The SuperHighway in the SkyThe SuperHighway in the Sky

ISOM 591

Understanding Modern Telecommunications, Chapter 6

March 13, 2000

SatellitesSatellites

• 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

SatellitesSatellites

• 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

SatellitesSatellites

• 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

Geosynchronous SatellitesGeosynchronous 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

Geosynchronous SatellitesGeosynchronous 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

WirelessWireless

• 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

WirelessWireless

• 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

Wireless: General ApplicationsWireless: 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)

Advantages of Microwave Advantages of Microwave TransmissionTransmission

• 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

Limitations of Microwave Limitations of Microwave TransmissionTransmission• 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

Mobile Satellite SystemsMobile 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

Low Earth-Orbiting SatellitesLow 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

Low Earth Orbiting Satellites Low Earth Orbiting Satellites (LEOs)(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

LEOS: “Teledesic: Internet in LEOS: “Teledesic: Internet in the Sky”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)

International SatellitesInternational 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

International SatellitesInternational 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

Satellite ApplicationsSatellite 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

Satellite ApplicationsSatellite 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

Satellite ApplicationsSatellite 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

Satellite ApplicationsSatellite 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!