1 / 42

Satellite and Space Networks

Satellite and Space Networks. OUTLINE. Satellites GEO, MEO, LEO High Altitude Platforms Integration Scenario Problem Definition & Solution Approach. SATELLITES. Distance: 378.000 km Period: 27.3 days. Basics. Satellites in circular orbits attractive force F g = m g (R/r)²

Download Presentation

Satellite and Space Networks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Satellite and Space Networks

  2. OUTLINE • Satellites • GEO, MEO, LEO • High Altitude Platforms • Integration Scenario • Problem Definition & Solution Approach

  3. SATELLITES Distance: 378.000 km Period: 27.3 days

  4. 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

  5. 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

  6. 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

  7. Inclination plane of satellite orbit satellite orbit perigee d inclination d equatorial plane

  8. Elevation Elevation: angle e between center of satellite beam and surface minimal elevation: elevation needed at least to communicate with the satellite e footprint

  9. 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

  10. 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

  11. Satellite Orbits

  12. Satellite Orbits 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

  13. 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

  14. 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

  15. 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.

  16. 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

  17. 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) • Currently second-generation satellites are being produced (by Thales Alenia Space) and 18 satellites launched in 2010 and 2011.

  18. Globalstar – Coverage Map

  19. 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. • Satellite collision (February 10, 2009).

  20. 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.

  21. 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

  22. 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

  23. HAP Coverage

  24. 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.

  25. An Integration Scenario • Integration of HAPs and mobile satellites • Establishment of optical links HAPs Satellites

  26. 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.

  27. 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

  28. Solution Approach • Example scenario • Three satellites & seven HAPs • Visible pairs are connected • Elevation angles are given on the links

  29. 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

  30. 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)

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

  32. 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

  33. Results - 2 εmin=-2

  34. Net gain function “Efficient Integration of HAPs and Mobile Satellites via Free-space Optical Links,” Computer Networks, 2011

  35. More on Satellites • Hand-over • Satellite-fixed / Earth-fixed footprints • Network Mobility Management • Routing

  36. Handover in satellite systems • 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.

  37. Satellite-fixed vs Earth-fixed Footprints Satellite Fixed Footprints (asynchronous handoff) Earth Fixed Footprints (synchronous handoff)

  38. 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

  39. 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

  40. 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.

  41. 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

  42. Thank You Any questions?

More Related