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• BLE (Bluetooth low energy) is mostly found in beacon based physical application.
• This Bluetooth beacon is used to manage indoor positioning system (the opposite of global positioning system (GPS)).

• There is this proximity beacon that is developer friendly, named Estimote http://estimote.com/.
• There is also this GitHub system to get indoor positioning for most WiFi enabled device (including Raspberry PI), https://github.com/schollz/find. I think this is the most relevant way to achieve proximity detection for my Sociometric Badge for now.
• Apple has a solution for this indoor positioning system called iBeacon. This was released in 2013 yet I never heard about this before, https://en.wikipedia.org/wiki/IBeacon.

• Here is an Apple official documentation about iBeacon, https://developer.apple.com/ibeacon/.
• There is this YouTube video that I got from a Reddit that thread I made, https://www.youtube.com/watch?v=Dw_7Q3LGuOI. Here is a link to my Reddit thread asking for a way to determine distance between two Raspberry PIs, https://www.reddit.com/r/raspberry_pi/comments/5ayn6q/hello_i_need_recommendation_on_detecting/.