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Comment by kibibyte | Hacker News Reader

GPS is a thing, so I can imagine that a phone would be able to receive satellite signals pretty easily. But for transmission, I can't imagine this working without having your phone put a ton of power into sending a powerful enough signal, perhaps more so than how much power you need to passively receive GPS signals in the absence of traditional cell towers?

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It sounds like the data rate is incredibly low. That allows for a lot of signal margin. GPS receivers are sensitive to something like -165dBm.

It's probably harder for the satellite to hear from the phone. Especially at a lower frequency like 700Mhz, a phone's tiny antenna is going to be a rather inefficient transmitter. Maybe 5g technology helps by allowing the phone to do some rudimentary beam forming using multiple antennas?

(How) Does beamforming work in a moving device?

I'm not an expert, but I believe a receiving radio can make note of the relative phase of the incoming signal across all antennas, and then simply transmit its signal with the same phase offsets. This can happen incredibly quickly on the time scale of milliseconds, and so the typical device really isn't moving that quickly to matter.

I believe modern wifi has periodic "sounding packets" specifically designed for devices to robustly measure that sort of information for beam forming and multiple spatial streams.

This is all roughly right except that when you say "incredibly quickly on the time scale of milliseconds" it's often actually microseconds.

This sort of work is done either in custom hardware or FPGAs - that's the only way you can precisely control the phase offset of signals (and measure it). They can respond very quickly.

There are cell phone sized thingies that can send text messages to iridium's constellation, which is (as I understand it) way further away than starlink. I use one while backpacking.

I'd be super excited to get the same capability in a standard cell phone.

The Iridium orbit heights are not much higher (781 km vs. 550 km). You can always trade bandwidth for increased sensitivity. The most extreme example is GPS. Your phone can receive it from 20000 km away with a tiny antenna, but the data rate is only 50 bits per second.

Within the limits of a 5G receiver, there are probably limits to how much sensitivity you can give up for bandwidth.

It works because the satellites will have a HUGE antenna, 5m by 5m. And to enable the launching of this many big satellites, SpaceX will launch (most of them) on Starship.

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