Sensors for Human Following Robot

what are sensor that can be used to make a human following robot.my team and i decide to make human following trolley.so what sensor that can be used to make it. is there a sensor that have a transmit and receive to make sure it will follow where ever the human goes??

MuhdAsri:
what are sensor that can be used to make a human following robot

This guy here has an ultrasound transmitter on the target, and 2x receivers on the robot. Radio on each synchs the time of ultrasound transmit, and the idea is the slightly different arrival time of the ultrasound at the 2x receivers will give a slightly different distance. That tells him if the target is to the left or right, and motors are driven to turn to the closer side.

Last I read, he hasn't got it working yet.