Cover Story: Satellite Radio: A Strong Signal, A Great Sound
by Peter J. Brown
Listen up. There is a strong chance that by 2003, the next new car you buy will come equipped not only with a built-in GPS unit but perhaps even a mobile satellite terminal that offers an almost unlimited supply of music in every possible genre.
Satellite engineers are devising new satellite payloads that are designed to handle traffic in the 1452 to 1492 MHz range (L-band) as well as in the 2320 to 2345 MHz (S-band) range. One result is a new generation of digital satellite radio services that is about to transform the American driving experience.
Two new Satellite Digital Audio Radio Service (SDARS) providers, Washington, DC-based XM Satellite Radio (XM) and New York City-based Sirius Satellite Radio (formerly CD Radio), must prove that there is a vast mobile audience in the United States willing to pay between $10 and $13 per month for their S-band services.
At the same time, Washington, DC-based Worldspace has a pair of L-band satellites, Afristar at 21 degrees E and Asiastar at 105 degrees E, beaming radio content and data down to fixed and portable receivers used by growing audiences in Africa and Asia.
Satellite radio and audio offerings will remain an essential ingredient in the broadband multimedia domain for emerging service providers such as Tokyo-based Mobile Broadcasting Corp. (MBC) of Japan, which has Space Systems/Loral (SS/L) designing and building its MBSat. Addressing the audio-related needs of customers in the 21st century is seen as a top priority.
What binds all of the ventures together? A shared requirement for some form of terrestrial component. The use of so-called “gap fillers” (aka terrestrial repeater technology by SDARS and 2 GHZ MSS providers) has generated considerable opposition in the United States today from groups like the Cellular Telecommunications and Internet Association (CTIA) and the National Association of Broadcasters (NAB). In mid-September the FCC granted XM and Sirius special temporary authority to use terrestrial repeaters.
More Affordable Pricing
With two of its three planned satellites now on station, Worldspace opened the door for a whole new category of global satellite service providers. Radio broadcasters are signing on quickly. The list includes the BBC, CNN, ABC Radio, Bloomberg and RFI. While services to audiences in Africa, Asia and Latin America have been planned for some time, Worldspace has also discovered that southern Europe is covered by the footprint of Afristar.
Alcatel Space performed the payload and end-to-end system design work and served as system prime contractor for the Astrium Eurostar 2000 L-band (1452 to 1492 MHz) payloads for Worldspace as well as serving as S-band payload supplier to Boeing for XM. In addition, Worldspace and Alcatel Space are jointly pursuing the creation of a Satellite-Digital Sound Broadcasting (S-DSB) consortium for Europe.
Worldspace is a push-centric broadcast entity, and it intends to stick to that model. It is not seeking to compete in the two-way market with Inmarsat, which has always been a successful provider of two-way satellite services on a global basis for users both on land and sea. Worldspace is not designed for two-way services, although it would be open to a terrestrial return capability.
Operators run their own segments of the Worldspace regional network by way of feeder link stations. Access to each Worldspace satellite for a so-called processed mission is accomplished via a Frequency Division Multiplex Access (FDMA) uplink using 7 GHz (X-band) 2.4-meter VSATs equipped with sub 100 W RF amplifiers. This complements the transparent half of the Worldspace capacity and is available in addition to the conventional TDM 7025 to 7075 MHz uplink beamed from a larger 6-meter earth station with an L-band TDM downlink.
Otherwise, digital audio coding is based on MP3 (the ISO MPEG-2 Layer 3 standard) with source bit rates ranging from 16 kbps for AM quality mono to 64 kbps for stereo, CD-like quality. Each Worldspace satellite operates in a triple beam configuration with a capacity anywhere between 50 to 200 programs per beam downlinked via TDM QPSK with high efficiency concatenated forward error correction (FEC) using a pair of 2.5-meter satellite antennas.
“There is a lack of audio services in our target market, and data is absent as well. Lots of customers need our type of service, including corporations eager to bypass poor telco infrastructure,” says Worldspace spokesman Nicholas Braden.
“We have 14 licensed receiver manufacturers including Matsushita, JVC, Hitachi and Sanyo,” adds Braden. “The price of the receivers continues to drop, and this is making them more affordable. Worldspace units cost $125 to $250 or more for the original receivers, and that has dropped to $75 to $100 out of the factory today.
“We have a good bouquet of content now, along with a 64 kbps closed user group system for corporations and non-governmental agencies (NGOs),” says Braden.
Recently, Worldspace began testing a weather data distribution and warning service for aircraft, and in-flight trials involving NASA, Rockwell Collins, and American Airlines have been under way on American flights between Chicago and Japan. Fishermen in remote areas such as along the coast of Gujurat, India, have also benefited from this service.
Worldspace intends to build its mobile vehicle business with the use of terrestrial repeaters, and is broadening its mix of services to include data and multimedia content as well. PC cards are undergoing tests. Worldspace has completed tests of terrestrial repeaters with prototype receivers in Pretoria, South Africa, using a system developed by Germany-based Fraunhofer (FhG) Institute.
“In terms of terrestrial repeater technology, we need to shrink the FhG solution into a chipset, following the same path as XM,” says Tarek Abdel-Nabi, vice president of space and mobile systems development. “There is a need to adapt the Multi-Carrier Modulation (MCM) technique for this application. After all, this Worldspace licensed technology which lies at the core of the system that XM has developed was optimized for S-band.”
According to Jean-Francois Gambart, vice president of marketing and business development at Toulouse, France-based Alcatel Space Industries, Olivier Courseille deserves credit as one of the inventors of both Worldspace and XM with conceptual input dating back to 1994. S. J. Campanella, executive vice president and chief scientist at Worldspace, is the chief architect of the entire system.
“The problem with SDARS (and S-DSB) is guaranteeing reception with an antenna the size of the palm of your hand,” says Gambart. “To achieve this goal, you need a very powerful RF signal beamed toward earth. It has to be far more powerful than any other type of satellite for telecom, DTH, GPS, or MSS because the necessary throughput of the digital stream which is in the range of 1 Mbps to 2 Mbps combined with the limited reception characteristics of the terminal.”
Whereas a typical Ku-band DTH satellite transponder uses RF power generally between 100 to 200 W in Ku-band, the six Worldspace transponders are rated 300 W in L-band, while the twin XM transponders run at 2,800 W in S-band, according to Gambart.
“This is a unique feature. XM operates a big Boeing 702 with 17 kW of power with only two very high power transponders associated with two large 5-meter reflector antennas. To achieve this required performance, Alcatel has developed unique techniques to parallel up to sixteen 216 W TWTAs for XM with an active phase control feedback loop and a clever algorithm to configure the transponder in case a single tube fails or performs out-of-spec,” says Gambart.
Redefining A Satellite
On June 28, Boeing Satellite Systems handed over the second of three Boeing 702 satellites to XM–the third is an on-ground spare–named XM Roll which was locked into geostationary orbit at 85 degrees W, joining XM Rock which had been deployed a few weeks earlier at 115 degrees W.
In September, Boeing revealed that an anomaly impacting the performance of solar arrays on its 702’s had been detected, and that this could impact the lifespan of satellites operated by several operators besides XM, including Thuraya, Telesat Canada and Panamsat.
As mentioned earlier, the XM’s S-band payload, which has been designed and developed by Alcatel Space Industries, uses twin sets of sixteen 216-watt TWTAs with six spares to drive a pair of transponders. The net result is approximately 2,800 to 3,000 W of RF signal power. XM has access to the upper portion of the FCC’s S-band downlink allocation ranging from 2332.5 to 2345 MHz.
“We have redefined what satellites can do. Using a 5-meter antenna on the satellite, we achieved an EIRP at full Conus in excess of 70 dBW, and nobody in the commercial realm has done this before,” says Derek de Bastos, vice president of space segment at XM Radio. “This level of power is necessary to get through various blockages. Unlike, Ku-band, our S-band downlink is immune to rain fade.”
According to de Bastos, XM Radio was also the first external customer for the new Boeing 702.
“We licensed Worldspace technology. However, we took that technology and made it mobile. Initially, we did not have such a high output power in our original payload design,” says de Bastos.
“Another difference is that the XM payload is entirely transparent whereas with Worldspace, transparent; and processed payloads are combined on a single payload,” he adds. “Otherwise, many units are exactly the same.” Telesat Canada provides all the monitoring and control (M&C) out of Ottawa for the Rock and Roll satellites as well as satellite fleet support services.
“Basically it is very simple, although paralleling up all the TWTAs to maintain a stable output can be a bit tricky. However, there is no reason to set gains everytime a new customer comes up,” says de Bastos.
XM estimates that it will cost more than $200 million to deploy its nationwide terrestrial repeater system in approximately 70 cities and metropolitan areas. In 1999, a contract was signed with LCC International Inc. for engineering and site preparation. That same year, XM signed another contract with Hughes Electronics Corp. for the hundreds of terrestrial repeaters necessary to fill any gaps in XM Radio’s satellite coverage.
The Sounds Of Geosynchronicity
The GEO realm includes lots of satellites in inclined orbits. Usually, this sets the stage for the final chapter in a satellite’s career. In this instance, we will examine the three satellites launched by Sirius, which are beginning their lifetimes in highly inclined, figure-8 or so-called geosynchronous orbits. The trio of Sirius S-band satellites–not to be confused with the Sirius fleet serving Scandinavia–are all SS/L FS1300s that were launched by ILS on Protons in Baikonur in 2000.
Sirius operates its payload uplinks in the 7060-7072.5 MHz (Earth-to-space) frequency band, and the payload downlink in the 2320-2332.5 MHz (space-to-Earth) frequency band. Telemetry, tracking and command (TT&C) functions are carried out in the 6422-6425 MHz (Earth-to-space) frequency band, and in the 4196.375-4197.125 MHz (space-to-Earth) frequency band.
“Each satellite consists of forty-eight 120 W S-band direct-radiating high-power traveling wave tubes (TWTs) with 32 operating, and the remaining 16 for redundancy,” says S. Paul Sharma, vice president of space segment at Sirius. “The 120-W TWTs are paired by a combination of coaxial and waveguide combiner design that allows for precise phase matching of the output of the TWTs.”
Each pair is referred to as a DTWTA or Dual TWTA, including the power supply. The DTWTAs are arranged into quadrants consisting of 6-for-4 ring redundant arrays of matched pairs, according to Sharma. Thus, if any two DTWTAs failed in any of the quadrants, the system can still operate at full capacity. The DTWTAs are phased combined into a single transponder output that generates approximately 3,840-W RF power.
“An automatic level control (ALC) is utilized to maintain a constant input to the DTWTAs. The ALC can accommodate a variation of 20 dB in the uplink power levels,” says Sharma.
The Sirius satellites resemble DBS satellites in the way that the TWTAs are paired. Again, each quad ring has redundancy, so the result is 16 DTWTAs.
“The trick is to combine these into one high-powered output. Our EIRP is much higher than your typical Ku-band satellite operating in the 45 to 55 dBW range. Our satellites offer full Conus coverage in excess of 60.3 dBW with a peak of 67 dBW,” says Sharma.
“The key is how to use existing standard power supply amps to come up with very large EIRP levels, and how to concentrate all available RF power into the very narrow–12.5 MHz- -S-band signal of the DARS system,” says Robert Prevaux, executive director for Sirius and MBSat at SS/L. MBSat is also based on the SS/L 1300, and shares the same topology as Sirius in terms of the number of TWTAs combined in phase. However, MBSat has the advantage of access to a full 25 MHz in its S-band allocation.
Readers need to keep in mind the fact that the mobile omnidirectional antennas used for DARS reception have a much lower gain than the typical fixed DBS antenna, for example. That is why the very high EIRP levels are required.
According to Sharma, when the company was formed under the CD Radio banner, a standard geostationary satellite deployment was planned. With the shift to the highly inclined orbit, the satellites traverse in gentle “figure 8’s” so that a pair of satellites is transmitting their payload signals 16 hours per day.
“This involved shifting to higher elevation angles. Beyond that, flying these satellites in geosynchronous orbit is not very different from the geostationary orbit that one sees with a satellite in a typical end-of-life inclined orbit. When it comes to stationkeeping, the in-plane and out-of-plane maneuvers are quite similar and thus far, we have had no problems,” says Sharma.
“The eclipse period is not standardized, and this helps from an operational perspective. As far as battery life is concerned, the orbital characteristics do not impose any additional power constraints. Everything in terms of batteries and solar arrays are standard,” he adds.
Among other things, Sirius had to convince the FCC that it would not adversely impact GSO FSS operations with respect to TT&C of its highly inclined NGSO. Sirius contended that its TT&C link has a low data rate requirement, and their transmissions will have the potential to interfere with GSO FSS operations only when the Sirius satellite is near the equatorial plane. According to Sirius documents on file at the FCC, the sole NGSO equatorial crossing points are at the nominal ascending node of 65.6 degrees W and the nominal descending node of 126.4 degrees W.
“The passage is in the north-south/south-north directions, respectively, and the duration of each passage is approximately 15 minutes per satellite, per crossing point, per day. Based on its analyses, Sirius asserts that its proposed use of the 4/6 GHz band for TT&C will be secondary to use of the bands by the FSS and it will accept any interference from FSS services provided in the 4/6 GHz bands. Moreover, Sirius has analyzed the operational capabilities of its NGSO design to ensure that its system will continue to operate,” the FCC document states.
“Generally, the Sirius satellites can be satisfactorily operated without 4/6 GHz TT&C during the short time periods for nodal crossings. It indicates that almost all commands especially critical commands occur well away from Sirius’ GSO nodal crossings. Uplink commands in the 6 GHz band are not continuous; they are normally sporadic, short, planned in advance, and they will be transmitted while the satellites are sufficiently far away from the GSO equatorial arc to preclude interference,” the document adds. “Continuous telemetry signals are not necessary to maintain control of the NGSO constellation. Sirius has incorporated into its spacecraft TT&C design the capability to terminate the 4 GHz transmissions during its NGSO equatorial crossing points without harm to the satellite transmitters.”
The FCC conditioned its authorization, requiring Sirius to coordinate with U.S. licensed satellites that are located within 5 degrees of Sirius’ GSO nodal crossings of the satellites’ final geosynchronous orbit. Sirius had to do the same for non-U.S. licensed satellites within 10 degrees S of its nodal crossings.
Sirius operates TT&C stations near Quito, Ecuador, and Utive, Panama, with an emergency backup TT&C facility available in Guaratiba, Brazil. The TT&C stations are remotely controlled from the company’s satellite operation centers (SOCs) in New York City, Hawley, PA, and Three Peaks, CA.
Make Way For Multimedia
In late 2003, a new mobile multimedia broadcasting satellite–an SS/L 1300–known as MBSat is scheduled to arrive in orbit. MBSat will be similar to the Sirius satellites as far as the number of TWTAs combined in phase, although MBSat will use a full 25 MHz for its services at 2630-2655 MHz (S-band) including more than 70 channels of audio, video and data services. This satellite includes a separate 54 dBW Ku-band TDM feed for its terrestrial S-band Code Division Multiplexing (CDM) gap fillers, and, Ku-band uplinking and TT&C.
According to Masaaki Igarashi, a spokesperson for Tokyo-based MBC, Toshiba, which is the largest of MBC’s 42 investors, created the overall MBSat system architecture back in 1995 and 1996. It will use digital signal conditioning technology with Advanced Audio Coding (AAC), MPEG-4 video encoding, and MPEG-2 multiplexing.
MBSat will tap 7400 kW to power twenty-four 120 W S-band transmitters with 2,400 W RF output. It will deploy a 12-meter unfurlable antenna to beam down a 2.6 GHz signal with an EIRP of 67 dBW.
“The satellite will have a somewhat asymmetric configuration with such a large appendage,” says Robert Prevaux, executive director for Sirius and MBSat at SS/L who reminds readers that despite the large size, this antenna reflector does not weigh anything in space. “This large antenna is similar in size and mass to a solar array. It does not really create a problem given our extensive experience with three-axis control systems, and our successful track record with asymmetric satellites such as GOES.”
MBSat will use wideband CDM, and is designed to offer mobile services at a data rate of more than 7 Mbps, doing so at a much lower cost than the evolving IMT-2000 3G system which operates at less than 384 kbps.
The Most Sound Solution
Unlike what unfolded with the earlier launch of DBS services in the United States, the FCC has mandated SDARS systems to be interoperable so that purchasers of automobiles equipped with SDARS receivers have a choice from the start. This will not happen immediately. First generation SDARS receivers will not be interoperable. However, second generation hardware is expected to comply with the FCC mandate.
XM and Sirius also face an enormous domestic radio industry in the United States which is in the midst of a steady and substantial digital transition including the looming rollout of so-called In-band, On-Channel (IBOC) technology. This will allow radio stations to add a digital transmission to existing analog broadcasts.
This mix of SDARS, Worldspace and tomorrow’s multimedia satellites shows how the satellite industry is doing its best to stay on top of a constantly evolving menu of new services for customers worldwide. Satellite executives have been saying for years that their solution represents the best way to reach the enormous audiences, and now the world can clearly hear that message.
Peter J. Brown is Via Satellite’s Senior Multimedia Editor. He lives on Mount Desert Island, ME.