Low−Earth−Orbit
Satellites (LEOs)
Overview
Quite a bit of discussion regarding wireless
communications has already been covered in earlier
chapters. However, a combination of two separate
services and technologies are merging as new services for broadband
communications. These two services include long haul communications and the use
of personal communications services (PCSs). The technologies include
the use of satellite and the cellular concepts combined. Worldwide
communications services can be achieved by these two combined services,
therefore some diligent effort should be made to understand just what is happening
in this arena. This truly brings home the concept of communications from
anywhere to anywhere. The thought of being out in the middle of a lake and receiving
a call, or rafting down a river and making a call, boggles the mind. This is
especially true when thinking about some of the more rural areas in the world
where no telephone service infrastructure exists today. Yet, in a matter of a
few years these remote locations, on mountaintops, in forests, in valleys, or
on the sea will all be reachable within a moment's notice. The infrastructure
of a wired world will not easily lend itself to this need, due to timing and
cost issues. Therefore, the use of a wireless transmission system is the
obvious answer.
Low Earth Orbit satellites must travel very quickly to resist the pull of gravity — approximately 17,000 miles per hour. Because of this, Lowe Earth Orbit satellies can orbit the planet in as little as 90 minutes.
Low Earth Orbit satellite systems require several dozen satellites to provide coverage of the entire planet.
Low Earth Orbit satellites typically operate in polar orbits.
Low Earth Orbit
satellites are used for applications where a short Round Trip Time (RTT) is
very important, such as Mobile Satellite Services (MSS).
Low Earth Orbit satellites have a typical service life expectancy of five to
seven years.
Orbit
|
Number Of
Competitors
|
Status
|
Low−Earth
|
8
|
Pending licenses granted Orbit
(LEO) based on very
specific areas of coverage
|
Mid−Earth
|
4
|
Experimental licenses Orbit
(MEO) granted for
specific areas of coverage
|
Geosynchronous
|
4
|
Licenses have already been Orbit
(GEO) issued for some, are experimental with others
|
Low−Earth
Orbit
In December 1990, Motorola filed an application with
the FCC for the purposes of constructing,
launching, and operating a LEO global mobile
satellite system known as Iridium. This was the hot button that sparked the world
into a frenzy. Iridium was a concept of launching a series of 66
satellites [1] Originally the Iridium proposal was
for 77 satellites, but Motorola amended this number after the World
Administrative Radio Council meeting in the spring of 1992. around the world to
provide global coverage for a mobile communications service operating in the
1.610 to 1.6265 GHz frequency bands. The concept was to use a portable or
mobile transceiver with low profile antennas to reach a constellation of 66
satellites. Each of the satellites would be interconnected to one another
through a radio communications system as they traversed the globe at 413
nautical miles above the earth in multiple polar orbits [2] The original
concept was to use 7 polar orbits with 11 satellites in each. This would
provide worldwide coverage, much similar to an orange slice concept .
In Figure 27−1, the concept of the LEO arrangement is
shown. In this particular case, the satellites
are traversing the earth's surface at a height of
400+ nautical miles above the earth, in a polar orbit. In the polar orbit, the
satellite moves around the earth's poles and passes over any specific point along
its path very quickly. The satellites move at approximately 7,400 meters per
second in different orbits. Therefore, as one target site moves out of view, a
new one comes into view at approximately the same time. A handoff will take
place between the individual satellites (using the Ka band).
The
Benefits of These Service Offerings
Motorola established a list of benefits from the
deployment of the Iridium network services, which at first glance may look
biased toward their services. Upon further evaluation, these benefits can be derived
from any network of this type. Therefore, generically these are addressed and
kept in the context of any LEO network. The benefits lean toward the end user
as shown. These are summarized as follows:
Ø Ubiquitous
services With
continuous and global coverage, any−to−any connections can occur. As users
travel either domestically or abroad, the service travels with them. It will eliminate
the need for special access arrangements and special numbers that must be dialed.
Users should never be out of range from their network. Remote areas with
limited demand and finances now have the capability to connect anywhere in the
world.
Ø Spectral
efficiency As
already mentioned, the frequency reuse patterns for the bandwidth
allocation will be significant. No
other satellite system has achieved these reuse ratios. Iridium was first to
claim this capability of efficiency. RDSS portion of the Iridium network is
contained in the same spectral
arrangement, freeing up 16.5 MHz of spectrum. This is a
quantum leap in the efficient use
of the spectrum.
Ø The
potential to save lives It's
common for the news media to publish stories of people stranded in remote areas
with no life support systems who died because of their inability to communicate.
The press today is filled with stories of cellular and PCS users notifying authorities
of casualties. If only people in remote areas had a means of notifying
authorities and/or rescue parties, their lives could be spared in the event
they get into a life−threatening situation.
Ø Capabilities
of the vendor Motorola
states that they are uniquely qualified to provide
these types of services due to
their background in the development and sales of other
ancillary
equipment that works in the wireless world. Specifically, they have been one of
the
Ø major developers in the
production, research, and development of private mobile−radio
services.
Ø LEO
deployment promotes international communications The LEO networks deliver
modern
digital−transmission services to remote areas of the world. The FCC and the
U.S. government are attempting to use telecommunications as a strategic and economic
tool to foster development in these areas. Their goals are to
·
Promote
the free flow of information worldwide.
·
Promote
the development of innovative, efficient, and cost−effective international
communications
services that meet the needs of users in support of commerce and
trade development.
·
Continuous
development and evolution of a communications services and networks
that can meet the
needs of all nations, and specifically those of developing nations.
References:
http://www.tech-faq.com/low-earth-orbit.html
Broadband Telecommunications Handbook
