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CSE475 Satellite and Space Networks. ÖMER KORÇAK omer . korcak @ marmara .edu.tr Marmara University Department of Computer Engineering Satellite Networks Research Laboratory (SATLAB) Department of Computer Engineering, Boğazi ç i University Istanbul, Turkey. OUTLINE. Satellites

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Cse475 satellite and space networks

CSE475SatelliteandSpace Networks



Marmara University

Department of ComputerEngineering

Satellite Networks Research Laboratory (SATLAB)

Department of Computer Engineering,

Boğaziçi University

Istanbul, Turkey


  • Satellites

    • GEO, MEO, LEO

  • High Altitude Platforms

  • Integration Scenario

  • Problem Definition & Solution Approach


Distance: 378.000 km

Period: 27.3 days


  • Satellites in circular orbits

    • attractive force Fg = m g (R/r)²

    • centrifugal force Fc = m r ²

    • m: mass of the satellite

    • R: radius of the earth (R = 6370 km)

    • r: distance to the center of the earth

    • g: acceleration of gravity (g = 9.81 m/s²)

    • : angular velocity ( = 2  f, f: rotation frequency)

  • Stable orbit

    • Fg = Fc

Satellite period and orbits
Satellite period and orbits



period [h]

velocity [ x1000 km/h]






synchronous distance

35,786 km




40 x106 m



  • elliptical or circular orbits

  • complete rotation time depends on distance satellite-earth

  • inclination: angle between orbit and equator

  • elevation: angle between satellite and horizon

  • LOS (Line of Sight) to the satellite necessary for connection

     high elevation needed, less absorption due to e.g. buildings

  • Uplink: connection base station - satellite

  • Downlink: connection satellite - base station

  • typically separated frequencies for uplink and downlink

    • transponder used for sending/receiving and shifting of frequencies

    • transparent transponder: only shift of frequencies

    • regenerative transponder: additionally signal regeneration


plane of satellite orbit

satellite orbit



inclination d

equatorial plane



angle e between center of satellite beam

and surface

minimal elevation:

elevation needed at least

to communicate with the satellite



Link budget of satellites
Link budget of satellites

  • Parameters like attenuation or received power determined by four parameters:

    • sending power

    • gain of sending antenna

    • distance between sender and receiver

    • gain of receiving antenna

  • Problems

    • varying strength of received signal due to multipath propagation

    • interruptions due to shadowing of signal (no LOS)

  • Possible solutions

    • Link Margin to eliminate variations in signal strength

    • satellite diversity (usage of several visible satellites at the same time) helps to use less sending power

L: Loss

f: carrier frequency

r: distance

c: speed of light

Atmospheric attenuation
Atmospheric attenuation

Attenuation of

the signal in %

Example: satellite systems at 4-6 GHz



rain absorption



fog absorption



atmospheric absorption






elevation of the satellite

Satellite orbits 2
SatelliteOrbits 2

GEO (Inmarsat)



LEO (Globalstar,Irdium)

inner and outer Van

Allen belts





ionized particles

2000 - 6000 km and

15000 - 30000 km

above earth surface



Geo satellites
GEO Satellites

  • No handover

  • Altitude: ~35.786 km.

  • One-way propagation delay: 250-280 ms

  • 3 to 4 satellites for global coverage

  • Mostly used in video broadcasting

    • Example: TURKSAT satellites

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

  • Advantage: well-suited for broadcast services

  • Disadvantages: Long delay, high free-space attenuation

Meo satellites
MEO Satellites

  • Altitude: 10.000 – 15.000 km

  • One-way propagation delay: 100 – 130 ms

  • 10 to 15 satellites for global coverage

  • Infrequent handover

  • Orbit period: ~6 hr

  • Mostly used in navigation

    • GPS, Galileo, Glonass

  • Communications: Inmarsat, ICO

Meo example gps
MEO Example: GPS

  • Global Positioning System

    • Developed by US Dept. Of Defence

    • Became fully operational in 1993

    • Currently 31 satellites at 20.200 km.

      • Last lunch: March 2008

  • It works based on a geometric principle

    • “Position of a point can be calculated if the distances between this point and three objects with known positions can be measured”

  • Four satellites are needed to calculate the position

    • Fourth satellite is needed to correct the receiver’s clock.

  • Selective Availability

  • Glonass (Russian):24 satellites, 19.100 km

  • Galileo (EU): 30 satellites, 23.222 km, under development (expected date: 2013)

  • Beidou (China): Currently experimental & limited.

Leo satellites
LEO Satellites

  • Altitude: 700 – 2.000 km

  • One-way propagation delay: 5 – 20 ms

  • More than 32 satellites for global coverage

  • Frequent handover

  • Orbit period: ~2 hr

  • Applications:

    • Earth Observation

      • GoogleEarth image providers (DigitalGlobe, etc.)

      • RASAT (First satellite to be produced solely in Turkey)

    • Communications

      • Globalstar, Iridium

    • Search and Rescue (SAR)



  • Satellite phone & low speed data comm.

  • 48 satellites (8 planes, 6 sat per plane) and 4 spares.

  • 52˚ inclination: not covers the polar regions

  • Altitude: 1.410 km

  • No intersatellite link: Ground gateways provide connectivity from satellites to PSTN and Internet.

  • Satellite visibility time: 16.4 min

  • Operational since February 2000.

  • 315.000 subscribers (as of June 2008)

  • Currentlysecond-generation satellites arebeingproduced (by Thales Alenia Space) and 18 satelliteslaunched in 2010 and 2011.


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

  • 86.4˚ inclination: full coverage

  • Altitude: 780 km

  • Intersatellite links, onboard processing

  • Satellite visibility time: 11.1 min

  • Satellites launched in 1997-98.

  • Initial company went into bankrupcy

    • Technologically flawless, however:

    • Very expensive; Awful business plan

    • Cannot compete with GSM

  • Now, owned by Iridium satellite LLC.

  • 280.000 subscribers (as of Aug. 2008)

  • Multi-year contract with US DoD.

  • Satellitecollision (February 10, 2009).

Cospas sarsat search and rescue satellite aided tracking
COSPAS-SARSAT(Search And Rescue Satellite Aided Tracking)

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

  • 4 GEO, 5 LEO satellites

  • Aircraft & maritime radiobeacons are automatically activated in case of distress.

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

  • Supporters are working to add a new capability called MEOSAR.

    • The system will put SAR processors aboard GPS and Galileo satellites

Since 1982, 30.713 persons rescued in 8.387 distress situation.

Satellites overview
Satellites - Overview

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

  • LEOs offer low latency, low terminal power requirements.

  • Inter-satellite links and on-board processing for increased performance and better utilization of satellites

    • From flying mirrors to intelligent routers on sky.

  • Major problem with LEOs: Mobility of satellites

    • Frequent hand-over

  • Another important problem with satellites:

    • Infeasible to upgrade the technology, after the satellite is launched

High altitude platforms haps
High Altitude Platforms (HAPs)

  • Aerial unmanned platforms

  • Quasi-stationary position (at 17-22 km)

  • Telecommunications & surveillance

  • Advantages:

    • Cover larger areas than terrestrial base stations

    • No mobility problems like LEOs

    • Low propagation delay

    • Smaller and cheaper user terminals

    • Easy and incremental deployment

  • Disadvantages:

    • Immature airship technology

    • Monitoring of the platform’s movement

Hap satellite integration
HAP-Satellite Integration

  • HAPs have significant advantages.

  • Satellites still represent the most attractive solution for broadcast and multicast services

  • Should be considered as complementary technologies.

An integration scenario
An Integration Scenario

  • Integration of HAPs and mobile satellites

  • Establishment of optical links



Optimal assignment of optical links
Optimal Assignment of Optical Links


  • A satellite and a HAP should have line of sight in order to communicate with each other.

    • Elevation angle between HAP and satellite should be larger than a certain εmin value.

  • Number of optical transmitters in satellites is limited.

    • A satellite can serve maximum of Hmax HAPs.

  • One to many relation between HAPs and satellites.


  • As much HAP as possible should be served (Maximum utilization)

  • Average of elevation angles between HAPs and satellites should be maximized.

Optimal assignment of optical links cont
Optimal Assignment of Optical Links (cont.)


  • This optimization problem can be represented as an Integer Linear Programming (ILP) problem

  • ILP solution approaches: Exclusive search, Branch-and-bound algorithms, etc.

    • Exponential time complexity

    • Not feasible for large networks

  • Optimization algorithm should be applied repeatedly

    • In periodic manner: every ∆t time unit

    • In event-driven manner: When a link becomes obsolete

  • Faster algorithm is necessary.

  • There exists a polynomial time solution approach

Solution approach
Solution Approach

  • Example scenario

  • Three satellites & seven HAPs

  • Visible pairs are connected

  • Elevation angles are given on the links

Solution approach cont
Solution Approach (cont.)

  • Bipartite graph

  • First group: Node for each satellite transmitter

  • Second group: Node for each HAP

  • Edge exists between a HAP and each transmitter of a satellite, if they are visible to each other.

  • Weights: Elevation angles

Maximum weighted maximum cardinality matching

Max weighted max cardinality matching
Max Weighted Max Cardinality Matching

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

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

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

  • Hungarian Algorithm: O(n3)


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 angle” may result in frequent optical link switching

    • Switching from one satellite to another is an expensive operation.

  • Reduction of switching = Increasing the link durations

  • Method: Favor existing links with a particular amount γ

    • Weights of utilized edges are incremented by γ

Results 2
Results - 2


Net gain function
Net gain function

“Efficient Integration of HAPs and Mobile Satellites

via Free-space Optical Links,” Computer Networks, 2011

More on satellites
More on Satellites

  • Hand-over

  • Satellite-fixed / Earth-fixedfootprints

  • Network MobilityManagement

  • Routing

Handover in satellite systems
Handover in satellitesystems

  • Several additional situations for handover in satellite systems compared to cellular terrestrial mobile phone networks caused by the movement of the satellites

    • Intra satellite handover

      • handover from one spot beam to another

      • mobile station still in the footprint of the satellite, but in another cell

    • Inter satellite handover

      • handover from one satellite to another satellite

      • mobile station leaves the footprint of one satellite

    • Gateway handover

      • Handover from one gateway to another

      • mobile station still in the footprint of a satellite, but gateway leaves the footprint

    • Inter system handover

      • Handover from the satellite network to a terrestrial cellular network

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

Satellite fixed vs earth fixed footprints
Satellite-fixed vs Earth-fixed Footprints

Satellite Fixed Footprints

(asynchronous handoff)

Earth Fixed Footprints

(synchronous handoff)

Cse475 satellite and space networks

Virtual Node

Routing is carried out without considering the movement of satellites

A satellite corresponds to a VN in a time.

  • Physical network topology is dynamic

  • Virtual Node topology is fixed


  • One solution: inter satellite links (ISL)

  • reduced number of gateways needed

  • forward connections or data packets within the satellite network as long as possible

  • only one uplink and one downlink per direction needed for the connection of two mobile phones

  • Problems:

  • more complex focusing of antennas between satellites

  • high system complexity due to moving routers

  • higher fuel consumption

  • thus shorter lifetime

  • Iridium and Teledesic planned with ISL

  • Other systems use gateways and additionally terrestrial networks

Features of satellite networks
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.

Routing network mm
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

Thank you

Thank You

Any questions?