Communication | Sending images with laser using sail as antenna

Images of the target planet could be transmitted by a 1Watt laser onboard the nanocraft, in a ‘burst mode’ which uses the energy storage unit to rapidly draw power for the power-intensive laser communications mode. Upon approach to the target, the sail would be used to focus the laser communication signal.

For a 4m sail, for example, the diffraction limit spot size on Earth would be on order of 1000m. A kilometer-scale receiving array would intercept 10-14 of the transmitted signal. The main challenge is to use the sail as diffraction limited optics for the laser communication system. This would be achieved by shaping the sail into a ‘Fresnel lens’ upon approach to the target. The sail structure could be different at the launch and communication phases. In order to maintain a high transmission through the Earth’s atmosphere, the communication would need to operate at a wavelength shorter than that used by the launch laser system, due to the Doppler shift of the nanocraft relative to the Earth.

Apr 14, 2016 13:52 Nemo in Nihilum Posted on: Breakthrough Initiatives

@roboturner:
most likely not. I only flew about the ideas for the sail yet, it's on my "to read" list, but what I saw till now is, that the sail will be far too ... thin and too light for any photovoltaics (of current state of the art at least. If you have an idea how to use the lightweight sail construction, which has to be flexible, serve several purposes and withstand the initial heating and fine grained impacts, for photovoltaics, that might change things of course. And to be honest, those kind of ideas are the reason for which we are here.).

@Karen:
Yes, definitely on board laser,... just estimate the remaining power of the earthbound beam when it reaches the 4m⌀ sail at Alpha Centauri.
We should make sure that error Anthony noted, does not make problems on other locations of the project.

@Adam and Marcus:
The idea of an interstellar relay poses several problems:

1. it adds points of failure. Given that these probes maybe cheap but not for free, I would expect, that financiers won't give us more money than for a single chain, maybe a double one.
Therefore relying on relays, just adds single (or double) points of failure, which I would want to risk as few as possible on such long term missions.

2. the current design needs the sail for the communication as a focus reflector. the following probes therefore are not equipped to receive transmissions from their direction of flight.

3. signal degradation must not be forgotten.
While we can receive and maybe try to improve a signal on earth, maybe intelligently adapt some filters and such, onto a stored signal, the following probes won't be able to make these intelligent decisions. Moreover, we can only draw the maximum out of a signal which we do receive. We can even try to correct or guess missing bytes.
If we receive the Xth iteration of a signal, we have no way of knowing where in the signal stream an error occurred and which effect to counter.
Has one probe had an error during transmission? Which parts of the signal have been weak and therefore maybe not correct? We don't know, since the relaying probes just transmitted what their logics interpreted to have received (not exactly but somewhat like "whisper down the lane").
Don't forget, we are not talking about IP communication, where defective packets can be resent upon request from the receiver.
This is a one way connection. The best you can get is maybe an ECC bit and sending the whole transmission twice. Maybe you could write a new protocol containing an exact error correction, but I guess you won't reduce such a signal to less than 150% of the original signal. And this kind of signal still can break if whole packets get lost or are unclear or if relays malfunction.
So we are back to my starting point of problem 3.

4. the following probes don't have a km scale receiving array, therefore you would need quite many of them, which makes points 1 and 3. worse.

@Karen
I would not expect the craft to be spinning, because a spinning craft brings us so many problems:
1. when we try to steer the craft, we receive nutation,
2. when we fly by the target, we not only have to match the camera to a flyby with 0,2c , we also have to match the camera to the rotation of the craft,
3. moreover the rotation costs us camera or sensor time during the fly by.
4. a rotation would put the construction of the sail under constant stress for decades, while a steady craft would experience no forces for most of the time (except for occasional mini impacts).



Well, before reading up here, I wanted to mention something:

We should take care our sender/receiver does not get molten by the propulsion lasers.
Since the sail is used as reflector for the communication, the sender/receiver will receive quite some energy, even if the sail is not focused on the sender/receiver unit during the acceleration phase.

Apr 16, 2016 09:09 advax@triumf.ca Posted on: Breakthrough Initiatives

If there are thousands of probes, I wondered if they could co-operate in a swarm to create a phased-array transmitter at radio wavelengths, with a much larger effective diameter than a single sail. Radio telescopes on earth have a much finer resolution than optical telescopes for this reason.

Apr 16, 2016 21:07 Karen Pease Posted on: Breakthrough Initiatives

New thought: any possibility of using gravitational lensing to refocus the diverging beam? Has anyone run the numbers for what sort of geometry would be needed for, say, a hypothetical earth-mass planet at Alpha Centauri, to lens the transmission beam to refocus it back on Earth? If you flyby directly ahead of the planet in its orbit then you should get an opportunity to have the planet between you and the Earth.

Obviously you'd get more of a lensing effect from a star, but then your signal would be utterly drown out by the light of the star.

Apr 17, 2016 06:02 john.hayden1@gmail.com Posted on: Breakthrough Initiatives

Doubt the gravitational lensing by the destination bodies is workable, given their relatively low mass and the positional precision required. The lens might only work at a small focus point, which might not be known in advance.

Apr 17, 2016 06:05 john.hayden1@gmail.com Posted on: Breakthrough Initiatives

IF the sail is used as a focusing lens for the laser, it must be designed to still work (optically precise shape, high reflectivity) even after sustaining damage during the 20yr 0.2c voyage. For shape: It might have to be designed in separate patches so that damage to any one region of the sail (that cripples any shaping mechanism) does not degrade the other zones of the sail.

Apr 17, 2016 06:07 john.hayden1@gmail.com Posted on: Breakthrough Initiatives

Also note, at the destination, the sail will experience illumination and a thrust vector from the destination star, which will affect course and pointing. The spaecraft may have to exert a balancing torque to maintain direction, which must be figured into the total energy/fuel budget.

Apr 19, 2016 05:42 marco palma Posted on: Breakthrough Initiatives

on the day that the project is ready perhaps quantum communication is accessible which facilitated all kinds of communication between earth and NanoCraft

Apr 21, 2016 00:40 David James Posted on: Breakthrough Initiatives

If we're Sending several thousand of these micro probes why not stop some of them along the way and use them as relays or repeaters? As a collective if you were to link them all together and have them scan the solar system. Possibly from lagrange points the whole system could be scanned and monitored. Not to mention the benefits of having several hundred of these in interstellar space.

Apr 21, 2016 00:46 David James Posted on: Breakthrough Initiatives

You could also put them in arrays to allow for transmission of a larger beam.

Apr 23, 2016 22:43 michael.million@sky.com Posted on: Breakthrough Initiatives

We can use them as relays and we can alter the velocity of each sail as appropriate.

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