CalinTamaian:
You could use 4 ultrasonic sensors ( one on top, one behind and one on each side). The ultrasonic sensors will report the distance to the first object. Depending on the readings you can triangulate the position. GPS works pretty much the same but at bigger scale.
Thanks for your reply.
The precision of ultrasonic module is ideal. The concern is that the ultrasonic module is very directional, like the infrared sensor. Perhaps the ultrasonic module is wider than the infrared sensor. It is still necessary to have many in order to have the good accuracy, since the space is about 8m x 5m.
Maybe the only solution is RSSI with XBee? I am now wondering, if the Xbees can determine the position through Trilateration :
http://forum.arduino.cc/index.php?topic=196681.0
Any kinds of your information or experiences would be appreciated.