, 90 degree opposed (orthogonal), planes (ie; Vertical and Horizontal).

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PSK - Phase Shift Key(ing) is the method of transferral of an electrical inpulse or differential, which represents a data bit, onto a Modulated IF carrier. Electrical pulses [Data] is presented to the PSK modulator for conversion to an IF carrier. In BPSK zeros and ones are represented by two phases of the RF carrier that differ by 180 degrees. The output rate of change (baud) is equal to the input rate (bits) ie symbol rate out = bit rate in. QPSK uses two modulators which are 90 degrees out of phase and only reflects a change when 2 bits are presented at the input, therefore the output rate of change (baud) = 1/2 the input rate, ie. symbol rate = 1/2 bit rate in. Consequently QPSK occupies 1/2 of the bandwidth of BPSK, for a given bit rate. BPSK is more tolerant to phase noise and adjacent channel interference than QPSK and typicaly used when bandwidth is not a consideration and in a Burst mode system. A BPSK reference carrier can be recovered up to five times faster than a QPSK carrier and requires less energy per bit for an equivalent BER performance.
In 2009, mainly thanks to ther DVB committe ratifying its use, 8PSK and 16QAM have become more common place in satellite communications. Most satellite operators now allow the use of 8PSK which is fast replacing QPSK as the modulation index of choice. 8PSK uses 1/3rd of the bandwidth that BPSK required however it is not as phase-tolerant as QPSK and has a slightly longer aquisition time. 8PSK when used in association with the modern FEC codes can offer similar Eb/No performance to the more robust QPSK techniques. 16QAM (Quadrature Amplitude Modulation) uses 1/4 of the bandwidth of BPSK however it is significantly more susceptable to phase noise disturbances and requires almost 3dB more Eb/No to offer the same BER as QPSK. Due to their physical properties these higher order modulation schema require a greater output back-off in amplifiers to supress the unwanted sidelobes (spectral regrowth) and Inter-Modulation products (IM3). They are also more susceptable to phase interference in pooly designed, high VSWR transmit/receive chains.

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QPSK - Quadrature Phase Shift Keying [or QuadriPhase SK] see PSK for more detail.

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Radio - Common American name for an SSPA, Upconvertor, Downconvertor package.[see Transceiver for more details].

RF - Radio frequency - Energy radiating from a given point measured in (a) Frequency, Hz or cycles per second, kHz 1000 cycles p/s, MHz 1,000,000 cycles p/s and GHz 1 Billion cycles [1,000,000,000] p/s and (b) Power, Watts [W] or milli "thousandths of" a Watt [mW]. The frequency of a carrier and the amount of data carried, is directly proportional to its bandwidth, the more cycles [or transitions] per second the greater the bandwidth occupied and consequently, the larger the amount of embedded data.

Rain Fade - Atmospheric Propogation [or rain] interferes with RF absorbing and reducing the power which in turn effects the BER of the link. When calculating a link budget provision must be made for the effect of rain both in the up(link) and down(link) path.

Redundant - An American term used in Satellite Communications to indicate that the baseband and/or SSPAs/HPAs are in a 1+1 hot standby condition. ie. If one unit fails the other will be automatically put into service restoring the link. To maintain a high level of link availability a redundant system is a requirement.

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SSPA - Solid State power Amplifier - Commonly used in digital uplink systems the SSPA amplifies the "Super High Frequency [SHF]" commonly 5.850GHz to 6.425GHz in C Band varieties and 14.00 to 14.5GHz in KU Band. SSPA's are rated in Watts of output power and come in a range of sizes 2,4,6,8,16Watt for KU and 2,5,10,20Watt for C Band. Other larger output models are available with some manufacturers offering Hundreds of Watts.
The SSPA is made up of a number of gallium arsenide [GaSa] metallic semiconductor field effect transistors [FETs] arranged in series and parallel to achieve the rated output power. This introduces a feature whereby the output power will reduce in the unlikely event of a single GaSaFET failure, [rather than disappear alltogether], this can be likened to an inherent element of built-in redundancy. If an SSPA has 4 paralleled devices and one should fail, only 2.5dB will be lost from the link, 8 devices = 1.25dB, 12 = 0.8dB. Most SSPA's are packaged with a power supply, an Upconvertor and a Downconvertor in a compact and waterproof package, and is known/referred to as the ODU, a Radio, or more correctly a Transceiver. See also BUC.

Saturation - A term relating to that point at which an amplifier cannot deliver more power despite the input levels being increased. At this point the amplifier is said to be "Saturated" [PS]. Amplifiers are rated at their 1dB compression point [P1], this is the point at which the output power becomes non-linear in relation to the input level. The transfer characteristics for an SSPA result in the P1 point being 1dB below the PS level. With an RF upconvertor typically the saturation point is when the output power level = 0dBm. Overdriving an amplifier, or trying to get more power out than the rated power, causes the SSPA to go into compression which actually reduces the output level. Other nasty effects are seen at this stage; see `intermodulation product' for a more detailed explanation.

Single Carrier (SCPC) - Single Channel per Carrier. Where each modulated carrier or data channel is allocated a separate frequency on the satellite.

Sps - Symbols per second is the term used to describe a digital carriers characteristics with relation to both data rate and modulation employed and is directly proportional to the carriers bandwidth (kSps = kHz). For an example of this relationship please refer to the table in the bandwidth section. [see modulation and PSK for more detail], you could read more on the Olympic Technologies [W] web site.

Sun Outage - Sun Outages occur when the energy from the sun becomes greater than [effectively blocking out] that transmitted from the satellite. This happens twice a year during the spring and autumnal equinox and effects all satellites in the geosynchronous orbit. The exact times are difficult to calculate, however PanAmSat offers an interesting insight to this on their Sun Outage definition page.

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Transceiver - A transmitting [Tx] and receiving [Rx] device. In satellite communication terms the transceiver is the equipment which accepts the Baseband RF signal from the Modem and upconverts it to a frequency which the SSPA amplifies prior to being fed to the Antenna for transmission to the satellite.
Typically the Transceiver is a Class B Gain Block which amplifies the input signal by its rated power. The upconvertor stage has approx 20 to 30dB of gain and the following SSPA further amplifies the signal to achieve the rated output power level
As an example a 2Watt SSE Technologies KStar has a fixed gain of +56dB [with a ±5dB adjustment range]. This is to say that the upconvertor section has a gain of +20dB and the SSPA has +33dB. Therefore to achieve the rated output of this particular type of Transceiver a Baseband RF @ -20dBm should be inserted. Note: Not all transceivers are the same gain block, some require a -30dB input to achieve Saturation. A transceiver is a wide band device and as such will produce a signal anywhere within the specific transmit band, ie. 14 to 14.5GHz in KU band operation. The exact output frequency is defined by the transceivers internal circuitry and the frequency of the presented IF. Typicaly the operator will set the transceiver [by means of dip switches or an RS232 interface], to transmit at a given frequency and adjust [to the required] in smaller steps using the modem. The Modem is also used to vary the stations radiated energy by varying the IF power level, for an explanation of uplink power calculation see EIRP.

TWTA - A Travelling Wave Tube Amplifier is a single, wide band device [typically 500MHz] used in satellite communication systems where high power and the ability to transmit several carriers simultaneously, across the entire band, is required. TWTAs are available in output powers rated @ Hundreds of Watts and are typically found in large earth stations.

TWTA - v - SSPA - One important thing to remember when contemplating a TWTA over an SSPA for use in digital uplink systems, is that they are less linear than a solid state amplifier and therefore require a larger Output Back Off to maintain most satellite operators spectral masking. Typicaly the resultant 3rd order intermodulation products [IM3] need to be around -25dBc [or better] with respect to the main carrier levels. Due to the physics of a TWTA the 1dB compression point [P1] is at least 3dB below saturation this means operating the amplifier with around 6 to 7dB OBO. An SSPA, on the other hand, will give -25dBc IM3 with only 3dB [or less] OBO. Also, due to their design, the output power at the flange of a TWTA is always less than the rated power of the tube by approximately 1dB ie. a 700Watt TWTA will only deliver 600Watts and a 400Watt around 330Watts. As an example; In a digital, QPSK modulated satellite link, a 170Watt TWTA will produce a similar [usable] output power to an 80Watt SSPA. Typically an SSPA only requires 40% of the power rating of its TWTA counterpart to offer equivalent system performance. If all of this is not enough reasoning an SSPA consumes 50% less power than the TWTA solution, requires less cooling, uses lower voltages and will last longer without detrimental ageing effects.

TI - Terrestrial Interference see interference sources for more information.

Turbo Coding - The latest FEC to emerge in 1999 is Turbo Coding. This FEC is a superior error correction technique providing better performance than other technologies. Unlike Reed Solomon, Turbo is not concatenated with a primary FEC and therefore encoding/decoding delays are reduced as the detailed interleaving/de-interleaving [of Reed Solomon] is no longer required. Turbo coding also supports new FEC rates of 0.325, 0.495 and 0.793, this later code rate exceeds the Intelsat specification for R1/2 Viterbi/Reed Solomon (V/RC) but only requires 27% of the bandwidth and provides a coding increase of 1.4dB over R7/8 V/RC. The 0.495 uses slightly less bandwidth than a R1/2 V/RS carrier and offers a 0.4dB advantage as well as a 30% decrease in latency. 0.325 coding is useful where bandwidth is available [it uses approx. 30% more than R1/2 V/RC] and where performance is critical the 0.325 rate offers a 0.8dB coding advantage over standard R1/2 V/RC. As we entered the 21st Century more powerful satellites have allowed the use of Turbo Coding and 8PSK modulation to become common. For more details please contact tech_help@sasltd.com.

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Up-Convertor - A unit which converts the IF to a higher frequency which is fed into the SSPA where it is amplified and guided [or fed] to the Tx port of a satellite uplink earth station. Care must be taken not to overload or saturate the upconvertor as its amplification stage can produce spurii which are then transmitted to the satellite polluting the space segment, and cause interference.

Uplink - A generic term for an earth station which has a modulator, upconvertor, HPA and Tx antenna configured to transmit to a satellite.

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VSat - Very Small Aperture terminal. A VSat is a complete terminal which is designed to interact with other VSats in a satellite delivered data network, commonly in a "star" configuration through a hub. Typically a VSat utilises a small antenna and the term VSat has [wrongly] become a euphemism for a small antenna [ie; 80cm - 1.2m]. The Small aperture actually relates to the occupied bandwith of the VSats transmitted carrier which is typically only a few kHz wide.
The VSat terminal will use a special and often proprietory modulation, scrambling and coding alogrithm which will allow the Hub or Network operator to control the system and present billing based on a data throughput, or other form of usage basis. VSats are used in a variety of applications and are designed as low cost units. Commonly several VSat networks are operated through the same hub [shared services] which reduces the initial installation/set up costs [for the user] and maximises profitable returns [for the operator]. The most obvious VSat network in the UK is operated by Camelot and provisions the UK Lottery. For a more detailed explanation of a VSat network the article written by Muhammad Ali Khan on the UK Satcoms.Org site is quite interesting.

VSWR - Voltage Standing Wave Ratio. This is an important and often unnoticed parameter which can have a detrimental effect on both transmit and (to a lesser extent) receive system. It is important to ensure that there is not a mismatch between the impedence of the modem, cable and amplifier especially when transmitting higher order modulation (ie 8PSK or 16QAM) and the resulting reflections (from the mismatch) will create phase errors in the signal, which in PSK modulated systems will result as bit errors in the data.

For an insight into the 50OHm / 75OHm issue click here to download a PDF (87kb) 50 and 75 OHm

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Watts - A measurement of power; used to describe the measured output power of an amplifier or SSPA. 1 Watt = +30dBm see dB for a more detailed explanation.
Waveguide - see Guided wave tube .

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X [Horizontal] - Linearly polarised satellite transponders are [in Europe] noted as X for Horizontal and Y for Vertical.

X-Pol - Cross Polar(isation) component. The "unwanted" element of the transmitted energy from an uplink. Expressed as a negative power ratio [-dB] the XPD [Cross Polar Discrimination] value is a function of the antenna, feed and OMT. In linear, orthogonal re-use satellite transmission systems X-Pol is always present and never wanted. Some antenna designs are better at reducing the XPD than others, typically an offset fed antenna of 0.6f/d will have a higher X-Pol component, which will cause interference in the opposite [or orthogonal] transponder, than a 0.8f/d or a dual optic antennae. When an uplink is first brought into service the satellite operator will test the station to ensure that the X-Pol is as low as possible and does not cause any interference. By rotating the feed the operator will align the ground station to the orthogonal plane of the satellite minimising unwanted X-Pol energy. Choosing a good quality and well designed antenna/feed/OMT package will (a) save money and (b) provide a better and cleaner environment for satellite communications.

In circular polarised systems the purity of the polarisation is specified in terms of an Axial Ratio, to obtain the XPD the following formula is used ;

XPD = 20log ((AR(n)+1) / (AR(n)-1))

Where :

AR (dB) =AR(n) =XPD (dB)

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Y [Vertical] - The opposite polarisation to X [Horizontal] as expressed in Europe. I believe that this form of notation is taken from a graphical representation where Y is up the side and X is along the base. In American the abbreviations are simplified to V and H (Note: that the letter H is also used to describe the magnetic field direction in a waveguide). Polarisation is always defined as the direction of the electric or E field.  In a rectangular waveguide the electric field exists between the two broad faces of the waveguide and if the waveguide is lying with its broad faces horizontal the polarisation is vertical. The magnetic field, called H, is at right angles to the electric or E field. If one thinks of a flexible waveguide the E plane is Easy to bend and the H plane Hard to bend.  The E plane tells you the polarisation.

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I must admit that I am struggling to think of a satellite term which begins with "Z".

Z Cal - [As submitted by: Zoller-Gritz, Robert S (CAP, AMR)] The term Zero Cal is used to describe the act of calibrating an earth station used in finding the distance to satellites (ranging). A series of phase shifted signals (tones) are fed from a source through the uplink chain to the antenna where they are looped back down to the receiver and the resulting phase changes are Zero'd out. Given that the phase delays of the satellite are provided by the manufacturer it is now possible to determine the exact distance of the satellite from the transmitting earth station. Robert explains that a Zero-Cal is required whenever equipment is changed in the "range path". As satellites are moved around by the gravitational pull of the earth, it is necessary to know the exact distance of the satellite, three dimensionally, to effect correct station keeping.

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Last Revised: 29/July/2010
Copyright © 2001-2010 S+AS Limited. All rights reserved.
We hope you found this page informative. If you have a question, which you would like answered, or wish to submit a question and/or answer for inclusion in this document please Email the FAQ desk at S+AS Limited.

Compiled by Mike Bartlett
EMail: mik (at) sasltd.com
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