3 ms·

Two quick questions that may or may not make any sense. If you've seen the Starlink dish teardown, would you suspect that it's built for just a single beam or

by jcims 6y ago

Two quick questions that may or may not make any sense. If you've seen the Starlink dish teardown, would you suspect that it's built for just a single beam or could it support multiple simultaneous beams? And what would you expect are the primary performance/capability differences between the 'dish' antenna and the one they fly in the satellite (space-hardening aside)?

matmatmatmat 6y ago
Not the OP. We have to clarify what you mean by "support multiple simultaneous beams". The SL receiver is half-duplex, so it can only send or receive at any given time, and, in theory anyway, it uses beamforming to communicate with just one bird at a time (if it couldn't beamform to within sufficient tolerance, then that would be an FCC no-no.) There's no theoretical reason, AFAIK, that it couldn't rapidly switch between birds, but the actual feasibility of that depends on terminal firmware, the ground network being setup to handle that, the arc between two birds, and whether or not the antenna's scan angle can encompass that arc. The primary difference between the ground terminals and the antennas in space is the amount of power passing through them, but also their sophistication (primarily the beamforming capability.)
- jcims 6y ago
  Ah, was actually wondering about the duplex situation as well, thanks for answering a bonus question :) Re: multiple beams I'm thinking about the ability to split the array up into 'subarrays' with independent beamforming capability. Probably doesn't work for reasons above my head but I was just trying to think about the mechanics of the transition between satellites. Having a chameleon mode with one eye pointed at one satellite and another pointed at the next seems like one way to ensure a seamless transfer. But realistically the dish probably knows where to steer next within an approximation of its beamwidth, so it probably just flips like a channel. Thanks for the answers!
  - matmatmatmat 6y ago
    Ah, OK, I understand better. Yes, in theory, a terminal with a phased array antenna can be built to emit more than one beam simultaneously and steer them independently. Doing so is currently still costly in terms of research, electronics, and power, so it's unlikely that the SL terminal can do it. It would also require there to be a second PHY chip somewhere in the terminal, which would also drive up cost. Also, it's not really necessary. Satcom already requires nanosecond-level timing accuracy, so I would think their terminal can probably steer its beams with at most ms lag.
- mNovak 6y ago
  Do you have a reference for the half-duplex note? That would seem a waste given that they already have FDMA. Also most satcom applications are full duplex.
  - matmatmatmat 6y ago
    https://twitter.com/TMFAssociates/status/1300438995843776514 https://twitter.com/TMFAssociates/status/1300438995843776514
mNovak 6y ago
Based on the teardown, it's impossible to tell; again largely comes down to what their ICs are capable of. But based on the patents, and what I've heard unofficially, it does support 2 simultaneous beams. The point of that isn't to double throughput or anything like that, it's to accomplish "make before break" handoff as one satellite moves towards the edge of view, and another is better. Don't know much about how they've set up their satellite terminals--for a typical GEO station, they'd support a huge number of static beams simultaneously to multiplex as much through the station as possible. With many birds in LEO, it's unclear how extreme they have to get about that.

by jcims 6y ago

matmatmatmat 6y ago

Not the OP. We have to clarify what you mean by "support multiple simultaneous beams". The SL receiver is half-duplex, so it can only send or receive at any given time, and, in theory anyway, it uses beamforming to communicate with just one bird at a time (if it couldn't beamform to within sufficient tolerance, then that would be an FCC no-no.) There's no theoretical reason, AFAIK, that it couldn't rapidly switch between birds, but the actual feasibility of that depends on terminal firmware, the ground network being setup to handle that, the arc between two birds, and whether or not the antenna's scan angle can encompass that arc. The primary difference between the ground terminals and the antennas in space is the amount of power passing through them, but also their sophistication (primarily the beamforming capability.)

jcims 6y ago
Ah, was actually wondering about the duplex situation as well, thanks for answering a bonus question :) Re: multiple beams I'm thinking about the ability to split the array up into 'subarrays' with independent beamforming capability. Probably doesn't work for reasons above my head but I was just trying to think about the mechanics of the transition between satellites. Having a chameleon mode with one eye pointed at one satellite and another pointed at the next seems like one way to ensure a seamless transfer. But realistically the dish probably knows where to steer next within an approximation of its beamwidth, so it probably just flips like a channel. Thanks for the answers!
- matmatmatmat 6y ago
  Ah, OK, I understand better. Yes, in theory, a terminal with a phased array antenna can be built to emit more than one beam simultaneously and steer them independently. Doing so is currently still costly in terms of research, electronics, and power, so it's unlikely that the SL terminal can do it. It would also require there to be a second PHY chip somewhere in the terminal, which would also drive up cost. Also, it's not really necessary. Satcom already requires nanosecond-level timing accuracy, so I would think their terminal can probably steer its beams with at most ms lag.
mNovak 6y ago
Do you have a reference for the half-duplex note? That would seem a waste given that they already have FDMA. Also most satcom applications are full duplex.
- matmatmatmat 6y ago
  https://twitter.com/TMFAssociates/status/1300438995843776514 https://twitter.com/TMFAssociates/status/1300438995843776514

mNovak 6y ago

Based on the teardown, it's impossible to tell; again largely comes down to what their ICs are capable of. But based on the patents, and what I've heard unofficially, it does support 2 simultaneous beams. The point of that isn't to double throughput or anything like that, it's to accomplish "make before break" handoff as one satellite moves towards the edge of view, and another is better. Don't know much about how they've set up their satellite terminals--for a typical GEO station, they'd support a huge number of static beams simultaneously to multiplex as much through the station as possible. With many birds in LEO, it's unclear how extreme they have to get about that.