Given how tricky it is to pick up accidental radio signals — “leakage” — from extraterrestrial civilizations, how hard would it be to communicate with our own probes once they’ve reached a system like Alpha Centauri? A front-runner for interstellar communications is the laser. JPL’s James Lesh analyzed the problem in a 1996 paper, concluding that a 20-watt laser system with a 3-meter telescope as the transmitting aperture could beam back all necessary data to Earth. It’s a system feasible right now.
Right now, that is, if we had some way to get the telescope, just a bit larger than the Hubble instrument, into Centauri space. But even though propulsion lags well behind laser technology for such a mission, we’re continuing to study how lasers can help closer to home. Their high frequencies allow far more data to be packed into the signal, but the highly focused beam also uses a fraction of the power of radio. Data return becomes less of a trickle and more of a flood (imagine high-definition moving video from Mars).
How to handle atmospheric effects that can hamper Earth-based receivers? It’s a problem even on cloud-free days because dust, dirt and water vapor can still scatter light and deflect parts of the beam. Listen to Penn State’s Mohsen Kavehrad: “Free space optical communications offer enormous data rates but operate much more at the mercy of the environment…All of the laser beam photons travel at the speed of light, but different paths make them arrive at different times.”
The result: data ‘echoes’ that confound accurate reception. But the project Kavehrad is working on, funded through the Defense Advanced Research Agency, aims at achieving almost 3 gigabytes per second of data over a distance of 6 to 8 miles through the atmosphere. What the Penn State team has done is to bring digital signal processing methods to bear on laser communications to make the optical link more reliable. They call their approach free-space optical communications. Here’s how a Penn State news release describes the system’s operation:
Using a computer simulation called the atmospheric channel model developed by Penn State’s CICTR, the researchers first process the signal to shorten the overlapping data and reduce the number of overlaps. Then the system processes the remaining signal, picking out parts of the signal to make a whole and eliminate the remaining echoes. This process must be continuous with overlap shortening and then filtering so that a high-quality, fiber optic caliber message arrives at the destination. All this, while one or both of the sender and receiver are moving.
The system works both for air-to-air and air-to-ground links, and provides fiber-optic quality signals. But extend the premise to the growing needs of the Deep Space Network to relieve spectrum overcrowding and provide reliable high-bandwidth links to spacecraft around the Solar System. We’re moving toward a future model of networked space vehicles, communicating not only with Earth but also with each other to coordinate data transfers that will one day be optical.
The bright future of optical communications relies on resolving complications like atmospheric distortion. NASA’s Table Mountain facility in the San Bernadino Mountains houses a one-meter laser telescope used as a testbed for refining data tracking in future space missions. That and a variety of space-borne tests have already demonstrated the viability of the concept. One day we may use it for deep space work and who knows, the reach of the laser may someday carry data from a distant star.
For those who want more details on the Alpha Centauri communications paper mentioned above, it’s Lesh et al., “Space Communications Technologies for Interstellar Missions,” Journal of the British Interplanetary Society 49 (1996): 7-14.
