Slide 1

Fall (2015)Lecture 1 &2 5th October, 20151EE 419Satellite CommunicationsCourse Outline by HEC2Introduction to Satellite Communication, Satellite Link Design, Propagation Characteristics of Satellite Links, Satellite systems: Space-segment and ground segment, Channel Modeling, Access Control Schemes, System Performance Analysis, System Design, Space standards, Satellite Applications such as earth observation, weather and communication. Books3Timothy Pratt, Charles W. Bostian and Jeremy E. Allnutt, Satellite Communications, Latest Edition, John Wiley & Sons, ISBN: 0471429120. Tri T Ha, Digital Satellite Communications, McGraw Hill publishing Company. Latest Edition.

Reference Books Tom Logsden, "Mobile Communication Satellites: Theory and Applications", McGraw-Hill, (Latest edition). Gerald M., Michel Bousquet, "Satellite Communication Systems: Systems, Techniques and Technologies", John Willey, (Latest Edition). Leon W. Couch, Digital & Analog Communication Systems, Latest Edition, Prentice Hall, ISBN: 0131424920. Theodore S. Rappaport, Wireless Communications: Principles and Practice, Latest Edition, Prentice Hall, ISBN: 0130422320. Jochen Schiller, Mobile Communications, Latest Edition, Addison-Wesley, ISBN: 0321123816. D. Roddy, Satellite Communications, McGraw-Hill Professional, 2001.

Contact Hours

4Credit Hours: 3

Section AMonday: 9am-11am (2hrs)Thursday:12am-1pm (1hr)

Section BFriday: 8am-10pm (2hrs)Wednesday: 10am-11am (1hr)

45Let us startOverview of Lecture6Background(evolution and growth)IntroductionBroadband Communication.Why use satellite systems?TypesBlock diagram, General structure of a satellite communication systemServicesAdvantagesHistoryFrequency Bands, International regulation and frequency coordination

Broadband Communication Systems7Coaxial Cable System

Terrestrial Microwave System

Tropo Scatter System

Satellite System

Fiber-Optic Cable SystemEarths atmosphereSource: All about GPS [www.kowoma.de]

8Radio wave Propagation9Ground wave/ surface waveIonospheric / sky waveTropospheric / forward scatter waveDirect/LOSOverviewSatellite technology has progressed tremendously over the last 50 years since Arthur C. Clarke first proposed its idea in 1945 in his article in Wireless World.

Today, satellite systems can provide a variety of services including broadband communications, audio/video distribution networks, maritime navigation, worldwide customer service and support as well as military command and control.

Satellite systems are also expected to play an important role in the emerging 4G global infrastructure providing the wide area coverage necessary for the realization of the Optimally Connected Anywhere, Anytime vision that drives the growth of modern telecom industry.Basics: How do Satellites WorkTwo Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means.

The two stations can use a satellite as a relay station for their communication

One Earth Station sends a transmission to the satellite. This is called a Uplink.

The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.Why use a satellite for Communications?12Why use a satellite for Communications?13Economical long distance communicationsBroadcast capabilityWide capabilityBroad coverageFreedom from natural barriersBetter coverage of rural and underdeveloped areasNew marketsNew common carriersNew servicesCustomer premises servicesAdvantages of SatellitesThe advantages of satellite communication over terrestrial communication are:

The coverage area of a satellite greatly exceeds that of a terrestrial system.

Transmission cost of a satellite is independent of the distance from the center of the coverage area.

Satellite to Satellite communication is very precise.

Higher Bandwidths are available for use.Disadvantages of SatellitesThe disadvantages of satellite communication:

Launching satellites into orbit is costly.

Satellite bandwidth is gradually becoming used up.

There is a larger propagation delay in satellite communication than in terrestrial communication. Focus16There have been many new developments in satellite communication technology, however the underlying principle of transmission of radio signals via satellites remains the same.

New applications have been developing for satellite communication.

New satellites and terminals have been implementing to built new systems.History and Evolution17Geo Satellite concept by Arthur Clark-1945SPUTNIK by USSR October 4, 1957 184 pounds 23 inches in diameter Began the space raceSCORE by US Air Force-1959ECHO 1 &2 by NASA and AT&T-1960TELESTAR 1 and 2-1962SYNCOM 1,2 and 3 by USA-1963EARLY BIRD, first commercial GEO satellite-1965Pioneers in Satellite CommunicationKonstantin Tsiolkovsky (1857 - 1935)Russian visionary of space flight First described the multi-stage rocket as means of achieving orbit.

Hermann Noordung (1892 - 1929)Postulated the geostationary orbit.

Arthur C. Clarke (1917 19 March 2008)Postulated the entire concept of international satellite telecommunications from geostationary satellite orbit including coverage, power, services, solar eclipse.Satellite History Calendar1957 October 4, 1957: - First satellite - the Russian Sputnik 01First living creature in space: Sputnik 021958First American satellite: Explorer 01First telecommunication satellite: This satellite broadcast a taped message: Score1959First meteorology satellite: Explorer 07

1960First successful passive satellite: Echo 1First successful active satellite: Courier 1BFirst NASA satellite: Explorer 08April 12, 1961: - First man in space1962First telephone communication & TV broadcast via satellite: Echo 1First telecommunication satellite, first real-time active, AT&T: Telstar 1First Canadian satellite: Alouette 1On 7th June 1962 at 7:53p the two-stage rocket; Rehbar-I was successfully launched from Sonmiani Rocket Range. It carried a payload of 80 pounds of sodium and soared to about 130 km into the atmosphere. With the launching of Rehbar-I, Pakistan had the honour of becoming the third country in Asia and the tenth in the world to conduct such a launching after USA, USSR, UK, France, Sweden, Italy, Canada, Japan and Israel. Rehbar-II followed a successful launch on 9th June 1962 1963Real-time active: Telstar 21964Creation of IntelsatFirst geostationary satellite, second satellite in stationary orbit: Syncom 3First Italian satellite: San Marco 1Satellite History Calendar1965Intelsat 1 becomes first commercial comsat: Early BirdFirst real-time active for USSR: Molniya 1A1967First geostationary meteorology payload: ATS 31968First European satellite: ESRO 2BJuly 21, 1969: - First man on the moon

1970First Japanese satellite: OhsumiFirst Chinese satellite: Dong Fang Hong 011971First UK launched satellite: ProsperoITU-WARC for Space Telecommunications INTELSAT IV Launched INTERSPUTNIK - Soviet Union equivalent of INTELSAT formed 1974First direct broadcasting satellite: ATS 61976MARISAT - First civil maritime communications satellite service started 1977EUTELSAT - European regional satellite ITU-WARC for Space Telecommunications in the Satellite Service 1979Creation of InmarsatSatellite History Calendar1980INTELSAT V launched - 3 axis stabilized satellite built by Ford Aerospace 1983ECS (EUTELSAT 1) launched - built by European consortium supervised by ESA 1984UK's UNISAT TV DBS satellite project abandoned First satellite repaired in orbit by the shuttle: SMM1985First Brazilian satellite: Brazilsat A1First Mexican satellite: Morelos 11988First Luxemburg satellite: Astra 1A1989INTELSAT VI - one of the last big "spinners" built by HughesCreation of Panamsat - Begins ServiceOn 16 July 1990, Pakistan launched its first experimental satellite, BADR-I from China 1990IRIDIUM, TRITIUM, ODYSSEY and GLOBALSTAR S-PCN projects proposed - CDMA designs more popular EUTELSAT II 1992OLYMPUS finally launched - large European development satellite with Ka-band, DBTV and Ku-band SS/TDMA payloads - fails within 3 years 1993INMARSAT II - 39 dBW EIRP global beam mobile satellite - built by Hughes/British Aerospace 1994INTELSAT VIII launched - first INTELSAT satellite built to a contractor's design Hughes describe SPACEWAY design DirecTV begins Direct Broadcast to Home1995Panamsat - First private company to provide global satellite services.21EIRP: Equivalent isotropically radiated power or Effective isotropic radiated power is the amount of power that a theoretical isotropic antenna (that evenly distributes power in all directions) would emit to produce the peak power density observed in the direction of maximum antenna gain. EIRP can take into account the losses in transmission line and connectors and includes the gain of the antenna. The EIRP is often stated in terms of decibels over a reference power emitted by an isotropic radiator with an equivalent signal strength.Satellite History Calendar1996INMARSAT III launched - first of the multibeam mobile satellites (built by GE/Marconi) Echostar begins Diresct Broadcast Service1997IRIDIUM launches first test satellites ITU-WRC'97 1999AceS launch first of the L-band MSS Super-GSOs - built by Lockheed Martin Iridium Bankruptcy - the first major failure? 2000Globalstar begins service Thuraya launch L-band MSS Super-GSO2001XM Satellite Radio begins servicePakistans 2nd Satellite, BADR-B was launched on 10 Dec 2001 at 9:15a from Baikonour Cosmodrome, Kazakistan 2002Sirius Satellite Radio begins servicePaksat-1, was deployed at 38 degrees E orbital slot in December 2002, Paksat-1, was deployed at 38 degrees E orbital slot in December 20022004Teledesic network planned to start operation2005Intelsat and Panamsat Merge VUSat OSCAR-52 (HAMSAT) Launched 2006CubeSat-OSCAR 56 (Cute-1.7) LaunchedK7RR-Sat launched by California Politechnic University2007Prism was launched by University of Tokyo 2008COMPASS-1; a project of Aachen University was launched from Satish Dawan Space Center, India. It failed to achieve orbit.IntelsatINTELSAT is the original "Inter-governmental Satellite organization".

It once owned and operated most of the World's satellites used for international communications, and still maintains a substantial fleet of satellites.INTELSAT is moving towards "privatization", with increasing competition from commercial operators (e.g. Panamsat, Loral Skynet, etc.).

INTELSAT Timeline:Interim organization formed in 1964 by 11 countries

Permanent structure formed in 1973

Commercial "spin-off", New Skies Satellites in 1998

Full "privatization" by April 2001 INTELSAT has 143 members.Intelsat Structure

EutelsatPermanent General Secretariat opened September 1978 Intergovernmental Conference adopted definitive statutes with 26 members on 14 May 1982Definitive organization entered into force on 1 September 1985

General Secretariat -> Executive Organ

Executive Council -> EUTELSAT Board of Signatories

Secretary General -> Director General

Current DG is Giuliano Berretta Currently almost 50 membersMoving towards "privatization"Limited company owning and controlling of all assets and activities

Also a "residual" intergovernmental organization which will ensure that basic principles of pan-European coverage, universal service, non-discrimination and fair competition are observed by the company

Eutelsat Structure

Communication SatelliteA Communication Satellite can be looked upon as a large microwave repeaterIt contains several transponders which listens to some portion of spectrum, amplifies the incoming signal and broadcasts it in another frequency to avoid interference with incoming signals.Motivation to use Satellites

Satellite MissionsSource: Union of Concerned Scientists [www.ucsusa.org]

29Satellite Microwave TransmissionSatellites can relay signals over a long distanceGeostationary SatellitesRemain above the equator at a height of about 22300 miles (geosynchronous orbits)Travel around the earth in exactly the same time, the earth takes to rotateFrequency BandsVery low Frequency (VLF)3kHz-30kHzLow Frequency (LF)30k-300kHzMedium Frequency (MF)300kHz-3MHzHigh Frequency (HF)3MHz-30MHzVery High Frequency(VHF)30MHz-300MHzUltra High Frequency (UHF)300MHz-3GHzSuper High Frequency (SHF)3GHz-30GHzExtremely High Frequency (EHF)30GHz-300GHzInfrared, Visible Light, UV10e3-10e7GHz31Frequency BandsDifferent kinds of satellites use different frequency bands.

LBand: 1 to 2 GHz, used by MSSS-Band: 2 to 4 GHz, used by MSS, NASA, deep space researchC-Band: 4 to 8 GHz, used by FSSX-Band: 8 to 12.5 GHz, used by FSS and in terrestrial imaging, ex: military and meteorological satellitesKu-Band: 12.5 to 18 GHz: used by FSS and BSS (DBS)K-Band: 18 to 26.5 GHz: used by FSS and BSSKa-Band: 26.5 to 40 GHz: used by FSS Types of Satellites33Satellites can be classified by their functions and these function leads to its designing.

AstronomyAtmosphericCommunicationNavigationRemote sensingSearch and RescueSpace explorationWeatherSatellite System Elements

Space SegmentSatellite Launching PhaseTransfer Orbit PhaseDeploymentOperationTT&C - Tracking Telemetry and Command StationSCC - Satellite Control Center, a.k.a.:OCC - Operations Control CenterSCF - Satellite Control FacilityRetirement PhaseGround SegmentCollection of facilities, Users and Applications

Earth Station = Satellite Communication Station (Fixed or Mobile)

Satellite Uplink and DownlinkDownlinkThe link from a satellite down to one or more ground stations or receiversUplinkThe link from a ground station up to a satellite.Some companies sell uplink and downlink services to television stations, corporations, and to other telecommunication carriers. A company can specialize in providing uplinks, downlinks, or both. Satellite Uplink and Downlink

Satellite CommunicationSource: Cryptome [Cryptome.org]When using a satellite for long distance communications, the satellite acts as a repeater.

An earth station transmits the signal up to the satellite (uplink), which in turn retransmits it to the receiving earth station (downlink).

Different frequencies are used for uplink/downlink.

39Satellite Transmission LinksEarth stations Communicate by sending signals to the satellite on an uplinkThe satellite then repeats those signals on a downlinkThe broadcast nature of downlink makes it attractive for services such as the distribution of TV programsDirect to User Services

One way Service (Broadcasting)Two way Service (Communication)Satellite SignalsUsed to transmit signals and data over long distancesWeather forecastingTelevision broadcastingInternet communicationGlobal Positioning SystemsSatellite Transmission BandsFrequency BandDownlinkUplinkC3,700-4,200 MHz5,925-6,425 MHzKu11.7-12.2 GHz14.0-14.5 GHzKa17.7-21.2 GHz27.5-31.0 GHzThe C band is the most frequently used. The Ka and Ku bands are reserved exclusively for satellite communication but are subject to rain attenuationTypes of Satellite based NetworksBased on the Satellite AltitudeGEO Geostationary Orbits36000 Km = 22300 Miles, equatorial, High latency MEO Medium Earth OrbitsHigh bandwidth, High power, High latency LEO Low Earth OrbitsLow power, Low latency, More Satellites, Small FootprintVSATVery Small Aperture SatellitesPrivate WANsSatellite System45

Block Diagram46

Three movements to be considered:47Earth is spinning.Time:______Distance:_______Earth is revolving around sun.Time:______Distance:______Our satellite is revolving around earth.Time:???Distance:???

Types of Orbits48AltitudeLEOMEOGEOShapeEOHEOCircularDirection/RotationRetrogradePrograde

MolniyaHAPsSatellite OrbitsSource: Federation of American Scientists [www.fas.org]

Geosynchronous Orbit (GEO): 36,000 km above Earth, includes commercial and military communications satellites, satellites providing early warning of ballistic missile launch.Medium Earth Orbit (MEO): from 5000 to 15000 km, they include navigation satellites (GPS, Galileo, Glonass).Low Earth Orbit (LEO): from 500 to 1000 km above Earth, includes military intelligence satellites, weather satellites.49Satellite Orbits

GEO - Geostationary OrbitIn the equatorial plane

Orbital Period = 23 h 56 m 4.091 s = 1 sidereal day*

Satellite appears to be stationary over any point on equator:Earth Rotates at same speed as SatelliteRadius of Orbit r = Orbital Height + Radius of EarthAvg. Radius of Earth = 6378.14 Km

3 Satellites can cover the earth (120 apart)51*More detail in next lecture: A sidereal day is the time between consecutive crossings of any particular longitude on the earth with reference toinertial space (or its own axis); I.e., in practice, with reference to any star other than the sun. This corresponds to a 360 degree rotation.

NGSO - Non Geostationary OrbitsOrbit should avoid Van Allen radiation belts:Region of charged particles that can cause damage to satelliteOccur at ~2000-4000 km and ~13000-25000 km

LEO - Low Earth OrbitsCircular or inclined orbit with < 1400 km altitudeSatellite travels across sky from horizon to horizon in 5 - 15 minutes => needs handoffEarth stations must track satellite or have Omni directional antennasLarge constellation of satellites is needed for continuous communication (66 satellites needed to cover earth)Requires complex architectureRequires tracking at ground

HEO - Highly Elliptical OrbitsHEOs (i = 63.4) are suitable to provide coverage at high latitudes (including North Pole in the northern hemisphere)Depending on selected orbit (e.g. Molniya, Tundra, etc.) two or three satellites are sufficient for continuous time coverage of the service area.All traffic must be periodically transferred from the setting satellite to the rising satellite (Satellite Handover)

Satellite OrbitsSource: Union of Concerned Scientists [www.ucsusa.org]

55Why Satellites remain in Orbits

Geostationary Earth Orbit (GEO)These satellites are in orbit 35,863 km above the earths surface along the equator.Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth.GEO (cont.)AdvantagesA GEO satellites distance from earth gives it a large coverage area, almost a fourth of the earths surface.GEO satellites have a 24 hour view of a particular area.These factors make it ideal for satellite broadcast and other multipoint applications.GEO (cont.)DisadvantagesA GEO satellites distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to point communication.GEO satellites, centered above the equator, have difficulty broadcasting signals to near polar regions

Low Earth Orbit (LEO)LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1,500 km above the surface.LEO satellites dont stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass.A network of LEO satellites is necessary for LEO satellites to be usefulLEO (cont.)AdvantagesA LEO satellites proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication.A LEO satellites smaller area of coverage is less of a waste of bandwidth.LEO (cont.)DisadvantagesA network of LEO satellites is needed, which can be costlyLEO satellites have to compensate for Doppler shifts cause by their relative movement.Atmospheric drag effects LEO satellites, causing gradual orbital deterioration.Medium Earth Orbit (MEO)A MEO satellite is in orbit somewhere between 8,000 km and 18,000 km above the earths surface. MEO satellites are similar to LEO satellites in functionality.MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to 8 hours.MEO satellites have a larger coverage area than LEO satellites. MEO (cont.)AdvantageA MEO satellites longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network.DisadvantageA MEO satellites distance gives it a longer time delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite. Other OrbitsMolniya Orbit SatellitesUsed by Russia for decades.Molniya Orbit is an elliptical orbit. The satellite remains in a nearly fixed position relative to earth for eight hours.A series of three Molniya satellites can act like a GEO satellite.Useful in near polar regions.Other Orbits (cont.)High Altitude Platform (HAP)One of the newest ideas in satellite communication.A blimp or plane around 20 km above the earths surface is used as a satellite.HAPs would have very small coverage area, but would have a comparatively strong signal.Cheaper to put in position, but would require a lot of them in a network. X-tics of different orbitsAltitudeSatellites Needed For Global CoverageLEO700km to 1400km40+MEO10,000km to 15,000km10-15GEO36,000 km

3-467Point of Consideration68Solar System (Planets revolving around sun). Our concern-EARTH.

Different layers outside and inner side of earth. Our concern-Outer layers.

Seven layers. Our concern-effects of these layers in propagation of our signal

LEO Aggregations69

Elliptical Orbit Aggregation70

Representation of earth in Longitude and Latitude scale.71

Review72Basics: How Satellites are usedService TypesFixed Service Satellites (FSS)Example: Point to Point CommunicationBroadcast Service Satellites (BSS)Example: Satellite Television/RadioAlso called Direct Broadcast Service (DBS).Mobile Service Satellites (MSS)Example: Satellite Phones73The Text Book makes an odd choice in calling DBS BSS (BSS is actually the abbreviation used for Boeing Satellite Service).

These distinctions arent set in stone. For example, FSS satellites used to be used for broadcasting.When to use SatellitesWhen the unique features of satellite communications make it attractive When the costs are lower than terrestrial routing When it is the only solution Examples:Communications to ships and aircraft (especially safety communications) TV services - contribution links, direct to cable head, direct to homeData services - private networks Overload traffic Delaying terrestrial investments 1 for N diversity Special events

741 for N Diversity: Where there is negligible likelihood of route failure, there is no need for route diversity protection and the type of protection used is known as "1 for N". In point to point radio systems it is (typically 7 : 1) throughout the world. If a worker section down a route fails, the traffic is switched to a stand-by section. After repair of the worker, traffic is returned to it after a suitable period of time. This period of time is that necessary for a stability test, to check that the fault has been genuinely cleared. Traffic loss due to section failure can typically be reduced by several hundred times by the use of "1-for-N" protection.

When to use TerrestrialPSTN - satellite is becoming increasingly uneconomic for most trunk telephony routes but, there are still good reasons to use satellites for telephony such as: thin routes, diversity, very long distance traffic and remote locations. Land mobile/personal communications - in urban areas of developed countries new terrestrial infrastructure is likely to dominate (e.g. GSM, etc.) but, satellite can provide fill-in as terrestrial networks are implemented, also provide similar services in rural areas and underdeveloped countries Advantages of Satellite CommunicationCan reach over large geographical areaFlexible (if transparent transponders) Easy to install new circuits Circuit costs independent of distance Broadcast possibilities Temporary applications (restoration) Niche applications Mobile applications (especially "fill-in") Terrestrial network "by-pass" Provision of service to remote or underdeveloped areas User has control over own network 1-for-N multipoint standby possibilities

76Transponders are microwave repeaters carried by communications satellites. Transparent transponders can handle any signal whose format can fit in the transponder bandwidth. No signal processing occurs other than that of heterodyning (frequency changing) the uplink frequency bands to those of the downlinks. Such a satellite communications system is referred to as a bent-pipe system. Connectivity among earth stations is reduced when multiple narrow beams are used. Hence, the evolution proceeded from the transparent transponder to transponders that can perform signal switching and format processing. Disadvantages of Satellite CommunicationLarge up front capital costs (space segment and launch) Terrestrial break even distance expanding (now approx. size of Europe) Interference and propagation delay Congestion of frequencies and orbits 77Breakeven Distance: As the cost of Satellite Circuit is independent of distance on the Earth between the two ends, whilst the cost of a terrestrial circuit is approximately directly proportional to that distance, the concept of a "breakeven" distance where the costs are equal has been used to determine where services should be routed via satellite. This breakeven distance varies according to the size of the route, growth rate, and any special networking requirements.When to use TerrestrialPSTN - satellite is becoming increasingly uneconomic for most trunk telephony routes but, there are still good reasons to use satellites for telephony such as: thin routes, diversity, very long distance traffic and remote locations. Land mobile/personal communications - in urban areas of developed countries new terrestrial infrastructure is likely to dominate (e.g. GSM, etc.) but, satellite can provide fill-in as terrestrial networks are implemented, also provide similar services in rural areas and underdeveloped countries Frequency Bands Allocated to the FSSFrequency bands are allocated to different services at World Radio-communication Conferences (WRCs). Allocations are set out in Article S5 of the ITU Radio Regulations. It is important to note that (with a few exceptions) bands are generally allocated to more than one radio services.CONSTRAINTS Bands have traditionally been divided into commercial" and "government/military" bands, although this is not reflected in the Radio Regulations and is becoming less clear-cut as "commercial" operators move to utilize "government" bands.

79FSS: Stands for Fixed Satellite Services. Satellite communications in the FSS frequency band were initially developed in order to provide transmission links between the public switched telephone networks (PSTNs) of different countries, first intercontinental and then regional (e.g. the Intelsat and Eutelsat systems respectively);Earths atmosphereSource: All about GPS [www.kowoma.de]

80Atmospheric LossesDifferent types of atmospheric losses can disturb radio wave transmission in satellite systems:Atmospheric absorptionAtmospheric attenuationTraveling ionospheric disturbances81Atmospheric AbsorptionEnergy absorption by atmospheric gases, which varies with the frequency of the radio waves.Two absorption peaks are observed (for 90 elevation angle):22.3 GHz from resonance absorption in water vapour (H2O)60 GHz from resonance absorption in oxygen (O2)For other elevation angles:[AA] = [AA]90 cosec

Source: Satellite Communications, Dennis Roddy, McGraw-Hill82Atmospheric AttenuationRain is the main cause of atmospheric attenuation (hail, ice and snow have little effect on attenuation because of their low water content).Total attenuation from rain can be determined by:A = L [dB]where [dB/km] is called the specific attenuation, and can be calculated from specific attenuation coefficients in tabular form that can be found in a number of publicationswhere L [km] is the effective path length of the signal through the rain; note that this differs from the geometric path length due to fluctuations in the rain density. 83Traveling Ionospheric DisturbancesTraveling ionospheric disturbances are clouds of electrons in the ionosphere that provoke radio signal fluctuations which can only be determined on a statistical basis.The disturbances of major concern are:Scintillation;Polarisation rotation.Scintillations are variations in the amplitude, phase, polarisation, or angle of arrival of radio waves, caused by irregularities in the ionosphere which change over time. The main effect of scintillations is fading of the signal.84What is Polarisation?Polarisation is the property of electromagnetic waves that describes the direction of the transverse electric field. Since electromagnetic waves consist of an electric and a magnetic field vibrating at right angles to each other.it is necessary to adopt a convention to determine the polarisation of the signal.Conventionally, the magnetic field is ignored and the plane of the electric field is used.85Types of PolarisationLinear Polarisation (horizontal or vertical):the two orthogonal components of the electric field are in phase;The direction of the line in the plane depends on the relative amplitudes of the two components.Circular Polarisation:The two components are exactly 90 out of phase and have exactly the same amplitude.Elliptical Polarisation:All other cases.

Linear PolarisationCircular PolarisationElliptical Polarisation86Satellite CommunicationsAlternating vertical and horizontal polarisation is widely used on satellite communicationsThis reduces interference between programs on the same frequency band transmitted from adjacent satellites (One uses vertical, the next horizontal, and so on)Allows for reduced angular separation between the satellites.

Information Resources for Telecommunication Professionals[www.mlesat.com]87user

Terrestrial Network

Earth station