by John Nordlie

For Commercialization of Space course

The dependance on communication by today's information society has pushed the state of the art in many communications technologies. One example of this is the cellular telephone. Little black antennas sprout from cars and trucks and some cellular phones are small enough to carry in a shirt pocket. The convenience of these high tech marvels is impressive, but what exactly is a cellular telephone?

As early as 1949, radio telephone systems for mobile use were available[1]. Such systems had many problems, however. Since each unit needed a separate frequency, much useful bandwidth went unused when the phone was not in service. Dependance on operators when placing calls made the phones inconvenient to use. The appeal of these systems to people who were mobile, such as travelling business people and some government workers, was great, but the limited radio spectrum made the cost of operating each unit prohibitively expensive.

The advent of microprocessors made possible a new method of controlling these phones, so that more units could function within the same area. Where as before, each telephone used its own unique frequency, and had to stay within the range of the one transmitting tower that served it, the new system let several phones use the same frequency, as long as they were far enough apart so as not to cause interference. Phones in the same cell use different frequencies to avoid interference. The "cellular" in cellular phones refers to the separation of an area into cells, or areas. Each area has a set of frequencies assigned to it, and when a call is placed, a computer in the phone transmits to a controlling computer to request a frequency. The control computer finds a currently unused frequency and assigns it to the phone. Should the user take the phone from one cell to another while having a conversation, the computer in the phone detects the fading in signal strength from the original transmitter, and requests a new frequency in the new cell. The controlling computer finds a new, unused frequency, switches the connection, informs the phone's computer, and returns the old frequency to unused status. All this takes place so quickly as to be undetectable to the user. This rather complex sounding frequency sharing system allows many cellular phones to be used in the same general area without interference and makes efficient use of radio spectrum space.

Cellular phone service grew from a fledgling industry in the early 70's into a $9 billion network in the early 90's[2]. Today, cellular phone service is available in most cities in the US, and a similar version of mobile communication is gaining acceptance in Europe.

Another fast-growing area in communications is small satellite systems. Small, light communications satellites in low orbits can be easily accessed using light-weight dishes and relatively inexpensive radio equipment. The applications found for these systems so far includes inventory assessment, credit card checks, freight and train tracking, and other business applications. Many retail stores these days sport small satellite antennas. The reduction in cost for these services has made satellite communication for routine business possible. Industry expects these small satellite systems, like their big brothers in geosynchronous orbit, to become immensely popular, and bring satellite communications to a much broader range of users. Businesses that could hardly afford to rent a transponder on a "bird" in geosync, can easily afford the modest fees of these new systems, and the smaller size of the antennas makes mounting them much more convenient.

The military has also begun to take notice of the rugged, portable nature of small satellite communications systems. Long haul, portable communications has been a goal of field commanders for years, and this technology offers it to them at a price not likely to inflate the national debt too much. The military enthusiasm in small satellite systems is relevant to business applications in that military contracts tends to push the state of the art in communications equipment, making it smaller, more powerful, and more robust. Personnel trained in the use of these systems by armed forces will also eventually enter the private sector, providing a wealth of expertise in the use and maintenance of these systems. Look for small satellite systems to experience a rapid growth in the coming decade.

Looking for ways to offer improved service, Motorola came upon the idea of marrying the two technologies of cellular phones and small satellite communications systems. In its proposal, later to be named "iridium", the company proposed to launch 77 small communications satellites and route cellular telephone traffic over them (the element iridium has an atomic number of 77, hence the name of the project)[3,4,5]. Ground users would communicate directly with the satellites, not with some local transmitter which would later pipe the communications through existing land-line systems. With the satellites acting in the role the ground-based cellular transmitters acted in cities, Motorola hopes to offer worldwide cellular phone service to anyone, anywhere at any time. With each satellite being capable of carrying 2,800 conversations at once, a total of 215,000 users could use the system at any given time6. The 77 satellites will travel in 7 different orbital planes, with 11 satellites in each orbit. The orbits are 413.5 nautical miles in height. The satellites in orbits 1, 3, 5, and 7 are in phase with one another, and those in orbits 2, 4, and 6 are halfway out of phase with those in orbits 1, 3, 5, and 7, while they are in phase with each other[5]. The "cells" in this system are 360 nautical miles in diameter, and there are 37 cells per satellite. This design minimizes cell size at the equator, and maximizes it at the poles, which is in keeping with average population density (lots of folks at the equator, and not many at the poles). The satellites will "crosslink" or talk to each other on frequencies in the 20 GHz band. The crosslinks allow users to talk directly to each other without accessing the public land-line phone system (for example, you can call a friend in another country now on your regular cellular phone, but the call is piped into AT&T's long distance switching network, the relayed via comsat to the foreign countries local phone system, then to your friend. Iridium lets you communicate from your phone to a satellite, crosslink to another satellite, then down to your friend). But what happens when you need to talk to a person on their regular phone? The ground stations proposed for use with iridium are of two types: control stations (very similar to control stations used today to control commercial communications satellites in geosynchronous orbit), and gateway stations. Gateways will provide access to telephone systems in their area. So, for example, if you want to call a friend in Europe, you place your call, it goes first to an iridium satellite, is crosslinked to another satellite over Europe, and is then downlinked via gateway to the local phone company you friend is connected to. While this will probably make the call more expensive, since the local phone company will want a share of the profits for use of its services, this connectivity increases the flexibility of the system immensely.

The building of this ambitious system will be shared among several contractors, who will provide satellite hardware, research, and launch services. Motorola auditioned various contractors, finally settling on Lockheed Missiles & Space to build the spacecraft[7,8,9]. The L-band antennas will be built by another contractor, Raytheon[10]. Other contractors being approached are Deutche Aerospace (Munich) for gallium arsenide solar panels, Matra (France) for power supply systems, British Aerospace (London) for spacecraft components, and Com Dev (Ontario) for intersatellite transmission links. Motorola says it "wants to keep the project as international as possible"[10]. Projected cost for the project, like many space projects, has more than doubled since its inception. Early estimates of $1 Billion have been updated to $3.1 Billion or more. Motorola also announced that the phones will use local cellular service if possible, switching to the iridium mode only if the person being called is unreachable by local cellular net. This will help keep the cost of the calls down.

The strongest demand for the iridium system is expected to come from business travelers and wealthy individuals who travel extensively[11]. These users will comprise an estimated 70 percent of the market, the remaining 30 percent being a mix of maritime subscribers, government employees, and residents or workers in sparsely populated areas. Motorola is looking for investors that will become part owners of Iridium Inc., which will manage the iridium system. Demand may, however, come from unexpected areas. When commercial communications satellites were first invented, the expected demand for them was grossly underestimated. Users such as cable companies, teleconferencing, business communications, and others helped make commercial communications satellites an incredibly fast growing industry, and currently the only space related industry that is quite profitable. One area that shows promise is digital data transfer over cellular networks. Due to the randomly switched nature of the nets, current systems for data transfer are slow or unreliable, but a new project to develop a standard for this type of communications is underway[2]. Data transfer rates of 19.2 Kilo Baud will be possible by using idle circuits of cellular nets. This technology is still in its infancy, but will, in my humble opinion, become an important aspect of cellular communications in the future.

While the prospects for iridium are good, not all has gone smoothly for Motorola. Expected lack of ground support equipment[12], as well as shy investors due to the high cost of the project[13] have caused delays and contract problems. Motorola is taking steps to reduce the risks of the project and attract investors, apparently with some success. Competition with rival communications companies is also making for rocky going. Inmarsat has a proposal for a satellite telephone network it calls Project 21, and is fighting for rights to the coveted L-band frequencies[13]. Other companies clamoring to compete with Motorola include Ellipsat Corp. of Washington, Mobile Satellite Corp., and Loral Cellular Systems Corp.[14]. These companies want not only part of the market, but rights to the limited parts of the spectrum that are ideal for these systems. The L band frequencies allow ground stations (phones) to use small antennas and low power to establish a reliable link, while not being affected by such things as rain and other adverse atmospheric conditions. Competition for these coveted bands for satellite communications is nothing new, as evidenced when commercial satellites systems were first experiencing their initial growth.

During the 1992 WARC (World Administrative Radio Conference) in March, small satellite companies won a big victory[15]. With band space allocated to the proposed use of low earth orbiting satellites, the real fray can begin over which companies will get FCC approval for their voice and data systems. During the WARC, some countries argued that operation of such systems might interfere with their local communications systems, such as Russia's Glonass system. All fears were apparently calmed, though, and the way is clear for the systems to be built.

Even though the systems will eventually be built, conflicts over billing and control still remain. Operators of phone services in Europe don't like the idea of systems that would bypass their equipment and, probably, taxes and fees. Some very complex negotiating will be needed to calm objections by such bodies, and no doubt some interesting compromises will be made. The iridium project's future is still uncertain, but looks rather promising as far as space ventures go. The use of such a system in helping promote the thought of the world as a "global village" deserves thought and, I think, respect. The global communications industry has grown like no one ever thought it would, and the future for iridium or an iridium-like system is, in the long run, assured.

[1] "Cellular Subscriber Technical Training Manual" Motorola Cellular North American subscriber division. Motorola Inc. 1990.
[2] "Pilot could give impetus to cellular data traffic" ComputerWorld April 27, 1992.
[3] "Motorola to enter small satellite business" Space News Volume 1 Number 21 June 4 - 10 1990
[4] "Motorola proposes 77 lightsats for global mobile phone service" Aviation Week & Space Technology July 2 1990
[5] "Iridium is in the works" Aerospace America February 1991
[6] "Calling the world" Flight International 28 August - 3 September 1991
[7] "Lockheed emerges as likely iridium builder" Space News Volume 2 Number 8 March 11 - 17 1991
[8] "Lockheed commits $50 million to iridium production" Space News Volume 2 Number 11 April 8 - 14 1991
[9] "Lockheed wins Motorola contract" Flight International 10 - 16 April 1991
[10] "Newsmaker Forum" Space News Volume 3 Number 6 February 17 - 23 1992
[11] "Fast start seen for global cellular system" Space News Volume 2 Number 38 November 4 - 10 1991
[12] "Changing world fits iridium project" Space News Volume 3 Number 2 January 13 - 26 1992
[13] "Iridium courts investors with move to lower risk" Space News Volume 3 Number 3 January 27 - February 2 1992
[14] "Alternate systems clamor to compete against iridium idea" Space News Volume 2 Number 21 June 10 - 16 1991
[15] "Delegates bestow mobile mandate" Space News Volume 3 Number 9 March 9 - 15 1992

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