Interesting. I didn't know that coverage was limited. I assumed satellite == pretty much all of earth.
The low orbits that give Starlink its low latency compared to geostationary satellite internet services also mean that each satellite can only see a small part of the earth at any given time. This is why they need so many satellites to provide reliable coverage.
Right now each satellite has to communicate directly to a uplink station, so it's only possible to provide coverage to areas where a satellite can simultaneously see the user and the uplink.
This is where SpaceX's planned inter-satellite link capability comes in to play, they claim they will be able to use lasers in a free-space optical network (think fiber without the fiber) to relay data directly from satellite to satellite, allowing service more than a single hop from a uplink station. This will also hypothetically allow for direct user to user connections over the satellite network that do not traverse the terrestrial internet, which would be huge for both military and business applications. Lots of words have been written about intercontinental high frequency trading for example.
Supposedly every satellite launched in 2022 has the capability but as far as I'm aware it hasn't been openly demonstrated to work yet. Making it work reliably within a single orbital ring is a hard problem and the claimed ability to cross-connect between adjacent rings is an absurdly hard problem. Neither are impossible, but I'll believe it when I see it.
This whole "yea inter-sat free space fiber links are totally going to happen" charade smacks of the same hype baiting as "full self driving by end of year" nonsense that Elon has been spouting since 2018.
The Starlink "team" did an AMA on reddit[0] last year and it was comical how empty the answers were. People asked about the space lasers and the answers were all "yea it's a really hard problem, BTW we're hiring!" which honestly felt like an admission from HR that they're looking for engineers willing/able to cash the checks marketing already wrote.
[0] https://www.reddit.com/r/Starlink/comments/jzozv3/every_answ...
Also seems likely that if they can find a way to make small sats with the ability to accurately point a laser at another sat, that would have pretty obvious implications to the defense dept. SpaceX is a military contractor after all.
Here's the design 101 from base principles:
In practice, you need a big-ish telescope on both ends to control beamspread and to collect light from a big aperture. Aperture is a given based on link budget (and you can trade off power to make the aperture smaller, but halving the aperture diameter on both sides means you'll need 16x the power); if you target F/2 then it'll be twice as long as the aperture. Maybe think about a 6cm aperture and 12cm long telescope for a starting point. This will get you a 6 arcsecond wide beam @ 1000nm.
Then, you need to slew this at pretty fast rates-- perhaps 15 degrees per second for acquisition, and control the pointing within 3 arcseconds while tracking at peak rates of a few degrees per second. Yeek! This pretty quickly takes you towards some kind of direct drive fork mount that is very gimbally-looking.
One bit of fun is that you need to have a lot of bandwidth on your reaction control system on the spacecraft, too-- because when you snap one of these telescopes around, the whole craft is going to want to counterrotate, so the reaction control wheels (and/or other telescopes for links in other directions) will need to react. Feedforward is advised.
You can't get the same degree of directivity. As wavelength decreases, you get more directivity for a given aperture. Light has 1/5000th the wavelength of plausible radio links, so both the sender and the receiver can have much higher gains. You also can have much more bandwidth, and thus you obtain many orders of magnitude higher data rates per unit of power used.
E.g. a 6cm telescope has 82dB of gain on each side for 1000nm light.
A 1 meter aperture (about what a 3.2x1.6x0.2m Starlink satellite can likely present to another satellite) has 53dB of gain on each side.
So for equivalent power, you have 6 orders of magnitude more signal strength, and you can occupy 10x the bandwidth, too, even if you have a very large phased array.
> The only reason you'd go for lasers here is thin civilian cover for developing a weapons platform.
This kind of system has very little in common with how I would build an anti-satellite laser system.
?
> 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.
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.
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.
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.
The precision required for aiming is directly related to the wavelength. Iridium NEXT satellites use Ka band with a wavelength around ten millimeters where anything light related has a wavelength measured in hundreds of nanometers.
The forward/backward links are a lot easier than the inter-plane links, but it's still not trivial because you're trying to hit an object the size of a small car with a laser from over 1000 miles away. Not impossible by any means, but there's not a lot of margin for error when they're looking to be able to transfer around 100 gigabits per second over this link. Other FSO systems work at significantly lower bandwidth and/or shorter range. That's not even getting in to the inter-plane links, where the target is constantly moving even in a relative sense.
Additionally there's the issue that their operating licenses don't allow inter-satellite communications.
This is of course a much easier problem to solve than putting a dynamic mesh network in space.
They did a test in late 2020[0], and all launches since June 2021 have been Starlink v1.5 with lasers[1].
0: https://wccftech.com/spacex-starlink-satellite-laser-test/
I'd even be satisfied by specific claims of test results that could be validated once the capability is officially activated.
6900km from the center of Earth. Figure you don't want the link to point within 150km (6500km) of Earth, to not pass through much atmosphere and to not see too much atmospheric glow (even with narrow filters, this matters).
Effectively you have an isosceles triangle with 6900km on the common side and an altitude of 6500km (tangent to "top of atmosphere" at 150km.
sqrt((6900^2 - 6500^2)) * 2 =~ 4600km
One interesting side-effect of the laser links is that they can open up connections between stock exchanges and trading houses that are faster than direct fiberoptic lines. Milliseconds count in high frequency trading.
So I think it's plausible for intercontinental links.