Hi there, As currently work for company that builds mobile apps I am looking into a way to measure whether a smartphone is outside a 5 meter range. As I am not an engineer I hope your expertise can help me out in order to come up with a right direction for further research. NFC, RFID, GPS, orthogonal coils, wifi ? I don't know where to begin :) Considerations: - Solution should be affordable (B2C) and scalable - 5 meter ranges should also be accurately determined through 1 or 2 normal size walls - Sender and receiver could be anything Application: This solution will be used for a client that want to use people's smartphones to automatically block scooter usage. I know smart car keys do something similar, can we use this technology in a phone? Hope this is something you guys can have a thought about!? Thanks in advance!