Technique offers high-bandwidth links to tactical users
An experimental hybrid
technology that combines both laser and radio frequency
communications into a single system may soon provide warfighters
with robust, high-bandwidth data networks. Software protocols will
allow nodes in these networks to switch automatically between the
two transmission modes based on the type of message sent and on
prevailing atmospheric conditions.
Mobile networking is
a vital part of the U.S. Defense Department’s future warfighting
plans. However, these overarching information grids currently are
incomplete and sometimes prone to failure. The weakest part of this
structure is at the tactical level where passing data between
individual aircraft or air and ground platforms can be compromised
by bad weather. A reliable communications grid that connects mobile
units in almost any circumstance would greatly enhance commanders’
situational awareness and help maintain operational tempo.
Managed by the U.S.
Air Force Research Laboratory, Dayton, Ohio, and the Defense
Advanced Research Projects Agency (DARPA), Arlington, Virginia, the
Optical and Radio Frequency Combined Link Experiment (ORCLE) program
will develop and test this dual technology for use in future
airborne and air-to-ground communications networks. According to
Stephen Griggs, ORCLE’s program manager, the technology is unique
because it combines both radio frequency (RF) and laser
communications in a single system.
To ensure network
reliability, the program is developing software-based techniques
such as clever networking and routing to make both technologies work
to their strengths while mitigating their weaknesses. “Optical has a
very high data rate, but its reliability in a mobile environment is
iffy. RF on the other hand is very reliable, but the data rate is
not quite there. By combining the two, we hope to make a better
whole,” Griggs says.
ORCLE is a follow-on
project of the Tera Hertz Operational Reachback (THOR) program. THOR
explored the use of lasers to provide warfighters with a
high-data-rate air-to-air and air-to-ground backbone. Although
existing optical systems can transmit large amounts of data, they
are more susceptible to environmental interference and therefore
less reliable than RF-based communications. Griggs notes that optics
can be used as point-to-point, high-data-rate links, but ORCLE has a
much larger goal. “We are trying to make a network where these beams
are pointing to different aircraft and ground targets. It’s by
adding RF that we get sufficient network reliability to make it a
reasonable thing to attempt,” he explains.
Reliability is
especially important for air-to-ground links. While air-to-air
communications are relatively free from interference, atmospheric
phenomena can complicate transmissions to and from ground nodes. “If
you want to operate in a wide range of environmental conditions, you
must be able to deal with clouds, fog and sand storms, all of which
are pretty bad for lasers,” Griggs explains.
Griggs emphasizes
that the system does not simply switch a transmission from RF to
laser. Depending on the size of a message and on prevailing
atmospheric conditions, ORCLE will choose the most efficient method
for sending it. “You use them both all the time. If you have a
channel that just loves data rate, then you send that data
optically. But at the same time, other data in the network are going
to the RF side. If you’re in an optical message mode, you don’t want
to turn around a 40-gigabit-per-second link for a 10-bit [RF]
acknowledgment and reply,” he adds.
Combining the two
techniques also greatly increases a communication network’s overall
capability, says John Wojnar, director of advanced programs business
development at Lockheed Martin’s Maritime Systems and Sensors
division in Akron, Ohio. He maintains that ORCLE offers a
thousand-fold increase in data rate over pure RF messaging, noting
that DARPA and the Air Force wanted a link channel availability of
at least 95 percent. Besides high reliability, program researchers
also wanted to push data rates into the gigabit-per-second range, up
from the current megabit-per-second rate. “They [DARPA] want to be
able to increase data rates 1,000, maybe 10,000 times more than what
exists right now,” Wojnar relates.
But achieving the
program’s objectives requires technical advances in areas such as
networking and laser beam steering applications. To make clever
networking operate seamlessly, researchers are using existing
protocols, such as Internet protocol, instead of developing new
ones. But Wojnar notes that maintaining high data rates and avoiding
latency remain major hurdles. “There are a lot of system challenges
being undertaken by ORCLE that have never been done before,” he
says.
Another network
management challenge is prioritizing message traffic. In an ORCLE
node, different types of signals will favor one kind of transmission
method over the other, Griggs explains. However, if one of the
transmission modes is lost, the platform-based nodes must know what
to do. “How much data do I store? How long do I store it? What do I
tell the person who sent me the data?” he asks.
Key to this
networking technique is the ability to select RF or optical
transmission modes automatically. “You don’t want the user to worry
about whether it’s an RF communications activity that’s taking place
because it’s cloudy or if it’s a laser transmission. It needs to be
totally transparent to the user,” Wojnar maintains.
Because reliability
is central to the program, ORCLE communication nodes will alert
users about a transmission’s status. If a transmission failure
should occur, each node can store 30 minutes of data at 10 gigabytes
per second. But managing these nodes in a large ad hoc mobile
network remains a challenge. “If I’ve got an optical link streaming
data at 10 gigabits per second and an RF channel operating at 50
megabits per second, and I lose the optical channel for a little
while, I’m now caching data at 10 gigabits per second
someplace—because it’s not going on the RF channel. So you have to
wonder which of the data that was going optical is important enough
to go RF. The other stuff you store. What did you just store? How
long do you store it? How long do you wait until you try to route it
some other way?” Griggs observes.
Laser-based systems
also present challenges. Because ORCLE nodes will be installed on a
variety of airborne platforms such as fighter jets and unmanned
aerial vehicles, researchers are developing systems that conform to
the skin of an aircraft for weight and drag reduction. Wojnar notes
that traditional gimbal-based laser aiming and reflecting devices,
while adequate for large command and control aircraft, are heavy and
create drag because they protrude into the air stream.
To address this
issue, the program is examining technologies such as
microelectromechanical mirrors—clusters of tiny mirrors that can be
tilted and panned to reflect light. But he adds that they are
currently very difficult to precisely control. Scientists also will
examine the effects of turbulence and other atmospheric phenomena
that occur around the skin of an aircraft. The goal of this research
is to compensate for these effects while transmitting and receiving
laser messages.
Other potential beam
steering technologies under consideration include liquid crystal
arrays and prisms. Prism-based systems have interesting features
because they can be both reflective and refractive. “Since you are
using a standard wavelength of only 1.55 microns, you can actually
use prisms to deflect your laser beam. That helps reduce drag on any
type of device that might protrude into the air stream,” he says.
As part of the
program, DARPA also awarded a series of contracts to develop new
technologies for the effort. This research explores applications
such as a combined RF and electro-optical aperture for
high-data-rate communications, transmissive liquid crystal optical
phased arrays for laser transmissions, adaptive spectral encoding
transceiver modems, and ultra short pulsed lasers and pulse shaping
for optical wireless communications.
ORCLE was launched
early this year with Lockheed Martin being awarded a $15 million
contract as the system integrator. The program calls for testing to
begin in the late summer or early fall of 2005. Based on the success
of the environmental tests at White Sands, DARPA and the Air Force
have the option to begin a 12-month flight test program using
high-altitude aircraft.
As the chief systems
integrator, Lockheed Martin will assemble all of the system’s
different components at its Akron, Ohio, facility. There, it also
will undergo a series of tests before being shipped to the White
Sands missile range in New Mexico for more evaluation.
- Signal Journal
