An adruino car person follower

I am trying to make a simple toy car which will follow me around.

I already know how to make the car itself and to use the sonar to avoid collisions.

But how can I make the car follow an external device (the car follows a transmitter device, it doesn't need to recognize with a camera anything).

I only need it to work in small places (10 - 15 meter radius).

I am looking for advice and reference to sources about parts I may require.
(What type of transmitter should I have, and which receiver should be on the car).

Has google told you anything about your project?
I googled "arduino follow me" and found many hits.