Overcrowding of the electromagnetic spectrum and information security
concerns are forcing the Defense Department to consider developing
and fielding a spaced-based, optical-data relay system that would
not be constrained by bandwidth availability.
The U.S. telecommunications industry—with 115 million customers—is
pushing the Pentagon to migrate out of the portion of the electromagnetic
spectrum in the 1,755 to 1,850 megahertz range. That segment of
the spectrum is sought aggressively by telecommunications providers
in order to expand wireless services.
The band that the Defense Department occupies now is being used,
meanwhile, by commercial providers outside the United States. That
means the security of the information could be compromised, particularly
if the Pentagon doesn’t take the time to encrypt it properly.
In June, for example, the Pentagon discovered that it had been
routing unencrypted satellite transmissions of spy-plane activity
to a worldwide viewing audience.
“European satellite TV viewers can watch live broadcasts
of peacekeeping and anti-terrorist operations being conducted by
U.S. spy-planes over the Balkans,” said an article in the
Guardian newspaper, based in London. “For more than six months,
live pictures from manned spy aircraft and drones have been broadcast
through a satellite over Brazil.
“The satellite, Telstar 11, is a commercial TV relay,”
said the Guardian. “The satellite feeds have also been connected
to the Internet, potentially allowing the missions to be watched
from around the globe.”
Pentagon officials are considering ways to avoid these problems.
One option would be to deploy a system of satellites employing optical
data links that communicate with ground stations. This system would
make it possible to reduce payload weight, would provide on board
energy and increase data transmission rates, experts said. There
also would be improvements in security.
Industry observers speculated that much of the experimentation
for such a communications network is being done by the U.S. government
in the secret, so-called “black” world.
However, recent activities by the European Space Agency and its
ARTEMIS program provide clues as to how such a system will operate
and why the Pentagon is so gung-ho about this technology.
“We are pursuing the development of laser communications
in space that has the potential to provide fiber optics-quality
broadband, secure communications anytime and anywhere U.S. forces
may operate,” said Deputy Secretary of Defense Paul Wolfowitz,
during a Senate hearing. “This capability could have a revolutionary
effect across many of our programs, because bandwidth limitations
are one of the key constraints on our ability to exploit unmanned
systems, networked information systems, and new surveillance capabilities.
“Laser communications is a good example of the synergistic
effects that capabilities in one area can have on others,”
he said.
The Pentagon’s 2003 budget includes $200 million to begin
research work that would lead to a laser satellite constellation.
Given the significant investment that the U.S. government made
in this technology in the past, it seems surprising that space-based
optical systems are not the norm, rather than the exception. A reason
for that, sources said, has been a decline in science and technology
funding throughout the 1990s, which has resulted in fewer technologies
mature enough for demonstration in space. Higher satellite and launch
costs also have caused a lag in the near term in space demonstrations.
NASA had been exploring spaced based optical relay systems as early
as the 1950s, by bouncing laser beams off satellites in orbit and
sending them to receiving stations. “In one experiment, a
technician who knew Morse code chopped the beam with his hand, sending
a message to a colleague at the remote receiver station,”
according to Ball Aerospace’s History of Laser Communications.
“Initially, the person at the receiving site thought something
was wrong with the equipment until he realized that there was a
pattern in the signal drop outs.”
The earliest attempt at laser communications from space was during
the Gemini 7 manned space mission in 1967. Astronaut James Lovell
peered out one of the space capsule windows and observed a beacon
laser sent from a ground station. Upon viewing the beacon, he placed
a handheld transmitter in the window and attempted to communicate
to a ground station. The experiment was unsuccessful.
One of the first demonstrations of the potential for laser communications
occurred in 1980, with the Airborne Flight Test System, sponsored
by the U.S. Air Force. A 1-gigabit per second data rate link was
established between a KC-135 aircraft and a ground station located
at White Sands, New Mexico. The program was successful enough that
the Air Force decided to proceed with production of satellite-to-satellite
communications.
In the 1980s, the Defense Advanced Research Projects Agency undertook
an effort to develop laser communications between satellites and
submerged submarines, but that program was cancelled for lack of
interest by the U.S. Navy. Oddly enough, in his June 2000 “pork
barrel” speech chiding the Defense Department for wasteful
spending, Senator John McCain (R-AZ) included laser communications
on his hit list and indicated that, along with similar programs,
“Only the cast of Star Trek could conceivably have looked
at a list of military funding shortfalls and concluded that had
to be in the fiscal year 2001 budget.”
Communicating by Light
A look at the success of the European Space Agency’s ARTEMIS
(Advanced Relay Technology Mission) program provides clues as to
why the Pentagon is promoting space-based optical systems and intends
to spend billions of dollars to get them into operation.
Space-based data relay systems have been around for many years.
For example, NASA’s Tracking and Data Relay Satellite System
(TDRSS) has been the premier space-based data relay system for a
wide variety of users. TDRSS operates in the S-band (2200-2300mhz),
KU-band (10.7-14.5ghz) and KA-band (26-30ghz) frequencies and transmits
hundreds of million bits of information each second from a user
satellite. But such a system linked by optics, with their shorter
wavelengths, would far surpass the S, KU and KA-band capabilities.
According to Gotthard Oppenhäuser, ARTEMIS program manager
at the European Space Agency, “The wavelength of the laser
used on ARTEMIS was 800 nanometers, roughly 10,000 times shorter
than the wavelength used on TRDSS. Such a short wavelength allows
optical energy to be focused into a very narrow beam. The high-energy
concentration also enables the use of much lower energy sources.
“Optical systems also are attractive because they are almost
interference free and practically impervious to interception by
an unauthorized user,” he wrote in the trade journal Aerospace
America.
In November 2001, the European Space Agency made history by creating
the first ever “laser data link” between two of its
satellites and an optical ground station. Launched in July 2001
on an Ariane 5, ARTEMIS, a satellite propelled by an ion thrust
engine, carried an optical data relay payload called a semiconductor
laser inter-satellite link experiment (SILEX).
This system can transmit data at rates up to 450 megabits per second
and provides an optical transmission link to a SPOT 4 satellite,
according to ESA documentation. ARTEMIS is in orbit at 31,000 kilometers
while SPOT 4 is at 832 kilometers.
According to Oppenhäuser, it’s an “artful”
task to point a beam of light at another spacecraft 42,000 kilometers
away and moving at a speed of 7,000 meters per second. Engineers
must, literally, play with light. “The time required for the
light to travel over this distance is not negligible. It takes about
140 milliseconds for the light to arrive where SPOT 4 was when the
light was transmitted. In this same period SPOT will have moved
about 1,000 meters from ARTEMIS and the beam will no longer hit
the target. ARTEMIS must calculate where the partner satellite will
be when the light arrives and point the beam accordingly.”
ARTEMIS proved its capability when SPOT 4 was taking pictures of
South America while ARTEMIS was located over the continent of Africa.
The system operated over an 18-minute period. “The image processing
center in Toulouse, France, received a complete sequence of pictures
covering French Guyana, Suriname, Brazil, Bolivia and Chile. The
delay between taking the pictures and their receipt in southern
France was less than one second.”
In an interview, Oppenhäuser suggested that the reason the
United States has not made as much progress as Europe in developing
laser satellites has to do with the lack of financial incentives
for companies to invest in this technology.
In the United States, he said, “there were plenty of programs,
very ambitious, sometimes unrealistically ambitious. But no program
went much further than the breadboard stage or qualification stage.”
The reason is that American industry saw little chance to make money
in the near future. And NASA has almost given up research and development
in the communication area, said Oppenhäuser.
“When the military people do not support the programs, then
nobody will do it,” he said. Another obstacle is the prevailing
skepticism based on the notion that, if “nobody has done it
yet, perhaps it does not work.”
He noted that SILEX would never be a commercial success. “But
it demonstrated the capabilities and may pave the way for an extended
application in a more developed form,” said Oppenhäuser.
“Space to space laser communication will grow, space to ground
will need some time, at least in non-military applications.”
Asked whether space-based laser to ground communications offers
a solution to the bandwidth problem, Oppenhäuser said it does,
“if it works. ... There is almost unlimited bandwidth available,
and the link can be established relatively easily when there are
no clouds.
“In clear sky conditions, optical links are easy to establish.
In the SILEX system, we transmit 50 megabits per second to a 1-megabit
ground station, using rather primitive modulation schemes and only
60 milliwatts of laser power. With more advanced modulation schemes
and higher laser power, that is now available, much higher data
rate can be reached.”
A light cloud, however, will interrupt the link or will prevent
establishment of the link. “A way out may be to have the ground
stations at places with favorable atmospheric conditions,”
he said. “When there are clouds over one station, you may
have clear sky over another station. This means that you receive
the data not where you need them.”
Oppenhäuser said that he does not believe that optical space-to-ground
communication will have a civilian application in the foreseeable
future, due to the low availability of the service. “However,
I believe in a fast development of optical inter-satellite communication.”
Space-to-ground optical communications are secure, he said. “The
reason is the extremely low width of the optical beam.” In
SILEX, the optical beam carrying the data has a width on the ground
of a few hundred meters only after having traveled through space
over a 40,000 km distance.
The art of establishing an optical link is to point an extremely
narrow beam towards the partner station. In fact, the probability
that an unwanted user is illuminated by the beam is very low. He
would have to be in the direct vicinity of the wanted user station,
said Oppenhäuser.
“In space, it is even impossible that another satellite ‘listens’
to the optical beam. The system will be so dynamic, that nobody
other than the wanted partner can follow the transmitter.”