Mobile Satellite Communications
If you believe the marketing hype, the world is about to be inundated with orbiting mobile communications satellites. The impending deployment of Motorola's Iridium scheme is about to start a new chapter in the communications revolution, one in which we can expect to see mobile cornmunications become ubiquitous.
Currently, there are about a dozen different schemes that have been proposed. Some of these will succeed and some may not even get off the drawing board - or CAD system monitor.
The first thoughts that come to mind are: 'Why now ?' and 'So what, we've had satellite comms since the sixties. What's new ?' To answer these questions, you need a better appreciation of the background to this situation. So, we will take a brief look at established satellite communications and their basic attributes.
The Geostationary Earth Orbit Satellite
The Gedstationary Earth Orbit (GEO) communications satellite was conceived several decades ago by physicist Arthur C Clarke. Clarke is best known as the author of '2001: A Space Oddysey'. He realised that if you hoisted a radio repeater into orbit, you get communications between two points on the surface of the earth that do not have a direct line of sight.
Clarke also realised that if the radio repeater is hoisted into an orbit with a rotational period identical to that of the Earth, the satellite will appear to hang motionless in the sky to an observer on the surface. These orbits are called 'Clarke' orbits. But, sadly for Clarke, it was not patented and he has not enjoyed the reward for his inspired thought.
While the mechanics of putting a satellite into GEO are quite fascinating, they exceed the scope of our discussion. It is sufficient to say that the satellite is boosted into a Low Earth Orbit (LEO) parking orbit, then boosted into a transfer orbit and finally into its operating orbit.
Once in orbit the satellite's stabilisation flywheels are spun up, the solar cell clad collectors are unfurled, its position is finely tuned, the antennas aligned and it goes to work. Today GEO satellites are a mainstay of global communications, carrying a substantial proportion of the world's voice, data and broadcast TV traffic.
Most GEO satellites in use today are what satellite engineers term 'bent pipe' repeaters, which take an incoming radio frequency (RF) carrier wave, heterodyne it to a slightly higher or lower frequency, and beam the signal back to earth. These devices are not intelligent - they generally do not understand the content of the data they carry. As a result, they offer very limited functionality apart from carrying multiplexed switched telephone traffic and broadcast TV video signal relays. While direct broadcast TV has been available for some years, it too relies on the GEO 'bent pipe' model.
Early this decade, it was becoming increasingly clear that the 'Clarke Belt' was becoming very busy. The number of physical slots available for new satellites in the most heavily used American and European longitudes was rapidly shrinking, as were the number of available carrier frequency slots. Congestion was setting in, but the market had yet to saturate with demand for the services provided by satellites.
Clearly the satellite had greater potential for global communications than originally realised by the GEO paradigm. But more significantly the dual pressures of market demand and the unavailability of orbital slots and frequencies meant that alternatives to GEO would become commercially viable. This together with important developments in microwave integrated circuits, which enabled the design of cheap receivers with small, fixed antennas, meant that the time had come for alternatives to the GEO model. Despite its popularity, the GEO satellite has some important limitations inherent in the fundamental idea. Some of which are particularly important in relation to the transport of computer traffic.
The first and foremost limitation of GEO satellites is round trip latency, a measure of the time taken for a signal to reach the satellite, turn around and send, and propagate to the receiver. This time delay, subject to the position of the ground stations and propagation delays through intermediate repeaters and or encoders / compressors/ decompressors and decoders, can be up to hundreds of milliseconds. It is an inherent property of the GEO geometry and unless somebody devises a warp speed radio wave, it is unavoidable.
While the GEO propagation latency may or may not compromise voice and videoconferencing performance, it has caused difficulties with packet oriented protocols in the past. It was the impetus for the RFC1323 large buffer size protocol revision. Happily many Unix implementations now support the RFC1323 model and no longer experience buffering problems due to GEO latency. Promoters of proprietary networking schemes have pushed this as an issue recently (for the record, this problem was solved some time ago).
The second limitation of GEO schemes is cost. Because the orbit is 36,000 km above the equator, you need to use a big satellite with powerful transmitters, high performance receivers and large antennas. The satellite, which could weight up to ten tons, then needs to be launched into orbit with a geostationary transfer booster attached. The more tons you are pushing out the Earth's gravity well, the more expensive it becomes, and the more limited the range of boosters capable of doing the job. This further exacerbates the cost problem.
This raise another essential limitation - with increasing complexity the potential for hardware failure increases. As a result, designers of GEO sets have been reluctant to include more sophisticated onboard hardware to decode and demultiplex data onboard the satellite. This explains the dominance of 'bent pipe' designs. Another limitation of GEO sets is poor performance in the polar regions. The slant range to the satellite is not only greater, but the radio waves to and from the sat must pass through a very thick layer of the troposphere, the lower 13 km of the atmosphere. Because the troposphere is laden with moisture, it absorbs radio waves rather well. Also, because the sat is viewed very low over the horizon, geographical obstacles such as mountains can actually block the view of the satellite.
The Iridium plan deploys multiple satellites communicating to a wide variety of devices.
One of the alternatives to the GEO model is the Soviet devised Molniya (Lightning) orbital model. The Molniya model was devised to provide satellite comms and TV broadcasts to remote Siberian sites. It uses an inclined elliptical orbit which sees the satellite rise gently above the horizon, and then drift down again, significantly reducing the tracking performance needs of a receiver antenna. A constellation of four Molniyas can provide 24 hours of uninterrupted coverage.
Another alternative is medium altitude satellite constellations, typically between the destructive charged particle ridden outer and inner Van Allen belts, or placed within the outer Van Allen belt. A MEO constellation has a number of geometrical advantages over the GEO model because they can cover large areas. They also have far lower propagation latencies and are far less demanding of booster performance. The downside is that they must be more electrically robust to handle the hostile particle environment and ground stations must know the orbit geometry to find the moving satellites to track and receive the signal. In the past, if you wanted to receive a signal from such a constellation, you would need a steerable dish on the roof with a clever box of smarts to drive it. Therefore MEO orbits have not been very popular.
The final alternative available to a mobile satcom network designer is the Low Earth Orbit (LEO) model. In the LEO model satellites are placed into a circular inclined orbit several hundred kilometres above the earth's surface. The advantages of this model are very low launch costs per satellite, modest demands on antenna, receiver and transmitter performance (and thus smaller weight and unit cost), and very low propagation latency times.
One limitation of an LEO constellation is the very limited footprint of each satellite, which means global coverage will require many sats. Also, you cannot build a LEO constellation to cover one part of the world alone, it's all or nothing. Even a modest LEO scheme will require several dozen sets to do the job. Another limitation is the limited life of the sats, which eventually dip into the upper atmosphere and burn up.
LEO and MEO orbital schemes provide a moving footprint for each satellite 'cell', thereby approximating the idea of mobile phone repeaters in orbit with 'fixed' users on the ground. A clever design is required to allow user terminals to 'handover' as one satellite disappears under the horizon and another becomes visible.
The current explosion in planned and proposed com sat schemes is driven by the LEO model. The first scheme to deploy is the Motorola Iridium. The most visible scheme, Teledesic, promoted by Bill Gates, relies on an architecture with several hundred satellites. Grandiose? Without a doubt, the launching of any LEO constellation is a gargantuan pursuit, expected to cost many billions of dollars. One overseas commentator made the apt observation that these schemes rival in scope the building of the pyramids - not an overstatement by any means!
One remaining and important technical point in these modern satcom schemes is the idea of crosslinks. A crosslink is a satellite to satellite high bandwidth microwave or laser link, which allows a satellite to route traffic to another satellite. Using crosslinks it is possible to wholly bypass the terrestrial fibre and copper networks. A user can beam their message up to an overhead sat, the message can then hop from satellite to satellite until it is above the recipient, from where it is then beamed down. This is a wonderful feature, especially if you want to cut out the terrestrialbound competition!
The downside of crosslinks is a significant increase in complexity, as the satellite must carry an onboard switch (or router) as well as steerable crosslink antennas and the associated receivers, transmitters and control hardware (and software). To gain an idea of the scope of this proposal, we will briefly survey some of the most notable schemes.
The Inmarsat network of GEO maritime communications satellites is well known. It is used for voice comms as well as search and rescue. More recently, Inmarsat proposed the Project21, renamed Inmarsat B and later moved to a new company, ICO. The ICO scheme involves a constellation of ten MEO sats at 10,355 km altitude, arranged in two 45 degree inclined planes of five satellites with two orbiting spares. The handsets will be dual mode, defaulting to the orbital link if a terrestrial link is unavailable.
The system uses TDMA techniques (like GSM phones). Each sat will support 4,500 phone channels. Based on the mature Hughes 601 GEO design, the satellites will weigh 2.6 tonnes each and will operate at 1.6/1.5 GHz, 1.6/2.4 GHz and around 2 GHz. The ICO scheme is primarily aimed at supporting voice channels. The ICO scheme should not be confused with the Inmarsat Mini-M scheme, which involves the use of a new generation of GEO satellites, with more powerful beams, and smaller laptop sized user terminals. It is a follow-on to the existing generation of Inmarsat orbital vehicles.
The proposed GlobalStar system is based on the model of 48 operational satellites in eight separate LEO planes, at 1414 Km altitude and 52 degrees inclination, with eight spares in orbit. GlobalStar uses Qualcomm's CDMA (Code Division Multiple Access) mobile phone technology. The GlobalStar model is tightly integrated with the terrestrial CDMA mobile phone network. It is assumed that local service providers will licence to users and deal with regulatory issues. The GlobalStar satellites are essentially bent pipe designs built to relay traffic between ground stations. Each vehicle weighs 450 Kg and uses GPS for attitude stabilisation.
Because CDMA allows multiple users to share bandwidth, by using spread spectrum techniques, it has advantages in robustness and lower transmit/receive power over simpler schemes such as TDMA (as used in GSM). GlobalStar is optimised for voice traffic, but will support low bandwidth data services at 1200, 2400, 4800 and 9600 kbps. GlobalStar links operate at the following frequencies terminal to satellite: L-band (1610-1626.5 MHz), satellite to terminal: Sband (2483.5-2500 MHz), gateway to satellite: C-band (5091-5250 MHz), and satellite to gateway: C-band (6875-7055 MHz).
The proposed Odyssey MEO constellation concept is similar to the ICO scheme. So much so, that Odyssey is now suing ICO for patent infringement. The Oddysey model employs 'bent pipe' repeaters and frequency division multiplexing of signals. The implementors of Oddysey are TRW Inc. in Redondo Beach, USA, and Montreal-based Teleglobe. At this time few details appear to be published on this scheme.
The first Motorola Iridium is sitting on a Cape Canaveral launch pad waiting for favourable weather and launch window, after an early January abort. Iridium is Motorola's biggest venture to date. It will involve a consortium with a large number of international partners and will be the first LEO constellation to be operational. The most simple description of Iridium is an orbiting GSM mobile phone network (see http://www.iridium.com).
Iridium will employ 66 satellites (originally 77, hence the name) in 778 Km high inclined orbits. Each 689 Kg satellite is essentially an orbiting telephone switch, with Ka-Band (~23GHz) crosslinks to connect to four adjacent satellites in the constellation. Links to user handsets will operate in the L-Band (1616-1626.5 MHz), whereas the Ka-Band (19.4-19.6 GHz for downlinks; 29.1-29.3 GHz for uplinks) is used between the satellite and the gateways and earth terminals.
The system uses a combination of FDMA and TDMA techniques, technically conservative but demanding fixed frequency band allocations. Unlike preceding designs, Iridium uses a more sophisticated model which routes traffic between satellites where terrestrial links are absent. As it is tightly integrated with the GSM protocol suite, it can easily integrate with existing GSM services. This is a tremendous marketing advantage, but it carries the performance and technical limitations of GSM. Iridium has provisions for handsets with serial RS-232C interfaces, but due its fundamentally voice traffic oriented circuit switched architecture, it will not be much more effective for data traffic than existing copper networks.
Proximity to Earth means LEOsats can communicate with hand-held devices.
Backed by Bill Gates, Teledesic is the most ambitious of the new satcom schemes. In simple terms Teledesic can be described as a proprietary broadband orbital competitor to the Internet, or 'Internet in the Sky' as one commentator noted, based on orbital protocol routers. The Teledesic model envisages no less than 840 (yes, 840!) satellites in multiple orbital planes, using a complex scheme of satellite crosslinks to wholly bypass the terrestrial infrastructure. Teledesic is data oriented and is intended to provide services from 16 Kbps up to E1 or 2.048 Mbps services.
The system will provide a sustainable useable capacity of 20,000 concurrent 2.048 Mbps connections, with a theoretical limit of 10^6 2.048 Mbps connections - all achieved with extremely low latency times and 10^-9 Bit Error Rates. Teledesic will employ an ATM-based model with adaptive routing to select the optimal path between satellites. Links will be encrypted and provisions will be made for 155.52 Mbps and 1.2 Gbps uplinks at selected sites. A complex scheme using FDMA, TDMA and SDMA (Space DMA), plus polarisation control, will be used to separate channels in adjacent ground cells.
The successful implementation of Teledesic will require some major technological advances in a number of areas. It is an interesting departure by Gates from his established paradigm of repackaging and dressing up mature technology for sale in the mass consumer market. With Teledesic, Gates is moving to a technically risky venture which will require frequent interfacing with highly technical customers. Whether Teledesic is successful, allowing Gates to monopolise the world's WAN services, or it becomes a black hole for investors' capital remains to be seen.
Clearly the central focus of most existing satellite mobile comms schemes is voice transmission with data treated as an ancillary service. With the exception of Teledesic, high performance data services will probably have to wait for the second generation of mobile satellite comms. These will probably appear in the latter first decade of the next century.
A substantial part of this webpage was aquired from an article by D. Carlo who is a PhD student at Monash University and a practicing Unix systems consultant. He can be reached at firstname.lastname@example.org or http://www.cs.monash.edu.au/~carlo
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Last revised: Sunday, 06 April 1997