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Comment by ctdonath | Hacker News Reader

Starlink animations depict each sat servicing a narrow cone below. Remember, these operate much closer to the ground than geosynchronous; they'll have a much better idea where the client is than just which hemisphere.

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Radio wave propagation is a little more complicated than what the animations depict. It would probably be easy to pretend to be somewhere else by pointing a more directional antenna at the horizon.

All Starlink communication is already extremely directional. The spot size of the beam coming from the satellite is ~1.5km on the ground, and the receiver is also directional with similar accuracy. The entire principle of Starlink is based on extreme SDMA.

If you try to communicate with a satellite on the horizon, you will not hear it, and it will not listen to you.

I checked it out, it says each satellite covers area with 550km diameter on the land. That's about a few countries at a time and possibly deep penetration into the larger ones.

They will have to turn off the sattelites over Austria to prevent connections in Turkey.

1.5km vs 550km is not necessarily mutually exclusive when using phased array antenna: the beam may be 1.5km wide, but it may be steered across a 550km area. Ergo sat could service both Austria and Turkey, without blurring - it knows which country it's "pointing" at during any millisecond, so could service each without legal conflicts.

Can you provide a source for the 1.5km beam claim?

I was actually wrong. The spot size is 1.5 degrees, or ~10km.

The source is the original Starlink FCC filing: https://cdn3.vox-cdn.com/uploads/chorus_asset/file/8174403/S...

But the point remains: The Starlink satellites do not just radiate omnidirectionally. Nor could they; the system is intended to eventually serve a meaningful fraction of the entire population on earth. This is not possible on non-steered RF. The only reason their plan has any sense is that every transmitter and receiver is directional, allowing efficient SDMA, where the same frequency can be used by multiple people simultaneously to communicate withe the same satellite, because the satellite can discriminate between their signals by the direction from which they were received.

Because of this, in order for Starlink to be able to communicate with you, it has to have a pretty good idea of where you are.

Can you quote the information? I couldn't find this in the document. I am sceptical that they can have that narrow beams and track the users(as the sattelite passes over them) from non-geostationary low earth orbit satellites. I would like to investigate this.

From A2: overall description:

> Advanced phased array beam-forming and digital processing technologies within the satellite payload give the system the ability to make highly efficient use of Ku- and Ka-band spectrum resources and the flexibility to share that spectrum with other licensed users. User terminals operating with the SpaceX System will use similar phased array technologies to allow for highly directive, steered antenna beams that track the system’s low-Earth orbit satellites. Gateway earth stations also apply advanced phased array technologies to generate high-gain steered beams to communicate with multiple NGSO satellites from a single gateway site.

Details of the downlink beams is found at A3:

> All Ku-band downlink spot beams on each SpaceX satellite are independently steerable over the full field of view of the Earth.

Just read the entire A3.1 for more details.

They use steerable beams. The spot size of the beam is just 1.5 km across.

(edit: I remembered wrong. 1.5 degrees, or at 550km, ~10 kilometers.)

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