Space-based optical comms could fix bandwidth problem.

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 Wolfowtiz, 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 rare 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 5, KU and KA-band capabilities.

According to Gotthard Oppenhauser, 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 pay-load 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 Oppenhauser, 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 raking 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 raking the pictures and their receipt in southern France was less than one second."

In an interview, Oppenhauser 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 Oppenhauser.

"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 Oppenhauser. "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, Oppenhauser. 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 our 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."

Oppenhauser 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 Oppenhauser.

"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."

RELATED ARTICLE: Navy, Air Force to Develop Twin-Mirror Laser-Retargeting Satellite Technology

The U.S. Navy and the Air Force are sponsoring a research laboratory focused on developing technology for a revolutionary new satellite, able to receive and re-target laser beams anywhere on earth.

The lab is located at the Naval Postgraduate School, in Monterey, Calif.

The school's superintendent, Rear Adm. David R. Ellison, said that this project is the "epitome of the joint interdisciplinary research efforts that will drive our nation's future military capabilities, and which none of us could do alone."

In the newly christened NPS/Air Force Research Lab Optical Relay Spacecraft Laboratory, researchers successfully demonstrated the laser tracking ability of a prototype twin-mirror Bifocal Relay Spacecraft designed to receive "up" beams and refocus them via a 'steering' mirror and second main mirror onto targets of choice on the ground.

If fielded, a constellation of 27 of the twin-mirror satellites will orbit at 715 kilometers sometime in the next decade.

"This is breakthrough work towards our goal of instantaneous global power with global reach," said R. Earl Good, director of the Directed Energy Directorate of the Air Force Research Laboratory, at Kirtland Air Force Base, N.M., which is co-sponsoring the project.

The Bifocal Relay Mirror Spacecraft project owes its very existence to NPS officer students. It started as a student spacecraft design project. In 2000, the NPS/AFRL team won the prestigious National Reconnaissance Office Director's Innovation Initiative Award, and with it $340,000, to further develop the technology.

During the next five years, the new laboratory and research effort will receive approximately $3.5 million in Air Force and Missile Defense Agency funding.

"What you're seeing is the only integrated spacecraft control/optical technology demonstration anywhere," said Air Force Capt. Mary Hartman, program manager for AFRL's relay mirror technology program.

AFRL scientists have the lead on the project's optics, while the Navy's corporate university has the responsibility for spacecraft control.

"The Bifocal Relay Mirror Spacecraft project marries the best expertise of our two groups--NPS's in spacecraft attitude and vibration control and AFRL's in high technology optics," said Aeronautics and Astronautics Prof. Brij Agrawal, director of the NPS Spacecraft Research and Design Center.

Good said the most likely future system would be a mix of ground-based, airborne, and space-based lasers in addition to the beam-retargeting space-based mirrors.

"This is not an either-or proposition," he said. "They're complementary."

According to project managers, the bifocal mirror tracking and targeting system is likely to first be tested on a lighter-than-air balloon or airship as a stepping stone to the ultimate space platform.

Agrawal stressed that the technology being developed for the space-mirror project is widely applicable to a number of other areas, such as reconnaissance, space optics, space communications, remote imaging enhancing night vision capabilities, camouflage detection and penetration, chemical warfare agent detection and identification, theater wind profiling, tunnel and underground structure detection, and cloud ceiling detection.

One of the most intriguing potential uses of the space mirrors would be as giant flashlights to light up future battlefields.

For more information on NPS Bifocal Relay Spacecraft Research Program, or on the NPS-AFRL Optical Relay Spacecraft Laboratory, contact Prof. Agrawal at [e-mail].--Barbara Honegger, Naval Postgraduate School Public Affairs Office