Following Robot

I am trying to get a robot to follow a human using only ultrasonic sensors. I trying to send out a signal from the robot and have a reciever on the human get the signal and then send out a reply signal to the robot. how can i encode the signal from the human so that the robot knows that the signal came from the human? Does anyone know of good ultrasonic sensors to use?

morse code , rather than just ping the output once and wait for a reply
send a series of output pulses with different intervals that uniquely identifies the bot.