Find other phone with the same app

Hello every ionic fans,
I have to create a social application IOS, Android and Windows Phone which should identify other applications that are within 250m.
And it should work even when the application is not launched.
I see how bluetooth, or else everything must be done on the server side, but there are other ways?
How to have the position of the phone if the application is not launched?
Thank you in advance for your help.