Does anybody know of any 3rd party RFID sensors that can determine the distance of signal from a particular card. This would seem to be an easy thing to design but I did not know how possible and how accurate. I have seen a device that can alert if a RFID device has gone beyond a 30 meter or 100 meter perimeter but I would want it to be 6" or better. Also I have noticed that there is some issue with directionality of the sensors, which could give false readings. Any thoughts?