Ch7: Satellite Communications

1. Wireless and Mobile Networks Ch7: Satellite Communications

2. Overview • Basics of Satellites • Types of Satellites • Capacity Allocation

3. Satellite Services & Applications • Voice/Video/Data Communications • Rural Telephony • News Gathering/Distribution • Internet Trunking • Corporate VSAT Networks • Tele-Medicine • Distance-Learning • Mobile Telephony • Videoconferencing • Business Television • Broadcast and Cable Relay • VOIP & Multi-media over IP • GPS/Navigation • Position Location • Timing • Search and Rescue • Mapping • Fleet Management • Emergency Services • Remote Sensing • Pipeline Monitoring • Infrastructure Planning • Forest Fire Prevention • Urban Planning • Flood and Storm watches • Air Pollution Management • Geo-spatial Services • Direct-To-Consumer • Broadband IP • Television • Digital Audio Radio • Interactive Entertainment & Games • Video & Data to handhelds

4. Important Milestones (1950’s) • 1956 - Trans-Atlantic cable opened (about 12 telephone channels – operator). • 1957 First man-made satellite launched by former USSR (Sputnik, LEO). • 1958 First US satellite launched (SCORE). First voice communication established via satellite (LEO, lasted 35 days in orbit after batteries failed).

5. Sputnik - I

6. Important Milestones (1960’s) • 1960 First communication satellite launched into space (Large balloons, Echo I and II). • 1962: First non-government active communication satellite launched Telstar I (MEO). • 1963: First satellite launched into geostationary orbit Syncom 1 (comms. failed). • 1964: International Telecomm. Satellite Organization (INTELSAT) created. • 1965 First communications satellite launched into geostationary orbit for commercial use Early Bird (re-named INTELSAT 1).

7. ECHO I

8. Telstar I

9. Basics: How do Satellites Work? • Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means. • The two stations can use a satellite as a relay station for their communication • One Earth Station sends a transmission to the satellite. This is called a Uplink. • The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.

10. Basics: Advantages of Satellites • The advantages of satellite communication over terrestrial communication are: • The coverage area of a satellite greatly exceeds that of a terrestrial system. • Transmission cost of a satellite is independent of the distance from the center of the coverage area. • Satellite to Satellite communication is very precise. • Higher Bandwidths are available for use.

11. Basics: Disadvantages of Satellites • The disadvantages of satellite communication: • Launching satellites into orbit is costly. • Satellite bandwidth is gradually becoming used up. • There is a larger propagation delay in satellite communication than in terrestrial communication.

12. Basics: How Satellites are used? • Service Types • Fixed Service Satellites (FSS) • Example: Point to Point Communication • Broadcast Service Satellites (BSS) • Example: Satellite Television/Radio • Also called Direct Broadcast Service (DBS). • Mobile Service Satellites (MSS) • Example: Satellite Phones

13. Major problems for satellites • Positioning in orbit • Stability • Power • Communications • Harsh environment

14. Positioning • This can be achieved by several methods • One method is to use small rocket motors • These use fuel - over half of the weight of most satellites is made up of fuel • Often it is the fuel availability which determines the lifetime of a satellite • Commercial life of a satellite typically 10-15 years

15. Stability • It is vital that satellites are stabilised • to ensure that solar panels are aligned properly • to ensure that communications antennae are aligned properly

16. Power • Modern satellites use a variety of power means • Solar panels are now quite efficient, so solar power is used to generate electricity • Batteries are needed as sometimes the satellites are behind the earth - this happens about half the time for a LEO satellite • Nuclear power has been used - but not recommended

17. Harsh Environment • Satellite components need to be specially “hardened” • Circuits which work on the ground will fail very rapidly in space • Temperature is also a problem - so satellites use electric heaters to keep circuits and other vital parts warmed up - they also need to control the temperature carefully

18. Alignment • There are a number of components which need alignment • Solar panels • Antennae • These have to point at different parts of the sky at different times, so the problem is not trivial

19. Satellite - satellite communication • It is also possible for satellites to communicate with other satellites • Communication can be by microwave or by optical laser

20. Orbits • Circular orbits are simplest • Inclined orbits are useful for coverage of equatorial regions • Elliptical orbits can be used to give quasi stationary behaviour viewed from earth • using 3 or 4 satellites • Orbit changes can be used to extend the life of satellites

21. Types of Satellites • Satellite Orbits • GEO • MEO • LEO • Frequency Bands • L–Band S-Band C-Band X-Band Ku-Band K-Band Ka-Band

22. Geostationary Earth Orbit (GEO) • These satellites are in orbit 35,863 km above the earth’s surface along the equator. • Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth.

23. GEO (cont.) • In the equatorial plane • Orbital Period = 23 h 56 min. 4.091 s = one Sidereal Day (defined as one complete rotation relative to the fixed stars) • Satellite appears to be stationary over a point on the equator to an observer • Radius of orbit, r, = 42,164.57 km NOTE: Radius = orbital height + radius of the earth Average radius of earth = 6,378.14 km

24. GEO (cont.) • Advantages • A GEO satellite’s distance from earth gives it a large coverage area, almost a fourth of the earth’s surface. • GEO satellites have a 24 hour view of a particular area. • These factors make it ideal for satellite broadcast and other multipoint applications.

25. GEO (cont.) • Disadvantages • A GEO satellite’s distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to point communication. • GEO satellites, centered above the equator, have difficulty broadcasting signals to near polar regions

26. Early satellites • Relay • 4000 miles orbit • Telstar • Allowed live transmission across the Atlantic • Syncom 2 • First Geosynchronous satellite

27. SYNCOM 2 • Picture from NASA

28. Geostationary Orbit Today

29. Low Earth Orbit (LEO) • LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1,500 km above the surface. • LEO satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass. • A network of LEO satellites is necessary for LEO satellites to be useful

30. LEO (cont.) • Advantages • A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication. • A LEO satellite’s smaller area of coverage is less of a waste of bandwidth.

31. LEO (cont.) • Disadvantages • A network of LEO satellites is needed, which can be costly • LEO satellites have to compensate for Doppler shifts cause by their relative movement. • Atmospheric drag effects LEO satellites, causing gradual orbital deterioration.

32. Medium Earth Orbit (MEO) • A MEO satellite is in orbit somewhere between 8,000 km and 18,000 km above the earth’s surface. • MEO satellites are similar to LEO satellites in functionality. • MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to 8 hours. • MEO satellites have a larger coverage area than LEO satellites.

33. MEO (cont.) • Advantage • A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network. • Disadvantage • A MEO satellite’s distance gives it a longer time delay and weaker signal than a LEO satellite, though not as bad as a GEO satellite.

34. Frequency Bands • Different kinds of satellites use different frequency bands. • L–Band: 1 to 2 GHz, used by MSS • S-Band: 2 to 4 GHz, used by MSS, NASA, deep space research • C-Band: 4 to 8 GHz, used by FSS • X-Band: 8 to 12.5 GHz, used by FSS and in terrestrial imaging, ex: military and meteorological satellites • Ku-Band: 12.5 to 18 GHz: used by FSS and BSS (DBS) • K-Band: 18 to 26.5 GHz: used by FSS and BSS • Ka-Band: 26.5 to 40 GHz: used by FSS

35. Capacity Allocation • FDMA • FAMA-FDMA • DAMA-FDMA • TDMA • Advantages over FDMA

36. FDMA • Satellite frequency is already broken into bands, and is broken in to smaller channels in Frequency Division Multiple Access (FDMA). • Overall bandwidth within a frequency band is increased due to frequency reuse (a frequency is used by two carriers with orthogonal polarization).

37. FDMA (cont.) • The number of sub-channels is limited by three factors: • Thermal noise (too weak a signal will be effected by background noise). • Intermodulation noise (too strong a signal will cause noise). • Crosstalk (cause by excessive frequency reusing).

38. FDMA (cont.) • FDMA can be performed in two ways: • Fixed-assignment multiple access (FAMA): The sub-channel assignments are of a fixed allotment. Ideal for broadcast satellite communication. • Demand-assignment multiple access (DAMA): The sub-channel allotment changes based on demand. Ideal for point to point communication.

39. TDMA • TDMA (Time Division Multiple Access) breaks a transmission into multiple time slots, each one dedicated to a different transmitter. • TDMA is increasingly becoming more widespread in satellite communication. • TDMA uses the same techniques (FAMA and DAMA) as FDMA does.

40. TDMA (cont.) • Advantages of TDMA over FDMA. • Digital equipment used in time division multiplexing is increasingly becoming cheaper. • There are advantages in digital transmission techniques. Ex: error correction. • Lack of intermodulation noise means increased efficiency.

41. Current Trends in Satellite Communications • Bigger, heavier, GEO satellites with multiple roles • More direct broadcast TV and Radio satellites • Expansion into Ka, Q, V bands (20/30, 40/50 GHz) • Massive growth in data services fueled by Internet • Mobile services: • May be broadcast services rather than point to point • Make mobile services a successful business?