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I am hoping that this qualifies for world-building based on the reasoning that the device is used in a reoccurring fight to the death competition created as a method of population control in the world of a story I am writing.

During the competition, teams of three participants are pitted against each other and each person member is equipped with a device that bilaterally alerts the wearer to the presence of an enemy target up within range. It is possible to remove the device to essentially go stealth but doing so means also permanently disabling the enemy alert feature. I'm trying to figure out if there is a real world equivalent to such a device or a mechanism with a similar concept that I can reference for research purposes? I feel like I am not using the right terminology to describe my concept which is why I'm getting stuck in my search.

Edit: Sorry, I forgot to make it clear that device does technically act as a GPS locate for the wearer, just to alert them when other are using it are nearby. It does not give an exact location. I was thinking that it would have three dots that would flash indicating proximity range like cold, warm and hot.

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    $\begingroup$ Isn't this an ordinary a smartphone with a specialized variant of one of those geosocial casual match making applications, of the kind popularized by Grindr? $\endgroup$
    – AlexP
    Nov 27, 2020 at 23:28
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    $\begingroup$ @AlexP I am loving the application of Grindr to this concept! The premise that there is a secret subpopulation of Highlander types (there can be only one! Ok, there can be 3 for this.) who are on Grindr to find similarly inclined individuals who wish to fight to the death. I wonder what you would put on your profile page? $\endgroup$
    – Willk
    Nov 27, 2020 at 23:39
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    $\begingroup$ In your world, is it a battle in the mass of people, or arenas are specifically made for this (battle-royale style)? It's not essential to the question, but still interesting! $\endgroup$
    – Tortliena
    Nov 27, 2020 at 23:47
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    $\begingroup$ @Tortliena the concepts not full fleshed out yet but I am thinking on an abandoned island, Battle Royale style. $\endgroup$
    – Myisha
    Nov 27, 2020 at 23:58
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    $\begingroup$ If they have a good hiding place, I think it would be worth disabling the device when they go there, and rely on more classic ways of detecting if anyone is near. Similarly,when they get one beep, they won't know if the other part fled / went looking the opposite way, or actually went into stealth mode and is about to shoot them. $\endgroup$
    – Ángel
    Nov 28, 2020 at 1:35

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Everyone thinks too difficult: use Bluetooth.

With the COVID crisis one explored option to discover who had contact with an infected person was to let people download an app that registers who came within bluetooth range. Bluetooth's limited range would let medical personel know who might have been in contact.

The problem with the system is that there wasnt a good way to tell range (or direction). For all the medical personel knew you could have passed each other on the freeway at max range or you could have been having an affair where you stuck your tongue down their throat.

Bluetooth, or a weaker/stronger version depending on the range you want, would be a solid option. No need for GPS, radar or similar. Just your phone telling you that there is another signal nearby.

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  • $\begingroup$ A simple and effective solution. I like it! $\endgroup$
    – Myisha
    Dec 5, 2020 at 18:24
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Yes, it exists on animals, ships and planes

They are respectively called echolocation and active radar systems. Why is that? These systems work by emitting sounds, light or electromagnetic waves and use the bounce back to find your prey or your opponent. However while these systems are really good at catching a target, they are emitters you can catch on your own system. so it's not ideal if you want to stay hidden. See in plane movies the alert the pilot always have when a missile close-in on them? Time to get counter-measures out. The flashlight you use to find a runaway? Pass by where they are not looking.

In case you want to sneak upon someone without them noticing, you use instead a passive system, like your good old eyes and ears, or systems to catch disturbations in the environment. Stealthier, but tends to be less accurate.

If you plan on making them more human-portable, you'd probably need to reduce the number of targets it can acquire to the competitors, typically by using a specific set of frequencies not used elsewhere in your arena. A bit like a radio station has its own frequency range, in some sort.

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    $\begingroup$ You would need a pretty sophisticated IFF system to pick up the three enemies out of the clutter of hundreds of neutrals $\endgroup$
    – AlexP
    Nov 27, 2020 at 23:37
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    $\begingroup$ @AlexP If they are planning this kind of competition, they would certainly make specific equipment that reduces the number of targets, like reducing the frequency range to one not used elsewhere. $\endgroup$
    – Tortliena
    Nov 27, 2020 at 23:45
  • $\begingroup$ @Tortliena Thank you, I kept getting stuck on thinking of this like a GPS system. I do think it would work like one in on the backend but since it does not give actual tracking features to the users then it does not truly function as one. I think it is more like a radar device as you've described! $\endgroup$
    – Myisha
    Nov 28, 2020 at 13:26
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    $\begingroup$ For simple presence detection, echolocation or radar are completely overkill. Active RFID works over warehouse ranges and can provide IFF easily $\endgroup$
    – Dragongeek
    Nov 29, 2020 at 9:27
  • $\begingroup$ @Dragongeek True enough; That's worth its own little answer, don't you think :)? $\endgroup$
    – Tortliena
    Nov 29, 2020 at 14:59
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They are in a competition with rules, right? So each of them gets a smartphone with GPS, linked to a smartwatch with GPS and an integrated heartbeat monitor.

  • The smartphone permanently tracks the box position by GPS. The phone also links by Bluetooth or a similar technology to the smartwatch.
  • The phone then transmits the phone position and the relative distance to the smartwatch to a control center by cellular phone.
  • The smartwatch also tracks the watch position by GPS and uploads it.
  • The control center calculates the map each participant should see and transmits it back to the phone.

According to the rules, putting the phone away is a legal move and blanks the watch position to the other participants (but not the observers in the control center). Putting the smartwatch away is not. If this is Hunger-Games-style, the smartwatch might be rather tough and mounted in a locked bracelet.

The control center can also tell if the participant is alive and moving or just dead.

Alternatively there is just one device, with a toggle to activate and deactivate stealth mode. That means one does not have to go back to the place where the phone was stashed to go active again.

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  • $\begingroup$ Thank you the suggestion. Sorry I just realized that I forgot to put that removal permanently damages the alert feature and that this device does technically act as a GPS locate for those using it just to alert them when other are using it are nearby. It good that you mentioned the heartbeat monitor. I was thinking that the competitors would be implanted with an internal heart monitor that also acts as an actual tracking device which is only visible to those monitoring the event. $\endgroup$
    – Myisha
    Nov 28, 2020 at 12:56
  • $\begingroup$ @Myisha, implanting it deeply would complicate removal, but solid steel around the wrist should do the job as well. Which contestant would dislocate a thumb to slip away from the monitors if that doesn't help survival? $\endgroup$
    – o.m.
    Nov 28, 2020 at 13:47
  • $\begingroup$ sorry for the confusion. The implant device is separate from the removable alert monitor. The monitor would be an external device possible like attached to the wrist. It is used by the competitions and can be removed but doing so damages and disables it permanently. The implant monitors the heart and gives exact GPS location for those monitoring the game so they can tell who is left and the exact location of the bodies so they can easily be retrieved. It is key that they are able to locate and retrieve the bodies for an important plot related conspiracy related to the competition. $\endgroup$
    – Myisha
    Nov 28, 2020 at 14:04
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The technology you are looking for is called beacons.

Bluetooth beacons are are already used for tracking patients in hospitals and such. Whatever electronics are inside could as easily detect other beacons in the vicinity. The latest BT specifications even allow for Angle of Attack - essentially allowing detection of the direction of source signal.

The disadvantage to this solution is low range - somewhere around 30m or so, the advantage is it being much cheaper.

The feature of turning it off for the remainder of the competition could easily be implemented using a button and appropriate firmware. Resetting it to default state would require the intervention of the organizers (who need special hardware to do it) or might be impossible (for example electrically destroying parts of the device).

One pitfall here is that anyone could take off their beacon to create a lure for a trap. Personally, I feel like it's a nice story point, but if you don't want that, you can as easily integrate ways to stop the "players" from this. For example using a heart rate sensor to detect heartbeat, maybe coupled with a capacitive sensor to detect skin (similar to how touch panels work) and throw in a tamper wire which detects when the bracelet is broken.

Most of this could easily be done using existing devices. You could, for example order Hexiwear and program it yourself to behave exactly that way.

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Nope, not RADAR or a beacon. Those are technologies that detect things or allow one to advertise one's location. But neither technology, by itself, performs a function that detects threats and notifies both parties. But such technology is common on commercial aircraft. It's called TCAS (Traffic Collision Avoidance System). TCAS allows participating aircraft to broadcast their location, and when the system detects that two aircraft are on a collision course it tells one pilot to go one way and the other pilot to go a different way so they avoid the collision. See https://en.wikipedia.org/wiki/Traffic_collision_avoidance_system

In your world, the personal TCAS would alert the both parties to the presence of the other. Depending on the type of beacon you "invent", the parties might be told precisely where the threat is. Or one user might switch his beacon to "presence only" and his opponent would only know of his presence but not his precise location and movement.

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