How can you tell if two phones have come close to each other? [closed]

I'm trying to implement a mobile phone app that raises a flag if two users come close to each other in the physical world.

I'm looking for a solution that does not require the apps to be actively open on the users phone to be able to make this determination and raise the flag.

Is this possible? Perhaps by bluetooth, or perhaps by using GPS & time data uploaded from each phone and have server determine if the users were sufficiently close together?

I'm looking for accuracy within <20ft.

https://stackoverflow.com/questions/65385947/how-can-you-tell-if-two-phones-have-come-close-to-each-other December 21, 2020 at 07:47AM