Lasers take on space communications

Optical communications systems, with their high bandwidths and information capacity, may soon be accessible in space applications. The National Aeronautic and Space Administration (NASA) and the European Space Agency (ESA), among others, are working rapidly to develop the technology for free-space laser communications in Earth orbit that would link satellites with each other and with ground communications systems. One long-term goal may be to replace radio-frequency communications with lasers, even for deep-space missions to the planets. Limiting factors remain the serious challenges related to pointing technology and laser size and efficiency.

In-orbit tests to begin

The ESA, in cooperation with the French space agency, has planned the first in-orbit test of free-space laser communications. The Advanced Relay and Technology Mission (ARTEMIS) data relay satellite should reach geosynchronous orbit in the first half of 2001.¹ The satellite carries the Optical Payload Experiment terminal of the intersatellite laser link. The other terminal is on the Earth Observation Satellite Spot 4 launched in 1998.

In ESA's Semiconductor laser Intersatellite Link Experiment (SILEX), a 60-mW laser operating at 800 to 850 nm links the two satellites. Information capacity is 50 Mbit/s. As with all laser space communications, the key problem is to accurately point the narrow beams, which typically have an angular divergence of only a few microradians, and hit a foot-wide target at a distance of 100 miles.

For its part, NASA is preparing to debut laser space communications on board the International Space Station in October 2002.² Its Optical Communication Demonstration and High-Rate Link facility will be a ground-to-satellite link with 2.5 Gbit/s of capacity. The system's 200-mW erbium-doped fiber amplifier, which operates at 1550 nm and must be funneled through a telescope 10-cm in diameter, should achieve a bit error rate of one in a million during clear weather across distances as great as 1000 km.

The facility's ground receiver station will use a commercial fiberoptic receiver package. To aid in the critical task of pointing the laser beam, the ground station will illuminate the space station with eight overlapping 980-nm laser beacons, each with 1.25-W output power. The overlapped beams have less scintillation or twinkle than a single beam would have. Their divergence is 200 µrad, much larger than the few microradians of the transmitted beam. If needed, an even wider, 1-mrad beam is possible using a pulsed Nd:YAG laser.

Pointing is key

The narrow beam width of laser communications is one of its primary advantages, since it allows a higher intensity at the receiver for the same power at the transmitter. That same narrow beam width, though, creates the main technical challenge to practical laser communications in space—the receiver must be acquired with the slender beam, and the connection must be maintained with accurate pointing throughout transmissions.

To develop accurate laser pointing systems, researchers at NASA's Jet Propulsion Laboratory (JPL; Pasadena, CA) are working with a prototype pointer-tracker terminal called the Optical Communications Demonstrator (OCD).³ For simplicity, the terminal uses just one fast-steering mirror for pointing and one focal-plane array for detecting a signal. During acquisition of the laser beacon, the control software scans the charge-coupled-device (CCD) camera image from the array. Once the beacon is found, the transmitting laser is turned on, and the CCD focuses on the pixels around the beam at a frame rate of 2 kHz. The control software generates pointing signals that move the transmitting beam to the desired location.

By accurately measuring the intensity in neighboring pixels on each side of the beam, scientists can determine the center of the beam to within a fraction of a pixel. Laboratory tests at JPL indicate that the system has an accuracy of about one-tenth of a pixel, or a little more than 1 µrad, good enough for almost any free-space application.

Communications between the ground and space, however, must operate through the atmosphere where scintillation makes the pointing problem more difficult. To examine this issue, another JPL team is testing the OCD between two mountains 47 km apart. The test situation is in one sense less favorable than the satellite case, since the mass of atmosphere between the two mountains is four times larger than that encountered looking straight up to a satellite. On the other hand, the pointing is greatly simplified, since the target is static, not moving like a satellite does.

So far, test results indicate that total errors due to turbulence, tracking errors, and other causes amount to 5 or 6 µrad—considerably larger than the 1 µrad seen in the laboratory, but still satisfactory for most applications. The JPL researchers do not think this is the limit of performance.

One potentially large cost-saver for laser communications for satellites is the capability to use commercial off-the-shelf technology developed for fiberoptic communications. Experiments by researchers at Ball Aerospace Technology Corp. (Boulder, CO) and Kirkland Air Force Base (NM) indicate that 1550-nm off-the-shelf equipment can provide the performance required in free-space communications, even after receiving a 100-krad dose of radiation typical of what might be experienced in space.

Lasers on Europa? Not yet

In Earth orbit, where satellites tend to be heavy and the weight of communications systems is not a major concern, the greater bandwidth of laser communications appears to be a big advantage. For NASA's deep-space probes to other planets and their satellites, the equation changes. These probes must be much smaller and the communications package mass becomes more of an issue.

Recently, JPL scientists carried out a systems study that compared laser and radio communications for a planned NASA mission to orbit Europa, one of the four major satellites of Jupiter.⁴ Europa became a major target of space exploration after the space probe Galileo sent back evidence that it has a vast ocean of water under several kilometers of ice. The Europa orbiter, by taking careful gravitational and radar measurements, should be able to provide a definitive answer.

The study found that deep-space laser communications face severe challenges. Instead of operating across thousands of kilometers, the systems must work at distances that could reach almost a billion kilometers from earth. Pointing requirements of a microradian or less would be very difficult to achieve, since the distance involved precludes the use of a laser beacon. Instead, the laser communications system would have to home in on the tiny, dim image of Earth itself or use stars for ultraprecise orientation.

Another issue involves signal interruption by the cloud cover on Earth. While this is also a problem for earth-to-orbit communications, redundant receiving stations to ensure clear skies somewhere are feasible. Redundant systems for deep-space systems would be too costly.

Another serious and unresolved difficulty involves the gaps in coverage that occur during solar conjunction, when Earth lies on the other side of the Sun from Europa. Since optical telescopes must be protected from pointing too close to the Sun, communications would be out for 48 days every 13 months. The outage is only 14 days for radio communications.

Although the scientists found they could implement a laser communication system for a Europa orbiter with a brute-force approach, its performance would barely improve on that of radio communications and would not be worth the extra effort.

Radio frequency still has significant advantages that counterbalance the frequency advantage of optical communications. At the moment, radio remains considerably more efficient than lasers, so transmitters are more powerful. Radio antennae also can be far lighter, both in space and on earth.

"History suggests that we move to higher frequencies, so NASA has a long-term interest in deep-space laser communications," says JPL researcher Chien Chen. "To make such equipment practical, though, we must increase efficiency, develop lighter optics, and improve pointing and tracking." In the meantime, the Europa Orbiter will use X-band RF communications.

While efficient deep-space laser communications may be far into the future, the high bandwidth of lasers makes it almost certain they will begin carrying part of the communications burden in Earth orbit within a few years.

REFERENCES

1. B. Demelenne et al., Proc. SPIE 3615, 2 (Jan. 1999).
2. J. V. Sandusky et al., Proc. SPIE 3615, 185 (Jan. 1999).
3. M. Jaganathan et al., Proc. SPIE 3615, 23 (Jan. 1999).
4. C. Chen et al., Proc. SPIE 3615, 142 (Jan. 1999).