Get Distances between mobile phone and object in real world

Hi guys! I am working on an enterprise which needs something a bit different as mobile App. They want an application who could measure the distance between you, holding the mobile phone and running Unity of course, and a object placed in real world. But without focusing the camera/mobile phone to the object, cause the idea is to do your job and get the distance at the same time.
I have tried with GPS, but they want a precission of 0.5m, so it doesnt work very well. And i have tried with AR but they need markers, and that means that you have to focus the camera/mobile phone to the object…
Any idea for this problem? Thank you so much and sorry for my english.

Why can’t you focus the camera? You don’t need to render the camerafeed to display, you could just run the camera in continuous focus mode (using vuforia which this sounds like) and still find your tracker and calculate the distance.

Yes, i think i will need bluetooth or wifi to get what we want. Basically the situation is the following one: one employee in an area, working with both hands in a nuclear plant (so cant hold the camera to focus anything), and some points in that area that emit radioactivity. So we need to check continuously the distance between the employee and those points.
The problem is that i dont know how to program the app to do that (connect two devices, or a device with other object with wifi or bluetooth).
I mean other object, cause we can put a beacon or any receptor on those points that emit radioactivity.
Thank you for ur answers guys, hope you can help me ^^

Humm, i will check all the thing you have said. Many thanks, hope we can build our app! Would be great, thanks again.