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The U.S. military is turning to
laser communications to help deliver the communications throughput
required for transformation. Most importantly, the services have
begun to develop laser technology for communication between
satellites and ground and air assets as part of the Transformational
Communications Architecture (TCA).
Despite their massive throughput,
high-speed links and other advantages, however, laser comms are not
expected to be widely used by the military until the next decade.
Technological limitations mean that laser comms will, for now at
least, supplement rather than replace traditional RF communications.
There is nothing new in laser
communications per se. Fiber optic cables today carry almost all the
global data networking and high bandwidth infrastructure. The cables
use photons to carry information rather than electrons used in
traditional RF solutions. Laser comms is sometimes called freespace
optical communications, because the photonic beam is transmitted
through the atmosphere rather than carried by a physical conduit, a
cable.
Commercial customers have been using
laser communications for some time. An example of the kind of
solution being used is the Supraconnect solution from LSA, which
provides a 155 megabits per second (Mbps) throughput with ranges of
up to 4.5 kilometers (km). Such systems are typically used on fixed
campus networks and metropolitan areas.
Laser communications technology was
used to support companies in New York after 9/11. The links could be
established quickly because laser communications systems are rapidly
deployable, and the linkage between systems is relatively
straightforward.
Laser comms have a number of
advantages over RF, not least in the area of security.
High-performance laser systems have an inherently high level of link
transmission security due to the very narrow transmitter beam width.
It is necessary to directly interrupt the beam in order to access
information, and this is both exceedingly difficult to achieve and
easily detectable. For the same reasons, it causes no interference
with nearby RF sources.
A further attraction of laser comms
is that it does not require Federal Communications Commission
licensing. Because lasers operate at a much higher frequency,
moreover, they are able to achieve an exponential data throughput
improvement. Transferring responsibility for throughput from satcom
frequencies and into the laser communications world will also free
up RF for other military users and for applications that laser comms
cannot meet.
To realize these advantages,
however, certain challenges need to be overcome. Environmental
conditions such as rain and foliage seriously impair or preclude
laser communications. In addition, a direct point-to-point link is
required, which limits military applications.
Space Segment
The satellite elements of the TCA
could be the first major military application for laser
communications, according to John Oglesby, vice president for
advanced programs, Raytheon Network Centric Systems.
In January, teams primed by Boeing
Integrated Defense Systems and Lockheed Martin and partner Northrop
Grumman were awarded 27-month contracts, valued at $472 million
each, for risk reduction and system definition in the
Transformational Satellite Space Segment. Boeing’s team comprises
Raytheon, Ball Aerospace, General Dynamics, IBM, L-3 Communications,
Cisco Systems, BBN Technologies, Hughes Network Systems, Lucent
Technologies, Harris, EMS Technologies and Alpha Informatics.
Rockwell Collins, General Dynamics
Advanced Information Systems, L-3 Communications, Stratogis, Cisco,
C&H Associates and ViaSat are all supporting the Lockheed
Martin/Northrop Grumman team. A single team, to be selected in 2006,
will begin putting satellites in place in 2011.
Laser communications are to be used
in the program to establish cross-links of up to 10 gigabits per
second between satellites. These satellites will receive information
from airborne platforms such as the E-10 and Global Hawk to be
relayed around the world. Practical work to understand the
requirement is now going on.
In May 2001, the National
Reconnaissance Office launched its GeoLITE (Geosynchronous
Lightweight Technology Experiment) satellite. This carried an
experimental laser communication payload to support future
Transformational Satellite Communications work.
Oglesby identified the additional
capabilities the military needs over and above those on the market
today: “The major difference is that the [transmission] path lengths
are very long. The whole thought process that [former Department of
Defense Chief Information Officer] John Stenbit had at the Office of
the Secretary of Defense for the TCA was to create the equivalent of
the Internet backbone infrastructure in space. That implies that you
do all this with freespace laser communications around and between
all of these satellite nodes. There is lot of difference between
going a few miles in an atmospheric environment to going between
satellites and getting that data off the satellites.”
The Air Force Laser Communication
Terminal program will provide the airborne terminus for these laser
links from the satellite. Getting information from the ground to the
satellite is expected to be via extremely high frequency (EHF)
links. While these links will be high capacity in terms of RF, at
perhaps 2 Mbps they will still be considerably under the capacity of
laser links.
Oglesby compared the laser
communications of today with the initial work on EHF communications.
“EHF was one set of developmental programs, followed by a horde of
terminal and source developments,” he said. “I believe that will
happen again with laser communications. It starts with a
path-finding program that proves the operational concepts and that
it can achieve the level of performance required, and will then be
followed by a series of programs by the services for different
capabilities of platforms.”
Joint Tactical Radio
The Joint Tactical Radio System
(JTRS) Joint Program Office is working toward fielding solutions
that can reach above the current 2 Ghz threshold. Commenting on the
role of laser communications for JTRS, Colonel Steven MacLaird, JTRS
program director, said, “The Software Communications Architecture
[SCA] is basically utilizing a standard capability to digitize what
used to be analog and some software waveforms into a specific
standard. That standard can be utilized by laser comms capabilities.
The DoD believes that the SCA is transferable to laser
communications.”
“The vision is that [laser
communications] will become another extension on the JTRS terminal
family,” Oglesby observed. “It becomes not nearly as unique as it
would have been in the past. If you look at JTRS as a set of
software functionalities and data capabilities, then this is just a
different aperture that goes on the front of the system.”
“Rockwell Collins has an optics
group that is working closely with our communications group. We see
the viability of optical communications and we see no barriers to
hosting optical communications on JTRS platforms,” commented Perry
Garrett, manager of the SATCOM department of Rockwell Collins
Government Systems.
In addition to ATL and other
programs that directly harness laser comms, the services also are
looking at the technology as a future upgrade. One such upgrade is
the Army-led High Capacity Communications Capability (HC3) program.
The program seeks to provide an overarching architecture to meet the
requirements for the high capacity MILSATCOM and line of sight data
links between the ground terminals and airborne and satellite
payloads, including an on-the-move capability.
The primary interface for HC3 will
be with the Warfighter Information Network-Tactical (WIN-T). “We do
not know yet how HC3 will evolve. What we are trying to do is gather
all the requirements for all services and from different domains.
There are different communities out there that have requirements for
high capacity communications—a trunk need for high capacity, a
satcom need for high capacity and an ISR need. We are trying to
capture than in one effort,” explained Colonel Thomas Cole, WIN-T
project manager.
The initial phase of HC3 comprises
two 18-month study contracts awarded Raytheon- and Boeing-led teams
by the WIN-T office.
Bill Iannacci, director of Army Air
Force satcom, Raytheon Network Centric Systems, offered his views on
how laser communications could support HC3. “In a post-2010
timeframe, at a minimum, the satellite payload-to-payload links will
be optical communications. Satellite payload links to aircraft or
UAVs may also be by laser communications.
“This ties into the HC3 architecture
of using unmanned aerial vehicles to extend line-of-sight
communications,” Iannacci continued. “There are other programs that
feed into HC3 that are part of the TCA, but from the point of view
for the ground terminals, the focus is not on the optical area. I do
not believe the technology is there yet for ground-to-payload
optical connections.”
Steered Agile Beam
The Defense Advanced Research
Projects Agency (DARPA) has been at the heart of the DoD’s push to
field this technology, with a series of research projects over
several decades. These projects include the recent Tera Hertz
Operational Reachback program, which explored the possibility of
using freespace optical communications to deliver “last mile”
communications between the point at which the terrestrial grid ends
and deployed forces in theater begin.
The Steered Agile Beam program, a
DARPA Microsystems Technology Office project, examined an electronic
means of steering optical energy. This would remove the need for
heavy, gimbaled systems and provide a lighter and more compact form,
in addition to being more energy efficient.
Lucent Technologies, through its
Bell Labs research arm, is working with DARPA and DoD on several
initiatives in this area. The company recently received a $13.4
million contract for the second phase of DARPA’s Coherent
Communications Imaging and Targeting (CCIT) program, working in
conjunction with the New Jersey Nanotechnology Consortium (NJNC).
The CCIT program essentially
involves a set of lenses that focus and receive the light more
effectively. This focus enables DARPA to build very high-speed links
in the multi-gigabit per-second range for communicating over long
distances from land, sea and airborne platforms to space. A further
goal of the program is to provide the facility to send
aberration-free three-dimensional imaging at distances of more than
1,000 km. The CCIT research is scheduled for completion by March
2006.
“For the CCIT program, spatial light
modulators are micro-electrical machine-based mirrors for
controlling the wave front of the optical beam. This is important
because you can correct for aberrations, and you can do optical beam
steering to better direct communications when your target moves.
Essentially, we are integrating photonics and high-speed electronics
into one system for a quantum leap in laser-based communications.
The system will digitally manipulate optical beams, like radio beams
are manipulated today, enabling better communications over farther
distances,” said Dave Bishop, vice president, nanotechnology
research for Bell Labs and president of NJNC.
Other than high-speed military
communications, Bishop identified a number of military and
non-military applications for this technology, including quantum
computing, optical radar, maskless lithography, astronomy, high
definition television, electronic ophthalmology and digital cinema.
The high data rates coming into the
wider military communications network by use of laser comms means
that improvements are needed across the system to cope with this
rise in data. Lucent is also aiding DARPA in the Integrated Router
Interconnected Spectrally (IRIS) program, receiving a $12.5 million
DARPA award earlier this year.
“For IRIS, we will build an
all-optical router superior in size, speed, and capacity to what is
available on the market,” said Martin Zirngibl, director of optical
research for Bell Labs. “Today’s switches cannot handle fully loaded
capacity; switching in the nodes is a fundamental problem. If you
look at a switch today, you need a lot of interconnect bandwidth.
You will need an optical interconnect system.
“Data traffic related to military
applications is growing every day, and current solutions are not
scalable. Like everyone else, the U.S. government wants to be able
to handle more traffic faster, on optical routers that are smaller
than current routers, but have more throughput and lower power
consumption,” Zirngibl added.
One of the developmental challenges
for any laser comms program is to find a flexible means of
interconnection. To achieve a laser comms link, connection is
critical. Commercial laser comms technology has required a
relatively simple point-to-point system, only having to deal with
slight movement between buildings due to wind. But clearly this
level of agility is inadequate for military use.
Oglesby believes, however, that
current laser technology can support more agile beams. “If you think
about the very agile, gimbaled, telescopic system in laser
designator targeting systems that are flown today, they are very
similar to what you need for laser comms. They have a lot of the
same physical characteristics, and are similar in that they have to
be agile and track and quickly move on a dynamic platform. The
technology has really been proven in that world. The big commercial
issue has always been cost. Customers do not want to pay for fast
steering optical systems for their inexpensive commercial systems.
The military however, is willing to pay for what is
required.” | 
