Dead reckoning of phone trajectory using inertial sensor (of the phone) + gps corrections ?

I don't see a dedicated camera on the drone (using gopro for machine vision is not ideal), nor a beefy cpu to handle person tracking onboard, but I could be mistaken.

You can get centimeter accuracy with gps. http://swift-nav.com/piksi.html Even with cheaper modules and phones that dont carrier phase rtk, you can get meter which would be plenty for this application.