Comment by piannucci | Hacker News Reader

Comment by piannucci | Hacker News Reader

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The particular signal they used can be captured with a $40 low-noise block w/ feedhorn (I’m using the Visiblewave Hybrid LNB), a 12-16 VDC biasing circuit, and a garden-variety SDR. I speculate, based on my own studies, that the signal they caught is the Starlink “Idle” tone. I don’t see that signal when the downlink is busy in my own experiments, but I can’t be certain yet. Different latitude, different equipment, different assumptions.

The broadband downlink signals I’m interested in require a bit more effort— I’m using a 75cm offset parabolic dish on an azimuth/elevation turret with some nice high-end servos w/ built-in 12-bit encoders. Up until yesterday, we could only capture 60 MHz of real-time bandwidth; but now we have the device we need to capture closer to the entire 2 GHz span of the 10.7-12.7 GHz downlink in one fell swoop.

With all that, I still only get about 11 dB of SNR. I’ve seen hints that I might be able to get closer to 20 dB, which I could trade off for a smaller dish.

11 dB is pretty impressive, especially if you are just looking at the Doppler shift in the carrier.

Which actually begs the question, can you decode anything (e.g. satellite ID) from the downlink/idle signal? If not, how do you figure out which satellite a given signal came from?

My first guess would be that you can fit an orbit to the Doppler shift, find the closest TLE set from space-track.org, and assume that the signal came from that satellite. Then propagate the orbit to the observation epoch and that should give you a fairly accurate guess of the satellite's actual location. Or do you do something completely different?

What you describe with finding the closest TLE is pretty much what they did. We use a highly directional antenna (~1-2 degree beam) and some tracking software, so it’s not ambiguous.

I’ve been able to demodulate and reconstruct about 100 subcarriers’ worth of signal, but until recently I couldn’t record the entire bandwidth. Now we’ve got our hands on one of those awesome RFSoCs with 2 Gsps ADC, so it might be possible to decode more. But unknown error correction, bit interleaving, so it’s a tall order. We’d have to get lucky.

I wonder if Starlink will ever implement a smallband broadcast channel, that can be received with a cheap antenna.

Some projects like othernet also tried this.

I imagine a selection of international news, weather warnings, etc. that everybody can receive.

I’ve asked them about this, but they said they were too busy rolling out the internet service to consider any alternatives at the moment. I’ll keep hounding them until they relent!

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