Tutorial Satellite Communication

By Kamran Ahmed

(kamrahmed@excite.com)

Course Contents• Overview of Satellite Systems

• Orbits & Launching Methods

• Orbital Mechanics

• Orbital Perturbations

• Satellite Visibility

• Radio Wave Propagation

• Polarization

• Antenna

• Link Budget

• Interference

• Channel Characterization

• Classical & Modern Applications

1. Overview of Satellite Systems

Contents• What is satellite communication• The Origin of Satellite• Elements of Satellite Communication• Key input data• Early Satellite Systems• System Design Considerations• Major Problems for Satellite• Limitation for Satellites• Advantages of Satellite • Different Applications• Frequency Allocation & Regulatory Aspects

• A communication satellite is basically an electronic communication package placed in orbit whose prime objective is to initiate or assist another through space.

• Satellite communication is one of the most impressive spin-offs from the space programs and has made a major contribution to the pattern of international communication.

• The information transferred most often correspondence to voice (telephone), video (Television) and digital data.

What is Satellite Communication…

Cont...

• Communication satellite are off-course only one means of telecommunication transmission. The traditional means include copper wire and microwave point- to-point links. Newer techniques involves use of optics either point-to-point infrared or fiber optics. Point-to-point radio system such as short wave radio may also be used.

The origin of satellite • The concept of using object in space to reflect signals for

communication was proved by Naval Research Lab in Washington D.C. when it use the Moon to establish a very low data rate link between Washington and Hawaii in late 1940’s.

• Russian started the Space age by successfully launching SPUTNIK the first artificial spacecraft to orbit the earth, which transmitted telemetry information for 21 days in Oct. 1957.

• The American followed by launching an experimental satellite EXPLORER In 1958.

• In 1960 two satellite were deployed “Echo” & “Courier”• In 1963 first GSO “Syncom”• The first commercial GSO (Intelsat & Molnya) in 1965

these provides video (Television) and voice (Telephone) for their audience

Elements of Satellite Communications

• The basic elements of a communication satellite service are divided between;

• Space Segment

• Ground Segment• The space segment consist of the spacecraft &

launch mechanism and ground segment comprises the earth station and network control center of entire satellite system.

Satellite Communications System

Uplink Down Link

Transmit Earth Station Receive Earth Station

IDU IDURFT RFT

RF

ConceptTransponder

Earth station (site A) Earth station(site B)

IRRADIUM

downlinkdownlink

uplinkuplink

downlinkdownlink

uplinkuplink

Propagation Delay

Single Hop 270 ms Double Hop 540 ms

Ground Station _ Anatomy

Indoor Unit(IDU)

Outdoor Unit(ODU)

Antenna Sub-System

IFL

70/140MHz

C/Ku

Satellite Services

• The ITU has grouped the satellite services in to three main groups

• Fixed Satellite Services (FSS)• Broadcast Satellite Services (BSS)• Mobile Satellite services (MSS)

Space Segment

• Space segment consist of a satellite in suitable orbit.

• Space segment classified on the basis of orbit;– LEO– MEO– HEO– GEO & GSO

Ground Segment

• The ground segment of each service has distinct characteristics.

• Services like;• FSS• BSS• MSS

– Maritime, Aeronautical & Land base

• DBS • Etc.

Satellite Footprints

Satellite beam their signals in a straight path to the earth. The satellite focus these microwaves signals onto the specified portions of the earth’s surface to most effectively use the limited power of their transponders. These focused signals create unique beam patterns called “footprints.”

Types of footprints:– Global beam footprint– Hemispheric Beam Footprint– Zone Beam Footprint

Satellite Footprints

Satellite Footprints

Satellite Footprints

Satellite Footprints

Bands:C-Band ( )Ku-Band ( )

Beams:Global ( )Hemi ( )Zone ( )Spot ( )

Key Input Data...

1 Anik, Canada 6 Telecom, France 11 Asiasat, East Asia2 Morelos, Mexico 7 Kopernikus, Germany 12 CS, Japan3 Panamsat, Americas 8 Italsat, Italy 13 Palapa, Indonesia 4 Brasilsat, Brazil 9 Arabsat, Arab League 14 Aussat, Australia5 Eutelsat, Europe 10 Insat, India

National and Regional Systems

Early Satellites

Satellite Launching Date Country/Organization Type Height (miles) Comments

RELAY 1962 USA/RCA & NASA Active Duplex 942-5303 4.2/1.7 GHz satellite designed to carry telephone signals.

SYNCOM 1963 USA/NASA Active Duplex Geostationary First Geostationary communication satellite used to transmit television signals from the Tokyo Olympics.

MOLNIYA 1965 U.S.S.R Active Duplex High altitude First Soviet communication satellite

elliptical used a high altitude elliptical orbit.

EARLY 1965 INTELSAT/COMSAT Active Geostationary First commercial communication

BIRD satellite; served the Atlantic ocean region; capacity to carry 240 voice channels

INTELSAT 2 1966 INTELSAT/COMSAT Active Geostationary First multiple access commercial satellite with multidestination capability

INTELSAT 3 1968 INTELSAT/COMSAT Active Geostationary 3 generation designed to carry 1200 voice circuits

Early Satellites

Satellite Launching Date Country/Organization Type Height (miles) Comments

Explorer 1958 USA/NASA Broadcast 110 to 920 Very short life; Noted for

re-broadcasting an on-board

taped message from president Eisnhour

ECHO 1960 USA/NASA Passive 1000 100-Foot diameter plastic balloon with an aluminum coating which

reflect radio signals

COURIER 1960 Department of defense Store & Repeat 600-700 First radio repeater satellite. It accepted and

stored upto 360,000 teletype words as it passed

overhead and then broadcast to ground stations further along the

orbit; only operated for 17 days.

TELSTAR 1962 USA/AT&T Active Duplex 682-4030 First satellite to receive and transmit simultaneously; Operated in 4/6 GHz band

Early Satellites

Satellite Launching Date Country/Organization Type Height (miles) Comments

INTELSAT 4 1971 INTELSAT/COMSAT Active Geostationary COMSAT’s 4th generation; designed to

carry 6000 voice circuits.

ANIK 1 1972 Canada/Telesat Active Geostationary World’s first domestic satellite; 5000 voice circuits capacity.

WESTAR 1974 USA/Western Union Active Geostationary First US domestic satellite

Early Satellites

• US Navy bounced messages off the moon

• ECHO 1 “balloon” satellite - passive

• ECHO 2 - 2nd passive satellite

• All subsequent satellites used active communications

ECHO 1

• Photo from NASA

Early Satellites

• Relay– 4000 miles orbit

• Telstar– Allowed live transmission across the Atlantic

• Syncom 2– First Geosynchronous satellite

TELSTAR

• Picture from NASA

SYNCOM 2

• Picture from NASA

Asiasat 2

System Design Consideration

• Services or Application• Selection of RF Band• Finance • Further technical design considerations are:-

– Optimal modulation, coding scheme, type of service, permitted earth station size and complexity, shape of service area, landing rights, state of prevailing technology related both to spacecraft and ground station.

Major problems for satellite

• Positioning in orbit in-term of Frequency & Orbit Selection

• Stability

• Power

• Communications

• Harsh environment

• Interference Problem

Limitation of Satellites

• High initial investment

• New investment require in Ground Segment

• Short life time

• Spectrum crowding

• Regulatory aspects (landing rights etc.)

• Launch vehicle reliability

Advantages of Satellite• Wide band capability• Wide area coverage readily possible• Distance-insensitive costs• Counter inflationary cost history• All user have same access possibilities• Point to point, point to multipoint (broadcast)

and multipoint to point (data collection) are all possible

• Inherently suited for mobile application.• Compatible with all new technologies• Service directly to the users premises

Applications• Communication

(truncking call)• Teleconference• Telemedicine• TV Broadcasting• Data communication• Telemetry(TEC,

remote sensing etc)

• Weather telecast• Navigation• GPS• Security/Calamity

monitoring• Standard Time• Military• Remote Sensing

Frequency Allocations & Regulatory Aspects

• Frequency bands for satellite services are shared with terrestrial services.

• Satellite signal strength is constrained to avoid interference by it to others.

• Thus a large antenna and sensitive receiver are needed at the earth station.

• Frequency sharing techniques are an important study area.• Many satellites have to share a limited frequency band (and

limited orbital arc) thus coordination in frequency and orbital location is important.

• Frequency allocation are done by international agreements

Electromagnetic Spectrum

The Frequency Spectrum and Typical Applications

102 104 106 108 1010 1012 1014 1016 1018 1020 Hz

PowerSystems Mittel

Welle

AM UKW

TV

Mobil Funk

MikroWelle IR Lamp

Sun Studio

X-Rays

AC Broadcast Microwave Infrared Ultraviolett X-Ray

SatTV

GPSGalileo

Glonass

Radio Frequency Bands

Band Number Band Name Frequency Range Metric Subdivision

4 VLF, Very low frequency 3-30 KHz Myriametric waves

5 LF, Low frequency 30-300 KHz Kilometric waves

6 MF, Medium frequency 300-3000 KHz Hectometric waves

7 HF, High frequency 3-30 MHz Decametric waves

8 VHF, Very high frequency 30-300 MHz Metric waves

9 UHF, Ultra high frequency 300-3000 MHz Decimetric waves

10 SHF, Super high frequency 3-30 GHz Centimetric waves

11 EHF, Extra high frequency 30-300 GHz Decimillimetric waves

Satellite Operating Frequency Bands

Frequency Range (GHz) Band Category

0.39-1.55 L MSS

1.55-5.2 S FSS & BSS

3.9-6.2 C FSS

5.2-10.9 X Military

10.9-36.0 K FSS & BSS

15.35-17.25 Ku FSS & BSS

18.3-31.0 Ka FSS

Frequency Allocation and Regulatory Aspects

• Domestice.g. Federal communication Commission (FCC)National Telecommunication and Information Administration (NITA)In Pakistan, PTA (Pakistan Telecommunication Authority)

• InternationalInternational Telecommunication Union (ITU)– Formed in 1932 from the International Telegraph Union– Consists of over 150 members nations– World Administrative Radio Conference (WARC)– International Radio Consultative Committee (CCIR)

consists of 13 study groups.

ITU Regions

ITU divides the surface area of the earth into three regions for the purpose of frequency allocation

• Region 1: Pacific Ocean RegionNorth and South America

Greenland

• Region 2: Atlantic Ocean RegionEuropeAfricaMiddle EastCentral Asia

ITU Regions (Continued)

• Region 3: Indian Ocean Region

Pakistan, India, Sub-continent , South East Asia & Australia

Frequency Allocations to Satellite Services

Examples of Satellite Radio Services:

- Fixed Satellite Service FSS

- Mobile Satellite Service MSS

- Broadcast Satellite Service BSS

- Radio Navigation Sat. Serv. RNSS

- Radio location Sat. Service RSS

- Space Operation Service SOS

- Earth observation Sat. Serv. ESS - . . .

In total more than 18 radio services

International Telecommunications Union

Artikel S5 der Radio Regulations

Region 1 Region 3Region 2

19.7 - 20.1 GHzFIXED-SATELLITE(space-to-earth)

Mobile-Satellite(space-to-earth)

S5.524

19.7 - 20.1 GHzFIXED-SATELLITE(space-to-earth)

MOBILE-SATELLITE(space-to-earth)

S5.524, S5.525, S5.526S5.527, S5.528, S5.529

19.7 - 20.1 GHzFIXED-SATELLITE(space-to-earth)

Mobile-Satellite(space-to-earth)

S5.524

International Telecommunications Union

A license is required by every operator in order to operate a satellite system nationally; a licence may only be acquired if:

- the operator can show that he has a contract with the system owner to be his service provider

- the frequencies for the system have been cleared / coordinated / notified

- that system is fully registered with the ITU

-the operator has workers registered as operators

A licence will be cancelled if:

- there are no more registered operators to work the system

- the service provider has breached ‘data protection laws’

International Telecommunications Union

2.2. Orbits and Launching Orbits and Launching MethodsMethods

Contents• Different Types of orbit• Satellite Orbits & Relative Periods• GEO• View & Coverage from GEO• Some GEO• Characteristics of GEO• Transfer Orbit• C & Ku Bands Satellites in Orbit• Mega LEO, MEO, HEO & GEO Projects• The Future • Broadband LEO• Launching • Launch Vehicle• Summary of Launchers• Types of Launches

Different Types of Orbits

• Circular orbits are simplest• Inclined orbits are useful for coverage of

equatorial regions• Elliptical orbits can be used to give quasi

stationary behaviour viewed from earth– using 3 or 4 satellites

• Orbit changes can be used to extend the life of satellites

Cont…

Several types

• LEOs - Low Earth Orbit

• MEOs - Medium Earth Orbit

• HEOs – Highly Elliptical Orbit

• GSO - Geostationary Earth Orbit

Cont…Cont…

LEO

• Low Earth Orbit

• 200-3,000 km

• High orbit speed

• Many satellites

• Predominately mobile

• Iridium, Globalstar

• (space shuttle orbit)

MEO

• Medium Earth Orbit

• 6,000 – 12,000km

• New generation

• About 12 satellites

• Voice and mobile

• ICO (Odyssey), Orbcomm,

Ellipso

Ellipso

Sub-Satellite Track of a HEOSub-Satellite Track of a HEO

The 24 h HEO of SiriusThe 24 h HEO of Sirius

The 24 h HEO of SiriusThe 24 h HEO of Sirius

HEOs: Molnya and TundraHEOs: Molnya and Tundra

Molnya Tundra

Period 12 h 24 h

Apogee 39 500 km 46 300 km

Perigee 1 000 km 25 300 km

Inclination 63.4° 63.4°

Satellite Orbits and PeriodsSatellite Orbits and Periods

Height Period Cell Visible Numbers Duration of of Orbit1 of Orbit Diameter Part of Earth of Satellite Over flight

(km) (h) (km) % * (min)

200 1.5 3 154 1.5 66 7

700 1.6 5 720 5.0 20 14

1000 1.8 6 719 6.8 15 18

1 414 1.9 7 806 9.1 11 22

10 000 5.8 14 935 30.5 4 130

20 000 11.9 16 922 37.9 3 300

35 786 24.0 18 100 42.4 3 24 h/d

1above the surface of the earth

*minimum necessary for 0° elevation and 0 redundancy

GEOs

• Originally proposed by Arthur C. Clarke• Circular orbits above the equator• Angular separation about 2 degrees -

allows 180 satellites• Orbital height above the earth about 23000

miles/35786.16km• Round trip time to satellite about 0.24

seconds

GEOs (2)

• GEO satellites require more power for communications

• The signal to noise ratio for GEOs is worse because of the distances involved

• A few GEOs can cover most of the surface of the earth

• Note that polar regions cannot be “seen” by GEOs

GEOs (3)

• Since they appear stationary, GEOs do not require tracking

• GEOs are good for broadcasting to wide areas

• Currently 329 GEO are in orbit(ref: web site provided by Johnston)

The original vision

• 1945 Arthur C Clark envisaged “extraterrestrial relays”

• # of Satellites: 03

• Period: 23 h 56 min 4.091 s

• Height: 36 000 km above

equator

• Speed of flight: 3.074 km/s

and then..

• 1957 Sputnik

• a rush of experimental satellites in many orbits

• Intelsat 1965 – 1st commercial GEO service

• over 800 objects registered so far

GEO - geostationary earth orbit

• characterised by:– delay (echo) ~0.5sec return– high power– 5-7 years life

• global and spot beams• C and K band (4-6Ghz and 12-14Ghz)• 2 – 3o spacing• Currently more than 200 GEO satellites in

operation

the view from 36,000km

Earth coverage with 2 spacecraft

Coverage of the inhabited world except for Polynesia

-170 -150 -130 -110 -90 -70 -50 -30 -10 10 30 50 70 90 110 130 150 170 190

90

70

50

30

10

-10

-30

-50

-70

-90

some GEO’s above us

• Optus * 3

• AsiaSat * 3

• PAS * 2

• Intelsat * 7

• Inmarsat * 2

• Palapa * 2and others

Some Service Providers:

Netspeed Austar Optus Telstra iHug Newskies MediaSat NTL Heartland Xantic Stratos

Characteristics of a Geostationary Satellite Orbit

• Eccentricity (e) 0

• Inclination of the orbital plane (i) 0º

• Period (T) 23h 56m 4s

• Semi-major axis (a) 42164 km

• Satellite altitude(R) 35786 km

• Satellite velocity (Vs) 3075 m/s

F=GMm/r2 T=2 a3/ =Gme=3.986x1014 m3/s2

e=c/a V= (2/r-1/a) m/s

The GEOThe GEO

d

pRo

Ro

Elevation , distance to the satelliteElevation , distance to the satellite

Kgrav = m Me G / r2 Kzent = m r 2, = m v2 / r

Angular velocity = 2 / T, T Period, v velocity

Kgrav = Kzent und

m Me g / r2 = m r 2 bzw. Me g / r2 = r 2

r 3 = Me g T2 / ( 2 )2

The period T of the circular orbit (r in km, m = 398 601.8 km3/s2) is ──── ──────T = 2 r 3 / m = 9.952 10-3 r 3 / km in Seconds

p = 6.611

d

pRo

Ro

lon = LongitudeE/S - LongitudeSatellite

lat = LatitudeE/S - LatitudeSatellite

Space angle : cos( ) = cos ( lon ) * cos( lat ) ───────────────────────────────────────

Distance d: d = Ro 6.6112 – 2 * 6.611 * cos + 1

Elevation : sin( ) = [ 6.6112 Ro2 – Ro

2 – d2 ) / ( 2 Ro d ) ]

Test: = 81.3° d = 41680 km and = 0°

= 0° d = 35787 km and = 90°

The GEOThe GEO

The inclination (1)

The equatorial plane

The inclined planeThe inclination: orbit remains geosynchroneous, 24 h; satellite moves North/South;inclination builds up 0.8°/year ifnot corrected contiuously

.

)

The inclination (2)

The equatorial plane

The inclined planeAfter 18 years some 15° of inclination will have built up; now the inclination reverses and decreases by 0.8°/year;satellites with <15° inclination are geostationary by law.

.

)

Transfer Orbits

C-Band satellites in GEO

Legende

im Orbit

im Bau

ITU Appl.

Legend

on orbit

under constr

ITU Appl.

(1995)

Ku-Band satellites in GEO

Legende

im Orbit

im Bau

ITU Appl.

Legend

on orbit

under constr

ITU Appl.

(1995)

C and Ku-Band satellites in America

Comparison ChartFeatures GEO MEO LEO

Height (km’s)

36,000 6,000-12,000

200-3000

Time per Orbit (hrs)

24 5-12 1.5

Speed (kms/ hr)

11,000 19,000 27,000

Time delay (ms)

250 80 10

Time in site of

Gateway

Always 2-4 hrs < 15 min

Satellites for Global Coverage

3 10-12 50-70

Mega LEOs, MEOs, HEOs, Mega LEOs, MEOs, HEOs, and GEOsand GEOs

1 TELEDESIC of microSoft with 288 LEOs at Ka-Band

2 V-Band Supplement of TELEDESIC/microSoft with 72 LEOs im Q-Band

3 GS-40 of Globalstar LP with 80 LEOs at Q-Band

4 M-Star of Mororola with 72 LEOs at Q-Band

5 LEO ONE of LEO ONE Corp. with 48 LEOs at Q-Band

6 ORBLINK of Orblink LLC with 7 MEOs in Q-Band

7 SkyBridge of ALCATEL witt 64 LEOs and 9 GEOs in Ku-Band

8 WEST of MATRA with 10 MEOs and 12 GEOs in Ka-Band

9 GESN of TRW with 15 MEOs and 4 GEOs in Q-Band

10 CELESTRI of Motorola MOT with 63 LEOs and 10 GEOs in Ka-Band

11 SpaceWay of Hughes Communications with 20 LEOs and 16 GEOs in Ka-

Band

12 StarLynx of Hughes Communications with 20 MEOs and 4 GEOs in Q-Band

13 DenAli Telecom LLC PenTriad in HEO im Ku-, Ka-, V- and W-Band

The Future

• given current-generation LEO’s and MEO’s are predominately used for mobile voice and low-speed data services (MPSS)– good voice coverage for remote regions– adjunct to GSM mobile networks ~ Globalstar

the future

• continual development in VSAT (GEO) technology– bandwidth gains– multiple services = choice

• Broadband LEOs – Teledesic

• fixed and transportable terminals• 64k – 2M – and above (Gb)• 288 satellites• 2005 launch??

– SkyBridge• 80 satellites• 2004

what is SkyBridge?• SkyBridge is an Alcatel controlled company planning to

establish a constellation of 80 satellites to provide broadband data communications direct to business & residential premises.

• Satellites are Low Earth Orbit (LEO) at an altitude of 1500 km• offers “last mile” broadband access from 2004

– no long-haul trunking capability - connects users to terrestrial gateway

• System cost is approx US$4.8bn

broadband LEO – low latency

GEO : 500ms Astrolink

Intelsat

Spaceway

LEO round-trip propagation time

comparable to terrestrial

1 500 km LEO : 30ms

SkyBridge

Teledesic

36 000 km

Launching

Step 1: satellite is released in the Low Earth Orbit by launch vehicle (click on the picture below)

Step 2: The Payload Assist Module (PAM) rocket fires to place the

satellite into the geostationary transfer orbit (GTO)

Launching (Continued)

Step 3: Several days after the satellite gets into the GTO the Apogee Kick Motor (AKM) fires to put the

satellite into a nearly circular orbit.

Launching (Continued)

Step 4: Orbital Adjustment by firing the AKM to achieve a circular geosynchronus orbit. (click on the picture below)

Launch Vehicles

Launch Atlas II Delta II Ariane-4 Proton Long H-2

Vehicles March-3

Country USA USA Europe Russia China JAPAN

Gross 460 t 680 t 202 t 260 T

Weight

Boast to 3636 Kg 1,819 Kg 2,200 Kg 2,000 Kg 650 Kg 2,200 kG

GTO

Launch Vehicle

Launch Vehicle

Summary of LaunchersSummary of Launchers

Sea LaunchSea Launch

At the Equator

equator

11 day travel, 3 days on site, 9 days back1. and 2. stage fueled on launch site; 3. stage and satellite fueled in Long Beach

Sea Launch

Lift-Off!Up to 6 t

3000 m deep water

Commander is 5 km away for launch

The Launch Service Alliance

ArianeSpace, Boeing Launch Services, and Mitsubishi Heavy Industries

↪ mutual backup to mitigate schedule risks, range issues, etc.

Summary of LaunchersSummary of LaunchersInternational Launch Services, ILSLockheed Martin, USA, Khrunichev, RUS, Energia, RUS

Atlas-IIARlo, Proton-Mhi

Baikonur Launch Site

Types of Launches

Land Launchsince the 60ies

Rail Launchsince the 70ies

Air Launchsince the 80ies

Sea Launchsince the 90ies

The Evolution:

Anatomy of a Satellite

A communication satellite consists of the following subsystems:• Antenna_For receiving and transmitting signals.• Transponder_It contains the electronics for receiving the

signals, amplifying them, changing their frequency and retransmitting them.

• Power Generation and conditioning subsystem_For creating power and converting the generated power into a usable form to operate the satellite.

• Command and Telemetry_For transmitting data about the satellite (status, health etc.) to the earth and receiving commands from earth.

• Thrust subsystem_For making the adjustments to the satellite orbital position and altitude.

• Stabilization subsystem_For keeping the satellite antennas pointing in exactly the right direction.

Common Abbreviations

Orbits:GEO = Geostationary Earth OrbitHEO = Highly inclined Elliptical OrbitMEO = Medium altitude Earth OrbitLEO = Low altitude Earth OrbitIGSO = Inclined Geo-Synchroneous OrbitHAP = High Altitude Platform

Services:BIG = Voice TelephonySuper = Voice telephony into mobiles from GEOLittle = Data only, typically store and forwardMega = Mega-bit/s servicesDBS = Direct Broadcast satellite television ServiceDab = Digital Audio Broadcast satellite serviceNav = Navigation service

glossaryGEO – geostationary earth orbit – 36,000kmMEO – Medium earth orbit – 6-12,000kmLEO – Low earth orbit – 200-3,000kmBroadcast – One to many simultaneous transmission,

usually associated with older style analogue transmission

Multicast – In communications networks, to transmit a message to multiple recipients at the same time. Multicast is a one-to-many transmission similar to broadcasting, except that multicasting means sending to specific groups, whereas broadcasting implies sending to everybody. When sending large volumes of data, multicast saves considerable bandwidth, because the bulk of the data is transmitted once from its source through major backbones and is multiplied, or distributed out, at switching points closer to the end users.

2-way – Infers forward and reverse transmission via the satellite, usually but not always asymmetric, i.e. high-speed download from the satellite and low speed from client to the satellite

latency – The time between initiating a request for data and the beginning of the actual data transfer. A GEO satellite has a latency of approx 256ms resulting in a round trip delay of about half a second (echo)

IP – Internet Protocol – the language of the Internet. The protocol stack is referred to as TCP / IP

Fixed – refers to a satellite receiver being attached as a permanent mounting, as opposed to tracking.

Mobile – Refers to a mobile satellite receiver such as a personal communicator or mobile phone. Usually associated with LEO and MEO services.

Broadband – high speed transmission. The threshold is arguable, but is construed as being faster than dial-up ~ 64kbps and upwards. Some conventions suggest the threshold starts at 1.5 or 2Mbps.

Orbit – The path of a celestial body or an artificial satellite as it revolves around another body.

One complete revolution of such a body

VSAT– Very small aperture terminal, refers to a small-dish service using a GEO satellite and a large central hub, usually 6 metres plus.

DTH – Direct to home. A service bypassing normal terrestrial infrastructure such as a satellite TV receiver. As opposed to community satellite service where local distribution from a satellite receiver is done by cable, radio or other means.

3.3. Orbital MechanicsOrbital Mechanics

Contents

• Kepler’s Laws• Orbital Elements• Epoch • Orbital Inclination • Right Ascension of Ascending Node (R.A.A.N.) • Argument of Perigee • Eccentricity • Mean Motion • Mean Anomaly • Drag (optional)• Apogee & Perigee Heights

Kepler’s Laws

• LAW 1: The orbit of a planet about the Sun is an ellipse with the Sun's center of mass at one focusLAW 2: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time

• LAW 3: The squares of the periods of the planets are proportional to the cubes of their semi-major axes

Kepler’s First Law

• LAW 1: The orbit of a planet about the Sun is an ellipse with the Sun's center of mass at one focus.

This is the equation for an ellipse:

Cont….

• Earth’s orbit has an eccentricity of 0.017 (nearly circular)

• Pluto’s orbit has an eccentricity of 0.248 (the largest in our solar system)

• Satellites also follow Kepler’s 1st Law – But Earth can replace sun at Focus

Kepler’s Second Law

• LAW 2: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time

Cont…

• So… Satellites go faster at Perigee than at Apogee

• Reason: conservation of specific mechanical energy;

i.e., З = KE + PE

Kepler’s Third Law

LAW 3: The period of an orbit depends on the

altitude of the orbit OR

The square of the period is proportional to the cube of its mean distance from primary focus

• Ta2 / Tb

2 = Ra3 / Rb

3

Cont…

• Low Earth orbit: 90 minutes– 186 miles, 17,684 mph

• Geosychronous: 24 hours– 22,236 miles, 6,857 mph

• Moon: 28 days (one month)– 238,330 miles, 2,259 mph

Orbital Elements

• The classic 'Keplerians' are the seven mathematical values which determine a spacecraft's orbit around the Earth.

• In practice there are additional values which are required because the Earth isn't a perfect sphere, and other anomalies.

Cont…• Seven numbers are required to define a satellite orbit.

This set of seven numbers is called the satellite orbital elements, or sometimes "Keplerian" elements (after Johann Kepler [1571-1630]), or just elements

• These numbers define an ellipse, orient it about the earth, and place the satellite on the ellipse at a particular time.

• In the Keplerian model, satellites orbit in an ellipse of constant shape and orientation. The Earth is at one focus of the ellipse, not the center (unless the orbit ellipse is actually a perfect circle)

Cont…The basic orbital elements are...1. Epoch 2. Orbital Inclination 3. Right Ascension of Ascending Node (R.A.A.N.) 4. Argument of Perigee 5. Eccentricity 6. Mean Motion 7. Mean Anomaly 8. Drag (optional) Note:Satellite keplerians are also distributed by NASA in a format called the NASA two-line

format.

Epoch• [aka "Epoch Time" or "T0"] • A set of orbital elements is a snapshot, at a particular

time, of the orbit of a satellite. Epoch is simply a number which specifies the time at which the snapshot was taken

Orbital Inclination• [aka "Inclination" or "I0"] • The orbit ellipse lies in a plane known as the orbital

plane. The orbital plane always goes through the center of the earth, but may be tilted any angle relative to the equator. Inclination is the angle between the orbital plane and the equatorial plane. By convention, inclination is a number between 0 and 180 degrees.

Right Ascension of Ascending Node• [aka "RAAN" or "RA of Node" or “RAAN", and

occasionally called "Longitude of Ascending Node"]

• Right ascension is another fancy word for an angle, in this case, an angle measured in the equatorial plane from a reference point in the sky where right ascension is defined to be zero. Astronomers call this point the vernal equinox.

• Finally, "right ascension of ascending node" is an angle, measured at the center of the earth, from the vernal equinox to the ascending node.

A few words about elliptical orbits... The point where the satellite is closest to the earth is called perigee, although it's sometimes called periapsis or perifocus. We'll call it perigee. The point where the satellite is farthest from earth is called apogee (aka apoapsis, or apifocus).

Apogee & Perigee

Argument of Perigee• If we draw a line from perigee to apogee, this line

is called the line-of-apsides (Sometimes the line-of-apsides is called the major-axis of the ellipse)

• The line-of-apsides passes through the center of the earth. We've already identified another line passing through the center of the earth: the line of nodes. The angle between these two lines is called the argument of perigee

• Where any two lines intersect, they form two supplementary angles, so to be specific, we say that argument of perigee is the angle (measured at the center of the earth) from the ascending node to perigee.

Cont…

• In simple words the polar angle locating the perigee point of a satellite in the orbital plane; drawn between the ascending node, geocenter and perigee and measured from ascending node in direction of satellite motion.

Eccentricity• [aka "ecce" or "E0" or "e"] • Eccentricity tells us the "shape" of the ellipse.

When e=0, the ellipse is a circle. When e is very near 1, the ellipse is very long and skinny.

Mean Motion• [aka "N0"] (related to "orbit period" and

"semimajor-axis") • Now we need to know the "size" of the orbit

ellipse. In other words, how far away is the satellite?

• Kepler's third law of orbital motion gives us a precise relationship between the speed of the satellite and its distance from the earth. Satellites that are close to the earth orbit very quickly. Satellites far away orbit slowly. This means that we could accomplish the same thing by specifying either the speed at which the satellite is moving, or its distance from the earth!

• Satellites in circular orbits travel at a constant speed. Simple. We just specify that speed, and we're done. Satellites in non-circular (i.e., eccentricity > 0) orbits move faster when they are closer to the earth, and slower when they are farther away. The common practice is to average the speed. You could call this number "average speed", but astronomers call it the "Mean Motion". Mean Motion is usually given in units of revolutions per day

• In this context, a revolution or period is defined as the time from one perigee to the next.

• Sometimes "orbit period" is specified as an orbital element instead of Mean Motion. Period is simply the reciprocal of Mean Motion. A satellite with a Mean Motion of 2 revs per day, for example, has a period of 12 hours.

• Sometimes semi-major-axis (SMA) is specified instead of Mean Motion. SMA is one-half the length (measured the long way) of the orbit ellipse, and is directly related to mean motion by a simple equation.

• Typically, satellites have Mean Motions in the range of 1 rev/day to about 16 rev/day

Mean Anomaly• [aka "M0" or "MA" or "Phase"] • Now that we have the size, shape, and orientation

of the orbit firmly established, the only thing left to do is specify where exactly the satellite is on this orbit ellipse at some particular time.

• Anomaly is yet another astronomer-word for angle. Mean anomaly is simply an angle that marches uniformly in time from 0 to 360 degrees during one revolution. It is defined to be 0 degrees at perigee, and therefore is 180 degrees at apogee.

Drag• [aka "N1"] • Drag caused by the earth's atmosphere causes

satellites to spiral downward. As they spiral downward, they speed up. The Drag orbital element simply tells us the rate at which Mean Motion is changing due to drag or other related effects. Precisely, Drag is one half the first time derivative of Mean Motion.

• Its units are revolutions per day per day. It is typically a very small number. Common values for low-earth-orbiting satellites are on the order of 10^-4. Common values for high-orbiting satellites are on the order of 10^-7 or smaller.

Kepler Orbital Parameters(Kepler Elements)

• Ω – right ascension of ascending node• i – inclination of orbital plane• ω – argument of perigee• a – semimajor axis of orbital ellipse• e – numerical eccentricity of ellipse• T0 – epoch of perigee passageRef:

www.amsat.org/amsat/keps/kepmodel.html#argp www.amsat.org/amsat/ftp/keps/current/amsat.all

Kepler Elements

4. Orbital Perturbation

Contents • Orbital perturbations• Types of Orbital Perturbations• The Non-Spherical Earth• Atmospheric Disturbances• Solar Radiation & Solar Winds• Third Body Interaction• Attitude Perturbations• Aerodynamic Pressure• Solar Pressure• Earth Magnetic Field

Orbital perturbations

• In this chapter we will discuss the most important disturbances. This is necessary to do because we want to know the lifetime of the satellite before it will tumble down to earth.

• We will also see how the orbit changes due to the different disturbances.

• One important thing to remember is that these calculations are for a cause to do the predicted orbit and lifetime more accurate.

Types of Orbital Perturbations• There are two types of Orbital

Perturbations– gravitational, when considering third body

interaction and the non-spherical shape of the earth.

– non-gravitational like atmospheric drag, solar-radiation pressure and tidal friction.

• These can also be classified as conservative or non-conservative disturbances forces. Where conservative forces depends only on the position, while non-conservative forces depends on both position and velocity.

The Non-Spherical Earth

• The earth is far away from perfectly spherical.

• One depends on the rotation, making the radius from center of the earth to the equator larger than from the center of the earth to the poles.– Gravitation potentialGravitation potential– Gravity harmonicsGravity harmonics– Force approachForce approach

Atmospheric Disturbances• Although the atmosphere is almost empty you

have to consider it. This is the most important disturbance, because it is the main cause in determining the lifetime of the satellite.

• The drag that can be calculated is an empirical function based on Cd which is a constant depending on the shape of the body.

• The also necessary density of the atmosphere depends on some different environmental factors such as the activity of the sun. The major part of the atmosphere below 1000 km consists of O2, N2, and He.

• The minor representative parts are O3, CO2, H2, NO,electrons, and both positive and negative ions.

• The difficulty to determine the density is because of the chemical reactions especially photochemical reactions. These are driven by the sunlight, and therefore the activity of the sun is important.

• The other chemical reaction in the atmosphere is diffusion. The minor constituents are controlled by photochemical processes and therefore the density depends on the sunlight.

• In this case we use a mean value of the density.

CD is the drag coefficient depending on the shape and surface but the best value is given in an actual test flight. But the value for a sphere is 2.2 and for a cylinder it is 3.0. Usually 2.2 is considered to give a conservative result.

Solar radiation and solar wind

• Solar radiation is all kind of electromagnetic field emitted by the sun, from X-rays to radio waves.

• The solar wind consists of particles emitted by the sun, mainly ionized nuclei and electrons.

• Because of the charged particles in the solar wind it does not penetrate the magnetopause, except at the magnetic poles. The magnetopause starts about 10 earth radii from the center of the earth (Re = 8371) km. Therefore, the sun is more or less active. It has an activity cycle of 22 years between two peaks of activity.

• Therefore the solar pressure is also not constant, but it fluctuate by < 1%. The pressure is, P0 = 4.7 · 10-6 [Pa]. The perturbing forces can be calculated by:

• The effect due to the solar radiation pressure is, for a LEO, not that big.

• The aerodynamic drag has a more disturbing effect. But at altitudes above 1000 km and an orbit close to the ecliptic plane it has a more distinct effect.

Third body interaction

• How do the other planets disturb the satellite?

Attitude Perturbations

• The disturbance in orientation or attitude is important to look at because we want to keep the orientation so it can perform the tasks

• Here we consider the atmospherically drag, the solar pressure and the magnetic disturbance.

• Aerodynamic Pressure– The pressure due to the atmosphere

affects the satellite, although one often think of space as a vacuum it has, or at least the environment where the satellite operates, has some kind of atmosphere. If the center of pressure of the body is different from the center of mass, the pressure acts on the body and the resultant of the forces is not through the center of mass and there are a torque due to the atmosphere. The force on a differential area can be expressed by;

Solar Pressure

• Just like the pressure from the atmosphere a torque due to solar pressure act on the satellite. The pressure of the the sun and the difference of the center of pressure and the center of mass causes a torque on the satellite. The force on a differential area can be described with;

The total torque can be found in the same way as for the atmospheric torque.

Earth Magnetic Field

• The magnetic field of the earth has two ways of disturbing the satellite. The first is when the satellite rotates in a magnetic field. The magnetic field induces eddy currents in the shell and due to the resistance of the shell it produces heat. The energy it takes to produce the heat is taken from the rotational energy but the effects are very small. In this case when we have a short life cycle of the satellite we do not have to take this aspect in our calculations. The torques due to eddy currents are;

• where ke is a constant depending on the satellite’s geometry (see table) and conductivity, B is the vector of the magnetic strength of the earth

5. Satellite Visibility

Contents

• When are satellites visible?

• Factors Affecting the satellite visibility

• Orbit & Attitude Inclination

• Earth Shadow

• Ground Track

• Other factors

Limit of Visibility

• When Are Satellites Visible? • Whether or not a satellite is visible to a

given observer is dependent upon many factors such as observer location, time of day, satellite altitude, and sky condition. Knowing these details may aid an observer in determining the most favorable times for sightings and is most certainly necessary

Factors Affecting Satellite Visibility

• Orbit Altitude And Inclination • Earth's Shadow • Ground Track • Other Factors

Orbit Altitude & Inclination

• GEO

• MEO

• LEO

• HEO

Earth's Shadow

• The Earth's shadow must also be considered. When eclipsed, a satellite is naturally not visible. Such events are dependent upon the satellite's altitude, inclination, the time of year, and the observer's location

Ground Track

• Precession Of course it is not simply a question of watching for a given satellite at the same time each night. Few satellites have an orbital period which is a simple fraction of one day, the geostationary satellites being the obvious exception. The orbital period is dictated by the satellite's altitude. The higher the altitude, the further it has to travel around the Earth and the longer it thus takes. Satellites in low Earth orbit complete one orbit in around 90 minutes, whereas at geostationary altitudes (about 36,000 km) one orbit takes 24 hours.

• Many satellites in low Earth orbit go through a similar cycle of visibility. The cycle varies with orbital inclination, altitude, and observer location.

Other Factors

• satellite suffers greater air resistance the lower its orbit. This bleeds off the orbital energy, lowering the orbit yet further as the satellite begins to brush the upper atmosphere at perigee.

• The forces on the satellite due to the Earth (and Moon, Sun, etc.) vary throughout its orbit giving rise to continual change in the orbit.

6. Radio Wave Propagation

Contents

• Introduction• Atmospheric Losses

– Beam-spreading Loss – Polarization Loss – Rayleigh fading– Scintillation Loss – Free-space loss– Weather Loss – Doppler Effect

• Rain Attenuation• Ionospheric Losses

• This section discusses the basic effects of the This section discusses the basic effects of the propagation anomalies as they influence the propagation anomalies as they influence the communication satellite system performancecommunication satellite system performance

• The greatest difference between the bands The greatest difference between the bands above 10 GHz and those between 1 and 10 Ghzabove 10 GHz and those between 1 and 10 Ghz

• The 1-10 GHZ range is already extensively The 1-10 GHZ range is already extensively used by both terrestrial microwave and satellite used by both terrestrial microwave and satellite services.although the noise level and services.although the noise level and attenuation are lower than the higher attenuation are lower than the higher frequencies, the potential for interference from frequencies, the potential for interference from terrestrial point-to-point services has limited terrestrial point-to-point services has limited earth station locations.earth station locations.

Introduction

• Above 10GHz the rain attenuation increases, Above 10GHz the rain attenuation increases, but the chances of interference with other but the chances of interference with other services are minimum.services are minimum.

• At certain wavelengths signals encounter At certain wavelengths signals encounter absorption bands due to atmospheric absorption bands due to atmospheric components (like water vapor and oxygen) components (like water vapor and oxygen) within the range of 1-10 GHzwithin the range of 1-10 GHz

• Frequencies above 30GHz have been Frequencies above 30GHz have been underutilized, there is spectrum available, underutilized, there is spectrum available, especially for services that do not pass through especially for services that do not pass through the atmosphere like ISL(Inter Satellite Link)the atmosphere like ISL(Inter Satellite Link)

• The fundamental equation for the free-space The fundamental equation for the free-space position of the slant range losses(Lposition of the slant range losses(Lrangerange) is;) is;

LLrangerange = (4 = (4S/S/))22

where;where;

S= Slant Range in mS= Slant Range in m

=Wavelength in m=Wavelength in m

• At 6GHz the slant range attenuation is about At 6GHz the slant range attenuation is about 200db200db

Atmospheric Losses

• In satellite communications, atmospheric losses results from the absorption of the Earth-satellite or satellite-Earth signals as they pass through the Earth's atmosphere. The value of the atmospheric loss is strongly dependent on frequency.

Atmospheric Losses

Atmospheric Losses

– Beam-spreading Loss – Polarization Loss – Rayleigh fading– Scintillation Loss – Free-space loss– Weather Loss – Doppler Effect

Beam-spreading loss

• In satellite communications, beam-spreading loss results from the spreading of the earth-satellite signals as they pass through the Earth's atmosphere

Scintillation loss

• In satellite communications, scintillation loss results from rapid variations in the signal’s amplitude and phase due to changes in the refractive index of the Earth's atmosphere.

Polarization loss

• In satellite communications, polarization loss results from a rotation of the polarization of the signal as it passes through the Earth's atmosphere

Rayleigh Fading

• Rayleigh fading is fading in a satellite communications channel due to the interference caused to the main signal by the same signal arriving over many different paths, resulting in out-of-phase components incident at the receiver.

• Rayleigh fading occurs commonly in wireless communications channels, including satellite communications channels.

Free Space Losses• In satellite communications, free-space loss is the major

loss suffered by signals in traveling over the Earth-satellite path. The loss is inversely proportional to the square of the distance traveled and inversely proportional to the square of the frequency used. That is, as the distance is doubled the received power is reduced by a factor of four. Similarly, as the frequency is doubled the received power is reduced by a factor of four.

• Free-space loss for geo-stationary satellite communications satellites varies between 190-210 dB depending on the frequency used

Weather Losses• In satellite communications, weather loss

results from attenuation of the Earth-satellite signals by hydrometers as they pass through the Earth's atmosphere

Brightness Temperature of the Earth

14 GHz (ESA/EUTELSAT-Modell)

Doppler Effect

• The Doppler effect in satellite communications is the change in frequency of an electromagnetic signal that results from the relative speed of the satellite and the Earth terminal. When the orbital parameters of a satellite are known, Doppler shift can be used to determine the position of the Earth terminal. When an Earth terminal's position is known, Doppler shift can be used to estimate the orbital parameters of a satellite. When the satellite (or the Earth station) is moving quickly, the Doppler effect is an important consideration in satellite communications

Atmospheric and Rain Attenuation

Rain Attenuation

• Rain is predominant loss element below 60GHz.

• Fog is shown has attenuation 0.1 g /m3

• The total link attenuation is the sum of the losses due to slant range , the atmosphere, precipitation and any additional losses(such as scintillation etc.)

Climatic Zones

A: is extremely dry climate, . . . P: extremely humid climate

Climatic Zones

PP

A

A

D

D

M

D

C

D

C

C

CE

E

E

EF

F

K K

P

K

KKE

E

E

H

H

P H

ME

H H

N

N

DP

Frequency in GHz

Equatorial Latitudes

MediumLatitudes

IonosphericDelay

Atmosph.Attenuation

Rain Attenuation20 mm/h

10 mm/h

Additional Attenuationin dB

10

1

100

100

10

1

5 GHz

Atmospheric and Rain Attenuation

Ionospheric Losses

• Al lower frequencies (e.g 1.5 and 2.5 GHz) ionospheric effect may be encountered, particularly scintillation.

• The magnitude of these losses vary considerably with the time of day and the sunspot activity level (the affect the ionosphere).

Ionospheric Losses

• All radio waves propagated over ionospheric paths undergo energy losses before arriving at the receiving site. As we discussed earlier, absorption in the ionosphere and lower atmospheric levels account for a large part of these energy losses.

• There are two other types of losses that also significantly affect the ionospheric propagation of radio waves. These losses are known as ground reflection loss and free space loss.

• The combined effects of absorption, ground reflection loss, and free space loss account for most of the energy losses of radio transmissions propagated by the ionosphere

Ionospheric Losses

7. Polarization

Contents• Polarization• Types of Polarization• Antenna polarization• Manual Polarization Switching• Polarization of satellite signals• Depolarization• Cross polarization discrimination• Ionospheric depolarization, rain & ice depolarization • XPD and Co-Polar Attenuation• Ionospheric Effect

Polarization

• The polarization of an electromagnetic wave is defined as the orientation of the electric field vector. Recall that the electric field vector is perpendicular to both the direction of travel and the magnetic field vector.

• The polarization is described by the geometric figure traced by the electric field vector upon a stationary plane perpendicular to the direction of propagation, as the wave travels through that plane.

Cont…

Cont…

• Polarization is also describe as the "direction of vibration" on the radio wave.

• It depends the orientation of elements of an antenna, when you set elements vertical, it generates vertical-polarized radio wave similarly when you set as horizontal, it generates horizontal-polarized.

• In the case of YAGI antenna, the direction of Electronic-Field is same as the direction of its elements.

• Radio stations have to set as a same direction of polarization for communication each other.

• An electromagnetic wave is frequently composed of (or can be broken down into) two orthogonal. This may be due to the arrangement of power input leads to various points on a flat antenna, or due to an interaction of active elements in an array, or many other reasons.

• The geometric figure traced by the sum of the electric field vectors over time is, in general, an ellipse as shown in Figure 2. Under certain conditions the ellipse may collapse into a straight line, in which case the polarization is called linear.

Types of Polarization

• In the other extreme, when the two components are of equal magnitude and 900 out of phase, the ellipse will become circular as shown in Figure 3. Thus linear and circular polarization are the two special cases of elliptical polarization. Linear polarization may be further classified as being vertical, horizontal, or slant.

Cont…

Polarization and its types

Cont…• Polarization makes the beam more concentrated• FSS satellites use horizontal and vertical

polarization, whereas DBS satellites use left- and right-hand circular polarization

• To use the channels that are available for satellite broadcast as efficiently as possible, both horizontal and vertical polarization (and left- and right-hand circular polarization) can be applied simultaneously per channel or frequency. In such cases the frequency of one of the two is slightly altered, to prevent possible interference

• Horizontal and vertical transmissions will therefore not interfere with each another because they are differently polarized. This means twice as many programs can be transmitted per satellite

• Consequently, via one and (almost) the same frequency the satellite can broadcast both a horizontal and a vertical polarized signal (H and V), or a left- and right-hand circular polarized signal (LH and RH).

Cont…

Radio stations have to set as a same direction of polarization for communication each other.

• When you try to hear the vertical-polarized wave with horizontal- polarized antenna, what will be happened? A theory tells it is impossible to receive. In fact, although it is possible, It becomes very difficult (very weak less than -20dB ). This is due to:-– The radio waves do not travels with pure-polarized

condition, and – There is no real antenna that has pure-polarized

character. Anyway, you should to adjust the polarization for better communication.

Is Circular Polarization better choice for satellite?

• Circular-polarization (CP) is another choice when you could not decide the polarization of your choice.

• CP is the special style of polarization, the direction of Electric-Field rotates one times par one cycle.

• The CP antenna can receive both horizontal and vertical polarized radio wave, even in the direction of slant-polarized.

• CP is very popular technique for satellite communication both commercial and amateur satellite systems.

Antenna Polarization

• Table 1 shows the theoretical ratio of power transmitted between antennas of different polarization. These ratios are seldom fully achieved due to effects such as reflection, refraction, and other wave interactions, so some practical ratios are also included.

Cont…

• The sense of antenna polarization is defined from a viewer positioned behind an antenna looking in the direction of propagation. The polarization is specified as a transmitting, not receiving antenna regardless of intended use.

• We frequently use "hand rules" to describe the sense of polarization. The sense is defined by which hand would be used in order to point that thumb in the direction of propagation and point the fingers of the same hand in the direction of rotation of the E field vector.

Cont…

• For example, referring to Figure 4, if your thumb is pointed in the direction of propagation and the rotation is counterclockwise looking in the direction of travel, then you have left hand circular polarization.

• The polarization of a linearly polarized horn antenna can be directly determined by the orientation of the feed probe, which is in the direction of the E-field.

Cont…

• In general, a flat surface or sphere will reflect a linearly polarized wave with the same polarization as received. A horizontally polarized wave may get extended range because of water and land surface reflections, but signal cancellation will probably result in "holes" in coverage. Reflections will reverse the sense of circular polarization.

Cont…

• For a linearly polarized antenna, the radiation pattern is taken both for a co-polarized and cross polarized response.

• The polarization quality is expressed by the ratio of these two responses. The ratio between the responses must typically be great (30 dB or greater) for an application such as cross polarized jamming

• For general applications, the ratio indicates system power loss due to polarization mismatch.

• For circularly polarized antennas, radiation patterns are usually taken with a rotating linearly polarized reference antenna.

Cont…

Manual Polarization Switching

• The CP antenna reduces QSB so it might be better for comfortable operation, but the CP antenna is bigger and more complicated than the simple linear-polarized antenna. Also the big and complicated antenna will be expensive. 3dB loss will be a problem with some limited conditions.

• There is another choice. Setup a pair of vertical/Horizontal polarized independent antenna and switch them at your shack. You select where either is better during its pass. This is the theory of "Divercity" Divercity" reception

• Applied for geo-stationary satellites• “Horizontal”polarization = parallel to the equatorial plane• “Vertical”polarization = parallel to the Earth's axis• Polarization angle at earth station

– r = local gravity direction– k = the direction of the wave propagation– p = unit polarization vector– f = k x r, normal to the reference plane– x = the angle between the reference plane

(r and k) and the polarization vector

Polarization of satellite signal

Depolarization

• The electric field E1 is depolarized after going through a depolarizing medium.

• The result is, as shown in the figure, an orthogonal (E12) component may be generated.

• E11 is called the co-polar component and E12 is called the cross-polar component.

• This phenomenon can cause interference.

Cont…

Cross-Polarization Discrimination (XPD)

• One measure to quantify the effects of polarization is called the cross-polarization discrimination (XPD)

Cross-polarization discriminationobservations - rain depolarization

• Looking at XPD as a function of the co-polar attenuation (A), it can be concluded that:– XPD degrades at a given co-polar attenuation as

the frequency decreases– XPD degrades with increasing co-polar attenuation– XPD for the Vertical Polarization wave is better

than that for Horizontal Polarization– XPD for the Vertical Polarization and the Horizontal

Polarization waves are better that the Circular Polarization

XPD and co-polar attenuation A

-> the elevation angle in degrees

the polarization tilt angle

for circular polarization

Ionospheric effects

• Faraday’s effects– The rotation of a linearly polarized

wave due to the earth’s magnetic field is called the Faraday’s effect. It is proportional to the 1/f2 factor.

• Ionospheric scintillation– Due to the refractive index variations

in the ionosphere caused by local concentrations of ionization. It is also proportional to the 1/f2 factor.

8. Antenna

Contents

• Antenna• Some Basic Definitions• Radiation Parameters• Radiation Patterns• Types of Radiation Patterns• Antenna Radiation Pattern Nulls & Lobes• Antenna Beamwidth• Types of Ground Station Antenna used in SatCom• Types of Space Segment Antenna used in SatCom

Antenna

• Antennas form a very important element in communication system, either terrestrial or extra terrestrial, depending on the mission type and requirements

• "That part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves".

• we use antennas to overcome our inability to lay a physical interconnection between two remote locations or an antenna can also be viewed as a transitional structure (transducer) between free-space and a transmission line (such as a coaxial line).

• Antennas cannot add power, instead they can only focus and shape the radiated power in space e.g. it enhances the power in some wanted directions and suppresses the power in other directions

Some Basic Definitions

• Suppose we have an antenna located at the origin of a spherical co-ordinate system, further assume that the antenna is transmitting and the observations are made for a very large distance;

• Let Po (Watts) be the accepted power in the antenna and Pr (Watts) be the radiated power, then the radiating efficiency ή as;

• ή = Pr / Po

z

rP

y

z

ç

θAntLocation

Radiation Intensity

• We define Radiation Intensity f (θ,Ф) or Θ(θ,Ф) (watts/steradians)

• The Average radiation intensity is;

Θavg = Pr / 4π

Pr =

Antenna Directivity(Measure of the focusing property of an antenna)

• "The directivity of an antenna is defined as the ratio of the radiation intensity in a given direction from the antenna, to the radiation intensity averaged over all directions.

• This average radiation intensity is equal to the total power of the antenna divided by (4 pi). If the direction is not specified, the directivity refers to the direction of maximum radiation intensity".

D (θ,Ф) = {Θ(θ,Ф) / Θavg}

or

D (θ,Ф) = 4π {Θ(θ,Ф) / Pr} is the elevation angle is the azimuth

• where D is the directivity. Generally D > 1, except in the case of an isotropic antenna for which D = 1. An antenna with directivity D >> 1 is called a directive antenna.

Cont…

Gain (Measure of Directivity)

• The Gain G(θ,ф) is the ability to concentrate the power accepted by the antenna in a particular direction. It is related to the Directivity and Power Radiation efficiency or in other words Power Radiation Intensity as follow;

G(θ,ф)= ή D(θ,ф)for loss less antenna ή =1

G(θ,ф)=4π{Θ(θ,Ф) / Pr}• With respect to the antenna's dimensions,

G= ή{4πA / λ2}A is the aperture area of the antenna

is the wavelength of the operational frequencyis the antenna efficiency (usually between 50%

and 70%)

Cont…

• Basically there are only two types of antennas:• dipole antenna (Hertzian)• vertical antenna (Marconi)

• All antennas can be broken down to one of these types (although some say that there is only one - the dipole)

• In addition to this we have a theoretical perfect antenna (non-existent) that radiates equally in all directions with 100% efficiency. This antenna is called an isotropic radiator.

Cont…(Basic Antenna types)

The gain can also be presented as a 3D gain. The radius of the spheroid is proportional to the antenna gain.

Gain presented as 3D gain

Gain in theory

• Since all real antennas will radiate more in some directions than in others, you can say that gain is the amount of power you can reach in one direction at the expense of the power lost in the others. When talking about gain it is always the main lobe that is discussed

• Gain may be expressed as dBi or dBd. The first is gain compared to the isotropic radiator and the second gain is compared to a half-wave dipole in free space (0 dBd=2.15 dBi)

Power Density• The power density P(θ,ф) is related to

radiation intensity as follows;

P(θ,ф)= {Θ(θ,Ф) / r2}or

P(θ,ф)= {G(θ,Ф) Po/ 4πr2}

• The factor Po/ 4πr2 represent the power density that results if the power accepted by the antenna were radiated by loss-less isotropic antenna

• The maximum power flux density at some distance “r” from a transmitting antenna of gain “G” is;

• An isotropic radiator with input power equal to GPS would produce the same flux density. Hence,

Equivalent Isotopic Radiated Power (EIRP)

Antenna Effective Area

• Measure of the effective absorption area presented by an antenna to an incident plane wave.

• Depends on the antenna gain and wavelength

• Aperture efficiency: a = Ae / AA: physical area of antenna’s aperture, (m2)

][m ),(4

22

GAe

Transmission losses

• Free Space Transmission [FSL]– More to follow

• Feeder Losses [RFL]– Between the receive antenna and the

receive proper

• Antenna Misalignment Losses [AML]• Fixed Atmospheric & Ionospheric Losses

– Absorption losses– Depolarization losses

Power transfer between two antennas

• For two antennas in free space separated by large distance R

• The received power is equal to a product of power density of the incident wave and the effective aperture area of the receiving antennas

Pr = PAe

or

Pr = {(GtPtGrλ2) / (16π2R2)}

Antenna Bandwidth• The bandwidth of an antenna is defined as ”The range of

frequencies within which the performance of the antenna, with respect to some characteristic, conforms to a specified standard”.

• The reason for this qualitative definition is that all the antenna parameters are changed with frequency and the importance of the different parameters as gain, return loss, beamwidth, side-lobe level etc. much depends on the application.

• For example, the bandwidth of an antenna for gain (-1dB from the maximum) is defined as

• where fU is the upper frequency, fL is the lower frequency, and fC is the center frequency. Another example is the bandwidth related to

the mismatch loss defined by the SWR.

Reciprocity

• ALL the major properties of a linear passive antenna are identical whether it is used in transmit or receive mode. There is only one exception to this rule called "reciprocity", and that is when the antenna contains magnetically biased magnetic materials such as ferrites with resonantly rotating electron spin systems.

• The physical reason for reciprocity is that the only difference between outgoing and incoming waves lies in the arrow of time. Since the electromagnetic equations are invariant except for the signs of magnetic fields and currents, under time reversal, there can be no difference between transmit and receive mode in the physical current and field distributions. However, if we have a magnet providing a steady bias field, under time reversed conditions we would have to reverse the direction of this bias field. But for incoming and outgoing waves, the bias field direction remains the same. Thus it is possible for the system to be non-reciprocal.

Cont…

• Of course, antennas containing amplifiers, or diodes, or spark gaps, may well not be reciprocal for obvious reasons. Also, practical antenna installations having metal-oxide-metal contacts, "rusty bolts", dry soldered joints and other electrical contact imperfections are also likely to behave differently under transmit and receive modes of operation

Radiation Parameters

• Radiation Pattern measurement– Graphical representation of the field magnitude at

a fixed distance from an antenna as a function of direction i.e. angular variation of the test antennas radiation.

• Gain measurement– Absolute measurement that gives the angular

variation of the test antenna’s radiation. Needed to fully characterize the radiation properties of the test antenna.

Radiation Parameters

• Polarization– Defined as the polarization of the electromagnetic

wave radiated by the antenna along a vector originating the antenna along the primary direction of propagation. The direction of the oscillating electrical field vector i.e. orientation of the E-filed.

– Four basic types of polarization

Vertical-, horizontal-linear polarization and Left-hand elliptical, Right-hand elliptical polarization.

Radiation Parameters

Radiation Pattern

• Radiation pattern characteristics/parameters:– Half-power beam width– Main lobe– Side lobes– Antenna directivity– Gain function– Boresight (Direction

of maximum gain)– Polarization– Distortion– XPD(cross polarization

Discrimination)

Radiation Pattern

• Antenna radiation pattern is three-dimensional, but is needed to describe them as two-dimensional paper. The most popular technique is to record signal level along great circle or conical cuts through the radiation pattern. In other words, one angular coordinate is held fixed, while the other is varies.

• Radiation Pattern = Radiation Intensity as function of the azimuth/ elevation angles

or In different words when power radiation intensity and power density are presented as relative scale, they are referred to as antenna radiation pattern.

• A family of such two-dimensional patterns then can be used to describe the complete three dimensional patterns

• The main lobe of the radiation pattern is in the direction of maximum gain

• There are many types of antenna radiation patterns, most common are;

• Omnidirectional (azimuthal plane) beam

• Pencil beam• Fan beam• Shaped beam

Types of Radiation Pattern

Omnidirectional Antenna and Coverage Patterns

                                                                                                            

The Omnidirectional beam is most popular in communication and broadcast applications. The azimuthal pattern is circular, but the elevation pattern will have some directivity to increase the gain in the horizontal directions

Pencil Beam

Pencil beam is applied to a highly directive antenna pattern consisting of a major lobe contained with in it cone of small solid angle. Usually the beam is circularly symmetric about the direction of peak intensity

Fan Beam

A fan beam is narrows in one direction and wide in the other. A typical use of a fan beam would be in search or surveillance radar

Shaped Beam

Shaped beams are also used in search and surveillance

Cont…• Radiation patterns generally defined as the far field power or field

strength produced by the antenna as a function of the direction (Azimuth and elevation) measured from the antenna position. The behavior of the fields is changed with the distance from the antenna, and generally three regions are defined:

• Reactive near-field region - The region in the space immediately surrounding the antenna in which the reactive field dominated the radiating field (d <λ/(2π)).

• Radiating near-field region - Beyond the former region and for which d <2D2/ λ where r is the distance from the antenna, D is the largest dimension of the antenna and λ is the wavelength. This region is called also Fresnel region. In this region the radiating field begins to dominate.

• Far-field region - Beyond this region, the reactive field become negligible and also the radial part of the fields. This region is called also Fraunhofer region.

– Generally measurements are taken in the far field region. In case of large planar antennas it is more convenient to make near field measurements and to calculate the far field.

Antenna Radiation Pattern Lobes and Nulls• A radiation lobe can be defined as a portion of

radiation pattern bounded by regions of relatively weak radiation intensity. The main lobe is a high radiating energy region. Other lobes are called sidelobes, and the lobe radiating in the counter direction to the desired radiation direction is called back lobe. Regions for which the radiation is very weak are called nulls.

Antenna Beamwidth.

• Antenna beamwidth is defined as the angle θ between half power points on the main beam. In case that we have a power pattern in [dB] units, it means that we measure the angle between two 3dB points.

Measuring E and H field of antenna

E field cut of dipole antenna

Half-power beam width

• It is the angular beam width at 3 dB. It can be approximated as,

• D is the antenna's diameter.is the operational wavelength.

Half-power beam width

Short Dipole in Free Space FF

0

1

0 90 180 270 360

Degrees

Re

lati

ve G

ain

H

V

Horizontal plane: GVi /GVimax = 1

Vertical plane: GHi /GHimax = |sin |

-11

Elements of Radiation Pattern

0-180 180

Emax

Emax /2

Beamwidth

Sidelobes

Nulls

Main lobe• Gain

• Beam width

• Nulls (positions)

• Side-lobe levels (envelope)

• Front-to-back ratio

Antenna Mask (Example 1)

• Typical relative directivity- mask of receiving antenna (Yagi ant., TV dcm waves)

[CCIR doc. 11/645, 17-Oct 1989)

-20

-15

-10

-5

0-1

80

-120 -6

0 0

60

120

180

Azimith angle, degrees

Isot

ropi

c ga

in, d

B

Antenna Mask (Example 2)

-50

-40

-30

-20

-10

0

0.1 1 10 100

Phi/Phi0

Rel

ativ

e ga

in (d

B)

RR/1998 APS30 Fig.9

COPOLAR

CROSSPOLAR

Reference pattern for co-polar and cross-polar components for satellite transmitting antennas in Regions 1 and 3 (Broadcasting ~12 GHz)

0dB

-3dB

Phi0/2

Phi

• Different satellite missions have different allotted frequency slots by ITU, each slot behaves differently between ground and earth segment in terms of dispersion, attenuation and noise accumulation

• Generally at frequencies below 1GHz, TTT&C are running, the antenna may then be arrays of dipoles, helices and yagi-uda arrays, such type of antenna systems have wider beamwidth and medium gain. Deploying them in an array pattern results in increased gain and fanned and shaped beams thus enabling them for comparatively easy tracking

• At frequencies above 1GHz the electromagnetic waves become highly directional but more susceptible to attenuation, fading and dispersion, therefore, horn and parabolic antennas are most commonly used. The most popular and widely used are the aperture antennas given bellow;

Types of Ground Antennas Used in Satellite Missions

• Axially Symmetric Fed Antenna– This is the most common type of antennas found on roof tops or

back yards of homes. They come in different configurations. Axis symmetric point focus feed. Front feed and Vortex feed

• Cassegrain Feed Antenna– The second common configuration used particularly in large

antennas is the Cassegrain antenna. Here the feed is located at the vertex of the parabolid and illuminates a hyperbolic shaped sub-reflector located at the focal area. The benefit here is that the electronics is located at a more accessible part of the antenna but

with some sacrifice in sidelobe level because of the blockage.

Types of Ground Antennas Used in Satellite Missions

• Gregorian Feed Antenna– In Gregorian configuration the feed is at the focal point

of an ellipse and the elliptical sub-reflector at its other focus. With this configuration there is an improvement in the far-outside lobe level

• Offset Aperture Antennas– These configurations indicate that the feed are on axis .

The same generic types may also be used with offset feeds. The removal of feed from a collimated beam improves the side lobe level and has better effect of reducing mutual interference from adjacent satellites.

Types of Ground Antennas Used in Satellite Missions

Reflector antennas

Crossed Yagi antennas for circular polarisation and

right-handed and left-handed helical antennas

Cassegrain Feed Antenna

Comparison between the measured antenna gain pattern and the predicted

one for small offaxis angles

Front Fed Antenna

A Front-Fed Offset Reflector Antenna with Multiple-Feed Horns (Courtesy Alenia Spazio)

Gregorian Feed Antenna

Offset Parabolic Reflector

Offset Parabolic Antennae

Satellite Antennas

• The physical dimensions of the spacecraft and the availability of limited power restrict use of large antennas.

• Medium gain antennas are used instead which include modified parabolic antennas for large area coverage

• In LEO missions, the satellite may be two axis stabilized, the rotation being on the axis with largest inertia, the antenna gain pattern may not remain uniform when received at the ground station. Therefore, a rotating antenna whose rotation is in the opposite direction of the satellite rotation is used, such type of antenna is called “Despun antenna”

• Circular polarization may employed for TT&C purposes or image transmission like weather satellite

• Helical antennas are used for circularly polarized EM wave pattern, these antennas has larger beamwidth, therefore, tracking by the ground station becomes easier

• In GEO satellites, DVB and VSAT applications are dominant

• In broadcast services satellite has to cover larger area , linearly polarized array antennas are used. For broadcast services the transmitting antennas may consist of array of Horn Antennas, Helical Antennas or Disk-on-Rod Antennas. Power beam form the antennas can be steered to cover specific area on the earth’s surface by switching on or off different antennas from the array on the satellite.

Satellite Antennas

18 dBi X-band pyramidal horn antenna

Helical Antenna

9. Link Budget

Contents• Introduction• General Architecture• Signal Power Calculation• EIRP• Noise Calculation• Thermal Noise• Effective Temperature• Noise Temperature• G/T• Link Analysis• Eb/No

• Carrier Parameters• BER• Rain Attenuation and Margin

Link Budget

IntroductionOverall design of a complete satellite communications system involves many complex trade-offs to obtain a cost-effective solutions

Factors which dominate are

–Downlink EIRP, G/T and SFD of Satellite

–Earth Station Antenna

–Frequency

–Interference

General Architecture

HPA / TransceiverLNA / LNB

G/T & SFD

EIRP down

UplinkDownlink

Uplink Path Loss

Rain Attenuation

Downlink Path Loss

Rain Attenuation

EIRP Up G/T ESGt

Pt

Transmit Earth Station– Antenna Gain– Power of Amplifier

Uplink– Path Loss– Rain Attenuation

Satellite

– G/T– EIRP (Equivalent Isotropic Radiated Power)

– SFD (Saturated Flux Density)

– Amplifier Characteristic

Downlink– Path Loss– Rain Attenuation

Receiving Earth Station– Antenna Gain– LNA /LNB Noise Temperature– Other Equipment

Signal Power Calculation

Antenna GainAntenna Gain

G = ( * d / ) 2 [dBi]

Where, = C / f , C = Speed of lightf = frequency of interest = efficiency of antenna (%), d = diameter

of antenna (m)

Signal Power Calculation

Antenna Beam widthAntenna Beam width

3dB = 70 * C / df [degrees]

Where,

C= 3x108 m/s (Velocity of Light)

EIRP

Is the effective radiated power from the transmitting side and is the product of the antenna gain and the transmitting power, expressed as

EIRP = Gt + Pt –Lf [dB]Where,

Lf is the Feed Losses

Signal Power (Pr)

Pr = EIRP – Path Loss + Gr (sat)

[dB]

Where,

Path Loss = (4D / ) 2

D is the Slant Range (m)

Noise Calculation

Thermal NoiseIs the noise of a system generated by the random

movement of electronics, expressed as

Noise Power = KTB

Where,

K= (-228.6 dBJ/K)

T= Equivalent Noise Temperature (K)

B= Noise Bandwidth of a receiver

Effective Temperature

Te = T1 + (T2/G1)

Where,

T1= Temperature of LNA

T2= Temperature of D/C

G1= Gain of LNA

Noise Temperature

Ts = Tant / Lf+(1-1/Lf)Tf

Where ,

Tant = Temperature of antenna

Lf = Feed Losses

Tf = Feed Temperature

Effective Temperature

Tsys = Ts + Te

• Being a first stage in the receiving chain, LNA is the major factor for the System Temperature Calculation

• Lower the noise figure of LNA lower the system temperature

• Antenna temperature depends on the elevation angle from the earth station to satellite

G/T (Gain to System Noise Temperature)

– This is the Figure of merit of any receiving system

– It is the ratio of gain of the system and system noise temperature

G/T = G-10log (Tsys)[dB/K]

Link AnalysisC/N Uplink

(C/N)u = (EIRP)e-(Path Loss)u+(G/T)sat-K-Noise BW [dB]

C/N Downlink

(C/N)d = (EIRP)sat-(Path Loss)d+(G/T)e-K-Noise BW [dB]

C/N Total

(C/N)T-1 = (C/N)u

-1 + (C/N)d-1 + [C/I)IM

-1 + [C/I]adj-1 + [C/I]xp

-1 [dB]

Eb/No (Energy per bit per Noise Power Density)

– Is the performance criterion for any desire BER

– It is the measure at the input to the receiver

– Is used as the basic measure of how strong the signal is

– Directly related to the amount of power transmitted from the uplink station

Eb/No = (C/N)T + Noise BW – Information Rate

Carrier Parameters • Solution - Carrier Performance:

– Eb/No Threshold– Bit Error Rate (BER)– Rain Attenuation

– Why is it used? - To represent the amount of errors occurring in a transmission

- To express the link quality– What is it? - BER is an equipment

characteristic- BER is directly related to Eb/No- BER improves as the Eb/No gets larger

P = 1/2 e -Eb/No (with P = Probability of error)

Bit Error Rate (BER)

• Data links: - BER threshold: 10-4

Carrier Parameters

• Performance:– Application specific

• Digital voice links:

– BER threshold 10-3

Eb/No for FEC 1/2 (dB)

Eb/No for FEC 3/4 (dB)

Eb/No for FEC 7/8 (dB)

BER

6.5 8.0 9.1 10-6 7.1 8.7 9.7 10-7 7.6 9.2 10.4 10-8 9.9 11.0 12.1 10-10

Carrier Parameters

• Performance:– Typical Eb/No values for different FEC

• Performance - Rain Attenuation:– Availability

• Rain Margins– Typically 99.60 % for Ku-Band – Typically 99.96 % for C-Band

• Performance - Additional Margins:– Adjacent Satellite Interference (ASI)– Interference Margins

E/S

TO SATELLITE

Rain AttenuationRain Attenuation

Summary; Transmission Parameter for Link Budgets

C = 10 log (c) in dBW c = 100.1 C in W

N = 10 log (n) in dBW n = 100.1 N in W

C-N = C - N in dB

EIRP = P + G - V in dBW

PL = FD + AD + RD in dB

G-T = G - T in dBi/K

N = T + K + B in dBW

C‑N = EIRP ‑ PL + G‑T - K ‑ B[dB] [dBW] [dB] [dBi/K] [dBWs/K] [dBHz]

EIRP = P + G - V in dBW, Equivalent Isotropic Radiated Power

G-T = G - T in dBi/K, Figure of Merit

PL = FD + AD + RD in dB, Pathlosses

N = T + K + B in dBW,Noise Power.

= No + B; No Noise Power Density dBW/Hz

C-N = C - N in dB, Signal to Noise Ratio

Eb-No =Energy per bit to noise power density, in dB

BER = Bit Error Rate, e.g.: 10-5

Cont...

10. Interference

Contents• Interference in Satellites• Interference Types• Sources of Interference• Causes of Interference• FM Interference• Cross Polarization Interference• Digital & CW Interference• Intermodulation Interference• Raised Noise Floor• Spikes & Unknown• Adjacent Satellite Interference• Adjacent Transponder Interference• Co-Channel Interference

Interference in Satellite

• Interference is mainly concern on;

– Interference Type– Sources of Interference– Causes of Interference

Interference

Interference Type:• Digital• Spike• Cross Polarization• TDMA • FM TV• Intermodulation• Unknown

InterferenceSource of Interference:

•Neighboring Customer

•Adjacent Satellite

•Self-Customer

•Opposite Polarization

•Others

External Factors: 40.22%

Internal Factors: 59.78%

Interference

Causes of Interference:

•Human Error: 29.89%

•Equipment Error: 21.74%

•Adjacent Satellite: 16.85%

•Customer Cooperation: 8.15%

•Others: 23.37%

Internal Factors: 59.78%

Types of Interference

• FM

• Cross Polarization

• Digital

• CW

• Intermodulation

• Raised Noise Floor

• TV/FM

• TDMA

• Spikes & Unknown

FM Interference

I

Base band Up converter HPAIF

70 MHz

RF

6 GHz

70 MHz 6 GHz

FM Radio Signal

FM signal:88 MHz to 108 MHz

FM InterferenceIIII

f (MHz) f (MHz)

70 88 90 108

+

IF

70 90

f (MHz)

6.0 6.09

RF

f (GHz)

FM Interference

III

Source:

• Terrestrial FM Radio Broadcast

• Introduced at the IF level of the Earth Station

FM Interference

IV

Cause:• Poor Connection between BB and RF

equipment, so FM broadcast is induced into the system and eventually transmitted to the satellite.

• Poor quality accessory between BB and RF• Poor grounding system

FM Interference

V

Prevention:• Select accessories with standard

specifications• Good Earth Station installation• Good grounding system• Coordinate with PCNS to perform UAT and

interference checking when a new station is installed

Cross Polarization Interfrence

Source:• If XPD level of an uplink antenna is less than

30 dB, antenna will transmit both vertical and horizontal polarizations

• Therefore, cross pole will occur at the other satellite or transponder with opposite pole and will interfere the existing carrier

Cross Polarization Interfrence

Cause:• Poor antenna pointing• Poor cross pole isolation• Sudden change in the antenna pointing due

to mistake or storm• Carrier uplink without performing proper UAT

with PCNS

Cross Polarization Interfrence

Prevention:• Do not uplink the carrier without

performing UAT with PCNS

• DO not uplink un-modulated carrier for UAT before PCNS’s directions

• Perform Regular Preventive maintenance

Digital & CW Interference

Source:

• Earth Station Equipment

Digital & CW Interference

Cause:• Transmission of wrong carrier frequency by

the user• Unauthorized access• Uplink CW for UAT before calling PCNS• Equipment malfunction

Digital & CW Interference

Prevention:• Verify U/L frequency before transponder

access• Do not uplink un-modulated carrier (CW)

before PCNS directions• Perform UAT• Request PCNS if customer wants to uplink a

new carrier for special purpose at some vacant slot

• Perform Preventive Maintenance periodically

Intermodulation Interference

Description:• If more than one carrier are transmitted by a

single HPA, mixing or Intermodulation (IM) processes take place

• This results in Intermodulation products which are displaced from the carriers at multiples of the difference frequencies

• The power level of the Intermodulation products are dependent on the relative power level of the carrier and the linearity of TWTA or SSPA

Intermodulation Interference

Description:

• The frequencies of the Intermodulation products are:

– 2f1-f2 f1: frequency of carrier #1

– 2f2-f1 f2: frequency of carrier #2

• It can occur at both E/S and Satellite

Intermodulation Interference

Cause:• U/L power level of the each carrier is set so high that

the Intermodulation occurs

• U/L power level is increased without considering the the possibility of intermodulation

• Increasing the U/L power without informing PCNS

Intermodulation Interference

How does it affects• It reduces the Eb/No of your carrier using at the

same frequency• May raise the Noise Floor of some slots• Existing uplink power at E/S would be used more

than normal• Therefore, you have to replace new RFT to get

more power when you would want to put new carriers into it

Intermodulation Interference

Prevention:• Verify the link budget of the station transmitting

more than one carrier before transponder access• Aggregate input back-off for HPA or RFT at E/S

must be defined and informed to up linker• Do not increase U/L power without informing

PCNS• Do not operate with overused power

Raised Noise Floor

Source:

• Earth Station Equipment

Raised Noise Floor

Cause:• E/S equipment configuration was not set up

properly• The gain of U/L equipment such as U/C or HPA

was not set suitably• The U/L power is too high

Raised Noise Floor

Prevention:• Use good E/S setup• Set suitable gain of E/S equipment• Do not increase the U/L power without informing

PCNS• Verify uplink noise level at the output of HPA

before transponder access

Spike and Unknown

Description:• Unpredictable Frequency, Bandwidth, Time• Some of them may occur at out of assigned

transponder

Spike and Unknown

Cause:• Most of them are caused by the U/L equipment

error (both base band and RF equipment)• It does not affect all carriers transmitted by itself

Spike and Unknown

Investigation:• Only RF equipment such as U/C, HPA, Transceiver

needs turning off

• Turning of Base band equipment such as Modem, Exciter, Modulator cannot prove the source of interference

Spike and Unknown

Prevention:• Perform Preventive Maintenance periodically

• Operate all U/L equipment under suitable conditions as directed by operational manual of the equipment

• Find out root cause if it disappeared with unknown reason or equipment reset in order to perform prevention

Txp 22/22

Txp 12/12

Wanted Carrier Unwanted Carrier

Sources of Interference

• Co-Channel Interference

Txp 12/12

Wanted Carrier Unwanted Carrier

… ...

Sources of Interference

• TWTA Intermodulation

E.I.R.P.Operation = E.I.R.P.Saturation - OBO

Transponder ParametersTransponder Parameters

• Intermodulation (IM)– What is it? - Potential source of noise– Why does it exist? - Different signals are sent

simultaneously – How is it avoidable? - By reducing the

saturation E.I.R.P.

RADIO LINKSATELLITE ANTENNA

SATELLITE SPACING

UNWANTED SIGNALSWANTED SIGNALS

Sources of Interference

• Adjacent Satellite Interference (ASI)

Satellite dish

Satellite dish

R C V R

W H

IM U X S M

1-2S S P A

S S P A 3-4

1 -2 3 -4 1 -2 1 -2

3-4 3-4

O M U X

Satellite dish

Satellite dish

W H

1-2

3-4

. . .

. . .

Sources of Interference

• Adjacent Transponder Interference (Multipath)

Sources of Interference

• Satellite:– Co-Channel Interference

– TWTA Intermodulation

– Adjacent Satellite Interference

– Adjacent Transponder Interference - “Multipath”

• Path Losses:– Up link thermal Noise

– Down link thermal Noise

• Earth Station:– HPA Intermodulation

• Outside:– Sun Interference

– Terrestrial Interference

11. Channel Characterization

Contents

• The sequence of signal processing and transmission• Multiplexing & Multiple Access• FDMA• TDMA• CDMA• Comparison in TDMA, FDMA & CDMA• Channel Coding & Modulation• Channel Reservation• Channel Coding• Modulation Techniques• The Baseband Eye Pattern

Transmission

Source Coding

Encryption

Multiplexing

Channel Coding

Interleaver

Modulation

Frequency Conversion

De-Interleaving

Channel Decoding

Demultiplexing

Decryption

Display

Frequency Conversion

Demodulation

Digitization

Source Decoding

The Sequence of Signal Processing and Transmission

Signal processing and transmission

Digitisation higher reliability, low cost, less susceptible to noise,

Source Coding to reduce bit rate for transmission

Encryption for communications privacy

Multiplexing for efficient transmission of multiple channels

ChannelCoding for error free transmission

Interleaving for robust error correction

Modulation imparting baseband information to a carrier

Frequency Conversion to operate at radio frequencies

Multiplexing and Multiple Access

• For the majority of data communications that take place, there is a requirement for several users to share a common channel resource at the same time.

• For multiple users to be able to share a common resource in a managed and effective way requires some form of access protocol that defines when or how the sharing is to take place and the means by which messages from individual users are to be identified upon receipt. These sharing process come to be known as multiplexing and multiple access in digital communications.

Multiple Access and Multiplexing

• Multiple Access:is the ability for several earth stations to transmit their respective carriers simultaneously into the same satellite transponder

• Multiplexing:is the reversible operation of

combining several information-bearing signals

to form a single, more complex signal.

Multiple Access and Multiplexing

Multiple Access Multiplexingat radio frequency at baseband

TDMA - TDM

FDMA - FDM

CDMA - CDM

FDMA

• Used extensively in the early telephone and wireless multi-user communication systems

• If a channel, such as a cable, has a transmission bandwidth W Hz, and individual users require B Hz to achieve their required information rate, then the channel in theory should be able to support W/B users

• Near-Far problem

Frequency Division Multiple Access; FDMA

Guard Band

Uplink Downlink

f1 f3..... fMf2

. . .

f2f1 f3 fM Frequency

TDMA

• The basic principle behind time division multiplexing is that the user has access to a modem operating at a rate several times that required to support his own data throughput, such that he can send his information in a time slot that is shorter than his own message transaction. Other users can then be assigned similar time slots on the same channel. Clearly if the data rate on the channel is w bits/second, and each individual user requires only b bits/second, then the system can support w/b simultaneous users.

• In TDM systems, users are assigned a time slot for the duration of their call whether they require it or not.

TDMA

Near – Far Effect in TDMA

TDMA

Example of a TDMA system

• The GSM digital cellular system is a very good example of a TDMA

Time Division Multiple Access; TDMA

Guard Time

Uplink

Downlink

t1 t3..... tMt2

. . .

T2t1 t3 tM Time

. . .

burst1 burst2 burst3 burstnto Joe to Bill to Tim to who?

Time Division Multiplexing

a coherent stream of data

Time Division Multiple Access; TDMA

CDMA

• In recent years, the interference immunity of CDMA for multi-user communications, together with its very good spectral efficiency characteristics, has been seen to offer distinct advantages for public cellular-type communications.

• There are two very distinct types of CDMA system,classified as direct sequence CDMA and frequency hopping CDMA. Both of these systems involve transmission bandwidths that are many times that required by an individual user, with the energy of each user's signal spread with time throughout this wide channel. Consequently these techniques are often referred to as spread spectrum systems.

Code Division Multiple Access; CDMA

Uplink

Downlink

c1 c3..... cMc2

CODE 1

CODE 2

CODE 3...CODE M

CODE

Sprectrum Spreading with PN Sequence

narrrowband signal

fff

PN-Sequence PN-Sequence

widebandsignal

narrrowband signal

PN-Syn-chronization

Sprectrum Spreading with PN Sequence

FDMA, TDMA, CDMA in bandwidth, power and time

time

power

bandwidth

time

power

bandwidth

ttime

power

bandwidth

FDMA

TDMA

CDMA

Througput in TDMA, FDMA and CDMA

Througput in %

1 10 100 Number of users

100

50

0

TDMA

FDMA

CDMA

Channel Coding & Modulation

Channel Reservation

No ReservationsSome Reserve´s Slotted ALOHAPure ALOHA

Rigorous PollingSelective Polling Co-Channel Request ChannelSpread Spectrum

Polling Request

No Reservation

Pre-Assignment

Some Reserve´s

DAMA*

Access Control

Channel Reservation

Pre-Assigment Resource is leased permanentlyDemand Assigment Resource is allocated on demandReservation Reservations are possible

Polling Polling of subscibers if they . . .Rigorous Polling Polling of all registered subscribersSelective Polling Polling according to statistics

Request Subsciber requests resource from systemRequest Channel Request on seperate request channelCo-Channel Request Request on communications channelSpread Spectrum Request per spread spectrum carrier

Pure Aloha Request at random in timeSlotted Aloha Request at random but in time slots

Channel Coding

• Every communication system operates in a noisy environment

• To develop a robust system one may choose:

– Transmit signals with higher power

– Repeat every signal

– Repeat only erroneous signals

– Apply forward error correction

Modulation Techniques

How can signals be employed to transmit information?

Sine Waves: Information is in amplitude, phase, or frequency

Pulses: Information is in amplitude, phase, position or pulse width

Analog signals modulation

schemes• Amplitude Modulation (AM)• Phase Modulation (PM)• Frequency Modulation (FM)

Amplitude modulation - AM

Cont…

• AM is not used for satellite communications because it is susceptible to signal fluctuation. (Long propagation distance)

• Difficult to achieve an acceptable C/N ratio.

• Two variants of AM are the Double side band suppressed carrier (DSBSC) and the SSB

Double-side-band suppressed carrier

(DSB-SC)

Single side band (SSB)

• The information is contained in either of the two side bands

• SSB utilizes the lower side band• A single side band is transmitted

DSB-SC, SSB and CSSB

• DSB-SC and SSB are not used in satellite communications– The carrier frequency is very low

compared to the frequencies used in satellites (More susceptible to noise)

• CSSB (Companded single side band)– A Technique in which the speech signal

levels are compressed before transmission, and, at the receiver are expanded again back to their original levels.

– It reduces idle noise

FDM telephony• Frequency-division multiplexing (FDM) is a technique

to combine several information channels into a single one.

• It utilizes the band limitations of the information signals.

• A basic voice channel occupies frequencies between 0.3 and 3.4 kHz, i.e. The bandwidth is 3.1 kHz.

• The different voice channels are modulated on frequencies separated by 4 kHz allowing some guard bands. Amplitude modulation is used.

• Practical implementation data (terrestrial networks)– Group – 12 VF-ch; fr. band: 60-108 kHz– Super-group – 60 VF-ch.; fr. band: 312-552 kHz– Basic master-group – 5x super-groups , 812-2044 kHz– Super master-group – 3x master-groups , 8516-12388 kHz

Frequency Modulation (FM)

Cont…

• The frequency modulated sinus signal is:

• The instantaneous frequency is:

• The maximum variation of the frequency is:

β is called modulation index.

Cont…

f is known as maximum frequency deviation.• Usually the frequency spectrum of the

frequency modulated signal is not band-limited. In practice the bandwidth is estimated by Carson's rule:

• In the general case f is the peak carrier deviation and fm is the highest frequency component in the baseband signal. The quantity

is called deviation ratio.

Pre-emphasis & De-emphasis

• Used to eliminate the effect of noise at the receiver’s output.– The transmitter amplifies (Pre-

emphasis) the modulating signal linearly as frequency increases.

– The receiver down convert the received signal in the reverse order (De-emphasis).

Cont…

Noise weighting

• By changing the shape of the noise spectrum we can gain signal-to noise ratio in certain frequency bands.

• Example of an weighting curve: CCIR (International Radio Consultative Committee) curve known as psophometric weighting curve. This curve is often used in telephone networks.

S/N & bandwidth for FDM/FMtelephony

• For the particular case of FDM/FM telephony we have receiver processing gain according to:

• In order to calculate BIF we need F:

• g is peak/rms factor depending on the number of channels. Usually between 10 and 13 dB. L is called loading factor.

Modulation Techniques

Eb-No =

9.6 dB 9.6 dB 12.9 dB 17.7 dB 13.5 dB dto

BPSK QPSK 8PSK 16PSK 16QAM dto

11

111

d

d

d

d

d

0 1 00 000 0000 0000

dI

Q

Eb/No vs BER

BER BPSK QPSK 8PSK 16PSK 16QAM 32QAM 64QAM

10-1 -1.0 -1.0 2.5 7.3 2.2 5.1 10-2 4.3 4.3 7.7 12.4 7.3 8.9 10-3 6.8 6.8 10.1 14.9 9.8 10.9 10-4 8.4 8.4 11.8 16.5 11.4 12.3 10-5 9.6 9.6 12.9 17.7 12.6 13.4 10-6 10.5 10.5 13.9 18.7 14.5 17.2 19.0 10-7 11.3 11.3 14.7 19.4 14.3 14.8 10-8 12.0 12.0 15.3 20.1 15.0 15.5 10-9 12.6 12.6 15.9 20.7 15.6 16.0 10-10 13.1 13.1 16.4 21.2 16.1 16.5 10-11 13.5 13.5 16.9 21.6 16.5 16.9 10-12 13.9 13.9 17.3 22.1 16.9 17.3 10-13 14.3 14.3 17.7 22.4 17.3 17.7 10-14 14.7 14.7 18.0 22.8 17.7 18.0 10-15 15.0 15.0 18.3 23.1 18.0 18.3

QPSK, MSK und GMSK Spectra

QPSKMSK (BT → ∞)

GMSK (BT = 0.3; employed in European GSM, e.g.)

GMSK (BT = 0.2)

0

-20 dB

-40 dB

-60 dB

-80 dB

-100 dB

fc-3/T fc-3/(2T) fc fc+3/(2T) fc+3/T

→ B is the 3 dB bandwidth of the baseband filter

→ T ist the bit period

* For a given T, a wide open baseband filter B leads to BT → ∞

↪ MSK has a wider main lobe but then is lower than QPSK

↪ GMSK is consistently lower than any of the other mod tech's

The Baseband Eye Pattern

die Zeit → time

Zustand "1" state "1"

Zustand "0" state “0"

several periods of the running signal superimposed on the oscilloscope the eye size is a measure of the quality of the signal

Augen-öffnung

eye size

→

→

Symboldauer period

Classical & Modern Applications

Classical Satellite Communication

• Classical in this context mean those satellite

systems and applications, already established

since a certain period of time.

• On the operator side these are the major

global and regional players such as:

INTELSAT

EUTELSAT

PANAMSAT

EUTELSAT-System

Classical Satellite Communication

• The typical or classical applications offered by operators can be summarized as

follows:

• Television Video Distribution

Video Backhaul

Satellite News Gathering

TV Broadcast

• Telephony Trunk Telephony

Thin Route Telephony/Rural

Telephony

• Data Corporate Networks

Internet Access

VSAT`s

Video Distribution Transmits video feeds around the world on a point-

to point basis Primary customers are the major television

channels Usually links between countries and continents

using major earth station facilities

Video Backhaul The term backhaul is used to desribe the transmission of

video feeds in raw format:

typical users are again the television channels and

some international organisations.

Satellite News Gathering

Satellite News Gathering (SNG) requires high mobility for the involved earth

stations. These could be either: Truck or Van mounted or Man portable Designed to be quickly deployed

TV Broadcast• The“Bread and Butter“ of all major satellite systems.

For the TV Broadcast major earth station requirements exist to

make the most effective use of the limited power resources of

the satellite

Can be a single uplink station at the studio or Teleports

Telephony

Trunk TelephonyHigh density routes will use full transponder TDMA

Telephony traffic will be digitised and multiplexed with

other traffic (data)

Satellites were previously used for restoration of cable

services when failures occur, but todays fibre

capacities are too great to be restored even by the

capacity of a full satellite

Telephony

Thin Route Telephony Direct satellite access from a remote telephone locaation or

to a village or small town with no direct access

Traffic is routed via the satellite to a gateway location where

traffic is switched to the PSTN

Local network may be extended with Wireless Local Loop

DATA

Corporate Data Networks

Primarily operated using VSAT technology

Some high bandwidth point to point links

IBS used as a basic regulatory model in the past

and today in more restricted markets

Rationalisation is often the driving force for

rootop-to-rooftop connections

Data

Internet Access

• Two major applications:

1. Direct access to the Internet via satellite (Direct-PC; Astra-net)

(Major problem area could be the number of users per transponder)

2. Access to Internet backbone by remote Internet Service Providers

Data

VSATThe nature of VSAT`s has changed significantly:

Originally VSAT were based on a star topology

Moved to support distributed, client-server environments

Evolved from legacy protocols to support all Ethernet and IP

LAN introduced in 1992 for most TDMA systems

High percentage of VSAT demand is now IP related

Intranet applications are moving demand back to a centralised

architecture

Demand for Extranet is increasing

Modern Satellite Communications

Modern and or future satellite communication can be characterised by three development trends:

Development of complex and powerful earth stations (VSAT`s)

Development of low cost terminals for mass production

Development of complex satellites with on board processing capabilies

The End