Show HN: Offline SOS signaling+recovery app for disasters/wars

> It can also be improved; I know BLE is not _that_ effective under rubble.

It's a tough problem to solve because you're up against the laws of physics and the very boring (and often counterintuitive) "Antenna Theory". Bluetooth is in the UHF band, and UHF isn't good for penetrating anything let a lone concrete rubble.

To penetrate rubble effectively you really want to be in the ELF-VLF bands, (That's what submarines/mining bots/underground seismic sensors use to get signals out).

Obviously that's ridiculous. Everything from ELF to even HF is impossible to use in a "under the rubble" situation because of physics[1]. Bluetooth (UHF) might be "better than nothing" but you're losing at least 25-30 dBs (which is like 99.99% signal) in 12 inches of concrete rubble. VHF (like a handheld radio) can buy you another 5 inches.

Honestly I think sound waves travel further in such medium than RF waves.

[1]: Your "standard reference dipole" antenna needs to be 1/2 or 1/4 your wave length to resonate. At ELF-VLF range you need an antenna that's 10k-1k feet long. You can play with inductors and loops to electrically lengthen your antenna without physically lengthening it, but you're not gonna get that below 500-200 feet. The length of a submarine is an important design consideration when deciding on what type of radio signal it needs to be able to receive/transmit vs how deep it needs to be for stealth.

You could make the phones communicate via sound, not sure how it would impact the battery nor its communication range. Lots of details would have to be worked out. You have to find the frequency that communicates the longest. Perhaps do short pulses of sound to save battery life.

Thanks for the amazing breakdown.

That means LoRa and cellular network won't help either... That's good to know.

Yeah, sound seems like a good option. There's two points I'm struggling with though: 1. Disasters often are accompanied with a lot of noise which I'm afraid would drown or interfere with the other sound waves. 2. Mobile devices might not be able to detect the sound waves that are broadcasted.

I'll need to do more homework about those.

We'll have more flexibility with recovery for sure, so maybe that can address the shortcomings of the sound signaling...?

It still has to be cheap-ish though. Because first responders already have MUCH better tools [1].

Again, thank you so much for this comment. If you have any other suggestions or feedback, please don't hesitate to open an issue on github. Your input is awesome.

[1]: https://www.dhs.gov/archive/detecting-heartbeats-rubble-dhs-...

> That means LoRa and cellular network won't help either... That's good to know.

It does. If anyone is buried under the rubble "why don't they just tweet/SMS/mesh-network they are stuck" might be the first question someone would have. A simple answer is that the nearest GSM tower is also under rubble. But that's not enough. Even if the nearest GSM tower is 100% fine (or their next LoRa mesh peer is also fine), you need to be less than 12-15 inches of concrete (in the most optimal/reflective way) to have any signal at all. At the end of the day you're up against physics. 3 questions physics mandates:

1. What is the wavelength you're transmitting?

2. How far is your receiver?

3. What is your medium?

The keyword "Antenna Theory" or "Antenna Design" is a good rabbit hole for this sort of thing. There is this funny saying among radio enthusiasts "everything is an antenna if you squint hard enough"[1].

There is also something that cooks call "The Human Element"[2] when they're trying to evaluate the appeal of some foods to people. The "Human Element" in an "under the rubble" situation is that you want something that a normal human being would have on them when the sudden situation of an earthquake happens and they find themselves under a rubble.

I don't know what that is. I live in a place that is pretty far removed from earthquakes as a major problem. Geologically speaking, we get a "major earthquake" every 500 years or so. However, that's an average and it's been 600 years for us but the max has been 3,000 years and the min has been 200 and the mean means nothing to the individual. However, a device of some sort that can emit low frequency, low bandwidth[3], long range message seems like a useful thing. I don't know if that would be the type of device every human would have "on their body" all the time. But maybe??

[1] Everything that's electrically conductive is an RF antenna. If you get buried in the right place next to a 100 feet long rebar, you could (theoretically) send a signal that anyone in 100 mile range could hear provided you had the right equipment of course.

[2] In cooking, it is this elusive thing that explains how some cultures/people love certain tastes or textures that other cultures don't. Slimy foods in Middle Eastern or Japanese/Asian cultures vs Western Pallet for example.

[3] A device that needs to send "S.O.S" needs maybe 1byte/second or even less. You can't communicate any meaningful information in a seismic-pulse-signal type device, but "SOS" is certainly doable.

What about muon imaging?

What about Rydberg sensors for VLF earth penetrating imaging, at least?

From "3D scene reconstruction in adverse weather conditions via Gaussian splatting" https://news.ycombinator.com/item?id=42900053 :

> Is it possible to see microwave ovens on the ground with a Rydberg antenna array for in-cockpit drone Remote ID signal location?

With a frequency range from below 1 KHz to multiple THz fwiu, Rydberg antennae can receive VLF but IDK about ELF.

IIRC there's actually also a way to transmit with or like Rydberg antennae now; probably with VLF if that's best for the application and there's not already enough backscatter to e.g. infer occlusion with? https://www.google.com/search?q=transmit+with+Rydberg+antenn....

Using an AirTag on my dog's collar and WifiMan on my phone to track BLE signals, I could pick it up (just... not enough to interact with it, but enough to detect it above -100dbM) about 8 - 10m with a clear line of sight, and about 2m when she was in dense scrub.

So yeah, I imagine concrete is going to be even worse than a shrubbery, that said, might get some interesting propagation paths if there's metal surfaces in the rubble to reflect waves.

My ham license let's me transmit on 2200 meter and 630 meter bands. I'm not allowed to use a 1100 meter or 315 meter dipole, either.

And I can only eirp 1000mW. I mean, legally. I can't eirp anywhere near that can you imagine the losses?

Unfortunately bass travels the furthest. I can hear cows a lot further away than frogs, and I can hear transport helicopters for miles (miles!)

The problem with bass sounds is ostensibly they're nondirectional. So the ideal sound for a phone would probably be the loudest pink noise they can make without blowing the speaker out.

TIL about Antenna Theory/Design.

Yeah the human element is the strictest here. From the little research/thinking I did, BLE felt like the most "commonly available" tool we have. But I'm wondering if manufacturing a cheap wearable could also be an option.

I definitely need to do a longer/deeper dive for this project. I overlooked it in the past, thinking people don't need it. But it seems like I just dropped the ball in distribution previously.

This is pretty cool! I need to learn more about both.

I imagine such fancier tools would be less available among common folks, and more among first responders.

NASA already has the tech to detect heartbeats under rubble using radar [1]. No additional equipment is needed by the rescued. The problem is emergency response can get overwhelmed in large disasters.

If Rydberg sensors would be more common, and new tech is added to mobile devices, this can seriously shift the playing field.

I will look into this, because we need out of the box solutions. Thank you!

[1]: https://www.dhs.gov/archive/detecting-heartbeats-rubble-dhs-...

I think you can use 5W eirp on 630 meter, at least that's consistent with my band plan :)

It looks that for 600m you could use a vertical wire with a lot of radials :) Maybe with a balloon?

But 2200 would be fun indeed.

"The Dark Knight" (Batman, 2009) is the one with the phone-based - is it wifi backscatter imaging - and the societal concerns.

FWIU there are contractor-grade imaging capabilities and there are military-grade see through walls and earth capabilities that law enforcement also have but there challenges with due process.

At the right time of day, with the right ambient temperature, it's possible to see the studs in the walls with consumer IR but only at a distance.

Also, FWIU it's possible to find plastic in the ground - i.e. banned mines - with thermal imaging at certain times of day.

Could there be a transceiver on a post in the truck at the road, with other flying drones to measure backscatter and/or transceiver emissions?

Hopefully NASA or another solvent company builds a product out of their FINDER research (from JPL).

How many of such heartbeat detection devices were ever manufactured? Did they ever add a better directional mic; like one that can read heartbeats from hundreds of meters awat? Is it mountable on a motorized tripod?

It sounds like there is a signal separation challenge that applied onboard AI could help with.

huh, 5W eirp. Either that changed or my memories did. cool. I also don't remember the 500W PEP ceiling, either. I thought you could drive it however hard you wanted to as long as your radiator didn't exceed [what i thought was 1000] 5000mW eirp.

Always remember that all these regulatory/legal limits don’t apply in any emergency or life preservation situations. The FCC, by law and their own rules, can’t/won’t come after you if you emit on any band with any power saying anything if you happen to be “under rubble”.

If someone is under rubble and needs to signal anyone, if they have the use of their hands, best thing they can do is get something solid and tap the SOS morse code on something. I thought that's what they did during the Kursk submarine accident but apparently that wasn't correct.

Anyway, SOS morse code will carry pretty well through concrete or steel or whatever.