|
|
 |
Knowledge Is Key
For Intelligent Decisions
Satellite Logic is a leading,
authoritative source of information in
the Satellite Industry. Located in the
heart of the Silicon Valley, Satellite
Logic provides one of the most
valuable and comprehensive
knowledge bases on the Satellite
market! This is a primary Worldwide
information center which enables our
clients to analyze, evaluate, inquire
and select their best tailored
solutions. Our company sets the
industry standards for targeted
buying leads, reflecting a dramatic
advance over traditional marketing
solutions.
|
|
|
 |
During the 1970s and 1980s the subject of satellite
communications was related as the data communications
technology of the future. In fact, many authors wrote about the
use of satellite communications technology eventually replacing
the use of most terrestrial communications. This conversion did
not occur completelly, primarily due to the delays associated
with transmitting a signal more than 25,000 miles into the
atmosphere and then having that signal retransmitted back to
earth. The delay time associated with the use of satellites
significantly affects their use for data transfer when data blocks
have to be periodically acknowledged. Due to the minimal effect
on voice conversations, the use of satellites has taken on a new
role as a mechanism for personal communication services.
The key difference between the evolving personal
communications satellite-based systems and existing cellular
telephones is in their method of communication. A cellular
telephone user communicates with a cell site connected via
terrestrial lines to the public switched telephone network.
In comparison, ground users can also subscribe to a low earth
orbit satellite-based communications service. Such subscriptions
enable ground users to make cellular calls or receive pages
through low earth orbiting satellites from just about every area
on earth.
Several companies are currently developing satellite systems
consisting of 12 to 60 or more non-geostationary low and middle
earth-orbiting satellites that will provide either continental or
near-global communications coverage. Such systems are being
developed to support a new type of wireless telephone service
that will transmit in the L and Ka-band frequency spectrums
and support high-quality voice service as well as facsimile and
paging signal transmission.
One of the first satellite-based personal communications services
is the Iridium system from Iridium, Inc. This firm represents a
consortium of telecommunication operators and industrial
companies originally sponsored by Motorola, Inc., which still has
a minority interest in this venture. The actual concept behind
Iridium dates to 1985 when Karen Bertiaer, the wife of a
Motorola executive, could not place a cellular call to the United
States when vacationing in the Caribbean. She convinced her
husband of the need for a global mobile wireless system.
The Iridium satellite network consists of 66 satellites in six
orbital planes approximately 480 miles above the earth.
Communications to Iridium satellites are via special Iridium
mobile telephones and via conventional telephone systems
through the use of satellite earth station gateways that convert
terrestrial signals to the Ka-band for transmission to an Iridium
satellite. In addition to Iridium, other major communications
firms provide competitive satellite-based personal
communications services. One such competitive service is
provided by ORBCOMM, a partnership owned by Orbital Sciences
corporation, Teleglobe, Inc. of Canada, and Technology
Resources Industries of Malaysia. ORBCOMM currently has 36
satellites in orbit and during 1998 received permission from the
FCC to add another 12 satellites to its constellation. ORBCOMM
provides a variety of two-way data and messaging
communications. Applications include monitoring of fixed assets
such as electric utility meters, pipelines, heavy equipment, rail
cars, and other vehicles.
Low earth orbit satellites are ideally suited for two-way cellular
and paging operations. This is because the low orbit of those
satellites requires less power to reach those satellites. Anyone
who has seen an old Tom Clancy made-for-TV movie in which
an intelligence agent opens a suitcase and constructs a small
earth station to bounce a signal off a satellite can now
recognize that the person was accessing a geostationary
satellite. In comparison, in the movie "Air Force One," Harrison
Ford was able to use a small handheld cellular phone because
he was accessing a low earth orbit satellite.
Satellite Communications Technology
Satellite communications systems are basically line-of-sight
microwave systems with a single repeater. The satellite is said
to be in geostationary orbit when the speed of the satellite is
matched to the rotation of the earth at the equator. Because of
the great distance of the satellite from the earth (about 22,300
miles) and antenna size limitations that limit focusing capability,
the cone of coverage for a single satellite transmitter can be as
large as the entire continental United States.
For those transmission services that originate at a single point
and flow to many points in one direction, such as television and
radio signals, the large area of coverage is ideal. The relatively
long delay between the instant a signal is sent and when it
returns to earth (about 240 milliseconds –) has no undesirable
effect when the signal is going only one way. However, for
signals such as data communications sessions and telephone
conversations, which go in both directions and are intended to
be received at only one other point, the large area of coverage
and the delay can cause problems.
Data and telephone conversations usually proceed as a series
of messages in one direction that are answered or
acknowledged by messages in the other direction. When the
delay between the message being sent and the reply or
acknowledgment is long, the transmission rate of information
slows down. In the case of voice messages, long delays
between utterance and reply make the speaker think he or she
has not been heard or understood. This leads to requests for
repeating (the equivalent of negative acknowledgment in the
data world) and increased frustration. There is also a serious
privacy issue with communications that are intended for only
one destination but are broadcast so that an entire continent
can receive them. One of the factors causing the rush to all-digital
transmission is the ease of encrypting information in digital form
so that when the inevitable interception of broadcast signals
occurs, the information intercepted is at least somewhat difficult
to decipher.
A satellite transmission system consists of one or more earth
stations and a geostationary satellite that can be seen by the
stations. Americatel is a regional telecommunications carrier that
provides voice, data, and facsimile services via a gateway and
switching center in Miami to common carriers and private
organizations throughout Central and South America. In Central
and South America, many subscribers to Americatel use Very
Small Aperture Terminals (VSATs) to communicate with North
America, bypassing the necessity to use terrestrial lines.
Through the use of VSAT facilities in remote areas, such as
mining locations, users can receive a modern communications
capability that was previously restricted to major metropolitan
locations. To facilitate satellite communications, as well as to
eliminate interference between transmission and reception,
standards govern the use of satellite frequencies. Separate
frequencies are assigned for sending to the satellite (the uplink)
and receiving from the satellite (the downlink).
Satellites are equipped with multiple repeater units called
transponders. Many systems have 10 or 12 transponders, but a
series of international satellites, called INTELSAT VI, has 46
transponders. Transponders are assigned different uses, but in
the case of those used for voice or voice-equivalent data
communications channels (nominal 4KHz bandwidth), the
transponder capacity can be as large as 3,000 channels.
Transponders operate at different carrier frequencies. Currently
seven frequency bands are used for most space communications
applications. The C-Band and Ku-Band are currently used
exclusively for broadcasting purposes, such as for providing
HBO, Showtime, and other television broadcasts to wide areas
of the world.
|
|
|
|
