Months after its July fly-by, New Horizons is still squeezing its images down pipe measured in bits-per-second – and that's a problem space boffins would like to solve in the future.
As we know from NASA's successful LADEE test, lasers are a viable and truly broadband space comms medium – but firing a laser from (say) Mars and receiving it on Earth would need a Bloody Big Telescope.
That's what boffins from Japan and Madrid reckon the world will have, once the proposed Cherenkov Telescope Array (CTA) is built, so in this ArXiv paper, they propose using that instrument for deep-space optical communications.
The CTA is going to have dozens of instruments of varying size (from 6m up to 24m) in both hemispheres, a requirement for deep space communications, and as Alberto Carrasco-Casado (National Institute of Information and Communications Technology in Tokyo) and collaborators José Manuel Sánchez-Pena and Ricardo Vergaz (University of Madrid) note, the instruments should be good enough to do double-service as communications receivers.
That's because handfuls of photons are exactly what the CTA receivers are designed to detect – the tiny flashes of light that cosmic rays and gamma rays produce when they collide with the upper atmosphere.
CTA telescopes should be able to pick up comms except
where the Sun is too bright (Mars conjunction, Lagrange 1)
Image: CTA telescopes as deep-space lasercom ground receivers at Arxiv
While the ideal solution, the boffins write, would be to modify some CTA telescopes for optical communications, shared operation (which would be financially attractive) is also possible.
The researchers say they've measured the reflectance of mirrors intended for use in the CTA, and found that they're highly efficient at the common-for-communications optical wavelength of 1550 nm: “the reflectance at FSOC [free space optical communications – The Register] wavelengths is even higher than in the Cherenkov region for current IACT mirrors, reaching over 90% at 1550 nm”, they write.
Because the comms detectors don't have to be good enough for imaging, they'll be simpler and smaller than the powerful cameras being designed for the CTA, the authors write.
Surprisingly, the researchers believe the receivers would even be able to detect some signals during daytime. If the field of vision is kept tight, atmospheric reflection of sunlight would add noise, but still leave enough signal for the detectors to work.
The system would work day or night from low-Earth-orbit to the Moon, the researchers estimate, and even Mars communications would work 24x7 when the Red Planet is in opposition.
It's also a relatively affordable proposal, the authors say: modifying the CTA instruments would be far cheaper than the cost of building dedicated telescopes for laser communications (which they estimate would reach €100 million for Mars-distance communications).
Wikipedia has an outline of the CTA project here. ®