Robot following person

Hi, I am working on a project , so that it can follow the person that is specified. So i want some help about that, what sensors should i use that are not too much expensive. Also i am not expert so i want suggestions which are at intermediate level.

Note: that the robot will follow one single person among the croud etc.


This has been answered in the last few days.

The answer is that it is not practical project, engineers have spend a lot of time and huge amounts of money to do it.