How does return a robot vacuumcleaner to his station?

Hi,
I'm wondering how does return a robot vacuum cleaner to his station?
does it use sound sensors, GPS ??

I can't find in google.

The following question is of course how could I make that with arduino.
Thanks.

From howstuffworks.com

If you have the self-charger, Roomba will return and connect to the charger all by itself when the battery power is low (the self-charger is sold as an add-on to the Roomba base model but comes included on most of the higher Discovery models). It accomplishes this using the infrared receiver on its front bumper. When the battery power gets low, the vacuum starts looking for the infrared signal emitted by the charger. Once it finds it, Roomba follows the signal and docks itself to the charger. Some robotic vacuums with this self-charging feature will head back out to resume cleaning once they're fully recharged.

I answered one of your questions, Think on the other by yourself and ask us if something goes wrong.

Greetings,
-Malhar

Denree:
I can't find in google.

I can easily find with Google how it works with German MIELE cleaning robots, this are a bit more expensive ones. They take several times per minute a picture from the ceiling, using a digital camera. And from the change between picture and next picture and next picture the robot decides for his position in the rom by picture processing from the different digital pictures taken between movements.