9_Line of Sight

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9. Line-of-Sight Systems: Fixed Radio Access, RadioRelays, and Satellites

9.1. IntroductionA line-of-sight system is a radio system where the transmitter and receiver antennae are

arranged such that they point directly at each other and where multipath interference can bekept at controlled levels. There is no obstacle in the transmission path between the twoantennae 1 and the signal attenuation follows the law of free space propagation (one divided bythe square of the distance between the antennae).

Three types of system are described below: fixed radio access (WiMax), radio relayand satellite. The major part of the chapter is concerned with satellite systems since thesesystems are considerably more complex than the other systems.

Line-of-sight systems are used both for access and transport. Fixed access systems are,of course, only applied in the access network. Radio relays are transport technologies whilesatellite systems are used both for transport and access as explained below.

9.2. Fixed Radio Access NetworksThe development of fixed radio access networks started more than 30 years ago but it is onlyrecently that such networks have been established, the reason being that the technology haspreviously been too expensive to compete with twisted pair and optical fibre. WiMax hasaltered this situation.

Fixed network TX/RX

Sectorialantenna

LAN(Ethernet or WiFi)

TX/RXUser

TX/RXUser

TX/RXUser

Directionalantenna

Hotspot

Userterminal

UserterminalBase station

Broadcast

Figure 9.1 Fixed radio-access network.

A fixed radio access network is illustrated in Figure 9.1. The system consists of basestations (TX/RX) connected to the fixed network and user equipment connected to eithersingle user terminals or local area networks. Wireless LANs accessible for everyone (forexample at a hotel or an airport) are often referred to as hotspots.

The base station antenna is radiating in a sector. Every user terminal in the line-of-sight of the base station will receive the signal. The antenna at the user terminal is directional.

The prevailing fixed radio access technology is WiMax. WiMax is not just a singlestandard but a set of standards that may be used for different purposes. The basic standardswere developed by IEEE and are contained in the 802.16 series of specifications. The systems

1 Edge diffraction may be used in certain radio relay systems. One example is a radio relay lin k in Greenlandusing edge diffraction across a nunatak on which it would be too expensive to place a repeater station.


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are designed for operation in the frequency range 2 to 66 GHz. The most popular bands arethe unlicensed bands between 2.4 and 2.7 GHz and between 3.4 and 3.6 GHz. WiMax offersbit rates in the range from 1.5 to more than 100 Mbps and is an alternative to ADSL on fixedconnections. The range is usually between 7 and 10 km but may be longer depending on thetopology. The large bandwidth can be achieved over long distances because the link is

stationary.The most interesting aspects of WiMax are commercial. WiMax enables newoperators to build cheap broadband access networks in stead of leasing the access from theincumbents (called local loop unbundling (LLUB) in the telecommunication regulatoryparlance) though the major commercial obstacle has been the cost of the base stations and thetransceivers at the user site. WiMax is also an alternative to optical fibres and ADSL in, forexample, new housing areas because of flexibility with regard to the number of users that canbe connected.

WiMax is connected to the internet and may offer any internet service including VoIP.The radio channel is organised in an orderly manner from the base station. From the userterminal, CSMA and collision avoidance contention control combined with capacity

reservation is used in more or less the same way as in the WLAN systems defined in IEEE802.11.

9.3. Radio RelaysIn line-of-sight radio relay systems, the signal is sent from one antenna to another by use of narrow beams of radio signals. Radio relays are used in the transport network but are not acompetitor but a supplement to optical fibres. Radio relays are used in mountainous and otherterrain where optical fibres are too expensive.

FibreEx Ex

Radio relay pathTXRX

Activerepeater

TXRX

Figure 9.2 Connection between exchanges (Ex) containing fibres and radio relays

The configuration of a typical connection containing radio relays is shown in Figure9.2. The narrow radio beam is obtained by using parabolic dish antennae at both thetransmitter and the receiver. The radio signal is called the carrier. The frequency range usedfor carrier frequencies is in the microwave range from 2 GHz to 40 GHz. This corresponds toa wavelength between 20 cm and 0.75 cm. Higher frequencies are used for communicationover very short paths.

The digital signal or baseband signal to be transferred is modulated on this carrierwave by use of phase shift keying (PSK) or amplitude-phase shift keying (APSK). The bitrates may range from 2 Mbps to 644 Mbps. The latter frequency corresponds to one of themodulation rates used on optical fibres (see SDH in Section 4.4).


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The basic component of a radio relay terminal is shown in Figure 9.3. The basebandsignal is delivered to the transmitter by an optical fibre (or coaxial cable). The transmittermodulates the signal onto the carrier and feeds the modulated signal to the antenna. The signalin the opposite direction is picked up by the antenna and demodulated by the receiver. Theresulting base-band signal is delivered to the fibre as shown.

Transmitter ReceiverDuplexer

Parabolic dishMicrowave feeder

Optical fibre

Horn

Sphericalwave

Planewave

Base band signal

Figure 9.3 Components of a radio relay

The simplest antenna 2 consists of a parabolic dish, a feeder and a horn. The hornradiates the signal as a spherical wave. The parabolic dish then collimates the wave so thatwhen the wave leaves the dish, it is confined to a thin pencil of rays along the axis of theparabola (almost plain wave). The radiating source (horn in the figure) is located in the focalpoint of the parabola. Similarly, a plane wave received by the parabolic dish is focused into aspherical wave having maximum intensity at the focal point where it is picked up by the horn.

The radio relay contains a single antenna that both transmits and receives signals. Thesignal therefore propagates in two directions in the antenna feeder. This is possible inwaveguides 3. A duplexer is used to separate the two directions of wave propagation in theguide so that the transmitted signal is fed to the antenna and not sent into the receiver. Theduplexer also directs the received signal to the receiver so that none of it is lost into thetransmitter chain.

A transmission system may consist of several radio relays in tandem. The system thenconsists of terminals at the end of the transmission chain and active repeaters along the pathwhere the signal is amplified before it is retransmitted. Sometimes passive reflectors are usedin order to lead the signal past obstacles as shown in Figure 9.4. The passive reflector ma yeither be a plain metallic sheet or two parabolic antennae placed back-to-back and connectedby waveguide. The passive reflector does not contain devices amplifying the signal and is acheap way of constructing a complex signal path, for example across a hill or along a windingvalley. Without the passive reflector, a full active repeater station had to be built.

The line-of-sight path is limited by several constraints. The path is naturally limited bythe curvature of the earth. The (geometric) line-of-sight distance between two antennae placed

2 More complex antennae exist: Cassegrain antenna with hyperbolic sub-reflector and where the feed is a wholein the centre of the parabolic dish; offset-feed reflector antenna where the dish is an off-axis part of the parabolaso that the feeder horn is outside the path of the radiated wave but still at the focal point of the parabola; andphase-array antenna consisting of many small antenna elements where the radiation pattern is built up by

controlling the phase of each antenna element.3 A waveguide is a rectangular or cylindrical tube made of metal with high conductivity in which theelectromagnetic signal propagates as a wave confined to the interior of the tube.


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in towers 30 m above the surface of the earth is 40 km. The distance is about 70 km if thetowers are 100 m high. The refractive index of the atmosphere tends to bend the wave towardthe surface of the Earth so tha t the path becomes longer than predicted above. In the examplesabove, we may add 10 to 20% to the geometric distance in order to determine the radiodistance.

The second constraint is the free space attenuation. The signal is attenuated by a factorthat is proportional to both the square of the distance and the square of the frequency (seeSection 9.9 below).

Active repeater(transceiver)

Passive reflector

Terminal

Figure 9.4 Path containing both active repeater and passive reflector

Rain attenuation is also important in the higher frequency bands used for radio relaysystems (above about 10 GHz). The statistics for the rain attenuation is directly given by thestatistics of rain intensity. Radio relay systems used in the long haul part of the transportnetwork carries many connections and require high reliability. Probability of outage due torain attenuation of only 0.01% is common for a radio relay hop. In this case, we may have to

take into account rain attenuation of 20 dB or more.Multipath attenuation occurs when the radio signal can follow several paths betweenthe transmitting and receiving antenna. Multipath propagation is usually caused by reflectionof the signal from the terrain and buildings 4. The multiple signals meet at the receivingantenna with different time delay causing the signals to interfere. Multipath attenuation isavoided by shielding the antennae so that only the main beam is allowed to pass. This can bedone by placing screens close to one of the antennae or using mountain ridges or rooftops asnatural shields.

Typical radio relay hops are about 50 km at frequencies in the frequency range 4 to 6GHz. The hop length is about 25 km for frequencies in the range of 10 to 12 GHz. In therange 18 to 23 GHz, the hop length is about 10 km. A communication path of 500 km may

then consist of 10 to more than 50 transceivers depending upon frequency and terrain.

9.4. Telecommunications Satellite ServicesA brief historyThe satellite systems exploit radio frequencies in the microwave range. The microwave rangespans the frequencies from about 2 GHz to about 200 GHz or, in terms of wavelength, from20 cm to 2 mm. Because of the maturity of the technology, frequencies above about 40 GHzare rarely used.

4 Sometimes the signal may follow different paths through the atmosphere because of rapid variations in thetemperature and moisture gradients of the atmosphere.


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The first satellite put into the geostationary orbit (SYNCOM II) was launched in 1963.The large distance between the Earth and the geostationary orbit gives rise to two problems: itis difficult to launch a satellite into the orbit that is so far away from the Earth, and thepropagation path suffers from huge propagation loss requiring large antennae, high-powermicrowave amplifiers and low-noise microwave receivers. The SYNCOM II experiment

proved the feasibility of the geostationary orbit in practical systems. SYNCOM II thusinaugurated the telecommunications satellite area and laid the foundation for the hugespacecraft industry that rapidly started evolving from about 1965.

Early Bird was the first commercial satellite of the telecommunications satellite era. Itwas launched in 1965 about half a year after the International Telecommunications SatelliteConsortium 5, INTELSAT, had been founded. The charter of INTELSAT was to provide andoperate international satellite communication for the member organisations (governments).Since 2001, INTELSAT (now Intelsat Ltd.) is a private company with headquarters inBermuda. This is just another result of the general privatisation of the telecommunicationsindustry. INTELSAT was an organisation owned and run by the old telecommunicationsmonopolies. The number of shares and hence the investment liability of an operator at any

time were determined from how much the operator actually used the satellites. This is nolonger a viable business model in the competitive telecommunications market.

Eight generations of INTELSAT satellites was put into orbit before the organisationwas sold. The evolution of the INTELSAT system is shown in Table 9.1.

Table 9.1 The INTELSAT system

42001997-Intelsat VIII 2

420081993-96Intelsat VII

420051989-91Intelsat VI

2000141980-89Intelsat V

1500111971-78Intelsat IV

29361968-70Intelsat III

87 and 19231967Intelsat II

3911965Intelsat I (Early Bird)

Approximate weight 1 (kg)Number of satellitesLaunch timeSystem

1 The satellites of one series are not identical so that the weight may vary somewhatfor different satellites. The table contains the most common value.2 Because of the privatisation of INTELSAT, detailed information about the operationsis no longer available.

How international satellite communication will evolve in the future is too early to

predict. Intelsat Ltd. offers intercontinental telecommunications services in competition withterrestrial optical fibre systems. Therefore, we may expect that the need for intercontinentalcommunications satellites will be smaller in the future as the optical network is built outfurther. Broadcast distribution on the terrestrial internet may also reduce the need for satellitebroadcast services.

The most noticeable evolution in the communication satellite business is the weight of the satellite. The weight of the Early Bird was only 39 kg. This was the carrying capacity of the original Delta rocket. The Atlas Centaur rocket, having a carrying capacity of about 2000kg was used between 1971 and 1988. Since 1989, the weight of the satellites has been inexcess of 4000 kg matching the carrying capacity of the European Ariane rocket.

5 From 1973 renamed as the International Telecommunications Satellite Organization.


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The telecommunications capacity of Intelsat VIII is about 120,000 64 kbps telephonechannels 6 or 22,000 telephone channels combined with 3 television broadcast channels.

The design lifetime of the early satellites was between 7 and 10 years. The designlifetime of Intelsat VIII is 14 to 17 years.

In 1969, a study in Norway showed that it was both technically and economically

feasible to offer geostationary satellite communication to ships. A study in the USA at thesame time also proved the feasibility of providing a similar service to aircraft. In both cases,the main motive was improved safety. The only radio communication system that could beused with ocean-going ships and aircraft on intercontinental flights was Morse telegraphy atHF (the frequency band between 3 and 30 MHz). HF propagation makes use of the reflectionof the radio waves by the ionosphere thereby providing worldwide coverage. The regionbetween the ionosphere and the ground can be modelled as a huge waveguide. However, thepropagation conditions at HF are complex and far from stable. Dead-zones with which long-range communication is not possible may form 7. A dead-zone may persist for several days ata time.

The event that triggered the study of maritime satellite communications was the

accident of the Norwegian ship Etnefjell in 1968, where 30 seamen lost their lives. When theship started burning, it was located in the ocean south of Africa in a region where there are nocommercial air-traffic routes so that the distress message was not picked up by aircraft 8. Inaddition, there were no other ships within radio distance and the radio-propagation conditionson HF were such that it was impossible to send messages to the distress centres in South-Africa or elsewhere.

The first maritime-mobile satellite system (MARISAT) was put into service in 1976-77 by the private US company COMSAT General. COMSAT General put an additionalcommercial transponder on a set of satellites they were to operate on behalf of the U. S. Navy.These satellites had the capacity to carry the commercial transponder in addition to their mainmission. The MARISAT system was taken over by the organisation Inmarsat in 1982, thesame year the Norwegian coast earth station was put into operation at Eik in Rogaland.Inmarsat was inaugurated in 1979 as an intergovernmental organisation for offering satellitecommunication to commercial vessels. Inmarsat later entered into aeronautical and landmobile satellite services and is now about to introduce broadband services with bit rates up to432 kbps. As of 2005 Inmarsat is traded on the stock exchange and, as INTELSAT, hasbecome a private company.

Since 1975, a large number of national satellite systems have been put into operationfor serving particular parts of the world. The first one was put in operation in 1976 in Norway(NORSAT) as a communication service for the oil installations in the North Sea. The systemused leased satellite transponders owned by INTELSAT. This system was later expanded to

6 Since each party of a speech conversation is actively speaking for only 40% of the time on average, only about120,000 0.4 = 48,000 physical channels are required for carrying 120,000 simultaneous conversations if amultiplexing technique called speech interpolation is used. Speech interpolation multiplexing means that thesatellite channel in one direction is assigned to a conversation only during the 40% of the time the channelactually contains a speech signal in this direction. During speech pauses, the channel is reallocated to otheractive conversations. When speech is again detected in the original conversation, the system allocates a newchannel for this conversation. If there is no idle channel, this may result in a phenomenon called freeze-outwhere one or two phonemes are lost at the beginning of the first utterance. However, the dynamics of speech issuch that freeze-out is no problem even if the satellite supports 2.5 times as many conversations than there arephysical channels.7

When and where such zones will form can actually be predicted from atmospheric and meteorologicalobservations.8 Aircraft and ships shared the same distress frequency at HF.


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provide telecommunications and broadcast services to Spitsbergen. Later, national satellitesystems were built in several countries (including Indonesia, Australia, Canada and Japan).

Several of these systems have met severe competition from optical fibres but some of them will continue into the future for the simple reason that all other telecommunicationssystem are likely to be too expensive. One particular system is called VSAT offering services

to remote locations such as hydropower stations and dams. VSAT systems are easy toreconfigure. This makes these systems particularly suitable as local area networks inenvironments that are temporary or likely to change rapidly such as construction sites. Inparticular in the USA, VSAT is used for interconnecting the offices of companies spread overlarge areas such as shopping chains and mining companies.

The idea behind the Iridium system was to employ low earth orbit (LEO) (see belowfor definition of the term) satellites in polar orbits. The system was planned with 66 satellitesin six polar orbits containing 11 satellites each 9. The business idea was to offer worldwideland mobile communication. The business idea was totally unrealistic and based on marketassessments without any rational foundation. The price of the Iridium terminals and thecommunication charges could not compete with terrestrial mobile systems such as GSM. The

traffic in areas not covered by GSM and similar systems was too small to form a soundcommercial basis for Iridium. The system neither offered any particular features that wouldmake it attractive for potential users. Therefore, the Iridium Consortium went bankrupt beforeit commenced operation.

However, the satellites were later sold to other companies re-establishing the services.The systems offer telecommunications to governments, oil industry, scientific explorations,relief operations and travellers. By the end of 2005, Iridium had 142,000 subscribers. The newIridium system is claimed to be profitable because most of the original debts could be writtenoff.

An even more ambitious project was Teledesic where a constellation of 824 LEOsatellite in polar orbits should offer global broadband data services independently of and incompetition with terrestrial systems. The project was scaled down to encompass 288 satellitesbefore it was eventually given up because of totally unrealistic market estimates: the projectwas even more pretentious and farther from reality than the Iridium project (the two projectswere even competing for the same customers). Bill Gates and Boeing were two of the majorsupporters of the project. Projects like Iridium and Teledesic must be viewed as twooffsprings of the overheated dot.com economy. However, systems like Teledesic may becomefeasible in the future. Therefore, the idea has not been abandoned completely.

Satellites are applied in numerous areas other than telecommunications. Examples areastronomy and astrophysics, meteorology, earth resource management and surveillance,navigation, space research and so on and so forth.

Satellite orbitsSatellite orbits can be classified as follows.

Low earth orbit (LEO) satellites are at an altitude above the surface of the Earthbetween 500 and 1000 km. The orbit periods of these satellites are between 1.6and 1.8 hours.

Medium earth orbit (MEO) satellites are at an altitude between 10,000 and 30,000km. The GPS satellites are MEO satellites at an altitude of 20,200 km.

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Originally the system concept consisted of 77 satellites. The system was christened Iridium because elementNo. 77 of the periodic system is Iridium. When the number of satellites was reduced to 66 satellites, the namewas not changed in the same vain because element No. 66 carries the more prosaic name dysprosium.


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Geostationary (GEO) satellites are located in an equatorial orbit at an altitude of 36,000 km. These satellites make one revolution in 24 hours, and since they rotatein the same direction as the Earth, they are always at a fixed location relative to apoint on the surface of the Earth. There is only one geostationary orbit.

The inclination of the satellite orbit is the angle the orbit makes with the equatorialplane of the Earth. The geostationary orbit has 0 inclination while a polar orbit has 90 inclination. The inclination of the GPS satellites is 55 .

The satellite orbit is elliptical where the Earth is located in one of the foci of theellipse. The eccentricity is the deviation from circularity of the orbit: the larger theeccentricity the more elliptical is the orbit. The eccentricity of an ellipse is a number in theinterval 0 < 1, where 0 corresponds to a circle 10 .

The ideal geostationary orbit is circular. However, the ideal geostationary orbit isunstable and gravitational perturbations caused by the Sun and the Moon and inhomogeneitiesin the gravitational field of the Earth converts the orbit into a helix winding around a slightlyelliptical path. Therefore, the position of the satellite is not entirely geostationary. The

satellite will drift along the orbit and move up and down vertically relative to the equatorialplane. The actual geometry of the geostationary orbit is very complex.The orbit position and the attitude of the satellite must be adjusted from time to time.

This activity is called station keeping . For this purpose, the satellite is equipped with severalsmall booster rockets that are used to move the satellite along the orbit, adjust the altitude andinclination of the orbit and alter the rotational speed of the body of the satellite.

Intercontinental satellite systems (INTELSAT), Inmarsat, broadcast satellites andmany national satellite systems exploit the geostationary orbit. The geostationary orbit is verycrowded and contains also much waste (remnants after earlier satellites). Now satellitesplaced in the geostationary orbit shall have a small remainder of propulsion fluid at their endof life so that they can be pushed out of the geostationary orbit (if the telemetry system still

works). There are no satellites at altitudes between 2000 an 8000 km because of the intenseelectromagnetic radiation and high particle activity (electrons, protons, ions and various forceparticles such as mesons) in this region (the Van Allen radiation belt). Even a short stay atthese altitudes may knock out the electronics in the space craft.

Transfer orbit is the orbit in which the satellite is placed before it is manoeuvred intothe final orbit via the deployment orbit . For simplicity, the deployment orbit in Figure 9.5 isshown as a single arch from the transfer orbit to the geostationary orbit. In reality, the orbitmay spiral towards the final orbit for several revolutions around the Earth. The transfer orbitof a GEO satellite is highly elliptical with the apogee (highest point of the orbit) close to thegeostationary orbit (altitude of 36,000-37,000 km). The lowest point (the perigee) is almost

touching the atmosphere at an altitude of about 300 km. The booster rockets are used to pushthe satellite from the deployment orbit and into its final position in the geostationary orbit.This operation may take several weeks. During this time, the satellite will pass through theVan Allen belt several times.

The orbital parameters for the geostationary orbit, the launch orbit and the deploymentorbit are shown in Figure 9.5. Placing the satellite in orbit is a complex process consisting of anumber of stages:

Launching.

10

The general formula for a conic can be written in the form y2

+ (1 2

) x2

= 4 px where is the eccentricity and p is a constant. If 0 < 1, the conic is an ellipse; if > 1, the conic is a hyperbola; if = 1, the conic is aparabola.


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Separation of the satellite from the launch vehicle and transfer of control from thelaunch control centre to the satellite control centre.

Orienting the satellite in the transfer orbit so that it is ready for injection into thefinal orbit.

Accurately controlling the apogee boost rockets in order to manoeuvre the rocketinto the final orbit and moving it along the orbit to the desired location.

Spin stabilise the satellite so that the communication antennae are pointing towardthe Earth.

Unfolding the solar panels and activating the sun acquisition subsystem. Testing all telemetry functions and the telecommunications platform 11 .

Perigee

Apogee

Transfer orbit

Geostationary orbit

Launch orbit

Deployment orbit

300- 500 km

Altitude 36,000 km

36,000-37,000 km

Figure 9.5 Bringing the satellite into the geostationary orbit

During eclipse the satellite is in the shadow of the Earth and does not receive lightfrom the Sun for its solar panels. The time the satellite is in eclipse depends on the orbit. Themaximum time the GEO satellite is in the shadow of the Earth is approximately one hour per24 hours, or about 4% of the time. The maximum occurs around equinox while aroundsummer and winter solstice there is no eclipse.

The maximum eclipse of a LEO satellite is about 30 minutes every100 minutes (therevolution time of the LEO) or 30% of the time. If the LEO orbit rotates such that the orbitalplane is vertical 8 to the direction from the Sun, there will be no eclipse. Such orbits arecalled sun synchronous 12 . The sun-synchronous orbits must obviously be polar orbits. Onlyone or two polar orbits of a global system such as Iridium can be sun synchronous. All otherorbits will experience eclipse between 0% and 30% of the time.

Full operation during eclipse is ensured by battery backup. The batteries are thenloaded when they receive sunlight.

11 One amusing event that really showed what could be done to a satellite took place in 1976. Thetelecommunications transponder of the first MARISAT satellite did not work properly. Investigations of thesecond satellite that was ready for launching showed that the problem could be caused by gold dust in one of themicrowave cavities in the transponder. The satellite was then shaken by firing booster rockets on opposite sides

of the satellite in rapid succession. This apparently caused the dust to settle somewhere in the cavity where it nolonger caused signal loss. After that the satellite worked without any further problems for many years.12 Such orbits are exploited in several scientific satellite missions.


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The battery capacity of a LEO satellite must be such that it allows full operationduring the maximum eclipse period of thirty minutes. The battery must be fast-charging sinceit must be fully charged in about one hour.

A final point is concerned with how the satellite can direct the communicationantennae toward the Earth and not point it in an arbitrary direction in space. This is called

attitude control . If the satellite body is spinning the principle is simple. A set of opticalsensors consisting of a telescope and a photo detector are mounted on the satellite body. Asthe body rotates the sensors will detect a low temperature (a few K) if the detector is pointingtoward empty space and a high temperature (about 300 K) if the sensor is pointing toward theEarth. This gives rise to pulse patterns from the sensors that can be used to determine exactlywhere the Earth is located relative to the sensor assembly. In fact, just two such sensors calledsouth-pointing and north-pointing Earth sensors are required for attitude control of a spinningsatellite 13 . The principle is shown in Figure 9.6 for four attitudes of the satellite.

Attitude Earth Sensor signals

Figure 9.6 Attitude control

Pairs of small booster rockets on the body of the satellite are fired for very shortperiods of time causing the satellite to rotate vertically to its spinning axis thereby altering theattitude of the satellite until the sensors point directly at the Earth. Once the satellite body is inthe correct position, servo loops ensures that the satellite retains its proper direction in spaceby firing the booster rockets when necessary. The attitude of the satellite can also be alteredby the satellite control centre (the TT&C centre) on Earth.

The antenna is stabilised by letting electrical motors spin the antenna in the oppositedirection of the satellite body and, in case of spot beam operation, even use standard antennatracking methods to rotate the antenna into position toward one earth station antenna.

The body of a three-axis stabilised satellite does not rotate regularly relative to theEarth. Three-axis stabilisation means that the attitude control keeps the whole body of thesatellite in a fixed orientation in space. In such systems, either spinning sensors can be used

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In addition, the satellite body and any flexible parts of it such as antenna platform and solar cell panels maycontain rate integrating gyros in order to keep track of the motion of the satellite body and the flexible parts. Thesatellite body may also contain sensors for gravitational sensing and sun tracking sensors.


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thus providing the same type of attitude control as spinning satellites, or a sensor array can beused to scan the environment electronically and thereby producing a signal patterndetermining the location of the Earth.

Frequency bands

The most commonly used frequency bands for satellite communication are (using theconvention uplink/downlink): Inmarsat and other international mobile satellite communication systems 1.6/1.5

GHz (uplink 1.610 to 1.660 GHz; downlink 1.525 to 1.610 GHz. These bandscontains sub-bands for maritime services, aeronautical services, land mobileservices and distress services)

Intelsat 6/4 GHz (uplink 5.925 to 6.425 GHZ; downlink 3.700 to 4.200 GHz) and14/12 GHz (uplink 14.000 to 14.500 GHz; downlink 11.700 to 12.200 GHz)

Fixed domestic systems 30/20 GHz NSTAR (Japanese mobile system) and other domestic and international systems

(Inmarsat) 2.5/2.0 GHz Broadcast 12 GHz (only downlink the uplink may be in the 6 GHz band)These bands are allocated on a global basis. Observe that the frequency of the

downlink is smaller than that of the uplink. The reason is that the higher the frequency, thelarger is the propagation loss. This is will become evident in Section 9.7.

9.5. Architecture of Communication Satellite NetworksBroadcast satellite systems

FDMA carrier withmultiplexed

broadcast channels

Commonbroadcastchannel

P

P

P

P

BB

FDMA carrier withmultiplexed

broadcast channels

Figure 9.7 Broadcast satellite system

In broadcast satellite systems, television, audio and data signals are fed to the satellite andbroadcast to the users. The primary mission of broadcast satellite systems is to offerpoint-to-multipoint services.

Several video, audio and data channels are multiplexed on each FDMA carrier orTDMA timeslot assigned to each feeder earth station. The users receive the signals directlyfrom the satellite. The user terminal consists of a parabola for receiving the signals from thesatellite and an electric module containing low noise amplifiers, demodulators, video andaudio channel demultiplexers and possibly decryption or descrambling equipment forextracting the individual channels. The advantage offered by the satellite is that the satellite


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covers a large area and many users. Though the satellite is expensive, the broadcast satellitesystem is much cheaper than a terrestrial broadcast system.

The system configuration is shown schematically in Figure 9.7. In the figure, Prepresents the producer of the information (for example, a broadcasting company) and Bbundles and multiplexes the broadcast channels received from the producers and presents

them to the satellite system.The receiver antennae at the homes are small parabolas with a diameter of about 60cm. The most commonly used frequency band on the downlink is in the 12 GHz range. Thecommon broadcast channel may be radiated in global beams or, what is more common, inspot beams. As we shall see soon, the higher the frequency the smaller antenna is needed todetect the signal. The 12 GHz band was chosen some thirty years ago for satellitebroadcasting services because the frequency is suitable for small antennae and is within thetechnological range where simple and cheap equipment can be designed (wavelength of about2.5 cm require microwave components with linear dimensions of about 1 cm).

Network

Broadband link

Narrowband link Server

Userterminal

Figure 9.8 Internet services over broadcast satellite

One particular application of broadcast satellites is shown in Figure 9.8. The satellitelink offers a one-way broadband channel but no return channel from the user via the satellite.Since the user usually only require narrowband services in the direction toward the network,the return link can be offered via the terrestrial network. This is one simple way for thebroadcast provider to become an internet service provider (ISP) in competition with IPSsoffering terrestrial internet services.

The receiver equipment at home (set-top) must then be equipped with additionalelectronics that picks out the data channel and be able to establish the return connection viathe fixed network. This technique is similar to that used by some cable television companiesin order to avoid investment in new infrastructure.

Fixed satellite systemsSatellites are used in the fixed telecommunications services in the transport network (point-to-point systems) where one advantage is that the cost of the transmission path islargely independent of distance and the number of earth stations connected to the satellite. Itis simple and cheap to add new earth stations to the network and to reconfigure the traffic


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The propagation characteristics and link planning of satellite systems is considered inSection 9.7.

Mobile satellite systemsMobile satellite systems offer access to users located in areas not feasible for terrestrial

technologies. Applications include maritime and aeronautical communication and land mobileservices for distress operation and access to isolated areas. The particular characteristics of these systems are considered in details in Section 9.9.

Very small aperture terminal (VSAT) systemsLocal area networks can be implemented using VSAT technology. VSAT networks have anumber of applications where other solutions are impractical or impossible. Examples are:

Interconnecting remote sites such as hydropower plants and dams with centraloperation, management and control systems.

Providing temporary telecommunications capabilities for construction sites. Setting up local area networks for companies with operations widely distributed

over large distances such as shopping chains.The most important advantage of VSAT systems is that they are easy to install, expand

and reconfigure.

Hub

Outstation

Videoconference

centre

LAN

Outstation

PBXOperation

&management

Telephonenetwork

InternetRouter

LAN

PBX

LAN

Videoconference

centre

Figure 9.10 VSAT system

The most common configuration of a VSAT system is a star network consisting of acentral station or hub and a number of outstations. Communication between outstations mustthen be relayed via the hub. The VSAT system can also be configured as a mesh network (oras a combination of star network and mesh network) where one of the stations is designated asthe network controlling station. In this configuration, the outstations may communicatedirectly with one another. The VSAT system may offer transmission of data, voice and videosignals. The typical configuration of a VSAT network is shown in Figure 9.10.

PBX designates a private branch exchange interconnecting the local telephone systemof outstations and hubs.


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TDMA is employed on the direction from outstation to hub where one or moretimeslots are assigned to each outstation. TDM on a single carrier is used in the direction fromthe hub.

The most common frequency bands are in the 14/11 GHz range; that is, in a bandaround 11 GHz for the downlink and around 14 GHz for the uplink. The antenna size is in the

range of 1.2 to 1.8 m.

9.6. Telecommunications Components of a SatelliteThe satellite is primarily a repeater (amplifier and signal regenerator) placed in orbit aroundthe Earth. The telecommunication payload of the satellite consists of equipment such asantenna, duplexer, low-noise receiver (LNR), signal regenerator (REG), possibly routing andswitching circuits, frequency converter (FQC), and high-power amplifier (HPA). Theconfiguration is shown in figure 9.11. Each path through the satellite is called a transponder.The satellite may contain several transponders.

Duplexer

LNR HPA

Optionalswitch

REG FQC

Transponder

Parabola, phase array

One or more antennae

Telemetry&control

ActuatorsControls

Measurements

Figure 9.11 Transponder

The telemetry & control subsystem allows the satellite to send measurement reportsconcerning the operational status of all its components to the telemetry, tracking & control(TT&C) station monitoring the operation of the satellite. The subsystem also receives anddistributes commands to actuators and control devices required for station keeping (orbitrepositioning), reconfiguration of the transponders, updating software and other controlfunctions.

The satellite may contain one or several communications antennae. These may beparabolas or multi-beam phase array antennae. The antenna may offer a single beam coveringapproximately one third of the Earth or several spot beams only covering a small area each.Spot beam systems are also referred to as space division multiple access systems (SDMA).

The multiple access technique may be frequency division multiple access (FDMA) ortime division multiple access (TDMA) or a combination of these methods. In some systems(e.g., those of Inmarsat), there may also be random access channels offering particularservices such as signalling for allocation of communication resources. Many complexcombinations of multiple access techniques are often exploited together in order to optimisethe satellite to the purpose it serves. Channels may be assigned on a permanent basis or ondemand (DAMA). See Chapter 5 for details.

The low-noise receiver amplifies the weak microwave signal before they are handledfurther. The further signal handling depends on the configuration of the demand assignment


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system and the antenna beam configuration. In the simplest case, the signal is fed to a signalrecovery device (REG) where the signal is amplified, resynchronised and reshaped. Then it isfed to the frequency converter (FQC) where the frequency is shifted to the downlink frequency. Finally, the signal is amplified by the high-power amplifier before if is fed to theantenna system.

If the satellite offers a complex multiple access scheme consisting of a mixture of TDMA, FDMA and SDMA, the regenerator may demodulate and demultiplex the receivedsignal in each transponder and feed it to a switching device where the individual channels aredistributed among all the transponders, remultiplexed, and remodulated before the signal ispresented to the transmitter chain.

9.7. Propagation Characteristics, Noise and Link Budgets

Attenuation

36,000 km

6,378 km42,000 km

Gaincontour

Max

Min

Radiation patternof parabolic antenna

Geometry of geostationary satellite

Figure 9.12 Antenna pattern and link configuration

The commonly used antenna in satellite communication is the parabola with a radiationpattern as shown in Figure 9.12. The parabola collimates the radiated signal into a narrowbeam as shown in the figure. The antenna gain expresses how effectively the antennacollimates the beam. The figure also shows that the shortest distance from the Earth to thegeostationary satellite is 36,000 km while the longest path corresponding to the edge of coverage is about 42,000 km. In the link budgets, the calculations are done for the longestpath and for the antenna gain in this direction (indicated by min in the figure; e.g., 1 dB belowmaximum).

Absorption and

scattering by airand precipitation

Receiver loss

Environmentalnoise from ground

Receiver noise

Pre-amplifier

Environmental noise from air and precipitation

EIRP

Figure 9.13 Loss and noise in radio systems


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Figure 9.13 shows possible sources of signal degradation along the propagationchannel.

The power of the signal p t fed to the antenna by the high power amplifier (HPA)multiplied by the antenna gain gt is called the equivalent isotropic radiated power (EIRP) 14 ;eirp = p t gt or in decibel: EIRP (dBW) = P t (dBW) + G t (dB), where we dBW means dB

relative to 1 W. Small letters will be used for the absolute value of a variable, and thecorresponding capital letter will be used for the same variable expressed in dB. If the antennagain is 13 dB and the power from the HPA is 4 W, then the EIRP = 13 dB + 10 log 4 dBW = 19 dBW or eirp = 80 W.

20 40 60 80 100

0.1

1

10

100

Frequency GHz

A t t e n u a

t i o n

d B / k m

H2O

O2

Figure 9.14 Absorption in the troposphere

The free space loss is ( /4 d )2 where d is the distance between the transmitter and thereceiver and is the wavelength. In decibels: L fs = 147.5 + 20 log d + 20 log f where f is the

frequency in Hz ( f = c / where c is the speed of light in vacuum) and the distance d is given inmetres.Absorption by the troposphere is shown in Figure 9.14. The path- length through the

troposphere is less than 10 km. For frequencies below 20 GHz, the attenuation is thereforeless then 10 km 0.05 dB/km = 0.5 dB. 0.5 dB may then be used as a conservative figure forthe atmospheric loss in the link budget.

The attenuation curves for rain and snow will differ from place to place and have aform as illustrated roughly in Figure 9.15. The best case corresponds to a dry climate whilethe worst case corresponds to a tropical climate with frequent and intense rain. It is impossibleto draw universal curves for the rain attenuation because of the large variation of climatebetween different locations: measurements over several years must be made at the particularantenna location in order to establish accurate statistics. However, there exist theoreticalmodels where fairly accurate rain attenuation curves can be estimated from climatologic data.

Here are a few rules of thumb. The rain and snow attenuation is more severe thehigher the frequency. The higher the rain or snow intensity, the larger is the loss. Theattenuation in snow depends strongly on how wet the snow is and how big the snowflakes are.The rules are that the wetter the snow, the larger the attenuation (high dielectric constant); andthe bigger the snowflakes, the bigger the attenuation (large radar cross section).

14 The EIRP is the power an isotropic antenna would have to radiate uniformly in all directions of space in orderthat the signal strength produced at the receiver is the same as that of the collimated antenna.


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10

1

0.1

0.01

0.001

P e r c e n t o f p e r i o d a t

t e n u a t

i o n e x c e e d e d

5 10 15 20 25 30

Attenuation in dB

Best

Worst

Figure 9.15 Typical rain attenuation curves

Note that 0.1% and 0.01 % of a year is 8.8 hours and 0.88 hours (or 53 minutes),respectively. Selecting a rain margin is thus the same as selecting the availability of the link.If an availability of 99.9% is sufficient, then 1 or 2 dB may be a safe margin. If theavailability shall be 99.99%, the margin in a dry climate may be 5 dB while that in a wetclimate may be as high as 15 dB or more. For a system with global coverage, a single rainmargin is often chosen for the entire system. The trade-off is then between offering lessavailability or increasing the size of the receiving antenna in wet climate. The antenna gain isproportional to the area, or, equivalently, to the square of the radius of the parabolic reflector.This means that a 3 dB increase of the antenna gain corresponds to about 40% increase inantenna radius. If a parabola with 20 m diameter is enough in dry climate, the diameter of anantenna in wet climate offering the same availability may then be 28 m or larger.

Interference from systems sharing the same part of the radio spectrum is another causeof signal deterioration. Radio relay systems and satellite systems share the 6/4 GHz band.Under particular circumstances, these systems may cause significant interference on eachother. Furthermore, circular polarisation is used in many satellite systems. Deviation fromcircularity causes loss in signal power. In systems with linear polarisation, Faraday rotation of the polarisation angle when the signal passes through the ionosphere causes loss in the sameway. A particular loss component in maritime satellite systems is multipath interferencecaused by reflections from the sea and the superstructure of the ship.

Let us call the total loss caused by all the sources discussed above (and possibly a fewmore) for M dB. Then the received signal is P r = EIRP L fs M . The loss M is a variable lossthat will be treated in a particular way in the link budget. EIRP and L fs are calculated for thelongest path at the edge of coverage.

NoiseIn order to calculate the signal-to-noise ratio at the receiver we must first determine the noise.The noise power N in a bandwidth B caused by thermal noise is given by N = kTB where k isBoltzmans constant ( = 1.38 10 23 W/sK (watt per second per kelvin)) and T is thetemperature in kelvin of the device producing the noise. T is also called the noise temperature.In this formula, B is often referred to as the noise bandwidth in order to distinguish it from the

signal bandwidth. In a linear system, the signal bandwidth and the noise bandwidth are equal


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but in nonlinear systems such as phase-locked loops and coherent demodulators signalbandwidth and noise bandwidth are different.

The signal-to-noise ratio , S / N , is defined as the ratio between the signal power and thenoise power at the same point in the system. The noise figure of an electrical circuit expressesthe change in signal-to-noise ratio when the signal passes through the circuit; that is

n f = (S / N )in /(S / N )out

A signal S that passes through an attenuator with loss l and embedded in a mediumwith temperature T is reduced by a factor l. Since the medium has a constant temperature, thenoise power is not affected by the loss but remains at the same value kTB at both the input andthe output. This means that the noise figure for an attenuator with loss l is n f = l since ( S / N )out = (Sin / l)/ N in = (1/ l) (S / N )in. Note that the noise produced by the medium is N in = kTB. We nowdefine an equivalent input noise temperature T in by the following formula N out = (input noiseproduced by the medium + input noise equivalent to that produced by the attenuator)/loss = (kTB + kT in B)/ l. Inserting this expression for N out , the expression for the input noise N in = kTB and Sout = Sin / l in the formula for the noise figure we get the following expression for the noisefigure and the equivalent input noise temperature of a circuit with loss l and temperature T

n f = l = 1 + T in / T or T in = T (l 1)

Similarly, we define the equivalent output noise temperature as T out = T in / l = T (1 1/ l). The equivalent input and output noise temperatures allow us to calculate the equivalentnoise temperature at any reference point in the system.

These formulae will now be used to derive an expression for the noise in the receiverof the satellite system. The configuration of the receiver system is shown in Figure 9.16.

EnvironmentTemperature T e

Loss l e

FeederTemperature T f

Loss l f

Equivalent antenna temperature T a

Clear skytemperature

T c

ReceiverReceiver noisetemperature T r

Figure 9.16 Equivalent antenna temperature of the Earth station

In order to calculate the signal-to-noise ratio in the Earth station we must determinethe signal power and the noise power at the same reference point. The reference point chosenin Figure 9.16 is the output of the antenna. The Universe and the dry atmosphere contributewith a noise temperature T c of approximately 100 K and 20 K at elevation angles of about 5 and 90 , respectively. The temperature may vary slightly with the solar activity. The antennawill always experience this temperature whether the sky is clear or there are clouds, rain orsnow.

The noise temperature of the Moon is about 300 K. Therefore, there is usually noproblem if a high gain receiving antenna points directly at the Moons surface. The noisetemperature of the sun at microwave frequencies is between 100,000 and 1,000,000 Kdepending on the sunspot activity. If the receiver antenna points directly at the sun,communication may be completely blocked. However, the noise contribution from the sun

depends on the gain of the receiving antenna. An antenna with gain exceeding 55 dB willhave an opening angle smaller than the diameter of the sun. The noise contribution is then


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between 100,000 K and 1,000,000 K so long as the antenna points directly at the sun. It iseasy to see from the orbital geometries of the satellite and the Earth and the seasonal rotationof the equatorial plane that such situations only occur seasonally for certain locations of theearth station antenna. The interference takes place only so long as the sun is within theantenna beam, that is, only for a few minutes a day for a few days per year. However, if the

antenna gain is less than 55 dB, the noise temperature caused by the sun decreasescorrespondingly. If the antenna gain is 25 dB, the noise temperature is reduced by a factorcorresponding to 55 25 = 30 dB = 1000; that is, the noise temperature is betweenapproximately 100 K and 1000 K. The noise contribution of the sun to an omnidirectional(isotropic) antenna is less than 3 K.

Noise generated by the sun is then only a problem for high-gain antennae.Around equinox, the sun may cause increase in the noise temperature of the satellite.

This is also easily seen from geometrical arguments. This interference is not severe.If there is rain, the ambient temperature is T e producing the equivalent noise

temperature at the antenna of T e(1 1/ le) where le is the loss due to the rain. If there is no rain,le = 1 and this noise component disappears. A typical value for T e is 290 K (20 C). We seethat the loss through rain enters twice in the performance calculation of the satellite link; thatis, both in the calculation of the noise temperature and as the variable loss component M defined above.

The feeder produces a noise temperature of T f (l f 1) at the reference point. The feedertemperature T f is set to 290 K (20 C). Finally, the equivalent noise temperature of the lownoise amplifier at the reference point is l f T r where T r is the noise temperature of the receiver 15 .Usually the noise produced by the preamplifier is given in terms of the noise figure of theamplifier: T r = (nr 1)T 0 where T 0 is a reference temperature usually taken to be 290 K and nr is the noise figure conventionally given in dB as N r = 10 log nr .

The equivalent noise temperature of the antenna is thus

T a = T c + T e(1 1/ le) + T f (l f 1) + l f T r

In this formula, all components except the rain contribution do not vary over time andcan be regarded as system constants. The noise temperature without rain contribution isreferred to as the system temperature T s

T s = T c + T f (l f 1) + l f (nr 1)T 0 and T a = T s + T e(1 1/ le)

where the noise produced by the preamplifier has been expressed in terms of the noise figureof the amplifier.

The rain contribution is usually treated in a particular way in the link budget just as theloss caused by rain.

The satellite is looking at the Earth having a noise temperature of 290 K. There is, of course, no increase because of rain since rain does not change the temperature of the Earthviewed from the satellite. However, the path loss caused by rain must be included in the totalloss also on the uplink.

Example 1Let us calculate the noise temperature for the following case:

T c = 100 K

15 An attenuator with loss l reduces a noise signal N i at the input in the same way as any other signal: N o = N i / l .Do not confuse this with the noise produced by the attenuator.


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T f = 290 K L f = 1 dB or l f = 1.26 N r = 1 dB or nr = 1.26 (high-performance low noise receiver) T 0 = 290 K

T e = 290 K Le = 3 dB or le = 2We find T s = 100 + 290 (1.26 1) + 1.26 (1.26 1) 290 = 270 K and T a = 270 +

290 (1 1/1.26) = 331 K. This figure is typical for a large Earth station in the Intelsatsystem.

Link budgetWe found above that the signal received at the remote antenna is given as p r = eirp / l fs in linearterms (we will go back to decibel in a moment). At the reference point at the output of thereceiver antenna, the signal is received with the strength p sr = g r eirp / l fs at the reference point,

where gr is the gain of the receiver antenna. The system noise power at the reference point iskT s B. The carrier-to-noise ratio is the ratio between the received signal power and the noise indecibel at the reference point is then C / N = 10 log ( g r eirp / kT s Bl fs), or

C / N = EIRP L fs + (G r /T s) 10 log B + 228.6 or

C / N = EIRP 20 log d 20 log f + (G r /T s) 10 log B + 376

where we have inserted L fs = 147.5 + 20 log d + 20 log f for the path loss. In this formula wehave introduced the figure of merit of the receiver antenna ( G r /T s) = G r 10 log T s. EIRP isthe equivalent isotropic radiated power of the transmitter in dBW, d is the longest distancebetween the satellite and Earth in metres, f is the carrier frequency in Hz and B is the noisebandwidth of the system in Hz.

The formula is used to calculate the carrier-to-noise ratio of the received signal. Fromthis figure we then can derive the bit error rate of the signal when we know the modulationand demodulation methods used.

From the formula we see that the carrier-to-noise ratio decreases as the carrierfrequency increases. Since the available power is limited in the satellite, the lowest frequencyis always used on the downlink (satellite-to-Earth), thereby reducing the path loss in thisdirection. Furthermore, the uplink is designed such that the carrier-to-noise level in thesatellite receiver is much larger than that of the downlink. Therefore, we may usually neglect

the noise contribution from the uplink in the overall link budget.Example 2Let us calculate the satellite EIRP at beam edge for a down link with the followingcharacteristics.

f = 4 GHz d = 40,000 km B = 1 MHz T s = 270 K from Example 1 G r = 60 dB C / N = 12.5 dB (bit error rate of 10 6 for 4PSK without error correction)


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Margin M = 5 dB (rain and other effects such as depolarization and pointing error)First we find that ( G r / T s) = 60 10 log 270 = 35.7 dB/K. Inserting all the values in the

equation EIRP = C / N + 20 log d + 20 log f (G r /T s) + 10 log B 376 + M we find EIRP = 10dBW or eirp = 10 W.

9.8. Trade-offsCostGeostationary satellites are used for broadcasting. This is why the parabola used for receivingsatellite television can be kept pointing in the same direction all the time, that is, pointing atthe fixed location of the geostationary satellite. With non-geostationary systems the antennamust track the satellite in order to maintain communications. This is an example of trade-off between using an expensive satellite orbit and offering a simple receiving system at eachhome. If a cheaper orbit (less expensive with regard to satellite launching) had been used, theearth segment would have become so expensive that the system would hardly be used. Thiscan be seen from the following equation. The cost per user ( C u) of a satellite system is givenby:

C u = C sat / N + C rec

The cost for the satellite system is C sat and the cost of the user terminal is C rec . N is thenumber of users. Both cost elements must be small. The cost of the satellite per user, C sat / N , issmall if there are many users (large N ). The cost of the receiver, C rec , is small if theelectronics and in particular the mechanics of the receiver is simple.

The cost of the receiver is also small if the receiver is equipped with omni-directionalantennae. An omni-directional antenna receives the signal equally well from all directions in

space. The gain of such an antenna is 1 (0 dB). However, from the link budget in Section 9.7,it follows directly that the satellite must then produce more energy in order to produce a highenough carrier-to-noise ratio in the receiver. Producing more energy makes the space segmentmore expensive.

Land mobile terminals, terminals for small vessels and terminals mounted on aircraftemploy antennae that are almost omni-directional.

In order not to make the first Inmarsat system too expensive the antenna at the shipterminal is a directive parabola with gain between 20 and 24 dB pointing at the satellite. Anelectronic control system compensates for roll, sway and pitch of the ship so that the antennaalways points in the direction of the satellite independently of the motions of the ship. TheEIRP of the satellite is 20 to 24 dB smaller compared to systems employing omni-directional

antenna at the ship.Let us now look at the cost of the satellite segment, C sat . It is particularly expensive toplace a geostationary satellite in orbit. However, only three or four satellites are required for aglobal system. It is comparatively cheap to place the satellite in a low-earth orbit. However,many satellites are required for global coverage. The Iridium system required 66 satellites andthe Teledesic system was planned with 840 satellites. The cost estimate for the satellites of theTeledesic system is $ 10 billions. In comparison, the geostationary satellite system owned byInmarsat offering worldwide communication to ships and aircraft costs about $ 2 billions. Thehigher cost of the LEO system is compensated for by a higher business potential than theGEO system. The failure of the Iridium system showed that this is certainly not always thecase.

The cost of a GEO satellite amounts to about one third of the overall cost. The cost of the launch is also one third. The final one third includes insurance, management and operation


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of the satellite. The cost of a satellite is roughly proportional to its weight. The weightrepresents also a complex trade-off concerning the accuracy of station keeping; that is, howaccurately the satellite is kept in its position in the orbit. The reason is that the spacecraft mustcontain rocket fuel for repositioning the satellite as it drifts away from its intended positionbecause of the gravitation pull of the Sun and the Moon and anomalies in the Earth's gravity

caused by mountains and ocean depths. The more rocket fluid, the more often can the orbit becorrected, but the more expensive is the spacecraft. On the other hand, repositioning mayprolong the usable lifetime of the system and hence reduce the time between investments.

Other trade-offsThere is also a trade-off between weight, transmitted power level, antenna size and beamshaping. Large radiated power-level may mean larger solar cell panels and more batterycapacity, again increasing the weight of the satellite. In the worst case, a larger rocket to getthe satellite into orbit may be required. On the other hand, larger solar cells may reduce thecost of the earth segment including user terminals since the receiver can be made simpler asexplained above for the satellite broadcast service.

There is also a trade-off between the cost of the infrastructure of the satellite systemand the cost of the user terminal. The LEO systems will have a high infrastructure cost and alow terminal cost. They need many users in order to be competitive with terrestrial systems.The Inmarsat system requires expensive user terminals. There is only a small number of usersso that both the investment cost and the usage cost are high for this system. On the other hand,satellite communication is the only available communication technology for ships ininternational waters and aircraft on intercontinental routes and the satellite operator can makeuse of monopoly advantages.

The trade-offs is also concerned with choice of operating frequencies, on-boardprocessing and several other items too numerous to include here.

The total economy of the system will include elements such as: cost of the different components of the system: satellite, launching, operation,

management, insurance, spare equipment, and earth segment; usage forecast including increase in number of users and churn rate (users lost to

or gained from competing systems); charges and charge fluctuations due to competition and market feedback; market segmentation and penetration rate depending on usage cost; variation of interest rate on debt; cost of in-orbit repair if applicable; cost of destroying satellites no longer in use 16 ; cost of delays.Note that the cost of the satellite system is sunk cost in the sense that system can

hardly be used for purposes other than those for which it was designed 17 . Since the investmentis huge, the economy of satellite communication requires deep and sober analysis before thedecision to implement it is taken.

16 Usually some rocket fluid is saved until the end of the lifetime of the satellite in order to push it out of orbit so

that it eventually burns up in the earth's atmosphere.17 The Iridium system looked as if it had no remaining value when it was terminated. However, a small amountof the cost was recovered from selling the satellites.


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9.9. Mobile Satellite Communication

The Inmarsat systemThe Inmarsat system is in all respects a very marginal system. Ship earth stations with verysmall antennae are to be served via geostationary satellites. The bandwidth per channel, or

equivalently the information rate, must then be small since the amount of power that thesatellite must radiate is proportional to the bandwidth of the signal (see the link budget inSection 9.7).

As mentioned above, the first study of maritime satellite communication wasundertaken in Norway in 1969 where it was demonstrated that communication with ships viageostationary satellites was indeed feasible both technically and economically. At about thesame time, an American study showed that communication with aircraft via geostationarysatellite was also feasible.

The first system, called Marisat, came into operation in 1976. The system wasdeveloped by the American company COMSAT and offered initially services to the AtlanticOcean and the Pacific Ocean. The whole operation was cheap because a set of satellites the

company had developed for military purposes had spare capacity to accommodate themaritime transponders. The Indian Ocean was covered in 1977. The Marisat system was takenover by the newly established International Maritime Satellite Organisation (Inmarsat) in1982. This system was named the Inmarsat-A system. The system is analogue usingfrequency modulation for the telephone and telex carriers. This was an unfortunate choicehampering the further development of the system severely. The Norwegian feasibility studyand the satellite system to the North Sea put in operation in 1975 had shown clearly that adigital system with moderate speech encoding had superior performance.

Inmarsat-A was designed for large ships in the commercial fleet. Inmarsat-A will bephased out during 2007 after 30 years. Note that it usually takes a long time to phase out asystem. The reason is that the technical and commercial lifetime of an Inmarsat-A terminal isat least 15 years. Replacing the system earlier, would be expensive for those ships that stillused Inmarsat-A.

Table 9.1 The Inmarsat systems

Up to 432kbps

4.8 kbps2.4 kbps64 kbps

4.8 kbps, 2.4kbps64 kbps (M4)

9.6 kbps,4.8 kbps,300 bps

600 bps16 kbps,9.6 kbps,300 bps,50 bps

Analogueand 50bps

Datarates

IPVoice anddata

Voice anddata

Voice anddata

Messagesand data

Voice,telex anddata

Voiceand telex 2

Services

MaritimeLand

Small andlarge shipsOffshore

Small vesselLand mobileAircraft (M4)

AircraftSmallvesselLandmobile

Largeship

Largeship

Usercategory

200620041993/20031990199119931982(19761)

Start of service

BroadbandFleetM/M4AeroCBASystem

1The satellites of the MARISAT system inaugurated in 1976 were taken over by Inmarsat in1982.2 Telex is a service that is no longer in general use. This is a text service operating at 50bps between teletypewriters.

Inmarsat commenced immediately the development a digital version called Inmarsat-B

for large ships and a terminal called Inmarsat-C that could be fitted on very small ships(fishing vessels, leisure boats). Soon afterwards Inmarsat-Aero for communication withcommercial aircraft and Inmarsat-M for land mobile communication were developed. The


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newest narrowband system is called Inmarsat-Fleet that also supports bit rates of 64 kbps and128 kbps. The 64 kbps service is used for extending GSM to commercial aircraft.

Presently, Inmarsat is introducing broadband services up to 432 kbps in its BroadbandGlobal Area Network (BGAN) made possible with its newest generation satellites (fourthgeneration). The first of these satellites was launched 11 March 2005. These satellites will

offer global and regional BGAN services on spot beams and global beams in addition to thecurrent narrowband services on global beams.All these systems share the same satellite resources and the same cost earth stations

and control station infrastructure.The Inmarsat systems are summarised in table 9.1. The table contains the main areas

of usage of the different systems, the main services offered by each system and a selection of the most important data rates.

Frequency bandsThe Inmarsat systems are using frequency bands around 1.53 GHz and 1.63 GHz for the link between satellite and SES (downlink) and SES and satellite (uplink), respectively. The uplink

and downlink frequencies between the CES and the satellite are in the 6 GHz and 4 GHzbands, respectively. These are the most commonly used frequency bands for satellitecommunication. The satellite transponder thus translates the frequencies from 1.63 GHz to 4GHz and from 6 GHz to 1.53 GHz.

Different frequency slots are allocated for maritime and aeronautic systems (19 MHzand 10 MHz, respectively). This allows the two systems to evolve independently of eachother. There is also a separate narrow slot between the maritime and aeronautical bands (1MHz) allocated to common distress and safety services.

Basic architecture and procedures

Fixed network

CESCES NCS

Request (random access)

Assignment requestAssignment

Conversation Release

Figure 9.17 Inmarsat architecture and request procedure

The architecture of the Inmarsat systems (except BGAN) is shown in Figure 9.17. The systemconsists of coast earth stations (CES) (or land earth stations (LES)) connecting ship earthstations (SES), aircraft earth stations (AES) or mobile earth stations (MES) to the public

network via the satellite. All CESs can communicate with all mobile earth stations (SES,AES, MES). In addition, one station acts as a network control station (NCS) for each satellite


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coverage area in charge of allocating communication resources to the CESs. The CEScommunicates with the NCS over the satellite using the same frequency bands as between theCES and the SES. The reason for this arrangement is that the number of communicationchannels is so small that all resources must be shared by all CESs on demand assignmentwithin one satellite coverage area.

Frequency division multiple access is used for all channels with a single channel percarrier (SCPC). This represents the most efficient way in which to utilise the limited numberof communications channels. This applies both on the links between the mobile terminal andthe satellite (at 1.6/1.5 GHz) and between the cost earth stations and the satellite (at 6/4 GHz).

Pure Aloha is used for the request message from the ship (the reason is explained inSection 5.9).

The procedure is as follows. The mobile terminal sends the request message on therandom access channel. The request is addressed to one particular coast earth station. Therequest also contains the required type of communication resource (voice, data) and priority tosupport pre-emptive handling of distress communication. Pre-emptive operation means that if there is no idle channel, a call without distress priority is forcedly released and the channel is

allocated to the distress call.The coast earth station sends a request for channel assignment to the network

coordination that assigns the channel, marks the channel as busy and returns the assignmentmessage. Since this message is sent on the 1.5 GHz downlink from the satellite, the messageis received by both the coast earth station and the mobile terminal. The earth station and themobile terminal tune to the assigned channel and commence conversation. When the call isreleased, the coast earth station hands back the channel to the network coordination stationthat marks the channel as idle and available for future assignment.

Antenna tracking in Inmarsat-A, -B and -AeroThe antenna gain of Inmarsat-A and -B ship earth stations is between 20 and 24 dB giving anaperture angle between 3 dB points somewhere in the range from 10 to 20 . An antenna withthis narrow beam must be mounted on a stable platform in order to compensate for the motionof the ship in rough sea.

Antenna stabilisation is done in two steps. First, the platform on which the antenna ismounted must be fixed relative to a Cartesian coordinate system x , y and z where z is in thedirection of zenith and x and y are the coordinates of the tangential plain of the Earth at thepoint where the ship is located. The y axis may point in an arbitrary direction, in the directionin which the ship is heading or (e.g.) toward the North Pole if an absolute reference system isprovided. An absolute reference system is required if program tracking of the antenna beam isapplied.

The second step consists in directing the antenna beam toward the satellite. The anglebetween the x- y-plane and the direction in which the antenna is pointing is called the elevationangle. The exact pointing direction of the antenna is thus given by the elevation angle and theangles between the beam and x and y axes.

A stable Cartesian reference system can be achieved in two ways. In the originalsystem designed in 1976, a spinning wheel platform was implemented. The platformconsisted of a plate on which the antenna is mounted. The plate balances on a pivot fastenedto the superstructure of the ship and contains two flywheels rotating in opposite directions.Because of the inertia, the flywheels act as a gyro stabilising the plate in the horizontal planewhichever movement the ship makes provided that the friction between the pivot and the plateis small. The technology is simple but bulky and expensive.

In more modern designs, the platform is stabilised by a servo system that uses thegyrocompass of the ship to determine the Cartesian reference coordinates. The gyrocompass


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provides both the coordinates for the course of the ship and the direction of zenith. Equippingthe platform with motors that rotate the platform around the three coordinate axes and sensorssuch as accelerometers and rate-integrating gyros that measure the instantaneous deviationfrom the reference coordinates, the servo system will keep the platform in a stable horizontalposition.

After having provided a stabilised platform for the antenna mount, we have to pointthe antenna toward the satellite. The pointing direction will change due to the ordinarymovements of the ship so that a servo system is required in order to compensate for thesemovements. The movement of the ship is usually a very slow process.

The beam can be kept in the direction of the satellite by employing either programtracking or step tracking.

The principle of program tracking is as follows. The direction toward the satellite canbe determined accurately from the orbital coordinates of the satellite and the geographiccoordinates of the ship. The satellite coordinates are the same during the entire lifetime of thesatellite and the coordinates of the four satellites need only to be read into the terminalcomputer during installation 18 . The geographical coordinates of the ship are provided by the

navigation system of the ship. It is then simple trigonometry to calculate the pointing anglesof the antenna and simple mechanical manoeuvre to make the antenna point in the desireddirection. Program tracking is also called open-loop tracking since a servo system withfeedback is not required in order to preserve the pointing direction.

The second method is called step-track. Step-track requires a servo system operatingas follows. Suppose that the mobile earth station is pointing directly at the satellite. As theship moves, the pointing becomes more and more inaccurate and the received power leveleventually drops with a certain amount, say, 0.5 dB. The servo system then moves the antennaa certain angle in an arbitrary direction. If the level drops further, the antenna is moved in theopposite direction until a maximum power level is detected. Then the process is continued inthe direction vertical to the initial one until the maximum power is detected.

In a geostationary system, the time between adjustment and the detection of the resultof that adjustment is about 250 ms (corresponding to the one-way delay over the satellite). Itcan be shown that if the loop shall remain stable, the time between adjustment andmeasurement must bee longer than 1 second (four times the delay). The time betweenadjacent steps must then be longer than one second.

Inmarsat-C does not require antenna tracking. Inmarsat-Aero uses programme trackingbased on the position of the satellite and data from the course computer of the aircraft. Theantenna gain is only 12 dB corresponding to a 3 dB opening angle of 50 . This implies thatpointing adjustment is not frequently required for normal motion of the aircraft. The Aeroantenna is an electronically steerable phase array antenna.

Land mobile terminals may also use program tracking combined with GPS.

A note on link budgetsTable 9.2 Characteristics of Inmarsat-B and -C

18 It is, of course, possible to alter this information in the terminal if new satellite positions are introduced.


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40,000 km40,000 km40,000 kmDistance d

1.5 GHz1.5 GHz1.5 GHzCarrier frequency f

5 dB5 dB8 dBC / N

1200 Hz1200 Hz24 kHzBandwidth B

21 dBK 23 dBK 4 dBKG / T

Inmarsat-C(3 dB gain)


Inmarsat-B

We shall compare the power requirements of Inmarsat-B and -C and see that the systems arecompatible in this way though they are very different in most other respects.

Table 9.2 shows the main characteristics of the Inmarsat- B and two versions of Inmarsat-C, one with zero antenna gain and one with gain corresponding to a crossed dipole.Inmarsat-B applies 3/4-rate convolutional error correction coding while Inmarsat-C applies1/2-rate coding reducing the carrier-to-noise threshold by about 3 dB for the same bit errorrate (10 3). Inmarsat-B supports telephony at 9.6 kbps giving a bandwidth of 24 kHz.Inmarsat-C supports only message services at bit rates up to 600 bps giving a total bandwidthof 1200 Hz.

The EIRP of the satellite is then (see the link budget above):

EIRP = C / N + 20 log d + 20 log f (G r /T s) + 10 log B 376 + M

Inserting the values in Table 9.2 and setting M = 3 we find the EIRP of the threesystems as shown in Table 9.3.

Table 9.3 shows that Inmarsat-B and Inmarsat-C require approximately the same

satellite EIRP and no particular precaution needs to be taken when allocating communicationresources to the two types of system. Similar considerations apply for Inmarsat-Aero andInmarsat-M. Equal EIRP in the satellite was an important design criterion for these systems.

Table 9.3 EIRP and transmitted power

67 W134 W67 WTransmitted power

18.3 dBW21.3 dBW18.3 dBWEIRP



Inmarsat-B

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9_Line of Sight