IRIDIUM:
CELLULAR PHONE VIA SATELLITE
by John Nordlie
For Commercialization of Space course
CELLULAR TELEPHONES
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.
SMALL SATELLITE SYSTEMS
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.
IRIDIUM
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.
APPLICATIONS & MARKETS
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.
PROBLEMS
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.
WARC
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.
CONCLUSIONS
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.
REFERENCES
[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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