Lecture 1

Introduction and Background

Feb 12, 2011

ELT3029Satellite Communications

HistoryMotivation to use the SkySpectrum AllocationSatellite Systems ApplicationsSystem ElementsSystem Design ConsiderationsCurrent Developments and Future Trends

Pratt, Bostian & Allnutt, Satellite Communications, Chapter 1Other references:

B. Elbert, Introduction to Satellite Communications, Artech-House, 1999.M. Richaria, Satellite Communication Systems, McGraw-Hilll, 1999.

Before the 1950’s – Putting the concepts together: 1000AD: Chinese invent rocket.1600 Tycho Brache’s experimental observations on planetary motion.1609-1619 Kepler’s laws on planetary motion1903 Russian teacher K. Tsiolkovsky publishes his ideas on space flight.1926 First liquid propellant rocket lauched by R.H. Goddard in the US.1927 First transatlantic radio link communication (HF).1942 First successful launch of a V-2 rocket in Germany.1945 Arthur Clarke publishes his ideas on geostationary satellites for worldwide communications (GEO concept).

1950’s – Putting the pieces together:1956 - Trans-Atlantic cable opened (about 12 telephone channels – operator).1957 First man-made satellite launched by former USSR (Sputnik, LEO).1958 First US satellite launched (SCORE). First voice communication established via satellite (LEO, lasted 35 days in orbit).

1960’s – First satellite communications:1960 First passive communication satellite (Large balloons, Echo I and II).1962: First active communication satellite (Telstar I , MEO).1963: First satellite into geostationary (GEO) orbit (Syncom 1, comms. failed).1964: International Telecomm. Satellite Organization (INTELSAT) created.1965 First successful communications GEO (Early Bird / INTELSAT 1).

1970’s – GEO Applications Development, DBS:1972 First domestic satellite system operational (Canada).1975 First successful direct broadcast experiment (USA-India).1977 A plan for direct broadcast sattellites (DBS) assigned by the ITU1979 International Mobile Satellite Organization (Inmarsat) established.

1980’s – GEO Applications Expanded, Mobile:1981 First reusable launch vehicle flight.1982 International maritime communications made operational.1984 First direct-to-home broadcast system operational (Japan).1987 Successful trials of land-mobile communications (Inmarsat).1989-90 Global mobile communication service extended to land mobile and aeronautical use (Inmarsat)

Trans -Atlantic Link750 kmhigh tower

In the equatorial planeOrbital period = 23 h 56 min. 4.091 s = one sidereal daySatellite appears to be stationary to an observer over a point on the equatorEarth rotates at same speed as satelliteRadius of orbit, r, = 42,164.57 km

Three satellites can cover the earth (120º apart)

NOTE: Radius = orbital height + radius of the earthAverage radius of earth = 6,378.14 km

Orbit should avoid Van Allen radiation belts

Region of charged particles that can cause damage to satelliteOccur at ~2000-4000 km and ~13000-25000 km

Van Allen radiation belt

Circular or inclined orbit with < 1400 km altitudeSatellite travels across sky from horizon to horizon in 5 - 15 minutes => needs handoffEarth stations must track satellite or have omnidirectional antennasLarge constellation of satellites is needed for continuous communication (66 satellites needed to cover earth)Requires complex architecture

Polar (LEO);Earth rotates about 23o each orbit;Useful for surveillance; large number of eclipses

Sun synchronous (LEO); Retrograde orbit; In plane of sun-earth axis on sun-side of orbit; few or no eclipses

Molniya (HEO) (USSR-1965); T ≈ 11h 38 min.;39,152 × 500 km;Orbit track repeats every other orbit

Satellite Orbital

OrbitalHeight (km)

Orbital Velocity (km/s)

System Periodh min s

INTELSAT 35,786.43 3.0747 23 56 4.091

ICO-Global 10,255 4.8954 5 55 48.4

Skybridge 1,469 7.1272 1 55 17.8

Iridium 780 7.4624 1 40 27.0

21 r

GMmF

r

mvF

2

2

Frequency: Rate at which an electromagnetic wave reverts its polarity (oscillates) in cycles per second or Hertz (Hz).Wavelength: distance between wavefronts in space. Given in meters as: λ= c/f Where: c = speed of light (3x108 m/s in vacuum)

f = frequency in Hertz

Frequency band: range of frequencies.Bandwidth: Size or “width” (in Hertz) of a frequency band.Electromagnetic Spectrum: full extent of all frequencies from zero to infinity.

RF Frequencies: Part of the electromagnetic spectrum ranging between 300 MHz and 300 GHz.

Efficient generation of signal powerRadiates into free spaceEfficient reception at a different point.

Differences depending on the RF frequency used:Propagation effects (diffraction, noise, fading)Antenna Sizes

Sub-range of the RF frequencies approximately from 1GHz to 30GHz. Main properties:

Line of sight propagation (space and atmosphere).Blockage by dense media (hills, buildings, rain)Wide bandwidths compared to lower frequency bands.Compact antennas, directionality possible.Reduced efficiency of power amplification as frequency grows:

Radio Frequency Power OUT

Direct Current Power IN

International Telecommunication Union (ITU):Members from practically all countries in the worldAllocates frequency bands for different purposes and distributes them around the planetCreates rules to limit RF Interference (RFI) between countries that reuse same RF bandsMediates disputes and deals with harmful RFI when it occursMeets biannually at the World Radiocommunication Conference (WRC) to discuss rules and allocations

Intelsat/Inmarsat: International groups that operate globalfixed/mobile networks

LEO satellites need lower RF frequencies Low distances between satellite and ground means lower antenna gains required => lower frequencies

GEO satellites need higher RF frequenciesLow frequencies occupied by terrestrial systemsApplications require high data rates => high bandwidths (many MHz) => high frequencies (GHz)

Note: High data rates mean high bandwidths are required but bandwidth is often hard to get (there’s always tradeoffs in satellite system design!)

Initial application – telephonyBroadcasting - mainly TV at present

DirectTV, PrimeStar, etc.Point to multi-point communications

Video distribution for Cable TVMobile servicesWeather observation

CommunicationsMilitary surveillanceWeatherAtmospheric studiesEarth observation

Polar icecap monitoringTracking plantation changes for harvests

GPS is a MEO satellite systemGPS satellites broadcast pulse trains with very accurate time signalsA receiver able to “see” four GPS satellites can calculate its position within 30 m anywhere in world24 satellites in clusters of four, 12 hour orbital period

LEO MEO GEO

Advantages Smaller handsets•Less required power•Low delay times•Frequency reuse•Suitable for Positioning

•Less handoff than with LEO•Less propagation delay than withGEO

Can cover almostentire world w/3satellites•Can continuouslymonitor one pointon earth’s surface•Good forbroadcasting

Disadvantages •Requires large numberof satellites•Complex handoff•Multiple satellite hops– large delays•Atmospheric drag

More satellitesrequired than withGEOs•Greater delaysand propagationlosses than withLEOs

Cannot cover highlatitudes or lowelevations

Telephone links via GEO satellites:A long way to travel (80,000 km each way)Round trip delay is 500 ms - very noticeable

Optical fibers, when available, areMuch cheaper to operate than satellitesHave 30-year lifetime - do not have to be relaunchedHave huge capacity - 2.7 Gbit/s per fiber is commonGuarantee lower Bit Error Rate (BER)

Satellite systems can be deployed in 4-5 years, whereas terrestrial systems take longer

One satellite can cover the same region that it would take multiple base stations of a terrestrial system to cover

Better for covering sparsely populated areas

Of course, satellite systems cost a lotCareful studies must be done to assure success – IRIDIUM

A satellite failure can have catastrophic results, but:Launch reliability – 99 %Most satellites proven to be reliable beyond predicted lifetime

Satellite Launching PhaseTransfer Orbit PhaseDeploymentOperation

TT&C - Tracking Telemetry and Command StationSSC - Satellite Control Center, a.k.a.:

OCC - Operations Control CenterSCF - Satellite Control Facility

Retirement Phase

Signals:Carried by wires as voltage or currentTransmitted through space as electromagnetic waves.Analog:

Voltage or Current proportional to signal. E.g. Telephone.

Digital: Generated by computers.Ex. Binary = 1 or 0 corresponding to +1V or –1V.

Sine wavesCarry no informationSine wave frequency is the carrier (center) frequency of the data

Data (information) is impressed onto the sine wave (carrier) by modulation

Results in signal (carrier plus data) occupying finite frequency band (bandwidth)

Modulation: Vary a parameter of the sine wave based on the information content

Amplitude Shift Keying (ASK)Frequency Shift Keying (FSK)

Uplink and Downlink:FDD: Frequency Division Duplexing.

f1 = Uplinkf2 = Downlink

TDD: Time Division Duplexing.t1=Up, t2=Down, t3=Up, t4=Down,….

PolarizationV & H linear polarizationRH & LH circular polarizations

Between Users or “Channels” (Multiple Access):FDMA: Frequency Division Multiple Access

f1 = User 1f2 = User 2f3 = User 3…

TDMA: Time Division Multiple Access.t1=User_1, t2=User_2, t3=User_3, t4 = User_1, ...

CDMA: Code Division Multiple AccessCode 1 = User 1Code 2 = User 2Code 3 = User 3…

Responsible for frequency translationFrom uplink (f1) to downlink (f2) (FDD)

Movement is from passive to active satellitesPassive: No on-board processing (only reflects signal)Active: On-board processing (e.g. signal amplification)

Linear and non-linear transponders

(Uplink) (Downlink)

High speed two way internet accessTwo way fixed satellite service to home terminal at Mbps ratesTwo way links to mobiles - but at what speed?

Higher power GEO satellites with multiple rolesMore direct broadcast TV and radio satellitesExpansion into Ka, Q, V bands (30/20, 50/40 GHz)Massive growth in data services fueled by internet demand (overtaking voice)Mobile services:

May be broadcast services rather than point to pointMake mobile services a successful business?

Growth requires new frequency bandsPropagation through rain and clouds becomes a problem as RF frequency is increasedC band (6/4 GHz)

Rain has little impact; 99.99% availability is possibleKu band (10-12 GHz) & Ka band (20 - 30 GHz)

Rain has significant impact, affects link availability

Low cost phased array antennas for mobiles are neededMobile systems are limited by use of omnidirectional antennasA self-phasing, self-steering phased array antenna with 6 dB gain can quadruple the capacity of a systemDirectional antennas allow frequency re-use

Expected revenues from all satellite communications services should reach $75 billion by the year 2005Satellite Direct-to-Home (DTH) Video and Internet services appear to be the major drivers