CSE475CSE475Satellite and Space NetworksSatellite and Space Networks

ÖMER KORÇAKÖMER KORÇAKomer.korcak@marmara.edu.tromer.korcak@marmara.edu.tr

Marmara UniversityMarmara UniversityDepartment of Computer EngineeringDepartment of Computer Engineering

Satellite Networks Research Laboratory Satellite Networks Research Laboratory (SATLAB)(SATLAB)

Department of Computer Engineering, Department of Computer Engineering, BoğaziBoğaziççi Universityi University

Istanbul, TurkeyIstanbul, Turkey

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OUTLINEOUTLINE

• SatellitesSatellites– GEO, MEO, LEOGEO, MEO, LEO

• High Altitude PlatformsHigh Altitude Platforms• Integration ScenarioIntegration Scenario• Problem Definition & Solution Problem Definition & Solution

ApproachApproach

SATELLITESSATELLITES

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Distance: 378.000 kmPeriod: 27.3 days

BasicsBasics• Satellites in circular orbitsSatellites in circular orbits

– attractive force Fattractive force Fgg = m g (R/r)² = m g (R/r)²

– centrifugal force Fcentrifugal force Fcc = m r = m r ²²

– m: mass of the satellitem: mass of the satellite– R: radius of the earth (R = 6370 km)R: radius of the earth (R = 6370 km)– r: distance to the center of the earthr: distance to the center of the earth– g: acceleration of gravity (g = 9.81 m/s²)g: acceleration of gravity (g = 9.81 m/s²) : angular velocity (: angular velocity ( = 2 = 2 f, f: rotation frequency) f, f: rotation frequency)

• Stable orbitStable orbit

– FFgg = F = Fcc

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2

)2( f

gRr

4

Satellite period and orbitsSatellite period and orbits

10 20 30 40 x106 m

24

20

16

12

8

4

radius

satellite period [h]velocity [ x1000 km/h]

synchronous distance35,786 km

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BasicsBasics elliptical or circular orbitselliptical or circular orbits complete rotation time depends on distance satellite-earthcomplete rotation time depends on distance satellite-earth inclination: angle between orbit and equatorinclination: angle between orbit and equator elevation: angle between satellite and horizonelevation: angle between satellite and horizon LOS (Line of Sight) to the satellite necessary for connectionLOS (Line of Sight) to the satellite necessary for connection

high elevation needed, less absorption due to e.g. high elevation needed, less absorption due to e.g. buildingsbuildings

Uplink: connection base station - satelliteUplink: connection base station - satellite Downlink: connection satellite - base stationDownlink: connection satellite - base station typically separated frequencies for uplink and downlinktypically separated frequencies for uplink and downlink

– transponder used for sending/receiving and shifting of transponder used for sending/receiving and shifting of frequenciesfrequencies

– transparent transponder: only shift of frequenciestransparent transponder: only shift of frequencies– regenerative transponder: additionally signal regenerative transponder: additionally signal

regenerationregeneration6

InclinationInclination

inclination

satellite orbit

perigee

plane of satellite orbit

equatorial plane

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ElevationElevation

Elevation:angle between center of satellite beam and surface

minimal elevation:elevation needed at leastto communicate with the satellite

footprint

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Link budget of satellitesLink budget of satellites

• Parameters like attenuation or received power determined Parameters like attenuation or received power determined by four parameters:by four parameters: sending powersending power gain of sending antennagain of sending antenna distance between sender distance between sender

and receiverand receiver gain of receiving antennagain of receiving antenna

• ProblemsProblems varying strength of received signal due to multipath propagation varying strength of received signal due to multipath propagation interruptions due to shadowing of signal (no LOS)interruptions due to shadowing of signal (no LOS)

• Possible solutionsPossible solutions Link Margin to eliminate variations in signal strength Link Margin to eliminate variations in signal strength satellite diversity (usage of several visible satellites at the same satellite diversity (usage of several visible satellites at the same

time) helps to use less sending powertime) helps to use less sending power

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c

frL

L: Lossf: carrier frequencyr: distancec: speed of light

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Atmospheric attenuationAtmospheric attenuationExample: satellite systems at 4-6 GHz

elevation of the satellite

5° 10° 20° 30° 40° 50°

Attenuation of the signal in %

10

20

30

40

50

rain absorption

fog absorption

atmospheric absorption

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Satellite OrbitsSatellite Orbits

Distance (km)

Period

Low Earth Orbit (LEO) 700 - 2000 ~2 hr

Medium Earth Orbit (MEO) 10.000 – 15.000

~6 hr

Geosynchronous Earth Orbit (GEO) 36.000 24 hr

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Satellite Satellite Orbits Orbits 22

earth

km35768

10000

1000

LEO (Globalstar,

Irdium)

HEO

inner and outer VanAllen belts

MEO (ICO)

GEO (Inmarsat)

Van-Allen-Belts:ionized particles2000 - 6000 km and15000 - 30000 kmabove earth surface

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GEO SatellitesGEO Satellites

• No handoverNo handover• Altitude: ~35.786 km.Altitude: ~35.786 km.• One-way propagation delay: 250-280 msOne-way propagation delay: 250-280 ms• 3 to 4 satellites for global coverage3 to 4 satellites for global coverage• Mostly used in video broadcastingMostly used in video broadcasting

– Example: TURKSAT satellitesExample: TURKSAT satellites

• Another applications: Weather forecast, global Another applications: Weather forecast, global communications, military applicationscommunications, military applications

• Advantage: well-suited for broadcast servicesAdvantage: well-suited for broadcast services• Disadvantages: Long delay, high free-space Disadvantages: Long delay, high free-space

attenuationattenuation13

MEO SatellitesMEO Satellites

• Altitude: 10.000 – 15.000 kmAltitude: 10.000 – 15.000 km• One-way propagation delay: 100 – 130 One-way propagation delay: 100 – 130

msms• 10 to 15 satellites for global coverage10 to 15 satellites for global coverage• Infrequent handoverInfrequent handover• Orbit period: ~6 hrOrbit period: ~6 hr• Mostly used in navigationMostly used in navigation

– GPS, Galileo, GlonassGPS, Galileo, Glonass

• Communications: Inmarsat, ICOCommunications: Inmarsat, ICO14

MEO Example: GPSMEO Example: GPS• Global Positioning SystemGlobal Positioning System

– Developed by US Dept. Of DefenceDeveloped by US Dept. Of Defence– Became fully operational in 1993Became fully operational in 1993– Currently 31 satellites at 20.200 km. Currently 31 satellites at 20.200 km.

• Last lunch: March 2008Last lunch: March 2008

• It works based on a geometric principleIt works based on a geometric principle– ““Position of a point can be calculated if the distances between this Position of a point can be calculated if the distances between this

point and thpoint and threeree objects with known positions can be measured” objects with known positions can be measured”

• Four satellites are needed to calculate the positionFour satellites are needed to calculate the position– Fourth satellite is needed to correct the receiver’s clock. Fourth satellite is needed to correct the receiver’s clock.

• Selective AvailabilitySelective Availability• Glonass (Russian):Glonass (Russian): 24 satellites, 19.100 km24 satellites, 19.100 km• Galileo (EU): 30 satellites, 23.222 km, under development Galileo (EU): 30 satellites, 23.222 km, under development

(expected date: 2013)(expected date: 2013)• Beidou (China): Currently experimental & limited. Beidou (China): Currently experimental & limited.

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LEO SatellitesLEO Satellites

• Altitude: 700 – 2.000 kmAltitude: 700 – 2.000 km• One-way propagation delay: 5 – 20 msOne-way propagation delay: 5 – 20 ms• More than 32 satellites for global coverageMore than 32 satellites for global coverage• Frequent handoverFrequent handover• Orbit period: ~2 hrOrbit period: ~2 hr• Applications:Applications:

– Earth Observation Earth Observation • GoogleEarth image providers (DigitalGlobe, etc.)GoogleEarth image providers (DigitalGlobe, etc.)• RASAT (First satellite to be produced solely in Turkey)RASAT (First satellite to be produced solely in Turkey)

– CommunicationsCommunications• Globalstar, IridiumGlobalstar, Iridium

– Search and Rescue (SAR)Search and Rescue (SAR)• COSPAS-SARSATCOSPAS-SARSAT

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GlobalstarGlobalstar• Satellite phone & low speed data Satellite phone & low speed data

comm.comm.• 48 satellites (8 planes, 6 sat per 48 satellites (8 planes, 6 sat per

plane) and 4 spares.plane) and 4 spares.• 52˚ inclination: not covers the polar 52˚ inclination: not covers the polar

regions regions • Altitude: 1.410 kmAltitude: 1.410 km• No intersatellite link: Ground No intersatellite link: Ground

gateways provide connectivity from gateways provide connectivity from satellites to PSTN and Internet.satellites to PSTN and Internet.

• Satellite visibility time: 16.4 minSatellite visibility time: 16.4 min• Operational since February 2000.Operational since February 2000.• 315.000 subscribers (as of June 315.000 subscribers (as of June

2008)2008)• Currently second-generation Currently second-generation

satellites are being produced (by satellites are being produced (by Thales Alenia Space) and 18 Thales Alenia Space) and 18 satellites launched in 2010 and 2011. satellites launched in 2010 and 2011.

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Globalstar – Coverage MapGlobalstar – Coverage Map

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IridiumIridium

• 66 satellites (6 planes, 11 sat per 66 satellites (6 planes, 11 sat per plane) and 10 spares.plane) and 10 spares.

• 86.4˚ inclination: full coverage 86.4˚ inclination: full coverage • Altitude: 780 kmAltitude: 780 km• Intersatellite links, onboard processingIntersatellite links, onboard processing• Satellite visibility time: 11.1 minSatellite visibility time: 11.1 min• Satellites launched in 1997-98.Satellites launched in 1997-98.• Initial company went into bankrupcyInitial company went into bankrupcy

– Technologically flawless, however:Technologically flawless, however:– Very expensive; Awful business plan Very expensive; Awful business plan – Cannot compete with GSMCannot compete with GSM

• Now, owned by Iridium satellite LLC.Now, owned by Iridium satellite LLC.• 280.000 subscribers (as of Aug. 2008)280.000 subscribers (as of Aug. 2008)• Multi-year contract with US DoD.Multi-year contract with US DoD.• Satellite collision (February 10, 2009).Satellite collision (February 10, 2009).

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COSPAS-SARSATCOSPAS-SARSAT(Search And Rescue Satellite Aided Tracking)(Search And Rescue Satellite Aided Tracking)

• An international satellite-based An international satellite-based SAR distress alert detection & SAR distress alert detection & information distribution system.information distribution system.

• 4 GEO, 5 LEO satellites4 GEO, 5 LEO satellites• Aircraft & maritime Aircraft & maritime

radiobeacons are automatically radiobeacons are automatically activated in case of distress.activated in case of distress.

• Newest beacons incorporate Newest beacons incorporate GPS receivers (position of GPS receivers (position of distress is transmitted)distress is transmitted)

• Supporters are working to add a Supporters are working to add a new capability called MEOSAR.new capability called MEOSAR.– The system will put SAR The system will put SAR

processors aboard GPS and processors aboard GPS and Galileo satellites Galileo satellites

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Since 1982, 30.713 persons Since 1982, 30.713 persons rescued in 8.387 distress rescued in 8.387 distress situation. situation.

Satellites - OverviewSatellites - Overview

• GEOs have good broadcasting capability, but long GEOs have good broadcasting capability, but long propagation delay.propagation delay.

• LEOs offer low latency, low terminal power LEOs offer low latency, low terminal power requirements.requirements.

• Inter-satellite links and on-board processing for Inter-satellite links and on-board processing for increased performance and better utilization of increased performance and better utilization of satellitessatellites– From flying mirrors to intelligent routers on sky. From flying mirrors to intelligent routers on sky.

• Major problem with LEOs: Mobility of satellites Major problem with LEOs: Mobility of satellites – Frequent hand-overFrequent hand-over

• Another important problem with satellites:Another important problem with satellites:– Infeasible to upgrade the technology, after the satellite is Infeasible to upgrade the technology, after the satellite is

launched launched 21

High Altitude Platforms High Altitude Platforms (HAPs)(HAPs)

• Aerial unmanned platformsAerial unmanned platforms• Quasi-stationary position (at 17-22 Quasi-stationary position (at 17-22

km)km)• Telecommunications & surveillance Telecommunications & surveillance • Advantages:Advantages:

– Cover larger areas than terrestrial Cover larger areas than terrestrial base stationsbase stations

– No mobility problems like LEOsNo mobility problems like LEOs– Low propagation delayLow propagation delay– Smaller and cheaper user terminalsSmaller and cheaper user terminals– Easy and incremental deploymentEasy and incremental deployment

• Disadvantages:Disadvantages:– Immature airship technologyImmature airship technology– Monitoring of the platform’s Monitoring of the platform’s

movementmovement22

HAP CoverageHAP Coverage

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HAP-Satellite IntegrationHAP-Satellite Integration

• HAPs have significant advantages.HAPs have significant advantages.• Satellites still represent the most attractive solution for broadcast Satellites still represent the most attractive solution for broadcast

and multicast servicesand multicast services• Should be considered as complementary technologies.Should be considered as complementary technologies.

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An Integration ScenarioAn Integration Scenario

• Integration of HAPs and mobile satellitesIntegration of HAPs and mobile satellites• Establishment of optical linksEstablishment of optical links

HAPs

Satellites

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Optimal Assignment of Optimal Assignment of Optical LinksOptical Links

CONSTRAINTS:CONSTRAINTS:• A satellite and a HAP should have line of sight in order to A satellite and a HAP should have line of sight in order to

communicate with each other. communicate with each other. – Elevation angle between HAP and satellite should be larger Elevation angle between HAP and satellite should be larger

than a certain than a certain εεminmin value. value.

• Number of optical transmitters in satellites is limited. Number of optical transmitters in satellites is limited. – A satellite can serve maximum of A satellite can serve maximum of HHmaxmax HAPs. HAPs.

• One to many relation between HAPs and satellites.One to many relation between HAPs and satellites.

AIMS:AIMS:• As much HAP as possible should be served (Maximum As much HAP as possible should be served (Maximum

utilization)utilization)• Average of elevation angles between HAPs and satellites Average of elevation angles between HAPs and satellites

should be maximized.should be maximized.

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Optimal Assignment of Optimal Assignment of Optical Links (cont.)Optical Links (cont.)

SOLUTION APPROACHESSOLUTION APPROACHES• This optimization problem can be represented as an This optimization problem can be represented as an

Integer Linear Programming (ILP) problemInteger Linear Programming (ILP) problem• ILP solution approaches: Exclusive search, Branch-and-ILP solution approaches: Exclusive search, Branch-and-

bound algorithms, etc.bound algorithms, etc.– Exponential time complexityExponential time complexity– Not feasible for large networksNot feasible for large networks

• Optimization algorithm should be applied repeatedlyOptimization algorithm should be applied repeatedly– In periodic manner: every ∆t time unitIn periodic manner: every ∆t time unit– In event-driven manner: When a link becomes obsoleteIn event-driven manner: When a link becomes obsolete

• Faster algorithm is necessary. Faster algorithm is necessary.

• There exists a polynomial time solution approachThere exists a polynomial time solution approach

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Solution ApproachSolution Approach

• Example scenarioExample scenario• Three satellites & Three satellites &

seven HAPsseven HAPs• Visible pairs are Visible pairs are

connectedconnected• Elevation angles are Elevation angles are

given on the links given on the links

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Solution Approach (cont.)Solution Approach (cont.)

• Bipartite graphBipartite graph• First group: Node for each First group: Node for each

satellite transmittersatellite transmitter• Second group: Node for Second group: Node for

each HAPeach HAP• Edge exists between a Edge exists between a

HAP and each transmitter HAP and each transmitter of a satellite, if they are of a satellite, if they are visible to each other.visible to each other.

• Weights: Elevation anglesWeights: Elevation angles

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Maximum weighted maximum cardinality matching

Max Weighted Max Cardinality MatchingMax Weighted Max Cardinality Matching

• Matching: Matching: A subset of edges, such that no two A subset of edges, such that no two edges share a common node.edges share a common node.

• Maximum cardinality matching:Maximum cardinality matching: Matching with Matching with maximum number of edges.maximum number of edges.

• Maximum weighted maximum cardinality Maximum weighted maximum cardinality matching:matching: Maximum cardinality matching where Maximum cardinality matching where the sum of the weights of the edges is maximum.the sum of the weights of the edges is maximum.

• Hungarian Algorithm: O(nHungarian Algorithm: O(n33))

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ResultsResults

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100 HAPs (height: 20 km)10 MEO Satellites: ICO (height: 10350 km, inclination: 45˚)Total time: 1 day, Δt=1 minute

100 HAPs (height: 20 km)10 MEO Satellites: ICO (height: 10350 km, inclination: 45˚)Total time: 1 day, Δt=1 minute

Increasing the Link Durations Increasing the Link Durations

• Matching with maximum “average elevation Matching with maximum “average elevation angle” may result in frequent optical link angle” may result in frequent optical link switchingswitching– Switching from one satellite to another is an expensive Switching from one satellite to another is an expensive

operation.operation.

• Reduction of switching = Increasing the link Reduction of switching = Increasing the link durationsdurations

• Method: Favor existing links with a particular Method: Favor existing links with a particular amount amount γγ– Weights of utilized edges are incremented by Weights of utilized edges are incremented by γγ

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Results - 2Results - 2

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εmin=-2

Net gain function

)0(

)0()()(

LD

LDLDG

)0(

)()0()(

avg

avgavg

A

AAL

)()1()()( LGNGF

“Efficient Integration of HAPs and Mobile Satellites via Free-space Optical Links,” Computer Networks, 2011

More on SatellitesMore on Satellites

• Hand-overHand-over• Satellite-fixed / Earth-fixed footprintsSatellite-fixed / Earth-fixed footprints• Network Mobility ManagementNetwork Mobility Management• RoutingRouting

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Handover in satellite systemsHandover in satellite systems• Several additional situations for handover in satellite systems compared to Several additional situations for handover in satellite systems compared to

cellular terrestrial mobile phone networks caused by the movement of the cellular terrestrial mobile phone networks caused by the movement of the satellitessatellites– Intra satellite handoverIntra satellite handover

• handover from one spot beam to anotherhandover from one spot beam to another• mobile station still in the footprint of the satellite, but in another mobile station still in the footprint of the satellite, but in another

cellcell– Inter satellite handoverInter satellite handover

• handover from one satellite to another satellitehandover from one satellite to another satellite• mobile station leaves the footprint of one satellitemobile station leaves the footprint of one satellite

– Gateway handoverGateway handover• Handover from one gateway to anotherHandover from one gateway to another• mobile station still in the footprint of a satellite, but gateway leaves mobile station still in the footprint of a satellite, but gateway leaves

the footprintthe footprint– Inter system handoverInter system handover

• Handover from the satellite network to a terrestrial cellular Handover from the satellite network to a terrestrial cellular networknetwork

• mobile station can reach a terrestrial network again which might be mobile station can reach a terrestrial network again which might be cheaper, has a lower latency etc.cheaper, has a lower latency etc.

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Satellite-fixed vs Earth-fixed Footprints

Satellite Fixed Footprints(asynchronous handoff)

Earth Fixed Footprints(synchronous handoff)

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　 Physical network topology is dynamic

　 Virtual Node topology is fixed

A satellite corresponds to a VN in a time.Routing is carried out without considering the movement of satellites

Virtual Node

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RoutingRouting

• One solution: inter satellite links (ISL)One solution: inter satellite links (ISL) reduced number of gateways needed reduced number of gateways needed forward connections or data packets within the satellite forward connections or data packets within the satellite

network as long as possiblenetwork as long as possible only one uplink and one downlink per direction needed for the only one uplink and one downlink per direction needed for the

connection of two mobile phones connection of two mobile phones • Problems:Problems: more complex focusing of antennas between satellites more complex focusing of antennas between satellites high system complexity due to moving routershigh system complexity due to moving routers higher fuel consumptionhigher fuel consumption thus shorter lifetimethus shorter lifetime• Iridium and Teledesic planned with ISLIridium and Teledesic planned with ISL• Other systems use gateways and additionally terrestrial Other systems use gateways and additionally terrestrial

networksnetworks39

Features of Satellite Networks

• Effects of satellite mobility– Topology is dynamic.– Topology changes are predictable and periodic.– Traffic is very dynamic and non-homogeneous.– Handovers are necessary.

• Limitations and capabilities of satellites– Power and onboard processing capability are limited.– Implementing the state-of-the-art technology is difficult.– Satellites have a broadcast nature.

• Nature of satellite constellations – Higher propagation delays.– Fixed number of nodes.– Highly symmetric and uniform structure.

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Routing & Network MM

• Considering these issues various routing & MM techniques are proposed. Main ideas are:– To handle dynamic topology changes with minimum

overhead.– To prevent an outgoing call from dropping due to link

handovers– To minimize length of the paths in terms of propagation

delay and/or number of satellite hops.– To prevent congestion of some ISLs, while others are idle

(Load balancing).– To perform traffic-based routing.– To provide better integration of satellite networks and

terrestrial networks.– To perform efficient multicasting over satellites.

“Exploring the Routing Strategies in Next-Generation Satellite Networks” IEEE Wireless Communications, 2007

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Thank YouThank You

Any questions?Any questions?