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CSE475 Satellite and Space NetworksPowerPoint Presentation

CSE475 Satellite and Space Networks

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### CSE475SatelliteandSpace Networks

### Thank You

ÖMER KORÇAK

omer.korcak@marmara.edu.tr

Marmara University

Department of ComputerEngineering

Satellite Networks Research Laboratory (SATLAB)

Department of Computer Engineering,

Boğaziçi University

Istanbul, Turkey

OUTLINE

- Satellites
- GEO, MEO, LEO

- High Altitude Platforms
- Integration Scenario
- Problem Definition & Solution Approach

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

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

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

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

- 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

Attenuation of

the signal in %

Example: satellite systems at 4-6 GHz

50

40

rain absorption

e

30

fog absorption

20

10

atmospheric absorption

5°

10°

20°

30°

40°

50°

elevation of the satellite

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

- 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

- 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

- 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

- 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

- Earth Observation

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

- 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)

- 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

- 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)

- 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

- 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

- Integration of HAPs and mobile satellites
- Establishment of optical links

HAPs

Satellites

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.)

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

- Example scenario
- Three satellites & seven HAPs
- Visible pairs are connected
- Elevation angles are given on the links

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

- 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

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

- 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

εmin=-2

Net gain function

“Efficient Integration of HAPs and Mobile Satellites

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

More on Satellites

- Hand-over
- Satellite-fixed / Earth-fixedfootprints
- Network MobilityManagement
- Routing

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.

- Intra satellite handover

Satellite-fixed vs Earth-fixed Footprints

Satellite Fixed Footprints

(asynchronous handoff)

Earth Fixed Footprints

(synchronous handoff)

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

- 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

- 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

- 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

Any questions?

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