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I would like to buy a Bluetooth distance tracker so that whenever my laptop is more than n feet distant from my Android phone the phone rings. I do not need GPS location, crowd finding etc.

Is today Bluetooth technology ready for this?
I read some Amazon reviews and it seems that, since Bluetooth connection is not stable, phone apps beep randomly due to Bluetooth signal drops or, even worse, there is long delay before the phone alerts you have left your stuff behind, so that you are warned when you have already got off the train, plane etc.

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Well yes, but actually no

From a software standpoint, I personally think the polish for Bluetooth location trackers has reached a point that it can be considered mature enough for "everyday" use. I personally don't have a ton of experience with them, but from what I have done and do know, they're in a good place.

The real problem is that all of these Bluetooth trackers use what's known as Received Signal Strength Indicator (RSSI.) Basically what this does is measure the signal strength and (along with some fancy things), uses the basic assumption that signal strength falls off as a function of distance to approximate the distance away. This is why an unstable (or too stable) connection or random signal drops due to interference leads to poor performance. This is inherent to the way that these trackers works.

There's a better way of tracking though: Time of Flight (ToF.) Since signals travel at a known speed (~the speed of light), this property can be used to accurately track distance, independent of signal strength (and therefore independent of any interference.) This is the way that GPS, 802.11v, and many other technologies work. If you lose your Apple Watch (if it's not GPS equipped), it can relatively accurately locate itself.

However, Bluetooth is fundamentally incapable of accurately using this technology. The problem is the Bluetooth bandwidth. Bluetooth operates between 2.402 GHz and 2.4800 GHz, (plus a 2 MHz bottom and 3.5 MHz top guard band), split into 79 channels of 1 MHz each. Speed of light / 1 MHz ≈ 300m (it's slightly more complicated than this, but close enough for our purposes.) This is so ridiculously inaccurate as to be completely impractical. Of course, it is possible to do better by averaging multiple runs and some of the newer features of Bluetooth, as done in this paper, but the fundamental restrictions of Bluetooth remain in place. With infinite time, would the law of large numbers end up getting you pretty good data? Sure. But at a more reasonable number of samples? Well, it's not really any better than RSSI. And at that point, why bother? Getting the RSSI value is so trivially easy that a middle schooler can (and has) done it. And if you're creating a product, how many people are really going to go through the trouble of, say, posting a question on a hardware recommendation forum? How many people who read an Amazon listing will know the difference between RSSI and ToF, much less care?

It's not like good location technology is out of reach anyways though. Like I said before, the Apple Watch simply uses more bandwidth, and thus can locate itself more accurately. It's not really a hard problem to solve from a technical standpoint. The issue is one of adoption: we've all got convenient little Bluetooth gadgets, everyone knows what they are, the market is small, how much do people really want to push for a better (consumer grade, at least) locator beacon?

Thus we find ourselves in the current state of affairs. Yes, they're polished, finished, marketable, and actually sometimes quite useful products. There is value to having them—especially if you just want to know if you're even in the same room as your wallet or keys. They're just not the magical locator beacon you might want.

Anyways, so glad someone asked this question! I did some work on this a number of years ago, and it feels good to share it to wider audience. It's also good to see new research is still coming out and pushing the boundaries of what we thought was possible.

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Tile's "Smart Alert" premium service can achieve the desired outcome. Trackr also has a similar capability called "Separation Alerts" which I believe is free.

Tile: https://tileteam.zendesk.com/hc/en-us/articles/360009109234-Keep-Calm-with-Smart-Alerts

Trackr: https://www.thetrackr.com/trackr-faq/ (Ctrl+F, search for separation)

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  • Doesn't the Tiles use both GPS and Bluetooth? I'm not fully sure how those work, and that's what I thought they did. I could be wrong tho. – Trevor Hummer Jan 28 at 17:18
  • The Tiles use Bluetooth to pair to your phone. They do have GPS, but it's just a backup for devices that are actually lost versus hiding somewhere in your house. So when I cannot find my TV remote, my Tile Sticker uses bluetooth to establish a connection to my phone and begins chirping. If I cannot find my phone in my house, I can press the button on the tile, which establishes the bluetooth connection, and then my phone rings. But if I leave my wallet at the grocery, the Tile app lets me use GPS to narrow my Tile Slim's last known location. – Evan Jan 28 at 18:41
  • As I understand, I buy a Bluetooth sticky gadget to be paired with my phone (much like a Bluetooth speaker). When my phone is too far and loses the connection with the paired gadget, it beeps (much like when the far speaker stops playing). Why do I need to have and pay a distinct subscription service to have my phone beeping on lost connections? – antonio Jan 29 at 22:48
  • @antonio I agree, it is a bit ridiculous that Tile charges for their Smart Alerts. Luckily Trackr's version called Separation Alerts are free. So if you decide to go with that manufacturer, your only expense will be the purchase of the bluetooth tracker. I personally use my Tile just to find my devices in my house, so not having access to Smart Alerts is not a big deal for me. But if I needed that feature, paying the subscription would be a non-option, I would just switch to Trackr. If you decide to give the Trackr a try, let me know your thoughts on it :) – Evan Jan 30 at 14:22

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