Communication | Pointing transmitter towards earth

Finding the Earth should be reasonably straightforward, given its proximity to the Sun, which would be bright from the vantage of Alpha Centauri. The on-board star tracker would also be useful, as would locking onto the Starshot laser system.

It may also be feasible to send commands and even reprogram the entire nanocraft via the Earth-based laser system. The angular diameter of a diffraction-limited beam, at a wavelength of 1 micron with a meter-class antenna, is on the order of 0.1 arcseconds. Pointing to this precision could be achieved by using the photon thrusters.

Comments (29)

  1. Aakash Rao:

    Hi, I am a student and not much of an expert but could we place something like a gyro, which always point towards the earth so that we could send signals later

  2. michael.million@sky.com:

    I think every now and then the laser should be fired so that at least some laser light could be used to allow the probe to correct its trajectory and allow better pointing towards Earth.

  3. John Gates:

    When the probes are 4.3 years out from PC fire up the laser used to propel them and send a beam at the collection of probes. As the probes reach PC they open up a set of retro-reflectors which can be modulated with far less power than an on-board transmitter would use. The beam from the laser array would arrive at the time of reflector opening and, then being modulated, would inherently send the data directly back toward Sol.

    Just a thought.

  4. Breakthrough Initiatives:

    RE:
    Feb 23, 2017 07:12 Aakash Rao Posted on: Breakthrough Initiatives

    Answer:
    Thanks for your question. We think that gyro would weigh to much to be carried on the star ship. We need to use and reuse everything many things. Our current thoughts are to use the camera onboard to look for the sun and other bright stars to be able to find our locations.

    - Pete Klupar, Breakthrough Starshot

  5. Breakthrough Initiatives:

    RE:
    Mar 12, 2017 10:39 michael.million@sky.com Posted on: Centauri Dreams

    Answer:
    Thanks for your input. This is a very good idea. We will use it

    - Pete Klupar, Breakthrough Starshot

  6. Breakthrough Initiatives:

    RE:
    Mar 13, 2017 19:50 John Gates Posted on: Breakthrough Initiatives

    Answer:
    Thanks for your input. This is indeed a good idea. We need to find retroreflectors that are light enough to meet our mass budget

    - Pete Klupar, Breakthrough Starshot

  7. Simon Dawson:

    Pete,

    I have already sent this in to you separately over a year ago but I will post it here as well:

    "OK, for data transmission back they assume they use the sail craft as a reflector (variously described as 1, 4, 10 and up to 30m in diameter) to focus the 1-10 W laser back to Earth and can thus support near HD video from Alpha Centauri if only for short time periods.

    They use the angle over which the beam spreads as the diffraction limit for the 30m dish which translates to 0.007 arc seconds (or 3.5E-8 radians).

    The trouble I have with that is that this then neglects the attitude control problem in its entirety which kind of kills that angular accuracy assumption. The problem is I am pointing a light back to Earth and I don’t know perfectly where the Earth is - relatively easy problem, I think - nor where I am pointing my light relative to that knowledge - much bigger problem and core of my post.

    **Assuming instead a 1 arc minute control** of this 30 m dish (WAG, really big WAG) then the beam size diameter after traveling 4.4 ly is not their 3.5E8 meters (diff limit at 1.06 microns for 30m dish) but closer to 3.7E11 m in diameter. That assumes a smooth distribution of the laser power of that disk at 4.4 ly distance. Also, it assumes that the 1 arc min is baking in the knowledge error and the knowledge error is small relative to the control problem of pointing a 30 m thin film dish. That feels reasonable but others may disagree.

    However, they are using the dish in terms of a diffraction limited optic so … using the Airy disk equations to work out how down in intensity the beam is all the way out at 1 arc min from center line knocks down the received power flux by another 10 to 11 orders of magnitude (!). In RF terms, they are way, way, way down in the side lobes and have a very, very pointy gain pattern. =>

    [If they did insist on the diff limited optics approach then they may want to worry about the (small but non-zero) chance of getting ‘lucky’ and viewing the intensity of the central Airy disk and knocking off line the receive electronics. 11 orders of dynamic range is kind of scary.]

    So, while they show data rates achievable by converting every 40 photons received in their 1E8 m^2 of receive area at around 70 Mbps, instead spreading it *smoothly* over the 1 arc min of angular control error (i.e., a diameter of 3.7E11 m) knocks that down to ~126 bps all else being equal in the link budget.

    Then, taking into account the additional knock down since the Airy disk angular size is way smaller than the attitude control error, and we could well be receiving it at the 1 arc minute point and nowhere near the Airy disk from that nice dish (statistically it is essentially guaranteed since diff limit angle <<<< ACS control error angle), then the data rate drops catastrophically to 3.4E-10 bps. Oops.

    The “solution”, I would suggest, is not to use 30 m sized diffraction limited optics to focus a beam back to Earth from 4.4 ly when you are instead dominated by ACS determination & control error when transmitting at Alpha Centauri. Rather, size the transmitted beam width to be comparable (a little wider) than the expected ACS determination and control error and live with the much lower data rates, i.e., not 70 Mbps and hi-def but a more leisurely ~100 bps.

    This solution may be incompatible with the reflector design requirements for the acceleration phase where a very tight tolerance on dish shape may well be required. BTW, presumably, pointing then during that phase is not going to be a walk in the park either, but I digress.

    So, how does one determine one’s attitude at Alpha Centauri relative to inertial space and how does one go about pointing a thin film dish of 1 - 30 m diameter with all of the other mass and power constraints? The first would seem easier to solve than the second.

    Thanks,

    Simon ... "

  8. Simon Dawson:

    " ... The angular diameter of a diffraction-limited beam, at a wavelength of 1 micron with a meter-class antenna, is on the order of 0.1 arcseconds. ..."

    OK, this is a lot looser than discussed elsewhere but 0.1 arc seconds seems out of bed with respect to attitude knowledge by at least an order of magnitude and maybe up to a factor of 50. Control may be OK here but I think you are dominated by STA accuracy.

  9. Breakthrough Initiatives:

    RE:
    Apr 13, 2017 02:12Simon Dawson Posted on: Breakthrough Initiatives

    Answer:
    You’re totally right about the pointing requirements being extremely demanding and the accuracy of current star trackers not being good enough. It’s a really hard problem, and the ultimate solution (and its performance) will probably be very different than what’s being proposed right now. I think we can close the link, but the achievable data rate is an open question.

    - Zac Manchester, Breakthrough Starshot

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