For those of us old enough to remember the days when only the Moon orbited the Earth, instantaneous global communication is something of a technological miracle. The space program may have been born of the Cold War, but since the launch of Telstar and the 1967 “Our World” broadcast, it has been increasingly difficult for news and information to remain sequestered because of geographic distances.
Many pieces
Today, global communication by satellite is often taken for granted. The engineering effort required to design, construct, implement, maintain and operate all subsystems of a satellite system is enormous. There’s the design and construction of the satellite, maintaining earth stations and deployment of transmit and receive dishes. And one other small detail, the rocket launch of the satellite into orbit!
The satellite and launch
Satellites have grown from relatively small sizes (less than 3x3ft) to the size of an SUV and larger. Launch operations have migrated from NASA to the private sector.
Equipment on a satellite can be divided into payload and bus. The physical vehicle, attitude control, thermal control and telemetry are among bus subsystems. The payload consists of all the resources between receive and transmit antennas and is referred to as the transponder.
Transponders, in a generic functional-block description, consist of redundant wideband receivers with a low noise amplifier (LNA), input demultiplexer, and a traveling wave tube (TWT) power amplifier. Frequency reuse takes advantage of RF carriers with opposite polarization to squeeze the maximum number of channels into transponder bandwidth.
The launch, once under government control by NASA, is now provided as a service. Companies such as Sea Launch, a division of Boeing, the European Space Agency and the Russian Satellite Communications Company routinely (if a rocket launch can ever be routine) place communication satellites in orbit.
The realization of Arthur C. Clark’s geostationary orbit concept is a cornerstone of communication satellite methodology. With the “bird” in a fixed position, defined portions of the earth are always able to send and receive signals. This enables content backhaul, contribution and direct-to-home broadcasting.
The Earth station
Large dishes are the most conspicuous portion of an earth station. Behind the scenes engineering technology is fine-tuned for satellite operation. Redundant signal paths with the ability to instantly switch in or out any component are mandatory and insure continuous operation. With the location of earth stations in geographically diverse locations, remote monitoring is a necessity. Notification, analysis and rectification of a malfunction can frequently be corrected without a trip to the “shack.”
A typical DTV satellite transmission system includes conditional access, error correction, modulation, RF upconversion, power boost and transmission. The typical processing chain of packetization and energy dispersal, Reed-Solomon forward error correction (FEC), interleaving, convolutional inner coding and mapping and baseband shaping and modulation is employed. Each of these processes is optimized for satellite environments.
Frequency bands
The conditioned signal is now ready for modulation on an intermediate frequency (IF), often 70MHz, carrier. Then the IF signal is heterodyned on an L band (1.0 – 2.0GHz) carrier.
Binary and quarternary phase shift key (BPSK, QPSK) digital modulation is frequently used in the final digital modulation step. Two BPSK or four QPSK different phase signals each represent one or two bits of data, respectively.
Dedicated frequency bands are used for satellite transmission. Different frequencies are used for uplink and downlink. The C band uses 6GHz for uplink and 4GHz for downlink, designated 6/4GHz. K band is 27/18GHz. Ku (under K) is 18/12GHz.
Direct to home
The use of the Ku band and the subsequent shrinking of the dish size necessary to receive a useable signal have facilitated satellite delivery of programming to the home. Gone are the 6ft-wide dishes in early adopter backyards of the 1980s.
EchoStar's DISH Network and DirecTV are trying to match features offered by cable system operators by introducing HD decoder boxes with DVR capabilities. The DISH Network ViP 622 is a multi-room HD satellite receiver that can record 25 hours of HD material. MPEG-4 decoding is a new feature. Two receivers are contained in the box, so it is really a “two room” receiver.
Delivery standards
European DVB-S2 and ATSC have specified satellite technical standards. Proprietary technologies are also in use.
DVB
Binary phase shift keying (BPSK) modulation is used in DVB satellite transmission systems. DVB-S specifies framing structure, channel coding and modulation for 10.7 - 12.75GHz satellite services.
DVB-S2, defines second generation framing structure, channel coding and modulation systems for broadcasting, interactive services, news gathering and other broadband satellite applications.
ASTC in orbit
The ATSC has not been idle about satellite broadcasting standards. Specifications for professional contribution and distribution as well as consumer direct-to-home implementations have been adopted.
A/80: Modulation and Coding Requirements for Digital TV (DTV) Applications Over Satellite defines modulation and coding of data delivered over satellite for digital television contribution and distribution applications. QPSK, 8PSK and 16 QAM modulation modes are included, as well as a range of forward error correction techniques.
A/81: Direct-to-Home Satellite Broadcast Standard describes the emission system for ATSC direct-to-home (DTH) satellite broadcast system and defines extensions to audio, video, transport, PSIP and data broadcasting subsystems as defined in ATSC Standards A/53B, A/65A and A/90.
Compression
Compression for satellite transmission has played a large role in enabling digital broadcasting. Adaptation of satellite compression schemes directly led to the realization of all-digital HDTV.
Today, many vendors provide compression equipment. Scopus Video Networks offers DVB-S2 digital broadcast platforms. The UE-9217/9218 features simultaneous MPEG-2 and MPEG-4 encoding with DVB-S2 modulation and upconversion. L-Band, ASI and IP outputs are concurrently available.
TANDBERG Television has implemented an HD MPEG-4 AVC compression engine in its soon-to-be-launched EN5940 unit that enables a transponder to deliver more HD channels. A 36MHz transponder with DVB-S2 and QPSK can fit more than six channels of full-resolution HD, and, with DVB-S2 and 8PSK, more than eight channels of full-resolution HD.
Issues
One problem with satellite transmission is the solar outage that occurs when the satellite lines up directly in front of the sun. The PamAmSat Web site, which is still active after its absorption by IntelSat, has a calculator that determines solar outage periods based on latitude, longitude, date and time of day. The site also explains how the calculations are done and why outages occur.
Orbital slots are limited and regulated by the FCC and ITU. The four-degree separation requirement has been reduced to two degrees because of transmission and satellite technology improvements.
At higher frequencies in the Ku, and K bands, signals become more susceptible to degraded performance from weather effects. Rain fade occurs because rain and snow absorb microwave RF above 11GHz. This can result in signal loss to DBS subscribers.
On the beam
Uptake of DBS service now totals about 23 percent of all U.S. households. Many of these subscribers have migrated away from cable. Local HD and SD OTA broadcast channels are available.
Besides being used for backhaul, satellite distribution of programming to affiliates and MSO headends is an established channel for content distribution. And satellite radio is having a transforming impact on the radio industry.
Links to Web sites that provide more information about all aspects of communications satellites can be found at the palowireless Satellite Resource Center.
The Satellite Industry Links Web site is a comprehensive listing of satellite manufacturers, launch services, space agencies, broadcasters and many other industry participants.
