Fullscreen HTML5Med Res (??MB)HTML4

Did you know there are more radio “broadcasts” out there than those you can receive on AM and FM? They’re called WorldSpace. They’re not intended for “us” but all you need is a satellite dish and a special receiver to open up to the world . . . WORLDSPACE RADIO BY SATELLITE IN AUSTRALIA by Garry Cratt W orldSpace Corporation was founded in 1990 by Noah A Samara and began with the vision of using direct audio broadcasting by satellite to stop the spread of AIDS in Africa. Now its vision is expanded somewhat, providing audio, data and multimedia services primarily to the emerging markets of (so far) Africa and Asia. The WorldSpace system can be used as a powerful tool for spreading knowledge, allowing users to become better educated, live a healthier lifestyle and to become more aware of the environment in which they live. To implement this vision, WorldSpace conceived and built the first-ever satellite radio infrastructure in the world. In the past 12 years, the company has built three and launched two satellites. The full system comprises geosynchronous satellites with coverage over Africa and the Middle East (AfriStar, launched in October 1998), AsiaPacific (AsiaStar, launched in March 2000) and Latin America (AmeriStar, yet to be launched). The three-satellite constellation has a potential audience of 4.6 billion people. The organisation has so far invested $US1.2 billion in the system. The WorldSpace system technology has been licensed and is being used in the USA by the XM Radio Company for the delivery of radio services in S-band across the USA. The WorldSpace satellites are based 56  Silicon Chip on a 3-axis stabilised Eurostar 2000 platform which carries 28-metre solar panels capable of supplying the 6kW required by the 2750 kg satellite. The AsiaStar satellite covering our part of the world was supplied to Alcatel Espace (the WorldSpace prime contractor) by Matra Marconi Space and launched aboard an Ariane V launcher. Signals are uplinked to the satellite on X band (7025-7075MHz) in FDMA (Frequency Division Multiple Access) mode. Unlike conventional C or Ku band systems, the WorldSpace satellite system uses the 1467-1492MHz “L” band spectrum, which was allocated for digital audio broadcasting by the ITU at the World Administrative Radio communication Conference of 1992. Audio and data content is transmitted in encoded 128Kbps MPEG 2.5 layer 3 format. The satellite signal utilises circular polarisation to minimise antenna pointing errors. Using powerful beams, the two existing satellites transmit three overlapping areas of approximately 14 million square kilometres each. The three beams allow for a mix of continent-wide and region-specific programming. Each beam can support up to 50 radio programs. It has been said that the use of digitalisation and audio data compression technologies, combined with satellite transmission, is the biggest single breakthrough since the advent of shortwave radio. Users can purchase a WorldSpace receiver and a “PC adaptor” to interface the receiver to a computer, allowing them to download Internet data, thus expanding the reception capabilities beyond audio to digital multimedia transmissions. This can be delivered by satellite to audiences located in areas where there is no, or poor Internet access. The WorldSpace receiver is also available as a plug in card, to be internally fitted to a personal computer. Listeners in the official coverage areas need only flip up the inbuilt 10cm antenna or place the 10cm external antenna on a windowsill to obtain near CD quality reception. For listeners in fringe areas, WorldSpace markets several Yagi antennas (which unfortunately we found to be of no use in Australia). WorldSpace satellites use onboard processing to allow broadcasters and The target areas for AfricaStar and AsiaStar and the proposed AmeriStar L-band digital audio broadcasting satellites. As you can see, theoretically there’s not much signal available for eastern Australia. But there is some . . . www.siliconchip.com.au This single-band, consolestyle WorldSpace receiver from Sanyo measures 260 x 80 x 180mm and weighs about 1kg. This receiver incorporates 36 memory presets and includes a clock radio and alarm. In target areas, the inbuilt antenna (the circular “dish” on top) is all that is required for reception. multimedia content providers to chose from two options for uplinking their broadcast signals. One option allows them to uplink their programs via a shared hub, while the second option allows for direct uplinking to the satellite using a transmitter, encoder and dish. The satellite is accessed in FDMA mode as this allows maximum flexibility when multiple independent uplink stations are used. In the studio, the broadcaster multiplexes the audio programs on a Broadcast Channel (BC). The uplink station splits the BC into Prime Rate Channels (PRC), each with a capacity of 16kbps for transmission to the satellite. The uplink has the capacity to accommodate up to 288 Prime Rate Channels. The digital processor on board the satellite demultiplexes and demodulates the Prime Rate Channels at baseband and converts them to TDM (Time Division Multiplexing) for Lband transmission of the signal to listeners. The satellite operates a pair of 150W travelling wave tube amplifiers operating in parallel. Within the 25MHz downlink band there are 82 carriers, labelled TDM 1-82. Each beam has two carriers and in the case of the AsiaStar southern beam, these are TDM54 and TDM59. Within each TDM there are 96 Prime Rate Channels of 16 kpbs. As can be seen from the AsiaStar satellite footprint, the signal officially reaches the northern part of Western Australia. However, there is sufficient signal spillover to allow reception in most parts of Australia. Reception in Karratha and Perth is possible with a 1.2m dish while a 2.3m dish is required in Sydney. Signals have been reported in Palau and Guam. The AsiaStar TCR (tracking, control and ranging) functions are provided by www.siliconchip.com.au the WorldSpace Regional operations centre (ROC) in Melbourne. This is backed up by a separate TCR centre in Mauritius. There has been a recent development in future WorldSpace technology with the introduction of a hybrid satellite/ terrestrial DBS delivery system concept. This new hybrid system has the ability to extend the performance of the digital system to deliver robust mobile reception. The system uses selective combining of digital signals from the satellite, with the same digital signal received and repeated by terrestrial stations of a single frequency network. In fact, the terrestrial transmission could take place on existing VHF or UHF bands, leading to the possibility of local traffic information and advertising insertion at the terrestrial transmitter site. The terrestrial delivery system is based on Multi Carrier Modulation (MCM), a multipath resistant Orthogonal Frequency Division Multiplex technique that has gained wide acceptance for high-quality terrestrial mobile reception. The MCM system uses multiple frequencies to avoid frequency selective fading and to narrow the receive signal bandwidth to minimise delay spread. A new second generation WorldSpace receiver will be required, one The “Afristar” satellite (a Eurostar 2000) launched atop an Ariane V launcher in Guiana, October 28 1998. that demodulates both the TDM signals from the satellite and the terrestrial MCM components. The receiver will be backward compatible with the present satellite service. The system was trialled in South Africa (using AfriStar) in late 2000 with successful results. What does all this mean for Australia? In 1993 the Australian government notified the ITU to reserve an orbital location at 155.5°E longitude for DBSTAR, a satellite to provide DSB services across Australia. This notification was revised in 1999 to provide enhanced coverage beyond Australia into the southwest Pacific. The WorldSpace hybrid DBS system could easily be used to provide Australia with universal coverage of all states and territories. All that is required is government support for the orbital allocation. The WorldSpace regional control/operating centre in Melbourne. April 2004  57 Receiving the “impossible” . . . Picking Up WorldSpace on the East Coast As you can see from the AsiaStar footprint, receiving its signal on the west coast of Australia, especially the upper west, should be a doddle. But the east coast, particularly at latitudes as far south as Sydney and Melbourne, should be next to impossible (well, officially at least). We like to take on challenges . . . from the tiny “dishes” required for in-target-area reception – but it goes to prove that if you want to go to the trouble, the signal is there. The patch antenna We set out to determine the minimum requirements for a system in Sydney and discovered that the combination of a WorldSpace patch antenna, homemade mounting bracket and a standard 2.3m TVRO dish gave quite good results on the southern beam. We could not receive the west or eastern beams of the satellite. Obviously a 2.3m dish is a far cry The patch antenna itself is an active device, powered by the receiver (3V) and contains antennas for RHCP and LHCP signals, a switching system and a preamplifier. It is fed with small diameter 75Ω coaxial cable and terminated with an F-type male plug. Extending the feed cable with quad shield RG6/U had no effect on the received signals. The dish does not have to be particularly accurate, as the L band signal is quite forgiving of reflector inaccuracies. AsiaStar is located at 105°E longitude and for Sydney this equates to a dish azimuth of 298° and an elevation of 27.23°. We used the metal plate supplied with most dishes to cover the hole in the centre of the dish (purely cosmetic) as a mounting platform for the patch antenna. By placing this plate at the focal point (where the scalar rings would normally be mounted in a sat- Here’s how we mounted the antenna to the backing plate – good ’ol Velcro! ellite receiving system), a convenient mounting position is created. The best method of securing the patch antenna to the plate is to use heavy duty “Velcro” strips. We found that two strips, 150mm long, provided enough support to hold the feed system in place. Prior to affixing the Velcro strips to the patch antenna, we found it was necessary to remove the swivel backing plate that is supplied with the antenna. This bracket is intended for use where the patch antenna is mounted on a window sill and is surplus to requirements in our application. The bracket can be removed using a Phillips screwdriver. The photo above right shows the internal components of the patch an- (Left): the WorldSpace Digital Receiver which we mounted as a “patch antenna” at the focal point of an old 2.3m mesh dish. We used the metal plate from the centre of the dish to mount the antenna on, as shown above and right. Incidentally, in prime (targeted) signal areas, you only need this digital receiver – no dish! 58  Silicon Chip www.siliconchip.com.au Here’s what the WorldSpace receiving antenna looks like inside – not much to it, is there? This antenna is all that is required in the targeted signal areas – here it is used in conjunction with a 2.3m dish to extract the very small signal available on the east coast. tenna. The metal plate at right houses the two antennas and the amplifier and switching circuitry (underside of plate). It is also a good idea to waterproof the antenna and this can be done using silicone sealant to cover the cable exit hole and those housing the screws securing the two halves of the patch antenna housing. We did try a combination of the WorldSpace LNA and various third party “coffee can” feeds with good re- sults, even though they were designed for GMS weather satellite reception at 1691MHz, some 200MHz away. Stepping through the available channels gave us the following free to air channels and our analysis of the content. The antenna is intended to be used indoors: one difficulty with using it outside is that it is not waterproof. Here we have applied a liberal dollop of silicone sealant to the output cable and a run of insulation tape around the outside – just in case. A program guide can be downloaded from the WorldSpace internet site (www.worldspace.com). No doubt there will be more channels as the systems gains popularity. SC A “coffee can” feed, intended for use on the GMS weather channels on 1691MHz . . . . . . here shown mounted to the same 2.3m dish. It too gave a good account of itself. www.siliconchip.com.au April 2004  59