←back to thread

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

(github.com)
158 points nizarmah | 5 comments | 01 Apr 25 21:58 UTC | HN request time: 0.613s | source

A couple of months ago, I built this app to help identify people stuck under rubble.

First responders have awesome tools. But in tough situations, even common folks need to help.

After what happened in Myanmar, we need something like this that works properly.

It has only been tested in controlled environments. It can also be improved; I know BLE is not _that_ effective under rubble.

If you have any feedback or can contribute, don't hold back.

Show context
eddythompson80 ◴[02 Apr 25 01:51 UTC] No.43553008[source]▶
> 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.

replies(5): >>43553050 #>>43553065 #>>43553294 #>>43553736 #>>43554202 #
1. nizarmah ◴[02 Apr 25 02:05 UTC] No.43553065[source]▶
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-...

replies(2): >>43553137 #>>43553229 #
2. ◴[02 Apr 25 02:21 UTC] No.43553137[source]▶
3. eddythompson80 ◴[02 Apr 25 02:40 UTC] No.43553229[source]▶
> 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.

replies(2): >>43554916 #>>43566447 #
4. nizarmah ◴[02 Apr 25 09:14 UTC] No.43554916[source]▶
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.

5. Cthulhu_ ◴[03 Apr 25 08:09 UTC] No.43566447[source]▶
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.