How Invisible Waves Have Changed the World
2023-12-19 [Petri]
I discussed the possibility of using lasers for spacecraft communications in my book. Some experiments were already done at the time, but now NASA has broken all earlier records with its Deep Space Optical Communications (DSOC) demonstrator. And as the Psyche probe that carries the demonstrator keeps moving further away from the Earth, there will be other new milestones in the future.
Receiving and transmitting radio signals from and to space probes has been the sole means of communications in all of the space missions. It is a proven technology, but it suffers from the seriously degrading bandwidth as the distance grows.
For example, when the New Horizons spacecraft flew past Pluto in the summer of 2015, it filled its internal memory with measurement data and images. But as the available data rate from that distance was roughly 2 kbps, or roughly seven times the speed of the early acoustical modems, it took about 15 months to get all that data beamed back to Earth.
Therefore, if the spacecraft had had any technical problems during this time, all the unsent data would have been lost forever.
Even though these data links use microwaves, the available bandwidth is dwarfed by what you can do by modulating light. This is the reason why the whole Earth is crisscrossed by optical cables: the advantages of using coherent laser light make it possible to shift the mind-boggling amounts of Internet data between continents that is happening at every second.
As a direct continuation to the laser-based space communications tests that I listed in my book, NASA did their first “beyond Moon distance” test on December 11th, sending a 4K cat video repetitively over a 267 Mbps data link with their Deep Space Optical Communications (DSOC) demonstrator.
Although the test distance of 31 million kilometers was already 80 times the distance to the Moon, it was still just about 1/8th of the average distance to Mars. But as the Psyche probe that carries the test setup goes continuously further out towards the asteroid Belt between Earth and Mars, additional tests will be made in the future to ensure that this concept remains viable also at much larger distances.
Psyche will reach a maximum distance of 2.7 Astronomical Units, and hence will be able to perform tests from further away than the largest possible distance between Earth and Mars.
The use of radio signals from deep space is not only hampered by the unavoidable inverse square law that dictates the signal attenuation by distance, but also by random interference from Earth-based sources that use the same or close-to-same frequencies.
NASA uses huge radio telescopes for deep space communications, and also requires massive transmission power of several tens of kilowatts to provide an uplink to the probes, as the probes can only have modest antennas onboard: for example, the main communications antenna of the New Horizons probe is just 2.1 meters in diameter.
When using laser light in combination of a very narrow aperture of a large optical telescope and NASA’s brand-new Superconducting Nanowire Single Photon Detector technology, the “data path” between the probe and the Earth is literally just empty space: there are no other signal sources interfering with the laser beam. And although theoretically the useful bandwidth of optical links would degrade faster than with radio links, this new photon counting technology is improving the odds considerably, together with the much higher initial bandwidth available.
Weight it everything when it comes to space probes, and the possibility of higher-bandwidth communications, combined with lower size and weight, is the promise that appears to be achievable, based on the ongoing tests.
Another laser-based test is gearing up much closer to home: NASA’s ILLUMA-T setup just went up to the International Space Station. It will beam high-bandwidth data from the ISS to a geostationary Laser Communications Relay Demonstrator, which then beams the data down to Earth. With this distance, the testing will attempt to achieve at least tenfold increase in bandwidth between ground stations and the ISS.
As I wrote in my book, it is highly likely that the future comms to and from a Mars colony are light-based, and these recent tests seem to have verified this possibility.
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Still from the "Space Cat" video stream [NASA/JPL-Caltech]
You can purchase A Brief History of Everything Wireless: How Invisible Waves Have Changed the World from Springer or from Amazon US, CA, UK, BR, DE, ES, FR, IT, AU, IN, JP. For a more complete list of verified on-line bookstores by country, please click here.
Earlier entries:
You can purchase A Brief History of Everything Wireless: How Invisible Waves Have Changed the World from Springer or from Amazon US, CA, UK, BR, DE, ES, FR, IT, AU, IN, JP. For a more complete list of verified on-line bookstores by country, please click here.
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