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Comment by lennartkoopmann | Hacker News Reader

Here is a video of me listening to the repeater while the ISS passes over Houston, TX at night: https://twitter.com/_lennart/status/1305685955370725376

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Absolutely love this capture.

The ISS is between 250-300 miles away in this picture, traveling so fast that a drop of sweat has the kinetic energy of a 44 magnum. And yet the short antenna and circuitry on that little $50 radio can see what amounts to a microwave lightbulb hanging off the ISS well enough to extract a one part in a million fluctuations in power four hundred and thirty seven million times a second.

Pretty wild

GPS receivers listening to 44W transmitters from 12,000 miles away can be heard saying hold my beer.

AND with postage stamp sized antennas. That one blows my mind regularly.

From under the noise floor. If anything that is the impressive feat.

I heard this in a podcast recently, imagine working at Bell Labs mid 20th century and calling a meeting with Nyquist, Hartley and Shannon.

Even if they didn’t show the invite would be a keepsake.

It would be hard to tell the names apart from the various tech bits and pieces named after the participants in the meeting. Shannon: "But what about the Nyquist frequency? Nyquist: "My trembling is much reduced, thank you." ...

Nothing special about the noise floor, it's just the point where you can only transmit around one bit per second per hertz

Link to the proof? Or at least, does the theorem has a well-known name?

The Shannon–Hartley theorem. It assumes additive white Gaussian noise (which is a good model for most kinds of thermal-ish noise), and provides a bound on the channel capacity that practical codes closely approach.

https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...

From an information-theoretic POV, yes, you can squeeze information from less, but the definition I found is based on signal levels not transmission capacity, and it seems to have other notable consequences.

AFAICT it's around this point that you can't tell whether there is a transmission, unless you know what it looks like; tuning requires decoding and/or fancy math, not a spectrometer; communication works just fine (with proper transmission modes), but there are nontrivial practical consequences as you approach or go below the noise floor.

I wouldn't expect to see analog equipment operating below the noise floor.

That's actually true for most electromagnetic telecoms. Ten orders of magnitude between the transmitted and received power is usually the easy case. With more careful techniques you can do even twenty or more orders of magnitude.

And yes, it baffles me as well.

There's a reason the word 'magic' is invoked around RF more than just about any technology.

My favorite is GPS. Not only below the noise floor, but significantly below it.. and all the satellites are on the same frequency. Gold codes are insane.

Power laws are powerful things.

This comment perfectly encapsulates what makes ham radio is so much fun. I'd argue that WSPR is even more fun than this.

If you enjoy WSPR, check out JS8Call. Its Slow mode has 30 second transmissions with extremely narrow bandwidth- very powerful even with low wattage, and it's a conversation mode and a beacon network mode :)

The other day there was an article here about "The ISS repeater on 437.8 Mhz". In the comments someone linked to an ISS tracker... I clicked it, and it said there was a pass near my house in 20 minutes, so I dug up my cheap-o RTL-SDR dongle and stuck the tiny aerial out on my fire escape.

I couldn't believe my eyes when I actually saw a signal pop up as it passed: https://imgur.com/a/jCOaFSk. It's apparently closer tonight, so I'm going up on my roof to see if I can get audio!

I was the guy who posted the "ISS repeater on 437.8 Mhz" article. Likewise, I was surprised that I managed to catch a minute or so of audio from the repeater using my handheld Baofeng BF-F8HP. I wasn't even really expecting it.. I'd had the radio out earlier in the day when I was programming the new repeater info in with Chirp, then I stuck it back on the charger and left it... unintentionally leaving it both "on" and on that channel. A few hours later I was in the other room and heard noise from the room where the radio was charging and jumped up and ran in there, and realized I was hearing people talking on the ISS repeater.

I grabbed the radio and ran outside to try and get a contact on the repeater myself, but no luck. :-(

Still, just hearing it was kinda cool in its own right.

Edit: as luck would have it, the ISS just passed over about 5 minutes ago. I went outside ahead of time to wait for it, and I caught a bit of audio, but wasn't able to make any contact myself. I think I waited a little to long to try, out of fear of walking on somebody else (per the earlier discussion, I don't have a radio with cross-band full duplex capability). Oh well... live and learn. And I'll buy a better radio at some point.

I know nothing about this. Who were you listening to? People on ISS? Or is it just a repeater that lets you listen to people at long distances on earth? What were they saying?

Exactly right on the repeater -- you communicate with the ISS on a VHF uplink and it rebroadcasts your communication on a UHF frequency (Earth stations typically run in a single band with a small offset). It's hard to talk to the ISS because it's moving so fast that there's a Doppler shift in the frequencies, but it's definitely possible with cheap gear (I've picked it up many times, but never tried talking through it). People aren't using it to communicate per se, more just to make brief long-distance (DX) contact with other amateur radio users.

There are also occasional opportunities to speak directly with people on the ISS (or more commonly, receive telemetry or slow-scan TV images).

That's awesome!

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