The SuperHighway in the Sky

1. The SuperHighway in the Sky ISOM 591 Understanding Modern Telecommunications, Chapter 6 March 13, 2000

2. Satellites • satellites simply reflect and amplify signals transmitted to them • the transponder provides this function and the typical unit is equivalent to 36 Mhz of bandwidth • a transponder receives a signal, amplifies it, and transmits it to Earth • signal to the satellite is called an uplink, from a satellite is called a downlink

3. Satellites • satellites simply reflect and amplify signals transmitted to them • the transponder provides this function and the typical unit is equivalent to 36 Mhz of bandwidth • a transponder receives a signal, amplifies it, and transmits it to Earth • signal to the satellite is called an uplink, from a satellite is called a downlink

4. Satellites • cross-strapping allows a satellite to receive a signal on one frequency and transmit it on another • the size of the footprint (area covered by the signal) can be controlled • intersatellite links can allow carriers to transmit signals between satellites • satellites can be characterized by their orbits

5. Geosynchronous Satellites • geosynchronous (or geostationary) • rotating at the same angular speed as earth • a stationary target for signals uplinked by Earth stations • footprint or coverage is about 1/4 to 1/3 of the earth • a single satellite can be used to satisfy demand from many nations • a 4 degree spacing required to keep satellites from interfering with one another, thus the number of satellites is limited

6. Geosynchronous Satellites • delay for signals travelling at the speed of light (186,00 miles/sec) to the satellite and back would be 2*22,300/186,000 = 0.24 sec. • latency or delay a real problem in data transmission • transaction-oriented applications and associated protocols with a large number of requests and responses result in too many roundtrips • Useful for broadcasting over large areas

7. Wireless • Unguided media for which an antenna radiates electromagnetic energy into the medium (air) and another antenna acts as the receptor • directional (point to point) and omnidirectional (like radio) • the dish is the most common type of microwave antenna • a series of microwave antennas can be used

8. Wireless • Major applications include long distance telecommunications (long distance international links), point to point trunks between telephone exchange offices, television distribution, direct broadcast satellite (ex: DirecTV), closed circuit TV, VSATs, and data links between LANs • in international telecom, used to bypass the local telephone company

9. Wireless: General Applications • microwave provides high capacity links in many established and emerging networks • geosynchronous orbiting satellites, low orbiting satellites • analog cellular phones • digital cellular phones promise more clarity and more capacity • wireless PCS (personal communication systems)

10. Advantages of Microwave Transmission • Common frequencies are in the range of 2 to 40 GHz, thus this has high frequency, high bandwidth, and a high potential data rate • global access to information for a mobile workforce • mobility within the workplace and home • ability to overcome environmental obstacles • cable is often impractical

11. Limitations of Microwave Transmission • Attenuation (repeaters or amplifiers need to be placed 10 to 100 km apart) • interference (ex: weather, airplanes) • disruption of radio frequency transmissions • security is a concern • transmissions areas overlap • expensive and limited availability

12. Mobile Satellite Systems • MSAT 1-2 GHz L-band frequency • American Mobile Satellite Corp., TeleSat Mobile, Inc. of Canada • mobile communications providers use MEO (middle earth orbit) at altitudes of about 10,000 km or LEO’s

13. Low Earth-Orbiting Satellites • inexpensive satellites which orbit the earth about 200 to 700 miles above the surface • signal is stronger than that of the geosynchronous satellites • coverage can be localized so less spectrum can be consumed • services include personal communication services, radiodetermination service (location services like GPS), two-way messaging, paging, fax, data

14. Low Earth Orbiting Satellites (LEOs) • for orbits closer to earth, less power is needed • these are not geosynchronous and orbital speed relative to earth is increased • footprints are smaller • thus, a constellation of satellites is needed to provide services like cellular

15. LEOS: “Teledesic: Internet in the Sky” • Teledesic, Motorola, Boeing, McCaw, Gates, Matra Marconi Space (France and UK) • http://www.teledesic.com/ • http://www.spacer.com/spacecast/news/ teledesic-97d.html • world’s first network to provide fiber-like access to telecom services including linking enterprise computing networks, broadband Internet access, videoconferencing and other digital needs • licensed in March 1997 by FCC and in November, 1997 by the ITU, service to begin in 2003 • other projects under development include Odyssey (TRW), GlobalStar (QualComm)

16. International Satellites • no single location or route needs to generate a sufficient volume of traffic to justify its investment since such a large area is covered • Satellites can be used for point-to-multipoint service like video distribution, broadcast and news services • medium for international telecommunications

17. International Satellites • until the middle 1980s, only the US, the European Space Agency, and the former Soviet Union had satellite launching technology • recently, China, Israel, Japan, Norway, Australia, to name a few, have launched satellites • lifetime expectation is about 15 years • satellites and submarine cables are complementary media

18. Satellite Applications • Globalstar is a low orbiting earth satellite network • 48 satellites in orbit covering 70 degrees north and south of the equator • acts as a communications network to ground stations which connect the user to the land network • this means you can roam the globe with your cellular phone • work sites in remote areas can use the Immarsat (International Maritime Satellite Organization) satellite to gain access to the global phone system

19. Satellite Applications • Teledesic • proposed by Microsoft and McCaw • global network of 840 low-earth (435 miles) satellites using ATM to carry voice, data, video, and other multimedia data • users can access this network using small hand-held devices

20. Satellite Applications • DirecTV • Hughes and USSB • the DirecTV satellite circumvents cable and broadcast TV to offer interactive television directly to subscribers • programming is transmitted as digital video with 16-bit stereo and audio, compatible with NTSB and HDTV • a back channel operates over a modem to make home shopping and interactive games available to the subscriber

21. Satellite Applications • Primestar, AlphaStar, EchoStar, USSB • American Sky Broadcasting system owned by US partners, including WorldCom • health applications such as broadcast of live surgery and diagnostics • to be delivered to the home • need for line-of-sight transmission and launch facilities and insurance!