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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

ÖMER KORÇAK

[email protected]

Marmara University

Department of ComputerEngineering

Satellite Networks Research Laboratory (SATLAB)

Department of Computer Engineering,

Boğaziçi University

Istanbul, Turkey

outline
OUTLINE
  • Satellites
    • GEO, MEO, LEO
  • High Altitude Platforms
  • Integration Scenario
  • Problem Definition & Solution Approach
satellites
SATELLITES

Distance: 378.000 km

Period: 27.3 days

basics
Basics
  • 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

24

satellite

period [h]

velocity [ x1000 km/h]

20

16

12

8

4

synchronous distance

35,786 km

10

20

30

40 x106 m

radius

basics1
Basics
  • 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
inclination
Inclination

plane of satellite orbit

satellite orbit

perigee

d

inclination d

equatorial plane

elevation
Elevation

Elevation:

angle e between center of satellite beam

and surface

minimal elevation:

elevation needed at least

to communicate with the satellite

e

footprint

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

50

40

rain absorption

e

30

fog absorption

20

10

atmospheric absorption

10°

20°

30°

40°

50°

elevation of the satellite

satellite orbits 2
SatelliteOrbits 2

GEO (Inmarsat)

HEO

MEO (ICO)

LEO (Globalstar,Irdium)

inner and outer Van

Allen belts

earth

1000

10000

Van-Allen-Belts:

ionized particles

2000 - 6000 km and

15000 - 30000 km

above earth surface

35768

km

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)
      • COSPAS-SARSAT
globalstar
Globalstar
  • 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.
iridium
Iridium
  • 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

HAPs

Satellites

optimal assignment of optical links
Optimal Assignment of Optical Links

CONSTRAINTS:

  • 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.

AIMS:

  • 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.)

SOLUTION APPROACHES

  • 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)
results
Results

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

εmin=-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)

slide38

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
routing
Routing
  • 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

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