Was listening to a story about a lost hiker, and it got me to thinking: how could you make it so that people could find you if you got lost?

The obvious answer is to have a smart phone or something, but what if it runs out of battery?

That got me to thinking about RFID, which provides passive identification and other information when it passes through a radio wave field.

And then it hit me: what if you could make some kind of passive item that, when the right radio frequencies hit it, would cause it to reflect back a signal, driven partly by the energy imparted to it by the radio wave?

I started visualizing the way reflectors and retro-reflective materials work. No matter what direction light hits them, light is reflected directly back.

If you could find a radio wave that could pass through walls, trees, and most rocks (basically, anything except a metal cage), and could have the item reflect a signal back, that could help with triangulation of the location of the owner, and allow you to find them.

Since it's activated by radio waves, you could perhaps use a low power, low frequency signal to pinpoint the location, then, if there were multiple answers, use a more directed high energy signal to activate its processor which would then send back a simple code allowing the searcher to identify the owner, and know if it's the person being searched for.

Obviously, not everyone would want this; only people who regularly go far afield, or who are going far away for the first time.

I imagine police, fire, and military might also include this in their gear.

Certainly, it could also be used for espionage, to track someone without their knowing. But there are many ways to do that.

What I don't know is if this would actually work.
-- simpleknight, Feb 12 2016

Personal EPIRB http://www.chsmith....k-PLB-with-GPS.html
Fits in your hand, weighs 130g. GPS linked so they don't even search for you, just send the helicopter to your location. Shelf life of 5+ years. [Custardguts, Feb 14 2016]

Passive - Aggressive Infra-Red Detector Passive_20-_20Aggre...nfra-Red_20Detector
Not quite the same. [8th of 7, Feb 14 2016]

Tactical Bacon http://gizmodo.com/...edible-for-10-years
[Bliss] - this pretty much meets the description you provided. [Custardguts, Feb 14 2016]

//If you could find a radio wave that could pass through walls, trees, and most rocks (basically, anything except a metal cage), and could have the item reflect a signal back,// There, I think, is the problem.

Given that wind-up radios exist, presumably someone could make a wind-up emergency transmitter?
-- MaxwellBuchanan, Feb 12 2016


I have often wondered about building a human-powered emergency transmitter. My idea was a sort of jam-jar lid that pops from concave to convex by pushing it, with a tuning fork and a magnet inside the jar, and a length of wire you could unreel as an antenna.
-- mitxela, Feb 12 2016


I thought the issue with transmitting radio signals any distance was juice. You can run your crystal radio off of a strong signal but to actually transmit a distance requires power. Your puny little thinger could make some sort of puny wave but how detectable would it be a mile away?

I think the human powered transmitted would have to be some sort of thing that you could pump or pedal, accumulating power, then expending it all over a second to send a help signal. Plus all the time you spend pumping the thing up would let you think of some message more helpful than just a recording of you screaming.
-- bungston, Feb 12 2016


Hmm, Wind up radios are a bit cumbersome, I wouldn't want to go hiking with one when I didn't need it. Now, old school radios used a capacitor bank and then a spark gap. The gap allows the build up of a large voltage before the resistance of the air breaks down and you get a big all-at- once signal*. Now, we don't need the spark gap if we can make high-voltage very quickly. It seems to me a that a whacking great piezo electric crystal should be able to do that job for you. A small tuned circuit, a long antenna to string up in a tree, a sturdy piezo electric thingy... then just bash it with a log until help comes.

*Nature was doing this with calcium billions of years ago.
-- bs0u0155, Feb 12 2016


// I wouldn't want to go hiking with one when I didn't need it.// Silly. You only have to carry it when you're going to get into trouble.
-- MaxwellBuchanan, Feb 12 2016


This is a very good idea, rather fortunately already well baked in the form of personal EPIRBs (except that they're not passive, but rather active, which is a good thing).

I've carried one for years on every hike, hunt, bikepacking trip, etc. I even take it in my pocket when on other people's boat. See link, not the model I have, mine is a bit older and is about the size of a packet of cigarettes, weighs 250g. The one I linked is a newer, even lighter model.

In case you don't know, EPIRBs are Emergency Position Indicating Radio Beacons. The older models operated at 121.5MHz, and required a nearby station or commercial air traffic to pick up the signal and trigger a search. The newer ones operate at 406Mhz, their signal is picked up by low Earth satellites, and have inbuilt GPS receivers so they chirp your location to SAR, and will be registered with your info, emergency contacts, etc. Basically, the postion is accurate enough that, depending on your location, they don't even send out search planes, etc - they just come and get you.

This is, for all intents and purposes, a pair of Dorothy's slippers that you can click three times and go home.
-- Custardguts, Feb 14 2016


If the shoe fits, ...
-- 8th of 7, Feb 14 2016


Read this as "Passive Emergency Radio Bacon", and it caught my interest. Hmmm, hungry now for bacon.
-- blissmiss, Feb 14 2016


[bliss] - see link.
-- Custardguts, Feb 14 2016


Ha. 10 years, hmmm, that sounds good. Open that baby up and slap 'er in the frying pan. Tac Bac. Yay.
-- blissmiss, Feb 15 2016


Well, if I wanted to find something to help me hiking, I would get an EPIRB, it sounds like a good idea.

However, the whole point is to not need power. I assumed that there was something battery powered that would do it.

A human powered unit would be closer if a radio reflector wouldn't work. Because then all you need to do is have it with you.

I find it unlikely that you couldn't do it with a small amount of power. Laser pointers travel pretty far, and even the most powerful don't need huge batteries.

It seems like there ought to be a way to make something human powered match that power.
-- simpleknight, Feb 15 2016


Right, well, I just wanted to link the EPIRBS because a lot of people think they're only for boats, and don't realise they are so small. Lots of people run into trouble on hikes, etc and don't have one - and I can only assume it's because they don't know about them.

The thing is, in order to communicate, you need to radiate some sort of signal, either by generating it or reflecting it. Generating a signal takes energy, and humans aren't a particularly good source.

Reflection is interesting. Here's an idea. Choose a network of low orbit satellites with good coverage - the Iridium network would probably do, although others would also suit. Now, you need to be able to see the satellite, and you can very often see the iridium satellites whizzing by, perhaps not often enough, but it's a start. Now your passive beacon needs only to be a corner reflector, fold out I suppose, tuned to the right size to reflect a particular wavelength back to the satellite. Maybe even something could be done to the corner reflector such that it reflects a specific wavelength only. So you spot the satellite, and aim the reflector by hand. So now the satellite is just looking for a sustained strong return on a particular wavelength - hell, maybe you could even signal SOS to it.

Not sure how the satellite could pinpoint your position, however.
-- Custardguts, Feb 15 2016


//Laser pointers travel pretty far, and even the most powerful don't need huge batteries//

Yes, but you've chosen the most directional device known to mankind. Literally any other device you choose will produce a more diffuse signal.

That said, you could probably jig up a laser pointer tuned to a specific low-atmospheric absorbtion frequency, and do the same as I have suggested above - point it at satellites. Thing is we'll need a whole new constellation of satellites with the capability of receiving this signal and doing something about it.

Eventually, a personal EPIRB is much cheaper - at least for the satellite company.
-- Custardguts, Feb 15 2016



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