4 ms·

it's extremely hard to do at those data rates. if it wasn't there would be existing examples. Facebook tried and failed.

mlyle 4y ago
> it's extremely hard to do at those data rates. ? > if it wasn't there would be existing examples. Let's see. NASA downlinked from the moon to the ground, through the atmosphere at OC-12 rates back in 2013-- so about 100x the distance, with the added penalty of traversing the atmosphere. NFIRE did 5.6 gigabit/sec LEO to ground (again through the atmosphere) in 2011-- shorter distances but higher angular rates which is the "hard part". And EDRS does 1.8gbit/sec over longer distances in geostationary orbit. Both flown and proven.
- shaklee3 4y ago
  are you really comparing the moon to the earth with two satellites at much closer distances moving rapidly?
  - mlyle 4y ago
    > are you really comparing the moon to the earth with two satellites at much closer distances moving rapidly? I'm comparing to LEO to ground, which has a higher rate of angular movement (e.g. harder to point at) than LEO-to-LEO in the same shell, among other things. I've built systems that point to sub-arcsecond precision at satellites in LEO. It's not quite an off-the-shelf controls problem (e.g. good luck getting a COTS motion controller to hit-a-fast-moving-target-at-a-chosen-time, rather than follow a track and not care about time) but it's not super hard, either.
    - shaklee3 4y ago
      you should probably have worked for Facebook a few years back when they threw billions at the problem and couldn't solve it.
      - mlyle 4y ago
        Citation needed, for Facebook: A) spent billions of dollars on this specific problem (free space optical links in space), and B) couldn't solve it. Given that there's systems that have successfully flown doing links from GEO to LEO (e.g. high angular rates again), using several year old conservative technology, it's not so bad. There's a million little details, of course. Just conduction cooling for fast optical transceivers is going to be annoying in space, for instance.
        shaklee3 4y ago
        SpaceX started a LEO system as ambitious as SpaceX for its time back in 2013. again, GEO to LEO is not LEO to LEO, and is also not moon to earth. there are no successful examples of 20+Gbps between LEO satellites. The million details is why SpaceX has yet to turn them on for production.
        mlyle 4y ago
        > SpaceX started a LEO system as ambitious as SpaceX for its time back in 2013. I don't understand what this means. If you mean Facebook-- Facebook didn't drop billions of dollars in to free space optical comms. > GEO to LEO is not LEO to LEO Yes, GEO-to-LEO is worse in every way (assuming the LEO satellites are in the same inclination and have the approximate same orbital period): * Longer link distances. (More path loss, worse link budgets) * Higher peak angular rates for pointing. LEO-to-Earth is mostly worse: * Shorter link path, but atmospheric dispersion (More path loss, worse link budgets, plus things like multipath). * Higher peak angular rates for pointing. * Less demanding pointing precision due to shorter path, though. The hard part isn't the optical comms in space. The hard part is fitting multiple precision-pointed transceivers into a tiny volume and mass budget.

mlyle 4y ago

> it's extremely hard to do at those data rates. ? > if it wasn't there would be existing examples. Let's see. NASA downlinked from the moon to the ground, through the atmosphere at OC-12 rates back in 2013-- so about 100x the distance, with the added penalty of traversing the atmosphere. NFIRE did 5.6 gigabit/sec LEO to ground (again through the atmosphere) in 2011-- shorter distances but higher angular rates which is the "hard part". And EDRS does 1.8gbit/sec over longer distances in geostationary orbit. Both flown and proven.

shaklee3 4y ago
are you really comparing the moon to the earth with two satellites at much closer distances moving rapidly?
- mlyle 4y ago
  > are you really comparing the moon to the earth with two satellites at much closer distances moving rapidly? I'm comparing to LEO to ground, which has a higher rate of angular movement (e.g. harder to point at) than LEO-to-LEO in the same shell, among other things. I've built systems that point to sub-arcsecond precision at satellites in LEO. It's not quite an off-the-shelf controls problem (e.g. good luck getting a COTS motion controller to hit-a-fast-moving-target-at-a-chosen-time, rather than follow a track and not care about time) but it's not super hard, either.
  - shaklee3 4y ago
    you should probably have worked for Facebook a few years back when they threw billions at the problem and couldn't solve it.
    - mlyle 4y ago
      Citation needed, for Facebook: A) spent billions of dollars on this specific problem (free space optical links in space), and B) couldn't solve it. Given that there's systems that have successfully flown doing links from GEO to LEO (e.g. high angular rates again), using several year old conservative technology, it's not so bad. There's a million little details, of course. Just conduction cooling for fast optical transceivers is going to be annoying in space, for instance.
      - shaklee3 4y ago
        SpaceX started a LEO system as ambitious as SpaceX for its time back in 2013. again, GEO to LEO is not LEO to LEO, and is also not moon to earth. there are no successful examples of 20+Gbps between LEO satellites. The million details is why SpaceX has yet to turn them on for production.
        mlyle 4y ago
        > SpaceX started a LEO system as ambitious as SpaceX for its time back in 2013. I don't understand what this means. If you mean Facebook-- Facebook didn't drop billions of dollars in to free space optical comms. > GEO to LEO is not LEO to LEO Yes, GEO-to-LEO is worse in every way (assuming the LEO satellites are in the same inclination and have the approximate same orbital period): * Longer link distances. (More path loss, worse link budgets) * Higher peak angular rates for pointing. LEO-to-Earth is mostly worse: * Shorter link path, but atmospheric dispersion (More path loss, worse link budgets, plus things like multipath). * Higher peak angular rates for pointing. * Less demanding pointing precision due to shorter path, though. The hard part isn't the optical comms in space. The hard part is fitting multiple precision-pointed transceivers into a tiny volume and mass budget.