Satellite Systems

1. IT351: Mobile & Wireless Computing Satellite Systems Objective: • To introduce satellite communications and provide details of the particulars of satellite systems design

2. Outline • Introduction • History • Basics • Categorization of satellite systems • Geostationary earth orbit (GEO) • Medium earth orbit (MEO) • Low earth orbit (LEO) • Routing • Localization

3. Overview of the main chapters Chapter 10: Support for Mobility Chapter 9: Mobile Transport Layer Chapter 8: Mobile Network Layer Chapter 4: Telecommunication Systems Chapter 5: Satellite Systems Chapter 6: Broadcast Systems Chapter 7: Wireless LAN Chapter 3: Medium Access Control Chapter 2: Wireless Transmission

4. Introduction • Satellite is a system that supports mobile communications • It offers global coverage without wiring costs for base stations and is almost independent of varying population densities • Two or more stations on Earth • Called ‘Earth Stations’ • One or more stations in Earth Orbit • Called ‘Satellites’ • Uplink = transmission to satellite • Downlink = transmission to earth station • The satellite converts uplink transmissions into downlink transmission via a ‘transponder’

5. History of satellite communication Satellite communication began after the Second World War when scientists knew that it was possible to build rockets that would carry radio transmitters into space. 1945 Arthur C. Clarke publishes an essay about “Extra Terrestrial Relays” 1957 first satellite SPUTNIK by Soviet Union during the cold war 1960 first reflecting communication satellite ECHO by US 1963 first geostationary satellite SYNCOM for news broadcasting 1965 first commercial geostationary satellite “Early Bird“ (INTELSAT I): 240 duplex telephone channels or 1 TV channel, 1.5 years lifetime 1976 three MARISAT satellites for maritime communication 1982 first mobile satellite telephone system INMARSAT-A 1988 first satellite system for mobile phones and data communication INMARSAT-C (data-rates about 600 bits/s) 1993 first digital satellite telephone system 1998 global satellite systems for small mobile phones

6. Applications • Traditionally • Weather forecasting: several satellites deliver pictures of the earth. • Radio and TV broadcast satellites: hundreds of radio and TV programs are available via satellite. This technology competes with cable in many places as it is cheap • Military satellites • Satellites for navigation and localization (e.g., GPS). Almost all ships and aircraft rely on GPS in addition to traditional navigation systems.

7. Applications • Telecommunication • Global telephone backbones: one of the first applications was the establishment of international telephone backbones. However, these satellites are increasingly being replaced by fiber optical cables crossing the oceans. • Connections for communication in remote places or underdeveloped areas • Global mobile communication: the latest trend is the support of global mobile data communication. Due to high latency, GEO satellites are not ideal for this task, but satellite in lower orbits are used. The purpose is not to replace the existing mobile phone network but to extend the area of coverage. • Satellite systems to extend cellular phone systems (e.g., GSM or AMPS)

8. 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 • Footprint: area on earth that is covered by satellite (where signals of satellite can be received) • 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

9. Inclination plane of satellite orbit satellite orbit d inclination d equatorial plane

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

11. Evolving of Satellite Systems • At the beginning satellite systems were simple transponders. • Transponders receive a signal on one frequency, amplify it and transmit on another frequency. • Only analog amplification was possible at the beginning • The use of digital signals allows for signal regeneration • The satellite decodes the signal into a bit stream and codes it again into a signal – higher quality of the received signal • Today’s communication satellites provides many functions of higher communication layers, e.g., inter-satellite routing and error correction.

12. Satellite Systems Inter Satellite Link (ISL) Mobile User Link (MUL) MUL Gateway Link (GWL) GWL base station or gateway footprint GSM ISDN PSTN User data PSTN: Public Switched Telephone Network

13. Link Problems of Satellites • Propagation delay • Propagation loss of signals depends on distance, angle and atmospheric condition • 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 • varying strength of received signal due to multipath propagation • interruptions due to shadowing of signal (no LOS) • Possible solutions • satellite diversity (usage of several visible satellites at the same time) helps to use less sending power

14. Satellite Communications • Categorisation • Coverage area: global, regional or national. Larger systems require more satellites • Service type: fixed satellite service (FSS), broadcast satellite service (BSS), or mobile satellite service (MSS)

15. Satellite Communications • Design considerations • Area/coverage; some satellites can cover almost 33% of earths surface, transmission cost becomes invariant of distance • Bandwidth; is a very limited resource. • Transmission quality; is usually very high, though delay can be up to ¼ second • Frequency bands: • C-band (4 and 6 GHz) • Ku-band (11 and 14 GHz • Ka-band (19 and 29 GHZ)

16. Satellite Communications • Orbit • Can be circular or elliptical around the center of earth • Can be in different (e.g. polar or equatorial) or same planes • Can be Geostationary (GEO), Medium (MEO) or Low (LEO) • Coverage is affected by objects such as buildings, by atmospheric attenuation, and electrical noise from earth GEO MEO LEO

17. Orbits Three different types of satellite orbits can be identified depending on diameter of the orbit: • GEO (Geostationary Earth Orbit), 36000 km above earth surface • LEO (Low Earth Orbit): 500 - 1500 km • MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit): 6000 - 20000 km GEO (Inmarsat) MEO (ICO) LEO (Globalstar,Irdium) inner and outer Van Allen belts earth 1000 10000 35768 km

18. Satellite Communications: GEO • Geostationary Earth Orbit (GEO) • Proposed by Arthur C Clarke in 1945 and have been operational since 1960s • Same speed as Earth • Appears to stay still • 35,863km above the Earth above Equator • Common for early applications like Weather and military

19. Geostationary Satellites (cont) • Orbit 35,786 km distance to earth surface, orbit in equatorial plane (inclination 0°) • complete rotation exactly one day, satellite is synchronous to earth rotation • fix antenna positions, no adjusting necessary • satellites typically have a large footprint (up to 34% of earth surface!), therefore difficult to reuse frequencies • bad elevations in areas with latitude above 60° due to fixed position above the equator • high transmit power needed • high latency due to long distance (0.24 sec) • not useful for global coverage for small mobile phones and data transmission, typically used for radio and TV transmission

20. Geostationary Satellites (cont) • GEO • Advantages • Relative stationary property means frequency changes are not a problem • Tracking by Earth stations is simple • Can ‘see’ huge areas, so less satellites needed • Disadvantages • 35,000km is a long way for signals to travel • Polar regions not well served • Long delay… (2 * 35,863)/300000 = 0.24s

21. Satellite Communications: LEO • Low Earth Orbit (LEO) • Circular or Elliptical orbit, under 2000km • Often in polar orbit at 500 to 1500 km altitude • Appear to move, usually 1.5 to 2 hours to orbit once • Coverage diameter about 8000km • Delay low, about 20ms • Only visible to Earth stations for about 20 minutes • Frequencies change with movement (Doppler shifts)

22. Low Earth Orbit (cont) • Requires many satellites in many planes for global coverage • Small foot-print, better frequency reuse • Satellites must communicate with each other to hand- over signals • More complex system • Cheaper kit with better signal strength, and bandwidth efficiency • Used in mobile communications systems, with increased use in 3G systems

23. Satellite Communications: MEO • Medium Earth Orbit (MEO) • Altitude 6000 to 20000km • 6 hour orbits • Coverage diameter 10000 to 15000km • Signal delay <80ms • Visible for a ‘few’ hours • Proposed for data communication services

24. MEO systems • comparison with LEO systems: • slower moving satellites • less satellites needed • simpler system design • for many connections no hand-over needed • higher latency, ca. 70 - 80 ms • higher sending power needed • special antennas for small footprints needed • Example: ICO (Intermediate Circular Orbit, Inmarsat) start 2000

25. Satellite Communications • Satellite Network Configurations • Point to Point • Two earth stations and one satellite • Broadcast Link • One earth transmitter, one satellite, many receivers

26. Satellite Communications • VSAT (Very Small Aperture Terminal) • Two-way communications via ground hub • Subscribers have low cost antennas • Subscribers communicate via hub

27. 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

28. Localization of mobile stations • Mechanisms similar to GSM • Gateways maintain registers with user data • HLR (Home Location Register): static user data • VLR (Visitor Location Register): (last known) location of the mobile station • SUMR (Satellite User Mapping Register): • satellite assigned to a mobile station • positions of all satellites • Registration of mobile stations • Localization of the mobile station via the satellite’s position • requesting user data from HLR • updating VLR and SUMR • Calling a mobile station • localization using HLR/VLR similar to GSM • connection setup using the appropriate satellite

29. Summary • The trend for communication satellite is moving away from big GEOs, towards the smaller MEOs and LEOs for the reason of lower delay. • Special problems of LEOs is the high system complexity and the relatively short lifetime • Most LEO satellites fly over non or sparsely populated areas- too few customers • A new application for satellite is the satellite digital multi-media broadcasting