Comment by mschuster91
1 year ago
> The lasers, which can sustain a 100Gbps connection per link
> Brashears also said Starlink’s laser system was able to connect two satellites over 5,400 kilometers (3,355 miles) apart. The link was so long “it cut down through the atmosphere, all the way down to 30 kilometers above the surface of the Earth,” he said, before the connection broke.
How do these tiny satellites achieve this kind of accuracy and link quality when they're shooting around Earth with 17.000 miles an hour?
(Meanwhile, me on Earth, has link quality issues due to a speck of dust on a fiber connector)
Relative to the origin satellite I would assume the others are in a fairly fixed position to it. Remember they try to keep them spaced out and even coverage. That means the things are not moving around wildly relative to each other. But to us they are wizzing by. For example I know I am relatively moving fairly quickly to the earths core and pretty fast around the center of the sun. But from my PoV everything around me looks stationary. Also there is not a lot of dirt up there.
3M just invented a new fiber interconnect thing to mitigate the dust issue: https://www.3m.com/3M/en_US/data-center-us/applications/inte...
Maybe the future of usb in 10 years :)
This says more about the link budget than anything else, it's much harder to keep tracking when satellites are close to each other moving at high relative velocities. At the distances in your example, movement of the laser link optical head is very slow, on the order of 0.01 - 0.1 deg/s. Optical heads also have a control loop which actively corrects for pointing errors once a positive link is established. Check out: https://www.sda.mil/wp-content/uploads/2022/04/SDA-OCT-Stand...
> (Meanwhile, me on Earth, has link quality issues due to a speck of dust on a fiber connector)
It's incredible really. I remember when I was a kid living with my mom on an island, we got broadband relatively late (compared to the rest of the country), as the island required antennas for getting mainland and the island linked, instead of cables. I think it was set up that way because of costs or something, remember it being expensive...
Regardless, the antennas were setup and we finally got broadband, but every time it got a bit windy and/or rainy, the links started to have huge issues, especially if the lake got lots of waves, then the connection simply disappeared.
And now it seems almost like magic to me how the same setup is literally done but way above our heads, in a really hostile environment like space.
In some important ways space is actually the least hostile environment. But yeah, it’s still amazing and has its own challenges.
The current Starlinks satellites aren't small. They are almost a ton and 13ft by 9ft.
Even the original ones weren't that small weighing 570lb.
Given this is Gen3 and Gen4 now being launched, it took some figuring out
The rate of change of their relative positions is what matters. At 5400km distance this is likely slowish so that tracking is not a big issue as long as position is well known, which it is.
Re. Link quality: laser, line of sight, most of the trip is in vacuum and the rest in very sparse atmosphere. So interferences are likely quite low.
Phased array antennas probably have a lot to do with this. You can aim the signal more accurately and faster than any mechanical system ever could.
Laser links are not using phased array antennas. It's a physically moving "turret" with a laser and another with a receiver. And they need to be separate units, because the speeds and distances involved are long enough that you are not receiving from the same direction as you are sending.
> that you are not receiving from the same direction as you are sending
It's a thrill to think about that. Starlink is really out there.
I bet this is lost on a lot of people. Not to patronize anyone, but what Tuna-Fish is pointing out is that due to the speed of light, the distance between satellites and their relatives velocities, when one satellite is beaming data to another satellite it must aim where the receiving satellite will be, as opposed to where it is now, when the light arrives. Further, the receiver must be looking at where the transmitter was back when the signal was sent, as opposed to where the transmitter is now. And because this is all bidirectional, each satellite must send and receive in different, continuously changing directions at the same time.
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Surely the lasers aren't phased arrays.
Their exact position in space can be calculated very precisely. The Starlink terminals do this as well.