Sat Def In At Ion

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. PanAmSats Buzz-Words section is well worth a visit if you are lost with satellite communication terminology and for those looking for an instructional reference click here for a link to Dr Leonards, educational, Satellite course . 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. Compile EMail: [email protected]

by

Copyright © 1999/2001 Updated 18/July/2001.

Mike

S+AS

Limited.

Bartlett

All

rights

reserved.

A 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 primary source, comprising the illumination horn, the associated reflector sub-assemblies and the non radiating components. 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. 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. A common name for the IDU or In-Door Units is the Baseband equipment. This generic term often encompasses the uplink and downlink sub-systems and the data processing equipment in a large earth station or Hub. 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. The following table shows actual bandwidth allocations applied by Eutelsat [as defined in ESOG Module:220 Vol II] for QPSK modulation schemes. Information Bit rate

Transmission rate

Transmitted Symbol rate

Allocated Bandwidth

kBps

FEC

kBps

Sps

kHz

64

R 1/2

128

64

84

64

R 3/4

85

43

56

512

R 1/2

1024

512

675

512

R 3/4

683

341

450

2048

R 1/2

4096

2048

2703

2048

R 3/4

2730

1365

1802

Click here to download an On-line copy of the document.

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. BPSK - Bi-Polar or Bi-Phase Shift Keying is a method of modulating or impressing a data stream onto an RF carrier used in satellite transmission systems. See PSK for more details. 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 depointed 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 bamnd 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.

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). By using this equation we can express the power differences between two sources ie. the input and output of an amplifier. Consequently; if an amplifier received a signal of 10Watts and increased its strength by a value of 100 to 1000Watts it would have a gain of 20dB. dB = 10log (1000/10) = 20dB. Similarly if a 10Watt signal was increased to 10 million Watts the amplifier would have to have a gain of 60dB. It becomes obvious in that, to say a satellite dish or LNB has a gain of 1 million is impracticable. For example in this style of annotation a 53.7dB gain 11GHz LNB would be described as having a gain of 233333.00 times. In satellite engineering, decibels are commonly expressed as a power relative to a referenced value, ie dBW [1 gain in dB relative to 1 Watt] or dBm [1 milliwatt], therefore, a

60dBW signal has a power of 1 million Watts. [dB, dBm and dBi can all be added together however, dBW must be converted to dBm before any manipulation]. The dB is also used by telecommunication engineers as the ratio of input and output voltages, the math is slightly different but the principle is the same. Click here [W] for a more detailed explanation.

DBi - In Antenna calculi the gain is given as relative to that of an isotropic antenna [one that radiates equally in all directions] and is written `dBi'. A simplified version of the math is; 10log [9.9 * (D/λ) 2] 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; C-band Down

3950 MHz

0.0759

C-band Up

6175 MHz

0.0486

KU Down (1)

11350 MHz

0.0264

KU Down (2)

12000 MHz

0.0025

KU Down (3)

12500 MHz

0.0024

KU Up (1)

14125 MHz

0.0211

KU Up (2)

17720 MHz

0.0169 \

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. Another way to calculate the gain is using the 3dB beamwidth method, see beamwidth for more information. 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. RS232 RS422 RS449 RS484 G.703 DS-1 X.21 X.25 V.35

balanced balanced Download

and and X.21

unbalanced unbalanced spec

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-InFirst-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. For example an S+AS Limited Eutelsat approved 2.4m antenna [SF24] has a transmit gain of 49.14 dBi and a system feed loss of 0.75dB [including baseball switch and all flexible and rigid waveguide in a 1+1 configuration] coupled with an SSET KStar 2Watt SSPA with an IF input of -20dBm. Maximum EIRP = (3.01) - (0.75) + (49.14) = 51.4dBW 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. In M phase modulation there are two common types of FEC, Viterbi and Sequential which are applied at different rates ie. FEC R1/2 where 2 bits are sent for every 1 and R3/4 where 4 bits are sent for every 3. R1/2 FEC would be used where power is a consideration and R3/4 would be used if bandwidth was a consideration, other rates exist ie. R4/5 and R7/8. For a more detailed example please see the table in the bandwidth section or, contact tech_help @S+AS Limited. 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.

Geosynchronous orbit - Arthur C Clarke and Dr Allen suggested and calculated that if an object was placed on a special orbit, in the equatorial plane at approximately 1/10th of the distance to the moon, gravitational attraction [0.22M/sec2] and the angular momentum of a satellite traveling in a circular orbit would coincide and cancel. At this point the object [or satellite] would appear to be stationary relative to a point on the earth, which would make an excellent place for a communications relay point. "Slots" or parts of this "Clarke Allen belt" some 22,238 Miles [35,786Km] above the earth are home to the many communication satellites in use today and are usually identified as X [number of] degrees East or West, of South. Unfortunately there are many forces which attempt to pull these satellites out of orbit and as a result, they are constantly moving and require fuel to keep them "on station". Some operators conserve fuel and allow the satellite to drift in a controlled gyration known as an "inclined orbit".

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. H HPA - High Powered Amplifier. A generic term for an SSPA or other form of Amplifier used in Satellite transmission systems. HUB - In some satellite delivered networks the data from each remote site [inbound] is processed and/or passed to another remote site [outbound] through a central Earth Station or Hub. The Hub comprises of a large antenna capable of discerning the many different inbound carriers and a complex intelligent baseband sub-system which will identify the requirements of the data for processing, re-direction and billing. The hub can also operate as a gateway to other services ie; ISDN or other terrestrial links for backhaul of data to the customers central office as well as being the originator of the outbound carrier which all VSats are tuned to. The outbound carrier from a hub contains several components ie; network control information, signaling and specific user data. Unlike VSats and other types of earth stations, the hub is a complex and expensive station which is probably manned 24hours a day, is configured as fully redundant and carries services for different applications on different networks etc. I 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 upconvertor].

Intermodulation product - Intermodulation arises from multiplication of signals due to nonlinearity 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. Interference sources - The successful operation of a satellite link is dependent upon many factors. The initial design of the link, the production of a link budget and the procurement and installation of good quality equipment are all within the control of the operator/user. If both the mechanical and electrical specifications are considered a successful link is established. However, outside the control of most is RF interference which corrupts the flow of data. This can occur either terrestrially, local to the ground segment [TI] or can be transmitted to, [or introduced] by others in the space segment [SSI]. Local TI can be identified and screened out [in most cases] and Satellite operators are very careful not to introduce intermediation effects or spurii onto their satellites, either by design or careless operation. Some users however re-transmit TI or exceed their power/bandwidth allocation. Any form of interference will corrupt a carriers integrity and result in symbol errors and data loss. TI commonly enters the uplink chain via ingress and or induction onto the IFL where it is consequently amplified and transmitted to the satellite, the interfering carrier can be anywhere within the bandwidth of the upconvertor/SSPA chain and will most likely be corrupting someone else's, rather than the originators, data. Other forms of obvious interference are generated by rogue carriers nominally caused by careless earth station operation. By far the most common form of interference is that transmitted to the orthogonally opposed transponder. All antenna systems transmit energy on both axis, the ratio of which is expressed as the XPD , in some cases this energy will effectively raise the noise floor of the transponder making it unusable. By physical law an offset antenna with an F/D of 0.6 will produce enough Xpolar component to interfere with the opposite transponder on modern satellites. If this type of antenna has a weak mechanical constitution, as it becomes miss-pointed, the XPD will increase to as much as -14dBc in the 3dB co-polar region effectively destroying the unfortunately placed carrier in the

opposing transponder. Lastly but by no means the least is the Sun. Twice a year, in the spring and autumn, the Sun will align itself directly behind the satellite your earth station is receiving from and will swamp the signal. The duration and timing of this "Sun outage" can be calculated. Ask your friendly systems integrator or satellite operator. 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. K kBps - Kilo-bits-per-second see bps for more details. L 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: 20/May/2002 Copyright © 1999/2002 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: [email protected]

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. The Modem, heart of a digital uplink system, is connected to the transceiver, and directly controls the uplink power and frequency. Please review the Radyne/Comstream section of our products page for more information, or [email protected]. 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 is now becoming common in commercial satellite links, even 16QAM (Quadrature Amplitude Modulation) 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. N 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). O 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).

P 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 should always be 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. Q QPSK - Quadrature Phase Shift Keying [or QuadriPhase SK] see PSK for more detail. R 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. S 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. 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. 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. T 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 [email protected]. U 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. V 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 [i.e.; 80cm - 1.2m]. The Small aperture actually relates to the occupied bandwidth 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.

W 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 . Return to Index or Top of page X 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 reuse 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 high X-Pol component which will cause interference in the opposite [or orthogonal] transponder. 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. 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. Y 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.

Z 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.

SATELLITES IN GENERAL Part 1 of Section 1 (SATELLITE COMMUNICATIONS - A SHORT COURSE) of SATELLITE COMMUNICATIONS, prepared by Dr. Regis Leonard for NASA Lewis Research Center What Keeps Objects in Orbit? For 10,000 years (or 20,000 or 50,000 or since he was first able to lift his eyes upward) man has wondered about questions such as "What holds the sun up in the sky?", "Why doesn't the moon fall on us?", and "How do they (the sun and the moon) return from the far west back to the far east to rise again each day?" Most of the answers which men put forth in those 10,000 or 20,000 or 50,000 years we now classify as superstition, mythology, or pagan religion. It is only in the last 300 years that we have developed a scientific description of how those bodies travel. Our description of course is based on fundamental laws put forth by the English genius Sir Isaac Newton in the late 17th century.

Please note, we say we have a "description" of how the sun and moon travel - not an "explanation." Even Sir Isaac, after publishing his theory of gravitation, made that distinction. Although his theory was an accurate description of how gravity works and was consistent with every bit of experimental evidence available at that time, he was careful to disavow any knowledge of why gravity worked that way.

The first of Newton's laws, which was a logical extension of earlier work by Johannes Kepler, proposed that every bit of matter in the universe attracts every other bit of matter with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between the two bits. That is, larger masses attract more strongly and the attraction gets weaker as the bodies are moved farther apart.

OPTIONAL FOR THE MATHEMATICALLY INCLINED Stated mathematically, Newton's law of gravity says that the magnitude of the attractive force (between the earth and the sun for example) is given by: F = G(Mearth Msun) / R2 :where Mearth is the mass of the earth Msun is the mass of the sun R is the distance between the sun and the earth, and G is a constant which was measured by Cavendish in the late 18th .century

Newton's law of gravity means that the sun pulls on the earth (and every other planet for that matter) and the earth pulls on the sun. Furthermore, since both are quite large (by our standards at least) the force must also be quite large. The question which every student asks (well, most students anyway) is, "If the sun and the planets are pulling on each other with such a large force, why don't the planets fall into the sun?" The answer is simply (are you ready for this?) THEY ARE! The Earth, Mars, Venus, Jupiter and Saturn are continuously falling into the Sun. The Moon is continuously falling into the Earth. Our salvation is that they are also moving "sideways" with a sufficiently large velocity that by the time the earth has fallen the 93,000,000 miles to the sun it has also moved "sideways" about 93,000,000 miles - far enough to miss the sun. By the time the moon has fallen the 240,000 miles to the earth, it has moved sideways about 240,000 miles - far enough to miss the earth. This process is repeated continuously as the earth (and all the other planets) make their apparently unending trips around the sun and the moon makes its trips around the earth. A planet, or any other body, which finds itself at any distance from the sun with no "sideways" velocity will quickly fall without missing the sun, will be drawn into the sun's interior and will be cooked to well-done. Only our sideways motion (physicists call it our "angular velocity" ) saves us. The same of course is true for the moon, which would fall to earth but for its angular velocity. This is illustrated in the drawing below.

The Earth Orbits the Sun With Angular Velocity People sometimes (erroneously) speak of orbiting objects as having "escaped" the effects of gravity, since passengers experience an apparent weightlessness. Be assured, however, that the force of gravity is at work. Were it suddenly to be turned off, the object in question would instantly leave its circular orbit, take up a straight line trajectory, which, in the case of the earth, would leave it about 50 billion miles from the sun after just one century. Hence the gravitational force between the sun and the earth holds the earth in its orbit. This is shown in the drawing below, where the earth was happily orbiting the sun until it reached point A, where the force of gravity was suddenly turned off.

The Earth No Longer Orbits the Sun if Gravity is Switched Off The apparent weightlessness experienced by the orbiting passenger is the same weightlessness which he would feel in a falling elevator or an amusement park ride. The earth orbiting the sun or the moon orbiting the earth might be compared to a rock on the end of a string which you swing in a circle around your head. The string holds the rock in place and is continuously pulling it toward your head. Because the rock is moving sideways however, it always misses your head. Were the string to be suddenly broken, the rock would be released from its orbit and fly off in a straight line, just as earth did in the drawing above. One question which one might ask is " Does the time required to complete an orbit depend on the distance at which the object is orbiting?" In fact, Kepler answered this question several hundred years ago, using the data of an earlier astronomer, Tycho Brahe.

Except for Kepler's analysis of his data, it is possible that Tycho Brahe would be best remembered today as a drinker and womanizer. However, without Brahe's unbelievably careful measurements of the planetary positions over many years, Kepler's revolutionary proposals would have been impossible.

After years of trial and error analysis (by hand - no computers, no calculators) , Kepler discovered that the quantity R3 / T2 was the same for every planet in our solar system. (R is the distance at which a planet orbits the sun, T is the time required for one complete trip around the sun.)Hence, an object which orbits at a larger distance will require longer to complete one orbit than one which is orbiting at a smaller distance. One can understand this at least qualitatively in terms of our "falling and missing" model. The planet which is at a larger distance requires longer to fall to where it would strike the sun. As a result, it takes a longer time to complete the ¼ trip around the sun which is necessary to make a circular orbit.

OPTIONAL FOR THE MATHEMATICALLY INCLINED Kepler's laws and the dependence of period on radius are simple consequences of Newton's second law of motion and Newton's law of gravitation. We know that the second law (which every physics student should recognize) says: F = MA We also know that the F, or force, in this case is the force of gravity, given to us by Newton: F = G(Mearth Msun) / R2 Finally, we know (or could show fairly easily) that the acceleration experienced by a body moving in a circle of radius R at constant speed (V) is given by A = V2 / R Putting these two expressions into the F = MA equation, one obtains: G(Mearth Msun) / R2 = MearthV2 / R or just GMsun/ R2=V2 / R But the velocity is simply the distance traveled in one orbit (2(pi)R) divided by the time required for one orbit (T). Inserting this quantity

(2(pi)R / T) for V, we obtain: GMsun/R2=(2(pi)R / T)2 / R - or T2 = 4(pi)2R3/ GMsun View Bumper Sticker

Can We Imitate Nature? (Artificial Satellites) Very soon after Newton's laws were published, people realized that in principle it should be possible to launch an artificial satellite which would orbit the earth just as the moon does. A simple calculation, however, using the equations which we developed above, will show that an artificial satellite, orbiting near the surface of the earth (R = 4000 miles) will have a period of approximately 90 minutes. This corresponds to a sideways velocity (needed in order to "miss" the earth as it falls), of approximately 17,000 miles/hour (that's about 5 miles/second) . To visualize the "missing the earth" feature, let's imagine a cannon firing a cannonball.

Launching an Artificial Satellite In the first frame of the cartoon, we see it firing fairly weakly. The cannonball describes a parabolic arc as we expect and lands perhaps a few hundred yards away. In the second frame, we bring up a little larger cannon, load a little more powder and shoot a little farther. The ball lands perhaps a few hundred miles away. We can see just a little of the earth's curvature, but it doesn't really affect anything. In the third frame, we use our super-shooter and the cannonball is shot hard enough that it travels several thousand miles. Clearly the curvature of the earth has had an effect. The ball travels much farther than it would have had the earth been flat. Finally, our mega-super-big cannon fires the cannonball at the unbelievable velocity of 5 miles/second or nearly 17,000 miles/hour. (Remember - the fastest race cars can make 250 miles/hour. The fastest jet planes can do a 2 or 3 thousand miles/hour.) The result of this prodigious shot is that the ball misses the earth as it falls. Nevertheless, the earth's gravitational pull causes it to continuously change direction and continuously fall. The result is a "cannonball" which is orbiting the earth. In the absence of gravity, however, the original throw (even the shortest, slow one) would have continued in a straight line, leaving the earth far behind. For many years, such a velocity was unthinkable and the artificial satellite remained a dream. Eventually, however, the technology (rocket engines, guidance systems, etc.) caught up with the concept, largely as a result of weapons research started by the Germans during the second World War. Finally, in 1957, the first artificial satellite, called Sputnik, was launched by the Soviets. Consisting of little more than a spherical case with a radio transmitter, it caused quite a stir. Americans were fascinated listening to the "beep. beep, beep" of Sputnik appear and then fade out as it came overhead every 90 minutes. It was also quite frightening to think of the Soviets circling overhead inasmuch as they were our mortal enemies.

Let's think about what would have happened to a "bomb" which would have been dropped from an orbiting Soviet satellite (America's worst nightmare in 1957). Simply "dropping" the bomb would do nothing. The bomb had a sideways velocity of 17,000 miles/hour when it was part of the spacecraft. Simply separating it from the spacecraft will not cause it to drop to earth. It still has its sideways velocity and will continue to miss the earth as it falls. In order to make it hit the earth, we must get rid of its sideways velocity - a task almost as challenging as imparting that sideways velocity in the first place.

After Sputnik, it was only a few years before the U.S. launched its own satellite; the Soviets launched Yuri Gagarin, the first man to orbit the earth; and the U.S. launched John Glenn, the first American in orbit. All of these flights were at essentially the same altitude (a few hundred miles) and completed one trip around the earth approximately every 90 minutes. People were well aware, however, that the period would be longer if they were able to reach higher altitudes. In particular Arthur Clarke pointed out in the mid-1940s that a satellite orbiting at an altitude of 22,300 miles would require exactly 24 hours to orbit the earth. Hence such an orbit is called "geosynchronous" or "geostationary." If in addition it were orbiting over the equator, it would appear, to an observer on the earth, to stand still in the sky. Raising a satellite to such an altitude, however, required still more rocket boost, so that the achievement of a geosynchronous orbit did not take place until 1963.

You may have heard of Arthur Clarke. He is the same gentleman who wrote "2001: A Space Odyssey" and lends his name to "Arthur Clarke's Mysterious World" - the television series.

COMMUNICATIONS SATELLITES Part 2 of Section 1 Why Satellites for Communications By the end of World War II, the world had had a taste of "global communications." Edward R. Murrow's radio broadcasts from London had electrified American listeners. We had, of course, been able to do transatlantic telephone calls and telegraph via underwater cables for almost 50 years. At exactly this time, however, a new phenomenon was born. The first television programs were being broadcast, but the greater amount of information required to transmit television pictures required that they operate at much higher frequencies than radio stations. For example, the very first commercial radio station (KDKA in Pittsburgh) operated ( and still does) at 1020 on the dial. This number stood for 1020 KiloHertz - the frequency at which the station transmitted. Frequency is simply the number of times that an electrical signal "wiggles" in 1 second. Frequency is measured in Hertz. One Hertz means that the signal wiggles 1 time/second. A frequency of 1020 kiloHertz means that the electrical signal from that station wiggles 1,020,000 times in one second.

The expressions "kilo", "mega", and "giga" are used by scientists as a shorthand way of expressing very large numbers. The prefix "kilo" in front of a unit means 1000 of that unit. "Kilo is abbreviated as k. For example, a kilogram (Kg) is 1000 grams. In the same way, "mega" means 1 million. Mega is abbreviated as M. A megawatt (MW) is 1,000,000 watts. The prefix "giga" stands for 1 billion. It is abbreviated as G. Hence a gigabit (Gbit) of data is 1,000,000,000 bits of data.

Television signals, however required much higher frequencies because they were transmitting much more information - namely the picture. A typical television station (channel 7 for example) would operate at a frequency of 175 MHz. As a result, television signals would not propagate the way radio signals did.

Both radio and television frequency signals can propagate directly from transmitter to receiver. This is a very dependable signal, but it is more or less limited to line of sight communication. The mode of propagation employed for long distance (1000s of miles) radio communication was a signal which traveled by bouncing off the charged layers of the atmosphere (ionosphere) and returning to earth. The higher frequency television signals did not bounce off the ionosphere and as a result disappeared into space in a relatively short distance. This is shown in the diagram below.

Radio Signals Reflect Off the Ionosphere; TV Signals Do Not Consequently, television reception was a "line-of-sight" phenomenon, and television broadcasts were limited to a range of 20 or 30 miles or perhaps across the continent by coaxial cable. Transatlantic broadcasts were totally out the question. If you saw European news events on television, they were probably delayed at least 12 hours, and involved the use of the fastest airplane available to carry conventional motion pictures back to the U.S. In addition, of course, the appetite for transatlantic radio and telephone was increasing rapidly. Adding this increase to the demands of the new television medium, existing communications capabilities were simply not able to handle all of the requirements. By the late 1950s the newly developed artificial satellites seemed to offer the potential for satisfying many of these needs. Low Earth-Orbiting Communications Satellites In 1960, the simplest communications satellite ever conceived was launched. It was called Echo, because it consisted only of a large (100 feet in diameter) aluminized plastic balloon. Radio and TV signals transmitted to the satellite would be reflected back to earth and could be received by any station within view of the satellite.

Echo Satellite Unfortunately, in its low earth orbit, the Echo satellite circled the earth every ninety minutes. This meant that although virtually everybody on earth would eventually see it, no one person, ever saw it for more than 10 minutes or so out of every 90 minute orbit. In 1958, the Score satellite had been put into orbit. It carried a tape recorder which would record messages as it passed over an originating station and then rebroadcast them as it passed over the destination. Once more, however, it appeared only briefly every 90 minutes - a serious impediment to real communications. In 1962, NASA launched the Telstar satellite for AT&T.

Telstar Communications Satellite Telstar's orbit was such that it could "see" Europe" and the US simultaneously during one part of its orbit. During another part of its orbit it could see both Japan and the

U.S. As a result, it provided real- time communications between the United States and those two areas - for a few minutes out of every hour. Geosynchronous Communications Satellites The solution to the problem of availability, of course, lay in the use of the geosynchronous orbit. In 1963, the necessary rocket booster power was available for the first time and the first geosynchronous satellite , Syncom 2, was launched by NASA. For those who could "see" it, the satellite was available 100% of the time, 24 hours a day. The satellite could view approximately 42% of the earth. For those outside of that viewing area, of course, the satellite was NEVER available.

Syncom II Communications Satellite However, a system of three such satellites, with the ability to relay messages from one to the other could interconnect virtually all of the earth except the polar regions. The one disadvantage (for some purposes) of the geosynchronous orbit is that the time to transmit a signal from earth to the satellite and back is approximately ¼ of a second - the time required to travel 22,000 miles up and 22,000 miles back down at the speed of light. For telephone conversations, this delay can sometimes be annoying. For data transmission and most other uses it is not significant. In any event, once Syncom had demonstrated the technology necessary to launch a geosynchronous satellite, a virtual explosion of such satellites followed. Today, there are approximately 150 communications satellites in orbit, with over 100 in geosynchronous orbit. One of the biggest sponsors of satellite development was Intelsat, an internationally-owned corporation which has launched 8 different series of satellites (4 or 5 of each series) over a period of more than 30 years. Spreading their satellites around the globe and making provision to relay from one satellite to another, they made it possible to transmit 1000s of phone calls between almost any two points on the earth. It was also possible for the first time, due to the large capacity of the satellites, to transmit live television pictures between virtually any

two points on earth. By 1964 (if you could stay up late enough), you could for the first time watch the Olympic games live from Tokyo. A few years later of course you could watch the Vietnam war live on the evening news.

A geosynchronous satellite must orbit at 22,300 miles altitude and it must be over the earth's equator. As a result, there are a limited number of "slots" for satellites. The allocation of these slots is carefully regulated by an international governing body. Needless to say, both processes are highly political inasmuch as (1) there are billions of dollars to be made, and (2) few things are more prestigious for a small, newly independent country than to be able to say, "We have our own satellite." To date (and for the foreseeable future) satellite communications is the biggest and virtually only money-making business in space.

COMPONENTS FOR COMMUNICATIONS SATELLITES Part 3 of Section 1 Basic Communications Satellite Components Every communications satellite in its simplest form (whether low earth or geosynchronous) involves the transmission of information from an originating ground station to the satellite (the uplink), followed by a retransmission of the information from the satellite back to the ground (the downlink). The downlink may either be to a select number of ground stations or it may be broadcast to everyone in a large area. Hence the satellite must have a receiver and a receive antenna, a transmitter and a transmit antenna, some method for connecting the uplink to the downlink for retransmission, and prime electrical power to run all of the electronics. The exact nature of these components will differ, depending on the orbit and the system architecture, but every communications satellite must have these basic components. This is illustrated in the drawing below.

Basic Components of a Communications Satellite Link Transmitters The amount of power which a satellite transmitter needs to send out depends a great deal on whether it is in low earth orbit or in geosynchronous orbit. This is a result of the fact that the geosynchronous satellite is at an altitude of 22,300 miles, while the low earth satellite is only a few hundred miles. The geosynchronous satellite is nearly 100 times as far away as the low earth satellite. We can show fairly easily that this means the higher satellite would need almost 10,000 times as much power as the low-orbiting one, if everything else were the same. (Fortunately, of course, we change some other things so that we don't need 10,000 times as much power.)

OPTIONAL FOR THE MATHEMATICALLY INCLINED In looking at the relative power requirements of satellites at different distances, it is useful to think of the total power (P0) radiated as spreading out and striking the surface of a sphere which is centered on the transmitter and has a radius equal to the distance between the transmitter and receiver. We know that the surface area of a sphere of radius R is given by A = 4(pi)R2 This means that if the power is emitted uniformly in all directions (isotropically) then the amount of power which strikes every square centimeter of the sphere is given by P = P0 / 4(pi)R2 If our receiver has an area of Ar square centimeters, then it will detect an amount of power Pr = Ar P0 / 4(pi)R2 If then R= 223 miles (it makes the arithmetic easier), Pr = Ar P0 / 4(pi)(223 miles)2 On the other hand, if R = 22,300 miles, Pr = Ar P0 / 4(pi)(22,300 miles)2 Which is 10,000 times smaller, so that in order to have the receiver detect the same amount of power, the transmitter power P0 must be 10,000 times larger for the geosynchronous system.

For either geosynchronous or low earth satellites, the power put out by the satellite transmitter is really puny compared to that of a terrestrial radio station. Your favorite rock station probably boasts of having many kilowatts of power. By contrast, a 200 watt transmitter would be very strong for a satellite.

Antennas One of the biggest differences between a low earth satellite and a geosynchronous satellite is in their antennas. As mentioned earlier, the geosynchronous satellite would require nearly 10,000 times more transmitter power, if all other components were the same. One of the most straightforward ways to make up the difference, however, is through antenna design. Virtually all antennas in use today radiate energy preferentially in some direction. An antenna used by a commercial terrestrial radio station, for example, is trying to reach people to the north, south, east, and west. However, the commercial station will use an antenna that radiates very little power straight up or straight down. Since they have very few listeners in those directions (except maybe for coal miners and passing airplanes) power sent out in those directions would be totally wasted. The communications satellite carries this principle even further. All of its listeners are located in an even smaller area, and a properly designed antenna will concentrate most of the transmitter power within that area, wasting none in directions where there are no listeners. The easiest way to do this is simply to make the antenna larger. Doubling the diameter of a reflector antenna (a big "dish") will reduce the area of the beam spot to one fourth of what it would be with a smaller reflector. We describe this in terms of the gain of the antenna. Gain simply tells us how much more power will fall on 1 square centimeter (or square meter or square mile) with this antenna than would fall on that same square centimeter (or square meter or square mile) if the transmitter power were spread uniformly (isotropically) over all directions. The larger antenna described above would have four times the gain of the smaller one. This is one of the primary ways that the geosynchronous satellite makes up for the apparently larger transmitter power which it requires.

OPTIONAL FOR THE MATHEMATICALLY INCLINED Antenna gains, like many power specifications are usually quoted in decibels (dB). The ratio of two power levels in decibels is defined as: R = 10 log10 (P1/P2) If the smaller of the two antenna mentioned above concentrated 100 times as much power on the receiver as would an antenna which

radiated isotropically, then the gain of the smaller antenna would be 10 log10(100) = 20 dB The larger antenna then concentrates 4 times as much power at the receiver as does the smaller one, which is 400 times as much as the one which radiates isotropically. Therefore its gain is 10 log10(400) 26 dB The power supplied by the larger is (400/100) = 4 times as great as the smaller, therefore its gain should be greater than the small one by 10 log10(4) 6 dB - which it is. Power levels are sometimes specified in dBW or dBm. These expressions indicate that the power level in question is being specified as a ratio to 1 watt or 1 milliwatt. For example, 13 dBW means that 10 log10(the power level in watts) = 13 In other words, the given power level is really about 20 watts. Similarly, 13 dBm would correspond to 20 milliwatts of power.

One other big difference between the geosynchronous antenna and the low earth antenna is the difficulty of meeting the requirement that the satellite antennas always be "pointed" at the earth. For the geosynchronous satellite, of course, it is relatively easy. As seen from the earth station, the satellite never appears to move any significant distance. As seen from the satellite, the earth station never appears to move. We only need to maintain the orientation of the satellite. The low earth orbiting satellite, on the other hand, as seen from the ground is continuously moving. It zooms across our field of view in 5 or 10 minutes. Likewise, the earth station, as seen from the satellite is a moving target. As a result, both the earth station and the satellite need some sort of tracking capability which will allow its antennas to follow the target during the time that it is visible. The only alternative is to make that antenna beam so wide that the intended receiver (or transmitter) is always within it. Of course, making the beam spot larger decreases the antenna gain as the available power is spread over a larger area , which in turn increases the amount of power which the transmitter must provide. Power Generation

You might wonder why we don't actually use transmitters with thousands of watts of power, like your favorite radio station does. You might also have figured out the answer already. There simply isn't that much power available on the spacecraft. There is no line from the power company to the satellite. The satellite must generate all of its own power. For a communications satellite, that power usually is generated by large solar panels covered with solars cells - just like the ones in your solarpowered calculator. These convert sunlight into electricity. Since there is a practical limit to the how big a solar panel can be, there is also a practical limit to the amount of power which can generated. In addition, unfortunately, transmitters are not very good at converting input power to radiated power so that 1000 watts of power into the transmitter will probably result in only 100 or 150 watts of power being radiated. We say that transmitters are only 10 or 15% efficient. In practice the solar cells on the most "powerful" satellites generate only a few thousand watts of electrical power. Satellites must also be prepared for those periods when the sun is not visible, usually because the earth is passing between the satellite and the sun. This requires that the satellite have batteries on board which can supply the required power for the necessary time and then recharge by the time of the next period of eclipse.

FUTURE COMMUNICATIONS SATELLITES Part 4 of Section 1 The nature of future satellite communications systems will depend on the demands of the marketplace (direct home distribution of entertainment, data transfers between businesses, telephone traffic, cellular telephone traffic, etc.); the costs of manufacturing, launching, and operating various satellite configurations; and the costs and capabilities of competing systems - especially fiber optic cables, which can carry a huge number of telephone conversations or television channels. In any case, however, several approaches are now being tested or discussed by satellite system designers. One approach, which is being tested experimentally, is the "switchboard in the sky" concept. NASA's Advanced Communications Technology Satellite (ACTS) consists of a relatively large geosynchronous satellite with many uplink beams and many downlink beams, each of which covers a rather small spot (several hundred miles across) on the earth. However, many of the beams are "steerable". That is to say, the beams can be moved to a different spot on the earth in a matter of milliseconds, so that one beam provides uplink or downlink service to a number of locations. Moving the beams in a regular scheduled manner allows the satellite to gather uplink traffic from a number of locations, store it on board, and then transmit it back to earth when a downlink beam comes to rest on the intended destination. The speed at which the traffic is routed and the agility with which the beams move make the momentary storage and routing virtually invisible to the user. The ACTS satellite is also unique in that it operates at frequencies of 30 GHz on the uplink and 20 GHz on the downlink. It

is one of the first systems to demonstrate and test such high frequencies for satellite communications. The ACTS concept involves a single, rather complicated, and expensive geosynchronous satellite. An alternative approach is to deploy a "constellation" of low earth orbiting satellites. By planning the orbits carefully, some number (perhaps as few as 20, perhaps as many as 250) of satellites could provide continuous contact with the entire earth, including the poles. By providing relay links between satellites, it would be possible to provide communications between any two points on earth, even though the user might only be able to see any one satellite for a few minutes every hour. Obviously, the success of such a system depends critically on the cost of manufacturing and launching the satellites. It will be necessary to mass produce communications satellites, so that they can turned out quickly and cheaply, the way VCRs are manufactured now. This seems a truly ambitious goal since until now the average communications satellite might require 6 months to 2 years to manufacture. Nevertheless, at the present time, several companies including Hughes Electronics, Motorola, and Teledesic, Inc., have indicated their intent to undertake such a system.

SUGGESTED CLASSROOM ACTIVITIES to accompany Part 1: Satellites in General Demonstrate that a force is necessary to hold an object in its orbit: •

Attach a tennis ball (or other convenient object) to the end of a string. Swing it round your head in a circular "orbit". At some point, cut the string (that is "turn off the gravity") and observe what happens to the ball, that is, what direction does it go?.

Ask students to verify Kepler's law: •

Look up the average radius (R) of each planet's orbit and its period (T). Is the quantity R3/T2 the same for all of the planets?

Students can prepare reports on subjects such as: • • • •

Sputnik Yuri Gagarin John Glenn The Mercury Astronauts

Conduct model rocket launches •

CAUTION: Be sure to observe proper safety precautions and be sure that there is adequate space for the launch, flight, and recovery. Almost any student (or pair of students) can build the commercially available model kits of "beginners' skill levels".

Part 2: Communications Satellites To relate to students' experience: •

What is the frequency of your favorite radio station? AM or FM? KiloHertz or MegaHertz?

•

Interview your parents (or, if necessary, your grandparents). What are their earliest memories of television?

•

Interview or visit your local cable company. Can they explain the route that your MTV (or HBO or Learning Channel) takes in traveling from its origin to your house? Can you visit their ground station?

•

How has satellite TV affected your life? What do you have that you didn't have otherwise? How has global TV affected our awareness of world events?

To Demonstrate the Geosynchronous Orbit: •

Have one student "portray" the earth; a second student the geosynchronous satellite. Have the "earth" stand in one place and rotate while the "satellite" revolves around the "earth", completing one revolution in exactly the time the "earth" completes one rotation. Ask the "earth" how the "satellite" looked during the rotation.

General Communications Activities: •

Ask class members to orally relay a message written on a piece of paper. Start at the back of the room. Ask the person at the front of the room to write the message (as he received it) on the blackboard. Compare it with the original written message.

•

Have students devise their own non-verbal, non-written communication system and use it to convey a message from one side of the room to the other.

•

Set up a point-to point telegraph system. A 6-volt power supply, a simple switch for a transmitter, and a 6-volt light bulb as a receiver are sufficient. Ask students to transmit a simple message of 6 or eight words, using the International Morse Code. Remind them that each "dot" and "dash" represents (approximately) one bit of transmitted data. Have them time their transmission and calculate a data rate in bits/second. Compare their efforts with the modern systems which can transmit 500,000,000 bits of information/second.

•

Make a list of advancements in communications systems, beginning with person-to-person voice communications. Keep advancements and inventions in chronological order

•

You are going to the park a block away. You are expecting an important phone call. How would you have someone from your home notify you when the call arrives? Consider solutions without electronics and with electronics.

Mathematical Activities: •

Communications satellites orbit at an altitude of 22,300 miles above the earth. How many meters is this?

•

Light and radio waves travel at 300,000,000 meters/second. How long will it take a radio signal to travel from the earth to a satellite in geosynchronous orbit and back?

Possible student reports include: • • • • • • • • •

ECHO satellite SYNCOM satellite NASA's experimental satellites (ATS-1 to ATS-6, CTS and ACTS) INTELSAT satellites The INTELSAT organization and network Arthur Clarke Rockets in general - how do they work NASA Launch Vehicles (rockets) - specific rockets and what they were used for. (Redstone, Atlas, Agena, Titan, Saturn) Wernher von Braun

Part 3: Components for Communications Satellites Collect pictures of satellite Build models of satellites •

From household materials - either specific real satellites or students' own designs.

Posters •

Illustrating the basic components of satellites or the structure of a specific satellite.

Collect pictures of antennas •

Where does each come from? What is it used for?

Demonstrate concept of electrically switched antenna beams •

Simulate antenna beams using a flashlight and a curved mirror. Moving the flashlight to a different location will cause the reflected beam to change direction. Using two flashlights, demonstrate how a different beam direction can be obtained simply by turning one beam on and the other off.

BACKGROUND MATERIALS FOR A VISIT TO NASA'S ADVANCED COMMUNICATIONS TECHNOLOGY SATELLITE (ACTS) FACILITY Part 1: General Description FACTS: The ACTS satellite was launched by means of the space shuttle Columbia on September 12, 1993. The satellite was intended as a test vehicle for new communications technology, and as such carries no commercial traffic. It is, however, available to any legitimate entity (government, corporation or university) who might wish to use its experimental capabilities. The cost of the ACTS program is approximately $500 million. This is largely because of the enormous amount of new technology which is being flown and demonstrated for the first time.

ACTS - An Experimental Communications Satellite ORBIT: ACTS was placed into a geosynchronous orbit (an altitude of 22,300 miles) at 100 degrees west longitude.

SPECIFICATIONS: • • • • •

Exclusive of its antennas and solar array, the satellite is a near cube with dimensions 80" x 84" x 75". Its solar array is 46.9' from tip to tip. The satellite weighs 3250 pounds. Its solar cells provide 1418 watts of electrical power. Its main antennas are 7.2 feet (uplink, receive) and 10.8 feet (downlink, transmit) in diameter.

Part 2: New Communications Technology Higher Frequencies The ACTS satellite receives information from ground stations at one of several frequencies near 30 GHz. Its downlink transmissions to the destination earth stations are at frequencies near 20 GHz. These frequencies are much higher than those currently used on satellite systems. Most commercial satellites presently in use operate with a 6 GHz uplink and a 4 GHz down link. By comparison, a typical AM radio station operates at 1000 KHZ (0.001 GHz) an FM radio station operates at 100 MHz (0.1 GHz). Channel 7 on your TV set is about 175 MHz. The higher frequencies used by ACTS have 4 significant effects: •

It demonstrates the feasibility of an entirely new resource. Just as you can only place a limited number of FM radio stations within the piece of the frequency spectrum allocated for that purpose, so you can only operate a certain number of "satellite stations" within any given frequency band. The lower frequencies which communications satellites have been using until now (4 - 6 GHz, known as the C-band and 11 - 14 GHz, known as the Kuband) are rapidly being filled up. Use of the 30 and 20 GHz bands will nearly double the amount of frequency space available for satellite communications.

•

The higher frequency means that an antenna of a certain size will have greater gain than it would at a lower frequency. The satellite designer can use this fact to his advantage in two ways. First, a higher gain means a smaller spot will be illuminated by the beam when it strikes the earth. As a result, the power which the satellite radiates is concentrated in a smaller area. This will either improve the quality of communications or will allow the designer to actually reduce the power which the satellite emits.The smaller beam spot also means that the satellite can be more precise in selecting the destination for its transmissions. In fact, it can use two small beams, each aimed at a different destination, and, if they do not overlap, it can transmit to both destinations at the same frequency, at the same time without interference. The ACTS satellite has a total of 5 uplink and 5 downlink beams, each with a width of a little less than 0.5 degrees, which corresponds to a "footprint" on earth approximately 150 miles across. Most previous satellites had beams which

covered at least half of the continental United States and frequently covered all of it or even the entire earth. •

A third effect of the higher frequencies at which ACTS receives and transmits is that it is capable of carrying much more information than would a similar satellite at lower frequency. Both the uplink and downlink frequency bands are approximately 2.5 GHz wide (from 27.5 to 30.0 GHz on the uplink; from 17.5 to 20.0 GHz on the downlink). This 2.5 GHz of bandwidth is enough for about 400 conventional television stations or 250,000 telephone calls, or 100,000 "high speed" data transfers (say from your "fast" 28K baud modem. As a demonstration version of a 30/20 GHz satellite, ACTS is not equipped to use the full data-carrying capability of the frequency band. However, it can handle a throughput of 1800 megabits/second, the equivalent of 450 television stations.

•

Finally, ACTS' operating at these higher frequencies means that many of its components had never been built before and required extensive engineering research. The development of these components (transmitters, receivers, antennas and switching devices of the required performance) represented major advances in electronics technology. Their development and demonstration in space means that the satellite communications industry will have relatively inexpensive "off-theshelf" components available when commercial use of these frequency bands becomes a reality.

Moveable Beams Another unique feature of the ACTS satellite is that not only are its beams smaller than previous satellites, but some of them are moveable. The moveability is accomplished not by physically reorienting the antenna system to point in another direction, but by electrically switching the signal. To understand how this works, we need to look at the satellite's antenna in a little more detail. Attached to the transmitter is a small radiating element which launches the signal into space. For the frequencies used here, the radiating element consists of a "horn", which "feeds" a large reflector or "dish". The reflector is shaped like a parabola and acts just like an optical mirror, focusing the energy emitted by the feed horn located at "A" and sending it all off in the same direction, shown as the beam "A" in the figure below. Just as with an optical mirror, if the source of radiation (the feedhorn) is relocated to "B", the focused and reflected beam will go off in a different direction, shown as beam "B" in the figure.

Beam Steering by Means of a Switch

The ACTS satellite has an array of feedhorns, each one corresponding to a different destination on the ground. Switches will route the transmitter signal to that horn which will reach the desired location. Because it is done by electrical switching rather than actually moving antenna parts, the beams can be rearranged in a matter of microseconds. If the beam moves through a series of preprogrammed locations, we say that the beam has "scanned" the entire area. A moveable or scanning beam on ACTS can visit 40 locations, covering a total of about 750,000 square miles, in 1/000 of a second. Two of ACTS' beams have this hopping capability. An operational satellite (that is, a money-maker, not an experiment) could use about 6 such "scanning" beams to provide service to the entire continental United States.

Switching and Routing Capabilities The ACTS satellite differs from all previous satellites in that switching devices on board the satellite (combined with ACTS' small spot beams) actually route messages,

data, or TV programs to a particular destination, rather than "broadcasting" them across the entire country. As a result, it is sometimes referred to as a "switchboard in the sky". ACTS uses two different concepts in switching, depending on whether the message has arrived through one of the fixed beams or one of the scanning beams.

Let's look first at how the satellite provides connections between its fixed beams. Let's think about it first in "slow motion". Suppose the satellite has three fixed beams, one on Atlanta, one on Cleveland, and one on Tampa. The satellite operates by switching the connections among the three beams, so that each beam is connected to each other beam at a very specific time for a very precise length of time. Let's say that between 12:00 and 1:00 the satellite connects the Cleveland uplink beam to the Atlanta downlink beam. Then from 1:00 to 2:00, the satellite connects the Cleveland uplink to Tampa and from 2:00 to 3:00, it connects Cleveland to itself. At 3:00, it repeats the pattern, again connecting Cleveland to Atlanta for an hour. The Cleveland station then operates by recording all incoming messages on a set of three tape recorders one for Atlanta-bound messages, one for Tampa-bound messages, and one for Cleveland-bound traffic. It would record Atlanta messages, for example, for three hours (say from12:00 to 3:00), after which, between 3:00 and 4:00, it would "burst" those 3 hours of Atlanta- bound messages up to the satellite. We might think of this as playing the tape recorder in the "fast forward" mode so that three hours of messages go out in one hour. On the satellite, the messages are immediately sent down the Atlanta beam to a ground station which will record them and then play them out to the intended listener at normal speed (which will take it three hours). By the time it has played all of the message, another segment should be arriving from the Cleveland station. The listener at the end of the line in Atlanta will never suspect that his message has been recorded, transmitted at fast forward speed, and then played back at normal speed - except of course that the entire message has been delayed three hours. Since many messages are only a few minutes long, or request a reply occasionally, the real ACTS system must work at a much higher speed. Connections between beams are provided every few milliseconds and will only last for a fraction of a millisecond, so that messages are delayed not by three hours but only by a few insignificant and practically undetectable milliseconds. This means, of course, that we can't really use tape recorders. They simply can't record, rewind, and fast forward in thousandths of a second. Instead, fast computer memories are used to store data until it is time to "burst" it to the satellite. The process now is really transparent to the end user.

The second mode of routing involves the use of the moveable or scanning beams. In this case, the ground station still needs to "tape record" the message which it wishes to send. It must now synchronize its fast-forward bursts of information with the arrival of the scanning beam at its location. The on board processor receives the message, stores it and retransmits it to the ground when the downlink beam is over the destination. Again, the receiving station will pass the message on to the intended recipient at a normal speed. He'll never suspect all of the skullduggery which has taken place. The moveable beam technique allows the satellite to provide flexibility in its service. If at any instant there are few or no transmissions from a particular beam spot, the satellite can reduce or even eliminate the time it spends on that particular spot, using the time for an area with more demand. Part 3: ACTS Ground Station The ACTS ground station at Lewis Research Center functions as the master control station, as well as being one of the "users". The computers which one can see in the control room provide the programming which determines where the hopping beams will travel, what interconnections will be made on the satellite, which ground stations will have access to the computer, exactly what times they can transmit and receive, and what data rates they can transmit. In addition, the facility here monitors the "health and welfare" of the satellite. These include things like whether the solar cells are providing the specified levels of power, whether the satellite is maintaining its position and orientation properly, and whether electronic systems in general are functioning properly. On the roof of the building which houses the control room, there are two large dish antennas (4.5 meters and 5 meters in diameter.) One is for controlling the satellite and the user network. The other is for a user's terminal. Their orientation can be adjusted. However, it is a slow process and should be unnecessary so long as the satellite maintains its position properly. These antennas are shown in the photograph below.

ACTS Antennas

Just below the roof is located the power equipment for the transmitter. It is located as close as possible to the antenna so as to lose as little power as possible in the cable which connects it to the antenna.

BACKGROUND MATERIALS FOR A VISIT TO NASA'S ADVANCED COMMUNICATIONS TECHNOLOGY SATELLITE (ACTS) FACILITY from SATELLITE COMMUNICATIONS, prepared by Dr. Regis Leonard for NASA Lewis Research Center

Part 4: Potential Applications of ACTS Technology and Concepts The greatest advantages which an ACTS type of system offers are its flexibility, it accessibility, and its capacity. For remote areas and for users on the move (like ships, airplanes, and autos) it offers a communications link which is available when you need it without having to pay for a satellite channel for those times when you don't need it. This feature has already been demonstrated in several ACTS experiments. For example, voice, video, and data were transmitted from a seismic exploration vessel in the Gulf of Mexico to a ground station here. Similarly, communication between a NASA jet and the ground through the ACTS satellite has been demonstrated. Such experiments demonstrate the feasibility of things like "distance learning", in which a number of remote classrooms can be given access to otherwise unavailable teachers or resources. In addition, the high data rates which are made possible by the higher frequencies mean that ACTS can be a useful link between a remote user and a modern supercomputer. Whereas conventional phone lines would severely limit the speed at which the user can obtain results or monitor progress , the ACTS high speed link, with gigabits of data/second allows the user to observe the supercomputer operation in near real time. This has been demonstrated in ACTS experiments in which engineers at Boeing in Seattle were able to watch the results of a computer simulation of a jet engine inlet, providing what they called a "virtual wind tunnel." Similarly, ACTS experiments have demonstrated the practicality of transmitting medical data, including high resolution X-rays or CAT scans via ACTS, thereby making possible diagnosis or consultation from otherwise inaccessible sources. Of course, high data rates are attractive to businesses which might transfer gigabits of data (things like your checking account balance) from one office to another every day. (Or maybe your grades from school to your home computer or TV set every day.)

End of Part 4 (POTENTIAL APPLICATIONS OF ACTS TECHNOLOGY AND CONCEPTS) of Section 3 (BACKGROUND MATERIALS FOR A VISIT TO NASA'S ADVANCED COMMUNICATIONS TECHNOLOGY SATELLITE (ACTS) FACILITY)

Go to Part 5 (QUESTIONS TO BE ANSWERED DURING A VISIT TO THE ACTS FACILITY) of Section 3 Go to Top of Part 4 (this document) Return to SATELLITE COMMUNICATIONS Home Page Responsible NASA Official: [email protected] Web page curator: [email protected] Glenn Research Center at Lewis Field

Part 5: Questions to be Answered During a Visit to the ACTS Facility 1. What word, meaning "goes around the earth once a day", is used to describe the orbit of ACTS ? 2. At what altitude does ACTS orbit? 3. To an observer on the earth, how does ACTS appear to move? 4. What frequencies does the ACTS system use? 5. A larger antenna will result in a (1) larger or (2) smaller beam spot? 6. As frequency increases, the size antenna required to obtain a certain performance becomes (1) larger or (2) smaller? 7. An advantage of the higher ACTS frequency is that a larger bandwidth is available. What does this mean in terms of the amount of data which the satellite can transmit?

8. How wide are the ACTS beam spots? 9. How long does it take the ACTS hopping beams to "visit" all of their destinations? 10. Which of the following are not advantages of an ACTS type of communications system? o It can provide communications to remote areas. o It can provide communications to moving terminals (ships, planes, autos). o It provides and charges for connection only as needed, and used - much like your telephone system. o It is easy to repair. 11. Explain how the satellite's moveable beams are made to move. 12. What do we mean when we call ACTS a "switchboard in the sky"? 13. What did ACTS cost to build? 14. How much money is made each year from the satellite communications business? 15. Explain why a larger antenna allows the satellite to use a lower transmitter power. 16. Name two applications where ACTS seems to offer advantages over conventional telephone. 17. How long does it take a message to travel from the earth to ACTS and back again?

Go to Top of Part 5 (this document) Return to SATELLITE COMMUNICATIONS Home Page Responsible NASA Official: [email protected] Web page curator: [email protected] Glenn Research Center at Lewis Field