As we move up the frequency ladder toward optical communications, each step takes us closer to the kind of data traffic we’ll need for deep space missions into the Kuiper Belt and beyond. The idea is to pack as much information as possible into the signal. A stream of data transmitted from an antenna spreads at a diffraction rate that is determined by the wavelength of the signal divided by the diameter of the antenna. Higher frequencies, then, give us a much narrower signal, alleviating bandwidth crowding. And a laser communications system makes fewer demands upon a spacecraft’s power sources than radio.
So watch developments like the recent experiment performed by the Japan Aerospace Exploration Agency (JAXA) with interest. The agency carried out a successful optical test using laser beams between its ‘Kirari’ satellite (also known as the Optical Inter-orbit Communication Engineering Test Satellite) and a mobile ground station in Germany. The downlink occurred with the satellite at about 600 kilometers altitude and lasted for three minutes.
We have much to do to iron out a laser communications infrastructure, but demonstrating communications with a mobile station on Earth points to a newfound flexibility in these operations. Lasers will give us data rates a hundred times faster than current radio systems, and will offer mission planners the ability to pack more and more high-resolution tools onto their vehicles for uses such as synthetic aperture radar and hyper-spectral imaging that are far more demanding than photographs. And someday, lasers will carry data from our first dedicated interstellar probes as they close on nearby stars.
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A light beam can carry several messages simultaneously by modulating frequency, amplitude, phase and polarity independently. (see “Contact” by Sagan.) It must also be possible to code multiple messages in the same modality (and sort them out at the receiver using fourier transform). The bandwidth of a single blip could be huge.
Yes, and communications at interstellar distance seem quite workable as well. JPL’s James Lesh has written (in JBIS) about a 20-watt laser system using a 3-meter telescope communicating with a 10-meter telescope on Earth. Lesh argues that if we could solve the propulsion problem, the communications issue would be solvable even today. And, of course, the thought of wide-bandwidth communications for Solar System missions and the data they could convey is mind-boggling.